Scientific Publications
by Hurricane Research Division personnel
2024
Aksoy, A. A Monte Carlo approach to understanding the impacts of initial-condition uncertainty, model uncertainty, and simulation variability on the predictability of chaotic systems: Perspectives from the one-dimensional logistic map. Chaos, 34(1):011102, https://doi.org/10.1063/5.0181705 2024
The predictability of the logistic map is investigated for the joint impact of initial-condition (IC) and model uncertainty (bias + random variability) as well as simulation variability. To this end, Monte Carlo simulations are carried out where IC bias is varied in a wide range of 10−15–10−3, and, similarly, model bias is introduced in comparable range. It is found that while the predictability limit of the logistic map can be continuously extended by reducing IC bias, the introduction of the model bias imposes an upper limit to the predictability limit beyond which further reductions in IC bias do not lead to an extension in the predictability limit, effectively restricting the feasible joint space spanned by the IC-model biases. It is further observed that imposing a lower limit to the allowed variability among ensemble solutions (so as to prevent the ensemble variability from collapse) results in a similar constraint in the joint IC-model-bias space; but this correspondence breaks down when the imposed variability limit is too high ( ∼>0.7 for the logistic map). Finally, although increasing the IC random variability in an ensemble is found to consistently extend the allowed predictability limit of the logistic map, the same is not observed for model parameter random variability. In contrast, while low levels of model parameter variability have no impact on the allowed predictability limit, there appears to be a threshold at which an abrupt transition occurs toward a distinctly lower predictability limit.
Alaka, G.J. Jr., J.A. Sippel, Z. Zhang, H.-S Kim, F. Marks, V. Tallapragada, A. Mehra, X. Zhang, A. Poyer, and S.G. Gopalakrishnan. Lifetime performance of the operational Hurricane Weather Research and Forecasting model (HWRF) for North Atlantic tropical cyclones. Bulletin of the American Meteorological Society, 105(6):E932-E961, https://doi.org/10.1175/BAMS-D-23-0139.1 2024
The Hurricane Weather Research and Forecasting (HWRF) model was the flagship hurricane model at NOAA’s National Centers for Environmental Prediction for 16 years and a state-of-the-art tool for tropical cyclone (TC) intensity prediction at the National Weather Service and across the globe. HWRF was a joint development between NOAA research and operations, specifically the Environmental Modeling Center and the Atlantic Oceanographic and Meteorological Laboratory. Significant support also came from the National Hurricane Center, Developmental Testbed Center, University Corporation for Atmospheric Research, universities, cooperative institutes, and the TC community. In the North Atlantic basin, where most improvement efforts focused, HWRF intensity forecast errors decreased by 45-50% at many lead times between 2007 and 2022. These large improvements resulted from increases in horizontal and vertical resolution, as well as advances in model physics and data assimilation. HWRF intensity forecasts performed particularly well over the Gulf of Mexico in recent years, providing useful guidance for a large number of impactful landfalling hurricanes. Such advances were made possible not only by significant gains in computing, but also through a substantial investment from the Hurricane Forecast Improvement Program.
Alford, A.A., B. Schenkel, S. Hernandez, J.A. Zhang, M.I. Biggerstaff, E. Blumenauer, T.N. Sandmæl, and S.M. Waugh. Examining outer band supercell environments in landfalling tropical cyclones using ground-based radar analyses. Monthly Weather Review, 152(10):2265-2285, https://doi.org/10.1175/MWR-D-23-0287.1 2024
Supercells in landfalling tropical cyclones (TCs) often produce tornadoes within 50 km of the coastline. The prevalence of TC tornadoes near the coast is not explained by the synoptic environments of the TC, suggesting a mesoscale influence is likely. Past case studies point to thermodynamic contrasts between ocean and land or convergence along the coast as a possible mechanism for enhancing supercell mesocyclones and storm intensity. This study augments past work by examining the changes in the hurricane boundary layer over land in the context of vertical wind shear. Using ground-based single- and dual-Doppler radar analyses, we show that the reduction in the boundary layer wind results in an increase in vertical wind shear/storm-relative helicity inland of the coast. We also show that convergence along the coast may be impactful to supercells as they cross the coastal boundary. Finally, we briefly document the changes in mesocyclone vertical vorticity to assess how the environmental changes may impact individual supercells.
Alvey, G.R. III, G.J. Alaka Jr., L. Gramer, and A. Hazelton. Evaluation of Hurricane Analysis and Forecast System (HAFS) error statistics stratified by internal structure and environmental metrics. Weather and Forecasting, https://doi.org/10.1175/WAF-D-24-0030.1 2024
This study quantifies tropical cyclone (TC) error statistics from the Hurricane Analysis and Forecast System (HAFS) across different environmental conditions (e.g., vertical wind shear) and inner core structural metrics. A particular focus is the evolution of poorly understood aspects of internal TC structure, including vortex tilt, and their impact on forecast errors. Although previous studies have demonstrated that vortex tilt, vertical wind shear, and precipitation processes impact TC intensity and track, this is the first known study to stratify these cooperative interactions to gain insights on their relationships with forecast errors. A three-year retrospective sample of forecasts in the North Atlantic basin from two HAFS configurations (-A and -B) demonstrates that TCs with larger tilt magnitudes have larger forecast track errors on average than smaller tilt TCs. Smaller tilt magnitudes have larger absolute intensity errors in short range forecasts, whereas larger tilt magnitudes tend to have larger negative intensity biases at medium range. TCs with a tilted vortex are shown to have both left of shear (maximizing DSL) and left of tilt oriented positional track biases. Furthermore, those cases with greater downshear biases tend to have more convection and larger positive intensity biases, highlighting the importance of the interplay between inner core characteristics and forecast errors.
Annane, B., and L.J. Gramer. Influence of CyGNSS L2 wind data on tropical cyclone analysis and forecasts in the coupled HAFS/HYCOM system. Frontiers in Earth Science, 12:1418158, https://doi.org/10.3389/feart.2024.1418158 2024
This study examines the influence of NASA Cyclone Global Navigation Satellite System (CyGNSS) Level 2-derived 10 m (near-surface) wind speed over the ocean on analyses and forecasts within the NOAA operational Hurricane Analysis and Forecast System (HAFS). HAFS is coupled with a regional configuration of the HYCOM ocean model. The primary advantages of data from the CyGNSS constellation of satellites include higher revisit frequency compared to polar-orbiting satellites, and the availability of reliable wind observations over the ocean surface during convective precipitation. CyGNSS data are available early in the life cycle of tropical cyclones (TCs) when aerial reconnaissance observations are not available. We focus on TCs whose forecasts were initialized when the TC was a depression or tropical storm. In the present study, we find first, that assimilation of CyGNSS near-surface winds improves storm track, intensity, and structure statistics in the analysis and early in the forecast, for many cases. Second, we find that assimilation of CyGNSS observations provides additional insights into the evolution of air-sea interaction in intensifying TCs: In effect, the ocean responds in the coupled model to modifications in the initial 10 m wind field, thereby impacting forecasts of intensity, storm structure, and sea surface height, as demonstrated by two case studies. We also discuss some forecasts where assimilating CYGNSS appears to degrade performance for either intensity or structure.
Bower, E., K.A. Reed, G.J. Alaka Jr., and A.T. Hazelton. Verification of operational forecast models in cases of extratropical transition of North Atlantic hurricanes. Weather and Forecasting, 39(11):1695-1714, https://doi.org/10.1175/WAF-D-24-0011.1 2024
Operational forecast models are necessary for the prediction of weather events in real time. Verification of these models must be performed to assess model skills and areas in need of improvement, particularly with different types of weather events that may occur. Despite the devastating impacts that can be caused by tropical cyclones (TCs) that undergo extratropical transition (ET) and become post-tropical cyclones (PTCs), these storms have not been extensively studied in the context of short-term weather prediction. This study completes the first analysis of the Global Forecast System (GFS) and a preoperational version of the newly operational Hurricane Analysis and Forecast System (HAFS) models in forecasting the occurrence of ET and the rainfall associated with ET storms in the North Atlantic basin. GFS’s skill exceeds that of HAFS in forecasting the occurrence of ET, but HAFS tends to have lower track and rain-rate errors in the fully tropical phase of ET storms’ life cycles. Both models simulate rain rates that are often too high near the storm center and fail to capture the larger area of moderate rain rates that greatly contributes to total rainfall accumulation. The discrepancies in rain rates between the models and Integrated Multi-satellitE Retrievals for GPM (IMERG) could be attributed to the models’ tendency to keep storms too intense and too compact with an overly strong warm core, even throughout the ET process.
Chen, X., and F.D. Marks. Parameterizations of boundary layer mass fluxes in high-wind conditions for tropical cyclone simulations. Journal of the Atmospheric Sciences, 81(1):77-91, https://doi.org/10.1175/JAS-D-23-0086.1 2024
Development of accurate planetary boundary layer (PBL) parameterizations in high-wind conditions is crucial for improving tropical cyclone (TC) forecasts. Given that Eddy-Diffusivity Mass-Flux (EDMF)-type PBL schemes are designed for non-hurricane boundary layers, this study examines the uncertainty of MF parameterizations in hurricane conditions by performing three-dimensional idealized simulations. Results show that the surface-driven MF plays a dominant role in the nonlocal turbulent fluxes and is comparable to the magnitude of downgradient momentum fluxes in the middle portion of TC boundary layers outside the radius of maximum wind (RMW); in contrast, the stratocumulus-top-driven MF is comparably negligible and exerts a marginal impact on TC simulations. To represent the impact of vertical wind shear on damping rising thermal plumes, a new approach of tapering surface-driven MF based on the surface stability parameter is proposed, aiming to retain the surface-driven MF only in unstable boundary layers. Compared to a traditional approach of MF tapering based on 10-m wind speeds, the new approach is physically more appealing as both shear and buoyancy forcings are considered and the width of the effective zone responds to diurnal variations of surface buoyancy forcing. Compared to the experiments retaining the original MF components, using either approach of MF tapering can lead to a stronger and more compact inner core due to enhanced boundary layer inflow outside the RMW; nevertheless, the radius of gale-force wind and inflow layer depth are only notably reduced using the new approach. Comparison to observations and further discussions on MF parameterizations in high-wind conditions are provided.
Chiodi, A.M., H. Hristova, G.R. Foltz, J.A. Zhang, C.W. Mordy, C.R. Edwards, C. Zhang, C. Meinig, D. Zhang, E. Mazza, E.D. Cokelet, E.F. Burger, F. Bringas, G. Goni, H.-S. Kim, S. Chen, J. Trinanes, K. Bailey, K.M. O’Brien, M. Morales-Caez, N. Lawrence-Slavas, S.S. Chen, and X. Chen. Surface ocean warming near the core of Hurricane Sam and its representation in forecast models. Frontiers in Marine Science, 10:1297974, https://doi.org/10.3389/fmars.2023.1297974 2024
On September 30, 2021, a saildrone uncrewed surface vehicle intercepted Hurricane Sam in the northwestern tropical Atlantic and provided continuous observations near the eyewall. Measured surface ocean temperature unexpectedly increased during the first half of the storm. Saildrone current shear and upper-ocean structure from the nearest Argo profiles show an initial trapping of wind momentum by a strong halocline in the upper 30 m, followed by deeper mixing and entrainment of warmer subsurface water into the mixed layer. The ocean initial conditions provided to operational forecast models failed to capture the observed upper-ocean structure. The forecast models failed to simulate the warming and developed a surface cold bias of ~0.5°C by the time peak winds were observed, resulting in a 12-17% underestimation of surface enthalpy flux near the eyewall. Results imply that enhanced upper-ocean observations and, critically, improved assimilation into the hurricane forecast systems, could directly benefit hurricane intensity forecasts.
Dorst, N.M. Before the hurricane hunters: Storm patrols and the lost hurricanes. Weatherwise, 77(1):42-52, https://doi.org/10.1080/00431672.2024.2276648 2024
Earl-Spurr, C., S. Langlade, D. Krahenbuhl, S.D. Aberson, M. Brunet, J. Chan, C. Fogarty, C.W. Landsea, B. Trewin, C. Velden, R.C. Balling, and R.S. Cerveny. New WMO certified tropical cyclone duration extreme: TD Freddy (04 February to 14 March 2023) lasting for 36.0 days. Bulletin of the American Meteorological Laboratory, https://doi.org/10.1175/BAMS-D-24-0071.1 2024
A World Meteorological Organization team has evaluated 2023's Tropical Cyclone Freddy's duration of 36.0 days (with 10-min average wind-speeds of 30 kt or higher) as the world record for longest tropical cyclone duration.
Fischer, M.S., R.F. Rogers, P.D. Reasor, and J.P. Dunion. An observational analysis of the relationship between tropical cyclone vortex tilt, precipitation structure, and intensity change. Monthly Weather Review, 152(1):203-225, https://doi.org/10.1175/MWR-D-23-0089.1 2024
This study uses a recently-developed airborne Doppler radar database to explore how vortex misalignment is related to tropical cyclone (TC) precipitation structure and intensity change. It is found that for relatively weak TCs, defined here as storms with a peak 10-m wind of 65 kt or less, the magnitude of vortex tilt is closely linked to the rate of subsequent TC intensity change, especially over the next 12–36 h. In strong TCs, defined as storms with a peak 10-m wind greater than 65 kt, vortex tilt magnitude is only weakly correlated with TC intensity change. Based on these findings, this study focuses on how vortex tilt is related to TC precipitation structure and intensity change in weak TCs. To illustrate how the TC precipitation structure is related to the magnitude of vortex misalignment, weak TCs are divided into two groups: small-tilt and large-tilt TCs. In large-tilt TCs, storms display a relatively large radius of maximum wind, the precipitation structure is asymmetric, and convection occurs more frequently near the mid-tropospheric TC center than the lower-tropospheric TC center. Alternatively, small-tilt TCs exhibit a greater areal coverage of precipitation inward of a relatively small radius of maximum wind. Greater rates of TC intensification, including rapid intensification, are shown to occur preferentially for TCs with greater vertical alignment and storms in relatively favorable environments.
Fung, K.Y., Z.-L. Yang, A. Martilli, E.S. Krayenhoff, and D. Niyogi. Prioritizing social vulnerability in urban heat mitigation. PNAS Nexus, 3(9):pgae360, https://doi.org/10.1093/pnasnexus/pgae360 2024
We utilized city-scale simulations to quantitatively compare the diverse urban overheating mitigation strategies, specifically tied to social vulnerability and their cooling efficacies during heatwaves. We enhanced the Weather Research and Forecasting model to encompass the urban tree effect and calculate Universal Thermal Climate Index for assessing thermal comfort. Taking Houston, Texas, U.S. as an example, the study reveals that equitably mitigating urban overheat is achievable by considering the city’s demographic composition and physical structure. Study results show that while urban trees may yield less cooling impact (0.27 K of Universal Thermal Climate Index in daytime) relative to cool roofs (0.30 K), the urban trees strategy can emerge as an effective approach for enhancing community resilience in heat stress-related outcomes. Social vulnerability-based heat mitigation was reviewed as Vulnerability-Weighted Daily Cumulative Heat Stress Change. Results underscore: (i) importance of considering the community resilience when evaluating heat mitigation impact, and (ii) the need to assess planting spaces for urban trees, rooftop areas, and neighborhood vulnerability when designing community-oriented urban overheating mitigation strategies.
Fung, K.Y., Z.-L. Yang, and D. Niyogi. Capturing urban heterogeneity enhances tropical cyclones simulation in Houston. Journal of Geophysical Research-Atmospheres, 129(19):e2024JD040807, https://doi.org/10.1029/2024JD040807 2024
Urbanization in the coastal region has increased socioeconomic losses from landfalling tropical cyclones (TCs). Although previous studies have explored the broad impacts of urbanization on TCs, the effect of heterogeneity caused by local intra-urban variability has not been examined. To address this gap, this study utilized the urban Local Climate Zone (LCZ) to capture the urban landscape heterogeneity and the impacts of this representation on the simulation of the post-landfall TCs characteristics in Houston. Taking the case of two recent TCs: Hurricane Harvey (occurred in 2017) and Tropical Storm Imelda (occurred in 2019), the study evaluated the impact of urban heterogeneity on 10-m winds, 2-m temperature, land surface temperature, and precipitation across Houston. The consideration of intra-urban heterogeneity using LCZ can improve the 10-m winds, 2-m temperature, and spatial pattern of land surface temperature. Although the cumulative rainfall remained largely similar within the experiments (with and without LCZ), incorporating intra-urban heterogeneity notably modified the spatial structure of urban rainfall, particularly in simulating heavy rainfall hot spots. These findings are consistent for both TCs and demonstrate the positive impact of incorporating intra-urban heterogeneity on the landfalling TC simulations over the Houston area.
Gramer, L.J., J. Steffen, M. Aristizabal, and H.-Y. Kim. The impact of coupling a dynamic ocean in the Hurricane Analysis and Forecast System. Frontiers in Earth Science, 12;1418016, https://doi.org/10.3389/feart.2024.1418016 2024
Coupling a three-dimensional ocean circulation model to an atmospheric model can significantly improve forecasting of tropical cyclones (TCs). This is particularly true of forecasts for TC intensity (maximum sustained surface wind and minimum central pressure), but also for structure (e.g., surface wind-field sizes). This study seeks to explore the physical mechanisms by which a dynamic ocean influences TC evolution, using an operational TC model. The authors evaluated impacts of ocean-coupling on TC intensity and structure forecasts from NOAA’s Hurricane Analysis and Forecast System v1.0 B (HFSB), which became operational at the NOAA National Weather Service in 2023. The study compared existing HFSB coupled simulations with simulations using an identical model configuration in which the dynamic ocean coupling was replaced by a simple diurnally varying sea surface temperature model. The authors analyzed TCs of interest from the 2020–2022 Atlantic hurricane seasons, selecting forecast cycles with small coupled track-forecast errors for detailed analysis. The results show the link between the dynamic, coupled ocean response to TCs and coincident TC structural changes directly related to changing intensity and surface wind-field size. These results show the importance of coupling in forecasting slower-moving TCs and those with larger surface wind fields. However, there are unexpected instances where coupling impacts the near-TC atmospheric environment (e.g., mid-level moisture intrusion), ultimately affecting intensity forecasts. These results suggest that, even for more rapidly moving and smaller TCs, the influence of the ocean response to the wind field in the near-TC atmospheric environment is important for TC forecasting. The authors also examined cases where coupling degrades forecast performance. Statistical comparisons of coupled versus uncoupled HFSB further show an interesting tendency: high biases in peak surface winds for the uncoupled forecasts contrast with corresponding low biases, contrary to expectations, in coupled forecasts; the coupled forecasts also show a significant negative bias in the radii of 34 kt winds relative to National Hurricane Center best track estimates. By contrast, coupled forecasts show very small bias in minimum central pressure compared with a strong negative bias in uncoupled. Possible explanations for these discrepancies are discussed. The ultimate goal of this work will be to enable better evaluation and forecast improvement of TC models in future work.
Hazelton, A., X. Chen, G.J. Alaka Jr., G.R. Alvey III, S. Gopalakrishnan, and F.D. Marks. Sensitivity of HAFS-B tropical cyclone forecasts to planetary boundary layer and microphysics parameterizations. Weather and Forecasting, 39(4):655-678, https://doi.org/10.1175/WAF-D-23-0124.1 2024
Understanding how model physics impact tropical cyclone (TC) structure, motion, and evolution is critical for the development of TC forecast models. This study examines the impacts of microphysics and planetary boundary layer (PBL) physics on forecasts using the Hurricane Analysis and Forecast System (HAFS), which is newly operational in 2023. The “HAFS-B” version is specifically evaluated, and 3 sensitivity tests (for over 400 cases in 15 Atlantic TCs) are compared with retrospective HAFS-B runs. Sensitivity tests are generated by 1) Changing the microphysics in HAFS-B from Thompson to GFDL, 2) turning off the TC-specific PBL modifications that have been implemented in operational HAFS-B, and 3) combining the PBL and microphysics modifications. The forecasts are compared through standard verification metrics, and also examination of composite structure. Verification results show that Thompson microphysics slightly degrades the Day 3-4 forecast track in HAFS-B, but improves forecasts of long-term intensity. The TC-specific PBL changes lead to a reduction in a negative intensity bias and improvement in RI skill, but cause some degradation in prediction of 34-knot wind radii. Composites illustrate slightly deeper vortices in runs with the Thompson microphysics, and stronger PBL inflow with the TC-specific PBL modifications. These combined results demonstrate the critical role of model physics in regulating TC structure and intensity, and point to the need to continue to develop improvements to HAFS physics. The study also shows that the combination of both PBL and microphysics modifications (which are both included in one of the two versions of HAFS in the first operational implementation) leads to the best overall results.
Kang, S.K., E.J. Kim, S. Kim, J. Cione, D. Lee, S. Landwehr, H.-W. Kang, K.-O. Kim, C.S. Hong, M.H. Kwon, K.H. Oh, J.H. Lee, S. Noh, J.K. So, D.-J. Kang, D. Kim, J.-H. Park, S. Nam, Y.K. Cho, B. Ward, and I. Ginis. Anomalously large latent heat fluxes in low to moderate wind conditions within the eddy-rich zone of the northwestern Pacific. Frontiers in Marine Science, 11:1298641, https://doi.org/10.3389/fmars.2024.1298641 2024
An air-sea interaction field campaign was conducted in September of 2017 within the warm and cold eddy region of the western Northwest Pacific (WNP) (17.5°-20.5°N, 127.5°E-133.5°E). Both near-surface oceanic and atmospheric conditions in addition to ocean heat content (OHC) were examined to better understand the mechanisms governing high heat flux magnitudes responsible for rapidly intensifying tropical cyclones. Observations from this experiment indicate that the latent heat flux (LHF) under modest wind conditions reached 400 W m-2 within the vicinity of a warm eddy, with OHC higher than 100 kJ cm-2 of warm eddy region being 2-3 times as large as that of cold eddy region. These high OHC and a resultant high LHF in the warm eddy, comparable to the magnitude of LHF in the North Equatorial Current, may explain the mechanism of why tropical cyclones over a warm eddy in eddy-rich zones often rapidly intensify in the WNP. A month later typhoon Lan rapidly intensified into a super typhoon, while passing over the boundary region of warm and cold eddies during the observation period. Results from this study illustrate that both the wind-normalized LHF and the difference (Qs-Qa) between the specific humidity at air (Qa) and at the sea surface (Qs), closely correlate with OHC patterns, which suggests that the ocean looks likely to control the spatial pattern of LHF. Overall, both the ocean and weather conditions govern the pattern of specific humidity difference between the air-sea interface, with large OHC over the warm eddy controlling higher Qs and the pattern of Qa depending on the pattern of wind direction. Qa as a factor impacting LHF magnitude is strongly linked to wind direction in the experimental area, that is, the drier northwesterly flow and southeasterly moist wind, resulting in the enhanced contrast of specific humidity at cold eddy region.
Kang, S.K., S.-H. Kim, I.-I. Lin, Y.-H. Park, Y. Choi, I. Ginis, J. Cione, J.Y. Shin, E.J. Kim, K.O. Kim, H.W. Kang, J.-H. Park, J.-R. Bidlot, and B. Ward. The North Equatorial Current and rapid intensification of super typhoons. Nature Communications, 15:1742, https://doi.org/10.1038/s41467-024-45685-2 2024
Super Typhoon Mangkhut, which traversed the North Equatorial Current (NEC; 8–17°N) in the western North Pacific in 2018, was the most intense Category-5 tropical cyclone (TC) with the longest duration in history—3.5 days. Here we show that the combination of two factors—high ocean heat content (OHC) and increased stratification — makes the NEC region the most favored area for a rapid intensification (RI) of super typhoons, instead of the Eddy Rich Zone (17–25°N), which was considered the most relevant for RI occurrence. The high OHC results from a northward deepening thermocline in geostrophic balance with the westward-flowing NEC. The stratification is derived from precipitation associated with the Inter-Tropical Convergence Zone in the summer peak typhoon season. These factors, which are increasingly significant over the past four decades, impede the TC-induced sea surface cooling, thus enhancing RI of TCs and simultaneously maintaining super typhoons over the NEC region.
Kim, H.-S., B. Liu, B. Thomas, D. Rosen, W. Wang, A. Hazelton, Z. Zhang, X. Zhang, and A. Mehra. Ocean component of the first operational version of Hurricane Analysis and Forecast System: Evaluation of HYbrid Coordinate Ocean Model and hurricane feedback forecasts. Frontiers in Earth Science, 12:1399409, https://doi.org/10.3389/feart.2024.1399409 2024
The first operational version of the coupled Hurricane Analysis and Forecast System (HAFSv1) launched in 2023 consists of the HYbrid Coordinate Ocean Model (HYCOM) and finite-volume cubed-sphere (FV3) dynamic atmosphere model. This system is a product of efforts involving improvements and updates over a 4-year period (2019–2022) through extensive collaborations between the Environmental Modeling Center at the US National Centers for Environmental Prediction (NCEP) and NOAA Atlantic Oceanography and Meteorology Laboratory. To provide two sets of numerical guidance, the initial operational capability of HAFSv1 was configured to two systems—HFSA and HFSB. In this study, we present in-depth analysis of the forecast skills of the upper ocean that was co-evolved by the HFSA and HFSB. We chose hurricane Laura (2020) as an example to demonstrate the interactions between the storm and oceanic mesoscale features. Comparisons performed with the available in situ observations from gliders as well as Argos and National Data Buoy Center moorings show that the HYCOM simulations have better agreement for weak winds than high winds (greater than Category 2). The skill metrics indicate that the model sea-surface temperature (SST) and mixed layer depth (MLD) have a relatively low correlation. The SST, MLD, mixed layer temperature (MLT), and ocean heat content (OHC) are negatively biased. For high winds, SST and MLT are more negative, while MLD is closer to the observations with improvements of about 8%–19%. The OHC discrepancy is proportional to predicted wind intensity. Contrarily, the mixed layer salinity (MLS) uncertainties are smaller and positive for higher winds, probably owing to the higher MLD. The less-negative bias of MLD for high winds implies that the wind-force mixing is less effective owing to the higher MLD and high buoyancy stability (approx. 1.5–1.7 times) than the observations. The heat budget analysis suggests that the maximum heat loss by hurricane Laura was O(< 3°C per day). The main contributor here is advection, followed by entrainment, which act against or with each other depending on the storm quadrant. We also found relatively large unaccountable heat residuals for the in-storm period, and the residuals notably led the heat tendency, meaning that further improvements of the subscale simulations are warranted. In summary, HYCOM simulations showed no systematic differences forced by either HFSA or HFSB.
Lin, G., Z. Wang, Y.F. Chu, C.L. Ziegler, X.M. Hu, M. Xue, B. Geerts, S. Paleri, A.R. Desai, K. Yang, M. Deng, and J. Degraw. Airborne measurements of scale-dependent latent heat flux impacted by water vapor and vertical velocity over heterogeneous land surfaces during the CHEESEHEAD19 campaign. Journal of Geophysical Research-Atmospheres, 129(3):e2023JD039586, https://doi.org/10.1029/2023JD039586 2024
The water vapor transport associated with latent heat flux (LE) in the planetary boundary layer (PBL) is critical for the atmospheric hydrological cycle, radiation balance, and cloud formation. The spatiotemporal variability of LE and water vapor mixing ratio (rv) are poorly understood due to the scale-dependent and nonlinear atmospheric transport responses to land surface heterogeneity. Here, airborne in situ measurements with the wavelet technique are utilized to investigate scale-dependent relationships among LE, vertical velocity (w) variance (σ2w), and rv variance (σ2HO2) over a heterogeneous surface during the Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors 2019 (CHEESEHEAD19) field campaign. Our findings reveal distinct scale distributions of LE, σ2w, and σ2HO2 at 100 m height, with a majority scale range of 120 m–4 km in LE, 32 m–2 km in σ2w, and 200 m–8 km in σ2HO2. The scales are classified into three scale ranges, the turbulent scale (8–200 m), large-eddy scale (200 m–2 km), and mesoscale (2–8 km) to evaluate scale-resolved LE contributed by σ2w and σ2HO2. The large-eddy scale in PBL contributes over 70% of the monthly mean total LE with equal parts (50%) of contributions from σ2w and σ2HO2. The monthly temporal variations mainly come from the first two major contributing classified scales in LE, σ2w, and σ2HO2. These results confirm the dominant role of the large-eddy scale in the PBL in the vertical moisture transport from the surface to the PBL, while the mesoscale is shown to contribute an additional ∼20%. This analysis complements published scale-dependent LE variations, which lack detailed scale-dependent vertical velocity and moisture information.
Lopez, H., S.-K. Lee, R. West, D. Kim, G.R. Foltz, G.J. Alaka Jr., and H. Murakami. Projected increase in the frequency of extremely active Atlantic hurricane seasons. Science Advances, 10(46):eadq7856, https://doi.org/10.1126/sciadv.adq7856 2024
Future changes to the year-to-year swings between active and inactive North Atlantic tropical cyclone (TC) seasons have received little attention, yet may have great societal implications in areas prone to hurricane landfalls. This work investigates past and future changes in North Atlantic TC activity, focusing on interannual variability and evaluating the contributions from anthropogenic forcing. We show that interannual variability of Atlantic TC activity has already increased, evidenced by an increase in the occurrence of both extremely active and inactive TC seasons. TC-resolving general circulation models project a 36% increase in the variance of North Atlantic TC activity, measured by accumulated cyclone energy, by the middle of the 21st century. These changes are the result of increased variability in vertical wind shear and atmospheric stability, in response to enhanced Pacific-to-Atlantic interbasin sea surface temperature variations. Robust anthropogenic-forced intensification in the variability of Atlantic TC activity will continue in the future, with important implications for emergency planning and societal preparedness.
Murray, E.J., J. Dunion, K.B. Karnauskus, Z. Wang, and J.A. Zhang. Cloud height distributions and the role of vertical mixing in the tropical cyclone eye derived from compact Raman lidar observations. Geophysical Research Letters, 51(14):e2024GL108515, https://doi.org/10.1029/2024GL108515 2024
The distribution of tropical cyclone (TC) eye cloud heights is documented for the first time using compact Raman lidar (CRL) measurements with high spatial resolution. These cloud heights act as tracers for low-level vertical mixing in the eye region. Cloud height distributions using all available data from nine Atlantic TCs in 2021 and 2022 show significant vertical variance, dispelling the notion of a flat stratiform eye cloud deck. Eye cloud widths are multiscale, with shallow convective clouds dominating CRL returns. Data from Hurricane Sam (2021) highlight the evolution of shallow convective clouds in the TC eye and their associated temperature inversions. The frequent appearance of convective eye clouds, along with observed vertical wind fluctuations, suggests that vertical mixing from the boundary layer frequently occurs in the TC eye, even beneath strong inversions. This strong vertical mixing should be accurately portrayed by TC simulations and forecasts.
Ramstrom, W., X. Zhang, K. Ahern, and S. Gopalakrishnan. Implementation of storm-following nest for the next generation Hurricane Analysis Forecast System (HAFS). Frontiers in Earth Science, 12:1419233, https://doi.org/10.3389/feart.2024.1419233 2024
Tropical cyclones models have long used nesting to achieve higher resolution of the inner core than was feasible for entire model domains. These high-resolution nests have been shown to better capture storm structures and improve forecast accuracy. The Hurricane Analysis and Forecast System (HAFS) is the new-generation numerical model embedded within NOAA’s Unified Forecast System (UFS). The document highlights the importance of high horizontal resolution (2 km or finer) in accurately simulating the small-scale features of tropical cyclones, such as the eyewall and eye. To meet this need, HAFS was developed by NOAA leveraging a high-resolution, storm-following nest. This nest moves with the cyclone, allowing better representation of small-scale features and more accurate feedback between the cyclone’s inner core and the larger environment. This hurricane following nest capability, implemented in the Finite-Volume Cubed-Sphere (FV3) dynamical core within the UFS framework, can be run both within the regional as well as global forecast systems. A regional version of HAFS with a single moving nest went into operations in 2023. HAFS also includes the first ever moving nest implemented within a global model which is currently being used for research. In this document we provide details of the implementation of moving nests and provide some of the results from both global and regional simulations. For the first time NOAA P3 flight data was used to evaluate the inner core structure from the global run.
Rios-Berrios, R., P.M. Finocchio, J.J. Alland, X. Chen, M.S. Fischer, S.N. Stevenson, and D. Tao. A review of the interactions between tropical cyclones and environmental vertical wind shear. Journal of the Atmospheric Sciences, 81(4):713-741, https://doi.org/10.1175/JAS-S-23-0022.1 2024
Tropical cyclone (TC) structure and intensity are strongly modulated by interactions with deep-layer vertical wind shear (VWS)—the vector difference between horizontal winds at 200 and 850 hPa. This paper presents a comprehensive review of more than a century of research on TC-VWS interactions. The literature broadly agrees that a TC vortex becomes vertically tilted, precipitation organizes into a wavenumber-one asymmetric pattern, and thermal and kinematic asymmetries emerge when a TC encounters an environmental sheared flow. However, these responses depend on other factors, including the magnitude and direction of horizontal winds at other vertical levels between 200 and 850 hPa, the amount and location of dry environmental air, and the underlying sea-surface temperature. While early studies investigated how VWS weakens TCs, an emerging line of research has focused on understanding how TCs intensify under moderate and strong VWS (i.e., shear magnitudes greater than 5 m s−1). Modeling and observational studies have identified four pathways to intensification: vortex tilt reduction, vortex reformation, axisymmetrization of precipitation, and outflow blocking. These pathways may not be uniquely different because convection and vortex asymmetries are strongly coupled to each other. Besides discussing these topics, this review presents open questions and recommendations for future research on TC-VWS interactions.
Rojas, B.S., A.C. Didlake Jr., and J.A. Zhang. Asymmetries during eyewall replacement cycles of Hurricane Ivan (2004). Monthly Weather Review, 152(8):1741-1761, https://doi.org/10.1175/MWR-D-23-0129.1 2024
The physical processes that govern eyewall replacement cycles (ERCs) in tropical cyclones (TCs) are not yet fully understood. In particular, asymmetric structures within the TC inner core have an uncertain role in ERC dynamics. This study analyzes the kinematic and precipitation asymmetric structures during successive ERCs in Hurricane Ivan (2004) using airborne Doppler radar observations. The azimuthal locations of these asymmetries are analyzed relative to the deep-layer (850-200 hPa) environmental wind shear vector. Two ERCs were analyzed at different stages of their evolution. During the concentric eyewall stage of the first ERC, the outer eyewall updrafts were strongest in the left-of-shear half, which also coincided with mesoscale descending inflow (MDI) just radially outward. Enhanced low-level convergence, updrafts, and MDI were collocated in an zone spiraling inward towards the strongest outer eyewall updrafts, suggesting that the vertical velocity asymmetry in the outer eyewall was possibly forced by a stratiform-induced cold pool similar to MDI impacts seen in past TC studies. During the final stage of the second ERC, the outer eyewall (now the singular primary eyewall) experienced an upwind shift in the precipitation and vertical velocity asymmetries. The updraft maximum shifted from the downshear-left quadrant to the downshear-right quadrant, and the precipitation maximum (downwind of the updraft maximum) shifted from left-of-shear to the downshear direction. This shift corroborates previous studies, which hypothesize that at the end of an ERC, the forcing mechanism that drives the eyewall vertical velocity asymmetry transitions from MDI/cold-pool processes to direct interaction with the environmental wind shear.
Rosencrans, M., E.S. Blake, C.W. Landsea, H. Wang, S.B. Goldenberg, R.J. Pasch, D.S. Harnos, and H. Lopez. The tropics: Tropical cyclones—Atlantic basin. In Chapter 4, State of the Climate in 2023. Bulletin of the American Meteorological Society, 105(8):S239-S245, https://doi.org/10.1175/BAMS-D-24-0098.1 2024
Shimada, U., P.D. Reasor, R.F. Rogers, M.S. Fischer, F.D. Marks, J.A. Zawislak, And J.A Zhang. Shear-relative asymmetric kinematic characteristics of intensifying hurricanes as observed by airborne Doppler radar Monthly Weather Review, 152(2):491-512, https://doi.org/10.1175/MWR-D-22-0340.1 2024
While recent observational studies of intensifying (IN) versus steady-state (SS) hurricanes have noted several differences in their axisymmetric and asymmetric structures, there remain gaps in the characterization of these differences in a fully three-dimensional framework. To address these limitations, this study investigates differences in the shear-relative asymmetric structure between IN and SS hurricanes using airborne Doppler radar data from a dataset covering an extended period of time. Statistics from individual cases show that IN cases are characterized by peak wavenumber-1 ascent concentrated in the upshear-left (USL) quadrant at ~12-km height, consistent with previous studies. Moderate updrafts (2–6 m s−1) occur more frequently in the downshear eyewall for IN cases than for SS cases, likely leading to a higher frequency of moderate to strong updrafts USL above 9-km height. Composites of IN cases show that low-level outflow from the eye region associated with maximum wavenumber-1 vorticity inside the radius of maximum wind (RMW) in the downshear-left quadrant converges with low-level inflow outside the RMW, forming a stronger local secondary circulation in the downshear eyewall than SS cases. The vigorous eyewall convection of IN cases produces a net vertical mass flux increasing with height up to ~5-km and then is almost constant up to 10 km, whereas the net vertical mass flux of SS cases decreases with height above 4 km. Strong USL upper-level ascent provides greater potential for the vertical development of the hurricane vortex, which is argued to be favorable for continued intensification in shear environments.
Sippel, J.A., S.D. Ditchek, K. Ryan, and C.W. Landsea. The G-IV inner circumnavigation: A story of successful organic interactions between research and operations at NOAA. Bulletin of the American Meteorological Society, 105(1):E218-E232, https://doi.org/10.1175/BAMS-D-23-0084.1 2024
This study describes both the research-to-operations process leading to a recent change in tropical cyclone (TC) reconnaissance sampling patterns, as well as observing-system experiments that evaluated the impact of that change on numerical weather prediction model forecasts of TCs. A valuable part of this effort was having close, multi-pronged connections between the TC research and operational TC prediction communities at the National Oceanic and Atmospheric Administration (NOAA). Related to this work, NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and National Hurricane Center (NHC) have a long history of close collaboration to improve TC reconnaissance. Similar connections between AOML and NOAA’s Environmental Modeling Center (EMC) also laid a foundation for the observing-system experiments conducted here. More specifically, AOML and NHC collaborated in 2018 to change how NHC uses NOAA’s Gulfstream-IV (G-IV) jet during TC synoptic surveillance missions. That change added a second circumnavigation at approximately 1.5 degrees from TC centers, when possible. Preliminary experiments suggest that the change improved track forecasts, although intensity results were more mixed. Despite the somewhat small sample size over a three-year period, the track improvement does agree with prior work. This effort has led to additional work to more fully examine G-IV sampling strategies.
Tang, B.H., R. Rios-Berrios, and J.A. Zhang. Diagnosing radial ventilation in dropsonde observations of Hurricane Sam (2021). Monthly Weather Review, 152(8):1725-1739, https://doi.org/10.1175/MWR-D-23-0224.1 2024
This study presents a method to diagnose radial ventilation, the horizontal flux of relatively low-θe air into tropical cyclones, from dropsonde observations. We used this method to investigate ventilation changes over three consecutive sampling periods in Hurricane Sam (2021), which underwent substantial intensity changes over three days. During the first and last periods, coinciding with intensification, the ventilation was relatively small due to a lack of spatial correlation between radial flow and θe azimuthal asymmetries. During the second period, coinciding with weakening, the ventilation was relatively large. The increased ventilation was caused by greater shear associated with an upper-level trough, tilting the vortex, along with dry, low-θe air wrapping in upshear. The spatial correlation of the radial inflow and anomalously low-θe air resulted in large ventilation at mid-to-upper levels. Additionally, at low-to-mid levels, there was evidence of mesoscale inflow of low-θe air in the stationary band complex. The location of these radial ventilation pathways and their effects on Sam’s intensity are consistent with previous idealized and real-case modeling studies. More generally, this method offers a way to monitor ventilation changes in tropical cyclones, particularly when there is full-troposphere sampling around and within a tropical cyclone’s core.
Wadler, J.B., J.J. Cione, S. Michlowitz, B. Jaimes de la Cruz, and L.K. Shay. Improving the statistical representation of tropical cyclone in-storm sea surface temperature cooling. Weather and Forecasting, 39(6):847-866, https://doi.org/10.1175/WAF-D-23-0115.1 2024
This study uses fixed buoy time series to create an algorithm for sea surface temperature (SST) cooling underneath a tropical cyclone (TC) inner-core. To build predictive equations, SST cooling is first related to single variable predictors such as the SST before storm arrival, ocean heat content (OHC), mixed layer depth, sea surface salinity and stratification, storm intensity, storm translation speed, and latitude. Of all the single variable predictors, initial SST before storm arrival explains the greatest amount of variance for the change in SST during storm passage. Using a combination of predictors, we created nonlinear predictive equations for SST cooling. In general, the best predictive equations have four predictors and are built with knowledge about the pre-storm ocean structure (e.g., OHC), storm intensity (e.g., minimum sea level pressure), initial SST values before storm arrival, and latitude. The best performing SST cooling equations are broken up into two ocean regimes: when the ocean heat content is less than 60 kJcm−2 (greater spread in SST cooling values) and when the ocean heat content is greater than 60 kJcm−2 (SST cooling is always less than 2 °C) which demonstrates the importance of initial oceanic thermal structure on the in-storm SST value. The new equations are compared to what is currently used in a statistical-dynamical model. Overall, since the ocean providing the latent heat and sensible heat fluxes necessary for TC intensification, the results highlight the importance for consistently obtaining accurate in-storm upper-oceanic thermal structure for accurate TC intensity forecasts.
Wang, W., J. Han, J. Shin, X. Chen, A. Hazelton, L. Zhu, H.-S. Kim, X. Li, B. Liu, Q. Liu, J. Steffen, R. Sun, W. Zheng, Z. Zhang, and F. Yang. Physics schemes in the first version of NCEP operational Hurricane Analysis and Forecast System (HAFS). Frontiers in Earth Science, 12:1379069, https://doi.org/10.3389/feart.2024.1379069 2024
This document summarizes the physics schemes used in two configurations of the first version of the operational Hurricane Analysis and Forecast System (HAFSv1) at NOAA NCEP. The physics package in HAFSv1 is the same as that used in NCEP global forecast system (GFS) version 16 except for an additional microphysics scheme and modifications to sea surface roughness lengths, boundary layer scheme, and the entrainment rate in the deep convection scheme. Those modifications are specifically designed for improving the simulation of tropical cyclones (TCs). The two configurations of HAFSv1 mainly differ in the adopted microphysics schemes and TC-specific modifications in addition to model initialization. Experiments are made to highlight the impacts of TC-specific modifications and different microphysics schemes on HAFSv1 performance. Challenges and developmental plans of physics schemes for future versions of operational HAFS are discussed.
Yu, H., G. Chen, W.K. Wong, J.L. Vigh, C.-K. Pan, X. Lu, J.A. Zhang, J. Tang, K. Zhao, P. Chen, Z. Yu, M. Yang, J. Dunion, Z. Fang, X. Lei, A. Tyagi, and L. Chen. WMO Typhoon Landfall Forecast Demonstration Project (2010-2022): A decade of transition from track forecasts to impact forecasts. Bulletin of the American Meteorological Society, 105(7):E1320-1349, https://doi.org/10.1175/BAMS-D-23-0085.1 2024
The Typhoon Landfall Forecast Demonstration Project (TLFDP) (2010–2022) was an international cooperative scientific project conducted under the framework of the WMO. The primary objectives of the TLFDP were to enhance the capability of tropical cyclone (TC) forecasters, and support related decision-makers in effective utilization of the most advanced forecasting techniques for the ultimate purpose of reducing and preventing disasters associated with TC landfall. Forty agencies/organizations/projects globally participated in the activities of the TLFDP following its inception in 2010, although the primary focus was on landfalling TCs in the western North Pacific. The TLFDP facilitated collaborations and workshops that realized notable achievements in four key areas: 1) the collection, production, and sharing of TC data; 2) the development and application of TC forecast verification metrics; 3) research on TC forecast skill; and 4) development of new techniques for TC forecasting. An obvious outcome was the shift from prediction of TC features, including track and intensity, toward prediction of TC impacts with more probabilistic conception. The final years of the project also promoted increasing application of artificial intelligence and machine learning techniques in various techniques for analysis and forecasting of TCs. Although the TLFDP ended in 2022, its core activities have continued to be extended through new WMO projects and regional cooperative initiatives.
Zeng, X., H. Su, S. Hristova-Veleva, D.J. Posselt, R. Atlas, S.T. Brown, R.D. Dixon, E. Fetzer, T.J. Galarneau Jr., M. Hardesty, J.H. Jiang, P.P. Kangaslahti, A. Ouyed, T.S. Pagano, O. Reitebuch, R. Roca, A. Stoffelen, S. Tucker, A. Wilson, L. Wu, and I. Yanovsky. Vientos—A new satellite mission concept for 3D wind measurements by combining passive water vapor sounders with Doppler wind lidar. Bulletin of the American Meteorological Society, 105(2):E357-E369, https://doi.org/10.1175/BAMS-D-22-0283.1 2024
It is challenging to accurately characterize the three-dimensional distribution of horizontal wind vectors (known as 3D winds). Feature-matching satellite cloud top or water vapor fields have been used for decades to retrieve atmospheric motion vectors, but this approach is mostly limited to a single and uncertain pressure level at a given time. Satellite wind lidar measurements are expected to provide more accurate data and capture the line-of-sight wind for clear skies, within cirrus clouds, and above thick clouds, but only along a curtain parallel to the satellite track. Here we propose Vientos—a new satellite mission concept that combines two or more passive water vapor sounders with Doppler wind lidar to measure 3D winds. The need for 3D wind observations is highlighted by inconsistencies in reanalysis estimates, particularly under precipitating conditions. Recent studies have shown that 3D winds can be retrieved using water vapor observations from two polar-orbiting satellites separated by 50 min, with the help of advanced optical flow algorithms. These winds can be improved through the incorporation of a small number of collocated higher-accuracy measurements via machine learning. The Vientos concept would enable simultaneous measurements of 3D winds, temperature, and humidity, and is expected to have a significant impact on scientific research, weather prediction, and other applications. For example, it can help better understand and predict the preconditions for organized convection. This article summarizes recent results, presents the Vientos mission architecture, and discusses implementation scenarios for a 3D wind mission under current budget constraints.
Zhao, B., L. Wu, G. Wang, J.A. Zhang, L. Liu, C. Zhao, Z. Zhuang, C. Xia, Y. Xue, X. Li, and F. Quao. A numerical study of tropical cyclone and ocean responses to air-sea momentum flux at high winds. Journal of Geophysical Research-Oceans, 129(7):e2024JC020956, https://doi.org/10.1029/2024JC020956 2024
The relationship between minimum sea level pressure (MSLP) and maximum wind speed, is a commonly employed metric for assessing tropical cyclone (TC) forecast skill. However, accurately reproducing this relationship in TC forecasts is challenging. By introducing a new air-sea momentum flux scheme considering both wave state and saturation (decrease) effect at high winds into a fully coupled atmosphere-ocean-wave model, our numerical results reveal that the maximum wind speed and MSLP respond oppositely to the air-sea momentum flux change. The simulated wind-pressure relationship aligns well with observations when the new flux scheme is used. This study highlights the large sensitivity of wind-pressure relationship to the air-sea momentum flux parameterization at high winds. Furthermore, the air-sea momentum flux has a significant effect on ocean wave characteristics and sea surface temperature (SST) cooling in TC simulations.
Zou, Z., J. Song, F. Qiao, D. Wang, and J.A. Zhang. The wave coherent stress and turbulent structure over swell waves. Journal of Physical Oceanography, 54(9):1933-1948, https://doi.org/10.1175/JPO-D-23-0144.1 2024
The generation of ocean surface waves by wind has been studied for a century, giving rise to wave forecasting and other crucial applications. However, the reacting force of swell waves on the turbulence in the marine Atmospheric Boundary Layer (ABL) remains unknown partly due to the unclear magnitude and profile of Wave Coherent (WC) stress. In this study, the intersection frequency between the energy-containing range and inertial subrange range in the turbulent spectra is identified based on the Attached Eddy Model (AEM), as the intersection modulated by swell wave could help to comprehend the physical process between the ocean surface wave and the marine ABL. Using observations from a fixed platform located in the South China Sea, this study shows that the intersection when the WC stress accounts for a lower proportion of the total wind stress (< 10%) follows U/(2πz) given by AEM, here U is wind speed, z is height. While the intersection depends on the drag coefficient of WC stress for the case when WC stress accounts for a large part of the total wind stress (> 10%). Considering the unclear magnitude and profile of WC stress, this study derives a new function to depict the WC stress.
2023
Aberson, S.D., J.A. Zhang, J. Zawislak, K. Sellwood, R. Rogers, and J.J. Cione. The NCAR GPS dropwindsonde and its impact on hurricane operations and research. Bulletin of the American Meteorological Society, 104(11):E2134-E2154, https://doi.org/10.1175/BAMS-D-22-0119.1 2023
The Global Positioning System dropwindsonde has provided thousands of high-resolution kinematic and thermodynamic soundings in and around tropical cyclones (TCs) since 1997. These data have revolutionized the understanding of TC structure, improved forecasts, and validated observations from remote-sensing platforms. About 400 peer-reviewed studies on TCs using these data have been published to date. This paper reviews the history of dropwindsonde observations, changes to dropwindsonde technology since it was first used in TCs in 1982, and how the data have improved forecasting and changed our understanding of TCs.
Al-Khaldi, M.M., S. Gleason, J.T. Johnson, R. Balasubramaniam, C.S. Ruf, D.S. McKague, B. Annane, T. Wang, A. Russel, and D. Twigg. Using synthetic cyclone models for high wind GNSS-R calibration, validation, and algorithm development: A CYGNSS case study. IEEE Transactions on Geoscience and Remote Sensing, 61:5801911, https://doi.org/10.1109/TGRS.2023.3294870 2023
This work reports a case study of the use of synthetic cyclone models for the development, assessment and validation of global navigation satellite system reflectometry (GNSS-R) wind speed remote sensing algorithms using a cyclone global navigation satellite system (CYGNSS) data record extending from 1 August 2018 to 31 December 2022. Synthetic cyclone models are shown to be useful in assessing the high wind speed sensitivity of CYGNSS’s v1.0, v2.1, v3.0, v3.1, and future v3.2 normalized bistatic radar cross Section (NBRCS) products due to the extended matchup dataset of high wind speed information that is obtained. The models are also shown useful in investigating the impacts of specific error corrections terms and in the development of level-2 geophysical model functions (GMFs) for the retrieval of ocean surface winds.
Apodoca, K., L. Cucurull, I. Genkova, R.J. Purser, and X. Su. Assessing the benefit of variational quality control for assimilating Aeolus Mie and Rayleigh wind profiles in NOAA’s Global Forecast System during tropical cyclones. Quarterly Journal of the Royal Meteorological Society, 149(756):2761-2783, https://doi.org/10.1002/qj.4530 2023
In this article, we show a “proof-of-concept” study to assess the utility of a variational quality control algorithm in increasing the number of assimilated Aeolus Mie-cloudy and Rayleigh-clear winds in National Oceanic and Atmospheric Administration (NOAA)'s global data assimilation and forecast system. The National Centers for Environmental Prediction (NCEP) Variational Quality Control (NCEP-VQC) algorithm was tuned and applied during the minimization process. This type of quality control uses optimal control theory principles to treat outliers in the probability density function (PDF) of observational departure statistics, assuming that the observation errors follow a family of logistic distributions. In the case of Aeolus Mie-cloudy and Rayleigh-clear winds, the NCEP-VQC algorithm permitted the relaxation of the gross error and one of the recommended ESA quality controls (reject Rayleigh-clear observations below 850 hPa), assigned adaptive observation weights ranging from 0 to 1, and led to an increase in the number of retained Aeolus observations for the calculation of global analyses, which in turn improved the verification statistics on analyzed tropical storms This article discusses the advantage of implementing the NCEP-VQC algorithm in the Aeolus data assimilation, the benefits of retaining more wind profiles that contribute to the analysis calculation, and shows improvements in the initialization and short-term forecasts on several tropical cyclone cases.
Chen, J.-H., A. Clark, G. Ge, L. Harris, K. Hoogewind, A. Jenson, H. Lopez, J. Mouallem, B. Zavadoff, X. Zhang, and L. Zhou. 2022-2023 global-nest initiative activity summary: Recent results and future plan. NOAA Technical Memorandum, OAR-GFDL 2023-001, 13 pp., https://doi.org/10.25923/yx20-3k04 2023
Chen, X., A. Hazelton, F.D. Marks, G.J. Alaka, and C. Zhang. Performance of an improved TKE-based eddy-diffusivity mass-flux (EDMF) PBL scheme in 2021 hurricane forecasts from Hurricane Analysis and Forecast System. Weather and Forecasting, 38(2):321-336, https://doi.org/10.1175/WAF-D-22-0140.1 2023
Continuous development and evaluation of planetary boundary layer (PBL) parameterizations in hurricane conditions are crucial for improving tropical cyclone (TC) forecasts. A turbulence kinetic energy (TKE)-based eddy-diffusivity mass-flux (EDMF-TKE) PBL scheme, implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS), was recently improved in hurricane conditions using large-eddy simulations. This study evaluates the performance of HAFS TC forecasts with the original (experiment HAFA) and modified EDMF-TKE (experiment HAFY) based on a large sample of cases during the 2021 North Atlantic hurricane season. Results indicate that intensity and structure forecast skill was better overall in HAFY than in HAFA, including during rapid intensification. Composite analyses demonstrate that HAFY produces shallower and stronger boundary layer inflow, especially within 1–3 times the radius of maximum wind (RMW). Stronger inflow and more moisture in the boundary layer contribute to stronger moisture convergence near the RMW. These boundary layer characteristics are consistent with stronger, deeper, and more compact TC vortices in HAFY than in HAFA. Nevertheless, track skill in HAFY is slightly reduced, which is in part attributable to the cross-track error from a few early cycles of Hurricane Henri that exhibited ~400 n mi track error at longer lead times. Sensitivity experiments based on HAFY demonstrate that turning off cumulus schemes notably reduces the track errors of Henri while turning off the deep cumulus scheme reduces the intensity errors. This finding hints at the necessity of unifying the mass fluxes in PBL and cumulus schemes in future model physics development.
Chen, X., C.M. Rozoff, R.F. Rogers, K.L. Corbosiero, D. Tao, J.-F. Gu, F. Judt, E.A. Hendricks, Y. Wang, M.M. Bell, D.P. Stern, K.D. Musgrave, J.A. Knaff, and J. Kaplan. Research advances on internal processes affecting tropical cyclone intensity change from 2018–2022. Tropical Cyclone Research and Review, 12(1):10-29, https://doi.org/10.1016/j.tcrr.2023.05.001 2023
This contribution summarizes the significant progress in a variety of topic areas related to internal tropical cyclone (TC) intensity change processes over 2018–2022 from the WMO Tenth International Workshop on Tropical Cyclones (IWTC-10). These topic areas include surface and boundary layer processes; TC internal structure and microphysical processes; and, radiation interactions with TCs. Recent studies better frame the uncertainty in the surface drag and enthalpy coefficients at high wind speeds. These parameters greatly impact TC intensity and it is therefore important that more direct measurements of these boundary layer parameters are made. Particularly significant scientific strides have been made in TC boundary layers. These advancements have been achieved through improved coupled models, large-eddy simulations, theoretical advancements, and detailed observations. It is now clear that the research field needs to better represent the eddy viscosity throughout the depth of the boundary layer. Furthermore, detailed study of coherent structures in TC boundary layers will likely be a propitious direction for the research community. Meanwhile, in-depth observational field campaigns and assiduous data analysis have made significant headway into verifying theory and modeling studies of intensification processes related to TC vortex alignment, efficient latent heating distributions, and overall 3D structure. Substantial efforts have also been made to better understand the intricate roles radiative processes play in TC evolution and intensity change. Finally, some promising progress has been made in the development of time-dependent theories of TC intensification and the predictability of the internal TC intensity change. Overall, there have been well-earned gains in the understanding of intensity change processes intrinsic to the TC system, but the journey is not complete. This paper highlights some of the most relevant and important research areas that are still shedding new light into internal factors governing TC intensity change.
Conroy, A., H. Titley, R. Rivett, X. Feng, J. Methven, K. Hodges, A. Brammer, A. Burton, P. Chakraborty, G. Chen, L. Cowan, P. Dunion, and A. Sarkar. Track forecast: Operational capability and new techniques—Summary from the Tenth International Workshop on Tropical Cyclones (IWTC-10). Tropical Cyclone Research and Review, 12(1):64-80, https://doi.org/10.1016/j.tcrr.2023.05.002 2023
In this paper, we summarize findings from the Tenth International Workshop on Tropical Cyclones (IWTC-10) subgroup on operational track forecasting techniques and capability. The rate of improvement in the accuracy of official forecast tracks (OFTs) appears to be slowing down, at least for shorter lead times, where we may be approaching theoretical limits. Operational agencies continue to use consensus methods to produce the OFT with most continuing to rely on an unweighted consensus of four to nine NWP models. There continues to be limited use of weighted consensus techniques, which is likely a result of the skills and additional maintenance needed to support this approach. Improvements in the accuracy of ensemble mean tracks is leading to increased use of ensemble means in consensus tracks. Operational agencies are increasingly producing situation-dependent depictions of track uncertainty, rather than relying on a static depiction of track forecast certainty based on accuracy statistics from the preceding 5 years. This trend has been facilitated by the greater availability of ensemble NWP guidance, particularly vortex parameter files, and improved spread in ensembles. Despite improving spread-skill relationships, most ensemble NWP systems remain under spread. Hence many operational centers are looking to leverage "super-ensembles" (ensembles of ensembles) to ensure the full spread of location probability is captured. This is an important area of service development for multi-hazard impact-based warnings as it supports better decision making by emergency managers and the community in the face of uncertainty.
Cucurull, L. Recent impact of COSMIC-2 with improved radio occultation data assimilation algorithms. Weather and Forecasting, 38(10):1829-1847, https://doi.org/10.1175/WAF-D-22-0186.1 2023
A Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) follow-on constellation, COSMIC-2, was successfully launched into equatorial orbit on June 24, 2019. With an increased signal-to-noise ratio due to improved receivers and digital beam-steering antennas, COSMIC-2 is producing about 5,000 high-quality radio-occultation (RO) profiles daily over the tropics and subtropics. The initial evaluation of the impact of assimilating COSMIC-2 into NOAA’s Global Forecast System (GFS) showed mixed results, and adjustments to quality control procedures and observation error characteristics had to be made prior to the assimilation of this dataset in the operational configuration in May 2020. Additional changes in the GFS that followed this initial operational implementation resulted in a larger percentage of rejection (~ 90 %) of all RO observations, including COSMIC-2, in the mid-lower troposphere. Since then, two software upgrades directly related to the assimilation of RO bending angle observations were developed. These improvements aimed at optimizing the utilization of COSMIC-2 and other RO observations to improve global weather analyses and forecasts. The first upgrade was implemented operationally in September 2021 and the second one in November 2022. This study describes both RO software upgrades and evaluates the impact of COSMIC-2 with this most recently improved configuration. Specifically, we show that the assimilation of COSMIC-2 observations has a significant impact in improving temperature and winds in the tropics, though benefits also extend to the extra-tropical latitudes.
Cucurull, L., and J. Purser. An improved one-dimensional bending angle forward operator for the assimilation of radio occultation profiles in the lower troposphere. Monthly Weather Review, 151(5):1093-1108, https://doi.org/10.1175/MWR-D-22-0073.1 2023
Under very large vertical gradients of atmospheric refractivity, which are typical at the height of the planetary boundary layer, the assimilation of radio-occultation (RO) observations into numerical weather prediction (NWP) models presents several serious challenges. In such conditions, the assimilation of RO bending angle profiles is an ill-posed problem, the uncertainty associated with the RO observations is higher, and the one-dimensional forward operator used to assimilate these observations has several theoretical deficiencies. As a result, a larger percentage of these RO observations are rejected at the NWP centers by existing quality control procedures, potentially limiting the benefits of this data type to improve weather forecasting in the lower troposphere. To address these problems, a new methodology that enables the assimilation of RO data to be extended to the lower moist troposphere has been developed. Challenges associated with larger atmospheric gradients of refractivity are partially overcome by a reformulation that has minimal effect at higher altitudes. As a first step towards this effort, this study presents both the theoretical development of this new methodology and a forecast impact assessment of it using the NCEP NWP system. Though using a conservative approach, benefits in the lower tropical troposphere are already noticeable. The encouraging results of this work open the potential for further exploitation and optimization of RO assimilation.
DesRosiers, A.J., M.M. Bell, P.J. Klotzbach, M.S. Fischer, and P.D. Reasor. Observed relationships between tropical cyclone vortex height, intensity, and intensification rate. Geophysical Research Letters, 50(8):e2022GL101877, https://doi.org/10.1029/2022GL101877 2023
As a tropical cyclone (TC) intensifies, the tangential wind field expands vertically and increases in magnitude. Observations and modeling support vortex height as an important TC structural characteristic. The Tropical Cyclone Radar Archive of Doppler Analyses with Recentering data set provides kinematic analyses for calculation of the height of the vortex (HOV) in observed storms. Analyses are azimuthally-averaged with tangential wind values taken along the radius of maximum winds. A threshold-based technique is used to determine the HOV. A fixed threshold HOV strongly correlates with current intensity. A dynamic HOV metric quantifies vertical decay of tangential wind with reduced dependency on intensity. Statistically significant differences are present between dynamic HOV values in groups of steady-state, intensifying, and rapidly-intensifying cases categorized by subsequent changes in pressure. A tall vortex is always observed in cases meeting a pressure-based rapid intensification definition. Taller vortices are also evident with slower intensification. Results suggest HOV may be a helpful predictor for TC intensification.
Ditchek, S.D., and J.A. Sippel. A comparison of the impacts of inner-core, over-vortex, and environmental dropsondes on tropical cyclone forecasts during the 2017-2020 hurricane seasons. Weather and Forecasting, 38(11):2169-2187, https://doi.org/10.1175/WAF-D-23-0055.1 2023
This study conducts the first large-sample comparison of the impact of dropsondes in the tropical cyclone (TC) inner core, vortex, and environment on NWP-model TC forecasts. We analyze six observing-system experiments, focusing on four sensitivity experiments that denied dropsonde observations within annuli corresponding with natural breakpoints in reconnaissance sampling. These are evaluated against two other experiments detailed in a recent parallel study: one that assimilated and another that denied dropsonde observations. Experiments used a basin-scale, multi-storm configuration of the Hurricane Weather Research and Forecasting model (HWRF) and covered active periods of the 2017–2020 North Atlantic hurricane seasons. Analysis focused on forecasts initialized with dropsondes that used mesoscale error covariance derived from a cycled HWRF ensemble, as these forecasts were where dropsondes had the greatest benefits in the parallel study. Some results generally support findings of previous research, while others are novel. Most notable was that removing dropsondes anywhere, particularly from the vortex, substantially degraded forecasts of maximum sustained winds. Removing in-vortex dropsondes also degraded outer-wind-radii forecasts in many instances. As such, in-vortex dropsondes contribute to a majority of the overall impacts of the dropsonde observing system. Additionally, track forecasts of weak TCs benefited more from environmental sampling, while track forecasts of strong TCs benefited more from in-vortex sampling. Finally, inner-core-only sampling strategies should be avoided, supporting a change made to the U.S. Air Force Reserve’s sampling strategy in 2018 that added dropsondes outside of the inner core.
Ditchek, S.D., J.A. Sippel, G.J. Alaka Jr., S.B. Goldenberg, And L. Cucurull. A systematic assessment of the overall dropsonde impact during the 2017-2020 hurricane seasons using the basin-scale HWRF. Weather and Forecasting, 38(6):789-816, https://doi.org/10.1175/WAF-D-22-0102.1 2023
This study marks the most comprehensive assessment of the overall impact of dropsondes on tropical cyclone (TC) forecasts to date. We compare two experiments to quantify dropsonde impact: one that assimilated and another that denied dropsonde observations. These experiments used a basin-scale, multi-storm configuration of the Hurricane Weather Research and Forecasting model (HWRF) and covered active North Atlantic basin periods during the 2017–2020 hurricane seasons. The importance of a sufficiently large sample size, as well as thoroughly understanding the error distribution by stratifying results, are highlighted by this work. Overall, dropsondes directly improved forecasts during sampled periods and indirectly impacted forecasts during unsampled periods. Benefits for forecasts of track, intensity, and outer wind radii were more pronounced during sampled periods. The forecast improvements of outer wind radii were most notable given the impact that TC size has on TC-hazards forecasts. Additionally, robustly observing the inner and near-core region was necessary for 64-kt-wind-radii forecasts. Yet, these benefits were heavily dependent on the data assimilation (DA) system quality. More specifically, dropsondes only improved forecasts when the analysis used mesoscale error covariance derived from a cycled HWRF ensemble, suggesting that it is a vital DA component. Further, while forecast improvements were found regardless of initial classification and in steady-state TCs, TCs undergoing intensity change had diminished benefits. The diminished benefits during intensity change probably reflects continued DA deficiencies. Thus, improving DA-system quality and observing system limitations would likely enhance dropsonde impacts.
Ditchek, S.D., J.A. Sippel, P.J. Marinescu, and G.J. Alaka, Jr. Improving best-track verification of tropical cyclones: A new metric to identify forecast consistency. Weather and Forecasting, 38(6):817-831, https://doi.org/10.1175/WAF-D-22-0168.1 2023
This paper introduces a new tool for verifying tropical cyclone (TC) forecasts. Tropical cyclone forecasts made by operational centers and by numerical weather prediction (NWP) models have been objectively verified for decades. Typically, the mean absolute error (MAE) and/or MAE skill are calculated relative to values within the operations center’s best track. Yet, the MAE can be strongly influenced by outliers and yield misleading results. Thus, this paper introduces an assessment of consistency between the MAE skill as well as two other measures of forecast performance. This “consistency metric” objectively evaluates the forecast-error evolution as a function of lead time based on thresholds applied to the: (1) MAE skill; (2) median absolute error (MDAE) skill; and (3) the frequency of superior performance (FSP), which indicates how often one forecast outperforms another. The utility and applicability of the consistency metric is validated by applying it to four research and forecasting applications. Overall, this consistency metric is a helpful tool to guide analysis and increase confidence in results in a straightforward way. By augmenting the commonly-used MAE and MAE skill with this consistency metric and creating a single scorecard with consistency-metric results for TC track, intensity, and significant-wind radii forecasts, the impact of observing systems, new modeling systems, or model upgrades on TC-forecast performance can be evaluated both holistically and succinctly. This could in turn help forecasters learn from challenging cases and accelerate and optimize developments and upgrades in NWP models.
Dunion, J.P., C. Davis, H. Titley, H. Greatrex, M. Yamaguchi, J. Methven, R. Ashrit, Z. Wang, H. Yu, A.-C. Fontan, A. Brammer, M. Kucas, M. Ford, P. Papin, F. Prates, C. Mooney, A. Kruczkiewicz, P. Chakraborty, A. Burton, M. DeMaria, R. Torn, and J.L. Vigh. Recommendations for improved tropical cyclone formation and position probabilistic forecast products. Tropical Cyclone Research and Review, 12(4):241-258, https://doi.org/10.1016/j.tcrr.2023.11.003 2023
Prediction of the potentially devastating impact of landfalling tropical cyclones (TCs) relies substantially on numerical prediction systems. Due to the limited predictability of TCs and the need to express forecast confidence and possible scenarios, it is vital to exploit the benefits of dynamic ensemble forecasts in operational TC forecasts and warnings. RSMCs, TCWCs, and other forecast centers value probabilistic guidance for TCs, but the International Workshop on Tropical Cyclones (IWTC-9) found that the “pull-through” of probabilistic information to operational warnings using those forecasts is slow. IWTC-9 recommendations led to the formation of the WMO/WWRP Tropical Cyclone-Probabilistic Forecast Products (TC-PFP) project, which is also endorsed as a WMO Seamless GDPFS Pilot Project. The main goal of TC-PFP is to coordinate across forecast centers to help identify best practice guidance for probabilistic TC forecasts. TC-PFP is being implemented in 3 phases: Phase 1 (TC formation and position); Phase 2 (TC intensity and structure); and Phase 3 (TC related rainfall and storm surge). This article provides a summary of Phase 1 and reviews the current state of the science of probabilistic forecasting of TC formation and position. There is considerable variability in the nature and interpretation of forecast products based on ensemble information, making it challenging to transfer knowledge of best practices across forecast centers. Communication among forecast centers regarding the effectiveness of different approaches would be helpful for conveying best practices. Close collaboration with experts experienced in communicating complex probabilistic TC information and sharing of best practices between centers would help to ensure effective decisions can be made based on TC forecasts. Finally, forecast centers need timely access to ensemble information that has consistent, user-friendly ensemble information. Greater consistency across forecast centers in data accessibility, probabilistic forecast products, and warnings and their communication to users will produce more reliable information and support improved outcomes.
Fischer, M.S., P.D. Reasor, B.H. Tang, K.L. Corbosiero, R.D. Torn, and X. Chen. A tale of two vortex evolutions: Using a high-resolution ensemble to assess the impacts of ventilation on a tropical cyclone rapid intensification event. Monthly Weather Review, 151(1):297-320, https://doi.org/10.1175/MWR-D-22-0037.1 2023
The multi-scale nature of tropical cyclone (TC) intensity change under moderate vertical wind shear was explored through an ensemble of high-resolution simulations of Hurricane Gonzalo (2014). Ensemble intensity forecasts were characterized by large short-term (36-h) uncertainty, with a forecast intensity spread of over 20 m s−1, due to differences in the timing of rapid intensification (RI) onset. Two subsets of ensemble members were examined, referred to as early-RI and late-RI members. The two ensemble groups displayed significantly different vortex evolutions under the influence of a nearby upper-tropospheric trough and an associated dry-air intrusion. Mid-upper-tropospheric ventilation in late-RI members was linked to a disruption of inner-core diabatic heating, a more tilted vortex, and vortex breakdown, as the simulated TCs transitioned from a vorticity annulus toward a monopole structure. A column-integrated moist static energy (MSE) budget revealed the important role of horizontal advection in depleting MSE from the TC core, while mesoscale subsidence beneath the dry-air intrusion acted to dry a deep layer of the troposphere. Eventually, the dry-air intrusion retreated from late-RI members as vertical wind shear weakened, the magnitude of vortex tilt decreased, and late-RI members began to rapidly intensify, ultimately reaching a similar intensity as early-RI members. Conversely, the vortex structures of early-RI members were shown to exhibit greater intrinsic resilience to tilting from vertical wind shear and early-RI members were able to fend off the dry-air intrusion relatively unscathed. The different TC intensity evolutions can be traced back to differences in the initial TC vortex structure and intensity.
Gramer, L.J., M. Soden, and J.C. Hendee. Operational ecoforecasting for coral reefs using artificial intelligence and integrated near real-time environmental data. Bulletin of Marine Science, 99(3):379-394, https://doi.org/10.5343/bms.2022.0012 2023
A synthesis of information products about environmental stressors provided in near real-time can serve environmental managers who seek to act decisively before stressors become unmanageable. We have created ecological forecasts, i.e., ecoforecasts, based on input from a variety of environmental sensors that report in near real-time, and we subsequently send those ecoforecasts to environmental managers. The application behind these ecoforecasts is Python-based software that uses an artificial intelligence (AI) inference engine called an expert system. The National Oceanic and Atmospheric Administration (NOAA) Environmental Information Synthesizer (NEIS), formerly the Environmental Information Synthesizer for Expert Systems (EISES), has been developed over two decades to meet the needs of environmental managers and scientists. NEIS integrates environmental data from multiple sources, including in situ and satellite sensors. The application produces ecoforecasts designed to identify environmental conditions conducive to mass coral bleaching and bleaching of specific coral species, as well as other marine environmental events such as algal blooms. This study evaluates the efficacy of coral bleaching ecoforecasts generated by NEIS for the Florida reef tract covering the years 2005–2017.
Hazelton, A., G.J. Alaka Jr., M. Fischer, R. Torn, and S. Gopalakrishnan. Factors influencing the track of Hurricane Dorian (2019) in the west Atlantic: Analysis of a HAFS ensemble. Monthly Weather Review, 151(1):175-192, https://doi.org/10.1175/MWR-D-22-0112.1 2023
Hurricane Dorian (2019), a category-five tropical cyclone (TC), was characterized by a large spread in track forecasts as it moved northwest. A set of 80 ensemble forecasts from the Hurricane Analysis and Forecast System (HAFS) was produced to evaluate Dorian’s track spread and the factors that contributed to it. Track spread was particularly critical at long lead times (5–7 days after initialization near the Lesser Antilles), due to the uncertainty in the location of landfall and hazards. Four clusters of members were analyzed based on the 7-day track, characterized by Dorian moving: 1) slowly near the northern Bahamas (closest to reality), 2) across the Florida Peninsula, 3) slowly into Florida’s east coast, and 4) quickly north of The Bahamas. Ensemble sensitivity techniques were applied to identify areas that were most critical for Dorian’s track. Key differences were found in the strength of the subtropical ridge over the western Atlantic with a weaker ridge and slower easterly steering flow in the offshore groups. Subtle differences in the synoptic pattern over the United States also appeared to affect the timing of Dorian’s northward turn, specifically the strength of a shortwave trough moving over the Ohio Valley. Despite some early track differences, the correlation between early and late track errors was not significant. An examination of four members further highlights the differences in steering and the strength of the subtropical ridge. This study demonstrates the utility of ensemble datasets for studying TC forecast uncertainty, and the importance of medium-range modeling of synoptic-scale steering features to accurately predict the track of tropical cyclones.
Hazelton, A., G.J. Alaka, Jr., L. Gramer, W. Ramstrom, S. Ditchek, X. Chen, B. Liu, Z. Zhang, L. Zhu, W. Wang, B. Thomas, J.H. Shin, C.-K. Wang, H.-S. Kim, X. Zhang, A. Mehra, F. Marks, and S. Gopalakrishnan. 2022 real-time hurricane forecasts from an experimental version of the Hurricane Analysis and Forecast System (HAFSV0.3S) Frontiers in Earth Science, 11:1264969, https://doi.org/10.3389/feart.2023.1264969 2023
During the 2022 hurricane season, real-time forecasts were conducted using an experimental version of the Hurricane Analysis and Forecast System (HAFS). The version of HAFS detailed in this paper (HAFSV0.3S, hereafter HAFS-S) featured the moving nest recently developed at NOAA AOML, and also model physics upgrades: TC-specific modifications to the planetary boundary layer (PBL) scheme and introduction of the Thompson microphysics scheme. The real-time forecasts covered a large dataset of cases across the North Atlantic and eastern North Pacific 2022 hurricane seasons, providing an opportunity to evaluate this version of HAFS ahead of planned operational implementation of a similar version in 2023. The track forecast results show that HAFS-S outperformed the 2022 version of the operational HWRF model in the Atlantic, and was the best of several regional hurricane models in the eastern North Pacific for track. The intensity results were more mixed, with a dropoff in skill at Days 4-5 in the Atlantic but increased skill in the eastern North Pacific. HAFS-S also showed some larger errors than the long-time operational Hurricane Weather Research and Forecasting (HWRF) model in the radius of 34-knot wind, but other radii metrics are improved. Detailed analysis of Hurricane Ian in the Atlantic highlights both the strengths of HAFS and opportunities for further development and improvement.
Holbach, H.M., O. Bousquet, L. Bucci, P. Chang, J. Cione, S. Ditchek, J. Doyle, J.-P. Duvel, J. Elston, G. Goni, K.K. Hon, K. Ito, Z. Jelenak, X. Lei, R. Lumpkin, C.R. McMahon, C. Reason, E. Sanabia, L.K. Shay, J.A. Sippel, A. Sushko, J. Tang, K. Tsuboki, H. Yamada, J. Zawislak, and J.A. Zhang. Recent advancements in aircraft and in situ observations of tropical cyclones. Tropical Cyclone Research and Review, 12(2):81-99, https://doi.org/10.1016/j.tcrr.2023.06.001 2023
Observations of tropical cyclones (TC) from aircraft and in situ platforms provide critical and unique information for analyzing and forecasting TC intensity, structure, track, and their associated hazards. This report, prepared for the tenth International Workshop on Tropical Cyclones (IWTC-10), discusses the data collected around the world in TCs over the past four years since the IWTC-9, improvements to observing techniques, new instruments designed to achieve sustained and targeted atmospheric and oceanic observations, and select research results related to these observations. In the Atlantic and Eastern and Central Pacific basins, changes to operational aircraft reconnaissance are discussed along with several of the research field campaigns that have taken place recently. The changes in the use and impact of these aircraft observations in numerical weather prediction models are also provided along with updates on some of the experimental aircraft instrumentation. Highlights from three field campaigns in the Western Pacific basin are also discussed. Examples of in-situ data collected within recent TCs such as Hurricane Ian (2022), also demonstrate that new, emerging technologies and observation strategies reviewed in this report, definitely have the potential to further improve ocean-atmosphere coupled intensity forecasts.
Kim, D., S.-K. Lee, H. Lopez, G.R. Foltz, C. Wen, R. West, and J. Dunion. Increase in Cape Verde hurricanes during Atlantic Niño. Nature Communications, 14:3704, https://doi.org/10.1038/s41467-023-39467-5 2023
At seasonal-to-interannual timescales, Atlantic hurricane activity is greatly modulated by El Niño–Southern Oscillation and the Atlantic Meridional Mode. However, those climate modes develop predominantly in boreal winter or spring and are weaker during the Atlantic hurricane season (June–November). The leading mode of tropical Atlantic sea surface temperature (SST) variability during the Atlantic hurricane season is Atlantic Niño/Niña, which is characterized by warm/cold SST anomalies in the eastern equatorial Atlantic. However, the linkage between Atlantic Niño/Niña and hurricane activity has not been examined. Here, we use observations to show that Atlantic Niño, by strengthening the Atlantic inter-tropical convergence zone rainband, enhances African easterly wave activity and low-level cyclonic vorticity across the deep tropical eastern North Atlantic. We show that such conditions increase the likelihood of powerful hurricanes developing in the deep tropics near the Cape Verde islands, elevating the risk of major hurricanes impacting the Caribbean islands and the U.S.
Ko, M.-C., X. Chen, M. Kubat, and S. Gopalakrishnan. The development of a consensus machine learning model for hurricane rapid intensification with Hurricane Weather Research and Forecasting (HWRF) data. Weather and Forecasting, 38(8):1253-1270, https://doi.org/10.1175/WAF-D-22-0217.1 2023
This study focused on developing a consensus machine learning (CML) model for tropical cyclone (TC) intensity-change forecasting, especially for rapid intensification (RI). This CML model was built upon selected classical machine learning models with the input data extracted from a high-resolution hurricane model, the Hurricane Weather Research and Forecasting (HWRF) system. The input data contained 21 or 34 RI-related predictors extracted from the 2018 version of HWRF (H218). This study found that TC inner-core predictors can be critical for improving RI predictions, especially the inner-core relative humidity. Moreover, this study emphasized that the importance of performing resampling on an imbalanced input dataset. Edited Nearest Neighbor and Synthetic Minority Oversampling Technique improved the Probability of Detection (POD) by ∼10% for the RI class. This paper also showed that the CML model has satisfactory performance on RI predictions compared to the operational models. CML reached 56% POD and 46% False Alarm Ratio (FAR), while the operational models had only 10 to 30% POD but 50 to 60% FAR. The CML performance on the non-RI classes was comparable to the operational models. The results indicated that, with proper and sufficient training data and RI-related predictors, CML has the potential to provide reliable probabilistic RI forecasts during a hurricane season.
Kurosawa, K., and J. Poterjoy. A statistical hypothesis testing strategy for adaptively blending particle filters and ensemble Kalman filters for data assimilation. Monthly Weather Review, 151(1):105-125, https://doi.org/10.1175/MWR-D-22-0108 2023
Particle filters avoid parametric estimates for Bayesian posterior densities, which alleviates Gaussian assumptions in nonlinear regimes. These methods, however, are more sensitive to sampling errors than Gaussian-based techniques such as ensemble Kalman filters. A recent study by the authors introduced an iterative strategy for particle filters that match posterior moments–where iterations improve the filter’s ability to draw samples from non-Gaussian posterior densities. The iterations follow from a factorization of particle weights, providing a natural framework for combining particle filters with alternative filters to mitigate the impact of sampling errors. The current study introduces a novel approach to forming an adaptive hybrid data assimilation methodology, exploiting the theoretical strengths of non-parametric and parametric filters. At each data assimilation cycle, the iterative particle filter performs a sequence of updates while the prior sample distribution is non-Gaussian, then an ensemble Kalman filter provides the final adjustment when Gaussian distributions for marginal quantities are detected. The method employs the Shapiro-Wilk test to determine when to make the transition between filter algorithms, which has outstanding power for detecting departures from normality. Experiments using low-dimensional models demonstrate that the approach has significant value, especially for non-homogeneous observation networks and unknown model process errors. Moreover, hybrid factors are extended to consider marginals of more than one co-located variables using a test for multivariate normality. Findings from this study motivate the use of the proposed method for geophysical problems characterized by diverse observation networks and various dynamic instabilities, such as numerical weather prediction models.
Li, M., J.A. Zhang, L. Matak, and M. Momen. The impacts of adjusting momentum roughness length on strong and weak hurricane forecasts: A comprehensive analysis of weather simulations and observations. Monthly Weather Review, 150(5):1287-1302, https://doi.org/10.1175/MWR-D-22-0191.1 2023
The momentum roughness length (z0) significantly impacts wind predictions in weather and climate models. Nevertheless, the impacts of z0 parameterizations in different wind regimes and various model configurations on the hurricane size, intensity, and track simulations have not been thoroughly established. To bridge this knowledge gap, a comprehensive analysis of 310 simulations of 10 real hurricanes using the Weather Research and Forecasting (WRF) model is conducted in comparison with observations. Our results show that the default z0 parameterizations in WRF perform well for weak (category 1-2) hurricanes; however, they underestimate the intensities of strong (category 3-5) hurricanes. This finding is independent of model resolution or boundary layer schemes. The default values of z0 in WRF agree with the observational estimates from dropsonde data in weak hurricanes while they are much larger than observations in strong hurricanes regime. Decreasing z0 close to the values of observational estimates and theoretical hurricane intensity models in high wind regimes (≳ 45 m s-1) led to significant improvements in the intensity forecasts of strong hurricanes. A momentum budget analysis dynamically explained why the reduction of z0 (decreased surface turbulent stresses) leads to stronger simulated storms.
Li, X., Z. Pu, J.A. Zhang, and Z. Zhang. A modified vertical eddy diffusivity parameterization in the HWRF model based on large eddy simulations and its impact on the prediction of two landfalling hurricanes. Frontiers in Earth Science, 11:1320192, https://doi.org/10.3389/feart.2023.1320192 2023
Vertical eddy diffusivity (VED) in the planetary boundary layer (PBL) has a significant impact on forecasts of tropical cyclone (TC) structure and intensity. VED uncertainties in PBL parameterizations can be partly attributed to the model's inability to represent roll vortices (RV). In this study, RV effects on turbulent fluxes derived from a large eddy simulation (LES) by Li and Pu (2021) are added to the VED parameterization of the PBL scheme within the operational Hurricane Weather Research and Forecasting (HWRF) model. RV contribution to VED is parameterized through a coefficient and varies with the RV intensity and velocity scale. A modification over land has also been implemented. This modified VED parameterization is compared with the original wind-speed-dependent VED scheme in HWRF. Retrospective HWRF forecasts of Hurricanes Florence (2018) and Laura (2020) are analyzed to evaluate the impacts of the modified VED scheme on landfalling hurricane forecasts.Results show that the modified PBL scheme with the RV effect leads to an improvement in 10-m maximum wind speed forecasts of 14%-31%, with a neutral to positive improvement for track forecasts. Improved wind structure and precipitation forecasts against observations are also noted with the modified PBL scheme. Further diagnoses indicate that the revised PBL scheme enhances moist entropy in the boundary layer over land, leading to improved TC intensity prediction compared to the original scheme.
Li, Z., A. Tiwari, X. Sui, J. Garrison, F. Marks, and D. Niyogi. Studying brown ocean re-intensification of Hurricane Florence using CYGNSS and SMAP soil moisture data and a numerical weather model. Geophysical Research Letters, 50(19):e2023GL105102, https://doi.org/10.1029/2023GL105102 2023
Hurricane Florence made landfall over the Carolinas 14 September 2018, bringing over 30 inches of rainfall. What remains understudied is the possible storm re-intensification by wet and warm antecedent soil moisture (ASM), known as the Brown Ocean Effect (BOE). This study investigates this effect with two approaches: (a) two satellite-based soil moisture (SM) data and (b) model simulation. The averaged Cyclone Global Navigation System and Soil Moisture Active Passive SM enables examination of land-atmosphere interaction at a sub-daily scale. Both observations and simulation results manifest positive feedback between ASM and rainfall intensity, with 3 days prior to landfall being the typical antecedent time scale. Wet (dry) ASM lead to intense (light) and concentrated (widespread) rains. We also found that soil temperature can modulate the BOE. This study aims to advance our understanding of land-atmosphere feedback and calls to acquire accurate antecedent land states to enhance forecast skills.
Ma, Z., Z. Li, J. Li, M. Min, J. Sun, X. Wei, T.J. Schmit, and L. Cucurull. An enhanced storm warning and nowcasting model in pre-convention environment. Remote Sensing, 15(10):2672, https://doi.org/10.3390/rs15102672 2023
A storm tracking and nowcasting model was developed for the contiguous US (CONUS) by combining observations from the advanced baseline imager (ABI) and numerical weather prediction (NWP) short-range forecast data, along with the precipitation rate from CMORPH (the Climate Prediction Center morphing technique). A random forest-based model was adopted by using the maximum precipitation rate as the benchmark for convection intensity, with the location and time of storms optimized by using optical flow (OF) and continuous tracking. Comparative evaluations showed that the optimized models had higher accuracy for severe storms with areas equal to or larger than 5000 km2 over smaller samples, and lower accuracy for cases smaller than 1000 km2, while models with sample-balancing applied showed higher possibilities of detection (PODs). A typical convective event from August 2019 was presented to illustrate the application of the nowcasting model on local severe storm (LSS) identification and warnings in the pre-convection stage; the model successfully provided warnings with a lead time of 1–2 h before heavy rainfall. Importance score analysis showed that the overall impact from ABI observations was much higher than that from NWP, with the brightness temperature difference between 6.2 and 10.3 microns ranking at the top in terms of feature importance.
Ming, J., J.A. Zhang, X. Li, Z. Pu, and M. Momen. Observational estimates of turbulence parameters in the atmospheric surface layer of landfalling tropical cyclones. Journal of Geophysical Research-Atmospheres, 128(17):e2022JD037768, https://doi.org/10.1029/2022JD037768 2023
This study analyzes observations collected by multilevel towers to estimate turbulence parameters in the atmospheric surface layer of two landfalling tropical cyclones (TCs). The momentum flux, turbulent kinetic energy (TKE) and dissipation rate increase with the wind speed independent of surface types. However, the momentum flux and TKE are much larger over land than over the coastal ocean at a given wind speed range. The vertical eddy diffusivity is directly estimated using the momentum flux and strain rate, which more quickly increases with the wind speed over a rougher surface. Comparisons of the eddy diffusivity estimated using the direct flux method and that using the friction velocity and height show good agreement. On the other hand, the traditional TKE method overestimates the eddy diffusivity compared to the direct flux method. The scaling coefficients in the TKE method are derived for the two different surface types to better match with the vertical eddy diffusivity based on the direct flux method. Some guidance to improve vertical diffusion parameterizations for TC landfall forecasts in weather simulations are also provided.
Osborne E., C. Martinez, S.D. Aberson, K. Nelson, S. Duncan, C. Ryals, F. Munoz, and T. Griffin-Elliott. Reimagining policies, practices, and culture to prevent and respond to sexual assault and sexual harassment at NOAA. Oceanography, 36(4):62-65, https://doi.org/10.5670/oceanog.2024.121 2023
Patel, P., K. Ankur, S. Jamshidi, A. Tiwari, R. Nadimpalli, N.K.R. Busireddy, S. Safaee, K.K. Osuri, S. Karmakar, S. Ghosh, D. Aliaga, J. Smith, F. Marks, Z.-L. Yang, and D. Niyogi. Impact of urban representation on simulation of hurricane rainfall. Geophysical Research Letters, 50(21):e2023GL104078, https://doi.org/10.1029/2023GL104078 2023
Taking the examples of Hurricane Florence (2018) over the Carolinas and Hurricane Harvey (2017) over the Texas Gulf Coast, the study attempts to understand the performance of slab, single-layer Urban Canopy Model (UCM), and Building Environment Parameterization (BEP) in simulating hurricane rainfall using the Weather Research and Forecasting (WRF) model. The WRF model simulations showed that for an intense, large-scale event such as a hurricane, the model quantitative precipitation forecast over the urban domain was sensitive to the model urban physics. The spatial and temporal verification using the modified Kling-Gupta efficiency and Method for Object based Diagnostic and Evaluation in Time Domain suggests that UCM performance is superior to the BEP scheme. Additionally, using the BEP urban physics scheme over UCM for landfalling hurricane rainfall simulations has helped simulate heavy rainfall hotspots.
Poyer, A., W. Komaromi, S. Gopalakrishnan, L. Wolf, F. Marks, G. Alaka Jr., J. Anderson, V. Tallapragada, M. Brennan, A. Mehra, X. Zhang, Z. Zhang, A. Hazelton, D.A. Zelinsky, J.L. Franklin, A. Aksoy, C. Alexander, M. Bender, L. Bernardet, M. Biswas, J. Cangialosi, M. DeMaria, R. Dunlap, M. Ek, G. Eosco, L. Gramer, L. Harris, J.S. Hilderbrand, E. Kalina, H.-S. Kim, P. Kucera, B. Liu, P. McCaslin, T. Marchok, J. Moskaitis, K. Musgrave, L. Nance, K. Newman, M. Onderlinde, W. Ramstrom, D. Rosen, J. Sims, J. Sippel, D. Stern, R. Torn, X. Wang, W. Wang, Y. Weng, B.C. Zachry, C. Zhang, M. Zhang, and L. Zhu. 2021-2022 HFIP R&D activities summary: Recent results and operational implementation. HFIP Technical Report, HFIP-2023-1, 73 pp., https://doi.org/10.25923/exgj-1n68 2023
Rajasree, P.M., X. Cao, H. Ramsay, K.M. Nunez-Ocasio, G. Kilroy, G.R. Alvey III, M. Chang, C.C. Nam, H. Fudeyasu, H.-F. Teng, and H. Yu. Tropical cyclogenesis: Controlling factors and physical mechanisms. Tropical Cyclone Research and Review, 12(3):165-181, https://doi.org/10.1016/j.tcrr.2023.09.004 2023
In this review, advances in the understanding of the controlling factors and physical mechanisms of tropical cyclogenesis (TCG) are summarized from recent (2018-2022) research on TCG, as presented in the Tenth International Workshop on Tropical Cyclones (IWTC-10). Observational, theoretical, and numerical modeling studies published in recent years have advanced our knowledge on the influence of large-scale environmental factors on TCG. Furthermore, studies have shown clearly that appropriate convective coupling with tropical equatorial waves enhances the development chances of TCG. More recently, illuminating research has been carried out on analyzing the mechanisms by which oscillations and teleconnections (El Niño Southern Oscillation (ENSO) in particular) modulate TCG globally, in association with changes in the sea surface temperature (SST). In addition to this, recent research has diligently addressed different aspects of TCG. Multiple studies have reported the applicability of unified theories and physical mechanisms of TCG in different ocean basins. Recently, research has been carried out on TCG under different flow pattern regimes, dry air intrusion, importance of marsupial pouch, genesis of Medicanes, wind shear, convection and vertical structure. Furthermore, studies have discussed the possibility of near equatorial TCG provided that there is enough supply of background vertical vorticity and relatively low vertical wind shear. Progress has been made to understand the role of climate change on global and regional TCG. However, there are still significant gaps which need to be addressed in order to better understand TCG prediction.
Rogers, R.F., and J.A. Zhang. Airborne Doppler radar observations of tropical cyclone boundary layer kinematic structure and evolution during landfall. Geophysical Research Letters, 50(23):e2023GL105548, https://doi.org/10.1029/2023GL105548 2023
Airborne Doppler radar observations of the wind field in the tropical cyclone boundary layer (TCBL) during the landfall of Hurricane Ida (2021) are examined here. Asymmetries in tangential and radial flow are governed by tropical cyclone (TC) motion and vertical wind shear prior to landfall, while frictional effects dominate the asymmetry location during landfall. Strong TCBL inflow on the offshore-flow side of Ida occurs during landfall, while the location of the peak tangential wind at the top of the TCBL during this period is located on the onshore-flow side. A comparison of these observations with a numerical simulation of TC landfall shows many consistencies with the modeling study, though there are some notable differences that may be related to differences in the characteristics of the land surface between the simulation and the observations here.
Rogers, R.F., J. Courtney, and K. Wood. The World Meteorological Organization Tenth International Workshop on Tropical cyclones (IWTC-10): A summary. Tropical Cyclone Research and Review, 12(1):1-9, https://doi.org/10.1016/j.tcrr.2023.04.001 2023
The Tenth International Workshop on Tropical Cyclones (IWTC-10) occurred from 5-9 December 2022 in Bali, Indonesia. This workshop continued the goal of the original IWTC, held in 1985 in Bangkok, Thailand, to bring together forecasters and researchers from countries around the world that are impacted by tropical cyclones (TCs) to discuss the latest research and forecast advances and share best practices to improve TC forecasts globally. The workshops have continued as a regular feature of WMO efforts to encourage the advancement of TC forecasting and improve ways of communicating TC hazards to the general public. Global TC forecasting efforts in the past 10-15 years have emphasized hazards and impacts of landfalling TCs beyond just track and intensity. Additionally, there has been a growing interest in improving the communication of these hazards and impacts, using concepts from social and behavioral sciences, in ways that can lead to effective decision-making from stakeholders (e.g., government officials, emergency managers, media, general public). As such, the theme for IWTC-10 was “Improved TC science and services for better decision-making.” More about this theme, how the workshop was structured around it, and key outcomes from the workshop are discussed in this summary article.
Rosencrans, M., E.S. Blake, C.W. Landsea, H. Wang, S.B. Goldenberg, R.J. Pasch, and D.S. Harnos. The tropics: Tropical cyclones—Atlantic basin. In Chapter 4, State of the Climate in 2022). Bulletin of the American Meteorological Society, 104(9):S232-S239, https://doi.org/10.1175/BAMS-D-23-0078.1 2023
Sellwood, K.J., J.A. Sippel, and A. Aksoy. Assimilation of Coyote small uncrewed aircraft system observations in Hurricane Maria (2017) using operational HWRF. Weather and Forecasting, 38(6):901-919, https://doi.org/10.1175/WAF-D-22-0214.1 2023
This study presents an initial demonstration of assimilating small Uncrewed Aircraft System (sUAS) data into an operational model with a goal to ultimately improve tropical cyclone (TC) analyses and forecasts. The observations, obtained using the Coyote sUAS in Hurricane Maria (2017), were assimilated into the operational Hurricane Weather Research and Forecast system (HWRF) as they could be in operations. Results suggest that the Coyote data can benefit HWRF forecasts. A single-cycle case study produced the best results when the Coyote observations were assimilated at greater horizontal resolution with more relaxed quality control (QC) than comparable flight-level high-density observations currently used in operations. The case study results guided experiments that cycled HWRF for a roughly four-day period that covered all Coyote flights into Maria. The cycled experiment that assimilated the most data improved initial inner-core structure in the analyses and better agreed with other aircraft observations. The average errors in track and intensity decreased in the subsequent forecasts. Intensity forecasts were too weak when no Coyote data was assimilated, and assimilating the Coyote data made the forecasts stronger. Results also suggest that a symmetric distribution of Coyote data around the TC center is necessary to maximize its benefits in the current configuration of operational HWRF. Although the sample size was limited, these experiments provide insight for potential operational use of data from newer sUAS platforms in future TC applications.
Stackhouse, S.D., S.E. Zick, C.J. Matyas, K.M. Wood, A.T. Hazelton, and G.J. Alaka Jr. Evaluation of experimental high-resolution model forecasts of tropical cyclone precipitation using object-based metrics. Weather and Forecasting, 38(10):2111-2134, https://doi.org/10.1175/WAF-D-22-0223.1 2023
Tropical cyclone (TC) precipitation poses serious hazards including freshwater flooding. High-resolution hurricane models predict the location and intensity of TC rainfall, which can influence local evacuation and preparedness policies. This study evaluates 0–72-hour precipitation forecasts from two experimental models, the Hurricane Analysis and Forecast System (HAFS) model and the Basin-scale Hurricane Weather Research and Forecasting (HWRF-B) model, for 2020 North Atlantic landfalling TCs. We use an object-based method that quantifies the shape and size of the forecast and observed precipitation. Precipitation objects are then compared for light, moderate, and heavy precipitation using spatial metrics (e.g., area, perimeter, elongation). Results show that both models forecast precipitation that is too connected, too close to the TC center, too enclosed around the TC center. Collectively, these spatial biases suggest that the model forecasts are too intense even though there is a negative intensity bias for both models, indicating there may be an inconsistency between the precipitation configuration and the maximum sustained winds in the model forecasts. The HAFS model struggles with forecasting stratiform versus convective precipitation and with the representation of lighter (stratiform) precipitation during the first six hours after initialization. No such spin-up issues are seen in the HWRF-B forecasts, which instead exhibit systematic biases at all lead times and systematic issues across all rain rate thresholds. Future work will investigate spin-up issues in the HAFS model forecast and how the microphysics parameterization affects the representation of precipitation in both models.
Stone, Z., G.R. Alvey III, J.P. Dunion, M.S. Fischer, D.J. Raymond, R.F. Rogers, S. Sentic, and J. Zawislak. Thermodynamic contribution to vortex alignment and rapid intensification of Hurricane Sally (2020). Monthly Weather Review, 151(4):931-951, https://doi.org/10.1175/MWR-D-22-0201.1 2023
As a part of the Tropical Cyclone Rapid Intensification Project (TCRI), observations were made of the rapid intensification of Hurricane Sally (2020) as it passed over the Gulf ofMexico. High-altitude dropsondes and radar observations from NOAA’s Gulfstream IV, radar observations from WP-3D aircraft, the WSR-88D ground radar network, satellite images and satellite-detected lightning strikes are used to apply recently developed theoretical knowledge about tropical cyclone intensification. As observed in many other tropical cyclones, strong, bottom-heavy vertical mass flux profiles are correlated with low (but positive) values of low to mid-level moist convective instability along with high column relative humidity. Such mass flux profiles produce rapid spinup at low levels and the environmental conditions giving rise to them are associated with an intense mid-level vortex. This low-level spinup underneath the mid-level vortex results in the vertical alignment of the vortex column which is a key step in the rapid intensification process. In the case of Sally, the spinup of low-level vortex resulted from vorticity stretching, while the spinup of the mid-level vortex at 6 km resulted from vorticity tilting produced by the interaction of convective ascent with moderate vertical shear.
Wadler, J.B., D.S. Nolan, J.A. Zhang, L.K. Shay, J.B. Olson, and J.J. Cione. The effect of advection on the three-dimensional distribution of turbulent kinetic energy and its generation in idealized tropical cyclone simulations. Journal of Advances in Modeling Earth Systems, 15(5):e2022MS003230, https://doi.org/10.1029/2022MS003230 2023
The distribution of turbulent kinetic energy (TKE) and its budget terms is estimated in simulated tropical cyclones (TCs) of various intensities. Each simulated TC is subject to storm motion, wind shear, and oceanic coupling. Different storm intensities are achieved through different ocean profiles in the model initialization. For each oceanic profile, the atmospheric simulations are performed with and without TKE advection. In all simulations, the TKE is maximized at low levels (i.e., below 1 km) and ∼0.5 km radially inward of the azimuthal-mean radius of maximum wind speed at 1-km height. As in a previous study, the axisymmetric TKE decreases with height in the eyewall, but more abruptly in simulations without TKE advection. The largest TKE budget terms are shear generation and dissipation, though variability in vertical turbulent transport and buoyancy production affect the change in the azimuthal-mean TKE distribution. The general relationships between the TKE budget terms are consistent across different radii, regardless of storm intensity. In terms of the asymmetric distribution in the eyewall, TKE is maximized in the front-left quadrant where the sea surface temperature (SST) is highest and is minimized in the rear-right quadrant where the SST is the lowest. In the category-5 simulation, the height of the TKE maximum varies significantly in the eyewall between quadrants and is between ∼400 m in the rear-right quadrant and ∼1,000 m in the front-left quadrant. When TKE advection is included in the simulations, the maximum eyewall TKE values are downwind compared to the simulations without TKE advection.
Wadler, J.B., J.E. Rudzin, B. Jaimes de la Cruz, J. Chen, M.S. Fischer, G. Chen, N. Qin, B. Tang, and Q. Li. A review of recent research progress on the effects of external influences on tropical cyclone intensity change. Tropical Cyclone Research and Review, 12(3):200-215, https://doi.org/10.1016/j.tcrr.2023.09.001 2023
Over the past four years, significant research has advanced our understanding of how external factors influence tropical cyclone (TC) intensity changes. Research on air-sea interactions shows that increasing the moisture disequilibrium is a very effective way to increase surface heat fluxes and that ocean salinity-stratification plays a non-negligible part in TC intensity change. Vertical wind shear from the environment induces vortex misalignment, which controls the onset of significant TC intensification. Blocking due to upper-level outflow from TCs can reduce the magnitude of vertical wind shear, making for TC intensification. Enhanced TC-trough interactions are vital for rapid intensification in some TC cases because of strengthened warm air advection, but upper-level troughs are found to limit TC intensification in other cases due to dry midlevel air intrusions and increased shear. Aerosol effects on TCs can be divided into direct effects involving aerosol-radiation interactions and indirect effects involving aerosol-cloud interactions. The radiation absorption by the aerosols can change the temperature profile and affect outer rainbands through changes in stability and microphysics. Sea spray and sea salt aerosols are more important in the inner region, where the aerosols increase precipitation and latent heating, promoting more intensification. For landfalling TCs, the intensity decay is initially more sensitive to surface roughness than soil moisture, and the subsequent decay is mainly due to the rapid reduction in surface moisture fluxes. These new insights further sharpen our understanding of the mechanisms by which external factors influence TC intensity changes.
Wadler, J.B., J.J. Cione, R.F. Rogers, and M.S. Fischer. On the distribution of convective and stratiform precipitation in tropical cyclones from airborne Doppler radar and its relationship to intensity change and environmental wind shear direction. Monthly Weather Review, 151(12):3209-3233, https://doi.org/10.1175/MWR-D-23-0048.1 2023
Airborne Doppler radar reflectivity data collected in hurricanes on the NOAA P-3 aircraft between 1997 and 2021 were parsed into different modes of precipitation: stratiform precipitation, shallow convection, moderate convection, and deep convection. Stratiform precipitation was the most frequent precipitation mode with 82.6% of all observed precipitation while deep convection was the most infrequent at 1.3%. When stratified by 12-hr intensity change, intensifying TCs had a greater areal coverage of total convection in the eyewall compared to weakening and steady-state TCs. The largest difference in the azimuthal distributions in the precipitation modes was in deep convection, which was mostly confined to the downshear-left quadrant in weakening and steady-state hurricanes and more symmetrically distributed in intensifying hurricanes. For all intensity change categories, the most symmetrically distributed precipitation mode was stratiform rain. To build upon the results of a recent thermodynamic study, the precipitation data were recategorized for hurricanes experiencing deep-layer wind shear with either a northerly-component or southerly-component. Like intensifying storms, hurricanes that experienced northerly-component shear had a more symmetric distribution of deep convection than southerly-component shear storms, which had a distribution of deep convection that resembled weakening storms. The greatest difference in the precipitation distributions between the shear direction groups were in major hurricanes experiencing moderate (4.5–11 m s−1) wind shear values. Consistent with previous airborne radar studies, the results suggest that considering the distribution of deep convection and the thermodynamic distributions associated with differing environmental wind shear direction could aid TC intensity forecasts.
Wang, W., Z. Zhang, J.P. Cangialosi, M. Brennan, L. Cowan, P. Clegg, T. Hosomi, I. Masaaki, A. Kumar Das, M. Mohapatra, M. Sharma, J. Knaff, J. Kaplan, T. Birchard, J. Doyle, J. Heming, J. Moskaitis, W. Komaromi, S. Ma, C. Sampson, L. Wu, and E. Blake. A review of recent advances (2018-2021) on tropical cyclone intensity change from operational perspectives, Part 2: Forecasts by operational centers. Tropical Cyclone Research and Review, 12(1):50-63, https://doi.org/10.1016/j.tcrr.2023.05.003 2023
This paper summarizes the progress and activities of tropical cyclone (TC) operational forecast centers during the last four years (2018-2021). It is part II of the review on TC intensity change from the operational perspective in the rapporteur report presented to the 10th International Workshop on TCs (IWTC) held in Bali, Indonesia, from Dec. 5 - 9, 2022. Part I of the review has focused on the progress of dynamical model forecast guidance. This part discusses the performance of TC intensity and rapid intensification forecasts from several operational centers. It is shown that the TC intensity forecast errors have continued to decrease since the 9th IWTC held in 2018. In particular, the improvement of rapid intensification forecasts has accelerated, compared with years before 2018. Consensus models, operational procedures, tools and techniques, as well as recent challenging cases from 2018-2021 identified by operational forecast centers are described. Research needs and recommendations are also discussed.
Wu, S.-N., B. Soden, and G.J. Alaka. The influence of radiation on the prediction of tropical cyclone intensification in a forecast model. Geophysical Research Letters, 50(2):e2022GL099442, https://doi.org/10.1029/2022GL099442 2023
This study examines the influence of radiative heating on the prediction of tropical cyclone (TC) intensification in the Hurricane Weather Research and Forecasting (HWRF) model. Previous idealized modeling and observational studies demonstrated that radiative heating can substantially modulate the evolution of TC intensity. However, the relevance of this process under realistic conditions remains unclear. Here, we use observed longwave radiative heating to explore the performance of TC forecasts in HWRF simulations. The performance of TC intensity forecasts is then investigated in the context of radiative heating forecasts. In observations and HWRF forecasts, high clouds near the TC center increase the convergence of radiative fluxes. A sharp spatial gradient (≥60 W/m2) in the flux convergence from the TC center outward toward the environment is associated with subsequent TC intensification. More accurate simulation of the spatial structure of longwave radiative heating is associated with more accurate TC intensity forecasts.
Zhang, C., G.R. Foltz, A.M. Chiodi, C.W. Mordy, C.R. Edwards, C. Meinig, D. Zhang, E. Mazza, E.D. Cokelet, E.F. Burger, F. Bringas, G.J. Goni, H.G. Hristova, H.-S. Kim, J.A. Trinanes, J.A. Zhang, K.E. Bailey, K.M. O’Brien, M. Morales-Caez, N. Lawrence-Slavas, R. Jenkins, S.S. Chen, and X. Chen. Hurricane observations by uncrewed systems. Bulletin of the American Meteorological Society, 104(10):E1893-E1917, https://doi.org/10.1175/BAMS-D-21-0327.1 2023
On 30 September 2021, a saildrone uncrewed surface vehicle (USV) was steered into Category 4 Hurricane Sam, the most intense storm of the 2021 Atlantic hurricane season. It measured significant wave heights up to 14 m (maximum wave height 27 m) and near-surface winds exceeding 55 m s−1. This was the first time in more than seven decades of hurricane observations that in real time a USV transmitted scientific data, images, and videos of the dynamic ocean surface near a hurricane’s eyewall. The saildrone was part of a five-saildrone deployment of the NOAA 2021 Atlantic Hurricane Observations Mission. These saildrones observed the atmospheric and oceanic near-surface conditions of five other tropical storms, of which two became hurricanes. Such observations inside tropical cyclones help to advance the understanding and prediction of hurricanes, with the ultimate goal of saving lives and protecting property. The 2021 deployment pioneered a new practice of coordinating measurements by saildrones, underwater gliders, and airborne dropsondes to make simultaneous and near-collocated observations of the air-sea interface, the ocean immediately below, and the atmosphere immediately above. This experimental deployment opened the door to a new era of using remotely piloted uncrewed systems to observe one of the most extreme phenomena on Earth in a way previously impossible. This article provides an overview of this saildrone hurricane observations mission, describes how the saildrones were coordinated with other observing platforms, presents preliminary scientific results from these observations to demonstrate their potential utility and motivate further data analysis, and offers a vision of future hurricane observations using combined uncrewed platforms.
Zhang, D., A.M. Chiodi, C. Zhang, G.R. Foltz, M.F. Cronin, C.W. Mordy, J. Cross, E.D. Cokelet, J.A. Zhang, C. Meinig, N. Lawrence-Slavas, P.J. Stabeno, and R. Jenkins. Observing extreme ocean and weather events using innovative saildrone uncrewed surface vehicles. Oceanography, 36(2-3):70-77, https://doi.org/10.5670/oceanog.2023.214 2023
Extreme ocean events and severe weather systems have large environmental impacts but are under-observed due to their harsh conditions and associated challenges with deployments of in situ observing platforms. Through a public-private partnership, the NOAA Pacific Marine Environmental Laboratory (PMEL) has developed the saildrone uncrewed surface vehicle (USV) into a viable air-sea interaction observing platform that can be utilized by the broader ocean research community. PMEL and the NOAA Atlantic Oceanographic and Meteorological Laboratory have demonstrated the potential of USVs for observing the Arctic marginal ice zone during the seasonal Arctic ice retreat and for observing the extreme ocean and weather conditions inside major hurricanes. These USVs will be an essential part of the Global Ocean Observing System, providing real-time data to improve prediction of rapid climate change and extreme ocean and weather events and to reduce their harmful impacts.
Zhang, J.A., R.F. Rogers, P.D. Reasor, and J. Gamache. The mean kinematic structure of the tropical cyclone boundary layer and its relationship to intensity change. Monthly Weather Review, 151(1):63-84, https://doi.org/10.1175/MWR-D-21-0335 2023
This study investigates the relationship between the azimuthally averaged kinematic structure of the tropical cyclone boundary layer (TCBL) and storm intensity, intensity change, and vortex structure above the BL. These relationships are explored using composites of airborne Doppler radar vertical profiles, which have a higher vertical resolution than typically used three-dimensional analyses and therefore better capture TCBL structure. Results show that the BL height, defined by the depth of the inflow layer, is greater in weak storms than in strong storms. The inflow layer outside the radius of maximum tangential wind speed (RMW) is deeper in intensifying storms than in non-intensifying storms at an early stage. The peak BL convergence inside the RMW is larger in intensifying storms than in non-intensifying storms. Updrafts originating from the TCBL are concentrated near the RMW for intensifying TCs, while updrafts span a large radial range outside the RMW for non-intensifying TCs. In terms of vortex structure above the BL, storms with a quickly-decaying radial profile of tangential wind outside the RMW (“narrow” vortices) tend to have a deeper inflow layer outside the RMW, stronger inflow near the RMW, deeper and more concentrated strong updrafts close to the RMW, and weaker inflow in the outer core region than those with a slowly-decaying tangential wind profile (“broad” vortices). The narrow TCs also tend to intensify faster than broad TCs, suggesting that a key relationship exists among vortex shape, the BL kinematic structure, and TC intensity change. This relationship is further explored by comparisons of absolute angular momentum budget terms for each vortex shape.
Zhang, X., S.D. Ditchek, K.L. Corbosiero, and W. Xu. Global and regional characteristics of radially outward propagating tropical cyclone diurnal pulses. Journal of Geophysical Research-Atmospheres, 128(7):e2022JD037660, https://doi.org/10.1029/2022JD037660 2023
The radially-outward propagating, cloud-top cooling, diurnal pulse (DP) is a prominent feature in tropical cyclones (TCs) that has important implications for changes in TC structure and intensity. By using an objective identification algorithm, this study characterizes DPs over various ocean basins and examines their environmental conditions and convective structures. DPs occur on 52% of TC days globally and the occurrence frequency exhibits significant regional variability. The Northwest Pacific (NWP) has the highest DP frequency (60%) and shares the largest fraction of DPs worldwide (34%).The median duration and propagation distance of DPs are 12–15 h and 500–600 km, respectively. Although the mean propagation speed of DPs is 11–13 m s-1, persistent DPs (lasting >15 h) mostly propagate at speeds similar to internal inertial gravity waves (5–10 m s-1). Additionally, the longer the pulse duration, the stronger the pulse amplitude. Further, most DPs initiate in the inner core overnight, in phase with inner-core deep convection. Inner-core cold clouds, precipitation, and lightning are all markedly enhanced on DP days compared to non-DP days. Interestingly, the DP signal significantly weakens and becomes slower while propagating through the 200–400-km annulus during 09–12 local time (LT). Finally, DPs are more likely to occur over warm sea surface temperatures (SSTs), in low shear, and with a moist mid- to upper-troposphere. SST plays an important role in DP development over all basins, while shear and humidity are less important in the NEP and NA basins.
Zhang, Z., W. Wang, J. Doyle, J. Moskaitis, W. Komaromi, J. Heming, L. Magnusson, J.P. Cangialosi, L. Cowan, M. Brennan, S. Ma, A. Kumar Dos, T. Hosomi, P. Clegg, T. Birchard, J. Knaff, J. Kaplan, M. Mohapatra, M. Sharma, I. Masaaki, and E. Blake. A review of recent advances (2018-2021) on tropical cyclone intensity change from operational perspectives, Part 1: Dynamical model guidance. Tropical Cyclone Research and Review, 12(1):30-49, https://doi.org/10.1016/j.tcrr.2023.05.004 2023
This review summarizes the rapporteur report on tropical cyclone (TC) intensity change from the operational perspective, as presented to the 10th International Workshop on TCs (IWTC-10) held in Bali, Indonesia, from Dec. 5 - 9, 2022. The accuracy of TC intensity forecasts issued by operational forecast centers depends on three aspects: real-time observations, TC dynamical model forecast guidance, and techniques and methods used by forecasters. The rapporteur report covers the progress made over the past four years (2018-2021) in all three aspects. This review focuses on the progress of dynamical model forecast guidance. The companion paper (Part II) summarizes the advance from operational centers. The dynamical model forecast guidance continues to be the main factor leading to the improvement of operational TC intensity forecasts. Here, we describe recent advances and developments of major operational regional dynamical TC models and their intensity forecast performance, including HWRF, HMON, COAMPS-TC, Met Office Regional Model, CMA-TYM, and newly developed HAFS. The performance of global dynamical models, including NOAA's GFS, Met Office Global Model (MOGM), JMA's GSM, and IFS (ECMWF), has also been improved in recent years due to their increased horizontal and vertical resolution as well as improved data assimilation systems. Recent challenging cases of rapid intensification are presented and discussed.
Zhu, P., J.A. Zhang, and F.D. Marks. On the lateral entrainment instability in the inner core region of tropical cyclones. Geophysical Research Letters, 50(8):e2022GL102494, https://doi.org/10.1029/2022GL102494 2023
Entrainment of dry moat air with low equivalent potential temperature laterally into the eyewall and rainbands is a unique turbulent process in the inner-core region of a tropical cyclone (TC). By analyzing in-situ aircraft measurements collected by the reconnaissance flights that penetrated the eyewalls and rainbands of Hurricanes Rita (2005), Patricia (2015), Harvey (2017), and Michael (2018), as well as numerical simulations of Hurricanes Patricia (2015), and Michael (2018), we show that the moat air entrained into the eyewall and rainbands meets the instability criterion, and therefore, sinks unstably as a convective downdraft. The resultant positive buoyancy fluxes are an important source for the turbulent kinetic energy (TKE) in the eyewall and rainband clouds. This mechanism of TKE generation via lateral entrainment instability should be included in the TKE-type turbulent mixing schemes for a better representation of turbulent transport processes in numerical forecasts of TCs.
2022
Ahern, K., R.E. Hart, and M.A. Bourassa. Asymmetric hurricane boundary layer structure during storm decay. Part 2: Secondary eyewall formation. Monthly Weather Review, 150(8):1915-1936, https://doi.org/10.1175/MWR-D-21-0247.1 2022
Three-dimensional hurricane boundary layer (BL) structure is investigated during secondary eyewall formation, as portrayed in a high-resolution, full-physics simulation of Hurricane Earl (2010). This is the second part of a study on the evolution of BL structure during vortex decay. As in part 1 of this work, the BL’s azimuthal structure was linked to vertical wind shear and storm motion. Measures of shear magnitude and translational speed in Earl were comparable to Hurricane Irma (2017) in part 1, but the orientation of one vector relative to the other differed, which contributed to different structural evolutions between the two cases. Shear and storm motion influence the shape of low-level radial flow, which in turn influences patterns of spinup and spindown associated with the advection of absolute angular momentum M. Positive agradient forcing associated with the import of M in the inner core elicits dynamically restorative outflow near the BL top, which in this case was asymmetric and intense at times prior to eyewall replacement. These asymmetries associated with shear and storm motion provide an explanation for BL convergence and spinup at the BL top outside the radius of maximum wind (RMW), which affects inertial stability and agradient forcing outside the RMW in a feedback loop. The feedback process may have facilitated the development of a secondary wind maximum over approximately two days, which culminated in eyewall replacement.
Aksoy, A., J.J. Cione, B.A. Dahl, and P.D. Reasor. Tropical cyclone data assimilation with Coyote uncrewed aircraft system observations, very frequent cycling, and a new online quality control technique. Monthly Weather Review, 150(4):797-829, https://doi.org/10.1175/MWR-D-21-0124.1 2022
A unique dataset obtained from the Coyote small uncrewed aircraft system (sUAS) in the inner-core boundary layer of Hurricane Maria (2017) is assimilated using NOAA’s Hurricane Ensemble Data Assimilation System (HEDAS) for data assimilation and Hurricane Weather Research and Forecast (HWRF) for model advances. The case of study is 1800 UTC 23 September 2017 when Maria was a Category-3 hurricane. In addition to the Coyote observations, measurements collected by the NOAA Lockheed WP-3D Orion and U.S. Air Force C-130 aircraft were also included. To support the assimilation of this unique dataset, a new online quality control (QC) technique in HEDAS scales the observation-background difference by the total uncertainty during data assimilation and uses the inter-quartile range outlier method to identify outlier observations. Experimental setup includes various very-frequent cycling scenarios for a Control that does not assimilate Coyote observations, assimilation of Coyote observations in addition to the Control observations, and the application of online QC. Findings suggest progressively improved analyses with more-frequent cycling, Coyote assimilation, and application of online QC. This applies to verification statistics computed at the locations of both Coyote and non-Coyote observations. In terms of the storm structure, only experiments that assimilated the Coyote observations were able to reproduce the double-eyewall structure that was observed at the time of the analysis, which is more consistent with the intensity of the storm according to the observations that were collected. Limitations of the study and future plans are also discussed.
Alaka, G.J. Jr., X. Zhang, and S.G. Gopalakrishnan. High-definition hurricanes: Improving forecasts with storm-following nests. Bulletin of the American Meteorological Society, 103(3):E680-E703, https://doi.org/10.1175/BAMS-D-20-0134.1 2022
To forecast tropical cyclone (TC) intensity and structure changes with fidelity, numerical weather prediction models must be “high definition”, i.e., horizontal grid spacing ≤ 3 km, so that they permit clouds and convection and resolve sharp gradients of momentum and moisture in the eyewall and rainbands. However, resolutions in operational global models remain too coarse to accurately predict these structures that are critical to TC intensity. Storm-following nests are a solution to this problem because they are computationally efficient at fine resolutions, providing a practical approach to improve TC intensity forecasts. Under the Hurricane Forecast Improvement Program, the operational Hurricane Weather Research and Forecasting (HWRF) system was developed to include telescopic, storm-following nests for a single TC per model integration. Subsequently, HWRF evolved into a state-of-the-art tool for TC predictions around the globe, although its single-storm nesting approach does not adequately simulate TC-TC interactions as they are observed. Basin-scale HWRF (HWRF-B) was developed later with a multi-storm nesting approach to improve the simulation of TC-TC interactions by producing high-resolution forecasts for multiple TCs simultaneously. In this study, the multi-storm nesting approach in HWRF-B was compared with a single-storm nesting approach using an otherwise identical model configuration. The multi-storm approach demonstrated TC intensity forecast improvements, including more realistic TC-TC interactions. Storm-following nests developed in HWRF and HWRF-B will be foundational to NOAA’s next-generation hurricane application in the Unified Forecast System.
Alvey, G.R., and A. Hazelton. How do weak, misaligned tropical cyclones evolve toward alignment? A case study using the Hurricane Analysis and Forecast System. Journal of Geophysical Research-Atmospheres, 127(10):e2022JD037268, https://doi.org/10.1029/2022JD037268 2022
This study simulates five initially weak, moderately sheared tropical cyclones (TCs) from the 2020–2021 North Atlantic basin hurricane seasons using the Hurricane Analysis and Forecast System (HAFS). Four of the five simulations rapidly evolve from misaligned vortices with asymmetric precipitation and thermodynamic distributions toward more aligned and symmetric configurations. The displaced low-level (LLC) and mid-level circulations (MLC) non-monotonically progress toward alignment with periods of reformation, precession, and advection. Beginning 12–18 hr pre-alignment, TCs have increasingly greater mid-tropospheric humidity and areal coverages of precipitation downshear left than the simulation that fails to align. Alignment precedes the most sustained symmetrization of favorable thermodynamics and precipitation, but deep convection (not necessarily symmetric) plays a critical role in alignment. Ida (2021), a high impact US storm, undergoes a vortex-scale evolution where an increase in areal coverage and intensity of deep convection promotes a reformation of the vortex into a smaller compact core with a closed MLC (a closed LLC does not immediately form). This convective behavior downtilt helps to reshape the irrotational velocity field in the lower troposphere toward the reformed vortex. The increasingly convergent flow of favorable boundary layer thermodynamics within the inflow region thereby increases the instability, which maintains the persistent intense convection. The confluent flow ultimately promotes an advection of the pre-existing LLC toward the reformed vortex resulting in alignment. Tilt reductions are also shown to be temporally linked with the diurnal cycle, wherein convection preferentially increases near the center during the early morning hours (local time).
Alvey, G.R., M. Fischer, P. Reasor, J. Zawislak, and R. Rogers. Observed processes underlying the favorable vortex repositioning early in the development of Dorian (2019). Monthly Weather Review, 150(1):253-273, https://doi.org/10.1175/MWR-D-21-0069.1 2022
Dorian’s evolution from a weak, disorganized tropical storm to a rapidly intensifying hurricane is documented through a unique multi-platform synthesis of NOAA’s P-3 tail-Doppler radar, airborne in situ data, and Meteo-France’s Martinique and Guadeloupe ground radar network. Dorian initially struggled to intensify with a misaligned vortex in moderate mid-tropospheric vertical wind shear that also allowed detrimental impacts from dry air near the inner core. Despite vertical wind shear eventually decreasing to less than 5 m/s and an increasingly symmetric distribution of stratiform precipitation, the vortex maintained its misalignment with asymmetric convection for 12 hours. Then, as the low-level circulation (LLC) approached St. Lucia, deep convection near the LLC’s center dissipated, the LLC broadened, and precipitation expanded radially outwards from the center temporally coinciding with the diurnal cycle. Convection then developed farther downtilt within a more favorable, humid environment and deepened appreciably at least partially due to interaction with Martinique. A distinct repositioning of the LLC towards Martinique is induced by spin-up of a mesovortex into a small, compact LLC. It is hypothesized that this somewhat atypical reformation event and the repositioning of the vortex into a more favorable environment, farther from detrimental dry mid-tropospheric air, increased its favorability for the rapid intensification that subsequently ensued. Although the repositioning resulted in tilt reducing to less than the scale of the vortex itself, the pre-existing broad mid-upper level cyclonic envelope remained intact with continued misalignment observed between the mid-level center and repositioned LLC even during the early stages of rapid intensification.
Barron, N.R., A.C. Didlake, and P.D. Reasor. Statistical analysis of convective updrafts in tropical cyclone rainbands observed by airborne Doppler radar. Journal of Geophysical Research-Atmospheres, 127(6):e2021JD035718, https://doi.org/10.1029/2021JD035718 2022
Ten years of airborne Doppler radar observations are used to study convective updrafts' kinematic and reflectivity structures in tropical cyclone (TC) rainbands. An automated algorithm is developed to identify the strongest rainband updrafts across 12 hurricane-strength TCs. The selected updrafts are then collectively analyzed by their frequency, radius, azimuthal location (relative to the 200–850 hPa environmental wind shear), structural characteristics, and secondary circulation (radial/vertical) flow pattern. Rainband updrafts become deeper and stronger with increasing radius. A wavenumber-1 asymmetry arises, showing that in the downshear (upshear) quadrants of the TC, updrafts are more (less) frequent and deeper (shallower). In the downshear quadrants, updrafts primarily have in-up-out or in-up-in secondary circulation patterns. The in-up-out circulation is the most frequent pattern and has the deepest updraft and reflectivity tower. Upshear, the updrafts generally have out-up-in or in-up-in patterns. The radial flow of the updraft circulations largely follows the vortex-scale radial flow shear-induced asymmetry, being increased low-level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) quadrants. It is hypothesized that the convective-scale circulations are significantly influenced by the vortex-scale radial flow at the updraft base and top altitudes. Other processes of the convective life cycle, such as bottom-up decay of aging convective updrafts due to increased low-level downdrafts, can influence the base altitude and, thus, the base radial flow of the updraft circulation. The findings presented in this study support previous literature regarding convective-scale patterns of organized rainband convection in a mature, sheared TC.
Casey, S.P.F., and L. Cucurull. The impact of data latency on operational global weather forecasting. Weather and Forecasting, 37(7):1211-1220, https://doi.org/10.1175/WAF-D-21-0149.1 2022
The impact of low data latency is assessed using observations assimilated into the NCEP Finite-Volume Cubed-Sphere Global Forecast System (FV3GFS). Operationally, a full dataset is used to generate short-term (9-h) forecasts used as the background state for the next cycle, and a limited dataset with fewer observations is used for long-term (16-day) forecasts due to time constraints that exist in an operational setting. In this study, the sensitivity of the global weather forecast skill to the use of the full and limited data sets in both the short- and long-term forecasts (out to 10 days only) is evaluated. The results show that using the full dataset for long-term forecasts yields a slight improvement in forecast skill, while using the limited dataset for short-term forecasts yields a significant degradation. This degradation is primarily attributed to a decrease of in-situ observations rather than remotely-sensed observations, though no individual observation type captures the amount of degradation noted when all observations are limited. Furthermore, limiting individual types of in-situ observations (aircraft, marine, rawinsonde) does not result in the level of degradation noted when limiting all in-situ observations, demonstrating the importance of data redundancy in an operational observational system.
Chen, S., F. Qiao, J.A. Zhang, Y. Xue, H. Ma, and S. Chen. Observed drag coefficient asymmetry in a tropical cyclone. Journal of Geophysical Research-Oceans, 127(9):e2021JC018360, https://doi.org/10.1029/2021JC018360 2022
The behavior of drag coefficient (CD) in two different motion-relative quadrants of Typhoon Mujigae (2015) is investigated through the flux observations conducted on a fixed platform over the coastal region in the northern South China Sea. Observations reveal that the variation of CD is closely related to the location relative to the tropical cyclone (TC) center. The CD presents an enhancement when the typhoon is away from the observational site. The spatial distribution of CD on the periphery of a TC is asymmetric, and the CD in the right rear quadrant is much larger than that in the right front quadrant for the same wind speed range. This asymmetric distribution of CD can be explained by the differences in wave properties between the two quadrants. CD is smaller in cross-swell conditions than that in the along-wind wave conditions. Observations also confirm that CD tends to level off and even attenuate with the increase of wind speed, and the critical wind speed for CD saturation over the coastal region (∼20 m/s) is much lower than that over the open ocean (∼30 m/s). The observational spatial distribution of CD in TC quadrants not only improves our understanding on the air-sea momentum flux but also provides a potential solution for the long-standing scientific bottleneck on TC intensity forecasting.
Chen, X. How do planetary boundary layer schemes perform in hurricane conditions? Journal of Advances in Modeling Earth Systems, 14(10):e2022MS003088, https://doi.org/10.1029/2022MS003088 2022
Parameterizations of turbulent processes in planetary boundary layer (PBL) schemes impact tropical cyclone (TC) forecasts. Existing PBL schemes are mostly designed for low-wind conditions, and assessing their uncertainties in hurricane conditions remains challenging, mostly due to very scarce observations. Using a recently developed framework based on large-eddy simulations (LES), this study evaluates K-profile parameterization (KPP) and high-order PBL schemes in hurricane conditions. Among KPP PBL schemes, the Global Forecast System (GFS) scheme tends to produce excessively deep inflow layers with large values of eddy viscosity (Km). Opposite results are found for the Yonsei University (YSU) scheme. Using LES results as a benchmark, the performance of YSU and GFS schemes is improved by modifying the “shape parameter” such that Km is maximized closer to the surface, and by using a new definition of boundary layer height tailored to high-wind conditions. The LES results also suggest an asymptotic mixing length of ∼40 m can improve the Louis-type parameterizations of the YSU scheme that operates above the boundary layer. Among high-order PBL schemes, the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme produces reasonably accurate vertical profiles of eddy viscosity, turbulent stress, and boundary layer winds under different high-wind conditions. Further analysis of MYNN supports a “three-layer” strategy for the mixing length parameterization for TCs that represents different types of turbulent regimes. In contrast, the high-order eddy-diffusivity mass-flux scheme produces excessive boundary-layer vertical mixing and a deeper inflow layer, partly attributable to a notable overestimation of the maximum allowable mixing length in the PBL code.
Chen, X., G.H. Bryan, A. Hazelton, F.D. Marks, and P. Fitzpatrick. Evaluation and improvement of TKE-based eddy-diffusivity mass-flux (EDMF) planetary boundary layer scheme in hurricane conditions. Weather and Forecasting, 37(6):935-951, https://doi.org/10.1175/WAF-D-21-0168.1 2022
Accurately representing boundary layer turbulent processes in numerical models is critical to improve tropical cyclone forecasts. A new turbulence kinetic energy (TKE)-based moist eddy-diffusivity mass-flux (EDMF-TKE) planetary boundary layer scheme has been implemented in NOAA’s Hurricane Analysis and Forecast System (HAFS). This study evaluates EDMF-TKE in hurricane conditions based on a recently-developed framework using large-eddy simulation (LES). Single-column modeling tests indicate that EDMF-TKE produces much greater TKE values below 500-m height than LES benchmark runs in different high-wind conditions. To improve these results, two parameters in the TKE scheme were modified to ensure a match between the PBL and surface-layer parameterizations. Additional improvements were made by reducing the maximum allowable mixing length to 40 m based on LES and observations, by adopting a different definition of boundary layer height, and by reducing nonlocal mass fluxes in high-wind conditions. With these modifications, the profiles of TKE, eddy viscosity, and winds compare much better with LES results. Three-dimensional idealized simulations and an ensemble of HAFS forecasts of Hurricane Michael (2018) consistently show that the modified EDMF-TKE tends to produce a stronger vortex with a smaller radius of maximum wind than the original EDMF-TKE, while the radius of gale-force wind is unaffected. The modified EDMF-TKE code produces smaller eddy viscosity within the boundary layer compared to the original code, which contributes to stronger inflow, especially within the annulus of 1-3 times of radius of maximum wind. The modified EDMF-TKE shows promise to improve forecast skill of rapid intensification in sheared environments.
Christophersen, H., J.A. Sippel, A. Aksoy, and N.L. Baker. Recent advances for tropical cyclone data assimilation. Annals of the New York Academy of Sciences, 1517(1):25-43, https://doi.org/10.1111/nyas.14873 2022
In this review, data assimilation (DA) techniques used for tropical cyclones (TCs) are briefly overviewed. The strength and weakness of variational methods, ensemble methods, hybrid methods, and particle filter methods are also discussed. Several global numerical weather prediction models and their corresponding DA systems frequently used for TC forecasting and verification are described first. The DA research and development efforts in the operational regional model from the National Centers for Environmental Prediction's Hurricane Weather Research and Forecasting are then discussed in greater detail. Focused remarks on TC observations from reconnaissance, ground-based radar, enhanced satellite-derived atmospheric motion vectors and all-sky satellite radiances and their impacts on TC analyses and forecasts are addressed. Recent TC DA advancements and challenges on better use of observations and more advanced DA methods for TC application are also briefly reviewed.
DeMaria, M.D., J.L. Franklin, R. Zelinsky, D.A. Zelinsky, M.J. Onderlinde, J.A. Knaff, S.N. Stevenson, J. Kaplan, K.D. Musgrave, G. Chirokova, and C.R. Sampson. The National Hurricane Center tropical cyclone model guidance suite. Weather and Forecasting, 37(11):2141-2159, https://doi.org/10.1175/WAF-D-22-0039.1 2022
The National Hurricane Center (NHC) uses a variety of guidance models for its operational tropical cyclone track, intensity, and wind structure forecasts and as baselines for the evaluation of forecast skill. A set of the simpler models, collectively known as the NHC guidance suite, is maintained by NHC. The models comprising the guidance suite are briefly described and evaluated, with details provided for those that have not been documented previously. Decay-SHIFOR is a modified version of the Statistical Hurricane Intensity FORecast (SHIFOR) model that includes decay over land; this modification improves the SHIFOR forecasts through about 96 h. T-CLIPER, a climatology and persistence model that predicts track and intensity using a trajectory approach, has error characteristics similar to those of CLIPER track and D-SHIFOR but can be run to any forecast length. The Trajectory and Beta model (TAB), another trajectory track model, applies a grid-point spatial filter to smooth winds from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) model. TAB model errors were 10%-15% lower than those of the Beta and Advection model (BAM), the model it replaced in 2017. Optimizing TAB’s vertical weights shows that the lower troposphere’s environmental flow provides a better match to observed tropical cyclone motion than does the upper troposphere and that the optimal steering layer is shallower for higher-latitude and weaker tropical cyclones. The advantages and disadvantages of the D-SHIFOR, T-CLIPER and TAB models relative to their earlier counterparts are discussed.
Dobbelaere, T., D.M. Holstein, E.M. Muller, L.J. Gramer, L. McEachron, S.D. Williams, and E. Hanert. Connecting the dots: Transmission of stony coral tissue loss disease from the Marquesas to the Dry Tortugas. Frontiers in Marine Science, 9:778938, https://doi.org/10.3389/fmars.2022.778938 2022
For the last 7 years, Florida’s Coral Reef (FCR) has suffered from widespread and severe coral loss caused by stony coral tissue loss disease (SCTLD). First observed off the coast of Miami-Dade County in 2014, the outbreak has since spread throughout the entirety of FCR and some areas of the Caribbean. However, the propagation of the disease through FCR seemed to slow down when it reached the western end of the Marquesas in August 2020. Despite being present about 30 km (∼20 miles) from the Dry Tortugas (DRTO), SCTLD was not reported in this area before May 2021. As SCTLD transmission is likely to be waterborne, here we suggest that this apparently delayed propagation is related to eddy activity near the DRTO under the influence of the Loop Current/Florida Current system. To quantify the impact of the local ocean circulation on the spread of SCTLD from the Marquesas and the DRTO, we evaluated the hydrodynamic-predicted connectivity between these two regions using a high-resolution hydro-epidemiological model between May 2018 and May 2021. Our results suggest that the Marquesas and the DRTO were not connected during February-October 2020 and January-May 2021. These periods coincided with either the occurrence of Tortugas gyres and mean circulation with an eastward component between the Marquesas and the DRTO or the presence of southward currents. Our results suggest that disease agents probably reached the DRTO in November 2020 and that they most likely originated from southern or northwestern reefs of the Marquesas. This study provides novel insight into the role played by the hydrodynamics in the spread of SCTLD within the western-most edge of FCR, and in propagating the disease to uninfected locations
Esmaili, R., C. Barnet, J. Dunion, M. Folmer, and J. Zawislak. Evaluating satellite sounders for monitoring the tropical cyclone environment in operational forecasting. Remote Sensing, 14(13):3189, https://doi.org/10.3390/ rs14133189 2022
Tropical cyclones can form over open ocean where in situ observations are limited, so forecasters rely on satellite observations to monitor their development and track. We explore the utility of an operational satellite sounding product for tropical forecasting by characterizing the products retrieval skill during research flights. Scientists from both the NOAA-Unique Combined Atmospheric Processing System (NUCAPS) research team and tropical cyclone communities collaborated to target relevant tropical cyclones during the campaign. This effort produced 130 dropsondes that are well-timed with satellite sounder overpasses over three different tropical cyclones and one Saharan Air Layer outbreak. For the combined infrared and microwave retrieval, the NUCAPS temperature has a root mean square error (RMSE) of 1.2 K near the surface (1000–600 mb) and 0.8 K in the mid-troposphere (600–300 mb), which is in line with global product requirements. The water vapor mixing ratio RMSE was 26% near the surface and 46% in the mid-troposphere. NUCAPS microwave-only retrievals can also be useful for cloudy scenes, with surface RMSE values of 4 K (temperature) and 23% (water vapor). Using information content analysis, we estimated that the vertical resolution near the surface was 1.7 km for the temperature retrievals and 2.2 km for the water vapor retrievals in this study. We discuss the feasibility of implementing NUCAPS in an operational forecasting setting, which requires rapid data delivery to forecaster software tools.
Fischer, M.S., P.D. Reasor, R.F. Rogers, and J.F. Gamache. An analysis of tropical cyclone vortex and convective characteristics in relation to storm intensity using a novel airborne Doppler radar database. Monthly Weather Review, 150(9):2255-2278, https://doi.org/10.1175/MWR-D-21-0223.1 2022
This analysis introduces a novel airborne Doppler radar database, referred to as the Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR). TC-RADAR is comprised of over 900 analyses from 273 flights into TCs in the North Atlantic, eastern North Pacific, and central North Pacific basins between 1997–2020. This database contains abundant sampling across a wide range of TC intensities, which facilitated a comprehensive observational analysis on how the three-dimensional, kinematic TC inner-core structure is related to TC intensity. To examine the storm-relative TC structure, we implemented a novel TC center-finding algorithm. Here, we show that TCs below hurricane intensity tend to have monopolar radial profiles of vorticity and a wide range of vortex tilt magnitudes. As TC intensity increases, vorticity becomes maximized within an annulus inward of the peak wind, the vortex decays more slowly with height, and the vortex tends to be more aligned in the vertical. The TC secondary circulation is also strongly linked to TC intensity, as more intense storms have shallower and stronger lower-tropospheric inflow as well as larger azimuthally-averaged ascent. The distribution of vertical velocity is found to vary with TC intensity, height, and radial domain. These results—and the capabilities of TC-RADAR—motivate multiple avenues for future work, which are discussed.
Foltz, G.R., C. Zhang, C. Meinig, J.A. Zhang, and D. Zhang. An unprecedented view inside a hurricane. EOS, 103(7):22-28, https://doi.org/10.1029/2022EO220228 2022
Gramer, L.J., J.A. Zhang, G. Alaka, A. Hazelton, and S. Gopalakrishnan. Coastal downwelling intensifies landfalling hurricanes. Geophysical Research Letters, 49(13):e2021GL096630 , https://doi.org/10.1029/2021GL096630 2022
This study demonstrates a link between coastal downwelling and tropical cyclone (TC) intensification. We show that coastal downwelling increases air-sea enthalpy (heat, moisture) fluxes ahead of TCs as they approach land, creating conditions conducive to intensification even in the presence of typically inhibiting factors like strong vertical wind shear. The study uses a coupled TC model (HWRF-B) and buoy observations to demonstrate that coastal downwelling developed as three TCs in 2020 approached land. Results show downwelling maintained warmer sea-surface temperatures over the ocean shelf, enhancing air-sea temperature/humidity contrasts. We found that in such cases resulting air-sea enthalpy fluxes can replenish the boundary-layer even when cool, dry air intrudes, as in sheared storms and storms approaching continental land-masses. The resulting warm, moist air is advected into the TC inner core, enhancing convective development, thus providing energy for TC intensification. These results indicate coastal downwelling can be important in forecasting TC intensity change before landfall.
Hazelton, A., J.A. Zhang, and S.G. Gopalakrishnan. Comparison of the performance of the observation-based hybrid EDMF and EDMF-TKE PBL schemes in 2020 tropical cyclone forecasts from the Global-nested Hurricane Analysis and Forecast System. Weather and Forecasting, 37(4):457-476, https://doi.org/10.1175/WAF-D-21-0124.1 2022
Better representation of the planetary boundary layer (PBL) in numerical models is one of the keys to improving forecasts of TC structure and intensity, including rapid intensification. To meet this goal, our recent work has used observations to improve the eddy-diffusivity mass flux with prognostic turbulent kinetic energy (EDMF-TKE) PBL scheme in the Hurricane Analysis and Forecast System (HAFS). This study builds on that work by comparing a modified version of EDMF-TKE (MEDMF-TKE) with the hybrid EDMF scheme based on a K-profile method (HEDMF-KP) in the 2020 HAFS-globalnest model. Verification statistics based on 101 cases in the 2020 season demonstrate that MEDMF-TKE improves track forecasts, with a reduction in a large right bias seen in HEDMF-KP forecasts. The comparison of intensity performance is mixed, but the magnitude of low bias at early forecast hours is reduced with the use of the MEDMF-TKE scheme, which produces a wider range of TC intensities. Wind radii forecasts, particularly the radius of maximum wind speed (RMW), are also improved with the MEDMF-TKE scheme. Composites of TC inner-core structure in and above the PBL highlight and explain differences between the two sets of forecasts, with MEDMF-TKE having a stronger and shallower inflow layer, stronger eyewall vertical velocity, and more moisture in the eyewall region. A case study of Hurricane Laura shows that MEDMF-TKE better represented the subtropical ridge and thus the motion of the TC. Finally, analysis of Hurricane Delta through a tangential wind budget highlights how and why MEDMF-TKE leads to faster spinup of the vortex and a better prediction of rapid intensification.
Hazelton, A., K. Gao, M. Bender, L. Cowan, G.J. Alaka Jr., A. Kaltenbaugh, L. Gramer, X. Zhang, L. Harris, T. Marchok, M. Morin, A. Mehra, Z. Zhang, B. Liu, and F. Marks. Performance of 2020 real-time Atlantic hurricane forecasts from high-resolution global-nested hurricane models: HAFS-globalnest and GFDL T-SHiELD. Weather and Forecasting, 37(1):143-161, https://doi.org/10.1175/WAF-D-21-0102.1 2022
The global-nested Hurricane Analysis and Forecast System (HAFS-globalnest) is one piece of NOAA’s Unified Forecast System (UFS) application for hurricanes. In this study, results are analyzed from 2020 real-time forecasts by HAFS-globalnest and a similar global-nested model, the Tropical Atlantic version of GFDL’s System for High-resolution prediction on Earth- to- Local Domains (T-SHiELD). HAFS-globalnest produced the highest track forecast skill compared to several operational and experimental models, while T-SHiELD showed promising track skill as well. The intensity forecasts from HAFS-globalnest generally had a positive bias at longer lead times primarily due to the lack of ocean coupling, while T-SHiELD had a much smaller intensity bias particularly at longer forecast lead times. With the introduction of a modified planetary boundary layer scheme and an increased number of vertical levels, particularly in the boundary layer, HAFS forecasts of storm size had a smaller positive bias than occurred in the 2019 version of HAFS-globalnest. Despite track forecasts that were comparable to the operational GFS and HWRF, both HAFS-globalnest and T-SHiELD suffered from a persistent right-of-track bias in several cases at the 4-5 day forecast lead times. The reasons for this bias were related to the strength of the subtropical ridge over the western North Atlantic and are continuing to be investigated and diagnosed. A few key case studies from this very active hurricane season, including Hurricanes Laura and Delta, were examined.
Holger, V., and J. Dunion. Aircraft dropsonde campaigns. In Field Measurements for Passive Environmental Remote Sensing: Instrumentation, Intensive Campaigns, and Satellite Applications, N.R. Nalli (ed). Elsevier, 185-194, https://doi.org/10.1016/B978-0-12-823953-7.00021-6 2022
Dropsondes are small meteorological devices similar to radiosondes, which are launched from aircraft or long duration balloons. These instruments are typically deployed over oceans to target special meteorological conditions or satellite overpass areas. Dropsondes provide high-resolution profiles of pressure, temperature, relative humidity, and winds from the aircraft altitude down to the ocean surface. The technology behind these measurements includes the aircraft infrastructure to launch the sondes and to receive the data by telemetry. Observations by dropsondes provide reference measurements for retrievals from satellites and support the validation and calibration of temperature, humidity, surface winds, and wind profiles. Dropsondes also measure the integrated water vapor column, which indirectly supports the validation of cloud parameters. Dropsondes may also be used in the calibration and validation of airborne remote sensors, which may act as transfer standards and which can provide more data than would be possible from dropsondes. In addition, dropsondes are heavily used in characterizing the atmospheric state and spatial variability, in particular in severe weather events, which may also challenge space-borne sensors.
Joe, P., J. Sun, N. Yussouf, S. Goodman, M. Riemer, K.C. Gouda, B. Golding, R. Rogers, G. Isaac, J. Wilson, P.-W.P. Li, V. Wulfmeyer, K. Elmore, J. Onvlee, P. Chong, and J. Ladue. Predicting the weather: A partnership of observation scientists and forecasters. In Toward the “Perfect” Weather Warning: Bridging Disciplinary Gaps through Partnership and Communication, B. Golding (ed.). Springer Publishing, 201-254 , https://doi.org/10.1007/ 978-3-030-98989-7_7 2022
Weather forecasts are the foundation of much of the information needed in the warnings we have been considering. To be useful, they require knowledge of the current atmospheric state as a starting point. In this chapter, we first look at the methods used to predict the weather and the resulting demands for observations. Then, we explore the wide variety of sensors and platforms used to obtain this information. There has been a long history of close working between sensor and platform designers and meteorologists that has produced spectacular advances in forecast accuracy. However, the latest high-resolution models require new approaches to obtaining observations that will require different collaborations. Examples are presented of partnerships in space observing and in aviation, a demonstration system from Canada, and the use of testbeds and observatories as environments for progress
Leighton, H., R. Black, X. Zhang, and F.D. Marks. The relationship between reflectivity and rainfall rate from rain size distributions observed in hurricanes. Geophysical Research Letters, 49(23):e2022GL099332, https://doi.org/10.1029/2022GL099332 2022
Raindrop size distributions collected by the DROPLET MEASUREMENT TECHNOLogies Precipitation imaging probe from 17 flights through 6 hurricanes during National Oceanic and Atmospheric Administration’s hurricane field program in 2020 are used to study reflectivity (Z) and rainfall rate (RR) (R) relationship (i.e., Z-R relationship). The results show that the Z-R distribution is highly scattered and the scatter increases with RR and reflectivity up to 48 dBZ or 25 mm hr−1, after which it decreases rapidly. The range of the estimated RR from a power-law Z-R relationship can be as large as 50 mm hr−1 at reflectivity of 40 dBZ. The result from random forest regression model demonstrates that including the information of mass-weighted-diameter (Dm) along with radar reflectivity improves the estimated RR significantly.
Li, X., Z. Pu, J.A. Zhang, and G.D. Emmitt. Combined assimilation of Doppler wind lidar and tail Doppler radar data over a hurricane inner core for improved hurricane prediction with the NCEP regional HWRF system. Remote Sensing, 14(10):2367, https://doi.org/10.3390/rs14102367 2022
Accurate specification of hurricane inner-core structure is critical to predicting the evolution of a hurricane. However, observations over hurricane inner cores are generally lacking. Previous studies have emphasized Tail Doppler radar (TDR) data assimilation to improve hurricane inner-core representation. Recently, Doppler wind lidar (DWL) has been used as an observing system to sample hurricane inner-core and environmental conditions. The NOAA P3 Hurricane Hunter aircraft has DWL installed and can obtain wind data over a hurricane’s inner core when the aircraft passes through the hurricane. In this study, we examine the impact of assimilating DWL winds and TDR radial winds on the prediction of Hurricane Earl (2016) with the NCEP operational Hurricane Weather Research and Forecasting (HWRF) system. A series of data assimilation experiments are conducted with the Gridpoint Statistical Interpolation (GSI)-based ensemble-3DVAR hybrid system to identify the best way to assimilate TDR and DWL data into the HWRF forecast system. The results show a positive impact of DWL data on hurricane analysis and prediction. Compared with the assimilation of u and v components, assimilation of DWL wind speed provides better hurricane track and intensity forecasts. Proper choices of data thinning distances (e.g., 5 km horizontal thinning and 70 hPa vertical thinning for DWL) can help achieve better analysis in terms of hurricane vortex representation and forecasts. In the analysis and forecast cycles, the combined TDR and DWL assimilation (DWL wind speed and TDR radial wind, along with other conventional data, e.g., NCEP Automated Data Processing (ADP) data) offsets the downgrade analysis from the absence of DWL observations in an analysis cycle and outperforms assimilation of a single type of data (either TDR or DWL) and leads to improved forecasts of hurricane track, intensity, and structure. Overall, assimilation of DWL observations has been beneficial for analysis and forecasts in most cases. The outcomes from this study demonstrate the great potential of including DWL wind profiles in the operational HWRF system for hurricane forecast improvement.
Liu, C., X. Li, J. Song, Z. Zou, J. Huang, J.A. Zhang, G. Jie, and J. Wang. Characteristics of the marine atmospheric boundary layer under the influence of ocean surface waves. Journal of Physical Oceanography, 52(6):1261-1276, https://doi.org/10.1175/JPO-D-21-0164.1 2022
The deviation of the mean wind profile from Monin-Obukhov similarity theory (MOST) within the wave boundary layer (WBL) is investigated by combining four levels of turbulence data measured on a fixed platform with wave measurements. The data suggest that the mean wind profile follows MOST under wind-sea conditions because the turbulence statistics and structure are consistent with the attached eddy model. However, pronounced swell-related peaks appeared in the velocity spectra and uw-cospectra under swell conditions. The upward wave-induced stress resulted in a large wind gradient within the WBL when light winds traveled with the swell, while the opposite result was found for the wind-opposite swell. These phenomena were analyzed based on the velocity spectra and turbulence variances. We found that the deviation of the wind profile was due to the longer (shorter) length of the f−1 scaling region appearing in the velocity spectra.
Marinescu, P.J., L. Cucurull, K. Apodaca, L. Bucci, and I. Genkova. The characterization and impacts of Aeolus wind profile observations in NOAA’s regional tropical cyclone model. Quarterly Journal of the Royal Meteorological Society, 148(749):3491-3508, https://doi.org/10.1002/qj.4370 2022
Observation system experiments (OSEs) are conducted to assess the potential impacts of horizontal line-of-sight wind profile observations from the Aeolus satellite on tropical cyclone (TC) forecasting. The OSEs utilize the operational Hurricane Weather and Research Forecasting (HWRF) model. The OSEs include 226 forecasts from seven TC cases in the Atlantic and Eastern Pacific basins. Comparisons between Aeolus and model background winds show that winds from Aeolus are consistently stronger than those from HWRF. Data assimilation statistics also demonstrate that the greatest potential impacts from the assimilation of Aeolus observations are likely to occur in the upper troposphere and lower stratosphere and within approximately 500 km from the TC centre. For TC forecasting applications, the assimilation of Aeolus observations improves TC intensity and size forecasts in the Eastern Pacific basin, while the results for track forecasts and results from the Atlantic basin are mixed. However, in both basins, the largest and most statistically significant, positive impacts from the assimilation of Aeolus observations occur when reconnaissance flight data are unavailable and during the early stages of TC development. The traditionally used forecast assessments of TC intensity, track and size are rooted in surface-based metrics, and an additional investigation above the surface demonstrated larger improvements from assimilating Aeolus observations on TC wind structure above 400 hPa as compared to the lower troposphere. Several, different assessments throughout this study demonstrate higher uncertainty and the need for special consideration associated with assimilation techniques within 500 km from the TC centre.
Ming J., R. Liu, J.A. Zhang, and R.F. Rogers. The shear-relative variation of inflow angle and its relationship to tropical cyclone intensification. Journal of Geophysical Research-Atmospheres, 127(16):e2022JD037280, https://doi.org/10.1029/2022JD037280 2022
Characterizing inflow structure is important to better represent tropical cyclone impacts in numerical models. While much research has considered the impact of storm translation on the distribution of inflow angle, comparatively less research has examined its distribution relative to the environmental wind shear. This study analyzes data from 3,655 dropsondes in 44 storms to investigate the radial and shear-relative distribution of surface inflow angle. Emphasis is placed on its relationship with intensity change. The results show that the radial variation in the inflow angle is small and not significantly dependent on the shear magnitude or intensity change rate. In contrast, the azimuthal distribution of the inflow angle shows a significant asymmetry, with the amplitude of the asymmetry increasing with shear magnitude. The maximum inflow angle is located in the downshear side. The degree of asymmetry is larger in the outer core than in the eyewall. Intensifying storms have a smaller degree of asymmetry than steady-state storms under moderate shear.
Poterjoy, J. Implications of multivariate non-Gaussian data assimilation for multiscale weather prediction. Monthly Weather Review, 150(6):1475-1493, https://doi.org/10.1175/MWR-D-21-0228.1 2022
Weather prediction models currently operate within a probabilistic framework for generating forecasts conditioned on recent measurements of Earth’s atmosphere. This framework can be conceptualized as one that approximates parts of a Bayesian posterior density estimated under assumptions of Gaussian errors. Gaussian error approximations are appropriate for synoptic-scale atmospheric flow, which experiences quasi-linear error evolution over time scales depicted by measurements, but are often hypothesized to be inappropriate for highly nonlinear, sparsely-observed mesoscale processes. The current study adopts an experimental regional modeling system to examine the impact of Gaussian prior error approximations, which are adopted by ensemble Kalman filters (EnKFs) to generate probabilistic predictions. The analysis is aided by results obtained using recently-introduced particle filter (PF) methodology that relies on an implicit non-parametric representation of prior probability densities—but with added computational expense. The investigation focuses on EnKF and PF comparisons over month-long experiments performed using an extensive domain, which features the development and passage of numerous extratropical and tropical cyclones. The experiments reveal spurious small-scale corrections in EnKF members, which come about from inappropriate Gaussian approximations for priors dominated by alignment uncertainty in mesoscale weather systems. Similar behavior is found in PF members, owing to the use of a localization operator, but to a much lesser extent. This result is reproduced and studied using a low-dimensional model, which permits the use of large sample estimates of the Bayesian posterior distribution. Findings from this study motivate the use of data assimilation techniques that provide a more appropriate specification of multivariate non-Gaussian prior densities or a multi-scale treatment of alignment errors during data assimilation.
Romdhani, O., J.A. Zhang, and M. Momen. Characterizing the impact of turbulence closures on real hurricane forecasts: A comprehensive joint assessment of grid resolution, turbulence models, and horizontal mixing length. Journal of Advances in Modeling Earth Systems, 14(9):e2021MS002796, https://doi.org/10.1029/2021MS002796 2022
Hurricanes are highly complex geophysical flows that have caused billions of dollars in damage in recent years. Despite the significance of these extreme weather events, the turbulence mechanisms that derive the dynamics of hurricane flow systems are poorly understood and ineffectively parameterized in numerical weather prediction (NWP) models. The objective of this study is to bridge these knowledge gaps by assessing the accuracy and deficiencies of existing horizontal turbulence models in NWPs for hurricane forecasts. In particular, the Weather and Research Forecasting (WRF) Model is employed to conduct 135 simulations of five real hurricanes by varying the grid resolution, turbulence models, and horizontal mixing length values. Decreasing the default horizontal mixing length values both in low and high resolution WRF simulations significantly improves the wind intensity forecasts. This result indicates that the existing horizontal diffusion parameterizations are overly dissipative for hurricane flows, and thus, generate a weaker vortex compared to observations. These deficiencies are shown to stem from the horizontal mixing-length parameterization in WRF that is prescribed as a function of grid size without considering the physics of the flows (e.g., rotation). The paper provides notable insights into the role of turbulent fluxes in simulated hurricane evolutions that can be useful to advance the turbulence parameterizations of NWP models for hurricane forecasts.
Rosencrans, M., E.S. Blake, C.W. Landsea, H. Wang, S.B. Goldenberg, and R.J. Pasch. The tropics: Atlantic basin. In State of the Climate in 2021, J. Blunden and T. Boyer (eds.). Bulletin of the American Meteorological Society, 103(8):S219-227, https://doi.org/10.1175/BAMS-D-22-0069.1 2022
Schwartz, C.S., J. Poterjoy, G.S. Romine, D.C. Dowell, J.R. Carley, and J. Bresch. Short-term convection-allowing ensemble precipitation forecast sensitivity to resolution of initial condition perturbations and central initial states. Weather and Forecasting, 37(7):1259-1286, https://doi.org/10.1175/WAF-D-21-0165.1 2022
Nine sets of 36-h, 10-member, convection-allowing ensemble (CAE) forecasts with 3-km horizontal grid spacing were produced over the conterminous United States for a 4-week period. These CAEs had identical configurations except for their initial conditions (ICs), which were constructed to isolate CAE forecast sensitivity to resolution of IC perturbations and central initial states about which IC perturbations were centered. The IC perturbations and central initial states were provided by limited-area ensemble Kalman filter (EnKF) analyses with both 15- and 3-km horizontal grid spacings, as well as from NCEP’s Global Forecast System (GFS) and Global Ensemble Forecast System. Given fixed-resolution IC perturbations, reducing horizontal grid spacing of central initial states improved ∼1–12-h precipitation forecasts. Conversely, for constant-resolution central initial states, reducing horizontal grid spacing of IC perturbations led to comparatively smaller short-term forecast improvements or none at all. Overall, all CAEs initially centered on 3-km EnKF mean analyses produced objectively better ∼1–12-h precipitation forecasts than CAEs initially centered on GFS or 15-km EnKF mean analyses regardless of IC perturbation resolution, strongly suggesting it is more important for central initial states to possess fine-scale structures than IC perturbations for short-term CAE forecasting applications, although fine-scale perturbations could potentially be critical for data assimilation purposes. These findings have important implications for future operational CAE forecast systems and suggest CAE IC development efforts focus on producing the best possible high-resolution deterministic analyses that can serve as central initial states for CAEs.
Scott, S.R., J.P. Dunion, M.L. Olson, and D.A. Gay. Lead isotopes in North American precipitation record the presence of Saharan dust. Bulletin of the American Meteorological Society, 103(2):E281-E292, https://doi.org/10.1175/BAMS-D-20-0212.1 2022
Atmospheric dust is an important mass transfer and nutrient supply process in Earth surface ecosystems. For decades, Saharan dust has been hypothesized as a supplier of nutrients to the Amazon Rain Forest and eastern North America. However, isotope studies aimed at detecting Saharan dust in the American sedimentary record have been ambiguous. A large Saharan dust storm emerged off the coast of Africa in June 2020 and extended into southeastern United States. This storm provided a means to evaluate the influence of Saharan dust in North America confirmed by independent satellite and ground observations. Precipitation samples from 17 sites within the National Atmospheric Deposition Program (NADP) were obtained from throughout the southeastern United States prior to, during, and after the arrival of Saharan dust. Precipitation samples were measured for their lead (Pb) isotopic composition, total Pb content, and 210Pb activity using multi-collector inductively coupled plasma mass spectrometry. We measured a significant isotopic shift (approximately 0.7 % in the 208Pb/206Pb relative to the 207Pb/206Pb) in precipitation that peaked in late June 2020 when the dust blanketed the southeastern US. However, the magnitude and short time period of the isotopic shift would make it difficult to detect in sedimentary records.
Shen, B.-W., R. Pielke, Sr., X. Zeng, J. Cui, S. Faghih-Naini, W. Paxson, A. Kesarkar, X. Zeng, and R. Atlas. The dual nature of chaos and order in the atmosphere. Atmosphere, 13(11):1892, https://doi.org/10.3390/atmos13111892 2022
In the past, the Lorenz 1963 and 1969 models have been applied for revealing the chaotic nature of weather and climate and for estimating the atmospheric predictability limit. Recently, an in-depth analysis of classical Lorenz 1963 models and newly developed, generalized Lorenz models suggested a revised view that “the entirety of weather possesses a dual nature of chaos and order with distinct predictability”, in contrast to the conventional view of “weather is chaotic”. The distinct predictability associated with attractor coexistence suggests limited predictability for chaotic solutions and unlimited predictability (or up to their lifetime) for non-chaotic solutions. Such a view is also supported by a recent analysis of the Lorenz 1969 model that is capable of producing both unstable and stable solutions. While the alternative appearance of two kinds of attractor coexistence was previously illustrated, in this study, multistability (for attractor coexistence) and monostability (for single type solutions) are further discussed using kayaking and skiing as an analogy. Using a slowly varying, periodic heating parameter, we additionally emphasize the predictable nature of recurrence for slowly varying solutions and a less predictable (or unpredictable) nature for the onset for emerging solutions (defined as the exact timing for the transition from a chaotic solution to a non-chaotic limit cycle type solution). As a result, we refined the revised view outlined above to: “The atmosphere possesses chaos and order; it includes, as examples, emerging organized systems (such as tornadoes) and time varying forcing from recurrent seasons”. In addition to diurnal and annual cycles, examples of non-chaotic weather systems, as previously documented, are provided to support the revised view.
Sippel, J.A., X. Wu, S.D. Ditchek, V. Tallapragada, and D. Kleist. Impacts of assimilating additional reconnaissance data on operational GFS tropical cyclone forecasts. Weather and Forecasting, 37(9):1615-2796, https://doi.org/10.1175/WAF-D-22-0058.1 2022
This study reviews the recent addition of dropwindsonde wind data near the tropical cyclone (TC) center as well as the first-time addition of high-density, flight-level reconnaissance observations (HDOBs) into the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). The main finding is that the additional data has profound positive impacts on subsequent TC track forecasts. For TCs in the North Atlantic (NATL) basin, statistically significant improvements in track extend through 4-5 days during reconnaissance periods. Further assessment suggests that greater improvements might also be expected at days 6-7. This study also explores the importance of comprehensively assessing data impact. For example, model or data assimilation changes can affect the so-called “early” and “late” versions of the forecast very differently. It is also important to explore different ways to describe the error statistics. In several instances the impacts of the additional data strongly differ depending on whether one examines the mean or median errors. The results demonstrate the tremendous potential for further improving TC forecasts. The data added here were already operationally transmitted and assimilated by other systems at NCEP, and many further improvements likely await with improved use of these and other reconnaissance observations. This demonstrates the need of not only investing in data assimilation improvements, but also enhancements to observational systems in order to reach next-generation hurricane forecasting goals.
Wadler, J.B., J.J. Cione, J.A. Zhang, E.A. Kalina, and J. Kaplan. The effects of environmental wind shear direction on tropical cyclone boundary layer thermodynamics and intensity change from multiple observational datasets. Monthly Weather Review, 150(1):115-134, https://doi.org/10.1175/MWR-D-21-0022.1 2022
The relationship between deep-layer environmental wind shear direction and tropical cyclone (TC) boundary layer thermodynamic structures is explored in multiple independent databases. Analyses derived from the tropical cyclone buoy database (TCBD) show that when TCs experience northerly-component shear, the 10-m equivalent potential temperature (θe) tends to be more symmetric than when shear has a southerly component. The primary asymmetry in θe in TCs experiencing southerly-component shear is radially outwards from twice the radius of maximum wind speed, with the left-of-shear quadrants having lower θe by 4–6 K than the right-of-shear quadrants. As with the TCBD, an asymmetric (symmetric) distribution of 10-m θe for TCs experiencing southerly-component (northerly-component) shear was found using composite observations from dropsondes. These analyses show that differences in the degree of symmetry near the sea surface extend through the depth of the boundary layer. Additionally, mean dropsonde profiles illustrate that TCs experiencing northerly-component shear are more potentially unstable between 500 m and 1000 m altitude, signaling a more favorable environment for the development of surface-based convection in rainband regions. Analyses from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) database show that subsequent strengthening (weakening) for TCs in the Atlantic Basin preferentially occurs in northerly-component (southerly-component) deep-layer environmental wind shear environments which further illustrates that the asymmetric distribution of boundary layer thermodynamics is unfavorable for TC intensification. These differences emphasize the impact of deep-layer wind shear direction on TC intensity changes which likely result from the superposition of large-scale advection with the shear-relative asymmetries in TC structure.
Wang, C., G. Zheng, X. Li, Q. Xu, B. Liu, and J.A. Zhang. Tropical cyclone intensity estimation from geostationary satellite imagery using deep convolutional neural networks. IEEE Transactions on Geoscience and Remote Sensing, https://doi.org/10.1109/TGRS.2021.3066299 2022
In this study, a set of deep convolutional neural networks (CNNs) was designed for estimating the intensity of tropical cyclones (TCs) over the Northwest Pacific Ocean from the brightness temperature data observed by the Advanced Himawari Imager onboard the Himawari-8 geostationary satellite. We used 97 TC cases from 2015 to 2018 to train the CNN models. Several models with different inputs and parameters are designed. A comparative study showed that the selection of different infrared (IR) channels has a significant impact on the performance of the TC intensity estimate from the CNN models. Compared with the ground truth Best Track data of the maximum sustained wind speed, with a combination of four channels of data as input, the best multicategory CNN classification model has generated a fairly good accuracy (84.8%) and low root mean square error (RMSE, 5.24 m/s) and mean bias (-2.15 m/s) in TC intensity estimation. Adding attention layers after the input layer in the CNN helps to improve the model accuracy. The model is quite stable even with the influence of image noise. To reduce the side-effect of the very unbalanced distribution of TC category samples, we introduced a focal_loss function into the CNN model. After we transformed the multiclassification problem into a binary classification problem, the accuracy increased to 88.9%, and the RMSE and the mean bias are significantly reduced to 4.62 and -0.76 m/s, respectively. The results show that our CNN models are robust in estimating TC intensity from geostationary satellite images.
Wu, Y.-C., M.-J. Yang, and R.F. Rogers. Examining terrain effects on the evolution of precipitation and vorticity of Typhoon Fanapi (2010) after departing the central mountain range on Taiwan. Monthly Weather Review, 150(6):1517-1540, https://doi.org/10.1175/MWR-D-21-0205.1 2022
Typhoon Fanapi (2010) made landfall in Hualien in Taiwan on 0100 UTC 19 September 2010 and left Taiwan on 1200 UTC 19 September 2010, producing heavy rainfall and floods. Fanapi’s eyewall was disrupted by the Central Mountain Range (CMR) and reorganized after leaving the CMR. High-resolution simulations (nested down to 1-km horizontal grid size) using the Advanced Research Weather Research and Forecast (WRF) model, one simulation using the full terrain (CTL) and another set of simulations where the terrain on Taiwan was removed, were analyzed. Precipitation areas were classified into different sub-regions by a convective-stratiform separation algorithm to assess the impact of precipitation structure on Fanapi’s eyewall evolution. The percentage of deep convection increased from 9% to 20% when Fanapi underwent an eyewall reorganization process while departing the CMR. In the absence of terrain, moderate convection occupied most of the convective regions during the period when Fanapi moved across Taiwan Island. The low-level total vorticity stretching within the convective, stratiform and weak echo regions in the no-terrain experiment were of similar magnitudes, but the total vorticity stretching within the convective region at low levels was dominant in the CTL experiment. Total vorticity stretching in the region of deep convection increased after eyewall reorganization, and later became stronger than that in the moderate convection region. In the absence of the CMR, total vorticity stretching in moderate convection dominated. The total vorticity stretching within the deep convective region in the CTL experiment played an essential role in the reorganization of Fanapi’s eyewall through a bottom-up process.
Zawislak, J., R.F. Rogers, S.D. Aberson, G.J. Alaka, G. Alvey, A. Aksoy, L. Bucci, J. Cione, N. Dorst, J. Dunion, M. Fischer, J. Gamache, S. Gopalakrishnan, A. Hazelton, H.M. Holbach, J. Kaplan, H. Leighton, F. Marks, S.T. Murillo, P. Reasor, K. Ryan, K. Sellwood, J.A. Sippel, and J.A. Zhang. Accomplishments of NOAA’S airborne hurricane field program and a broader future approach to forecast improvement. Bulletin of the American Meteorological Society, 103(2):E311-E338, https://doi.org/10.1175/BAMS-D-20-0174.1 2022
Since 2005, NOAA has conducted the annual Intensity Forecasting Experiment (IFEX), led by scientists from the Hurricane Research Division at NOAA’s Atlantic Oceanographic and Meteorological Laboratory. They partner with NOAA’s Aircraft Operations Center, who maintain and operate the WP-3D and G-IV Hurricane Hunter aircraft, and NCEP’s National Hurricane Center and Environmental Modeling Center, who task airborne missions to gather data used by forecasters for analysis and forecasting and for ingest into operational numerical weather prediction models. The goal of IFEX is to improve tropical cyclone (TC) forecasts using an integrated approach of analyzing observations from aircraft, initializing and evaluating forecast models with those observations, and developing new airborne instrumentation and observing strategies targeted at filling observing gaps and maximizing the data’s impact in model forecasts. This summary article not only highlights recent IFEX contributions towards improved TC understanding and prediction, but also reflects more broadly on the accomplishments of the program during the 16 years of its existence. It describes how IFEX addresses high-priority forecast challenges, summarizes recent collaborations, describes advancements in observing systems monitoring structure and intensity, as well as in assimilation of aircraft data into operational models, and emphasizes key advances in understanding of TC processes, particularly those that lead to rapid intensification. The article concludes by laying the foundation for the “next generation” of IFEX as it broadens its scope to all TC hazards, particularly rainfall, storm-surge inundation, and tornadoes, that have gained notoriety during the last few years after several devastating landfalling TCs.
Zhao, B., G. Wang, J.A. Zhang, L. Liu, J. Liu, J. Xu, H. Yu, C. Zhao, X. Yu, C. Sun, and F. Qiao. The effects of ocean surface waves on tropical cyclone intensity: Numerical simulations using a regional atmosphere-ocean-wave coupled model. Journal of Geophysical Research-Oceans, 127(11):e2022JC019015, https://doi.org/10.1029/2022JC019015 2022
Tropical cyclones (TCs), especially landfalling intense storms often pose serious threats to life and property in coastal areas. Although TC track forecast skill has been improved in the past decades, the progress of advancing the intensity forecast lags that of the track forecast. One possible limiting factor is the absence of ocean surface waves in forecast systems. To better represent the interaction of TC and underlying ocean, a regional atmosphere-ocean-wave coupled model is employed in this study. Twenty-one TCs of a whole year in 2013 are retrospectively simulated through twin simulations, a Control and a Fully coupled run. Results show that TC intensity bias has been significantly reduced in the fully coupled simulation, in which five ocean surface wave related physical processes are considered, including wave modulation of momentum flux, sea spray effect on enthalpy flux, surface current and Stokes drift on air sea flux, non-breaking wave induced mixing in the upper ocean as well as rain induced ocean surface cooling. A case study approach is used to diagnose the effect of individual surface wave related physical process on TC simulations. Similar to the effect of sea spray, surface waves also act as positive feedback on TC intensification by modulating air-sea momentum flux. Absolute angular momentum budget analysis suggests that larger radial inflows and stronger updrafts near the eyewall promote the radial and vertical advections of absolute angular momentum and in turn lead to a stronger TC in Fully coupled simulation. The TC structure and size agree better with observations in Fully coupled simulation.
Zhao, Z., R. Gao, J.A. Zhang, Y. Zhu, C. Liu, P.W. Chan, and Q. Wan. Observations of boundary layer wind and turbulence of a landfalling tropical cyclone. Nature Scientific Reports, 12:11056, https://doi.org/10.1038/s41598-022-14929-w 2022
This study investigates the atmospheric boundary layer structure based on multiple-level tower observations with a height of 350 m during the landfall of Super Typhoon Mangkhut (2018). Results show a layer of log wind profile outside of the radius of maximum wind speed with a height of 100 m or larger. The log layer height increases with the wind speed. The height of the constant flux layer reaches ~ 300 m for 10-m wind speeds less than 13 m s−1 while this height decreases with the wind speed. Momentum fluxes and turbulent kinetic energy increase with the wind speed at all vertical levels. The drag coefficient and surface roughness length estimated at the tower location have values of 7.3 × 10–3 and 0.09 m, respectively, which are independent of wind speed. The estimated vertical eddy diffusivity and mixing length increase with height up to ~ 160 m and then slowly decrease with height. The vertical eddy diffusivity increases with the wind speed while the vertical mixing length has no dependence on the wind speed. Comparing our results with previous work indicates that the vertical eddy diffusivity is larger over land than over ocean at a given wind speed range.
2021
Aberson, S.D. Serial correlation of tropical cyclone track and intensity forecasts. NOAA Technical Memorandum, OAR-AOML-107, 6 pp., https://doi.org/10.25923/m0ah-bh98 2021
Statistical significance tests can inform whether differences between two samples are real or due to sampling error. Forecasts are serially correlated because the first guess for each model cycle is a forecast from the previous one, and this must be accounted for in statistical tests on the impact of new observing systems or model system techniques. Prior studies showed that tropical cyclone track forecasts created every 24 h or 12 h were serially correlated so that only every other forecast was independent of the others. Forecasts are now initialized more frequently than those used in the earlier studies (every 6 h), requiring a reassessment of the serial correlation. The current study calculates the effective time between independent samples based on two distinct techniques for both tropical cyclone track and intensity forecasts. The calculated effective time varies by storm, forecast, and technique, though it appears that the separation times for both track and intensity are about 12 h/18 h/24 h from lead times 12-26 h/42-96 h/102-120 h, respectively. These calculations may be used to best calculate whether differences between tropical cyclone track and intensity forecasts from various models are statistically significant, and to inform the efficient design of tests of new systems.
Ahern, K., R.E. Hart, and M.A. Bourassa. Asymmetric hurricane boundary layer structure during storm decay. Part 1: Formation of descending inflow. Monthly Weather Review, 149(11):3851-3874, https://doi.org/10.1175/MWR-D-21-0030.1 2021
In this first part of a two-part study, the three-dimensional structure of the inner-core boundary layer (BL) is investigated in a full-physics simulation of Hurricane Irma (2017). The BL structure is highlighted during periods of intensity change, with focus on features and mechanisms associated with storm decay. The azimuthal structure of the BL is shown to be linked to the vertical wind shear and storm motion. The BL inflow becomes more asymmetric under increased shear. As BL inflow asymmetry amplifies, asymmetries in the low-level primary circulation and thermodynamic structure develop. A mechanism is identified to explain the onset of pronounced structural asymmetries in coincidence with external forcing (e.g., through shear) that would amplify BL inflow along limited azimuth. The mechanism assumes enhanced advection of absolute angular momentum along the path of the amplified inflow (e.g., amplified downshear), which results in local spinup of the vortex and development of strong supergradient flow downwind and along the BL top. The associated agradient force results in the outward acceleration of air immediately above the BL inflow, affecting fields including divergence, vertical motion, entropy advection, and inertial stability. In this simulation, descending inflow in coincidence with amplified shear is identified as the conduit through which low-entropy air enters the inner-core BL, thereby hampering convection downwind and resulting in storm decay.
Bell, G.D., M. Rosencrans, E.S. Blake, C.W. Landsea, H. Wang, S.B. Goldenberg, and R.J. Pasch. The tropics: Tropical cyclones—Atlantic basin. In State of the Climate in 2020, J. Blunden, and T. Boyer (eds.). Bulletin of the American Meteorological Society, 102(8):S224-S230, https://doi.org/10.1175/BAMS-D-21-0080.1 2021
Bucci, L.R., S.J. Majumdar, R. Atlas, G.D. Emmitt, and S. Greco. Understanding the response of tropical cyclone structure to the assimilation of synthetic wind profiles. Monthly Weather Review, 149(6):2031-2047, https://doi.org/10.1175/MWR-D-20-0153.1 2021
This study examines how varying wind profile coverages in the tropical cyclone (TC) core, near-environment and broader synoptic environment affect the structure and evolution of a simulated Atlantic hurricane through data assimilation. Three sets of observing system simulation experiments (OSSEs) are examined in this paper. The first experiment establishes a benchmark for the case study specific to the forecast system used by assimilating idealized profiles throughout the parent domain. The second presents how TC analyses and forecasts respond to varying the coverage of swaths produced by polar-orbiting satellites of idealized wind profiles. The final experiment assesses the role of TC inner-core observations by systematically removing them radially from the center. All observations are simulated from a high-resolution regional “Nature Run” of a hurricane and the tropical atmosphere, assimilated an Ensemble Square-Root Kalman Filter and the Hurricane Weather and Research Forecast (HWRF) regional model. Results compare observation impact to the analyses, domain-wide and TC centric error statistics, and TC structural differences among the experiments. The study concludes that the most accurate TC representation is a result of the assimilation of collocated and uniform thermodynamic and kinematics observations. Intensity forecasts are improved with increased inner core wind observations, even if the observations are only available once daily. Domain-wide root-mean-square errors are significantly reduced when the TC is observed during a period of structural change, like rapid intensification. The experiments suggest the importance of wind observations and the role of inner-core surveillance when analyzing and forecasting realistic TC structure.
Chen, N., T. Tang, J.A. Zhang, L.-M. Ma, and H. Yu. On the distribution of helicity in the tropical cyclone boundary layer from dropsonde composites. Atmospheric Research, 249:105298, https://doi.org/10.1016/j.atmosres.2020.105298 2021
This study analyzes GPS dropsonde data in multiple tropical cyclones from 1997 to 2017 to investigate the boundary layer structure with a focus on helicity distribution. A helicity-based method for boundary layer height is developed and evaluated by comparing it to other boundary layer height scales including the inflow layer depth, height of the maximum tangential wind speed and thermodynamic mixed layer depth. Our dropsonde composites confirmed the radial variations of these boundary layer heights seen in previous studies. The results show that the boundary layer height defined by the maximum vertical gradient of helicity is closest to the height of the maximum tangential wind speed or jet height and is located between the inflow layer depth and thermodynamic mixed layer height in all intensity groups. All three kinematic height scales generally decrease with storm intensity at a given radius. These kinematic height scales converge in the major hurricane group, while the inflow layer depth is much larger than the other two height scales in the tropical storm group. The maximum normalized helicity is located at 100–200 m altitude which is close to the height of the maximum inflow. Both front-back and downshear-upshear asymmetries are observed in the 0–1 km layer integrated helicity in the inner core region of a storm, and the helicity on the front and downshear sides is larger in all intensity groups. The results also show that the helicity magnitude is generally larger in the boundary layer of stronger storms. Application of helicity to quantify turbulent characteristics in the boundary layer is discussed.
Chen, X., and G.H. Bryan. Role of advection of parameterized turbulence kinetic energy in tropical cyclone simulations. Journal of the Atmospheric Sciences, 78(11):3593-3611, https://doi.org/10.1175/JAS-D-21-0088.1 2021
Horizontal homogeneity is typically assumed in the design of planetary boundary layer (PBL) parameterizations in weather prediction models. Consistent with this assumption, PBL schemes with predictive equations for subgrid turbulence kinetic energy (TKE) typically neglect advection of TKE. However, tropical cyclone (TC) boundary layers are inhomogeneous, particularly in the eyewall. To gain further insight, this study examines the effect of advection of TKE using the Mellor-Yamada-Nakanishi-Niino (MYNN) PBL scheme in idealized TC simulations. The analysis focuses on two simulations, one that includes TKE advection (CTL) and one that does not (NoADV). Results show that relatively large TKE in the eyewall above 2 km is predominantly attributable to vertical advection of TKE in CTL. Interestingly, buoyancy production of TKE is negative in this region in both simulations; thus, buoyancy effects cannot explain observed columns of TKE in TC eyewalls. Both horizontal and vertical advection of TKE tends to reduce TKE and vertical viscosity (Km) in the near-surface inflow layer, particularly in the eyewall of TCs. Results also show that the simulated TC in CTL has slightly stronger maximum winds, slightly smaller radius of maximum wind (RMW), and ~5% smaller radius of gale-force wind than in NoADV. These differences are consistent with absolute angular momentum being advected to smaller radii in CTL. Sensitivity simulations further reveal that the differences between CTL and NoADV are more attributable to vertical advection (rather than horizontal advection) of TKE. Recommendations for improvements of PBL schemes that use predictive equations for TKE are also discussed.
Chen, X., G.H. Bryan, J.A. Zhang, J.J. Cione, and F.D. Marks. A framework for simulating the tropical-cyclone boundary layer using large-eddy simulation and its use in evaluating PBL parameterizations. Journal of the Atmospheric Sciences, 78(11):3559-3574, https://doi.org/10.1175/JAS-D-20-0227.1 2021
Boundary layer turbulent processes affect tropical cyclone (TC) structure and intensity change. However, uncertainties in the parameterization of the planetary boundary layer (PBL) under high-wind conditions remain challenging, mostly due to limited observations. This study presents and evaluates a framework of numerical simulation that can be used for a small-domain [O(5 km)] large-eddy simulation (LES) and single-column modeling (SCM) to study the TC boundary layer. The framework builds upon a previous study that uses a few input parameters to represent the TC vortex and adds a simple nudging term for temperature and moisture to account for the complex thermodynamic processes in TCs. The reference thermodynamic profiles at different wind speeds are retrieved from a composite analysis of dropsonde observations of mature hurricanes. Results from LES show that most of the turbulence kinetic energy and vertical momentum flux is associated with resolved processes when horizontal grid spacing is O(10 m). Comparison to observations of turbulence variables such as momentum flux, effective eddy viscosity, and turbulence length scale show that LES produces reasonable results but highlight areas where further observations are necessary. LES results also demonstrate that compared to a classic Ekman-type boundary layer, the TC boundary layer is shallower, develops steady conditions much quicker, and exhibits stronger wind speed near the surface. The utility of this framework is further highlighted by evaluating a first-order PBL parameterization, suggesting that an asymptotic turbulence length scale of 40 m produces a good match to LES results.
Chen, X., J.-F. Gu, J.A. Zhang, F.D. Marks, R.F. Rogers, and J.J. Cione. Boundary layer recovery and precipitation symmetrization preceding rapid intensification of tropical cyclones under shear. Journal of the Atmospheric Sciences, 78(5):1523-1544, https://doi.org/10.1175/JAS-D-20-0252.1 2021
This study investigates the precipitation symmetrization preceding rapid intensification (RI) of tropical cyclones (TCs) experiencing vertical wind shear by analyzing numerical simulations of Typhoon Mujigae (2015) with warm (CTL) and relatively cool (S1) sea surface temperatures (SSTs). A novel finding is that precipitation symmetrization is maintained by the continuous development of deep convection along the inward flank of a convective precipitation shield (CPS), especially in the downwind part. Beneath the CPS, downdrafts flush the boundary layer with low-entropy parcels. These low-entropy parcels do not necessarily weaken the TCs; instead, they are “recycled” in the TC circulation, gradually recovered by positive enthalpy fluxes, and develop into convection during their propagation toward a downshear convergence zone. Along-trajectory vertical momentum budget analyses reveal the predominant role of buoyancy acceleration in the convective development in both experiments. The boundary layer recovery is more efficient for warmer SST, and the stronger buoyancy acceleration accounts for the higher probability of these parcels developing into deep convection in the downwind part of the CPS, which helps maintain the precipitation symmetrization in CTL. In contrast, less efficient boundary layer recovery and less upshear deep convection hinder the precipitation symmetrization in S1. These findings highlight the key role of boundary layer recovery in regulating the precipitation symmetrization and upshear deep convection, which further accounts for an earlier RI onset timing of the CTL TC. The inward rebuilding pathway also illuminates why deep convection is preferentially located inside the radius of maximum wind of sheared TCs undergoing RI.
Chen, X., M. Xue, B. Zhou, J. Feng, J.A. Zhang, and F.D. Marks. Effect of scale-aware planetary boundary layer schemes on tropical cyclone intensification and structural changes in the gray zone. Monthly Weather Review, 149(7):2079-2095, https://doi.org/10.1175/MWR-D-20-0297.1 2021
Horizontal grid spacings of numerical weather prediction models are rapidly approaching O (1 km) and have become comparable with the dominant length scales of flows in the boundary layer; within such “gray-zone”, conventional planetary boundary layer (PBL) parameterization schemes start to violate basic design assumptions. Scale-aware PBL schemes have been developed recently to address the gray-zone issue. By performing WRF simulations of Hurricane Earl (2010) at sub-kilometer grid spacings, this study investigates the effect of the scale-aware Shin-Hong (SH) scheme on the tropical cyclone (TC) intensification and structural changes in comparison to the non-scale-aware YSU scheme it is built upon. Results indicate that SH tends to produce a stronger TC with a more compact inner core than YSU. At early stages, the scale-aware coefficients in SH gradually decrease as the diagnosed boundary layer height exceeds the horizontal grid spacing. This scale-aware effect is most prominent for the nonlocal subgrid-scale vertical turbulent fluxes, in the non-precipitation regions radially outside of the convective rainband, and from the early stage through the middle of rapid intensification (RI) phase. Both the scale awareness and different parameterization of the nonlocal turbulent heat flux in SH reduce the parameterized vertical turbulent mixing, which further induces stronger radial inflows and helps retain more water vapor in the boundary layer. The resulting stronger moisture convergence and diabatic heating near the TC center account for the faster inner-core contraction before RI onset and the higher intensification rate during the RI period. Potential issues of applying these two PBL schemes in TC simulations and suggestions for improvements are discussed.
Christophersen, H.W., B.A. Dahl, J.P. Dunion, R.F. Rogers, F.D. Marks, R. Atlas, and W.J. Blackwell. Impact of TROPICS radiances on tropical cyclone prediction in an OSSE. Monthly Weather Review, 149(7):2279-2298, https://doi.org/10.1175/MWR-D-20-0339.1 2021
As part of the NASA Earth Venture-Instrument program, the Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission, to be launched in January 2022, will deliver unprecedented rapid-update microwave measurements over the tropics that can be used to observe the evolution of the precipitation and thermodynamic structure of tropical cyclones (TCs) at meso- and synoptic scales. TROPICS consists of six CubeSats, each hosting a passive microwave radiometer that provides radiance observations sensitive to atmospheric temperature, water vapor, precipitation, and precipitation-size ice particles. In this study, the impact of TROPICS all-sky radiances on TC analyses and forecasts is explored through a regional mesoscale observing system simulation experiment (OSSE). The results indicate that the TROPICS all-sky radiances can have positive impacts on TC track and intensity forecasts, particularly when some hydrometeor state variables and other state variables of the data assimilation system that are relevant to cloudy radiance assimilation are updated. The largest impact on the model analyses is seen in the humidity fields, regardless of whether or not there are radiances assimilated from other satellites. TROPICS radiances demonstrate large impact on TC analyses and forecasts when other satellite radiances are absent. The assimilation of the all-sky TROPICS radiances without default radiances leads to a consistent improvement in the low- and mid-tropospheric temperature and wind forecasts throughout the five-day forecasts, but only up to 36 h lead time in the humidity forecasts at all pressure levels. This study illustrates the potential benefits of TROPICS data assimilation for TC forecasts and provides a potentially streamlined pathway for transitioning TROPICS data from research to operations post-launch.
Cucurull, L., and S.P.F. Casey. Improved impacts in observing system simulation experiments of radio occultation observations as a result of model and data assimilation changes. Monthly Weather Review, 149(1):207-220, https://doi.org/10.1175/MWR-D-20-0174.1 2021
As global data assimilation systems continue to evolve, Observing System Simulation Experiments (OSSEs) need to be updated to accurately quantify the impact of proposed observing technologies in weather forecasting. Earlier OSSEs with radio occultation (RO) observations have been updated and the impact of the originally proposed Constellation Observing Satellites for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) mission, with a high-inclination and low-inclination component, has been investigated by using the operational data assimilation system at NOAA and a 1-dimensional bending angle RO forward operator. It is found that the impact of the low-inclination component of the originally planned COSMIC-2 mission (now officially named COSMIC-2) has significantly increased as compared to earlier studies, and significant positive impact is now found globally in terms of mass and wind fields. These are encouraging results as COSMIC-2 was successfully launched in June 2019 and data have been recently released to operational weather centers. Earlier findings remain valid indicating that globally distributed RO observations are more important to improve weather prediction globally than a denser sampling of the tropical latitudes. Overall, the benefits reported here from assimilating RO soundings are much more significant than the impacts found in previous OSSEs. This is largely attributed to changes in the data assimilation and forecast system and less to the more advanced 1-dimensional forward operator chosen for the assimilation of RO observations.
DeMaria, M., J.L. Franklin, M.J. Onderlinde, and J. Kaplan. Operational forecasting of tropical cyclone rapid intensification at the National Hurricane Center. Atmosphere, 12(6):683, https://doi.org/10.3390/atmos12060683 2021
Although some recent progress has been made in operational tropical cyclone (TC) intensity forecasting, the prediction of rapid intensification (RI) remains a challenging problem. To document RI forecast progress, deterministic and probabilistic operational intensity models used by the National Hurricane Center (NHC) are briefly reviewed. Results show that none of the deterministic models had RI utility from 1991 to about 2015 due to very low probability of detection, very high false alarm ratio, or both. Some ability to forecast RI has emerged since 2015, with dynamical models being the best guidance for the Atlantic and statistical models the best RI guidance for the eastern North Pacific. The first probabilistic RI guidance became available in 2001, with several upgrades since then leading to modest skill in recent years. A tool introduced in 2018 (DTOPS) is currently the most skillful among NHC’s probabilistic RI guidance. To measure programmatic progress in forecasting RI, the Hurricane Forecast Improvement Program has introduced a new RI metric that uses the traditional mean absolute error but restricts the sample to only those cases where RI occurred in the verifying best track or was forecast. By this metric, RI forecasts have improved by ~20–25% since the 2015–2017 baseline period.
Domingues, R., M. Le Henaff, G. Halliwell, J.A. Zhang, F. Bringas, P. Chardon, H.-S. Kim, J. Morell, and G. Goni. Ocean conditions and the intensification of three major Atlantic hurricanes of 2017. Monthly Weather Review, 149(5):1265-1286, https://doi.org/10.1175/MWR-D-20-0100.1 2021
Major Atlantic hurricanes Irma, Jose, and Maria of 2017 reached their peak intensity in September while traveling over the tropical North Atlantic Ocean and Caribbean Sea, where both atmospheric and ocean conditions were favorable for intensification. In-situ and satellite ocean observations revealed that conditions in these areas exhibited: (i) sea surface temperatures above 28°C; (ii) upper-ocean heat content above 60 kJ cm-2; and (iii) the presence of low-salinity barrier layers associated with a larger-than-usual extension of the Amazon and Orinoco riverine plumes. Proof-of-concept coupled ocean-hurricane numerical model experiments demonstrated that the accurate representation of such ocean conditions led to an improvement in the simulated intensity of Hurricane Maria for the 3 days preceding landfall in Puerto Rico, when compared to an experiment without the assimilation of ocean observations. Without the assimilation of ocean observations, upper-ocean thermal conditions were generally colder than observations, resulting in reduced air-sea enthalpy fluxes - enthalpy fluxes are more realistically simulated when the upper-ocean temperature and salinity structure is better represented in the model. Our results further showed that different components of the ocean observing system provide valuable information in support of improved TC simulations, and that assimilation of underwater glider observations alone provided the largest improvement of the total improvement over the 24-hour time frame before landfall. Our results therefore indicated that ocean conditions were relevant for more realistically simulating Hurricane Maria’s intensity. However, further research based on a comprehensive set of hurricane cases is required to confirm robust improvements to forecast systems.
Gopalakrishnan, S., A. Hazelton, and J.A. Zhang. Improving hurricane boundary layer parameterization scheme based on observations. Earth and Space Science, 8(3):e2020EA001422 , https://doi.org/10.1029/2020EA001422 2021
Flight‐level data and global positioning system dropwindsonde observations collected from more than 187 flights into 19 tropical cyclones were used to examine why different planetary boundary layer parameterization schemes applied to hurricane models produce diverse forecasts of structure and intensity change. Two popular, yet diverse, physics schemes, namely, the GFS K‐Profile and a 1.5‐order turbulence kinetic energy closure parameterization from NOAA's next‐generation FV3‐based Hurricane Analysis and Forecast System were used. It was found that uncertainty related to some key variables used in the parameterization of the eddy diffusivity, Km, led to diverse solutions. For a given grid resolution, both parameterization schemes converged to a similar forecast state provided those uncertainties could be identified and improved based on observations. This is important for providing a generalized framework for the development and evaluation of parameterization schemes in operational models that resemble reality. This study also indicates that the shape of the Km profile is equally important as its maximum value. The smaller the Km near the surface, the stronger the inflow in the boundary layer.
Green, A., S.G. Gopalakrishnan, G.J. Alaka, and S. Chiao. Understanding the role of mean and eddy momentum transport in the rapid intensification of Hurricane Irma (2017) and Hurricane Michael (2018). Atmosphere, 12(4):492, https://doi.org/10.3390/atmos12040492 2021
The prediction of rapid intensification (RI) in tropical cyclones (TCs) is a challenging problem. In this study, the RI process and factors contributing to it are compared for two TCs: an axis-symmetric case (Hurricane Irma, 2017) and an asymmetric case (Hurricane Michael, 2018). Both Irma and Michael became major hurricanes that made significant impacts in the United States. The Hurricane Weather Research and Forecasting (HWRF) Model was used to examine the connection between RI with forcing from the large-scale environment and the subsequent evolution of TC structure and convection. The observed large-scale environment was reasonably reproduced by HWRF forecasts. Hurricane Irma rapidly intensified in an environment with weak-to-moderate vertical wind shear (VWS), typically favorable for RI, leading to the symmetric development of vortical convective clouds in the cyclonic, vorticity-rich environment. Conversely, Hurricane Michael rapidly intensified in an environment of strong VWS, typically unfavorable for RI, leading to major asymmetries in the development of vortical convective clouds. The tangential wind momentum budget was analyzed for these two hurricanes to identify similarities and differences in the pathways to RI. Results suggest that eddy transport terms associated with convective processes positively contributed to vortex spin up in the early stages of RI and inhibited spin up in the later stages of RI in both TCs. In the early stages of RI, the mean transport terms exhibited notable differences in these TCs; they dominated the spin-up process in Irma and were of secondary importance to the spin-up process in Michael. Favorable aspects of the environment surrounding Michael appeared to aid in the RI process despite hostile VWS.
Hazelton, A., G. Alaka, L. Cowan, M. Fischer, and S. Gopalakrishnan. Understanding the processes causing the early intensification of Hurricane Dorian through an ensemble of the Hurricane Analysis and Forecast System (HAFS). Atmosphere, 12(1):93, https://doi.org/10.3390/atmos12010093 2021
The early stages of a tropical cyclone can be a challenge to forecast, as a storm consolidates and begins to grow based on the local and environmental conditions. A high-resolution ensemble of the Hurricane Analysis and Forecast System (HAFS) is used to study the early intensification of Hurricane Dorian, a catastrophic 2019 storm in which the early period proved challenging for forecasters. There was a clear connection in the ensemble between early storm track and intensity: stronger members moved more northeast initially, although this result did not have much impact on the long-term track. The ensemble results show several key factors determining the early evolution of Dorian. Large-scale divergence northeast of the tropical cyclone (TC) appeared to favor intensification, and this structure was present at model initialization. There was also greater moisture northeast of the TC for stronger members at initialization, favoring more intensification and downshear development of the circulation as these members evolved. This study highlights the complex interplay between synoptic and storm scale processes in the development and intensification of early-stage tropical cyclones.
Hazelton, A., Z. Zhang, B. Liu, J. Dong, G. Alaka, W. Wang, T. Matchok, A. Menra, S. Gopalakrishnan, X. Zhang, M. Bender, V. Tallapragada, and F. Marks. 2019 Atlantic hurricane forecasts from the Global-Nested Hurricane Analysis and Forecast System (HAFS): Composite statistics and key events. Weather and Forecasting, 36(2):519-538, https://doi.org/10.1175/WAF-D-20-0044.1 2021
NOAA’s Hurricane Analysis and Forecast System (HAFS) is an evolving FV3-based hurricane modeling system that is expected to replace the operational hurricane models at the National Weather Service. Supported by the Hurricane Forecast Improvement Program (HFIP), global-nested and regional versions of HAFS were run in real-time in 2019 to create the first baseline for the HAFS advancement. In this study, forecasts from the global-nested configuration of HAFS (HAFS-globalnest) are evaluated and compared with other operational and experimental models. The forecasts by HAFS-globalnest covered the period from July through October during the 2019 hurricane season. Tropical cyclone (TC) track, intensity, and structure forecast verifications are examined. HAFS-globalnest showed track skill superior to several operational hurricane models and comparable intensity and structure skill, although the skill in predicting rapid intensification was slightly inferior to the operational model skill. HAFS-globalnest correctly predicted that Hurricane Dorian would slow and turn north in the Bahamas and also correctly predicted structural features in other TCs such as a sting jet in Hurricane Humberto during extratropical transition. Humberto was also a case where HAFS-globalnest had better track forecasts than a regional version of HAFS (HAFS-SAR) due to a better representation of the large-scale flow. These examples and others are examined through comparisons with airborne tail Doppler radar from the NOAA WP-3D to provide a more detailed evaluation of TC structure prediction. The results from this real-time experiment motivates several future model improvements, and highlights the promise of HAFS-globalnest for improved TC prediction.
Homeyer, C.R., A.O. Fierro, B.A. Schenkel, A.C. Didlake, G.M. McFarquhar, J. Hu, A.V. Ryzhkov, J.B. Basara, A.M. Murphy, and J. Zawislak. Polarimetric signatures in landfalling tropical cyclones. Monthly Weather Review, 149(1):131-154, https://doi.org/10.1175/MWR-D-20-0111.1 2021
Polarimetric radar observations from the NEXRAD WSR-88D operational radar network in the contiguous United States, routinely available since 2013, are used to reveal three prominent microphysical signatures in landfalling tropical cyclones: (1) hydrometeor size sorting within the eyewall convection; (2) vertical displacement of the melting layer within the inner core; and (3) dendritic growth layers within stratiform regions of the inner core. Size sorting signatures within eyewall convection are observed with greater frequency and prominence in more intense hurricanes, and are observed predominantly within the deep-layer environmental wind shear vector-relative quadrants that harbor the greatest frequency of deep convection (i.e., downshear and left-of-shear). Melting layer displacements are shown to exceed 1 km in altitude compared to melting layer altitudes in outer rainbands and are complemented by analyses of archived dropsonde data. Dendritic growth and attendant snow aggregation signatures in the inner core are found to occur more often when echo top altitudes are low (≤10 km ASL), nearer the –15°C isotherm commonly associated with dendritic growth. These signatures, uniquely observed by polarimetric radar, provide greater insight into the physical structure and thermodynamic characteristics of tropical cyclones, which are important for improving rainfall estimation and the representation of tropical cyclones in numerical models.
Huang, F., J.L. Garrison, S.M. Leidner, B. Annane, R.N. Hoffman, G. Grieco, and A. Stoffelen. A forward model for data assimilation of GNSS ocean reflectometry delayed-Doppler maps. IEEE Transactions on Geoscience and Remote Sensing, 59(3):2643-2656, https://doi.org/10.1109/TGRS.2020.3002801 2021
Delay-Doppler maps (DDMs) are generally the lowest level of calibrated observables produced from global navigation satellite system reflectometry (GNSS-R). A forward model is presented to relate the DDM, in units of absolute power at the receiver, to the ocean surface wind field. This model and the related Jacobian are designed for use in assimilating DDM observables into weather forecast models. Given that the forward model represents a full set of DDM measurements, direct assimilation of this lower level data product is expected to be more effective than using individual specular-point wind speed retrievals. The forward model is assessed by comparing DDMs computed from hurricane weather research and forecasting (HWRF) model winds against measured DDMs from the Cyclone Global Navigation Satellite System (CYGNSS) Level 1a data. Quality controls are proposed as a result of observed discrepancies due to the effect of swell, power calibration bias, inaccurate specular point position, and model representativeness error. DDM assimilation is demonstrated using a variational analysis method (VAM) applied to three cases from June 2017, specifically selected due to the large deviation between scatterometer winds and European Centre for Medium-Range Weather Forecasts (ECMWF) predictions. DDM assimilation reduced the root-mean-square error (RMSE) by 15%, 28%, and 48%, respectively, in each of the three examples.
Huang, F., J.L. Garrison, S.M. Leidner, G. Grieco, A. Stoffelen, B. Annane, and R.N. Hoffman. Assimilation of GNSS reflectometry delay-Doppler maps with a two-dimensional variational analysis of global ocean surface winds. Quarterly Journal of the Royal Meteorological Society, 147(737):2469-2489, https://doi.org/10.1002/qj.4034 2021
Direct remote‐sensing observations (e.g., radar backscatter, radiometer brightness temperature, or radio occultation bending angle) are often more effective for use in data assimilation (DA) than the corresponding geophysical retrievals (e.g., ocean surface winds, soil moisture, or atmospheric water vapor). In the particular case of Global Navigation Satellite System Reflectometry (GNSS‐R), the lower‐level delay‐Doppler map (DDM) observable shows a complicated relationship with the ocean surface wind field. Prior studies have demonstrated DA using GNSS‐R wind retrievals inferred from DDMs. The complexity of the DDM dependence on winds, however, suggests that the alternative approach of ingesting DDM observables directly into DA systems, without performing a wind retrieval, may be beneficial. We demonstrate assimilation of DDM observables from the NASA Cyclone Global Navigation Satellite System (CYGNSS) mission into global ocean surface wind analyses using a two‐dimensional variational analysis method. Bias correction and quality‐control methods are described. Several models for the required observation‐error covariance matrix are developed and evaluated, with the conclusion that a diagonal matrix performs as well as a fully populated matrix empirically tuned to a large ensemble of CYGNSS observation data. The 10‐m surface winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) operational forecast are used as the background (i.e., prior in the variational analysis). Results are compared with independent scatterometer (the advanced scatterometer (ASCAT), the oceansat‐2 Scatterometer (OSCAT)) winds. For one month (June 2017) of data, the root‐mean‐square difference (RMSD) was reduced from 1.17 to 1.07 m·s−1 and bias from −0.14 to −0.08 m·s−1 for the wind speed at the specular point. Within a 150‐km wide swath along the specular point track, the RMSD was reduced from 1.20 to 1.13 m·s−1. These RMSD and bias statistics are smaller than other CYGNSS wind products available at this time.
Huang, J., Z. Zou, Q. Zeng, P. Li, J. Song, L. Wu, J.A. Zhang, S. Li, and P-W. Chan. The turbulent structure of the marine atmospheric boundary layer during and before a cold front. Journal of the Atmospheric Sciences, 78(3):863-875, https://doi.org/10.1175/JAS-D-19-0314.1 2021
The turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind, sea, and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments for flat land surfaces. The results reveal that the velocity spectra follow a k-1 law within the intermediate wavenumber (k) range immediately below the inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the Attached Eddy Model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k-1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions.
Jaimes de la Cruz, B., L.K. Skay, J.B. Wadler, and J.E. Rudzin. On the hyperbolicity of the bulk air-sea heat flux functions: Insights into the efficiency of air-sea moisture disequilibrium for tropical cyclone intensification. Monthly Weather Review, 149(5):1517-1534, https://doi.org/10.1175/MWR-D-20-0324.1 2021
Sea-to-air heat fluxes are the energy source for tropical cyclone (TC) development and maintenance. In the bulk aerodynamic formulae, these fluxes are a function of surface wind speed (U10) and air-sea temperature and moisture disequilibrium (ΔT and Δq, respectively). While many studies have explained TC intensification through the mutual dependence between increasing U10 and increasing sea-to-air heat fluxes, recent studies found TC intensification can occur through deep convective vortex structures that obtain their local buoyancy from sea-to-air moisture fluxes, even under relatively low-wind conditions. Herein, a new perspective on the bulk aerodynamic formulae is introduced to evaluate the relative contribution of wind-driven (U10) and thermodynamically-driven (ΔT and Δq) ocean heat-uptake. Previously unnoticed salient properties of these formulae, reported here, are: (1) these functions are hyperbolic; and, (2) increasing Δq is an efficient mechanism for enhancing the fluxes. This new perspective was used to investigate surface heat fluxes in six TCs during phases of steady state intensity (SS), slow intensification (SI), and rapid intensification (RI). A capping of wind-driven heat-uptake was found during periods of SS, SI, and RI. Compensation by larger values of Δq>5 g kg-1 at moderate values of U10 led to intense inner-core moisture fluxes >600 W m-2 during RI. Peak values in Δq preferentially occurred over oceanic regimes with higher sea surface temperature (SST) and upper-ocean heat content. Thus, increasing SST and Δq is a very effective way to increase surface heat fluxes—this can be easily achieved as a TC moves over deeper warm oceanic regimes.
Kalina, E.A., M.K. Biswas, J.A. Zhang, and K.M. Newman. Sensitivity of an idealized tropical cyclone to the configuration of the Global Forecast System–eddy diffusivity mass flux planetary boundary layer scheme. Atmosphere, 12(2):284, https://doi.org/10.3390/atmos12020284 2021
The intensity and structure of simulated tropical cyclones (TCs) are known to be sensitive to the planetary boundary layer (PBL) parameterization in numerical weather prediction models. In this paper, we use an idealized version of the Hurricane Weather Research and Forecast system (HWRF) with constant sea-surface temperature (SST) to examine how the configuration of the PBL scheme used in the operational HWRF affects TC intensity change (including rapid intensification) and structure. The configuration changes explored in this study include disabling non-local vertical mixing, changing the coefficients in the stability functions for momentum and heat, and directly modifying the Prandtl number (Pr), which controls the ratio of momentum to heat and moisture exchange in the PBL. Relative to the control simulation, disabling non-local mixing produced a ~15% larger storm that intensified more gradually, while changing the coefficient values used in the stability functions had little effect. Varying Pr within the PBL had the greatest impact, with the largest Pr (~1.6 versus ~0.8) associated with more rapid intensification (~38 versus 29 m s−1 per day) but a 5–10 m s−1 weaker intensity after the initial period of strengthening. This seemingly paradoxical result is likely due to a decrease in the radius of maximum wind (~15 versus 20 km), but smaller enthalpy fluxes, in simulated storms with larger Pr. These results underscore the importance of measuring the vertical eddy diffusivities of momentum, heat, and moisture under high-wind, open-ocean conditions to reduce uncertainty in Pr in the TC PBL.
Kren, A C., and R.A. Anthes. Estimating error variances of a microwave sensor and dropsondes aboard the Global Hawk in hurricanes using the three-cornered hat method. Journal of Atmospheric and Oceanic Technology, 38(2):197-208, https://doi.org/10.1175/JTECH-D-20-0044.1 2021
This study estimates the random error variances and standard deviations (STDs) for four data sets: Global Hawk (GH) dropsondes (DROP), the High-Altitude Monolithic Microwave Integrated Circuit Sounding Radiometer (HAMSR) aboard the GH, the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis, and the Hurricane Weather Research and Forecasting (HWRF) model, using the three-cornered hat (3CH) method. These estimates are made during the 2016 Sensing Hazards with Operational Unmanned Technology (SHOUT) season in the environment of four tropical cyclones from August to October. For temperature and specific and relative humidity, the ERA5, HWRF, and DROP data sets all have similar magnitudes of errors, with ERA5 having the smallest. The error STDs of temperature and specific humidity are less than 0.8 K and 1.0 g kg-1 over most of the troposphere, while relative humidity error STDs increase from less than 5% near the surface to between 10 and 20% in the upper troposphere. The HAMSR bias-corrected data have larger errors, with estimated error STDs of temperature and specific humidity in the lower troposphere between 1.5 and 2.0 K and 1.5 and 2.5 g kg-1. HAMSR’s relative humidity error STD increases from approximately 10% in the lower troposphere to 30% in the upper troposphere. The 3CH method error estimates are generally consistent with prior independent estimates of errors and uncertainties for the HAMSR and dropsonde data sets, although they are somewhat larger, likely due to the inclusion of representativeness errors (differences associated with different spatial and temporal scales represented by the data).
Kurosawa, K., and J. Poterjoy. Data assimilation challenges posed by nonlinear operators: A comparative study of ensemble and variational filters and smoothers. Monthly Weather Review, 149(7):2369-2389, https://doi.org/10.1175/MWR-D-20-0368.1 2021
The ensemble Kalman Filter (EnKF) and the 4D variational method (4DVar) are the most commonly used filters and smoothers in atmospheric science. These methods typically approximate prior densities using a Gaussian and solve a linear system of equations for the posterior mean and covariance. Therefore, strongly nonlinear model dynamics and measurement operators can lead to bias in posterior estimates. To improve the performance in nonlinear regimes, minimization of the 4DVar cost function typically follows multiple sets of iterations, known as an “outer loop”, which helps reduce bias caused by linear assumptions. Alternatively, “iterative ensemble methods” follow a similar strategy of periodically re-linearizing model and measurement operators. These methods come with different, possibly more appropriate, assumptions for drawing samples from the posterior density, but have seen little attention in numerical weather prediction (NWP) communities. Lastly, particle filters (PFs) present a purely Bayesian filtering approach for state estimation, which avoids many of the assumptions made by the above methods. Several strategies for applying localized PFs for NWP have been proposed very recently. The current study investigates intrinsic limitations of current data assimilation methodology for applications that require nonlinear measurement operators. In doing so, it targets a specific problem that is relevant to the assimilation of remotely-sensed measurements, such as radar reflectivity and all-sky radiances, which pose challenges for Gaussian-based data assimilation systems. This comparison includes multiple data assimilation approaches designed recently for nonlinear/non-Gaussian applications, as well as those currently used for NWP.
Le Hénaff, M., R. Domingues, G. Halliwell, J.A. Zhang, H.-S. Kim, M. Aristizabal, T. Miles, S. Glenn, and G. Goni. The role of the Gulf of Mexico ocean conditions in the intensification of Hurricane Michael (2018). Journal of Geophysical Research–Oceans, 126(5):e2020JC016969, https://doi.org/10.1029/2020JC016969 2021
Hurricane Michael formed on October 7, 2018, in the Northwestern Caribbean Sea, and quickly traveled northward through the Gulf of Mexico, making landfall on the Florida panhandle as a devastating Category 5 hurricane only 3 days later. Before landfall, Michael underwent rapid intensification, despite unfavorable atmospheric conditions. Using observations, we characterized the key ocean features encountered by Michael along its track, which are known for favoring hurricane intensification: high sea surface temperatures, upper ocean heat content and low salinity barrier layer conditions. Ocean observations were consistent with suppressed hurricane-induced upper ocean cooling, which could only be observed by underwater gliders, and showed that Hurricane Michael constantly experienced sea surface temperatures above 28°C. We carried out ocean Observing System Experiments, which demonstrate that the combined assimilation of in situ and satellite ocean observations into a numerical ocean model led to the most realistic representation of the ocean conditions. They also suggest that, when using the Cooper-Haines (1996) method to assimilate altimetry observations, assimilating temperature observations is necessary to constrain the model upper ocean vertical structure. We also performed coupled hurricane-ocean simulations to assess the impact of ocean initial conditions on forecasting Michael. These simulations demonstrate that the ocean conditions, in particular the high sea surface temperatures north of 24°N, played a crucial role in the intensification of Michael. Coupled simulations initialized with the most realistic ocean conditions, constrained by field and satellite observations, show a ∼56% error reduction in wind intensity prior to landfall compared to simulations initialized without data assimilation.
Lin, I.-I., R.F. Rogers, H.-C. Huang, Y.-C. Liao, D. Herndon, J.-Y. Yu, Y.-T. Chang, J.A. Zhang, C.M. Patricola, I.-F. Pun, and C.-C. Lien. A tale of two rapidly-intensifying supertyphoons: Hagibis (2019) and Haiyan (2013). Bulletin of the American Meteorological Society, 102(9):E1645-E1664, https://doi.org/10.1175/BAMS-D-20-0223.1 2021
Devastating Japan in October 2019, Supertyphoon (STY) Hagibis was an important typhoon in the history of the Pacific. A striking feature of Hagibis was its explosive rapid intensification (RI). In 24 h, Hagibis intensified by 100 kt, making it one of the fastest-intensifying typhoons ever observed. After RI, Hagibis’s intensification stalled. Using the current typhoon intensity record holder, i.e., STY Haiyan (2013), as a benchmark, this work explores the intensity evolution differences of these two high-impact STYs. We found that the extremely high pre-storm sea surface temperature reaching 30.5°C, deep/warm pre-storm ocean heat content reaching 160 kJ cm−2, fast forward storm motion of ~8 ms−1, small during-storm ocean cooling effect of ~ 0.5°C, significant thunderstorm activity at its center, and rapid eyewall contraction were all important contributors to Hagibis’s impressive intensification. There was 36% more air-sea flux for Hagibis’s RI than for Haiyan’s. After its spectacular RI, Hagibis’s intensification stopped, despite favorable environments. Haiyan, by contrast, continued to intensify, reaching its record-breaking intensity of 170 kt. A key finding here is the multiple pathways that storm size affected the intensity evolution for both typhoons. After RI, Hagibis experienced a major size expansion, becoming the largest typhoon on record in the Pacific. This size enlargement, combined with a reduction in storm translational speed, induced stronger ocean cooling that reduced ocean flux and hindered intensification. The large storm size also contributed to slower eyewall replacement cycles (ERCs), which prolonged the negative impact of the ERC on intensification.
Ma, Z., Z. Li, J. Li, T.J. Schmit, L. Cucurull, R. Atlas, and B. Sun Enhance low level temperature and moisture profiles through combining NUCAPS, ABI observations, and RTMA analysis. Earth and Space Science, 8(6):e2020EA001402, https://doi.org/10.1029/2020EA001402 2021
Thermodynamic information from low levels in the atmosphere is crucial for operational weather forecasts and meteorological researchers. The NOAA Unique Combined Atmospheric Processing System (NUCAPS) sounding products have been proven beneficial to fill the data gap between synoptic radiosonde observations (RAOBs). However, compared with the upper troposphere, the accuracy of NUCAPS soundings in the low levels still needs improvement. In this study, a deep neural network (DNN) is applied to fuse multiple data sources to enhance the NUCAPS temperature and moisture profiles in the lower atmosphere. The network is developed by combining satellite observations, including NUCAPS sounding retrievals and high resolution geostationary satellite observations from the Advanced Baseline Imager, and surface analysis from the Real-Time Mesoscale Analysis (RTMA) as inputs, while collocated soundings from ECMWF re-analysis version 5 are used as the benchmark for the training. The performance of the model is evaluated by using the independent testing data set, data from a different year, as well as collocated RAOBs, showing improvement to the temperature and moisture profiles by reducing the root-mean-squared-error (RMSE) by more than 30% in the lower atmosphere (from 700 hPa to surface) in both clear sky and partially cloudy conditions. A convective event from June 18, 2017 is presented to illustrate the application of the enhanced low level soundings on high impact weather events. The enhanced soundings from fused data capture the large surface-based convective available potential energy structures in the preconvection environment, which is very useful for severe storm nowcasting and forecasting applications.
Miles, T.N., D. Zhang, G.R. Foltz, J.A. Zhang, C. Meinig, F. Bringas, J. Trinanes, M. Le Henaff, M.F. Aristizabal Vargas, S. Coakley, C.R. Edwards, D. Gong, R.E. Todd, M.J. Oliver, W.D. Wilson, K. Whilden, B. Kirkpatrick, P. Chardon-Maldonado, J.M. Morell, D. Hernandez, G. Kuska, C.D. Stienbarger, K. Bailey, C. Zhang, S.M. Glenn, and G.J. Goni. Uncrewed ocean gliders and saildrones support hurricane forecasting and research. Oceanography 34(4):78-81, https://doi.org/10.5670/oceanog.2021.supplement.02 2021
Mueller, M.J., B. Annane, S.M. Leidner, and L. Cucurull. Impact of CYGNSS-derived winds on tropical cyclone forecasts in a global and regional model. Monthly Weather Review, 149(10):3433-3447, https://doi.org/10.1175/MWR-D-21-0094.1 2021
An observing system experiment (OSE) was conducted to assess the impact of wind products derived from the Cyclone Global Navigation Satellite System (CYGNSS) on tropical cyclone (TC) track, maximum 10-m wind speed (Vmax), and minimum sea level pressure forecasts. The experiment used a global data assimilation and forecast system and the impact of both CYGNSS-derived scalar and vector wind retrievals was investigated. The CYGNSS-derived vector wind products were generated by optimally combining the scalar winds and a gridded a priori vector field. Additional tests investigated the impact of CYGNSS data on a regional model through the impact of lateral boundary and initial conditions from the global model during the developmental phase of Hurricane Michael (2018). In the global model, statistically significant track forecast improvements of 20-40 km were found in the first 60 h. Vmax forecasts showed some significant degradations of ~2 kts at a few lead times, especially in the first 24 h. At most lead times, impacts were not statistically significant. Degradations in Vmax for Hurricane Michael in the global model were largely attributable to a failure of the CYGNSS-derived scalar wind test to produce rapid intensification in the forecast failure of the CYGNSS-derived scalar wind test to produce rapid intensification in the forecast symmetrical compared to the control and CYGNSS-derived vector wind test. The regional model used initial and lateral boundary conditions from the global control and CYGNSS scalar wind tests. The regional forecasts showed large improvements in track, Vmax, and minimum sea level pressure.
Poterjoy, J., G.J. Alaka, and H.R. Winterbottom. The irreplaceable utility of sequential data assimilation for numerical weather prediction system development: Lessons learned from an experimental HWRF system. Weather and Forecasting, 36(2):661-677, https://doi.org/10.1175/WAF-D-20-0204.1 2021
Limited-area numerical weather prediction models currently run operationally in the United States follow a “partially-cycled” schedule, where sequential data assimilation is periodically interrupted by replacing model states with solutions interpolated from a global model. While this strategy helps overcome several practical challenges associated with real-time regional forecasting, it is no substitute for a robust sequential data assimilation approach for research-to-operations purposes. Partial cycling can mask systematic errors in weather models, data assimilation systems, and data pre-processing techniques, since it introduces information from a different prediction system. It also adds extra heuristics to the model initialization steps outside the general Bayesian filtering framework from which data assimilation methods are derived. This study uses a research-oriented modeling system, which is self-contained in the operational Hurricane Weather Research and Forecasting (HWRF) model package, to illustrate why next-generation modeling systems should prioritize sequential data assimilation at early stages of development. This framework permits the rigorous examination of all model system components—in a manner that has never been done for the HWRF model. Examples presented in this manuscript show how sequential data assimilation capabilities can accelerate model advancements and increase academic involvement in operational forecasting systems at a time when the United States is developing a new hurricane forecasting system.
Rogers, R.F. Recent advances in our understanding of tropical cyclone intensity change processes from airborne observations. Atmosphere, 12(5):650, https://doi.org/10.3390/atmos12050650 2021
Recent (past ~15 years) advances in our understanding of tropical cyclone (TC) intensity change processes using aircraft data are summarized here. The focus covers a variety of spatiotemporal scales, regions of the TC inner core, and stages of the TC lifecycle, from preformation to major hurricane status. Topics covered include (1) characterizing TC structure and its relationship to intensity change; (2) TC intensification in vertical shear; (3) planetary boundary layer (PBL) processes and air–sea interaction; (4) upper-level warm core structure and evolution; (5) genesis and development of weak TCs; and (6) secondary eyewall formation/eyewall replacement cycles (SEF/ERC). Gaps in our airborne observational capabilities are discussed, as are new observing technologies to address these gaps and future directions for airborne TC intensity change research.
Ryglicki, D.R., C.S. Velden, P.D. Reasor, D. Hodyss, and J.D. Doyle. Observations of atypical rapid intensification characteristics in Hurricane Dorian (2019). Monthly Weather Review, 149(7):2131-2150, https://doi.org/10.1175/MWR-D-20-0413.1 2021
Multiple observation and analysis datasets are used to demonstrate two key features of the Atypical Rapid Intensification (ARI) process that occurred in Atlantic Hurricane Dorian (2019): (1) precession and nutations of the vortex tilt; and (2) blocking of the impinging upper-level environmental flow by the outflow. As Dorian came under the influence of an upper-level anticyclone, traditional methods of estimating vertical wind shear all indicated relatively low values were acting on the storm; however, high-spatiotemporal-resolution atmospheric motion vectors (AMVs) indicated that the environmental flow at upper levels was actually impinging on the vortex core, resulting in a vertical tilt. We employ a novel ensemble of centers of individual swaths of dual-Doppler radar data from WP-3D aircraft to characterize the precession and wobble of the vortex tilt. This tilting and wobbling preceded a sequence of outflow surges that acted to repel the impinging environmental flow, thereby reducing the shear and permitting ARI. We then apply prior methodology on satellite imagery for distinguishing ARI features. Finally, we use the AMV dataset to experiment with different shear calculations and show that the upper-level cross-vortex flow approaches zero. We discuss the implication of these results with regards to prior works on ARI and intensification in shear.
Shen, B.-W., R.A. Pielke, X. Zeng, J.-J. Baik, S. Faghih-Naini, J. Cui, and R. Atlas. Is weather chaotic? Coexistence of chaos and order within a generalized Lorenz model. Bulletin of the American Meteorological Society, 102(1):E148-E158, https://doi.org/10.1175/BAMS-D-19-0165.1 2021
Over 50 years since Lorenz’s 1963 study and a follow-up presentation in 1972, the statement "weather is chaotic" has been well accepted. Such a view turns our attention from regularity associated with Laplace’s view of determinism to irregularity associated with chaos. In contrast to single type chaotic solutions, recent studies using a generalized Lorenz model (GLM) have focused on the coexistence of chaotic and regular solutions that appear within the same model using the same modeling configurations but different initial conditions. The results, with attractor coexistence, suggest that the entirety of weather possesses a dual nature of chaos and order with distinct predictability. In this study, based on the GLM, we illustrate the following two mechanisms that may enable or modulate two kinds of attractor coexistence and, thus, contribute to distinct predictability: (1) the aggregated negative feedback of small-scale convective processes that can produce stable non-trivial equilibrium points and, thus, enable the appearance of stable steady-state solutions and their coexistence with chaotic or nonlinear oscillatory solutions, referred to as the 1st and 2nd kinds of attractor coexistence; and (2) the modulation of large-scale time varying forcing (heating) that can determine (or modulate) the alternative appearance of two kinds of attractor coexistence. Based on our results, we then discuss new opportunities and challenges in predictability research with the aim of improving predictions at extended-range time scales, as well as sub-seasonal to seasonal time scales.
Tang, J., J.A. Zhang, P. Chan, K. Hon, X. Lei, and Y. Wang. A direct aircraft observation of helical rolls in the tropical cyclone boundary layer. Scientific Reports, 11:18771, https://doi.org/10.1038/s41598-021-97766-7 2021
Helical rolls are known to play a significant role in modulating both the mean and turbulence structure of the atmospheric boundary layer in tropical cyclones. However, in-situ measurements of these rolls have been limited due to safety restrictions. This study presents analyses of data collected by an aircraft operated by the Hong Kong Observatory in Typhoon Kalmaegi (1415) and Typhoon Nida (1604). Examination of the flight-level data at ~ 600 m altitude confirmed the existence of sub-kilometer-scale rolls. These rolls were mostly observed in the outer-core region. Turbulent momentum fluxes were computed using the eddy correlation method. The averaged momentum flux of flight legs with rolls was found to be ~ 2.5 times that of legs without rolls at a similar wind speed range. This result suggests that rolls could significantly modulate turbulent transfer in the tropical cyclone boundary layer. This roll effect on turbulent fluxes should be considered in the planetary boundary layer parameterization schemes of numerical models simulating and forecasting tropical cyclones.
Wadler, J.B., D.S. Nolan, J.A. Zhang, and L.K. Shay. Thermodynamic characteristics of downdrafts in tropical cyclones as seen in idealized simulations of different intensities. Journal of the Atmospheric Sciences, 78(11):3503-3524, https://doi.org/10.1175/JAS-D-21-0006.1 2021
The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye-eyewall mixing below 2-km altitude. Additionally, a layer of higher θe between 1 and 2 km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θe air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θe deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θe by 10-14 K, showing that air-sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θe mid-tropospheric air towards the inflow layer. Here, the low-θe air entrains into the updraft in less than five minutes with a mean θe deficit of 8.2 K. In general, θe recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.
Wadler, J.B., J.A. Zhang, R.F. Rogers, B. Jaimes, and L.K. Shay. The rapid intensification of Hurricane Michael (2018): Storm structure and the relationship to environmental and air-sea interactions. Monthly Weather Review, 149(1):245-267, https://doi.org/10.1175/MWR-D-20-0145.1 2021
The spatial and temporal variation in multiscale structures during the rapid intensification of Hurricane Michael (2018) are explored using a coupled atmospheric and oceanic dataset obtained from NOAA WP-3D and G-IV aircraft missions. During Michael’s early lifecycle, the importance of ocean structure is studied to explore how the storm intensified despite experiencing moderate vertical shear. Michael maintained a fairly symmetric precipitation distribution and resisted lateral mixing of dry environmental air into the circulation upshear. The storm also interacted with an oceanic eddy field leading to cross-storm sea surface temperature (SST) gradients of ~2.5 °C. This led to the highest enthalpy fluxes occurring left-of-shear, favoring the sustainment of updrafts into the upshear quadrants and a quick recovery from low-entropy downdraft air. Later in the lifecycle, Michael interacted with more uniform and higher SSTs that were greater than 28 °C, while vertical shear imposed asymmetries in Michael’s secondary circulation and distribution of entropy. Mid-level (~4–8 km) outflow downshear, a feature characteristic of hurricanes in shear, transported high entropy air from the eyewall region outwards. This outflow created a cap which reduced entrainment across the boundary layer top, protecting it from dry mid-tropospheric air out to large radii (i.e. > 100 km), and allowing for rapid energy increases from air-sea enthalpy fluxes. Upshear, low-level (~0.5–2 km) outflow transported high-entropy air outwards which aided boundary layer recovery from low-entropy downdraft air. This study underscores the importance of simultaneously measuring atmospheric and oceanographic parameters to understand tropical cyclone structure during rapid intensification.
Wang, X., H. Jiang, X. Li, and J.A. Zhang. Observed shear-relative rainfall asymmetries associated with landfalling tropical cyclones. Advances in Meteorology, 2021:4676713, https://doi.org/10.1155/2021/4676713 2021
This study examines the shear-relative rainfall spatial distribution of tropical cyclones (TCs) during landfall based on the 19-year (1998–2016) TRMM satellite 3B42 rainfall estimate dataset and investigates the role of upper-tropospheric troughs on the rainfall intensity and distribution after TCs make a landfall over the six basins of Atlantic (ATL), eastern and central Pacific (EPA), northwestern Pacific (NWP), northern Indian Ocean (NIO), southern Indian Ocean (SIO), and South Pacific (SPA). The results show that the wavenumber 1 perturbation can contribute ∼ 50% of the total perturbation energy of total TC rainfall. Wavenumber 1 rainfall asymmetry presents the downshear-left maxima in the deep-layer vertical wind shear between 200 and 850 hPa for all the six basins prior to making a landfall. In general, wavenumber 1 rainfall tends to decrease less if there is an interaction between TCs and upper-level troughs located at the upstream of TCs over land. The maximum TC rain rate distributions tend to be located at the downshear-left (downshear) quadrant under the high (low)-potential vorticity conditions.
Wu, D., F. Zhang, X. Chen, A. Ryzhkov, K. Zao, M.R. Kumjian, X. Chen, and P-W. Chan. Evaluation of microphysics schemes in tropical cyclones using polarimetric radar observations: Convective precipitation in an outer rainband. Monthly Weather Review, 149(4):1055-1068, https://doi.org/10.1175/MWR-D-19-0378.1 2021
Cloud microphysics significantly impact tropical cyclone precipitation. A prior polarimetric radar observational study by Wu et al. (2018) revealed the ice-phase microphysical processes as the dominant microphysics mechanisms responsible for the heavy precipitation in the outer rainband of Typhoon Nida (2016). To assess the model performance regarding microphysics, three double-moment microphysics schemes (i.e., Thompson, Morrison, and WDM6) are evaluated by performing a set of simulations of the same case. While these simulations capture the outer rainband’s general structure, microphysics in the outer rainbands are strikingly different from the observations. This discrepancy is primarily attributed to different microphysics parameterizations in these schemes, rather than the differences in large-scale environments due to cloud-environment interactions. An interesting finding in this study is that the surface rain rate or liquid water content is inversely proportional to the simulated mean raindrop sizes. The mass-weighted raindrop diameters are overestimated in the Morrison and Thompson schemes and underestimated in the WDM6 scheme, while the former two schemes produce lower liquid water content than WDM6. Compared with the observed ice water content based on a new polarimetric radar retrieval method, the ice water content above the environmental 0 °C level in all simulations is highly underestimated, especially at heights above 12 km MSL where large concentrations of small ice particles are typically prevalent. This finding suggests that the improper treatment of ice-phase processes is potentially an important error source in these microphysics schemes. Another error source identified in the WDM6 scheme is overactive warm-rain processes that produce excessive concentrations of smaller raindrops.
Zhang, B., Z. Zhu, W. Perrie, J. Tang, and J.A. Zhang. Estimating tropical cyclone wind structure and intensity from spaceborne radiometer and synthetic aperture radar. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 14:4043-4050, https://doi.org/10.1109/JSTARS.2021.3065866 2021
We present a relatively simple method to estimate tropical cyclone (TC) surface wind structure (34-, 50-and 64-kt wind radii) and intensity (maximum wind speed, MWS) from wind fields acquired from the L-band SMAP radiometer and C-band Sentinel-1A/B and RADARSAT-2 synthetic aperture radar (SAR) between 2015 and 2020. The radiometer and SAR-derived wind radii and MWS are systematically compared with the best-track estimates. The root-mean-square errors (RMSEs) of R34, R50 and R64 are 31.2, 21.8 and 17.0 n mi (1 n mi =1.852 km) for radiometer, and 21.7, 16.5 and 16.3 n mi for SAR, respectively. These error values are smaller than the averaged best-track uncertainty estimates for the three wind radii. Compared to best-track reports, the bias and RMSE for the MWS estimates are-0.2 m/s and 5.8 m/s for radiometer, and 4.4 m/s and 9.1 m/s for SAR, respectively. These results are for the wind speeds in the range of 17-80 m/s. For the two typical TCs (Lionrock and Noru) in the Northwest Pacific Ocean, our results show that a combination of the radiometer and SAR wind data acquired within a very short time interval has the potential to simultaneously obtain reasonable measurements of the wind radii and intensity parameters. Moreover, for a TC with long lifecycle, such as Typhoon Noru, we demonstrate that high-resolution and multi-temporal synergistic observations from SAR and radiometer are valuable for studying fine-scale features of the wind field and characteristics of wind asymmetry associated with intensity change, as well as the evolution of TC surface wind structure and intensity.
Zhang, G., X. Li, W. Perrie, and J.A. Zhang. Tropical cyclone winds and inflow angle asymmetry from SAR imagery. Geophysical Research Letters, 48(20):e2021GL095699, https://doi.org/10.1029/2021GL095699 2021
This study developed a morphological model for tropical cyclone (TC) wind and inflow angle asymmetry based on sea surface wind fields derived from spaceborne synthetic aperture radar (SAR) images. The model extracts the standard TC morphological information (center, intensity, and radius of the maximum wind) and decomposes the SAR-derived winds into vortex rotation winds and motion vector, making the reconstruction of the entire TC structure reliable, even in areas not mapped by SAR. The derived wind speeds and inflow angles are verified with aircraft measurements by stepped-frequency microwave radiometer and dropsondes, obtaining root-mean-square errors of 4.32 m/s and 16.04 m/s, respectively. A systematic analysis of 130 SAR TCs images, collected by RADARSAT-2 and SENTINEL-1, reveals that the model can capture the main asymmetrical TC structure. Both TC asymmetry and eye size decrease as TC intensity increases.
Zhang, Z., J.A. Zhang, G.J. Alaka, Jr., K. Wu, A. Mehra, and V. Tallapragada. A statistical analysis of high frequency track and intensity forecasts from NOAA’s Operational Hurricane Weather Research and Forecast (HWRF) modeling system. Monthly Weather Review, 149(10):3325-3339, https://doi.org/10.1175/MWR-D-21-0021.1 2021
A statistical analysis is performed on the high-frequency (3 1/3 s) output from NOAA’s cloud-permitting, high-resolution operational Hurricane Weather Research and Forecasting (HWRF) model for all tropical cyclones (TCs) in the North Atlantic basin over a 3-year period (2017-2019). High-frequency HWRF forecasts of TC track and 10-m maximum wind speed (Vmax) exhibited large fluctuations that were not captured by traditional low-frequency (6 h) model output. Track fluctuations were inversely proportional to Vmax with average values of 6-8 km. Vmax fluctuations were as high as 20 kt in individual forecasts and were a function of maximum intensity, with a standard deviation of 5.5 kt for category 2 hurricanes and smaller fluctuations for tropical storms and major hurricanes. The radius of Vmax contracted or remained steady when TCs rapidly intensified in high-frequency HWRF forecasts, consistent with observations. Running mean windows of 3-9 h were applied at synoptic times to smooth the high-frequency HWRF output to investigate its utility to operational forecasting. Smoothed high-frequency HWRF output improved Vmax forecast skill by up to 8% and produced a more realistic distribution of 6-h intensity change when compared with low-frequency, instantaneous output. Furthermore, the high-frequency track forecast output may be useful for investigating characteristics of TC trochoidal motions.
Zhu, P., A. Hazelton, Z. Zhang, F. Marks, and V. Tallapragada. The role of eyewall turbulent transport in the pathway to intensification of tropical cyclones. Journal of Geophysical Research-Atmospheres, 126(17):e2021JD034983, https://doi.org/10.1029/2021JD034983 2021
In a tropical cyclone (TC), turbulence not only exists in the planetary boundary layer (PBL) but also can be generated above the PBL by the cloud processes in the eyewall and rainbands. It is found that the Hurricane Analysis and Forecast System (HAFS), a new multi-scale operational model for TC prediction, fails to capture the intense turbulent mixing in eyewall and rainband clouds due to a poor estimation of static stability in clouds. The problem is fixed by including the effects of multi-phase water in the stability calculation. Simulations of 21 TCs and tropical storms in the North Atlantic basin of 2016–2019 hurricane seasons totaling 118 forecast cycles show that the stability correction substantially improves HAFS's skill in predicting storm track and intensity. Analyses of HAFS's simulations of Hurricane Michael (2018) show that the positive tendency of vortex's tangential wind resulting from the radially inward transport of absolute vorticity dominates the eddy correlation tendencies induced by the model-resolved asymmetric eddies and serves as a main mechanism for the rapid intensification of Michael. The sub-grid scale (SGS) turbulent transport above the PBL in the eyewall plays a pivotal role in initiating a positive feedback among the eyewall convection, mean secondary overturning circulation, vortex acceleration via the inward transport of absolute vorticity, surface evaporation, and radial convergence of moisture in the PBL. Without the SGS transport above the PBL, the model-resolved vertical transport alone may not be sufficient in initiating the positive feedback underlying the rapid intensification of TCs.
2020
Aberson, S.D., and J. Kaplan. The relationship between the Madden-Julian Oscillation and tropical cyclone rapid intensification. Weather and Forecasting, 35(10):1865-1870, https://doi.org/10.1175/WAF-D-19-0209.1 2020
The relationship between the Madden-Julian Oscillation (MJO) and tropical cyclone rapid intensification in the northern basins of the western hemisphere is examined. All rapid-intensification events in the north/western hemisphere and the MJO phase and amplitude are compiled from 1974 to 2015. Rapid intensification events and the MJO tend to move in tandem with each other from west to east across the hemisphere, though rapid intensification appears most likely during a neutral MJO phase. The addition of this information to an operational statistical rapid intensification forecasting scheme does not significantly improve forecasts.
Alaka, G.J., D. Sheinin, B. Thomas, L. Gramer, Z. Zhang, B. Liu, H.-S. Kim, and A. Mehra. A hydrodynamical atmosphere/ocean coupled modeling system for multiple tropical cyclones. Atmosphere, 11(8):869, https://doi.org/10.3390/atmos11080869 2020
The goal of this paper is to introduce a new multi-storm atmosphere/ocean coupling scheme that was implemented and tested in the Basin-Scale Hurricane Weather Research and Forecasting (HWRF-B) model. HWRF-B, an experimental model developed at the National Oceanic and Atmospheric Administration (NOAA) and supported by the Hurricane Forecast Improvement Program, is configured with multiple storm-following nested domains to produce high-resolution predictions for several tropical cyclones (TCs) within the same forecast integration. The new coupling scheme parallelizes atmosphere/ocean interactions for each nested domain in HWRF-B, and it may be applied to any atmosphere/ocean coupled system. TC forecasts from this new hydrodynamical modeling system were produced in the North Atlantic and eastern North Pacific from 2017–2019. The performance of HWRF-B was evaluated, including forecasts of TC track, intensity, structure (e.g., surface wind radii), and intensity change, and simulated sea-surface temperatures were compared with satellite observations. Median forecast skill scores showed significant improvement over the operational HWRF at most forecast lead times for track, intensity, and structure. Sea-surface temperatures cooled by 1–8 °C for the five HWRF-B case studies, demonstrating the utility of the model to study the impact of the ocean on TC intensity forecasting. These results show the value of a multi-storm modeling system and provide confidence that the multi-storm coupling scheme was implemented correctly. Future TC models within NOAA, especially the Unified Forecast System’s Hurricane Analysis and Forecast System, would benefit from the multi-storm coupling scheme whose utility and performance are demonstrated in HWRF-B here.
Alford, A.A., J.A. Zhang, M.I. Biggerstaff, P. Dodge, F.D. Marks, and D.J. Bodine. Transition of the hurricane boundary layer during the landfall of Hurricane Irene (2011). Journal of the Atmospheric Sciences, 77(10):3509-3531, https://doi.org/10.1175/JAS-D-19-0290.1 2020
The hurricane boundary layer (HBL) has been observed in great detail through aircraft investigations of tropical cyclones over the open ocean, but the coastal transition of the HBL has been less frequently observed. During the landfall of Hurricane Irene (2011), research and operational aircraft over water sampled the open ocean HBL simultaneously with ground-based research and operational Doppler radars onshore. The location of the radars afforded 13 hours of dual-Doppler analysis over the coastal region. Thus, the HBL from the coastal waterways, through the coastal transition, and onshore was observed in great detail for the first time. Three regimes of HBL structure were found. The outer bands were characterized by temporal perturbations of the HBL structure with attendant low-level wind maxima in the vicinity of rainbands. The inner core, in contrast, did not produce such perturbations, but did see a reduction of the height of the maximum wind and a more jet-like HBL wind profile. In the eyewall, a tangential wind maximum was observed within the HBL over water as in past studies and above the HBL onshore. However, the transition of the tangential wind maximum through the coastal transition showed that the maximum continued to reside in the HBL through 5 km inland, which has not been observed previously. It is shown that the adjustment of the HBL to the coastal surface roughness discontinuity does not immediately mix out the residual high momentum jet aloft. Thus, communities closest to the coast are likely to experience the strongest winds onshore prior to the complete adjustment of the HBL.
Alvey, G.R., E. Zipser, and J. Zawislak. How does Hurricane Edouard (2014) evolve toward symmetry before rapid intensification? A Cloud-resolving ensemble study. Journal of the Atmospheric Sciences, 77(4):1329-1351, https://doi.org/10.1175/JAS-D-18-0355.1 2020
A 14-member high-resolution ensemble of Edouard (2014), a moderately sheared tropical storm that underwent rapid intensification (RI), is used to determine causes of vortex alignment and precipitation symmetry prior to RI. Half the members intensify similarly to the NHC’s best track, while the other 7 ensemble members fail to reproduce intensification. Analyses of initial conditions (vertical wind shear, sea surface temperatures, relative humidity, vortex structure) reveal that lower humidity and weaker, more tilted vortices in non-intensifying members likely increase their susceptibility to boundary layer flushing episodes. As the simulations progress, vortex tilt, inner core humidity, and azimuthal variations in the structure of precipitation best differentiate the two ensemble subsets. Although all members initially are slowly intensifying asymmetric storms, the RI members are unique in that they have more persistent deep convection downshear, which favors vortex alignment via the stretching term and/or precession. As deep convection transitions to stratiform precipitation and anvil clouds in the upshear quadrants, evaporation and sublimation of condensate advected from the downshear quadrants moistens the mid-upper troposphere. This is hypothesized to promote an increase in precipitation symmetrization, a necessary precursor for RI.
Balaguru, K., G.R. Foltz, L.R. Leung, J. Kaplan, W. Xu, N. Reul, and B. Chapron. Pronounced impact of salinity on rapidly intensifying Atlantic hurricanes. Bulletin of the American Meteorological Society, 101(9):e1497-e1511, https://doi.org/10.1175-BAMS-D-19-0303.1 2020
Tropical Cyclone (TC) rapid intensification (RI) is difficult to predict and poses a formidable threat to coastal populations. A warm upper ocean is well-known to favor RI, but the role of ocean salinity is less clear. This study shows a strong inverse relationship between salinity and TC RI in the eastern Caribbean and western tropical Atlantic due to near-surface freshening from the Amazon-Orinoco River system. In this region, rapidly intensifying TCs induce a much stronger surface enthalpy flux compared to more weakly intensifying storms, in part due to a reduction in SST cooling caused by salinity stratification. This reduction has a noticeable positive impact on TCs undergoing RI, but the impact of salinity on more weakly intensifying storms is insignificant. These statistical results are confirmed through experiments with an ocean mixed layer model, which show that the salinity-induced reduction in SST cold wakes increases significantly as the storm’s intensification rate increases. Currently, operational statistical-dynamical RI models do not use salinity as a predictor. Through experiments with a statistical RI prediction scheme, it is found that the inclusion of surface salinity significantly improves the RI detection skill, offering promise for improved operational RI prediction. Satellite surface salinity may be valuable for this purpose, given its global coverage and availability in near real-time.
Bell, G.D., E.S. Blake, C.W. Landsea, M. Rosencrans, H. Wang, S.B. Goldenberg, and R.J. Pasch. The tropics: Tropical cyclones—Atlantic basin. In State of the Climate in 2019, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 101(8):S204-S209, https://doi.org/10.1175/BAMS-D-20-0077.1 2020
Bhalachandran, S., D.R. Chavas, F.D. Marks, S. Dubey, A. Shreevastava, and T.N. Krishnamurti. Characterizing the energetics of vortex-scale and sub-vortex-scale asymmetries during tropical cyclone rapid intensity changes. Journal of the Atmospheric Sciences, 77(1):315-336, https://doi.org/10.1175/JAS-D-19-0067.1 2020
Our collective understanding of azimuthally-asymmetric features within the coherent structure of a tropical cyclone (TC) continues to improve with the availability of more detailed observations and high-resolution model outputs. However, a precise understanding of how these asymmetries impact TC intensity changes is lacking. Prior attempts at investigating the asymmetric impacts follow a mean-eddy partitioning that condenses the effect of all the asymmetries into one term and fails to highlight the differences in the role of asymmetries at different scales. In this study, we present a novel energetics-based approach to analyze the asymmetric impacts at multiple length-scales during periods of rapid intensity changes. Using model outputs of TCs under low and high shear, we compute the different energy pathways that enhance/suppress the growth of multi-scale asymmetries in the wavenumber (WN) domain. We then compare and contrast the energetics of the mean flow field (WN 0) with that of the persistent, coherent vortex-scale asymmetric structures (WNs 1,2) and the more local, transient, sub-vortex-scale asymmetries (WNs ≥ 3). We find in our case-studies that the dominant mechanisms of growth/decay of the asymmetries are the baroclinic conversion from available potential to kinetic energy at individual scales of asymmetries and the transactions of kinetic energy between the asymmetries of various length-scales, rather than the barotropic mean-eddy transactions as is typically assumed. Our case-study analysis further shows that the growth/decay of asymmetries is largely independent of the mean. Certain aspects of eddy energetics can potentially serve as early-warning indicators of TC rapid intensity changes.
Biswas, M.K., J.A. Zhang, E. Grell, E. Kalina, K. Newman, L. Bernardet, L. Carson, J. Frimel, and G. Grell. Evaluation of the Grell-Freitas convective scheme in the Hurricane Weather Research and Forecasting (HWRF) model. Weather and Forecasting, 35(3):1017-1033, https://doi.org/10.1175/WAF-D-19-0124.1 2020
The Developmental Testbed Center (DTC) tested two convective parameterization schemes in the Hurricane Weather Research and Forecasting (HWRF) model and compared them in terms of performance of forecasting tropical cyclones (TCs). Several TC forecasts were conducted with the scale aware Simplified Arakawa Schubert (SAS) and Grell-Freitas (GF) convective schemes over the Atlantic basin. For this sample of over 100 cases, the storm track and intensity forecasts were superior for the GF scheme compared to SAS. A case study showed improved storm structure for GF when compared with radar observations. The GF run had increased inflow in the boundary layer which resulted in higher angular momentum. An angular momentum budget analysis shows that the difference in the contribution of the eddy transport to the total angular momentum tendency is small between the two forecasts. The main difference is in the mean transport term, especially in the boundary layer. The temperature tendencies indicate higher contribution from the microphysics and cumulus heating above the boundary layer in the GF run. A temperature budget analysis indicated that both the temperature advection and diabatic heating were the dominant terms and they were larger near the storm center in the GF run than in the SAS run. The above results support the superior performance of the GF scheme for TC intensity forecast.
Chen, S., F. Qiao, J.A. Zhang, H. Ma, Y. Xue, and S. Chen. Swell modulation on wind stress in the constant flux layer. Geophysical Research Letters, 47(20):e2020GL089883, https://doi.org/10.1029/GL089883 2020
The impact of swell on wind stress is investigated through direct three‐layer flux measurements taken by a fixed tower in the marine atmospheric boundary layer. Observations confirm that the assumption of constant momentum flux layer is valid under swell‐dominated conditions around the reference height of 10 m. The swell can modulate the total wind stress to be less than the turbulent stress derived from the first‐order closure method, and the extent of this modulation decreases with height. The critical layer that represents the top of the layer affected by stronger swells is estimated to reach 45‐m altitude, and the depth of this layer decreases as the swells weaken and the wind speed increases. Furthermore, a simple swell correction scheme for the total stress calculation is developed, showing good performance against observations.
Cione, J.J., G.H. Bryan, R. Dobosy, J.A. Zhang, G. de Boer, A. Aksoy, J.B. Wadler, E.A. Kalina, B.A. Dahl, K. Ryan, J. Neuhaus, E. Dumas, F.D. Marks, A.M. Farber, T. Hock, and X. Chen. Eye of the storm: Observing hurricanes with a small Unmanned Aircraft System. Bulletin of the American Meteorological Society, 101(2):E186-E205, https://doi.org/10.1175/BAMS-D-19-0169.1 2020
Unique near-surface observations were collected in hurricanes using a small unmanned aircraft system deployed from NOAA’s hurricane hunter aircraft. Unique data from seven flights of the Coyote small Unmanned Aircraft System (sUAS) were collected in Hurricanes Maria (2017) and Michael (2018). Using NOAA's P-3 reconnaissance aircraft as a deployment vehicle, the sUAS collected high-frequency (> 1 Hz) measurements in the turbulent boundary layer of hurricane eyewalls, including measurements of wind speed, wind direction, pressure, temperature, moisture, and sea surface temperature, which are valuable for advancing knowledge of hurricane structure and the process of hurricane intensification. This study presents an overview of the sUAS system and preliminary analyses that were enabled by these unique data. Among the most notable results are measurements of turbulence kinetic energy and momentum flux for the first time at low levels (< 150 m) in a hurricane eyewall. At higher altitudes and lower wind speeds, where data were collected from previous flights of the NOAA P-3, the Coyote sUAS momentum flux values are encouragingly similar, thus demonstrating the ability of an sUAS to measure important turbulence properties in hurricane boundary layers. Analyses from a large-eddy simulation (LES) are also used to place the Coyote measurements into context of the complicated high-wind eyewall region. Thermodynamic data are also used to evaluate the operational HWRF model, showing a cool, dry, and thermodynamically unstable bias near the surface. Preliminary data assimilation experiments also show how sUAS data can be used to improve analyses of storm structure. These results highlight the potential of sUAS operations in hurricanes, and suggest opportunities for future work using these promising new observing platforms.
Cucurull, L., and M.J. Mueller. An analysis of alternatives for the COSMIC-2 constellation in the context of global Observing System Simulation Experiments. Weather and Forecasting, 35(1):51-66, https://doi.org/10.1175/WAF-D-19-0185.1 2020
Observing System Simulation Experiments (OSSEs) were conducted to evaluate the potential impact of the six Global Navigation Satellite System (GNSS) radio occultation (RO) receiver satellites in equatorial orbit from the initially proposed Constellation Observing Satellites for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) mission, known as COSMIC-2A. Furthermore, the added value of the high-inclination component of the proposed mission was investigated by considering a few alternatives architecture designs, including the originally proposed polar constellation of six satellites (COSMIC-2B), a constellation with a reduced number of RO receiving satellites, and a constellation of six satellites but with fewer observations in the lower troposphere. The 2015-year version of the operational three-dimensional ensemble-variational data assimilation system of the National Centers for Environment Prediction (NCEP)’s was used to run the OSSEs. Observations were simulated and assimilated using the same methodology and their errors assumed uncorrelated. The largest benefit from the assimilation of COSMIC-2A, with denser equatorial coverage, was to improve tropical winds, and its impact was found overall neutral in the extratropics. When soundings from the high-inclination orbit were assimilated in addition to COSMIC-2A, positive benefits were found globally, confirming that a high-inclination orbit constellation of RO receiving satellites is necessary to improve weather forecast skill globally. The largest impact from reducing COSMIC-2B from six to four satellites was to slightly degrade weather forecast skill in the northern hemisphere extratropics. The impact of degrading COSMIC-2B to COSMIC level of accuracy, in terms of penetration into the lower troposphere, was mostly neutral.
Dobbelaere, T., E.M. Muller, L.J. Gramer, D.M. Holstein, and E. Hanert. Coupled epidemio-hydrodynamic modeling to understand the spread of a deadly coral disease in Florida. Frontiers in Marine Science, 7:591881, https://doi.org/10.3389/fmars.2020.591881 2020
For the last six years, the Florida Reef Tract (FRT) has been experiencing an outbreak of the Stony Coral Tissue Loss Disease (SCTLD). First reported off the coast of Miami-Dade County in 2014, the SCTLD has since spread throughout the entire FRT with the exception of the Dry Tortugas. However, the causative agent for this outbreak is currently unknown. Here we show how a high-resolution bio-physical model coupled with a modified patch Susceptible-Infectious-Removed epidemic model can characterize the potential causative agent(s) of the disease and its vector. In the present study, the agent is assumed to be transported within composite material (e.g., coral mucus, dying tissues, and/or resuspended sediments) driven by currents and potentially persisting in the water column for extended periods of time. In this framework, our simulations suggest that the SCTLD is likely to be propagated within neutrally buoyant material driven by mean barotropic currents. Calibration of our model parameters with field data shows that corals are diseased within a mean transmission time of 6.45 days, with a basic reproduction number slightly above 1. Furthermore, the propagation speed of the disease through the FRT is shown to occur for a well-defined range of values of a disease threshold, defined as the fraction of diseased corals that causes an exponential growth of the disease in the reef site. Our results present a new connectivity-based approach to understand the spread of the SCTLD through the FRT. Such a method can provide a valuable complement to field observations and lab experiments to support the management of the epidemic as well as the identification of its causative agent.
Dong, J., B. Liu, Z. Zhang, W. Wang, A. Mehra, A.T. Hazelton, H.R. Winterbottom, L. Zhu, K. Wu, C. Zhang, V. Tallapragada, X. Zhang, S. Gopalakrishnan, and F. Marks. The evaluation of real-time Hurricane Analysis and Forecast System (HAFS) Stand-Alone Regional (SAR) model performance in 2019 Atlantic hurricane season. Atmosphere, 11(6):617, https://doi.org/10.3390/atmos11060617 2020
The next generation Hurricane Analysis and Forecast System (HAFS) has been developed recently in the National Oceanic and Atmospheric Administration (NOAA) to accelerate the improvement of tropical cyclone (TC) forecasts within the Unified Forecast System (UFS) framework. The finite-volume cubed sphere (FV3) based convection-allowing HAFS Stand-Alone Regional model (HAFS-SAR) was successfully implemented during Hurricane Forecast Improvement Project (HFIP) real-time experiments for the 2019 Atlantic TC season. HAFS-SAR has a single large 3-km horizontal resolution regional domain covering the North Atlantic basin. A total of 273 cases during the 2019 TC season are systematically evaluated against the best track and compared with three operational forecasting systems: Global Forecast System (GFS), Hurricane Weather Research and Forecasting model (HWRF), and Hurricanes in a Multi-scale Ocean-coupled Non-hydrostatic model (HMON). HAFS-SAR has the best performance in track forecasts among the models presented in this study. The intensity forecasts are improved over GFS, but show less skill compared to HWRF and HMON. The radius of gale force wind is over-predicted in HAFS-SAR, while the hurricane force wind radius has lower error than other models.
Fan, S., B. Zhang, A.A. Mouche, W. Perrie, and J.A. Zhang. Estimation of wind direction in tropical cyclones using C-band dual-polarization synthetic aperture radar. IEEE Transactions on Geoscience and Remote Sensing, 58(2):1450-1462, https://doi.org/10.1109/TGRS.2019.2946885 2020
Under extreme weather conditions, the imprints of kilometer-scale marine atmospheric boundary layer roll vortices on the ocean surface are clearly visible in synthetic aperture radar (SAR) images of storms. Therefore, information about wind direction in storms can be obtained by analyzing SAR image features caused by boundary layer rolls. VH-polarized SAR imagery captures the structural features of storms well and shows prominent image gradients along the radial directions of the storm. The signal-to-noise ratios of VH-polarized images are small in low wind speed areas, but they are large in the same regions of VV-polarized images. The capability of retrieving the atmospheric rolls orientation in VV-polarization is also found to be sensitive to incidence angle, with better performances for larger incidence angles. Thus, there is the potential to retrieve the storm’s wind directions using a combination of the VH- and VV-polarized SAR observations. In this article, we use the local gradient method to estimate tropical cyclone (TC) wind directions from C-band RADARSAT-2 and Sentinel-1A dual-polarization (VV + VH) SAR imagery. As a case study, wind directions with a spatial resolution of 25 km are derived by using both wide-swath VV- and VH-polarized SAR imagery over two hurricanes (Earl and Bertha) and one typhoon (Meranti). We compare wind directions derived from ten dual-polarization SAR images with collocated wind directions from buoys, Global Positioning System (GPS) dropsondes, a scatterometer, and a radiometer. Statistical comparisons show that the wind direction bias and root-mean-square error are, respectively, -0.54° and 14.78° for VV-polarization, 0.38° and 14.25° for VH-polarization, 0.20° and 13.30° for VV- and VH-polarization, suggesting dual-polarization SAR is more suitable for the estimation of TC wind directions than VV- or VH-polarization SAR.
Feng, J., X. Wang, and J. Poterjoy. A comparison of two local moment-matching nonlinear filters: Local particle filter (LPF) and local nonlinear ensemble transform filter (LNETF). Monthly Weather Review, 148(11):4377-4395, https://doi.org/10.1175/MWR-D-19-0368.1 2020
The local particle filter (LPF) and the local nonlinear ensemble transform filter (LNETF) are two moment-matching nonlinear filters to approximate the classical particle filter (PF). They adopt different strategies to alleviate filter degeneracy. LPF and LNETF localize observational impact but use different localization functions. They assimilate observations in a partially sequential and a simultaneous manner, respectively. In addition, LPF applies the resampling step, whereas LNETF applies the deterministic square root transformation to update particles. Both methods preserve the posterior mean and variance of the PF. LNETF additionally preserves the posterior correlation of the PF for state variables within a local volume. These differences lead to their differing performance in filter stability and posterior moment estimation. LPF and LNETF are systematically compared and analyzed here through a set of experiments with a Lorenz model. Strategies to improve the LNETF are proposed. The original LNETF is inferior to the original LPF in filter stability and analysis accuracy, particularly for small particle numbers. This is attributed to both the localization function and particle update differences. The LNETF localization function imposes a stronger observation impact than the LPF for remote grids and thus is more susceptible to filter degeneracy. The LNETF update causes an overall narrower range of posteriors that excludes true states more frequently. After applying the same localization function as the LPF and additional posterior inflation to the LNETF, the two filters reach similar filter stability and analysis accuracy for all particle numbers. The improved LNETF shows more accurate posterior probability distribution but slightly worse spatial correlation of posteriors than the LPF.
Fischer, M.S., R.F. Rogers, and P.D. Reasor. The rapid intensification and eyewall replacement cycles of Hurricane Irma (2017). Monthly Weather Review, 148(3):981-1004, https://doi.org/10.1175/MWR-D-19-0184.1 2020
The initiation of a rapid intensification (RI) event for a tropical cyclone (TC) at major hurricane intensity is a rare event in the North Atlantic basin. This study examined the environmental and vortex-scale processes related to such an RI event observed in Hurricane Irma (2017) using a combination of flight-level and airborne radar aircraft reconnaissance observations, microwave satellite observations, and model environmental analyses. The onset of RI was linked to an increase in sea surface temperatures and ocean heat content toward levels more commonly associated with North Atlantic RI episodes. Remarkably, Irma’s RI event was comprised of two rapidly-evolving eyewall replacement cycle (ERC) episodes that each completed in less than 12 h. The two ERC events displayed different secondary eyewall formation (SEF) mechanisms and vortex evolutions. During the first SEF event, a secondary maximum in ascent and tangential wind was observed at the leading edge of a mesoscale descending inflow jet. During the ensuing ERC event, the primary eyewall weakened and ultimately collapsed, resulting in a brief period of weakening. The second SEF event displayed characteristics consistent with unbalanced boundary layer dynamics. Additionally, it is plausible both SEF events were affected by the stagnation and axisymmeterization of outward propagating vortex Rossby waves. During the second ERC event, the TC continued to rapidly intensify, which is a stark contrast to the ERC paradigm described in the literature. The differing ERC evolutions appear linked to the vortex response to changing environmental conditions. The results presented here underscore the utility of frequent aircraft reconnaissance observations for an improved understanding of TC dynamics.
Gopalakrishnan, S., D. Koch, S. Upadhayay, M. DeMaria, F. MARKS, E.N. Rappaport, A. Mehra, V. Tallapragada, Y. Jung, G. Alaka, C. Alexander, M. Bender, L. Bernardet, M. Biswas, T. Black, M. Brennan, J. Cangialosi, J. Dong, R. Dunlap, M. Ek, J.L. Franklin, L. Gramer, G. Hallliwell, L. Harris, A. Hazelton, J.S. Hilderbrand, E. Kalina, H.S. Kim, P. Kucera, N. Lett, B. Liu, T. Marchok, P. McCaslin, K. Musgrave, L. Nance, K. Newman, M. Onderlinde, A. Penny, W. Ramstrom, J. Sippel, R. Torn, X. Wang, W. Wang, J. Whitaker, H. Winterbottom, D.A. Zelinsky, F. Zhang, C. Zhang, X. Zhang, Z. Zhang, and L. Zhu. 2019 Hurricane Forecast Improvement Project R&D activities summary: Recent results and operational implementation. HFIP Technical Report, HFIP2020-1, 45 pp., https://doi.org/10.25923/qzd3-m787 2020
This technical report describes the activities and results of the Hurricane Forecast Improvement Program (HFIP) that occurred in 2019. The major development focus in 2019 was on building the next generation hurricane model—the Hurricane Analysis and Forecast System (HAFS)—primarily for track and intensity predictions. This report summarizes the progress in 2019, including model developments and the first year of progress made towards transforming it into the next generation of HFIP.
Guimond, S.R., P.D. Reasor, G.M. Heymsfield, and M.M. McLinden. The dynamics of vortex Rossby waves and secondary eyewall development in Hurricane Matthew (2016): New insights from radar measurements. Journal of the Atmospheric Sciences, 77(7):2349-2374, https://doi.org/10.1175/JAS-D-19-0284.1 2020
The structure of vortex Rossby waves (VRWs) and their role in the development of a secondary eyewall in Hurricane Matthew (2016) is examined from observations taken during the NOAA Sensing Hazards with Operational Unmanned Technology (SHOUT) field experiment. Radar measurements from ground-based and airborne systems, with a focus on the NASA High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) instrument on the Global Hawk aircraft, revealed the presence of ~12–15-km wavelength spiral bands breaking from the inner core eyewall in the down shear-right quadrant. The vorticity characteristics and calculations of the intrinsic phase speeds of the bands are shown to be consistent with sheared VRWs. A new angular momentum budget methodology is presented that allows an understanding of the secondary eyewall development process with narrow swath radar measurements. Filtering of the governing equations enables explicit insight into the nonlinear dynamics of scale interactions and the role of the VRWs in the storm structure change. The results indicate that the large-scale (scales>15-km) vertical flux convergence of angular momentum associated with the VRWs dominates the time tendency with smaller effects from the radial flux term. The small-scale (scales≤15-km) vertical term produces weak, but non-negligible nonlinear forcing of the large scales primarily through the Reynolds and cross-stress components. The projection of the wave kinematics onto the low-wavenumber (zero and one) fields appears to be the more significant dynamic process. Flight-level observations show secondary peaks in tangential winds in the radial region where the VRW forcing signatures are active, connecting them with the secondary eyewall formation process.
Harris, L., L. Zhou, S.-J. Lin, J.-H. Chen, X. Chen, K. Gao, M. Morin, S. Rees, Y. Sun, M. Tong, B. Xiang, M. Bender, R. Benson, K.-Y. Cheng, S. Clark, O.D. Elbert, A. Hazelton, J.J. Huff, A. Kaltenbaugh, Z. Liang, T. Marchok, H.H. Shin, and W. Stern. GFDL SHiELD: A unified system for weather-to-seasonal prediction. Journal of Advances in Modeling Earth Systems, 12(10):e2020MS002223, https://doi.org/10.1029/2020MS002223 2020
We present the System for High‐resolution prediction on Earth‐to‐Local Domains (SHiELD), an atmosphere model developed by the Geophysical Fluid Dynamics Laboratory (GFDL) coupling the nonhydrostatic FV3 Dynamical Core to a physics suite originally taken from the Global Forecast System. SHiELD is designed to demonstrate new capabilities within its components, explore new model applications, and to answer scientific questions through these new functionalities. A variety of configurations are presented, including short‐to‐medium‐range and subseasonal‐to‐seasonal prediction, global‐to‐regional convective‐scale hurricane and contiguous U.S. precipitation forecasts, and global cloud‐resolving modeling. Advances within SHiELD can be seamlessly transitioned into other Unified Forecast System or FV3‐based models, including operational implementations of the Unified Forecast System. Continued development of SHiELD has shown improvement upon existing models. The flagship 13‐km SHiELD demonstrates steadily improved large‐scale prediction skill and precipitation prediction skill. SHiELD and the coarser‐resolution S‐SHiELD demonstrate a superior diurnal cycle compared to existing climate models; the latter also demonstrates 28 days of useful prediction skill for the Madden‐Julian Oscillation. The global‐to‐regional nested configurations T‐SHiELD (tropical Atlantic) and C‐SHiELD (contiguous United States) show significant improvement in hurricane structure from a new tracer advection scheme and promise for medium‐range prediction of convective storms.
Hazelton, A.T., X. Zhang, S. Gopalakrishnan, W. Ramstrom, F. Marks, and J.A. Zhang. High-resolution ensemble HFV3 forecasts of Hurricane Michael (2018): Rapid intensification in shear. Monthly Weather Review, 148(5):2009-2032, https://doi.org/10.1175/MWR-D-19-0275.1 2020
The FV3GFS is the current operational Global Forecast System (GFS) at the National Centers for Environmental Prediction (NCEP), which combines a finite volume cubed sphere dynamical core (i.e. abbreviated as FV3) and GFS physics. In this study, FV3GFS is used to gain understanding of rapid intensification (RI) of tropical cyclones (TCs) in shear. The analysis demonstrates the importance of TC structure in a complex system like Hurricane Michael, which intensified to a Category 5 hurricane over the Gulf of Mexico despite over 20 kt (10 m s-1) of vertical wind shear. Michael’s RI is examined using a global-nest FV3GFS ensemble with the nest at 3-km resolution. The ensemble shows a range of peak intensities from 77 to 159 kt (40 to 82 m s-1). Precipitation symmetry, vortex tilt, moisture, and other aspects of Michael’s evolution are compared through composites of stronger and weaker members. The 850–200 hPa vertical shear is 22 kt (11 m s-1) in the mean of both strong and weak members during the early stage. Tilt and moisture are two distinguishing factors between strong and weak members. The relationship between vortex tilt and humidification is complex, and other studies have shown both are important for sheared intensification. Here, it is shown that tilt reduction leads to upshear humidification and is thus a driving factor for intensification. A stronger initial vortex and early evolution of the vortex also appear to be the key to members that are able to resist the sheared environment.
Hendee, J., N. Amornthammarong, L. Gramer, and A. Gomez. A novel low-cost, high-precision sea temperature sensor for coral reef monitoring. Bulletin of Marine Science, 96(1):97-110, https://doi.org/10.5343/bms.2019.0050 2020
The role of elevated sea temperatures in coral bleaching has been well documented. Many of the sea temperature records utilized for purposes of widespread, multi-species bleaching predictions in recent publications have been acquired through satellite remote sensing. Satellites estimate sea temperatures at only a narrow range of depths near the surface of the ocean and may, therefore, not adequately represent the true temperatures endured by the world’s coral ecosystems. To better characterize sea temperature regimes that coral reef ecosystems experience, as well as better define the individual thresholds for each species that bleaches, in situ sea temperature sensors are required. Commercial sensors are expensive in large quantities, however, reducing the capacity to conduct large-scale research programs to elucidate the range of significant scales of temperature variability. At the National Oceanic and Atmospheric Administration’s (NOAA) Atlantic Oceanographic and Meteorological Laboratory (AOML), we designed a low-cost (roughly US $9 in parts) and high-precision sea temperature sensor that uses an Arduino microprocessor board and a high accuracy thermistor. This new temperature sensor autonomously records temperatures onto a memory chip and provides better accuracy (+0.05°C) than a comparable commercial sensor (+0.2°C). Moreover, it is not difficult to build—anyone who knows how to solder can build the temperature sensor. In March 2019, students at middle and high schools in Broward County, Florida built close to 60 temperature sensors. During 2019, these sensors will be deployed by Reef Check, a global-scale coral reef monitoring organization, as well as by other programs, to determine worldwide sea temperature regimes through the Opuhala Project (https://www.coral.noaa.gov/opuhala). This paper chronicles results from the initial proof-of-concept deployments for these AOML-designed sensors.
Hristova-Veleva, S., P.P. Li, B. Knosp, Q. Vu, F.J. Turk, W.L. Poulsen, Z. Haddad, B. Lambrigtsen, B.W. Stiles, T.-P. Shen, N. Niamsuwan, S. Tanelli, O. Sy, E.-K. Seo, H. Su, D.G. Vane, Y. Chao, P.S. Callahan, R.S. Dunbar, M. Montgomery, M. Boothe, V. Tallapragada, S. Trahan, A.J. Wimmers, R. Holz, J.S. Reid, F. Marks, T. Vukicevic, S. Bhalachandran, H. Leighton, S. Gopalakrishnan, A. Navarro, and F.J. Tapiador. An eye on the storm: Integrating a wealth of data for quickly advancing the physical understanding and forecasting of tropical cyclones. Bulletin of the American Meteorological Society, 101(10):e1718-e1742, https://doi.org/10.1175/BAMS-D-19-0020.1 2020
The JPL Tropical Cyclone Information System integrates observations with model forecasts, allowing interrogation of a large number of variables, to help better understand the multi-scale non-linear interactions that lead to storm development, and to evaluate models. Tropical cyclones (TCs) are among the most destructive natural phenomena with huge societal and economic impact. They form and evolve as the result of complex multi-scale processes and non-linear interactions. Even today the understanding and modeling of these processes is still lacking. A major goal of NASA is to bring the wealth of satellite and airborne observations to bear on addressing the unresolved scientific questions and improving our forecast models. Despite their significant amount, these observations are still underutilized in hurricane research and operations, due to the complexity associated with finding and bringing together semi-coincident and semi-contemporaneous multi-parameter data that are needed to describe the multiscale TC processes. Such data are traditionally archived in different formats, with different spatio-temporal resolution, across multiple databases, and hosted by various agencies. To address this shortcoming, NASA supported the development of the Jet Propulsion Laboratory (JPL) Tropical Cyclone Information System (TCIS) - a Data Analytic Framework that integrates model forecasts with multiparameter satellite and airborne observations, providing interactive visualization and on-line analysis tools. TCIS supports interrogation of a large number of atmospheric and ocean variables, allowing for quick investigation of the structure of the tropical storms and their environments. This paper provides an overview of the TCIS’s components and features. It also summarizes recent pilot studies, providing examples how the TCIS has inspired new research, helping to increase our understanding of TCs. The goal is to encourage more users to take full advantage of the novel capabilities. TCIS allows atmospheric scientists to focus on new ideas and concepts rather than painstakingly gathering data scattered over several agencies.
Johns, E.M., R. Lumpkin, N.F. Putman, R.H. Smith, F.E. Muller-Karger, D. Rueda-Roa, C. Hu, M. Wang, M.T. Brooks, L.J. Gramer, and F. E. Werner. The establishment of a pelagic Sargassum population in the tropical Atlantic: Biological consequences of a basin-scale long distance dispersal event. Progress in Oceanography, 182:102269, https://doi.org/10.1016/j.pocean.2020.102269 2020
Starting in 2011, coastal areas of the Caribbean Sea and tropical Atlantic Ocean began to experience extraordinary yearly accumulations of pelagic Sargassum brown alga. Historical reports place large quantities of Sargassum only in the North Atlantic (mostly in the Gulf of Mexico and the Sargasso Sea). Accumulations of Sargassum in the tropical Atlantic have continued. We used a numerical particle-tracking system, wind and current reanalysis data, drifting buoy trajectories, and satellite imagery to determine the origin of the Sargassum that is now found persistently in the tropical Atlantic. Our analyses suggest that during the extreme negative phase of the winter 2009-2010 North Atlantic Oscillation (NAO), unusually strong and southward-shifted westerly winds explain the transport of Sargassum from the Sargasso Sea (∼20-40°N, 80-20°W) into the far eastern North Atlantic. Our hindcast Sargassum distributions agree with surface current simulations with the inclusion of “windage”. Windage is the additional, wind-induced drift of material floating at the free surface resulting from direct wind forcing on the sea surface, as well as on floating or partially-submerged objects. In our simulations, windage is included as an added vector (speed and direction) to the model-computed surface ocean currents equivalent to 1% of surface wind velocities. Lagrangian analysis of the regional circulation suggests that (1) part of the Sargassum subsequently drifted to the southwest in the North Equatorial Current (NEC) and entered the central tropical Atlantic, arriving in the Caribbean by the spring of 2011, with (2) another portion continuing southward along the coast of Africa in the Canary Current, eventually joining the seasonally-varying system of tropical Atlantic currents and thereby delivering a large Sargassum population to the tropical Atlantic. Since then, Sargassum patches aggregate from March to September in massive windrows along the Inter-Tropical Convergence Zone (ITCZ) under the action of converging winds. The windrows follow the ITCZ in its seasonal northward migration in the central tropical Atlantic. They are stretched across the central tropical Atlantic as the ITCZ crosses the latitude of the seasonal formation of the North Equatorial Counter Current (NECC). These patches and windrows are exposed to high sunlight and open-ocean upward flux of nutrients due to eddy and wind-driven mixing in the central tropical Atlantic. During the northern spring and summer, as the Sargassum drifts farther north with the ITCZ, large portions of the population are advected into the eastern Caribbean Sea. Some of these patches remain dispersed as the ITCZ migrates southward, and re-aggregate into new windrows as the ITCZ intensifies the following March-April. If wind mixing is strong and the mixed layer is deeper than about 50-60 m in the southern tropical Atlantic at this time, the Sargassum will bloom and form a massive windrow. Otherwise, the bloom will be inhibited. The extreme 2009-2010 NAO wind anomaly could be considered as triggering a biosphere “tipping point” that caused important ocean-scale ecosystem changes in the tropical Atlantic, with significant recurrent social and economic consequences. Understanding whether this new expanded geographic range of massive Sargassum blooms is temporary or whether it will revert to its pre-2009 distribution requires sustained monitoring and research.
Ko, M.-C., F.D. Marks, G.J. Alaka, and S.G. Gopalakrishnan. Evaluation of Hurricane Harvey (2017) rainfall in deterministic and probabilistic HWRF forecasts. Atmosphere, 11(6):666, https://doi.org/10.3390/atmos11060666 2020
Rainfall forecast performance was evaluated for the first time for the Hurricane Weather Research and Forecasting (HWRF) model. This study focused on HWRF performance in predicting rainfall from Hurricane Harvey in 2017. In particular, two configurations of the 2017 version of HWRF were investigated: a deterministic version of the Basin-scale HWRF (HB17) and an ensemble version of the operational HWRF (H17E). This study found that HB17 generated reasonable rainfall patterns and rain-rate distributions for Hurricane Harvey, in part due to accurate track forecasts. However, the estimated rain rates near the storm center (within 50 km) were slightly overestimated. In the rainband region (150 to 300 km), HB17 reproduced heavy rain rates and underestimated light rain rates. The accumulated rainfall pattern successfully captured Harvey’s intense outer rainband with adequate spatial displacement. In addition, the performance of H17E on probabilistic rainfall has shown that the ensemble forecasts can potentially increase the accuracy of the predicted locations for extreme rainfall. Moreover, the study also indicated the importance of high-resolution dynamical models for rainfall predictions. Although statistical models can generate the overall rainfall patterns along a track, extreme rainfall events produced from outer rainbands can only be forecasted by numerical models, such as HWRF. Accordingly, the HWRF models have the capability of simulating reasonable quantitative precipitation forecasts and providing essential rainfall guidance in order to further reduce loss of life and cost to the economy. here.
Kren, A.C., L. Cucurull, and H. Wang. Addressing the sensitivity of forecast impact to flight path design for targeted observations of extratropical winter storms: A demonstration in an OSSE framework. Meteorological Applications, 27(4):e1942, https://doi.org/10.1002/met.1942 2020
Few studies have examined the forecast uncertainties brought about from varying aircraft flight track patterns in targeted observations for extratropical winter storms. To examine the degree of uncertainty in downstream forecasts caused by different aircraft flight patterns, a series of observing system simulation experiments (OSSEs) are performed and demonstrated for two extratropical winter storms identified in the European Centre for Medium‐Range Weather Forecasts (ECMWF) T511 Nature Run using the National Centers for Environmental Prediction Global Data Assimilation System and Global Forecast System (Q1FY15). Winter storms were chosen to support operational Pacific Ocean targeting strategies using unmanned aircraft. For these two storms, objective and composite flight tracks are generated as they could occur in an operational field mission to sample sensitive areas and meteorologically important regions, and then the changes in downstream forecasts across the various flight tracks are evaluated. The forecast impact downstream is sensitive to flight track orientation and shows case‐dependent results, with some flight patterns leading to significant improvements, while others result in neutral to degraded forecasts. The degree of downstream uncertainty in the verification region can vary up to 8% from the different flight paths, depending on the metric used and the atmospheric variables analysed. Although the study is a demonstration of the technique and is limited to only two case studies, it suggests that uncertainty in flight path design should not be neglected in future field missions. Some guidance for mitigating this uncertainty is also discussed.
Leighton, H., R. Black, X. Zhang, F.D. Marks, and S.G. Gopalakrishnan. Ice particle size distribution from composites of microphysics observations collected in tropical cyclones. Geophysical Research Letters, 47(15):e2020GL088762, https://doi.org/10.1029/2020GL088762 2020
Ice microphysics observations collected from eight flights into tropical cyclones (TCs) were analyzed to examine the performance of exponential versus gamma functions in representing the particle size distributions (PSDs) for cloud ice, snow, and graupel. Eighty‐four percent (87%) of cloud ice (snow) PSDs are above the correlation threshold of 0.9 between observations and the corresponding fitted curves by gamma fits, while only 43% (55%) of cloud ice (snow) PSDs by exponential fits. Sixteen percent of graupel PSDs are above the threshold by gamma fits but none by exponential fits. The intercept, slope, and shape in gamma functions are mutually dependent. When one among the three parameters is prescribed, the other two can be empirically rendered from the mutual‐dependence relationship. Counterintuitively, temperature does not play a conspicuous role in controlling ice PSDs in the TC environment but horizontal winds do, especially for snow, through the breakup process.
Li, J., J. Li, C. Velden, P. Wang, T.J. Schmit, and J. Sippel. Impact of rapid‐scan‐based dynamical information from GOES‐16 on HWRF hurricane forecasts. Journal of Geophysical Research-Atmospheres, 125(3):e2019JD031647, https://doi.org/10.1029/2019JD031647 2020
Observations of dynamical information in the upper levels of tropical cyclones at high spatiotemporal resolutions are rare but very important to the analysis and prediction of the storm evolution and landfall impacts. These observations are now becoming routinely available from the new generation of geostationary weather satellites. Understanding and optimizing the utilization of that information in numerical weather prediction models is a vital step toward simulating tropical cyclone behavior and improving forecasts. The Advanced Baseline Imager (ABI) onboard GOES‐16 is providing high spatial and temporal resolution images that can be targeted on North Atlantic tropical cyclones. In addition to a full‐disk scan every 10 min and a CONUS scan every 5 min, the ABI also has a flexible “mesoscale scan” mode featuring limited moving domains at 1‐min intervals. The mesosector can focus on a targeted storm center with a 10°×10° domain coverage that follows the storm movement. Using this 1‐min ABI imagery to track cloud motions, automated algorithms have been developed to produce enhanced, high‐resolution atmospheric motion vectors (AMVs) during a targeted tropical cyclone event. These high spatiotemporal AMVs represent estimates of the wind field around the storm and can provide critical dynamical information on the targeted storm and its near environment. This information can help improve the representation of the initialized vortex in numerical model analyses. To study the impact of the enhanced AMV observations on numerical weather prediction, the Hurricane Weather Research Forecast (HWRF) model is used in a series of assimilation and forecast experiments. Three destructive Atlantic hurricane cases from 2017, Harvey, Irma, and Maria, are chosen as case studies. The results show that the assimilation of the enhanced AMVs from GOES‐16 consistently improves the HWRF hurricane track and size forecasts, and have mixed impacts on intensity forecasts. These results augment previously published studies on optimizing the quantitative use of new generation geostationary satellite rapid‐scan observations for improving high impact weather forecasts.
Liu, Q., X. Zhang, M. Tong, Z. Zhang, B. Liu, W. Wang, L. Zhu, B. Zhang, X. Xu, S. Trahan, L. Bernardet, A. Mehra, and V. Tallapragada. Vortex initialization in the NCEP operational hurricane models. Atmosphere, 11(9):968, https://doi.org/10.3390/atmos11090968 2020
This paper describes the vortex initialization (VI) currently used in NCEP operational hurricane models (HWRF and HMON, and possibly HAFS in the future). The VI corrects the background fields for hurricane models: it consists of vortex relocation, and size and intensity corrections. The VI creates an improved background field for the data assimilation and thereby produces an improved analysis for the operational hurricane forecast. The background field after VI can be used as an initial field (as in the HMON model, without data assimilation) or a background field for data assimilation (as in HWRF model).
McFarquhar, G.M., E. Smith, E.A. Pillar-Little, K. Brewster, P.B. Hilson, T.R. Lee, S. Waugh, N. Yussouf, X. Wang, M. Xue, G. de Boer, J.A. Gibbs, C. Fiebrich, B. Baker, J. Brotzge, F. Carr, H. Christophersen, M. Fengler, P. Hall, T. Hock, A. Houston, R. Huck, J. Jacob, R. Palmer, P.K. Quinn, M. Wagner, Y. Zhang, and D. Hawk. Current and future uses of UAS for improved forecasts/warnings and scientific studies. Bulletin of the American Meteorological Society, 101(8):e1322-1328, https://doi.org/10.1175/BAM-D-20-0015.1 2020
Moradi, I., K.F. Evans, W. McCarty, M. Cordero-Fuentes, R. Gelaro, and R.A. Black. Assimilation of satellite microwave observations over the rainbands of tropical cyclones. Monthly Weather Review, 148(12):4729-4245, https://doi.org/10.1175/MWR-D-19-0341.1 2020
A novel Bayesian Monte Carlo Integration (BMCI) technique was developed to retrieve geophysical variables from satellite microwave radiometer data in the presence of tropical cyclones. The BMCI technique includes three steps: generating a stochastic database, simulating satellite brightness temperatures using a radiative transfer model, and retrieving geophysical variables such as profiles of temperature, relative humidity, and cloud liquid and ice water content from real observations. The technique also provides uncertainty estimates for each retrieval and can output the error covariance matrix of selected parameters. The measurements from Advanced Technology Microwave Sounder (ATMS) onboard Suomi National Polar-orbiting Partnership (NPP) and the Global Precipitation Measurement (GPM) Microwave Imager (GMI) were used as input. A new technique was developed to correct the ATMS and GMI observations for the beam-filling effect, which is due to small scale variability of precipitation and clouds compared with the instrument footprint and also the non-linear relation between the brightness temperature and precipitation. In addition, the assimilation of the BMCI retrievals into the NASA GEOS model are discussed for Hurricane Maria. The results show that assimilating the BMCI retrievals can influence the dynamical features of the cyclone, including a stronger warm core, a symmetric eye, and vertically aligned wind columns. Two possible factors that may limit the impact of the BMCI retrievals include, the resolution of the model (about 25 km) which was too coarse to show the potential of the BMCI data in improving the representation of tropical storms in the model forecast and also the data assimilation system not being able to consider vertically correlated observation errors.
Mueller, M.J., A.C. Kren, L. Cucurull S.P.F. Casey, R.N. Hoffman, R. Atlas, and T.R. Peevey. Impact of refractivity profiles from a proposed GNSS-RO constellation on tropical cyclone forecasts in a global modeling system. Monthly Weather Review, 148(7):3037-3057, https://doi.org./10.1175/MWR-D-19-0360.1 2020
A global Observing System Simulation Experiment (OSSE) was used to assess the potential impact of a proposed Global Navigation Satellite System (GNSS) radio occultation (RO) constellation on tropical cyclone (TC) track, maximum 10-m wind speed (Vmax), and integrated kinetic energy (IKE) forecasts. The OSSE system was based on the 7-km NASA nature run and simulated RO refractivity determined by the spatial distribution of observations from the original planned (i.e., including both equatorial and polar orbits) Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2). Data was assimilated using the NOAA operational weather analysis and forecasting system. Three experiments generated global TC track, Vmax, and IKE forecasts over 6 weeks of the North Atlantic hurricane season in the North Atlantic, East Pacific, and West Pacific basins. Confidence in our results was bolstered because track forecast errors were similar to those of official National Hurricane Center forecasts, and Vmax errors and IKE errors showed similar results. GNSS-RO assimilation did not significantly impact global track forecasts, but did slightly degrade Vmax and IKE forecasts in the first 30-60 h of lead time. Global forecast error statistics show adding or excluding explicit random errors to RO profiles made little difference to forecasts. There was large forecast–to–forecast variability in RO impact. For two cases studied in depth, track and Vmax improvements and degradations were traced backwards through the previous 24 h of assimilation cycles. The largest Vmax degradation was traced to particularly good control analyses rather than poor analyses caused by GNSS-RO.
Pan, X., M. Dresner, B. Mantin, and J.A. Zhang. Pre-hurricane consumer stockpiling and post-hurricane product availability: Empirical evidence from natural experiments. Production and Operations Management, 29(10):2350-2380, https://doi.org/10.1111/poms13230 2020
The provision of essential supplies is a key service provided by retailers when demand spikes due to consumer stockpiling during environmental emergencies. Moreover, it is important for retailers to quickly recover from these events by replenishing the stock of essential supplies to meet the continuing needs of local residents. The main purpose of this research is to study consumer precautionary stockpiling behavior prior to the onset of hurricane landfalls and determine the impact of this behavior on in‐store product availability for various formats of retail store outlets. Specifically, we focus on the bottled water product category, an essential emergency category in hurricane preparedness. This study combines an event analysis methodology with econometric models using archival retail scanner data from 60 US retail chains located in 963 counties and real‐time data from four recent US continental hurricanes. We find that supply‐side characteristics (retail network and product variety), demand‐side characteristics (hurricane experience and household income), and disaster characteristics (hazard proximity and hazard intensity) significantly affect consumer stockpiling propensity as the hurricanes approach. The increased consumer stockpiling has immediate and longer‐term impacts on retail operations, namely, in‐store product availability. Among various retail formats, drug stores are associated with the highest consumer stockpiling propensity before hurricanes, while dollar stores and discount stores are associated with the lowest in‐store product availability following hurricanes. Our study points to the need for retailers and policymakers to carefully monitor factors affecting consumer stockpiling behavior that will allow for better allocation of critical supplies during the hurricane season.
Prasanth, S., D.R. Chavas, F.D. Marks, S. Dubey, A. Shreevastava, and T.N. Krishnamurti. Characterizing the energetics of vortex-scale and sub-vortex-scale asymmetries during tropical cyclone rapid intensity changes. Journal of the Atmospheric Sciences, 77(1):315-336, https://doi.org/10.1175/JAS-D-19-0067.1 2020
Our collective understanding of azimuthally asymmetric features within the coherent structure of a tropical cyclone (TC) continues to improve with the availability of more detailed observations and high-resolution model outputs. However, a precise understanding of how these asymmetries impact TC intensity changes is lacking. Prior attempts at investigating the asymmetric impacts follow a mean–eddy partitioning that condenses the effect of all the asymmetries into one term and fails to highlight the differences in the role of asymmetries at different scales. In this study, we present a novel energetics-based approach to analyze the asymmetric impacts at multiple length scales during periods of rapid intensity changes. Using model outputs of TCs under low and high shear, we compute the different energy pathways that enhance/suppress the growth of multiscale asymmetries in the wavenumber (WN) domain. We then compare and contrast the energetics of the mean-flow field (WN 0) with that of the persistent, coherent vortex-scale asymmetric structures (WNs 1 and 2) and the more local, transient, sub-vortex-scale asymmetries (WNs ≥ 3). We find in our case studies that the dominant mechanisms of growth/decay of the asymmetries are the baroclinic conversion from available potential to kinetic energy at individual scales of asymmetries and the transactions of kinetic energy between the asymmetries of various length scales, rather than the barotropic mean–eddy transactions as is typically assumed. Our case study analysis further shows that the growth/decay of asymmetries is largely independent of the mean. Certain aspects of eddy energetics can potentially serve as early-warning indicators of TC rapid intensity changes.
Ren, Y., J.A. Zhang, J.L. Vigh, P. Zhu, H. Liu, X. Wang, and J.B. Wadler. An observational study of the symmetric boundary layer structure and tropical cyclone intensity. Atmosphere, 11(2):158, https://doi.org/10.3390/atmos11020158 2020
This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.
Rogers, R.F., P.D. Reasor, J.A. Zawislak, and L.T. Nguyen. Precipitation processes and vortex alignment during intensification of a weak tropical cyclone in moderate vertical shear. Monthly Weather Review, 148(5):1899-1929, https://doi.org/10.1175/MWR-D-19-0315.1 2020
The mechanisms underlying the development of a deep, aligned vortex, and the role of convection and vertical shear in this process, are explored by examining airborne Doppler radarand deep layer dropsonde observations of the intensification of Hurricane Hermine (2016), a long-lived tropical depression that intensified to hurricane strength in the presence of moderate vertical wind shear. During Hermine’s intensification the low-level circulation appeared to shift toward locations of deep convection that occurred primarily downshear. Hermine began to steadily intensify once a compact low-level vortex developed within a region of deep convection in close proximity to a midlevel circulation, causing vorticity to amplify in the lower troposphere primarily through stretching and tilting from the deep convection. A notable transition of the vertical mass flux profile downshear of the low-level vortex to a bottom-heavy profile also occurred at this time. The transition in the mass flux profile was associated with more widespread moderate convection and a change in the structure of the deep convection to a bottom-heavy mass flux profile, resulting in greater stretching of vorticity in the lower troposphere of the downshear environment. These structural changes in the convection were related to a moistening in the middle troposphere downshear, a stabilization in the lower troposphere, and the development of a mid- to upper-level warm anomaly associated with the developing midlevel circulation. The evolution of precipitation structure shown here suggests a multiscale cooperative interaction across the convective and mesoscale that facilitates an aligned vortex that persists beyond convective time scales, allowing Hermine to steadily intensify to hurricane strength.
Stechman, D.M., G.M. McFarquar, R.M. Rauber, B.F. Jewett, and R.A. Black. Composite in situ microphysical analysis of all spiral vertical profiles executed within BAMEX and PECAN mesoscale convective systems. Journal of the Atmospheric Sciences, 77(7):2541-2565, https://doi.org/10.1175/JAS-D-19-0317.1 2020
Vertical profiles of temperature, relative humidity, cloud particle concentration, median mass dimension, and mass content were derived using instruments on the NOAA P-3 aircraft for 37 spiral ascents/descents flown within five mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) project, and 16 spiral descents of the NOAA P-3 within 10 MCSs during the 2003 Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The statistical distribution of thermodynamic and microphysical properties within these spirals is presented in context of three primary MCS regions—the transition zone (TZ), enhanced stratiform rain region (ESR), and the anvil region (AR)—allowing deductions concerning the relative importance and nature of microphysical processes in each region. Aggregation was ubiquitous across all MCS zones at subfreezing temperatures, where the degree of ambient subsaturation, if present, moderated the effectiveness of this process via sublimation. The predominately ice-supersaturated ESR experienced the least impact of sublimation on microphysical characteristics relative to the TZ and AR. Aggregation was most limited by sublimation in the ice-subsaturated AR, where total particle number and mass concentrations decreased most rapidly with increasing temperature. Sublimation cooling at the surface of ice particles in the TZ, the driest of the three regions, allowed ice to survive to temperatures as high as +6.8°C. Two spirals executed behind a frontal squall line exhibited a high incidence of pristine ice crystals, and notably different characteristics from most other spirals. Gradual meso- to synoptic scale ascent in this region likely contributed to the observed differences.
Stow, J.P., M.A. Bourassa, and H.M. Holbach. Analyzing gaps and hurricane rain coverage to inform NASA satellite proposals. Remote Sensing, 12(17):2673, https://doi.org/10.3390/rs12172673 2020
This study assesses where tropical cyclone (TC) surface winds can be measured as a function of footprint sizes and wavelengths (Ka- Ku- and C-band). During TCs, most high-resolution surface observations are impeded by considerable "rain contamination." Under these conditions, high-resolution surface observations typically come from operational aircraft. Other techniques that provide high-resolution surface observations through rain are also hindered somewhat by rain contamination and are very sparse in space and time. The impacts of rain are functions of the remotely sensed wavelength and rain–drop size. Therefore, relative long wavelengths have been used to observe the surface, but at the cost of a larger footprint. We examine how smaller footprint sizes could be used to observe through gaps between moderate to heavy rainbands that circulate around the main low-pressure center of a TC. Aircraft data from the National Oceanic and Atmospheric Administration’s (NOAA’s) WP-3D turboprop aircraft will be used to create realistic maps of rain. Our results provide information on the satellite instrument characteristics needed to see the surface through these gaps. This information is expected to aid in developing hurricane-related applications of new higher-resolution satellites.
Tymochko, S., E. Munch, J. Dunion, K. Corbosiero, and R. Torn. Using persistent homology to quantify a diurnal cycle in hurricanes. Pattern Recognition Letters, 133:137-143, https://doi.org/10.1016/j.patrec.2020.02.022 2020
The diurnal cycle of tropical cyclones (TCs) is a daily cycle in clouds that appears in satellite images and may have implications for TC structure and intensity. The diurnal pattern can be seen in infrared (IR) satellite imagery as cyclical pulses in the cloud field that propagate radially outward from the center of nearly all Atlantic-basin TCs. These diurnal pulses, a distinguishing characteristic of this diurnal cycle, begin forming in the storm’s inner core near sunset each day, appearing as a region of cooling cloud-top temperatures. The area of cooling takes on a ring-like appearance as cloud-top warming occurs on its inside edge and the cooling moves away from the storm overnight, reaching several hundred kilometers from the circulation center by the following afternoon. The state-of-the-art TC diurnal cycle measurement in IR satellite imagery has a limited ability to analyze the behavior beyond qualitative observations. We present a method for quantifying the TC diurnal cycle using one-dimensional persistent homology, a tool from Topological Data Analysis, by tracking maximum persistence and quantifying the cycle using the discrete Fourier transform. Using Geostationary Operational Environmental Satellite IR imagery from Hurricanes Felix and Ivan, our method is able to detect an approximate daily cycle.
Wang, X., H. Jiang, J.A. Zhang, and K Peng. Satellite-observed warm-core structure in relation to tropical cyclone intensity change. Atmospheric Research, 240:104931, https://doi.org/10.1016/j.atmosres.2020.104931 2020
Using a 13-year dataset of Atmospheric Infrared Sounder (AIRS) retrieved temperature profiles including 5019 AIRS overpasses in 1061 tropical storm through category-2 tropical cyclones (TCs) in global basins during 2002–2014, this study examines the relationship between the warm-core structure and TC intensity change with a focus on rapid intensification (RI). The AIRS TC overpasses are classified into RI, slowly intensifying (SI), neutral (N), and weakening (W) categories. The effect of the warm-core structure upon TC intensification is entangled with that upon TC intensity. It is necessary to exclude the weakening category in order to single out the relationship between TC intensification and warm-core structure from a statistical method. The composite warm-core maximum temperature anomaly is the strongest in RI storms (~7 K), followed by W (~6 K), SI (~5 K), and N (~ 4 K) storms. RI storms have the highest equivalent potential temperature (θe) and convective available potential energy (CAPE) in the eye among all intensity change categories. The warm-core structure of RI storms is asymmetric relative to shear, with the higher temperature anomaly and CAPE located in the down-shear quadrant. When only considering samples with intensification rates ≥0, a significant and positive correlation is found between the warm-core strength and TC intensification rate. The warm-core height is also positively correlated with the TC intensification rate at a high confidence level. The AIRS-derived warm-core temperature anomaly greater than 4 K and weighted warm-core height higher than 450 hPa are the necessary conditions for RI.
Wick, G.A., J.P. Dunion, P.G. Black, J.R. Walker, R.D. Torn, A.C. Kren, A. Aksoy, H. Christophersen, L. Cucurull, B. Dahl, J.M. English, K. Friedman, T.R. Peevey, K. Sellwood, J.A. Sippel, V. Tallapragada, J. Taylor, H. Wang, R.E. Hood, and P. Hall. NOAA’s Sensing Hazards with Operational Unmanned Technology (SHOUT) Experiment: Observations and forecast impacts. Bulletin of the American Meteorological Society, 101(7):E698-E987, https://doi.org/10.1175/BAMS-D-18-0257.1 2020
Field operations and data impact studies examine how observations from high-altitude unmanned aircraft can improve forecasts of tropical cyclones and other high-impact weather events. The National Oceanic and Atmospheric Administration’s (NOAA) Sensing Hazards with Operational Unmanned Technology (SHOUT) project evaluated the ability of observations from high-altitude unmanned aircraft to improve forecasts of high-impact weather events like tropical cyclones or mitigate potential degradation of forecasts in the event of a future gap in satellite coverage. During three field campaigns conducted in 2015 and 2016, the National Aeronautics and Space Administration (NASA) Global Hawk, instrumented with GPS dropwindsondes and remote sensors, flew 15 missions sampling 6 tropical cyclones and 3 winter storms. Missions were designed using novel techniques to target sampling regions where high model forecast uncertainty and a high sensitivity to additional observations existed. Data from the flights were examined in real time by operational forecasters, assimilated in operational weather forecast models, and applied post-mission to a broad suite of data impact studies. Results from the analyses spanning different models and assimilation schemes, though limited in number, consistently demonstrate the potential for positive forecast impact from the observations, both with and without a gap in satellite coverage. The analyses with the then-operational modeling system demonstrated large forecast improvements near 15% for tropical cyclone track at a 72-h lead time when the observations were added to the otherwise complete observing system. While future decisions regarding use of the Global Hawk platform will include budgetary considerations, and more observations are required to enhance statistical significance, the scientific results support the potential merit of the observations. This article provides an overview of the missions flown, observational approach, and highlights from the completed and ongoing data impact studies.
Worku, L.Y., A. Mekonnen, and C.J. Schreck. The impact of MJO, Kelvin, and equatorial Rossby waves on the diurnal cycle over the maritime continent. Atmosphere, 11:711, https://doi.org/10.3390/atmos11070711 2020
The impacts of the Madden–Julian Oscillation (MJO), Kelvin waves, and Equatorial Rossby (ER) waves on the diurnal cycle of rainfall and types of deep convection over the Maritime Continent are investigated using rainfall from the Tropical Rainfall Measurement Mission Multisatellite Precipitation Analysis and Infrared Weather States (IR–WS) data from the International Satellite Cloud Climatology Project. In an absolute sense, the MJO produced its strongest modulations of rainfall and organized deep convection over the islands, when and where convection is already strongest. The MJO actually has a greater percentage modulation over the coasts and seas, but it does not affect weaker diurnal cycle there. Isolated deep convection was also more prevalent over land during the suppressed phase, while organized deep convection dominated the enhanced phase, consistent with past work. This study uniquely examined the effects of Kelvin and ER waves on rainfall, convection, and their diurnal cycles over the Maritime Continent. The modulation of convection by Kelvin waves closely mirrored that by the MJO, although the Kelvin wave convection continued farther into the decreasing phase. The signals for ER waves were also similar but less distinct. An improved understanding of how these waves interact with convection could lead to improved subseasonal forecast skill.
Wu, S.-N., B.J. Soden, and G.J. Alaka. Ice water content as a precursor to tropical cyclone rapid intensification. Geophysical Research Letters, 47(21):e2020GL089669, https://doi.org/10.1029/2020GL089669 2020
This study examines how the structure and amount of cloud ice water content are related to rates of tropical cyclone (TC) intensification using CloudSat profiling radar measurements and simulations from the Hurricane Weather Research and Forecasting (HWRF) model. Observational studies have demonstrated the signal of TC intensification in the passive satellite measurements of frozen water concentration. However, the vertical and horizontal resolution of passive satellite observations are limited. CloudSat measurements and HWRF simulations provide high‐resolution data sets of ice water content to better understand its relationship with the rate of TC intensification. It is found that rapidly intensifying TCs have larger ice water content compared to TCs with slower intensification rates. Similar results are obtained even after accounting for the effect of initial TC intensity. Such precursors of rapidly intensifying TCs may be used to better understand and improve the prediction of TC intensification.
Xian, P., P.J. Klotzbach, J.P. Dunion, M.A. Janiga, J.S. Reid, P.R. Colarco, and Z. Kipling. Revisiting the relationship between Atlantic dust and tropical cyclone activity using aerosol optical depth reanalyses: 2003-2018. Atmospheric Chemistry and Physics, 20(23):15,357-15,378, https://doi.org/10.5194/acp-20-15357-2020 2020
Previous studies have noted a relationship between African dust and Atlantic tropical cyclone (TC) activity. However, due to the limitations of past dust analyses, the strength of this relationship remains uncertain. The emergence of aerosol reanalyses, including the Navy Aerosol Analysis and Prediction System (NAAPS) aerosol optical depth (AOD) reanalysis, NASA Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), and ECMWF Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA), enables an investigation of the relationship between African dust and TC activity over the tropical Atlantic and Caribbean in a consistent temporal and spatial manner for 2003–2018. Although June–July–August (JJA) 550 nm dust AOD (DAOD) from all three reanalysis products correlates significantly over the tropical Atlantic and Caribbean, the difference in DAOD magnitude between products can be as large as 60 % over the Caribbean and 20 % over the tropical North Atlantic. Based on the three individual reanalyses, we have created an aerosol multi-reanalysis consensus (MRC). The MRC presents overall better root mean square error over the tropical Atlantic and Caribbean compared to individual reanalyses when verified with ground-based AErosol RObotic NETwork (AERONET) AOD measurements. Each of the three individual reanalyses and the MRC have significant negative correlations between JJA Caribbean DAOD and seasonal Atlantic accumulated cyclone energy (ACE), while the correlation between JJA tropical North Atlantic DAOD and seasonal ACE is weaker. Possible reasons for this regional difference are provided. A composite analysis of 3 high-JJA-Caribbean-DAOD years versus 3 low-JJA-Caribbean-DAOD years reveals large differences in overall Atlantic TC activity. We also show that JJA Caribbean DAOD is significantly correlated with large-scale fields associated with variability in interannual Atlantic TC activity including zonal wind shear, mid-level moisture, and sea surface temperature (SST), as well as the El Niño–Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM), implying confounding effects of these factors on the dust–TC relationship. We find that seasonal Atlantic DAOD and the AMM, the leading mode of coupled Atlantic variability, are inversely related and intertwined in the dust–TC relationship. Overall, DAOD in both the tropical Atlantic and Caribbean is negatively correlated with Atlantic hurricane frequency and intensity, with stronger correlations in the Caribbean than farther east in the tropical North Atlantic.
Zawislak, J. Global survey of precipitation properties observed during tropical cyclogenesis and their differences compared to nondeveloping disturbances. Monthly Weather Review, 148(4):1585-1606, https://doi.org/10.1175/MWR-D-18-0407.1 2020
This study evaluates precipitation properties involved in tropical cyclogenesis by analyzing a multi-year, global database of passive microwave overpasses of the pre-genesis stage of developing disturbances and nondeveloping disturbances. Precipitation statistics are quantified using brightness temperature proxies from the 85–91-GHz channels of multiple spaceborne sensors, as well as retrieved rain rates. Proxies focus on the overall raining area, areal coverage of deep convection, and the proximity of precipitation to the disturbance center. Of interest are the differences in those proxies for developing versus nondeveloping disturbances, how the properties evolve during the pre-genesis stage, and how they differ globally. The results indicate that of all of the proxies examined, the total raining area and rain volume near the circulation center are the most useful precipitation-related predictor for genesis. The areal coverage of deep convection also differentiates developing from nondeveloping disturbances and, similar to the total raining area, generally also increases during the pre-genesis stage, particularly within a day of genesis. As the threshold convective intensity is increased, pre-genesis cases are less distinguishable from nondeveloping. Compared to the western Pacific and Indian Oceans, the Atlantic and eastern North Pacific have less precipitation and deep convection observed during genesis and the smallest differences between developing and nondeveloping disturbances. This suggests that the total raining area and areal coverage of deep convection associated with tropical disturbances are better predictors of tropical cyclogenesis fate in the Pacific and Indian Oceans than in the Atlantic and eastern North Pacific.
Zeng, X., R. Atlas, R.J. Birk, F.H. Carr, M.J. Carrier, L. Cucurull, W.H. Hooke, E. Kalnay, R. Murtugudde, D.J. Posselt, J.L. Russell, D.P. Tyndall, R.A. Weller, and F. Zhang. Use of Observing System Simulation Experiments in the United States. Bulletin of the American Meteorological Society, 101(8):e1427-e1438, https://doi.org/10.1175/BAMS-D-19-0155.1 2020
We briefly review the use of Observing System Simulation Experiments in the U.S. and discuss their values and limitations, leading to an expert consensus on five recommendations for moving forward. The NOAA Science Advisory Board appointed a Task Force to prepare a white paper on the use of Observing System Simulation Experiments (OSSEs). Considering the importance and timeliness of this topic and based on this white paper, here we briefly review the use of OSSEs in the U.S., discuss their values and limitations, and develop five recommendations for moving forward: national coordination of relevant research efforts; acceleration of OSSE development for Earth system models; consideration of the potential impact on OSSEs of deficiencies in the current data assimilation and prediction system; innovative and new applications of OSSEs; and extension of OSSEs to societal impacts. OSSEs can be complemented by calculations of forecast sensitivity to observations, which simultaneously evaluate the impact of different observation types in a forecast model system.
Zhang, J.A., E.A. Kalina, M.K. Biswas, R.F. Rogers, P. Zhu, and F.D. Marks. A review and evaluation of planetary boundary layer parameterizations in Hurricane Weather Research and Forecasting model using idealized simulations and observations. Atmosphere, 11(10):1091, https://doi.org/10.3390/atmos11101091 2020
This paper reviews the evolution of planetary boundary layer (PBL) parameterization schemes that have been used in the operational version of the Hurricane Weather Research and Forecasting (HWRF) model since 2011. Idealized simulations are then used to evaluate the effects of different PBL schemes on hurricane structure and intensity. The original Global Forecast System (GFS) PBL scheme in the 2011 version of HWRF produces the weakest storm, while a modified GFS scheme using a wind-speed dependent parameterization of vertical eddy diffusivity (Km) produces the strongest storm. The subsequent version of the hybrid eddy diffusivity and mass flux scheme (EDMF) used in HWRF also produces a strong storm, similar to the version using the wind-speed dependent Km. Both the intensity change rate and maximum intensity of the simulated storms vary with different PBL schemes, mainly due to differences in the parameterization of Km. The smaller the Km in the PBL scheme, the faster a storm tends to intensify. Differences in hurricane PBL height, convergence, inflow angle, warm-core structure, distribution of deep convection, and agradient force in these simulations are also examined. Compared to dropsonde and Doppler radar composites, improvements in the kinematic structure are found in simulations using the wind-speed dependent Km and modified EDMF schemes relative to those with earlier versions of the PBL schemes in HWRF. However, the upper boundary layer in all simulations is much cooler and drier than that in dropsonde observations. This model deficiency needs to be considered and corrected in future model physics upgrades.
Zhang, J.A., J.P. Dunion, and D.S. Nolan. In situ observations of the diurnal variation in the boundary layer of mature hurricanes. Geophysical Research Letters, 47(3):e2019GL086206, https://doi.org/10.1029/2019GL086206 2020
Recent studies have suggested that the structure of tropical cyclones (TCs), especially the upper‐level clouds as indicated by satellite infrared brightness temperatures and precipitation, fluctuates with the diurnal cycle. The diurnal cycle of the low‐level structure, including the boundary layer, has not yet been investigated with observations. This study analyzes data from 2242 GPS dropsondes collected in mature hurricanes to investigate the diurnal variation of the mean boundary layer structure. A composite analysis is conducted to compare the kinematic and thermodynamic structure during nighttime (0–6 local time) versus in the afternoon (12–18 local time). The composites show that much stronger inflow occurs during nighttime and the moist entropy is also larger than that in the daytime. Grouping the dropsonde data into 6‐h time windows relative to the local time shows a clear diurnal signal of boundary layer inflow. The amplitude of the diurnal signal is largest at a radius of 250–500 km.
Zhao, Z., P.W. Chan, N. Wu, J.A. Zhang, and K.K. Hon. Aircraft observations of turbulent characteristics in the tropical cyclone boundary layer. Boundary-Layer Meteorology, 174(3):493-511, https://doi.org/10.1007/s10546-019-00487-8 2020
The Hong Kong Observatory conducted six flights in the atmospheric boundary layer of five tropical cyclones: tropical storm Jebi (1309), typhoon Kalmaegi (1415), severe tropical storm Linfa (1510), typhoon Mujigae (1522), and severe typhoon Nida (1604). Three-dimensional wind data with a 20-Hz sampling rate were available for a height range of 500–670 m, with the mean wind speed from these low-level flights ranging from 10 to 62 m s−1. The turbulent momentum flux and turbulence kinetic energy (e) are measured using the eddy-correlation method, while horizontal scales of turbulent eddies, vertical eddy diffusivity (K), and the vertical turbulent mixing length scale are estimated indirectly. The dependence of the momentum flux, e, K, and the vertical mixing length on wind speed and height are compared with previous studies. Both the momentum flux and turbulent kinetic energy increase with the wind speed, although the rate of increase is smaller for higher wind speeds. It is also found that K increases with wind speed according to a power law up to 40 m s−1 before levelling off, while the vertical mixing length is nearly constant at 100 m. The results serve as a reference for evaluating and improving the turbulent parametrizion in tropical-cyclone models, while the observed large turbulent mixing near the top of the inflow layer of the eyewall region should not be neglected in numerical models.
Zou, Z., S. Li, J. Huang, P. Li, J. Song, J.A. Zhang, and Z. Wan. Atmospheric boundary layer turbulence in the presence of swell: Turbulent kinetic energy budget, Monin-Obukhov similarity theory and inertial dissipation method. Journal of Physical Oceanography, 50(5):1213-1225, https://doi.org/10.1175/JPO-D-19-0136.1 2020
Turbulence over the mobile ocean surface has distinct properties compared to turbulence over land. Thus, findings that are based on the turbulent kinetic energy (TKE) budget and Monin–Obukhov similarity theory (MOST) over land may not be applicable to conditions over ocean partly because of the existence of a wave boundary layer (the lower part of atmospheric boundary layer including effects of surface waves; we used the term "WBL" in this article for convenience), where the total stress can be separated into turbulent stress and wave coherent stress. Here the turbulent stress is defined as the stress generated by wind shear and buoyancy, while the wave coherent stress accounts for the momentum transfer between ocean waves and atmosphere. In this study, applicability of the turbulent kinetic energy (TKE) budget and the inertial dissipation method (IDM) in the context of the MOST within the WBL are examined. It was found that turbulent transport terms in the TKE budget should not be neglected when calculating the total stress under swell conditions. This was confirmed by observations made on a fixed platform. The results also suggested that turbulent stress, rather than total stress, should be used when applying the MOST within the WBL. By combining the TKE budget and MOST, our study showed that the stress computed by the traditional IDM corresponds to the turbulent stress rather than the total stress. The swell wave coherent stress should be considered when applying the IDM to calculate the stress in the WBL.
2019
Ahren, K., M.A. Bourassa, R.E. Hart, J.A. Zhang, and R.F. Rogers. Observed kinematic and thermodynamic structure in the hurricane boundary layer during intensity change. Monthly Weather Review, 147(8):2765-2785, https://doi.org/10.1175/MWR-D-18-0380.1 2019
The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥ 20 kt (24 h)−1], weakening [WE; intensity tendency < −10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3,091 dropsondes were composited for analysis below 2.5 km elevation—1,086 during IN, 1,042 during WE, and 963 during SS. In non-intensifying hurricanes, the lowlevel tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I) at those radii for non-intensifying hurricanes. Differences in tangential wind structure (and I) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Non-intensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of non-intensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.
Alaka, G.J., X. Zhang, S.G. Gopalakrishnan, Z. Zhang, F.D. Marks, and R. Atlas. Track uncertainty in high-resolution ensemble forecasts of Hurricane Joaquin. Weather and Forecasting, 34(6):1889-1908, https://doi.org/10.1175/WAF-D-19-0028.1 2019
Hurricane Joaquin (2015) was characterized by high track forecast uncertainty when it approached the Bahamas from 29 September 2015 to 01 October 2015, with five-day track predictions ranging from landfall on the United States to east of Bermuda. The source of large track spread in Joaquin forecasts is investigated using an ensemble prediction system (EPS) based on the Hurricane Weather Research and Forecasting (HWRF) model. For the first time, a high-resolution analysis of an HWRF-based EPS is performed to isolate the factors that control tropical cyclone (TC) track uncertainty. Differences in the synoptic-scale environment, the TC vortex structure, and the TC location are evaluated to understand the source of track forecast uncertainty associated with Joaquin, especially at later lead times when U.S. landfall was possible. EPS members that correctly propagated Joaquin into the central North Atlantic are compared with members that incorrectly predicted U.S. landfall. Joaquin track forecasts were highly dependent on the evolution of the environment, including weak atmospheric steering flow near the Bahamas and three synoptic-scale systems: a trough over North America, a ridge to the northeast of Joaquin, and an upper-tropospheric trough to the east of Joaquin. Differences in the steering flow were associated with perturbations of the synoptic-scale environment at the model initialization time. Ultimately, members that produced a more progressive mid-latitude synoptic-scale pattern had reduced track errors. Joaquin track forecast uncertainty was not sensitive to the TC vortex structure or the initial TC position.
Anthes, R.A., M.W. Maier, S. Ackerman, R. Atlas, L.W. Callahan, G.J. Dittberner, R. Edwing, P.G. Emch, M. Ford, W.B. Gail, M. Goldberg, S. Goodman, C. Kummerow, T. Onsager, K. Schrab, C. Velden, T. von der Haar, and J.G. Yoe. Developing priority observational requirements from space using multi-attribute utility theory. Bulletin of the American Meteorological Society, 100(9):1753-1793, https://doi.org/10.1175/BAMS-D-18-0180.1 2019
This paper describes an analysis and prioritization process for a future NOAA observational system from space, with emphasis on operational applications. Over a 2-year period beginning in 2015, a panel of subject matter experts, the Space Platform Requirements Working Group (SPRWG), carried out an analysis and prioritization of different space-based observations supporting NOAA’s operational services in the areas of weather, oceans, and space weather. NOAA leadership used the SPRWG analysis of space-based observational priorities in different mission areas, among other inputs, to inform the Multi-Attribute Utility Theory (MAUT) based value model and the NOAA Satellite Observing Systems Architecture (NSOSA) study (Volz et al., 2016; NOAA, 2018). The goal of the NSOSA study is to develop candidate satellite architectures for the era beginning in approximately 2030. The SPRWG analysis included a prioritized list of observational objectives together with the quantitative attributes of each objective at three levels of performance, a threshold level of minimal utility, an intermediate level that the community expects by 2030, and a maximum effective level, a level for which further improvements would not be cost effective. This process is believed to be unprecedented in the analysis of long-range plans for providing observations from space. This paper describes the process for developing the prioritized objectives and their attributes and how they were combined in the EDR (Environmental Data Record) Value Model (EVM). The EVM helped inform NOAA’s assessment of many potential architectures for its future observing system within the NSOSA study. However, neither the SPRWG nor its report represents official NOAA policy positions or decisions and the responsibility for selecting and implementing the final architecture rests solely with NOAA senior leadership.
Banos, I.H., L.F. Sapucci, L. Cucurull, C.F. Bastarz, and B.B. Silveira. Assimilation of GPSRO bending angle profiles into the Brazilian Global Atmospheric Model. Remote Sensing, 11(3):256, https://doi.org/10.3390/rs11030256 2019
The Global Positioning System (GPS) Radio Occultation (RO) technique allows valuable information to be obtained about the state of the atmosphere through vertical profiles obtained at various processing levels. From the point of view of data assimilation, there is a consensus that less processed data are preferable because of their lowest addition of uncertainties in the process. In the GPSRO context, bending angle data are better to assimilate than refractivity or atmospheric profiles; however, these data have not been properly explored by data assimilation at the CPTEC (acronym in Portuguese for Center for Weather Forecast and Climate Studies). In this study, the benefits and possible deficiencies of the CPTEC modeling system for this data source are investigated. Three numerical experiments were conducted, assimilating bending angles and refractivity profiles in the Gridpoint Statistical Interpolation (GSI) system coupled with the Brazilian Global Atmospheric Model (BAM). The results highlighted the need for further studies to explore the representation of meteorological systems at the higher levels of the BAM model. Nevertheless, more benefits were achieved using bending angle data compared with the results obtained assimilating refractivity profiles. The highest gain was in the data usage exploring 73.4% of the potential of the RO technique when bending angles are assimilated. Additionally, gains of 3.5% and 2.5% were found in the root mean square error values in the zonal and meridional wind components and geopotencial height at 250 hPa, respectively.
Bell, G.D., E.S. Blake, C.W. Landsea, H. Wang, S.B. Goldenberg, and R.J. Pasch. Tropical cyclones: Atlantic basin. In State of the Climate in 2018, J. Blunden and D.S. Arndt (eds.) Bulletin of the American Meteorological Society, 100(9):S113-S119, https://doi.org/10.1175/2019BAMSStateoftheClimate.1 2019
Bhalachandran, S., P.S.C. Rao, and F.D. Marks. A conceptual framework for the scale-specific stochastic modeling of transitions in tropical cyclone intensities. Earth and Space Science, 6(6):972-981, https://doi.org/10.1029/2019EA000585 2019
At any given time, a tropical cyclone (TC) vortex has multiple intensity pathways that are possible. We conceptualize this problem as a scenario where each of the TC's intensity pathways is a distinct attractor basin, and a combination of several external and internal factors across multiple scales dictates as to which of the many pathways the TC vortex actually takes. As with any complex system, it is difficult to know the details of the multiscale processes that cause or initiate the tipping of the TC vortex into an attractor basin. A stochastic shock arising from any of the various scales within a TC vortex and the subsequent cross‐scale energy transactions may rapidly increase the probability of the vortex intensifying or weakening. To address this problem and apply our conceptual framework to actual TC case studies, we formulate a novel scale‐specific stochastic model that examines the multiscale energetics at and across individual wave numbers within the TC vortex. The stochastic term is modeled in a realistic manner in that the lower and higher wave numbers are treated differently. High‐resolution Hurricane Weather and Research Forecast model outputs of two Bay of Bengal TCs, Phailin (intensifying) and Lehar (weakening), are used as case studies. An ensemble of intensity pathways is generated, and the nonstationary probability distributions of the intensity transitions at each time are examined. Our approach is another step toward an improved understanding of the stochastic dynamics of multiscale transitions of a TC vortex.
Bhalachandran, S., R. Nadimpalli, K.K. Osuri, F.D. Marks, S. Gopalakrishnan, S. Subramanian, U.C. Mohanty, and D. Niyogi. On the processes influencing rapid intensity changes of tropical cyclones over the Bay of Bengal. Scientific Reports, 9:3382, https://doi.org/10.1038/s41598-019-40332-z 2019
We present a numerical investigation of the processes that influenced the contrasting rapid intensity changes in Tropical Cyclones (TC) Phailin and Lehar (2013) over the Bay of Bengal. Our emphasis is on the significant differences in the environments experienced by the TCs within a few weeks and the consequent differences in their organization of vortex-scale convection that resulted in their different rapid intensity changes. The storm-relative proximity, intensity, and depth of the subtropical ridge resulted in the establishment of a low-sheared environment for Phailin and a high-sheared environment for Lehar. Our primary finding here is that in Lehar’s sheared vortex, the juxtaposition in the azimuthal phasing of the asymmetrically distributed downward eddy flux of moist-entropy through the top of the boundary layer, and the radial eddy flux of moist-entropy within the boundary layer in the upshear left-quadrant of Lehar (40–80 km radius) establishes a pathway for the low moist-entropy air to intrude into the vortex from the environment. Conversely, when the azimuthal variations in boundary layer moist-entropy, inflow, and convection are weak in Phailin’s low-sheared environment, the inflow magnitude and radial location of boundary layer convergence relative to the radius of maximum wind dictated the rapid intensification.
Bhalachandran, S., Z.S. Haddad, S.M. Hristova-Veleva, and F.D. Marks. The relative importance of factors influencing tropical cyclone rapid intensity changes. Geophysical Research Letters, 46(4):2282-2292, https://doi.org/10.1029/2018GL079997 2019
Predicting rapid intensity changes in tropical cyclones (TCs) is a major challenge due to the influence of multiple competing processes within the vortex and in the TC environment. We present an empirical framework that quantifies the relative importance of the various factors that influence critical transitions in TC intensities. Our analysis of model simulations of recent TCs over the Bay of Bengal identifies the following variables within the vortex as the biggest influence on TC rapid intensity changes: the amplitudes of wave number 1 of 700‐ to 850‐mb horizontal moisture flux convergence and precipitation in the rainband region and the amplitude of wave number 0 of precipitation within the radius of maximum winds. Likewise, the most important environmental variables identified are the angle between the driest air and the shear vector and the magnitude of vertical wind shear. These findings provide guidance on guidance for future observational efforts and data assimilation into TC forecasting models.
Bourassa, M.A., T. Meissner, I. Cerovecki, P.S. Chang, X. Dong, G. De Chiara, C. Donlon, D. Dukhovskoy, J. Elya, A. Fore, M.R. Fewings, R.C. Foster, S.T. Gille, B.K. Haus, S. Hristova-Veleva, H.M. Holbach, Z. Jelenak, J.A. Knaff, S.A. Kranz, A. Manaster, M. Mazloff, C. Mears, A. Mouche, M. Portabella, N. Reul, L. Ricciardulli, E. Rodriguez, C. Sampson, D. Solis, A. Stoffelen, M.R. Stukel, B. Styles, D. Weissman, and F. Wentz. Remotely sensed winds and wind stresses for marine forecasting and ocean modeling. Frontiers in Marine Science, 6:443, https://doi.org/10.3389/fmars.2019.00443 2019
Strengths and weakness of remotely sensed winds are discussed, along with the current capabilities for remotely sensing winds and stress. Future missions are briefly mentioned. The observational needs for a wide range of wind and stress applications are provided. These needs strongly support a short list of desired capabilities of future missions and constellations.
Chen, X., J.A. Zhang, and F.D. Marks. A thermodynamic pathway leading to rapid intensification of tropical cyclones in shear. Geophysical Research Letters, 46(15):9241-9251, https://doi.org/10.1029/2019GL083667 2019
Understanding physical processes leading to rapid intensification (RI) of tropical cyclones (TCs) under environmental vertical wind shear (VWS) is key to improving TC intensity forecasts. This study analyzes the thermodynamic processes that help saturate the TC inner core before RI onset using a column‐integrated moist static energy (MSE) framework. Results indicate that the nearly‐saturated inner core in the lower‐middle troposphere is achieved by an increase in the column‐integrated MSE, as column water vapor accumulates while the mean column temperature cools. The sign of the column‐integrated MSE tendency depends on the competition between surface enthalpy fluxes, radiation, and VWS‐induced ventilation effect. The reduction of ventilation above the boundary layer due to vertical alignment is crucial to accumulate the energy within the inner‐core region. A comparison of the RI simulation with a null simulation further highlights the impact of vortex structure on the thermodynamic state adjustment and TC intensification.
Cui, Z., Z. Pu, V. Tallapragada, R. Atlas, and C.S. Ruf. A preliminary impact study of CYGNSS ocean surface wind speeds on numerical simulations of hurricanes. Geophysical Research Letters, 46(5):2984-2992, https://doi.org/10.1029/2019GL082236 2019
The NASA Cyclone Global Navigation Satellite System (CYGNSS) was launched in December 2016, providing an unprecedented opportunity to obtain ocean surface wind speeds including wind estimates over the hurricane inner‐core region. This study demonstrates the influence of assimilating an early version of CYGNSS observations of ocean surface wind speeds on numerical simulations of two notable landfalling hurricanes, Harvey and Irma (2017). A research version of the National Centers for Environmental Prediction operational Hurricane Weather Research and Forecasting model and the Gridpoint Statistical Interpolation‐based hybrid ensemble three‐dimensional variational data assimilation system are used. It is found that the assimilation of CYGNSS data results in improved track, intensity, and structure forecasts for both hurricane cases, especially for the weak phase of a hurricane, implying potential benefits of using such data for future research and operational applications.
de Boer, G., B. Argrow, J. Cassano, J. Cione, E. Frew, D. Lawrence, G. Wick, and C. Wolff. Advancing unmanned aerial capabilities for atmospheric research. Bulletin of the American Meteorological Society, 100(3):ES105-ES108, https://doi.org/10.1175/BAMS-D-18-0254.1 2019
Domingues, R., A. Kuwano-Yoshida, P. Chardon-Maldonado, R.E. Todd, G. Halliwell, H.-S. Kim, I.-I. Lin, K. Sato, T. Narazaki, L.K. Shay, T. Miles, S. Glenn, J.A. Zhang, S.R. Jayne, L. Centurioni, M. Le Henaff, G. Foltz, F. Bringas, M.M. Ali, S.F. DiMarco, S. Hosoda, T. Fukuoka, B. LaCour, A. Mehra, E.R. Sanabia, J.R. Gyakum, J. Dong, J.A. Knaff, and G. Goni. Ocean observations in support of studies and forecasts of tropical and extratropical cyclones. Frontiers in Marine Science, 6:446, https://doi.org/10.3389/fmars.2019.00446 2019
Over the past decade, measurements from the climate-oriented ocean observing system have been key to advancing the understanding of extreme weather events that originate and intensify over the ocean, such as tropical cyclones (TCs) and extratropical bomb cyclones (ECs). In order to foster further advancements to predict and better understand these extreme weather events, a need for a dedicated observing system component specifically to support studies and forecasts of TCs and ECs has been identified, but such a system has not yet been implemented. New technologies, pilot networks, targeted deployments of instruments, and state-of-the art coupled numerical models have enabled advances in research and forecast capabilities and illustrate a potential framework for future development. Here, applications and key results made possible by the different ocean observing efforts in support of studies and forecasts of TCs and ECs, as well as recent advances in observing technologies and strategies are reviewed. Then a vision and specific recommendations for the next decade are discussed.
Dunion, J.P., C.D. Thorncroft, and D.S. Nolan. Tropical cyclone diurnal cycle signals in a hurricane nature run. Monthly Weather Review, 147(1):363-388, https://doi.org/10.1175/MWR-D-18-0130.1 2019
The diurnal cycle of tropical convection and tropical cyclones (TCs) has been previously described in observational, satellite, and modeling based studies. The main objective of this work is to expand on these earlier studies by identifying signals of the TC diurnal cycle (TCDC) in a hurricane nature run, characterize their evolution in time and space, and better understand the processes that cause them. Based on previous studies that identified optimal conditions for the TCDC, a select period of the hurricane nature run is examined when the simulated storm was intense, in a low shear environment, and sufficiently far from land. When analyses are constrained by these conditions, marked radially propagating diurnal signals in radiation, thermodynamics, winds, and precipitation that affect a deep layer of the troposphere become evident in the model. These propagating diurnal signals, or TC diurnal pulses, are a distinguishing characteristic of the TCDC and manifest as a surge in upper-level outflow with underlying radially propagating tropical squall line-like features. The results of this work support previous studies that examined the TCDC using satellite data and have implications for numerical modeling of TCs and furthering our understanding of how the TCDC forms, evolves, and possibly impacts TC structure and intensity.
Gopalakrishnan, S.G., K.K. Osuri, F.D. Marks, and U.C. Mohanty. An inner-core analysis of the axisymmetric and asymmetric intensification of tropical cyclones: Influence of shear. Mausam: Quarterly Journal of Meteorology, Hydrology and Geophysics, 70(4):667-690, 2019
The state-of-the-art in intensity forecasting is provided using the Hurricane Weather Research and Forecasting (HWRF) modeling system as the basis. A brief review of existing axisymmetric theory for the intensification of tropical cyclones (TCs) is also provided. Two cloud-resolving simulations from HWRF, one from an idealized case and another from a retrospective forecast of Super Cyclone Phalin (2013), are used to understand the axisymmetric, rapid intensification (RI) process. However, TCs are rarely axisymmetric. The asymmetric structure in the inner core of a TC may be generated by both internal dynamics and external forcing due to environmental factors such as shear and moisture. We use a retrospective HWRF run from Atlantic Hurricane Earl (2010) to understand the role of shear-induced asymmetries on the RI of TCs. We seek to address the following questions: How do TCs rapidly intensify in a sheared environment? What is the role of eddy fluxes on TC intensification? Is the well-accepted theoretical framework of TC intensification still valid for sheared storms undergoing RI? Our findings show that eddy radial vorticity fluxes play a significant role in controlling TC intensity changes in sheared storms. In the case of Earl, despite persistent environmental shear and a lack of symmetric convection, a positive eddy vorticity flux in the middle to upper troposphere created by mesoscale convective complexes had a profound influence in accelerating the TC spin-up process. RI does not occur until persistent convective bursts and the collocated vertical vorticity are concentrated in the downshear-left quadrant about 50 km from the surface center, followed by the propagation of these complexes in an upshear direction. When convective bursts reach the upshear-left quadrant, a nearly symmetric pattern of eddy radial vorticity flux surrounds the center, indicating vorticity anomalies have merged to create a stronger mean vortex wherein the upper and lower level circulations are better aligned. This process is different from that of an idealized vortex. An idealized vortex intensifies in a shear-free environment where individual vortical hot plumes converge and stretch ambient low-level vorticity into a small-scale anomaly and multiple mergers of these plumes lead to a single stronger vortex in a vorticity rich environment. In the idealized case, the major spin-up of the vortex occurs in the TC boundary layer and eyewall region, while spin-up in the case of Earl appears to be top-down during the early stages of RI. Nevertheless, evidence of stronger spin-up is observed in the boundary layer after Earl’s initial RI phase. Although a fully three-dimensional model is required to understand the real TC intensification problem, when viewed from an axially-averaged framework, the basic axisymmetric theory of intensification is still valid for all cases.
Jin, S., X. Li, X. Yang, J.A. Zhang, and D. Shen. Identification of tropical cyclone centers in SAR imagery based on template matching and particle swarm optimization algorithms. IEEE Transactions on Geoscience and Remote Sensing, 57(1):598-608, https://doi.org/10.1109/TGRS.2018.2863259 2019
Synthetic aperture radar (SAR) has emerged as a new tool for tropical cyclone (TC) monitoring by providing information on the location of TC centers. However, SAR does not usually cover the entire TC domain due to its limited swath width. In this paper, we develop a procedure to identify the location of the center of a TC when an SAR image only covers the rain band portion of the TC but not the eye. The algorithm is based on both an image processing procedure and the available knowledge of the inherent rain-band structure of a TC. The three-step algorithm includes: 1) applying a Canny edge detector to find the curves associated with rain bands; 2) defining two filter criteria to select the spiral curves that resemble the estimation based on a TC rain-band model; 3) searching for the optimal matching solution using the particle swarm optimization algorithm. Numerical experiments with images without TC eye information show that the proposed method can effectively locate the centers of TCs. We compare the experimental results with the best track data to indicate the accuracy. Then, we compare the inflow angle model and the logarithmic spiral model and find that the inflow angle model is more accurate for TC center identification.
Klotz, B.W., and D.S. Nolan. SFMR surface wind undersampling over the tropical cyclone lifecycle. Monthly Weather Review, 147(1):247-268, https://doi.org/10.1175/MWR-D-18-0296.1 2019
Surface wind speeds in tropical cyclones are important for defining current intensity and intensification. Traditionally, airborne observations provide the best information about the surface wind speeds with the Stepped Frequency Microwave Radiometer (SFMR) providing a key role in obtaining such data. However, the flight patterns conducted by hurricane hunter aircraft are limited in their azimuthal coverage of the surface wind field, resulting in an undersampling of the wind field and consequent underestimation of the peak 10-m wind speed. A previous study provided quantitative estimates of the average underestimate for a very strong hurricane. However, no broader guidance on applying a correction based on undersampling has been presented in detail. To accomplish this task, a modified observing system simulation experiment with five hurricane simulations is used to perform a statistical evaluation of the peak wind speed underestimate over different stages of the tropical cyclone lifecycle. Analysis of numerous simulated flights highlights prominent relationships between wind speed undersampling and storm size, where size is defined by the radius of maximum wind speed (RMW). For example, an intense hurricane with small RMW needs negligible correction while a large-RMW tropical storm requires a 16-19% change. A look-up table of undersampling correction factors as a function of peak SFMR wind speed and RMW is provided to assist the tropical cyclone operations community. Implications for hurricane best track intensity estimates are also discussed using real data from past Atlantic hurricane seasons.
Li, Z., J. Li, T.J. Schmit, P. Wang, A. Lim, J. Li, F.W. Nagle, W. Bai, J.A. Otkin, R. Atlas, R.N. Hoffman, S.-A. Boukabara, T. Zhu, W.J. Blackwell, and T.S. Pagano. The alternative of CubeSat-based advanced infrared and microwave sounders for high impact weather forecasting. Atmospheric and Oceanic Science Letters, 12(2):80-90, https://doi.org/10.1080/16742834.2019.1568816 2019
The advanced infrared (IR) and microwave (MW) sounding systems have been providing atmospheric sounding information critical for nowcasting and improving weather forecasts through data assimilation in numerical weather prediction. In recent years, advanced IR and MW sounder systems are being proposed to be onboard CubeSats that are much more cost efficient than traditional satellite systems. An impact study using a regional Observing System Simulation Experiment on a local severe storm (LSS) was carried out to evaluate the alternative of using advanced MW and IR sounders for high-impact weather forecasting in mitigating the potential data gap of the Advanced Technology Microwave Sounder (ATMS) and the Cross-track Infrared Sounder (CrIS) on the Suomi-NPP (SNPP) or Joint Polar Satellite System (JPSS). It was found that either MicroMAS-2 or the CubeSat Infrared Atmospheric Sounder (CIRAS) on a single CubeSat was able to provide a positive impact on the LSS forecast, and more CubeSats with increased data coverage yielded larger positive impacts. MicroMAS-2 has the potential to mitigate the loss of ATMS, and CIRAS the loss of CrIS, on SNPP or JPSS, especially when multiple CubeSats are launched. There are several approximations and limitations to the present study, but these represent efficiencies appropriate to the principal goal of the study—gauging the relative values of these sensors.
Martinez, J., M.M. Bell, R.F. Rogers, and J.D. Doyle Axisymmetric potential vorticity evolution of Hurricane Patricia (2015). Journal of the Atmospheric Sciences, 76(7):2043-2063, https://doi.org/10.1175/JAS-D-18-0373.1 2019
Operational numerical models failed to predict the record-setting rapid intensification and rapid over-water weakening of Hurricane Patricia (2015) in the eastern North Pacific basin, resulting in large intensity forecast errors. In an effort to better understand the mesoscale processes contributing to Patricia’s rapid intensity changes, we analyze high-resolution aircraft observations collected on 22–23 October. Spline-based variational analyses are created from observations collected via in situ measurements, Doppler radar, and full-tropospheric dropsonde profiles as part of the Office of Naval Research Tropical Cyclone Intensity (TCI) experiment and the National Oceanic and Atmospheric Administration Intensity Forecasting Experiment (IFEX). We present the first full-tropospheric calculation of the dry, axisymmetric Ertel’s potential vorticity (PV) in a tropical cyclone without relying on balance assumptions. Detailed analyses reveal the formation of a “hollow tower” PV structure as Patricia rapidly approached its maximum intensity, and a subsequent breakdown of this structure during Patricia’s rapid over-water weakening phase. Transforming the axisymmetric PV analyses from radius-height to potential radius-isentropic coordinates reveals that Patricia’s rapid intensification was closely related to the distribution of diabatic heating and eddy mixing. During Patricia’s rapid over-water weakening phase, eddy mixing processes are hypothesized to be the primary factor rearranging the PV distribution near the eye-eyewall region, diluting the PV previously confined to the hollow tower while approximately conserving the absolute circulation.
Mayer, D.A., J.A. Zhang, and R.H. Weisberg. Surface layer turbulence parameters derived from 1-s wind observations on the West Florida Shelf. Journal of Geophysical Research-Atmospheres, 124(4):1992-2007, https://doi.org/10.1029/2018JD029392 2019
One‐second wind data on the West Florida Shelf were used to examine turbulent scales from large eddies to small eddies in the atmospheric surface layer within a frequency band from 0.02 to 0.3 Hz (periods from ~1 min to ~3 s). Data were collected at two at‐sea locations spanning 6.5 months. Three events in three wind ranges were examined in exploring the one‐dimensional turbulent power spectra: >14 m/s, wind range I; those between 10 and 14 m/s, wind range II; and those between 5 and 10 m/s, wind range III. Events consisted of ensembles of abutting 30‐min subsets spanning 5.5 to 23 hr. The mean vector wind time scale of T0 = 30 min was found to be reasonable for the West Florida Shelf region. The first wind range provided the best results, more or less in line with a Kolmogorov −5/3 power law whose mean vector wind speed over 21 subsets (10.5 hr) was nearly 15 m/s. The one‐dimensional turbulent power spectra provided an estimate of the dissipation rate (ε) from which other turbulent quantities could be computed: u*, τ, and Cd (the frictional velocity, the surface stress, and the drag coefficient, respectively). The salient point here is that these quantities were larger than those from previous observational studies. Where the power law was not operant intrinsic turbulent spatial scales ranged from 1 to 0.1 m and provide evidence of anisotropy for frequencies greater than 0.1 Hz.
Mears, C.A., J. Scott, F.J. Wentz, L. Ricciardulli, S.M. Leidner, R. Hoffman, and R. Atlas. A near-real-time version of the Cross-Calibrated Multiplatform (CCMP) ocean surface wind velocity dataset. Journal of Geophysical Research-Oceans, 124(10):6997-7010, https://doi.org/10.1029/2019JC015367 2019
The Cross‐Calibrated Multiplatform (CCMP) ocean surface wind data set was originally developed by Atlas and coworkers to blend cross‐calibrated satellite winds, in situ data, and wind analyses from numerical weather prediction. CCMP uses a variational analysis method to smoothly blend these data sources into a gap‐free gridded wind estimate every 6 hr. CCMP version 2.0 is currently produced by Remote Sensing Systems using consistently cross‐calibrated satellite winds, in situ data from moored buoys, and background winds from the ERA‐Interim reanalysis. The reanalysis fields are only available after a delay of several months, making it impossible to produce CCMP 2.0 in near real time. Measurements from in situ sources such as moored buoys are also often delayed. To overcome these obstacles and produce a near‐real‐time (NRT) version of CCMP (CCMP‐NRT), two changes are made to the input data sets: The background winds are now the operational 0.25‐degree NCEP analysis winds, and no in situ data are used. This allows CCMP‐NRT to be routinely processed with a latency of less than 48 hr. An intercomparison of the CCMP‐NRT results with CCMP 2.0, and independent measurements from moored buoys shows that CCMP‐NRT provides a modest improvement over the background wind from NCEP in regions where satellite data are available. Analysis shows that the inclusion of in situ measurement in CCMP improves the agreement with these measurements, artificially reducing estimates of the error.
Molinari, J., J.A. Zhang, R.F. Rogers, and D. Vollaro. Repeated eyewall replacement cycles in Hurricane Frances (2004). Monthly Weather Review, 147(6):2009-2022, https://doi.org/10.1175/MWR-D-18-0345.1 2019
Hurricane Frances (2004) represented an unusual event that produced three consecutive overlapping eyewall replacement cycles (ERCs). Their evolution followed some aspects of the typical ERC. The strong primary eyewalls contracted and outward sloping secondary eyewalls formed near three times the radius of maximum winds. Over time these secondary eyewalls shifted inward, became more upright, and replaced the primary eyewalls. In other aspects, however, the ERCs in Hurricane Frances differed from previously described composites. The outer eyewall wind maxima became stronger than the inner in only 12 hours, versus 25 hours for average ERCs. More than 15 m s-1 outflow peaked in the upper troposphere during each ERC. Relative vorticity maxima peaked at the surface but extended to middle and upper levels. Mean 200 hPa zonal velocity was often from the east, whereas ERC environments typically have zonal flow from the west. These easterlies were produced by an intense upper anticyclone slightly displaced from the center and present throughout the period of multiple ERCs. Inertial stability was low at almost all azimuths at 175 hPa near the 500 km radius during the period of interest. It is hypothesized that the reduced resistance to outflow associated with low inertial stability aloft induced deep upward motion and rapid intensification of the secondary eyewalls. The annular hurricane index of Knaff et al. (2008) showed that Hurricane Frances met all the criteria for annular hurricanes, which make up only 4% of all storms. It is argued that the annular hurricane directly resulted from the repeated ERCs following Wang’s (2008) reasoning.
Nguyen, L.T., R. Rogers, J. Zawislak, and J.A. Zhang. Assessing the influence of convective downdrafts and surface enthalpy fluxes on tropical cyclone intensity change in moderate vertical wind shear. Monthly Weather Review, 147(10):3519-3534, https://doi.org/10.1175/MWR-D-18-0461.1 2019
The thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TC) were investigated using dropsonde observations collected from 1996–2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 hours following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.
Obura, D.O., G. Aeby, N. Amornthammarong, W. Appeltans, N. Bax, J. Bishop, R.E. Brainard, S. Chan, P. Fletcher, T.A.C. Gordon, L. Gramer, M. Gudka, J. Halas, J. Hendee, G. Hodgson, D. Huang, M. Jankulak, A. Jones, T. Kimura, J. Levy, P. Miloslavich, L. Ming Chou, F.E. Muller-Karger, K. Osuka, M. Samoilys, S.D. Simpson, K. Tun, and S. Wongbusarakum. Coral reef monitoring, reef assessment technologies, and ecosystem-based management. Frontiers in Marine Science, 6:580, https://doi.org/10.3389/fmars.2019.00580 2019
Coral reefs are exceptionally biodiverse and human dependence on their ecosystem services is high. Reefs experience significant direct and indirect anthropogenic pressures, and provide a sensitive indicator of coastal ocean health, climate change, and ocean acidification, with associated implications for society. Monitoring coral reef status and trends is essential to better inform science, management and policy, but the projected collapse of reef systems within a few decades makes the provision of accurate and actionable monitoring data urgent. The Global Coral Reef Monitoring Network has been the foundation for global reporting on coral reefs for two decades, and is entering into a new phase with improved operational and data standards incorporating the Essential Ocean Variables (EOVs) (www.goosocean.org/eov) and Framework for Ocean Observing developed by the Global Ocean Observing System. Three EOVs provide a robust description of reef health: hard coral cover and composition, macro-algal canopy cover, and fish diversity and abundance. A data quality model based on comprehensive metadata has been designed to facilitate maximum global coverage of coral reef data, and tangible steps to track capacity building. Improved monitoring of events such as mass bleaching and disease outbreaks, citizen science, and socio-economic monitoring have the potential to greatly improve the relevance of monitoring to managers and stakeholders, and to address the complex and multi- dimensional interactions between reefs and people. A new generation of autonomous vehicles (underwater, surface, and aerial) and satellites are set to revolutionize and vastly expand our understanding of coral reefs. Promising approaches include Structure from Motion image processing, and acoustic techniques. Across all systems, curation of data in linked and open online databases, with an open data culture to maximize benefits from data integration, and empowering users to take action, are priorities. Action in the next decade will be essential to mitigate the impacts on coral reefs from warming temperatures, through local management and informing national and international obligations, particularly in the context of the Sustainable Development Goals, climate action, and the role of coral reefs as a global indicator. Mobilizing data to help drive the needed behavior change is a top priority for coral reef observing systems.
Poterjoy, J., L. Wicker, and M. Buehner. Progress toward the application of a localized particle filter for numerical weather prediction. Monthly Weather Review, 147(4):1107-1126, https://doi.org/10.1175/MWR-D-17-0344.1 2019
A series of papers published recently by the first author introduce a nonlinear filter that operates effectively as a data assimilation method for large-scale geophysical applications. The method uses sequential Monte Carlo techniques adopted by particle filters, which make no parametric assumptions for the underlying prior and posterior error distributions. The filter also treats the underlying dynamical system as a set of loosely coupled systems to effectively localize the effect observations have on posterior state estimates. This property greatly reduces the number of particles – or ensemble members – required for its implementation. For these reasons, the method is called the local particle filter. The current manuscript summarizes algorithmic advances made to the local particle filter following recent tests performed over a hierarchy of dynamical systems. The revised filter uses modified vector weight calculations and probability mapping techniques from earlier studies, and new strategies for improving filter stability in situations where state variables are observed infrequently with very accurate measurements. Numerical experiments performed on low-dimensional data assimilation problems provide evidence that supports the theoretical benefits of the new improvements. As a proof of concept, the revised particle filter is also tested on a high-dimensional application from a real-time weather forecasting system at the NOAA National Severe Storms Laboratory (NSSL). The proposed changes have large implications for researchers applying the local particle filter for real applications, such as data assimilation in numerical weather prediction models.
Ren, Y., J.A. Zhang, S.R. Guimond, and X. Wang. Hurricane boundary layer height relative to storm motion from GPS dropsonde composites. Atmosphere, 10(6):339, https://doi.org/10.3390/atmos10060339 2019
This study investigates the asymmetric distribution of hurricane boundary layer height scales in a storm-motion-relative framework using global positioning system (GPS) dropsonde observations. Data from a total of 1916 dropsondes collected within four times the radius of maximum wind speed of 37 named hurricanes over the Atlantic basin from 1998 to 2015 are analyzed in the composite framework. Motion-relative quadrant mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to increase with increasing radius in all four motion-relative quadrants. It is also found that the thermodynamic mixed layer depth and height of maximum tangential wind speed are within the inflow layer in all motion-relative quadrants. The inflow layer depth and height of the maximum tangential wind are both found to be deeper in the two front quadrants, and they are largest in the right-front quadrant. The difference in the thermodynamic mixed layer depth between the front and back quadrants is smaller than that in the kinematic boundary layer height. The thermodynamic mixed layer is shallowest in the right-rear quadrant, which may be due to the cold wake phenomena. The boundary layer height derived using the critical Richardson number ( Ric" role="presentation" id="MathJax-Element-1-Frame">Ric ) method shows a similar front-back asymmetry as the kinematic boundary layer height.
Rogers, R.F., C.S. Velden, J. Zawislak, and J.A. Zhang. Tropical cyclones and hurricanes: Observations. Reference Module in Earth Systems and Environmental Sciences, 25 pp., https://doi.org/10.1016/B978-0-12-409548-9.12065-2 2019
This article describes advances in airborne, spaceborne, and ground-based systems and technologies used to observe tropical cyclones (TCs), as well as the applications and products that are derived from them. These descriptions include new instrumentation onboard manned aircraft and the development and use of unmanned aircraft, new spaceborne platforms with extremely high spatial and temporal resolution and the plethora of products that exploit these capabilities, and advances in the observations of TC structure changes that accompany landfall through the use of ground-based remote sensing and in situ technologies. An exciting recent development is the increasing use of these technologies in TC-prone regions throughout the world. This global network of observations helps to improve the understanding and prediction of TCs by providing an improved specification of the atmospheric state as well as a robust evaluation of numerical models; improving situational awareness of the location, intensity, and structure of TCs; and developing and testing theories on TC structure and evolution.
Rosales, S.M., C. Sinigalliano, M. Gidley, P.R. Jones, and L.J. Gramer. Oceanographic habitat and the coral microbiomes of urban-impacted reefs. PeerJ, 7:e7552, https://doi.org/10.7717/peerj.7552 2019
Coral reefs are in decline worldwide. In response to this habitat loss, there are efforts to grow, outplant, and restore corals in many regions. The physical oceanographic habitat of corals—such as sea temperature, waves, ocean currents, and available light—is spatially heterogeneous. We therefore hypothesize that outplant location may affect microbiomes, and ultimately, coral health and restoration success. We evaluated the influence of the physical oceanographic habitat on microbes in wild Porites astreoides and Siderastrea siderea. Tissue samples were collected at four Florida reefs in March, June, and September of 2015. We estimated oceanographic conditions from moored instruments, diver observations, remote sensing data, and numerical models. We analyzed microbiomes using amplicon 16S rRNA high-throughput sequencing data. We found microbial alpha-diversity negatively correlated with in situ sea temperature (which represented both the annual cycle and upwelling), as well as modeled alongshore currents, in situ sea-level, and modeled tide. Microbial beta-diversity correlated positively with significant wave height and alongshore currents from models, remotely-sensed relative turbidity, and in situ temperature. We found that archaea from the order Marine Group II decrease with increases in significant wave height, suggesting that this taxon may be influenced by waves. Also, during times of high wave activity, the relative abundance of bacteria from the order Flavobacteriales increases, which may be due to resuspension and cross-shelf transport of sediments. We also found that bacteria from the order SAR86 increase in relative abundance with increased temperature, which suggests that this taxon may play a role in the coral microbiome during periods of higher temperature. Overall, we find that physical oceanographic variability correlates with the structure of these coral microbiomes in ways that could be significant to coral health.
Ryan, K., L. Bucci, R. Atlas, J. Delgado, and S. Murillo. Impact of Gulfstream-IV dropsondes on tropical cyclone prediction in a regional OSSE system. Monthly Weather Review, 147(8):2961-2977, https://doi.org/10.1175/MWR-D-18-0157.1 2019
Aircraft reconnaissance missions remain the primary means of collecting direct measurements of marine atmospheric conditions affecting tropical cyclone formation and evolution. The National Hurricane Center tasks the NOAA G-IV aircraft to sample environmental conditions that may impact the development of a tropical cyclone threatening to make landfall in the United States or its territories. These aircraft data are assimilated into deterministic models and used to produce real-time analyses and forecasts for a given tropical cyclone. Existing targeting techniques aim to optimize the use of reconnaissance observations and partially rely on regions of highest uncertainty in the Global Ensemble Forecast System. Evaluating the potential impact of various trade-offs in the targeting process is valuable for determining the ideal aircraft flight track for a prospective mission. AOML’s Hurricane Research Division has developed a system for performing regional Observing System Simulation Experiments (OSSEs) to assess the potential impact of proposed observing systems on hurricane track and intensity forecasting. This study focuses on improving existing targeting methods by investigating the impact of proposed aircraft observing system designs through various sensitivity studies. G-IV dropsonde retrievals were simulated from a regional Nature Run, covering the life cycle of a rapidly intensifying Atlantic hurricane. Results from sensitivity studies provide insight into improvements for real-time operational synoptic surveillance targeting for hurricanes and tropical storms, where dropsondes released closer to the vortex-environment interface provide the largest impact on the track forecast. All dropsonde configurations provide a positive 2-day impact on intensity forecasts by improving the environmental conditions known to impact tropical cyclone intensity.
Sinigalliano, C.D., I.C. Enochs, S.J. Stamates, P.R. Jones, C.M. Featherstone, M.L. Gidley, S.M. Rosales, L.J. Gramer, C. Staley, and T.P. Carsey. Water quality and coral reef monitoring along the southeast Florida coast. NOAA Technical Report, OAR-AOML-47, https://doi.org/10.25923/aanj-0912 2019
This 3-year project was designed to assist in providing data for use in the development of nutrient numeric criteria, as required by the Florida Department of Environmental Protection. Researchers with AOML's Ocean Chemistry and Ecosystems Division conducted field work during the first 2 years of the project, followed by the development of various deliverables, including this final report, which describes in detail four separate efforts: (1) water quality cruises; (2) ocean current measurements; and (3) coral assessments; and (4) microbiological assessments.
Smith, A.W., B.K. Haus, and J.A. Zhang. Stability and sea state as limiting conditions for TKE dissipation and dissipative heating. Journal of the Atmospheric Sciences, 76(3):689-706, https://doi.org/10.1175/JAS-D-18-0142.1 2019
This study analyzes high-resolution ship data collected in the Gulf of Mexico during the LAgrangian Submesoscale ExpeRiment (LASER) from January-February 2016 to produce the first reported measurements of dissipative heating in the explicitly non-hurricane atmospheric surface layer. Although typically computed from theory as a function of wind speed cubed, the dissipative heating directly estimated via the turbulent kinetic energy (TKE) dissipation rate is also presented. The dissipative heating magnitude agreed with a previous study that estimated the dissipative heating in the hurricane boundary layer using in-situ aircraft data. Our observations that the 10-meter neutral drag coefficient parameterized using TKE dissipation rate approaches zero slope as wind increases suggests that TKE dissipation and dissipative heating are constrained to a physical limit. Both surface-layer stability and sea state were observed to be important conditions influencing dissipative heating, with the stability determined via TKE budget terms and sea state determined via wave steepness and age using direct shipboard measurements. Momentum and enthalpy fluxes used in the TKE budget are determined using the eddy-correlation method. It is found that the TKE dissipation rate and the dissipative heating are largest in a non-neutral atmospheric surface layer with a sea surface comprised of steep windsea and slow swell waves at a given surface wind speed, whereas the ratio of dissipative heating to enthalpy fluxes is largest in near-neutral stability where the turbulent vertical velocities are near zero.
Sun, Z., B. Zhang, J.A. Zhang, and W. Perrie. Examination of surface wind asymmetry in tropical cyclones over the northwest Pacific Ocean using SMAP observations. Remote Sensing, 11(22):2604, https://doi.org/10.3390/rs11222604 2019
Tropical cyclone (TC) surface wind asymmetry is investigated by using wind data acquired from an L-band passive microwave radiometer onboard the NASA Soil Moisture Active Passive (SMAP) satellite between 2015 and 2017 over the Northwest Pacific (NWP) Ocean. The azimuthal asymmetry degree is defined as the factor by which the maximum surface wind speed is greater than the mean wind speed at the radius of the maximum wind (RMW). We examined storm motion and environmental wind shear effects on the degree of TC surface wind asymmetry under different intensity conditions. Results show that the surface wind asymmetry degree significantly decreases with increasing TC intensity, but increases with increasing TC translation speed, for tropical storm and super typhoon strength TCs; whereas no such relationship is found for typhoon and severe typhoon strength TCs. However, the degree of surface wind asymmetry increases with increasing wind shear magnitude for all TC intensity categories. The relative strength between the storm translation speed and the wind shear magnitude has the potential to affect the location of the maximum wind speed. Moreover, the maximum degree of wind asymmetry is found when the direction of the TC motion is nearly equal to the direction of the wind shear.
Zhang, J.A., and R.F. Rogers. Effects of parameterized boundary layer structure on hurricane rapid intensification in shear. Monthly Weather Review, 147(3):853-871, https://doi.org/10.1175/MWR-D-18-0010.1 2019
This study investigates the role of the parameterized boundary-layer structure in hurricane intensity change using two retrospective HWRF forecasts of Hurricane Earl (2010) in which the vertical eddy diffusivity (Km) was modified during physics upgrades. Earl undergoes rapid intensification (RI) in the low-Km forecast as observed in nature, while it weakens briefly before resuming a slow intensification at the RI onset in the high-Km forecast. Angular momentum budget analysis suggests that Km modulates the convergence of angular momentum in the boundary layer, which is a key component of the hurricane spin-up dynamics. Reducing Km in the boundary layer causes enhancement of both the inflow and convergence, which in turn leads to stronger and more symmetric deep convection in the low-Km forecast than in the high-Km forecast. The deeper and stronger hurricane vortex with lower static stability in the low-Km forecast is more resilient to shear than that in the high-Km forecast. With a smaller vortex tilt in the low- Km forecast, downdrafts associated with the vortex tilt are reduced, bringing less low-entropy air from the mid-levels to the boundary layer, resulting in a less stable boundary layer. Future physics upgrades in operational hurricane models should consider this chain of multiscale interactions to assess their impact on model RI forecasts.
Zhu, P., B. Tyner, J.A. Zhang, E. Aligo, S. Gopalakrishnan, F.D. Marks, A. Mehra, and V. Tallapragada. Role of eyewall and rainband eddy forcing in tropical cyclone intensification. Atmospheric Chemistry and Physics, 19(22):14,289-14,310, https://doi.org/10.5194/acp-19-14289-2019 2019
While turbulence is commonly regarded as a flow feature pertaining to the planetary boundary layer (PBL), intense turbulent mixing generated by cloud processes also exists above the PBL in the eyewall and rainbands of a tropical cyclone (TC). The in-cloud turbulence above the PBL is intimately involved in the development of convective elements in the eyewall and rainbands and consists of a part of asymmetric eddy forcing for the evolution of the primary and secondary circulations of a TC. In this study, we show that the Hurricane Weather Research and Forecasting (HWRF) model, one of the operational models used for TC prediction, is unable to generate appropriate sub-grid-scale (SGS) eddy forcing above the PBL due to a lack of consideration of intense turbulent mixing generated by the eyewall and rainband clouds. Incorporating an in-cloud turbulent-mixing parameterization in the vertical turbulent-mixing scheme notably improves the HWRF model's skills in predicting rapid changes in intensity for several past major hurricanes. While the analyses show that the SGS eddy forcing above the PBL is only about one-fifth of the model-resolved eddy forcing, the simulated TC vortex inner-core structure, secondary overturning circulation, and the model-resolved eddy forcing exhibit a substantial dependence on the parameterized SGS eddy processes. The results highlight the importance of eyewall and rainband SGS eddy forcing to numerical prediction of TC intensification, including rapid intensification at the current resolution of operational models.
Zou, Z., J. Song, P. Li, J. Huang, J.A. Zhang, Z. Wan, and S. Li. Effects of swell waves on atmospheric boundary layer turbulence: A low wind field study. Journal of Geophysical Research-Oceans, 124(8)5671-5685, https://doi.org/10.1029/2019JC015153 2019
The effect of swell waves on atmospheric boundary layer turbulence under low winds was explored using data from a fixed platform located in the South China Sea. The wind spectra, cospectra, and Ogive curve measured at a height of 8 m above the mean sea surface provided direct evidence that wind stress was affected by swell waves. To interpret such phenomena, an improved approach was derived based on the fact that the total wind stress was the vector sum of turbulent stress and wave‐coherent stress. Different from the approaches of earlier studies, our approach did not align the turbulent stress with the mean wind speed. The influence of swell waves on the magnitude and direction of the total wind stress was analyzed using our approach. The results showed that the wave‐coherent stress derived from our data accounted for 32% of the total wind stress. The magnitude and angle of the wind stress changed by swell waves depended on the relative angle between the turbulent stress and swell direction.
2018
Annane, B., B. McNoldy, S.M. Leidner, R. Hoffman, R. Atlas, and S.J. Majumdar. A study of the HWRF analysis and forecast impact of realistically simulated CYGNSS observations assimilated as scalar wind speeds and as VAM wind vectors. Monthly Weather Review, 146(7):2221-2236, https://doi.org/10.1175/MWR-D-17-0240.1 2018
In preparation for the launch of the NASA Cyclone Global Navigation Satellite System (CYGNSS), a variety of observing system simulation experiments (OSSEs) were conducted to develop, tune, and assess methods of assimilating these novel observations of ocean surface winds. From a highly detailed and realistic hurricane nature run (NR), CYGNSS winds were simulated with error characteristics that are expected to occur in reality. The OSSE system makes use of NOAA’s HWRF model and GSI data assimilation system in a configuration that was operational in 2012. CYGNSS winds were assimilated as scalar wind speeds and as wind vectors determined by a Variational Analysis Method (VAM). Both forms of wind information had positive impacts on the short-term HWRF forecasts, as shown by key storm and domain metrics. Data assimilation cycle intervals of 1, 3, and 6 hours were tested, and the 3-h impacts were consistently best. One day forecasts from CYGNSS VAM vector winds were the most dynamically consistent with the NR. The OSSEs have a number of limitations, most noteworthy that this is a case study and static background error covariances were used.
Bell, G.D., E.S. Blake, C.W. Landsea, S.B. Goldenberg, and R.J. Pasch. The tropics—Atlantic basin. In State of the Climate in 2017, J. Blunden, D.S. Arndt, and G. Hartfield (eds.). Bulletin of the American Meteorological Society, 99(8):S114-S118, https://doi.org/10.1175/2018BAMSStateoftheClimate.1 2018
Bhalachandran, S., Z.S. Haddad, S. Hristova-Veleva, and F.D. Marks. A low-wavenumber analysis of the environmental and vortex-scale variables responsible for rapid intensity changes in landfalling tropical cyclones. Proceedings, SPIE Symposium on Remote Sensing and Modeling of the Atmosphere, Oceans, and Interactions, Honolulu, HI, September 24-26, 2018. International Society for Optics and Photonics, SPIE Vol. 10782, https://doi.org/10.1117/12.2500290 2018
Forecasting rapid intensity changes in tropical cyclones (TCs) is hard as the factors responsible span many scales. External and internal dynamical and thermodynamical variables act simultaneously in a nonlinear fashion, either complementing, amplifying, inhibiting or not impacting the TC intensity at all. We try to address the following question: What is the relative importance of the external and vortex-scale variables that influence rapid intensity changes within a TC? Further, which of these variables must be prioritized from an observational standpoint? To answer these questions, a systematic analysis was conducted on a large number of representative TCs to make statistically significant conclusions using discriminant analyses of wavenumber (WN) - filtered fields, with a principal component analysis to detect over-fitting and identify the subset of variables (from the environment and the vortex) consistently correlated with rapid intensity change. Our analyses indicate that a small number of variables wield the most influence on TC rapid intensity changes. The most important variables within the vortex are the WN 0 of precipitation within the radius of maximum winds, the amplitudes of WN 1 of precipitation and the mid-level horizontal moisture flux convergence in the rain band region. Likewise, the most important environmental variables are the angle of the driest air from the shear vector and the magnitude of environmental wind shear. These variables must be prioritized in future observational and consequent data assimilation efforts.
Blackwell, W.J., S. Braun, R. Bennartz, C. Velden, M. DeMaria, R. Atlas, J. Dunion, F. Marks, R. Rogers, B. Annane, and R.V. Leslie. An overview of the TROPICS NASA Earth Venture mission. Quarterly Journal of the Royal Meteorological Society, 141(S1):16-26, https://doi.org/10.1002/qj.3290 2018
The Time‐Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission was selected by NASA as part of the Earth Venture‐Instrument (EVI‐3) program. The overarching goal for TROPICS is to provide nearly all‐weather observations of 3‐D temperature and humidity, as well as cloud ice and precipitation horizontal structure, at high temporal resolution to conduct high‐value science investigations of tropical cyclones. TROPICS will provide rapid‐refresh microwave measurements (median refresh rate better than 60 minutes for the baseline mission) that can be used to observe the thermodynamics of the troposphere and precipitation structure for storm systems at the mesoscale and synoptic scale over the entire storm lifecycle. TROPICS comprises six CubeSats in three low‐Earth orbital planes. Each CubeSat will host a high performance radiometer to provide temperature profiles using seven channels near the 118.75 GHz oxygen absorption line, water vapor profiles using three channels near the 183 GHz water vapor absorption line, imagery in a single channel near 90 GHz for precipitation measurements (when combined with higher resolution water vapor channels), and a single channel near 205 GHz that is more sensitive to precipitation‐sized ice particles. This observing system offers an unprecedented combination of horizontal and temporal resolution to measure environmental and inner‐core conditions for tropical cyclones on a nearly global scale and is a major leap forward in the temporal resolution of several key parameters needed for assimilation into advanced data assimilation systems capable of utilizing rapid‐update radiance or retrieval data. Launch readiness is currently projected for late 2019.
Boukabara, S.-A., K. Ide, N. Shahroudi, Y. Zhou, T. Zhu, R. Li, L. Cucurull, R. Atlas, S.P.F. Casey, and R.N. Hoffman. Community global Observing System Simulation Experiment (OSSE) package (CGOP): Perfect observations simulation validation. Journal of Atmospheric and Oceanic Technology, 35(1):207-226, https://doi.org/10.1175/JTECH-D-17-00771 2018
The simulation of observations—a critical CGOP component—is validated first by comparison of error-free simulated observations for the first 24 h at the start of the nature run (NR) to the real observations for those sensors that operated during that period. Sample results of this validation are presented here for existing low earth orbit (LEO) infrared (IR) and microwave (MW) brightness temperature (BT) observations, for radio occultation (RO) bending angle observations, and for various types of conventional observations. For sensors not operating at the start of the NR, a qualitative validation is obtained by comparing geographic and statistical characteristics of observations over the initial day for such a sensor and an existing similar sensor. Comparisons agree, with no significant unexplained bias, and to within the uncertainties due to real observation errors, time and space collocation differences, radiative transfer uncertainties, and differences between the NR and reality. To validate channels of a proposed future MW sensor with no equivalent existing spaceborne sensor channel, multiple linear regression is used to relate these channels to existing similar channels. The validation then compares observations simulated from the NR to observations predicted by the regression relationship applied to actual real observations of the existing channels. Overall, the CGOP simulations of error-free observations from conventional and satellite platforms that make up the global observing system are found to be reasonably accurate and suitable as a starting point for creating realistic simulated observations for OSSEs. These findings complete a critical step in the CGOP validation, thereby reducing the caveats required when interpreting the OSSE results.
Boukabara, S.-A., K. Ide, Y. Zhou, N. Shahroudi, R.N. Hoffman, K. Garrett, V. Krishna Kumar, T. Zhu, and R. Atlas. Community Global Observing System Simulation Experiment (OSSE) package (CGOP): Assessment and validation of the OSSE system using an OSSE-OSE intercomparison of summary assessment metrics. Journal of Atmospheric and Oceanic Technology, 35(10):2061-2078, https://doi.org/10.1175/JTECH-D-18-0061.1 2018
Observing systems simulation experiments (OSSEs) are used to simulate and assess the impacts of new observing systems planned for the future or the impacts of adopting new techniques for exploiting data or for forecasting. This study focuses on impacts of satellite data on global numerical weather prediction (NWP) systems. Since OSSEs are based on simulations of nature and observations, reliable results require that the OSSE system be validated. This validation involves cycles of assessment and calibration of the individual system components, as well as the complete system, with the end goal of reproducing the behavior of real-data observing systems experiments (OSEs). This study investigates the accuracy of the calibration of an OSSE system—here, the Community Global OSSE Package (CGOP) system—before any explicit tuning has been performed by performing an intercomparison of the OSSE summary assessment metrics (SAMs) with those obtained from parallel real-data OSEs. The main conclusion reached in this study is that, based on the SAMs, the CGOP is able to reproduce aspects of the analysis and forecast performance of parallel OSEs despite the simplifications employed in the OSSEs. This conclusion holds even when the SAMs are stratified by various subsets (i.e., the tropics only, temperature only, ...).
Bowers, G.S., D.M. Smith, N.A. Kelley, G.F. Martinez-McKinney, S.A. Cummer, J.R. Dwyer, S. Heckman, R.H. Holzworth, F. Marks, P. Reasor, J. Gamache, J. Dunion, T. Richards, and H.K. Rassoul. A terrestrial gamma-ray flash inside the eyewall of Hurricane Patricia. Journal of Geophysical Research-Atmospheres, 123(10):4977-4987, https://doi.org/10.1029/2017JD027771 2018
On 23 October 2015 at ~1732 UTC, the Airborne Detector for Energetic Lightning Emissions (ADELE) flew through the eyewall of Hurricane Patricia aboard the National Oceanic and Atmospheric Administration’s Hurricane Hunter WP‐3D Orion, aircraft, observing the first terrestrial gamma‐ray flash (TGF) ever seen in that context and the first ever viewed from behind the forward direction of the main TGF gamma‐ray burst. ADELE measured 184 counts of ionizing radiation within 150 μs, coincident with the detection of a nearby lightning flash. Lightning characteristics inferred from the associated radio signal and comparison of the gamma‐ray energy spectrum to simulations suggest that this is the first observation of a reverse beam of positrons predicted by the leading TGF production model, relativistic runaway electron avalanches. This paper presents the first experimental evidence of a previously predicted second component of gamma‐ray emission from TGFs. The brightest emission, commonly observed from orbit, is from the relativistic runaway electron avalanche bremsstrahlung; the second, fainter component reported here is from the bremsstrahlung of positrons propagating in the reverse direction. This reverse gamma‐ray beam penetrates to low enough altitudes to allow ground‐based detection of typical upward TGFs from mountain observatories.
Brammer, A., C.D. Thorncroft, and J.P. Dunion. Observations and predictability of a nondeveloping tropical disturbance over the eastern Atlantic. Monthly Weather Review, 146(9):3079-3096, https://doi.org/10.1175/MWR-D-18-0065.1 2018
A strong African easterly wave (AEW) left the west African coast in early September 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern Atlantic, despite the presence of a large region of dry air northwest of the disturbance. Analysis and in-situ observations show that after leaving the coast, the closed circulation associated with the AEW trough was not well aligned vertically and therefore low-level or mid-level dry air was advected below or above (respectively) areas of closed circulation. GPS dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant. This advection of dry air constrains the spatial extent of deep convection within the AEW trough, leading to the vortex decaying. As the column continues to be displaced horizontally, losing vertical alignment, this enables increased horizontal advection of dry air into the system further limiting convective activity. Ensemble forecasts indicate that short-term errors in precipitation rate and vorticity generation can lead to an over intensified and well aligned vortex which then interacts less with the unfavorable environment, allowing for further convection and intensification. The stronger vortex provides more favorable conditions for precipitation through a more vertically coherent closed circulation and thus a positive feedback loop is initiated. The short-term forecasts of precipitation were shown to be sensitive to lower tropospheric moisture anomalies around the AEW trough through ensemble sensitivity analysis from Global Ensemble Forecast System real-time forecasts.
Bucci, L.R., C. O’Handley, G.D. Emmitt, J.A. Zhang, K. Ryan, and R. Atlas. Validation of an airborne Doppler wind lidar in tropical cyclones. Sensors, 18(12):4288, https://doi.org/10.3390/s18124288 2018
This study presents wind observations from an airborne Doppler Wind Lidar (ADWL) in 2016 tropical cyclones (TC). A description of ADWL measurement collection and quality control methods is introduced for the use in a TC environment. Validation against different instrumentation onboard the National Oceanographic and Atmospheric Administration’s WP-3D aircraft shows good agreement of the retrieved ADWL measured wind speed and direction. Measurements taken from instruments such as the global positioning system dropsonde, flight-level wind probe, tail Doppler radar, and Stepped Frequency Microwave Radiometer are compared to ADWL observations by creating paired datasets. These paired observations represent independent measurements of the same observation space through a variety of mapping techniques that account for differences in measurement procedure. Despite high correlation values, outliers are identified and discussed in detail. The errors between paired observations appear to be caused by differences in the ability to capture various length scales, which directly relate to certain regions in a TC regime. In validating these datasets and providing evidence that shows the mitigation of gaps in 3-dimensional wind representation, the unique wind observations collected via ADWL have significant potential to impact numerical weather prediction of TCs.
Cheung, K., Z. Yu, R.L. Elsberry, M. Bell, H. Jiang, T.C. Lee, K.-C. Lu, Y. Oikawa, L. Qi, R.F. Rogers, and K. Tsuboki. Recent advances in research and forecasting of tropical cyclone rainfall. Tropical Cyclone Research and Review, 7(2):106-127, https://doi.org/10.6057/2018TCRR02.03 2018
In preparation for the Fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-IV), a summary of recent research studies and the forecasting challenges of tropical cyclone (TC) rainfall has been prepared. The extreme rainfall accumulations in Hurricane Harvey (2017) near Houston, Texas and Typhoon Damrey (2017) in southern Vietnam are examples of the TC rainfall forecasting challenges. Some progress is being made in understanding the internal rainfall dynamics via case studies. Environmental effects such as vertical wind shear and terrain-induced rainfall have been studied, as well as the rainfall relationships with TC intensity and structure. Numerical model predictions of TC-related rainfall have been improved via data assimilation, microphysics representation, improved resolution, and ensemble quantitative precipitation forecast techniques. Some attempts have been made to improve the verification techniques as well. A basic forecast challenge for TC-related rainfall is monitoring the existing rainfall distribution via satellite or coastal radars, or from over-land rain gauges. Forecasters also need assistance in understanding how seemingly similar landfall locations relative to the TC experience different rainfall distributions. In addition, forecasters must cope with anomalous TC activity and landfall distributions in response to various environmental effects.
Christophersen, H., A. Aksoy, J. Dunion, and S. Aberson. Composite impact of Global Hawk unmanned aircraft dropwindsondes on tropical cyclone analyses and forecasts. Monthly Weather Review, 146(7):2297-2314, https://doi.org/10.1175/MWR-D-17-0304.1 2018
The impacts of Global Hawk (GH) dropwindsondes on tropical cyclone (TC) analyses and forecasts are examined over a composite sample of missions flown during the NASA Hurricane and Severe Storm Sentinel (HS3) and the NOAA Sensing Hazards with Operational Unmanned Technology (SHOUT) field campaigns. An ensemble Kalman filter is employed to assimilate the dropwindsonde observations at the vortex scale. With the assimilation of GH dropwindsondes, TCs generally exhibit less position and intensity errors, a better wind-pressure relationship, and improved representation of integrated kinetic energy in the analyses. The resulting track and intensity forecasts with all the cases generally show a positive impact when GH dropwindsondes are assimilated. The impact of GH dropwindsondes is further explored with cases stratified for intensity change and presence of crewed aircraft data. GH dropwindsondes demonstrate a larger impact for non-steady-state TCs (non-SS; 24-h intensity change larger than 20 kt) than for steady state (SS) TCs. The relative skill from assimilating GH dropwindsondes ranges between 25-35% for either the position or intensity improvement in the final analyses overall, but only up to 10% for SS cases alone. The resulting forecasts for non-SS cases show higher skill for both track and intensity than SS cases. In addition, the GH dropwindsonde impact on TC forecasts varies in the presence of crewed aircraft data. An increased intensity improvement at long lead times is seen when crewed aircraft data are absent. This demonstrates the importance of strategically designing flight patterns to exploit the sampling strengths of the GH and crewed aircraft in order to maximize data impacts on TC prediction.
Christopherson, H., R. Atlas, A. Aksoy, and J. Dunion. Combined use of satellite observations and Global Hawk unmanned aircraft dropwindsondes for improved tropical cyclone analyses and forecasts. Weather and Forecasting, 33(4):1021-1031, https://doi.org/10.1175/WAF-D-17-0167.1 2018
This study demonstrates that Global Hawk unmanned aircraft system dropwindsondes and Atmospheric Infrared Sounder (AIRS) observations can be complementary in sampling a tropical cyclone (TC). The assimilation of both datasets in a regional ensemble data assimilation system shows that the cumulative impact of both datasets is greater than either one alone due to the presence of mutually independent information content. The experiment that assimilates both datasets has smaller position and intensity errors in the mean analysis than the ones with individual datasets. The improvements in track and intensity forecasts that result from combining both datasets also indicate synergistic benefits. Overall, superior track and intensity forecasts are evident. This study suggests that polar-orbiting satellite spatial coverage should be considered in operational reconnaissance mission planning in order to achieve further improvements in TC analyses and forecasts.
Cucurull, L., R. Atlas, R. Li, M.J. Mueller, and R.N. Hoffman. An observing system simulation experiment with a constellation of radio occultation satellites. Monthly Weather Review, 146(12):4247-4259, https://doi.org/10.1175/MWR-D-18-0089.1 2018
Experiments with a global Observing System Simulation Experiment (OSSE) system based on the recent 7-km resolution NASA nature run known as the G5NR were conducted to determine the potential value of proposed Global Navigation Satellite System (GNSS) radio occultation (RO) constellations in current operational numerical weather prediction systems. The RO observations were simulated with the geographic sampling expected from the original planned Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) system, with 6 equatorial (total of ~6,000 soundings/day) and 6 polar (total of ~6,000 soundings/day) receiver satellites. The experiments also accounted for the expected improved vertical coverage provided by the Jet Propulsion Laboratory RO receivers onboard COSMIC-2. Except that RO observations were simulated and assimilated as refractivities, the 2015-year version of the NCEP’s operational data assimilation system was used to run the OSSEs. The OSSEs quantified the impact of RO observations on global weather analyses and forecasts, and the impact of adding explicit errors to the simulation of perfect RO profiles. The inclusion or exclusion of explicit errors had small statistically insignificant impacts on results. The impact of RO observations was found to increase the length of the useful forecasts. In experiments with explicit errors, these increases were found to be 0.6 hours in the Northern Hemisphere extratropics (a 0.4% improvement), 5.9 hours in the Southern Hemisphere extratropics (a significant 4.0% improvement), and 12.1 hours in the Tropics (a very substantial 28.4% improvement).
Didlake, A.C., P.D. Reasor, R.F. Rogers, and W.-C. Lee. Dynamics of the transition from spiral rainbands to a secondary eyewall in Hurricane Earl (2010). Journal of the Atmospheric Sciences, 75(9):2909-2929, https://doi.org/10.1175/JAS-D-17-0348.1 2018
Airborne Doppler radar captured the inner core of Hurricane Earl during the early stages of secondary eyewall formation (SEF), providing needed insight into the SEF dynamics. An organized rainband complex outside of the primary eyewall transitioned into an axisymmetric secondary eyewall containing a low-level tangential wind maximum. During this transition, the downshear-left quadrant of the storm exhibited several notable features. A mesoscale descending inflow (MDI) jet persistently occurred across broad stretches of stratiform precipitation in a pattern similar to previous studies. This negatively buoyant jet travelled radially inward and descended into the boundary layer. Radially inward, enhanced low-level inflow and intense updrafts appeared. The updraft adjacent to the MDI was likely triggered by a region of convergence and upward acceleration (induced by the negatively buoyant MDI) entering the high-θe boundary layer. This updraft and the MDI in the downshear-left quadrant accelerated the tangential winds in a radial range where the axisymmetric wind maximum of the secondary eyewall soon developed. This same quadrant eventually exhibited the strongest overturning circulation and wind maximum of the forming secondary eyewall. Given these features occurring in succession in the downshear-left quadrant, we hypothesize that the MDI plays a significant dynamical role in SEF. The MDI within a mature rainband complex persistently perturbs the boundary layer, which locally forces enhanced convection and tangential winds. These perturbations provide steady low-level forcing that projects strongly onto the axisymmetric field, and forges the way for secondary eyewall development via one of several SEF theories that invoke axisymmetric dynamical interactions.
Dougherty, E.M., J. Molinari, R.F. Rogers, J.A. Zhang, and J.P. Kossin. Hurricane Bonnie (1998): Maintaining intensity during high vertical wind shear and an eyewall replacement cycle. Monthly Weather Review, 146(10):3383-3399, https://doi.org/10.1175/MWR-D-18-0030.1 2018
Hurricane Bonnie (1998) was an unusually resilient hurricane that maintained a steady-state intensity while experiencing strong (12–16 m s−1) vertical wind shear and an eyewall replacement cycle. This remarkable behavior was examined using observations from flight-level data, microwave imagery, radar, and dropsondes over the two-day period encompassing these events. Similar to other observed eyewall replacement cycles, Bonnie exhibited the development, strengthening, and dominance of a secondary eyewall while a primary eyewall decayed. However, Bonnie’s structure was highly asymmetric due to the large vertical wind shear, in contrast to the more symmetric structures observed in other hurricanes undergoing eyewall replacement cycles. It is hypothesized that the unusual nature of Bonnie’s evolution arose due to an increase in vertical wind shear from 2 to 12 m s−1 even as the storm intensified to a major hurricane in the presence of high ambient sea-surface temperatures. These circumstances allowed for the development of outer rainbands with intense convection downshear, where the formation of the outer eyewall commenced. In addition, the circulation broadened considerably during this time. The secondary eyewall developed within a well-defined beta skirt in the radial velocity profile, consistent with earlier theory. Despite the large ambient vertical wind shear, the outer eyewall steadily extended upshear, supported by 35 % larger surface wind speed upshear than downshear. The larger radius of maximum winds during and after the eyewall replacement cycle might have aided Bonnie’s resiliency directly, but also increased the likelihood that diabatic heating would fall inside the radius of maximum winds.
Guimond, S.R., J.A. Zhang, J.W. Sapp, and S.J. Frasier. Coherent turbulence in the boundary layer of Hurricane Rita (2005) during an eyewall replacement cycle. Journal of the Atmospheric Sciences, 75(9):3071-3093, https://doi.org/10.1175/JAS-D-17-0347.1 2018
The structure of coherent turbulence in an eyewall replacement cycle in Hurricane Rita (2005) is presented from novel airborne Doppler radar observations using the Imaging Wind and Rain Airborne Profiler (IWRAP). The IWRAP measurements and three-dimensional (3D) wind vector calculations at a grid spacing of 250 m in the horizontal and 30 m in the vertical reveal the ubiquitous presence of organized turbulent eddies in the lower levels of the storm. The data presented here, and the larger collection of IWRAP measurements, currently are the highest resolution Doppler radar, 3D wind vectors ever obtained in a hurricane over the open ocean. Coincident data from NOAA airborne radars, the stepped frequency microwave radiometer and flight level data help to place the IWRAP observations into context and provide independent validation. The typical characteristics of the turbulent eddies are the following: radial wavelengths of ~01–3 km (mean value is ~2 km), depths from the ocean surface up to flight level (~1.5 km), aspect ratio of ~1.3 and horizontal wind speed perturbations of 10–20 m s-1. The most intense eddy activity is located on the inner edge of the outer eyewall during the concentric eyewall stage with a shift to the inner eyewall during the merging stage. The evolving structure of the vertical wind shear is connected to this shift and together these characteristics have several similarities to boundary layer roll vortices. However, eddy momentum flux analysis reveals that high momentum air is being transported upwards, in contrast with roll vortices, with large positive values (~150 m2 s-2) found in the turbulent filaments. In the decaying, inner eyewall, elevated tangential momentum is also being transported radially outward to the intensifying, outer eyewall. These results indicate that the eddies may have connections to potential vorticity waves with possible modifications due to boundary layer shear instabilities.
Hoffman, R.N., V.K. Kumar, S.-A. Boukabara, K. Ide, F. Yang, and R. Atlas. Progress in forecast skill at three leading global operational NWP centers during 2015-2017 as seen in Summary Assessment Metrics (SAMs). Weather and Forecasting, 33(6):1661-1679, https://doi.org/10.1175/WAF-D-18-0117.1 2018
The summary assessment metric (SAM) method is applied to an array of primary assessment metrics (PAMs) for the deterministic forecasts of three leading numerical weather prediction (NWP) centers for the years 2015-2017. The PAMs include anomaly correlation, RMSE, and absolute mean error (i.e., the absolute value of bias) for different forecast times, vertical levels, geographic domains, and variables. SAMs indicate that in terms of forecast skill ECMWF is better than NCEP, which is better than but approximately the same as UKMO. The use of SAMs allows a number of interesting features of the evolution of forecast skill to be observed. All three centers improve over the three year period. NCEP short-term forecast skill substantially increases during the period. Quantitatively, the effect of the 2016 May 11 NCEP upgrade to the 4D-ensemble variational (4DEnVar) system is a 7.37% increase in the probability of improved skill relative to a randomly chosen forecast metric from 2015-2017. This is the largest SAM impact during the study period. However, the observed impacts are within the context of slowly improving forecast skill for operational global NWP as compared to earlier years. Clearly the systems lagging ECMWF can improve, and there is evidence from SAMs in addition to the 4DEnVar example that improvements in forecast and data assimilation systems are still leading to forecast skill improvements.
Hoffmann, R.N. The effect of thinning and superobservations in a simple one-dimensional data analysis with mischaracterized error. Monthly Weather Review, 146(4):1181-1195, https://doi.org/10.1175/MWR-D-17-0363.1 2018
A one-dimensional (1d) analysis problem is defined and analyzed to explore the interaction of observation thinning or superobservation with observation errors that are correlated or systematic. The general formulation might be applied to a 1d analysis of radiance or radio occultation observations in order to develop a strategy for the use of such data in a full data assimilation system, but is applied here to a simple analysis problem with parameterized error covariances. Findings for the simple problem include the following. For a variational analysis method that includes an estimate of the full observation error covariances, the analysis is more sensitive to variations in the estimated background and observation error standard deviations than in the corresponding correlation length scales. Furthermore, if everything else is fixed, the analysis error increases with decreasing true background error correlation length scale and with increasing true observation error correlation length scale. For a weighted least squares analysis method that assumes the observation errors are uncorrelated, best results are obtained for some degree of thinning and/or tuning of the weights. Without tuning, the best strategy is superobservation with a spacing approximately equal to the observation error correlation length scale.
Holbach, H.M., E.W. Uhlhorn, and M.A. Bourassa. Off-nadir SFMR brightness temperature measurements in high-wind conditions. Journal of Atmospheric and Oceanic Technology, 35(9):1865-1879, https://doi.org/10.1175/JTECH-D-18-0005.1 2018
Wind and wave-breaking directions are investigated as potential sources of an asymmetry identified in off-nadir, remotely sensed measurements of ocean surface brightness temperatures obtained by the Stepped-Frequency Microwave Radiometer (SFMR) in high-wind conditions, including in tropical cyclones. Surface wind speed, which dynamically couples the atmosphere and ocean, can be inferred from SFMR ocean surface brightness temperature measurements using a radiative transfer model and inversion algorithm. The accuracy of the ocean surface brightness temperature to wind speed calibration relies on accurate knowledge of the surface variables that are influencing the ocean surface brightness temperature. Previous studies have identified wind direction signals in horizontally polarized radiometer measurements in low to moderate (0-20 m s-1) wind conditions over a wide range of incidence angles. This study finds that the azimuthal asymmetry in the off-nadir SFMR brightness temperature measurements is also likely a function of wind direction and extends the results of these previous studies to high-wind conditions. The off-nadir measurements from the SFMR provide critical data for improving the understanding of the relationships between brightness temperature, surface wave-breaking direction, and surface wind vectors at various incidence angles, which are extremely useful for the development of geophysical model functions for instruments like the Hurricane Imaging Radiometer (HIRad).
Huang, L., X. Li, B. Liu, J.A. Zhang, D. Shen, Z. Zhang, and W. Yu. Tropical cyclone boundary layer rolls in synthetic aperture radar imagery. Journal of Geophysical Research-Oceans, 123(4):2981-2996, https://doi.org/10.1029/2018JC013755 2018
Marine atmospheric boundary layer (MABL) roll plays an important role in the turbulent exchange of momentum, sensible heat, and moisture throughout MABL of tropical cyclone (TC). Hence, rolls are believed to be closely related to a TC’s development, intensification, and decay processes. Spaceborne synthetic aperture radar (SAR) provides a unique capability to image the sea surface imprints of quasi‐linear streaks induced by the MABL rolls within a TC. In this study, 16 SAR images, including three images acquired during three major hurricanes—Irma, Jose, and Maria in the 2017 Atlantic hurricane season—were used to systematically map the distribution and wavelength of MABL rolls under the wide range of TC intensities. The images were acquired by SAR onboard RADARSAT‐1/2, ENVISAT, and SENTINEL‐1 satellites. Our findings are in agreement with the previous one case study of Hurricane Katrina (2005), showing the roll wavelengths are between 600 and 1,600 m. We also find that there exist roll imprints in the eyewall and rainbands, although the boundary layer heights are shallower there. Besides, the spatial distribution of roll wavelengths is asymmetrical. The roll wavelengths are found to be the shortest around the storm center, increase and then decrease with distance from storm center, reaching the peak values in the range of d*‑ 2d*, where d* is defined as the physical location to TC centers normalized by the radius of maximum wind. These MABL roll characteristics cannot be derived using conventional aircraft and land‐based Doppler radar observations.
Kieu, C., K. Keshavamurthy, V. Tallapragada, S. Gopalakrishnan, and S. Trahan. On the growth of intensity forecast errors in the operational Hurricane Weather Research and Forecasting (HWRF) model. Quarterly Journal of the Royal Meteorological Society, 144(715):1803-1819, https://doi.org/10.1002/qj.3344 2018
This study examines the growth of tropical cyclone (TC) intensity forecast errors and related intensity predictability for the NOAA operational Hurricane Weather Research and Forecasting (HWRF) model. Using operational intensity forecasts during the 2012 to 2016 seasons, two conditions for a limited range of TC intensity predictability are demonstrated, which include (a) the existence of an intensity error saturation limit, and (b) the dependence of the intensity error growth rate on storm intensity during TC development. By stratifying intensity errors based on different initial intensity bins, it is shown that TC intensity error growth rate is relatively small (∼0.3 kt h−1) at the early stage of TC development, but it quickly increases to ∼1 kt h−1 during TC intensification. Of further importance is that the intensity error saturation varies in the range of 14–18 kt in different ocean basins, thus suggesting the potential dependence of the intensity predictability on large‐scale environment. Additional idealized experiments with the HWRF model confirm the saturation of intensity errors, even under a perfect model scenario. The existence of the intensity error saturation together with the finding of a faster error growth rate for higher intensity suggests that the TC dynamics possesses an inherent limited predictability, which prevents us from reducing the intensity errors in TC dynamical models below a certain threshold.
Kren, A.C., L. Cucurull, and H. Wang. Impact of UAS Global Hawk dropsonde data on tropical and extratropical cyclone forecasts in 2016. Weather and Forecasting, 33(5):1121-1141, https://doi.org/10.1175/WAF-D-18-0029.1 2018
A preliminary investigation into the impact of dropsonde observations from the Global Hawk (GH) on tropical and extratropical forecasts is performed using the National Centers for Environmental Prediction (NCEP) Global Data Assimilation System (GDAS). Experiments are performed during high-impact weather events which were sampled as part of the Sensing Hazards with Operational Unmanned Technology (SHOUT) field campaigns in 2016: (1) three extratropical systems in February 2016, and (2) Hurricanes Matthew and Nicole in the western Atlantic. For these events, the benefits of GH observations under a satellite data gap scenario are also investigated. It is found that the assimilation of GH dropsondes reduces the track error for both Matthew and Nicole; the improvements are as high as 20% beyond 60 hours. Additionally, the localized dropsondes reduce global forecast track error for four tropical cyclones by up to 9%. Results are mixed under a satellite gap scenario, where only Hurricane Matthew is improved from assimilated dropsondes. The improved storm track is attributed to a better representation of the steering flow and atmospheric mid-level pattern. For all cases, dropsondes reduce the root-mean-squared error in temperature, relative humidity, wind, and sea-level pressure by 3-8% out to 96 hours. Additional benefits from GH dropsondes are obtained in precipitation, with higher skill scores over the southeastern United States versus control forecasts of up to 8%, as well as for low-level parameters important for severe weather prediction. The findings from this study are preliminary and, therefore, more cases are needed for statistical significance.
Leidner, S.M., B. Annane, B. McNoldy, R. Hoffman, and R. Atlas. Variational analysis of simulated ocean surface winds from the Cyclone Global Navigation Satellite System (CYGNSS) and evaluation using a regional OSSE. Journal of Atmospheric and Oceanic Technology, 35(8):1571-1584, https://doi.org/10.1175/JTECH-D-17-0136.1 2018
A positive impact of adding directional information to observations from the CYclone Global Navigation Satellite System (CYNGSS) constellation of microsatellites is observed in simulation using a high-resolution nature run of an Atlantic hurricane for a 4-day period. Directional information is added using a 2-dimensional variational analysis method (VAM) for near-surface vector winds that blends simulated CYGNSS wind speeds with an a priori background vector wind field at 6-hour analysis times. The resulting wind vectors at CYGNSS data locations are more geophysically self-consistent when using high-resolution, 6-hour forecast backgrounds from a Hurricane Weather Research and Forecast (HWRF) Control Observing System Simulation Experiment (OSSE) compared to low-resolution 6-hour forecasts from an associated Global Forecast System (GFS) model Control OSSE. An important contributing factor is the large displacement error in the center of circulation in the GFS background wind fields that produces asymmetric circulations in the associated VAM analyses. Results of a limited OSSE indicate that CYGNSS winds reduce forecast error in hurricane intensity in 0-48 hour forecasts compared to using no CYGNSS data. Assimilation of VAM-CYGNSS vector winds reduces maximum wind speed error by 2-5 kts and reduces minimum central pressure error by 2-5 hPa. The improvement in forecast intensity is notably larger and more consistent than the reduction in track error. Assimilation of VAM-CYGNSS wind vectors constrains analyses of surface wind field structures during OSSE more effectively than wind speeds alone. Due to incomplete sampling and the limitations of the data assimilation system used, CYGNSS scalar winds produce unwanted wind/pressure imbalances and asymmetries more often than the assimilation of VAM-CYGNSS data.
Leighton, H., S. Gopalakrishnan, J.A. Zhang, R.F. Rogers, Z. Zhang, and V. Tallapragada. Azimuthal distribution of deep convection, environmental factors and tropical cyclone rapid intensification: A perspective from HWRF ensemble forecasts of Hurricane Edouard (2014). Journal of the Atmospheric Sciences, 75(1):275-295, https://doi.org/10.1175/JAS-D-17-0171.1 2018
Forecasts from the operational Hurricane Weather Research and Forecasting (HWRF) based ensemble prediction system for Hurricane Edouard (2014) are analyzed to study the differences in both the tropical cyclone inner-core structure and large-scale environment between rapidly intensifying (RI) and non-intensifying (NI) ensemble members. An analysis of the inner-core structure reveals that as deep convection wraps around from the downshear side of the storm to the upshear-left quadrant for RI members, vortex tilt and asymmetry reduce rapidly and RI occurs. For NI members, deep convection stays trapped in the downshear/downshear-right quandrant, and storms do not intensify. The budget calculation of tangential wind tendency reveals that the positive radial eddy vorticity flux for RI members contributes significantly to spinning up the tangential wind in the middle and upper levels and reduces vortex tilt. The negative eddy vorticity flux for NI members spins down the tangential wind in the middle and upper levels and does not help the vortex become vertically aligned. An analysis of the environmental flow shows that the cyclonic component of the storm-relative upper-level environmental flow in the left-of-shear quadrants aids the cyclonic propagation of deep convection and helps establish the configuration that leads to the positive radial vorticity flux for RI members. In contrast, the anticyclonic component of the storm-relative middle-and-upper-level environmental flow in the left-of-shear quadrants inhibits the cyclonic propagation of deep convection and suppresses the positive radial eddy vorticity flux for NI members. Environmental moisture in the downshear-right quadrant is also shown to be important for the formation of deep convection for RI members.
Leroux, M.-D., K. Wood, R.L. Elsberry, E.O. Cayanan, E. Hendricks, M. Kucas, P. Otto, R. Rogers, B. Sampson, and Z. Yu. Recent advances in research and forecasting of tropical cyclone track, intensity, and structure at landfall. Tropical Cyclone Research and Review, 7(2):85-105, https://doi.org/10.6057/2018TCRR02.02 2018
This review prepared for the fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-4) summarizes the most recent (2015-2017) theoretical and practical knowledge in the field of tropical cyclone (TC) track, intensity, and structure rapid changes at or near landfall. Although the focus of IWTCLPIV was on landfall, this summary necessarily embraces the characteristics of storms during their course over the ocean prior to and leading up to landfall. In the past few years, extremely valuable observational datasets have been collected for TC forecasting guidance and research studies using both aircraft reconnaissance and new geostationary or low-earth orbiting satellites at high temporal and spatial resolution. Track deflections for systems near complex topography such as that of Taiwan and La Reunion have been further investigated, and advanced numerical models with high spatial resolution necessary to predict the interaction of the TC circulation with steep island topography have been developed. An analog technique has been designed to meet the need for longer range landfall intensity forecast guidance that will provide more time for emergency preparedness. Probabilistic track and intensity forecasts have also been developed to better communicate on forecast uncertainty. Operational practices of several TC forecast centers are described herein and some challenges regarding forecasts and warnings for TCs making landfall are identified. This review concludes with insights from both researchers and forecasters regarding future directions to improve predictions of TC track, intensity, and structure at landfall.
Li, Z., J. Li, P. Wang, A. Lim, J. Li, T.J. Schmit, R. Atlas, S.-A. Boukabara, and R.N. Hoffman. Value-added impact of geostationary hyperspectral infrared sounders on local severe storm forecasts—via a quick regional OSSE. Advances in Atmospheric Sciences, 35(10):1217-1230, https://doi.org/10.1007/s00376-018-8036-3 2018
Accurate atmospheric temperature and moisture information with high temporal/spatial resolution are two of the key parameters needed in regional numerical weather prediction (NWP) models to reliably predict high-impact weather events such as local severe storms (LSSs). High spectral resolution or hyperspectral infrared (HIR) sounders from geostationary orbit (GEO) provide an unprecedented source of near time-continuous, three-dimensional information on the dynamic and thermodynamic atmospheric fields—an important benefit for nowcasting and NWP-based forecasting. In order to demonstrate the value of GEO HIR sounder radiances on LSS forecasts, a quick regional OSSE (Observing System Simulation Experiment) framework has been developed, including high-resolution nature run generation, synthetic observation simulation and validation, and impact study on LSS forecasts. Results show that, on top of the existing LEO (low earth orbit) sounders, a GEO HIR sounder may provide value-added impact [a reduction of 3.56% in normalized root-mean-square difference (RMSD)] on LSS forecasts due to large spatial coverage and high temporal resolution, even though the data are assimilated every 6 h with a thinning of 60 km. Additionally, more frequent assimilations and smaller thinning distances allow more observations to be assimilated and may further increase the positive impact from a GEO HIR sounder. On the other hand, with denser and more frequent observations assimilated, it becomes more difficult to handle the spatial error correlation in observations and gravity waves due to the limitations of current assimilation and forecast systems (such as a static background error covariance). The peak reduction of 4.6% in normalized RMSD is found when observations are assimilated every 3 h with a thinning distance of 30 km.
Ming, J., and J.A. Zhang. Direct measurements of momentum flux and dissipative heating in the surface layer of tropical cyclones during landfalls. Journal of Geophysical Research-Atmospheres, 123(10):4926-4938, https://doi.org/10.1029/2017JD028076 2018
This study analyzes high‐frequency wind data collected by research towers in the surface layer of Typhoons Hagupit (2008) and Chanthu (2010) to investigate the characteristics of the momentum flux, turbulent kinetic energy (TKE), drag coefficient, and dissipative heating (DH) during landfalls. It is found that the momentum flux TKE and DH increase with the wind speed up to the maximum observed wind speed (~40 m/s), in agreement with previous studies that presented eddy correlation flux data in a similar condition but with lower maximum observed wind speed. However, the momentum flux, TKE, drag coefficient, and DH are found to be substantially larger in Typhoon Chanthu (2010) than those in Typhoon Hagupit (2008) at a given wind speed, likely due to much rougher surface conditions surrounding the tower deployed in Typhoon Chanthu (2010). Furthermore, the DH is calculated using two different methods: (1) based on surface‐layer theory; and (2) based on the standard turbulent spectra method. It is found that the first method tends to overestimate the value of DH compared to the second method, and the overestimation of the DH by the first method is much smaller over rougher underlying surface than over the smoother underlying surface. Our analysis shows that the magnitude of the DH over land is as large as the sensible heat flux (~100 W/m2) previously observed over the ocean, which should not be neglected in numerical models simulating tropical cyclones during landfalls.
Morzfeld, M., D. Hodyss, and J. Poterjoy. Variational particle smoothers and their localization. Quarterly Journal of the Royal Meteorological Society, 144(712):806-825, https://doi.org/10.1002/qj.3256 2018
Given the success of 4D‐variational methods (4D‐Var) in numerical weather prediction, and recent efforts to merge ensemble Kalman filters with 4D‐Var, we revisit how one can use importance sampling and particle filtering ideas within a 4D‐Var framework. This leads us to variational particle smoothers (varPS) and we study how weight‐localization can prevent the collapse of varPS in high‐dimensional problems. We also discuss the relevance of (localized) weights in near‐Gaussian problems. We test our ideas on the Lorenz'96 model of dimensions n = 40, n = 400, and n = 2,000. In our numerical experiments the localized varPS does not collapse and yields results comparable to ensemble formulations of 4D‐Var, while tuned EnKFs and the local particle filter lead to larger estimation errors. Additional numerical experiments suggest that using localized weights may not yield significant advantages over unweighted or linearized solutions in near‐Gaussian problems.
Munsell, E.B., F. Zhang, S.A. Braun, J.A. Sippel, and A.C. Didlake. The inner-core temperature structure of Hurricane Edouard (2014): Observations and ensemble variability. Monthly Weather Review, 146(1):135-155, https://doi.org/10.1175/MWR-D-17-0095.1 2018
The inner-core thermodynamic structure of Hurricane Edouard (2014) is explored, primarily through an examination of both high-altitude dropsondes deployed during NASA’s Hurricane and Severe Storm Sentinel (HS3) campaign and a 60-member convection-permitting ensemble initialized with an ensemble Kalman filter. The 7-day forecasts are initialized coincident with Edouard’s tropical depression designation and include Edouard’s significant intensification to a major hurricane. Ten-member ensemble groups are created based on timing of near rapid intensification (RI) onset, and the associated composite inner-core temperature structures are analyzed. It is found that at Edouard’s peak intensity, in both the observations and the simulations, the maximum inner-core perturbation temperature (~10–12 K) occurs in the mid-levels (~4–8 km). In addition, in all composite groups that significantly intensify, the evolution of the area-averaged inner-core perturbation temperatures indicate that weak to moderate warming (at most 4 K) begins to occur in the low- to mid-levels (~2–6 km) ~24–48 h prior to RI, and this warming significantly strengthens and deepens (up to ~8 km) ~24 h after RI has begun. Despite broad similarities in the evolution of Edouard’s warm core in these composites, variability in the height and strength of the maximum perturbation temperature and in the overall development of the inner-core temperature structure are present amongst the members of the composite groups (despite similar intensity time series). This result and concomitant correlation analyses suggest that the strength and height of the maximum perturbation temperature is not a significant causal factor for RI onset in this ensemble. Fluctuations in inner-core temperature structure occur either in tandem with or after significant intensity changes.
Nystrom, R.G., F. Zhang, E.B. Munsell, S.A. Braun, J.A. Sippel, Y. Weng, and K. Emanuel. Predictability and dynamics of Hurricane Joaquin (2015) explored through convection-permitting ensemble sensitivity experiments. Journal of the Atmospheric Sciences, 75(2):401-424, https://doi.org/10.1175/JAS-D-17-0137.1 2018
Real-time ensemble forecasts from the PSU WRF-EnKF system for Hurricane Joaquin (2015) are examined in this study. The ensemble forecasts from early in Joaquin’s lifecycle displayed large track spread, with nearly half of the ensemble members tracking Joaquin towards the United States east coast and the other half tracking Joaquin out to sea. The ensemble forecasts also displayed large intensity spread with many of the members developing into major hurricanes and other ensemble members not intensifying at all. Initial condition differences from the regions greater than (less than) 300 km were isolated by effectively removing initial condition differences in desired regions through relaxing each ensemble member to GFS (APSU) initial conditions. The regions of initial condition errors contributing to the track spread were examined, and the dominant source of track errors arose from the region greater than 300 km from the tropical cyclone center. Further examination of the track divergence revealed that the region between 600 and 900 km from the initial position of Joaquin was found to be the largest source of initial condition errors that contributed to this divergence. Small differences in the low-level steering flow, originating from perturbations between 600 and 900 km from the initial position, might have resulted in the bifurcation of the forecast tracks of Joaquin. The initial condition errors north of the initial position of Joaquin were also shown to contribute most significantly to the track divergence. The region inside of 300 km, specifically the initial intensity of Joaquin, was the dominant source of initial condition errors contributing to the intensity spread.
Peevey, T.R., J.M. English, L. Cucurull, H. Wang, and A.C. Kren. Improving winter storm forecasts with Observing System Simulation Experiment (OSSEs). Part 1: An idealized case study of three US storms. Monthly Weather Review, 146(5):1341-1366, https://doi.org/10.1175/MWR-D-17-0160.1 2018
Severe weather events can have a significant impact on local communities due to the loss of life and property. Forecast busts associated with high-impact weather events have been attributed to initial condition errors over data sparse regions such as the Pacific Ocean. Numerous flight campaigns have found that targeted observations over these areas can improve forecasts. To better understand the impacts of measurement type and sampling domains on forecast performance, Observing System Simulation Experiments are performed using the National Centers for Environmental Prediction Global Forecast System (GFS) with Hybrid 3DEnVar data assimilation and the ECMWF T511 Nature Run. First, three types of simulated perfect dropsonde observations (temperature, specific humidity, and wind) are assimilated into the GFS system over a large idealized sampling domain covering the Pacific Ocean. For three winter storms studied, forecast error was found to be significantly reduced with all three types of measurements providing the most benefit (~ 5-15% reduction in error). Instances when forecasts are not improved are investigated and concluded to be due to challenging meteorological structures such as cutoff lows and interactions with atmospheric structures outside the sampling domain. Second, simulated dropsondes are assimilated over sensitive areas and flight tracks established using the Ensemble Transform Sensitivity (ETS) technique. For all three winter storms, forecast error is reduced up to 5%, which is less than that found using an idealized domain. These results suggest that targeted observations over the Pacific Ocean may provide a small improvement to winter storm forecasts over the United States.
Rogers, R.F., K. Cheung, R.L. Elsberry, N. Kohno, M.-D. Leroux, and P. Otto. The World Meteorological Organization Fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-IV): A summary. Tropical Cyclone Research and Review, 7(2):77-84, https://doi.org/10.6057/2018TCRR02.01 2018
The Fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-4) was held in Macao, China from 5-7 December 2017. The workshop was organized by the World Meteorological Organization (WMO) Expert Team on Tropical Cyclone Landfall Processes in partnership with the WMO Tropical Cyclone Program. The workshop provided a forum for discussion between researchers and forecasters on the current status of tropical cyclone landfall processes and on priorities and opportunities for research. More than 60 leading research scientists and warning specialists working on topics related to tropical cyclone landfall examined current knowledge, forecasting, and research trends from an integrated global perspective. The workshop offered a number of recommendations for future forecasting studies and research with special regard to the varying needs of different tropical cyclone affected regions. The recommendations emanating from the workshop will be presented at the upcoming Ninth International Workshop on Tropical Cyclones (IWTC-9) (Hawaii, USA, 3-7 December 2018).
Steward, J.L., J.E. Roman, A. Lamas Davina, and A. Aksoy. Parallel direct solution of the covariance-localized ensemble square root Kalman filter equations with matrix functions. Monthly Weather Review, 146(9):2819-2836, https://doi.org/10.1175/MWR-D-18-0022.1 2018
Recently, the serial approach to solving the Square-Root Ensemble Kalman Filter (ESRF) equations in the presence of covariance localization was found to depend on the order of observations. As shown previously, correctly updating the localized posterior covariance in serial requires additional effort and computational expense. A recent work, Steward et al. (2017), details an all-at-once direct method to solve the ESRF equations in parallel. This method uses the eigenvectors and eigenvalues of the forward observation covariance matrix to solve the difficult portion of the ESRF equations. The remaining assimilation is easily parallelized, and the analysis does not depend on the order of observations. While this allows for long localization lengths that would render local analysis methods inefficient, in theory an eigenpair-based method scales as the cube number of observations, making it infeasible for large numbers of observations. In this work, we extend this method to use the theory of matrix functions to avoid eigenpair computations. The Arnoldi process is used to evaluate the covariance localized ESRF equations on the reduced-order Krylov subspace basis. This method is shown to converge quickly and apparently regains a linear scaling with the number of observations. The method scales similarly to the widely-used serial approach of Anderson and Collins (2007) in wall-time but not in memory usage. To improve the memory usage issue, this method potentially can be used without an explicit matrix. In addition, hybrid ensemble and climatological covariances can be incorporated.
Tang, J., J.A. Zhang, C. Kieu, and F.D. Marks. Sensitivity of hurricane intensity and structure to two types of planetary boundary layer parameterization schemes in idealized HWRF simulations. Tropical Cyclone Research and Review, 7(4):201-211, https://doi.org/10.6057/2018TCRR04.01 2018
This paper investigates the sensitivity of simulated hurricane intensity and structure to two planetary boundary layer (PBL) schemes in the Hurricane Weather and Research Forecast model including (1) the GFS scheme (control run) that uses the K-profile method to parameterize turbulent fluxes, and (2) the MYJ scheme that is based on a turbulent kinetic energy (TKE) budget for turbulent closure. Idealized simulations with these two PBL schemes show that the storm in the TKE run is stronger than that in the control run after 3 days into simulation. Multi-scale structures are evaluated and compared between the control and the TKE runs prior to the divergence of the model-simulated intensity to elucidate the mechanism underlying such a difference in the intensity between the two runs. It is found that the storm in the TKE run has (i) a shallower boundary layer with a stronger PBL inflow, (ii) stronger boundary layer convergence closer to the storm center, (iii) higher vorticity and inertial stability inside the RMW, (iv) stronger and deeper updrafts in regions further inward from the radius of maximum wind (RMW), and (v) more convective bursts located near the RMW as compared to the control run. Angular momentum budget analysis suggests that the convergence of angular momentum in the boundary layer is much stronger in the TKE run than in the control run, which is responsible for faster spin-up of the hurricane vortex in the TKE run.
Tang, J., J.A. Zhang, S.D. Aberson, F.D. Marks, and X. Lei. Multilevel tower observations of vertical eddy diffusivity and mixing length in the tropical cyclone boundary layer during landfalls. Journal of the Atmospheric Sciences, 75(9):3159-3168, https://doi.org/10.1175/JAS-D-17-0353.1 2018
This study analyzes the fast-response (20-Hz) wind data collected by a multi-level tower during the landfalls of Tropical Storm Lionrock (1006), Typhoon Fanapi (1011) and Typhoon Megi (1015) in 2010. Turbulent momentum fluxes are calculated using the standard eddy-correlation method. Vertical eddy diffusivity (Km) and mixing length are estimated using the directly measured momentum fluxes and mean-wind profiles. It is found that the momentum flux increases with wind speed at all four levels. The eddy diffusivity calculated using the direct-flux method is compared to that using a theoretical method in which the vertical eddy diffusivity is formulated as a linear function of the friction velocity and height. It is found that below ~ 60 m, Km can be approximately parameterized using this theoretical method, though this method overestimates Km for higher altitude, indicating that the surface-layer depth is close to 60 m in the tropical cyclones studied here. It is also found that Km at each level varies with wind direction during landfalls: Km estimated based on observations with landward fetch is significantly larger than that estimated using data with seaward fetch. This result suggests that different parameterizations of Km should be used in the boundary-layer schemes of numerical models forecasting tropical cyclones over land versus over the ocean.
Tong, M., J.A. Sippel, V. Tallapragada, E. Liu, C. Kieu, I.-H. Kwon, W. Wang, Q. Liu, Y. Ling, and B. Zhang. Impact of assimilating aircraft reconnaissance observations on tropical cyclone initialization and prediction using operational HWRF and GSI ensemble-variational hybrid data assimilation. Monthly Weather Review, 146(12):4155-4177, https://doi.org/10.1175/MWR-D-17-0380.1 2018
This study evaluates the impact of assimilating high-resolution inner-core reconnaissance observations on tropical cyclone initialization and prediction in the 2013 version of the operational Hurricane Weather Research and Forecasting (HWRF) model. The 2013 HWRF data assimilation system is a GSI-based hybrid ensemble-variational system that in this study uses the Global Data Assimilation System ensemble to estimate flow-dependent background error covariance. Assimilation of inner-core observations improves track forecasts and reduces intensity error after 18-24 h. The positive impact on the intensity forecast is mainly found in weak storms, where inner-core assimilation produces more accurate tropical cyclone structures and reduces positive intensity bias. Despite such positive benefits, there is degradation in short-term intensity forecasts that is attributable to spin-down of strong storms, which has also been seen in other studies. There are several reasons for the degradation of intense storms. First, a newly-discovered interaction between model biases and the HWRF vortex initialization procedure causes the first-guess wind speed aloft to be too strong in the inner core. The problem worsens for the strongest storms, leading to a poor first-guess fit to observations. Though assimilation of reconnaissance observations results in analyses that better fit the observations, it also causes a negative intensity bias at the surface. In addition, the covariance provided by the NCEP global model is inaccurate for assimilating inner-core observations, and model physics biases result in a mismatch between simulated and observed structure. The model ultimately cannot maintain the analysis structure during the forecast, leading to spin-down.
Tratt, D.M., J.A. Hackwell, B.L. Valant-Spaight, R.L. Walterscheid, L.J. Gelinas, J.H. Hecht, C.M. Swenson, C.P. Lampen, M.J. Alexander, L. Hoffman, D.S. Nolan, S.D. Miller, J.L. Hall, R. Atlas, F.D. Marks, and P.T. Partain. GHOST: A satellite mission concept for persistent monitoring of stratospheric gravity waves induced by severe storms. Bulletin of the American Meteorological Society, 99(9):1813-1828, https://doi.org/10.1175/BAMS-D-17-0064.1 2018
GHOST would continuously monitor storm-induced gravity waves, observing their development through complete storm life-cycles in order to elucidate causal relationships between storm phenomena linked to latent heating and gravity-wave production. The prediction of tropical cyclone rapid intensification is one of the most pressing unsolved problems in hurricane forecasting. The signatures of gravity waves launched by strong convective updrafts are often clearly seen in airglow and carbon dioxide thermal emission spectra under favorable atmospheric conditions. By continuously monitoring the Atlantic hurricane belt from the main development region to the vulnerable sections of the continental U.S. at high cadence it will be possible to investigate the utility of storm-induced gravity wave observations for the diagnosis of impending storm intensification. Such a capability would also enable significant improvements in our ability to characterize the 3D, transient behavior of upper atmospheric gravity waves, and point the way to future observing strategies that could mitigate the risk to human life due to severe storms. This paper describes a new mission concept involving a mid-infrared imager hosted aboard a geostationary satellite positioned at approximately 80°W longitude. The sensor’s 3-km pixel size ensures that gravity wave horizontal structure is adequately resolved, while a 30-s refresh rate enables improved definition of the dynamic intensification process. In this way the transient development of gravity wave perturbations caused by both convective and cyclonic storms may be discerned in near real-time.
Wadler, J.B., J.A. Zhang, B. Jaimes, and L.K. Shay. Downdrafts and the evolution of boundary layer thermodynamics in Hurricane Earl (2010) before and during rapid intensification. Monthly Weather Review, 146(11):3545-3565, https://doi.org/10.1175/MWR-D-18-0090.1 2018
Using a combination of NOAA P-3 aircraft tail Doppler radar, NOAA and NASA dropsondes, and buoy and drifter based sea surface temperature data, different types of downdrafts and their influence on boundary layer (BL) thermodynamics are examined in Hurricane Earl (2010) during periods prior to rapid intensification (RI; a 30 knot increase in intensity over 24 hours) and during RI. Before RI, the BL was generally warm and moist. The largest hindrances for intensification are convectively driven downdrafts inside the radius of maximum winds (RMW) and upshear-right quadrant and vortex-tilt induced downdrafts outside the RMW in the upshear-left quadrant. Possible mechanisms for overcoming the low entropy (θe) air induced by these downdrafts are BL recovery through air-sea enthalpy fluxes and turbulent mixing by atmospheric eddies. During RI, convective downdrafts of varying strengths in the upshear-left quadrant had differing effects on the low-level entropy and surface heat fluxes. Interestingly, the stronger downdrafts corresponded with maximums in 10-m θe. It is hypothesized that the large amount of evaporation in a strong (>2 m s-1) downdraft underneath a precipitation core can lead to high amounts of near-surface specific humidity. By contrast, weaker downdrafts corresponded with minimums in 10-m θe, likely because they contained lower evaporation rates. Since weak and dry downdrafts require more surface fluxes to recover the low entropy air than strong and moist downdrafts, they are greater hindrances to storm intensification. This study emphasizes how different types of downdrafts are tied to hurricane intensity change through their modification of BL thermodynamics.
Wadler, J.B., R.F. Rogers, and P.D. Reasor. The relationship between spatial variations in the structure of convective bursts and tropical cyclone intensification using airborne Doppler radar. Monthly Weather Review, 146(3):761-780, https://doi.org/10.1175/MWR-D-17-0213.1 2018
The relationship between radial and azimuthal variations in the composite characteristics of convective bursts (CBs), i.e., regions of the most intense upward motion in tropical cyclones (TCs), and TC intensity change is examined using NOAA P-3 tail Doppler radar. Aircraft passes collected over a 13-year period are examined in a coordinate system rotated relative to the deep-layer vertical wind shear vector and normalized by the low-level radius of maximum winds (RMW). The characteristics of CBs are investigated to determine how the radial and azimuthal variations of their structures are related to hurricane intensity change. In general, CBs have elevated reflectivity just below the updraft axis, enhanced tangential wind below and radially outward of the updraft, enhanced vorticity near the updraft, and divergent radial flow at the top of the updraft. When examining CB structure by shear-relative quadrant, the downshear-right (upshear-left) region has updrafts at the lowest (highest) altitudes and weakest (strongest) magnitudes. When further stratifying by intensity change, the greatest differences are seen upshear. Intensifying storms have updrafts on the upshear side at a higher altitude and stronger magnitude than steady-state storms. This distribution provides a greater projection of diabatic heating onto the azimuthal mean, resulting in a more efficient vortex spin-up. For variations based on radial location, CBs located inside the RMW show stronger updrafts at a higher altitude for intensifying storms. Stronger and deeper updrafts inside the RMW can spin up the vortex through greater angular momentum convergence and a more efficient vortex response to the diabatic heating.
Wang, W., J.A. Sippel, S. Abarca, L. Zhu, B. Liu, Z. Zhang, A. Mehra, and V. Tallapragada. Improving NCEP HWRF simulations of surface wind and inflow angle in the eye area. Weather and Forecasting, 33(3):887-898, https://doi.org/10.1175/WAF-D-17-0115.1 2018
This technical note describes a modification of the boundary layer parameterization scheme in the Hurricane Weather Research and Forecasting (HWRF) model, which improves the simulations of low-level wind and surface inflow angle in the eyewall area and has been implemented in the HWRF system and used in the operational system since 2016. The modification is on an observation-based adjustment of eddy diffusivity previously implemented in the model. It is needed because the previous adjustment resulted in a discontinuity in the vertical distribution of eddy diffusivity near the surface-layer top, which increases the friction within the surface layer and compromises the surface-layer constant-flux assumption. The discontinuity affects the simulation of storm intensity and intensification, one of the main metrics of model performance, particularly in strong tropical cyclones. This issue is addressed by introducing a height-dependent adjustment so that the vertical profile of eddy diffusivity is continuous throughout the boundary layer. It is shown that the implementation of the modification results in low-level winds and surface inflow angles in the storm’s eyewall region closer to observations.
Weatherhead, E.C., B.A. Wielicki, V. Ramaswamy, M. Abbott, T.P. Ackerman, R. Atlas, G. Brasseur, L. Bruhwiler, A.J. Busalacchi, J.H. Butler, C.T.M. Clack, R. Cooke, L. Cucurull, S.M. Davis, J.M. English, D.W. Fahey, S.S. Fine, J.K. Lazo, S. Liang, N.G. Loeb, E. Rignot, B. Soden, D. Stanitski, G. Stephens, B.D. Tapley, A.M. Thompson, K.E. Trenberth, and D. Wuebbles. Designing the climate observing system of the future. Earth’s Future, 6(1):80-102, https://doi.org/10.1002/2017EF000627 2018
Climate observations are needed to address a large range of important societal issues including sea level rise, droughts, floods, extreme heat events, food security, and fresh water availability in the coming decades. Past, targeted investments in specific climate questions have resulted in tremendous improvements in issues important to human health, security, and infrastructure. However, the current climate observing system was not planned in a comprehensive, focused manner required to adequately address the full range of climate needs. A potential approach to planning the observing system of the future is presented in this paper. First, this paper proposes that priority be given to the most critical needs as identified within the World Climate Research Program as Grand Challenges. These currently include seven important topics: melting ice and global consequences; clouds, circulation and climate sensitivity; carbon feedbacks in the climate system; understanding and predicting weather and climate extremes; water for the food baskets of the world; regional sea-level change and coastal impacts; and near-term climate prediction. For each Grand Challenge, observations are needed for long-term monitoring, process studies and forecasting capabilities. Second, objective evaluations of proposed observing systems, including satellites, ground-based and in situ observations, as well as potentially new, unidentified observational approaches, can quantify the ability to address these climate priorities. And third, investments in effective climate observations will be economically important as they will offer a magnified return on investment that justifies a far greater development of observations to serve society’s needs.
Wick, G.A., T.F. Hock, P.J. Neiman, H. Vomel, M.L. Black, and J.R. Spackman. The NCAR/NOAA Global Hawk dropsonde system. Journal of Atmospheric and Oceanic Technology, 35(8):1585-1604, https://doi.org/10.1175/JTECH-D-17-0225.1 2018
A new remotely controlled AVAPS® dropsonde system has been developed for and deployed on the NASA Global Hawk (GH) unmanned aircraft. Design, fabrication, and operation of the system was led by the National Center for Atmospheric Research (NCAR) with support from the National Oceanic and Atmospheric Administration (NOAA) Unmanned Aircraft Systems (UAS) program. The system has employed the NRD94 dropsonde, a smaller and lighter version of the standard RD94 dropsonde deployed from manned aircraft but with virtually identical sensors. The dropsondes provide in situ atmospheric profiles of temperature, pressure, and humidity at a 2 Hz data rate, and wind speed and direction at 4 Hz. The system is capable of carrying up to 90 dropsondes and can support 8 simultaneous soundings. Operation from the GH means that the dropsondes can be deployed from altitudes up to 19.8 km during flights in excess of 24-hour duration. Dropsonde launch is commanded directly by an operator on the ground in coordination with the aircraft commander. Over 2700 total dropsondes have been deployed from the GH during four major campaigns since 2011. Data are processed in near-real-time and have been employed by forecasters, for assimilation in numerical weather prediction models, and in diverse research studies. Intercomparison studies suggest the performance of the GH NRD94 dropsondes is similar to those deployed from manned aircraft. This paper describes the components and operation of the system and illustrates its unique capabilities through highlights of data application to research on the arctic atmosphere, atmospheric rivers, and tropical cyclones.
Zhang, J.A., F.D. Marks, J.A. Sippel, R.F. Rogers, X. Zhang, S.G. Gopalakrishnan, Z. Zhang, and V. Tallapragada. Evaluating the impact of improvement in the horizontal diffusion parameterization on hurricane prediction in the operational Hurricane Weather Research and Forecasting (HWRF) model. Weather and Forecasting, 33(1):317-329, https://doi.org/10.1175/WAF-D-17-0097.1 2018
Improving physical parameterizations in forecast models is essential for hurricane prediction. This study documents the upgrade of horizontal diffusion parameterization in the Hurricane Weather Research and Forecasting (HWRF) model and evaluates the impact of this upgrade on hurricane forecasts. The horizontal mixing length (Lh) was modified based on aircraft observations and extensive idealized and real-case numerical experiments. Following Zhang and Marks (2015), who focused on understanding how the horizontal diffusion parameterization worked in HWRF and its dynamical influence on hurricane intensification using idealized simulations, a series of sensitivity experiments was conducted to simulate Hurricane Earl (2010) in which only Lh was varied. Results from the Earl forecasts confirmed the findings from previous theoretical and idealized numerical studies, in that both the simulated maximum intensity and intensity change rate are dependent on Lh. Comparisons between the modeled and observed structure of Hurricane Earl, such as storm size, boundary layer heights, warm-core height and temperature anomaly, and eyewall slope, suggested that the Lh used in the HWRF model should be decreased. Lowering Lh in HWRF has a positive impact on hurricane prediction based on over 200 retrospective forecasts of 10 Atlantic storms. Biases in both storm intensity and storm size are significantly reduced with the modified Lh.
Zhang, J.A., R. Atlas, G.D. Emmitt, L. Bucci, and K. Ryan. Airborne Doppler wind lidar observations of the tropical cyclone boundary layer. Remote Sensing, 10(6):825, https://doi.org/10.3390/rs10060825 2018
This study presents a verification and an analysis of wind profile data collected during Tropical Storm Erika (2015) by a Doppler Wind Lidar (DWL) instrument aboard a P3 Hurricane Hunter aircraft of the National Oceanic and Atmospheric Administration (NOAA). DWL-measured winds are compared to those from nearly collocated GPS dropsondes and show good agreement in terms of both the wind magnitude and asymmetric distribution of the wind field. A comparison of the DWL-measured wind speeds versus dropsonde-measured wind speeds yields a reasonably good correlation (r2 = 0.95), with a root mean square error (RMSE) of 1.58 m s−1 and a bias of −0.023 m s−1. Our analysis shows that the DWL complements the existing P3 Doppler radar, in that it collects wind data in rain-free and low-rain regions where Doppler radar is limited for wind observations. The DWL observations also complement dropsonde measurements by significantly enlarging the sampling size and spatial coverage of the boundary layer winds. An analysis of the DWL wind data shows that the boundary layer of Erika was much deeper than that of a typical hurricane-strength storm. Streamline and vorticity analyses based on DWL wind observations explain why Erika maintained intensity in a sheared environment. This study suggests that DWL wind data are valuable for real-time intensity forecasts, basic understanding of the boundary layer structure and dynamics, and offshore wind energy applications under tropical cyclone conditions.
Zou, Z., D. Zhao, J.A. Zhang, S. Li, Y. Cheng, H. Lv, and X. Ma. Influence of swell on the atmospheric boundary layer under nonneutral conditions. Journal of Physical Oceanography, 48(4):925-936, https://doi.org/10.1175/JPO-D-17-0195.1 2018
The anomalous phenomena induced by the prevailing swell at low wind speeds prevent a complete understanding of air–sea interaction processes. Many studies have considered this complex problem, but most have focused on near-neutral conditions. In this study, the influence of the swell on the atmospheric boundary under nonneutral conditions was addressed by extending the turbulent closure models of Makin and Kudryavtsev and the Monin–Obukhov similarity theory (MOST; Monin and Yaglom) to the existence of swell and nonneutral conditions. It was shown that wind profiles derived from these models were consistent with each other and both departed from the traditional MOST. At low wind speeds, a supergeostrophic jet appeared on the upper edge of the wave boundary layer, which was also reported in earlier studies. Under nonneutral conditions, the influence of buoyancy was significant. The slope of the wind profile increased under stable conditions and became smoother under unstable conditions. Considering the effects of buoyancy and swell, the wind stress derived from the model agreed quantitatively with the observations.
2017
Aberson, S.D., J.A. Zhang, and K. Nunez-Ocasio. An extreme event in the eyewall of Hurricane Felix on 2 September 2007. Monthly Weather Review, 145(6):2083-2092, https://doi.org/10.1175/MWR-D-16-0364.1 2017
During a routine penetration into Hurricane Felix late on 2 September 2007, NOAA-42 encountered extreme turbulence and graupel, flight-level horizontal wind gusts of over 83 m s-1, and vertical wind speeds varying from 10 m s-1 downward to 31 m s-1 upward and back to nearly 7 m s-1 downward within 1 min. This led the plane to rise nearly 300 m and then return to its original level within that time. Though a dropwindsonde was released during this event, the radars and data systems on board the aircraft were rendered inoperable, limiting the amount of data obtained. The feature observed during the flight is shown to be similar to that encountered during flights into Hurricanes Hugo (1989) and Patricia (2015), and by a dropwindsonde released into a misovortex in Hurricane Isabel (2014). This paper describes a unique dataset of a small-scale feature that appears to be prevalent in very intense tropical cyclones, providing new evidence for eye-eyewall mixing processes that may be related to intensity change.
Aberson, S.D., K.J. Sellwood, and P.A. Leighton. Calculating dropwindsonde location and time from TEMP-DROP messages for accurate assimilation and analysis. Journal of Atmospheric and Oceanic Technology, 34(8):1673-1678, https://doi.org/10.1175/JTECH-D-17-0023.1 2017
Current practice is to transmit dropwindsonde data from aircraft using the TEMP-DROP format, which provides only the release location and time with 0.1° latitude-longitude (about 11 km), and 1-h resolutions, respectively. The current dropwindsonde has a fall speed of approximately 15 ms-1, so the instrument will be advected faster horizontally than it will descend if the wind speed exceeds this value. Where wind speeds are greatest, such as in tropical cyclones, this will introduce large errors in the location of the observations, especially near the surface. A technique to calculate the correct time and location of each observation in the TEMP-DROP message is introduced. Mean differences between the calculated and reported locations are about 0.5 km for distance and 15 sec for time, or <1% of the error size for distance and <10% for time.
Aksoy, A., J.A. Zhang, B.W. Klotz, E.W. Uhlhorn, and J.J. Cione. Axisymmetric initialization of the atmosphere and ocean for idealized coupled hurricane simulations. Journal of Advances in Modeling Earth Systems, 9(7):2672-2695, https://doi.org/10.1002/2017MS000977 2017
A new vortex-scale initialization scheme is presented for idealized coupled hurricane simulations. The atmospheric scheme involves construction of azimuthally averaged kinematic and thermodynamic initial fields based on historical composite datasets from hurricane reconnaissance aircraft. For ocean initialization, a statistical scheme is proposed to construct regression models among atmospheric and ocean fields in the hurricane inner core. For the numerical model, the Hurricane Weather Research and Forecasting (HWRF) model coupled with a one-dimensional, diffusive ocean model is used with modifications to initialize with the observation-based vortex and to ensure that the storm environment remains approximately steady. The primary goal in these simulations is to obtain steady-state hurricanes of category-1 intensity with characteristics typically observed during the hurricane season of the western Atlantic and Caribbean Sea regions. It is demonstrated that this is successfully achieved in the simulations. In an azimuthally averaged sense, regression models are found to capture about 70% of total variance for sea-surface temperature cooling and up to 55% of total variance for mixed-layer depth perturbation in the hurricane inner core. Furthermore, within the inner core of a hurricane vortex, it is found that storm speed contributes most to upper-ocean perturbations, whereas characteristics of the atmospheric vortex contribute very little. The importance of storm speed in controlling upper-ocean perturbations is strongest near the storm center, diminishing gradually toward no measurable impact beyond the immediate inner core.
Alaka, G.J., and E.D. Maloney. Internal intraseasonal variability of the West African Monsoon in WRF. Journal of Climate, 30(15):5805-5813, https://doi.org/10.1175/JCLI-D-16-0750.1 2017
The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden-Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30-90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30-90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.
Alaka, G.J., X. Zhang, S.G. Gopalakrishnan, S.B. Goldenberg, and F.D. Marks. Performance of basin-scale HWRF tropical cyclone track forecasts. Weather and Forecasting, 32(3):1253-1271, https://doi.org/10.1175/WAF-D-16-0150.1 2017
The Hurricane Weather Research and Forecasting model (HWRF) is a dynamical model that has shown annual improvements to its tropical cyclone (TC) track forecasts as a result of various modifications. This study focuses on an experimental version of HWRF, called the “basin-scale” HWRF (HWRF-B), configured with: (1) a large, static outer domain to cover multiple TC basins; and (2) multiple sets of high-resolution movable nests to produce forecasts for several TCs simultaneously. Although HWRF-B and the operational HWRF produced comparable average track errors for the 2011-2014 Atlantic hurricane seasons, strengths of HWRF-B are identified and linked to its configuration differences. HWRF-B track forecasts were generally more accurate compared to the operational HWRF when at least one additional TC was simultaneously active in the Atlantic or East Pacific basins and, in particular, when additional TCs were greater than 3500 km away. In addition, at long lead times, HWRF-B average track errors were lower than for the operational HWRF for TCs initialized north of 25°N or west of 60°W, highlighting the sensitivity of TC track forecasts to the location of the operational HWRF outermost domain. A case study, performed on Hurricane Michael, corroborated these HWRF-B strengths. HWRF-B shows potential to serve as an effective bridge between regional modeling systems and next generational global efforts.
Atlas, R., G.D. Emmitt, L. Bucci, K. Ryan, and J.A. Zhang. Application of Doppler wind lidar observations to hurricane analysis and prediction. Proceedings, SPIE Symposium on Lidar Remote Sensing for Environmental Monitoring, San Diego, CA, August 6-10, 2017. International Society for Optics and Photonics, SPIE Vol. 10406, 8 pp., 2017
One of the most important applications of a space-based Doppler Wind Lidar (DWL) would be to improve atmospheric analyses and weather forecasting. Since the mid-1980s, Observing System Simulation Experiments (OSSEs) have been conducted to evaluate the potential impact of space-based DWL data on numerical weather prediction (NWP). All of these OSSEs have shown significant beneficial impact on global analyses and forecasts. In more recent years, a limited number of experiments have been conducted to evaluate the potential impact of DWL data on hurricane forecasting and also to begin to evaluate the impact of real airborne DWL observations. These latest studies suggest that DWL can complement existing hurricane observations effectively and have the potential to contribute to improved hurricane track and intensity forecasting.
Bell, G.D., E.S. Blake, C.W. Landsea, C. Wang, J. Schemm, T. Kimberlain, R.J. Pasch, and S.B. Goldenberg. Tropical cyclones: Atlantic basin. In State of the Climate in 2016, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 98(8):S108-S112, 2017
Bryan, G.H., R.P. Worsnop, J.K. Lundquist, and J.A. Zhang. A simple method for simulating wind profiles in the boundary layer of tropical cyclones. Boundary-Layer Meteorology, 162(3):475-502, https://doi.org/10.1007/s10546-016-0207-0 2017
A method to simulate characteristics of wind speed in the boundary layer of tropical cyclones in an idealized manner is developed and evaluated. The method can be used in a single-column modelling set-up with a planetary boundary-layer parametrization or within large-eddy simulations (LES). The key step is to include terms in the horizontal velocity equations representing advection and centrifugal acceleration in tropical cyclones that occur on scales larger than the domain size. Compared to other recently developed methods, which require two input parameters (a reference wind speed and radius from the centre of a tropical cyclone), this new method also requires a third input parameter: the radial gradient of reference wind speed. With the new method, simulated wind profiles are similar to composite profiles from dropsonde observations; in contrast, a classic Ekman-type method tends to overpredict inflow-layer depth and magnitude, and two recently developed methods for tropical cyclone environments tend to overpredict near-surface wind speed. When used in LES, the new technique produces vertical profiles of total turbulent stress and estimated eddy viscosity that are similar to values determined from low-level aircraft flights in tropical cyclones. Temporal spectra from LES produce an inertial subrange for frequencies ≳0.1 Hz, but only when the horizontal grid spacing ≲20 m.
Christophersen, H., A. Aksoy, P. Dunion, and K. Sellwood. The impact of NASA Global Hawk unmanned aircraft dropwindsonde observations on tropical cyclone track, intensity and structure: Case studies. Monthly Weather Review, 145(5):1817-1830, https://doi.org/10.1175/MWR-D-16-0332.1 2017
The impact of Global Hawk dropwindsondes on tropical cyclone analyses and forecasts is evaluated in an ensemble-based vortex-scale data assimilation system. Two cases from Hurricane Edouard (2014) are presented. In the first case, inner-core observations were exclusively provided by Global Hawk (GH) dropwindsondes, while in the second case, GH dropwindsondes were concentrated in the storm’s near environment and were complemented by an extensive number of inner-core observations from other aircraft. It is found that when GH dropwindsondes are assimilated, a positive impact on the minimum sea-level pressure (MSLP) forecast persists for most lead times in the first case, conceivably due to the better representation of the initial vortex structure, such as the warm-core anomaly and primary and secondary circulations. The verification of the storm’s kinematic and thermodynamic structure in the forecasts of the first case is carried out relative to the time of the appearance of a secondary wind maximum (SWM) using the tail Doppler radar and dropwindsonde composite analyses. A closer-to-observed wavenumber-zero wind field in the experiment with GH dropwindsondes is seen before the SWM is developed, which likely contributes to the superior intensity forecast up to 36 h. The improvement in the warm-core anomaly in the forecasts from the experiment with GH dropwindsondes is believed to have also contributed to the consistent improvement in the MSLP forecast. For the latter case, a persistent improvement in the track forecast is seen, which is consistent with a better representation of the near-environmental flow obtained from GH data in the same region.
Cucurull, L., R. Li, and T.R. Peevey. Assessment of radio occultation observations from the COSMIC-2 mission with a simplified Observing System Simulation Experiment configuration. Monthly Weather Review, 145(9):3581-3597, https://doi.org/10.1175/MWR-D-16-0475.1 2017
The mainstay of the global Radio Occultation (RO) system, the COSMIC constellation of six satellites launched in April 2006, is already past the end of its nominal lifetime and the number of soundings are rapidly declining because the constellation is degrading. For about the last decade, COSMIC profiles have been collected and their retrievals assimilated in numerical weather prediction systems to improve operational weather forecasts. The success of RO in increasing forecast skill and COSMIC’s aging constellation have motivated planning for the COSMIC-2 mission, a 12-satellite constellation to be deployed in two launches. The first six satellites (COSMIC-2A) are expected to be deployed in December 2017 in a low-inclination orbit for dense equatorial coverage, while the second six (COSMIC-2B) are expected to be launched later in a high inclination orbit for global coverage. In order to evaluate the potential benefits from COSMIC-2, we have used an earlier version of the NCEP’s operational forecast model and data assimilation system to conduct a series of Observing System Simulation Experiments with simulated soundings from the COSMIC-2 mission. In agreement with earlier studies using real RO observations, the benefits from assimilating COSMIC-2 observations are found to be most significant in the Southern Hemisphere. No or very little gain in forecast skill is found by adding COSMIC-2A to COSMIC- 2B, making the launch of COSMIC-2B more important for terrestrial global weather forecasting than that of COSMIC-2A. Furthermore, results suggest that further improvement in forecast skill might better be obtained with the addition of more RO observations with global coverage and other types of observations.
Didlake, A.C., G.M. Heymsfield, P.D. Reasor, and S.R. Guimond. Concentric eyewall asymmetries in Hurricane Gonzalo (2014) observed by airborne radar. Monthly Weather Review, 145(3):729-749, https://doi.org/10.1175/MWR-D-16-0175.1 2017
Two eyewall replacement cycles were observed in Hurricane Gonzalo by the NOAA P3 tail radar and the recently developed NASA HIWRAP radar. These observations captured detailed precipitation and kinematic features of Gonzalo’s concentric eyewalls both before and after the outer eyewall’s winds became the vortex maximum winds. The data were analyzed relative to the deep-layer environmental wind shear vector. During the beginning eyewall replacement cycle stages, the inner and outer eyewalls exhibited different asymmetries. The inner eyewall asymmetry exhibited significant low-level inflow, updrafts, and positive tangential acceleration in the downshear quadrants, consistent with observational and theoretical studies. The outer eyewall asymmetry exhibited these features in the left-of-shear quadrants, further downwind from those of the inner eyewall. It is suggested that the low-level inflow occurring at the outer but not at the inner eyewall in the downwind regions signals a barrier effect that contributes to the eventual decay of the inner eyewall. Toward the later eyewall replacement stages, the outer eyewall asymmetry shifts upwind, becoming more aligned with the asymmetry of the earlier inner eyewall. This upwind shift is consistent with the structural evolution of eyewall replacement as the outer eyewall transitions into the primary eyewall of the storm.
Doyle, J.D., J.R. Moskaitis, J.W. Feldmeier, R.J. Ferek, M. Beaubien, M.M. Bell, D.L. Cecil, R.L. Creasey, P. Duran, R.L. Elsberry, W.A. Komaromi, J. Molinari, D.R. Ryglicki, D.P. Stern, C.S. Velden, X. Wang, T. Allen, B.S. Barrett, P.G. Black, J.P. Dunion, K.A. Emanuel, P.A. Harr, L. Harrison, E.A. Hendricks, D. Herndon, W.Q. Jeffries, S.J. Majumdar, J.A. Moore, Z. Pu, R.F. Rogers, E.R. Sanabia, G.J. Tripoli, and D.-L. Zhang. A view of tropical cyclones from above: The Tropical Cyclone Intensity Experiment. Bulletin of the American Meteorological Society, 98(10):2113-2134, https://doi.org/10.1175/BAMS-D-16-0055.1 2017
Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes.
Evans, C., K.M. Wood, S.D. Aberson, H.M. Archambault, S.M. Milrad, L.F. Bosart, K.L. Corbosiero, C.A. Davis, J.R. Dias Pinto, J. Doyle, C. Fogarty, T.J. Galarneau, C.M. Grams, K.S. Griffin, J. Gyakum, R.E. Hart, N. Kitabatake, H.S. Lentink, R. McTaggart-Cowan, W. Perrie, J.F.D. Quinting, C.A. Reynolds, M. Riemer, E.A. Ritchie, Y. Sun, and F. Zhang. The extratropical transition of tropical cyclones, Part 1: Cyclone evolution and direct impacts. Monthly Weather Review, 145(11):4317-4344, https://doi.org/10.1175/MWR-D-17-0027.1 2017
Extratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. ET is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary-scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and Western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, modeling the direct impacts of ET-related wind, waves, and precipitation are noted. Improved understanding of structural evolution throughout the transformation stage of ET fostered in large part by novel aircraft observations collected in several recent ET events is highlighted. Predictive skill for operational and numerical model ET-related forecasts is discussed along with environmental factors influencing post-transition cyclone structure and evolution. Operational ET forecast and analysis practices and challenges are detailed. In particular, some challenges of effective hazard communication for the evolving threats posed by a tropical cyclone during and after transition are introduced. This review concludes with recommendations for future work to further improve understanding, forecasts, and hazard communication.
Goni, G.J., R.E. Todd, S.R. Jayne, G.R. Halliwell, S. Glenn, J. Dong, R. Curry, R. Domingues, F. Bringas, L. Centurioni, S.F. DiMarco, T. Miles, J. Morell, L. Pomales, H.-S. Kim, P.E. Robbins, G.G. Gawarkiewicz, J. Wilkin, J. Heiderich, B. Baltes, J.J. Cione, G. Seroka, K. Knee, and E.R. Sanabia. Autonomous and Lagrangian ocean observations for Atlantic tropical cyclone studies and forecasts. Oceanography, 30(2):85-95, https://doi.org/10.5670/oceanog.2017.227 2017
The tropical Atlantic basin is one of seven global regions where tropical cyclones (TCs) commonly originate, intensify, and affect highly populated coastal areas. Under appropriate atmospheric conditions, TC intensification can be linked to upper-ocean properties. Errors in Atlantic TC intensification forecasts have not been significantly reduced during the last 25 years. The combined use of in situ and satellite observations, particularly of temperature and salinity ahead of TCs, has the potential to improve the representation of the ocean, more accurately initialize hurricane intensity forecast models, and identify areas where TCs may intensify. However, a sustained in situ ocean observing system in the tropical North Atlantic Ocean and Caribbean Sea dedicated to measuring subsurface temperature, salinity, and density fields in support of TC intensity studies and forecasts has yet to be designed and implemented. Autonomous and Lagrangian platforms and sensors offer cost-effective opportunities to accomplish this objective. Here, we highlight recent efforts to use autonomous platforms and sensors, including surface drifters, profiling floats, underwater gliders, and dropsondes, to better understand air-sea processes during high-wind events, particularly those geared toward improving hurricane intensity forecasts. Real-time data availability is key for assimilation into numerical weather forecast models.
Halliwell, G.R., M. Mehari, L.K. Shay, V.H. Kourafalou, H. Kang, H.-S. Kim, J. Dong, and R. Atlas. OSSE quantitative assessment of rapid-response pre-storm ocean surveys to improve coupled tropical cyclone prediction. Journal of Geophysical Research-Oceans, 122(7):5729-5748, https://doi.org/10.1002/2017JC012760 2017
Ocean fields that initialize coupled TC prediction models must accurately represent the dynamics of mesoscale features and the associated distribution of upper ocean temperature and salinity. They must also provide unbiased realizations of upper ocean heat content and stratification. Ocean Observing System Simulation Experiments (OSSEs) are performed for three storms: Isaac, 2012; Edouard, 2014; and Gonzalo, 2014. These OSSEs assess the impact of rapid-response prestorm ocean profile surveys on improving ocean model initialization. Two types of surveys are evaluated: airborne deployments of expendable profilers and deployments of in situ thermistor chains along lines intersecting predicted storm paths. Assimilation of the existing ocean observing system substantially constrains mesoscale structure in dynamical fields, primarily because of the four available altimeters. However, these observations only modestly constrain mesoscale structure and bias in upper ocean thermal fields. Adding rapid-response airborne surveys to these observing systems produces substantial additional correction in thermal fields, but minimal additional correction in dynamical fields. Without altimetry assimilation, rapid-response profiles produce large additional correction in both dynamical and thermal fields. Airborne CTDs sampling temperature and salinity over 1000 m versus XBTs sampling temperature over 400 m produce additional correction for dynamical fields, but not for upper ocean thermal fields. Airborne surveys are generally more effective than thermistor chain deployments because they can sample a larger area at higher horizontal resolution and because the latter only measures temperature over the upper ∼100 m. Both airborne profile surveys and thermistor chain deployments effectively reduce upper ocean thermal biases.
Halliwell, G.R., M. Mehari, M. Le Henaff, V. Kourafalou, I. Androulidakis, H. Kang, and R. Atlas. North Atlantic Ocean OSSE system: Evaluation of operational ocean observing system components and supplemental seasonal observations for potentially improving tropical cyclone prediction in coupled systems. Journal of Operational Oceanography, 10(2):154-175, https://doi.org/10.1080/1755876X.2017.1322770 2017
Observing System Simulated Experiments (OSSEs) performed during the 2014 North Atlantic hurricane season quantify ocean observing system impacts with respect to improving ocean model initialization in coupled tropical cyclone (TC) prediction systems. The suitability of the OSSE system forecast model (FM) with respect to the previously validated Nature Run is demonstrated first. Analyses are then performed to determine the calibration required to obtain credible OSSE impact assessments. Impacts on errors and biases in fields important to TC prediction are first quantified for three major components of the existing operational ocean observing system. Satellite altimetry provides the greatest positive impact, followed by Argo floats and sea surface temperature measurements from both satellite and in-situ systems. The OSSE system is then used to investigate observing system enhancements, specifically regional underwater glider deployments during the 2014 hurricane season. These deployments resulted in modest positive impacts on ocean analyses that were limited by (1) errors in the horizontal structure of the increment field imposed by individual gliders and (2) memory loss in the spreading of these corrections by nonlinear model dynamics. The high-resolution, three-dimensional representation of the truth available in OSSE systems allows these issues to be studied without high-density ocean observations.
Hazelton, A.T., R.E. Hart, and R.F. Rogers. Analyzing simulated convective bursts in two Atlantic hurricanes. Part II: Intensity change due to bursts. Monthly Weather Review, 145(8):3095-3117, https://doi.org/10.1175/MWR-D-16-0268.1 2017
This paper investigates convective burst (CB) evolution in Weather Research and Forecasting (WRF) simulations of two tropical cyclones (TCs), focusing on the relationship between CBs and TC intensity change. Analysis of intensity change in the simulations shows that there are more CBs inside the radius of maximum winds (RMW) during times when the TCs are about to intensify, while weakening/steady times are associated with more CBs outside the RMW, consistent with past observational and theoretical studies. The vertical mass flux distributions show greater vertical mass flux at upper levels both from weaker updrafts and CBs for intensifying cases. The TC simulations are further dissected by past intensity change, and times of sustained intensification have more CBs than times when the TC has been weakening but then intensifies. This result suggests that CB development may not always be predictive of intensification, but rather may occur as a result of ongoing intensification and contribute to ongoing intensification. Abrupt short-term intensification is found to be associated with an even higher density of CBs inside the RMW than slower intensification. Lag correlations between CBs and intensity reveal a broad peak, with the CBs leading pressure falls by 0-3 hours. These relationships are further confirmed by analysis of individual simulation periods, although the relationship can vary depending on environmental conditions and the previous evolution of the TC. These results show that increased convection due to both weak updrafts and CBs inside the RMW are favorable for sustained TC intensification, and show many details of the typical short-term response of the TC core to CBs.
Hazelton, A.T., R.F. Rogers, and R.E. Hart. Analyzing simulated convective bursts in two Atlantic hurricanes. Part I: Burst formation and development. Monthly Weather Review, 145(8):3073-3094, https://doi.org/10.1175/MWR-D-16-0267.1 2017
Understanding the structure and evolution of the tropical cyclone (TC) inner core remains an elusive challenge in tropical meteorology, especially the role of transient asymmetric features such as localized strong updrafts known as convective bursts (CBs). This study investigates the formation of CBs and their role in TC structure and evolution using high-resolution simulations of two Atlantic hurricanes (Dean 2007 and Bill, 2009) with the Weather Research and Forecasting (WRF) model. Several different aspects of the dynamics and thermodynamics of the TC inner-core region are investigated with respect to their influence on TC convective burst development. Composites with CBs show stronger radial inflow in the lowest 2 km, and stronger radial outflow from the eye to the eyewall around z = 2‑4 km, than composites without CBs. Asymmetric vorticity associated with eyewall mesovortices appears to be a major factor in leading to some of the radial flow anomalies that lead to CB development. The anomalous outflow from these mesovortices, along with outflow from supergradient parcels above the boundary layer, favors low-level convergence and also appears to mix high-θe air from the eye into the eyewall. Analysis of individual CBs and parcel trajectories show that parcels are pulled into the eye, and briefly mix with the eye air. The parcels then rapidly move outward into the eyewall, and quickly ascend in CBs, in some cases with vertical velocity over 20 ms-1. These results support the importance of horizontal asymmetries in forcing extreme asymmetric vertical velocity in tropical cyclones.
Hoffman, R.H., N. Prive, and M. Bourassa. Comments on ‟Reanalysis and observations: What’s the difference?” Bulletin of the American Meteorological Society, 98(11):2455-2459, https://doi.org/10.1175/BAMS-D-17-0008.1 2017
Hoffman, R.N., S.-A. Boukabara, V.K. Kumar, K. Garrett, S.P.F. Casey, and R. Atlas. An empirical cumulative density function approach to defining summary NWP forecast assessment metrics. Monthly Weather Review, 145(4):1427-1435, https://doi.org/10.1175/MWR-D-16-0271.1 2017
The empirical cumulative density function (ECDF) approach can be used to combine multiple, diverse assessment metrics into summary assessment metrics (SAMs) to analyze the results of impact experiments and pre-operational implementation testing with numerical weather prediction (NWP) models. The main advantages of the ECDF approach are that it is amenable to statistical significance testing and produces results that are easy to interpret because the SAMs for various subsets tend to vary smoothly and in a consistent manner. In addition, the ECDF approach can be applied in various contexts thanks to the flexibility allowed in the definition of the reference sample. The interpretations of the examples presented here of the impact of potential future data gaps are consistent with previously reported conclusions. An interesting finding is that the impact of observations decreases with increasing forecast time. This is interpreted as being caused by the masking effect of NWP model errors increasing to become the dominant source of forecast error.
Holbach, H.M., and M.A. Bourassa. Platform and across-swath comparison of vorticity spectra from QuikSCAT, ASCAT-A, OSCAT, and ASCAT-B scatterometers. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10(5):2205-2213, https://doi.org/10.1109/JSTARS.2016.2642583 2017
In the last few years there has been tremendous improvement in the calibration of ocean surface vector winds from scatterometers and polarimetric radiometers. This is the first detailed investigation of across-swath consistency in scatterometer-derived (i.e., QSCAT, ASCAT-A, OSCAT, and ASCAT-B) vorticity (curl of the ocean surface vector winds). Spatial derivatives of the wind fields are very important for atmospheric boundary-layer processes, upper ocean forcing, and deep ocean forcing. Improvements in wind calibration imply improvements in derivatives of these winds; however, it does not imply consistency. This study demonstrates near consistency in across-swath vorticity and near consistency between platforms.
Jin S., S. Wang, X. Li, L. Jiao, and J.A. Zhang. Tropical cyclone center location in SAR images based on feature learning and visual saliency. In Hurricane Monitoring with Spaceborne Synthetic Aperture Radar, X. Li (ed.). Springer Singapore, 141-181, https://doi.org/10.1007/978-981-10-2893-9_8 2017
Synthetic aperture radar (SAR), with its high spatial resolution, large areal coverage, day/night imaging capability, and penetrating cloud capability has been used as an important tool for tropical cyclone monitoring. The accuracy of locating tropical cyclone centers has a large impact on the accuracy of tropical cyclone track prediction. This study focuses on the center location of tropical cyclones in SAR images. Based on an analysis of the characteristics of tropical cyclone SAR images, combined with the theory and methods of SAR image segmentation and computer vision, center location methods for both tropical cyclones with eyes in SAR images and tropical cyclones without eyes in SAR images are presented in this chapter. The main work is as follows: (1) For a tropical cyclone with its eye in the SAR image, the eye area in the image appears as black or dark gray for there being no rain and little wind in the eye area. However, the gray level contrast is not always obvious. There may be no complete or clear eye when a tropical cyclone is in the development period or the recession period. The eye area in a tropical cyclone SAR image may appear as a light gray area at these periods. It is, therefore, necessary to enhance the gray level contrast before image segmentation. Additionally, denoising the speckle noise is also necessary for SAR image processing. A tropical cyclone eye extraction method based on a non-local means method and labeled watershed algorithm is given. A PPB filter is used to denoise the speckle noise. The top-hat transform is then used to enhance the contrast. Finally, the tropical cyclone eye is extracted by the labeled watershed algorithm. The eye area extracted with this method is computed to compare it with the eye area extracted manually. The comparison indicates the accuracy of the extraction accuracy. (2) Generally speaking, the center of the tropical cyclone without its eye is located with the template matching method for a single image. The spiral cloud band of the tropical cyclone without its eye is information that can be fully used in the tropical cyclone SAR image. By taking advantage of the simple background with little textural information, a center location method of the tropical cyclone without its eye in the SAR image based on feature learning and visual saliency detection is proposed. Spiral cloud bands appear as light and dark spiral structures in tropical cyclone SAR images, containing rich directional information. Therefore, a salient region map that takes advantage of the gray contrast feature and orientation feature is built. The salient region map makes the spiral cloud bands outstanding and excludes the irrelevant clouds. The morphology method is then used to extract the spiral bands in the salient region map, and the skeleton lines of spiral cloud bands are extracted. Finally, the tropical cyclone center is estimated with the inflow angle model and the particle swarm optimization algorithm. The estimation results are compared with the best track data, confirming the validity of the algorithm.
Jin, S., S. Wang, X. Li, L. Jiao, J.A. Zhang, and D. Shen. A salient region detection and pattern matching-based algorithm for center detection of a partially covered tropical cyclone in a SAR image. IEEE Transactions on Geoscience and Remote Sensing, 55(1):280-291, https://doi.org/10.1109/TGRS.2016.2605766 2017
Spaceborne microwave synthetic aperture radar (SAR), with its high spatial resolution, large area coverage, day/night imaging capability, and penetrating cloud capability, has been used as an important tool for tropical cyclone monitoring. The accuracy of locating tropical cyclone centers has a large impact on the accuracy of tropical cyclone track prediction. Usually, the center of a tropical cyclone can be accurately located if the tropical cyclone eye is fully covered by a SAR image. In some cases, due to the limited coverage of the SAR, only a part of a tropical cyclone can be imaged without the eye. From a SAR image processing point of view, these facts make the automatic center location of tropical cyclones a challenging work. This paper addresses the problem by proposing a semiautomatic center location method based on salient region detection and pattern matching. A salient region detection algorithm is proposed, in which the salient region map contains mainly the rain bands of a tropical cyclone in a SAR image. The pattern matching problem is transformed into an optimization problem solved by using the particle swarm optimization algorithm to search for the best estimated center of a tropical cyclone. To estimate the accuracy of the located center, we compare the results with the NOAA-National Hurricane Center's best track data. Experiments demonstrate that the proposed method achieves good accuracy for locating the centers of tropical cyclones from SAR images that do not contain a distinguishable eye signature.
Kalina, E.A., S.Y. Matrosov, J.J. Cione, F.D. Marks, J. Vivekenandan, R.A. Black, J.C. Hubbert, M.M. Bell, D.E. Kingsmill, and A.B. White. The ice water paths of small and large ice species in Hurricane Arthur (2014) and Irene (2011). Journal of Applied Meteorology and Climatology, 56(5):1383-1404, https://doi.org/10.1175/JAMC-D-16-0300-1 2017
Dual-polarization scanning radar measurements, air temperature soundings, and a polarimetric radar-based particle identification scheme are used to generate maps and probability density functions (PDFs) of the ice water path (IWP) in Hurricanes Arthur (2014) and Irene (2011) at landfall. The IWP is separated into the contribution from small ice (i.e., ice crystals), termed small-particle IWP, and large ice (i.e., graupel and snow), termed large-particle IWP. Vertically profiling radar data from Hurricane Arthur suggest that the small ice particles detected by the scanning radar have fall velocities mostly greater than 0.25 m s-1 and that the particle identification scheme is capable of distinguishing between small and large ice particles in a mean sense. The IWP maps and PDFs reveal that the total and large-particle IWPs range up to 10 kg m-2, with the largest values confined to intense convective precipitation within the rainbands and eyewall. Small-particle IWP remains mostly < 4 kg m-2, with the largest small-particle IWP values collocated with maxima in the total IWP. PDFs of the small-to-total IWP ratio have shapes that depend on the precipitation type (i.e., intense convective, stratiform, or weak echo precipitation). The IWP ratio distribution is narrowest (broadest) in intense convective (weak echo) precipitation and peaks at a ratio of about 0.1 (0.3).
Klotz, B.W., and H. Jiang. Examination of surface wind asymmetries in tropical cyclones: Part I. General structure and wind shear impacts. Monthly Weather Review, 145(10):3989-4009, https://doi.org/10.1175/MWR-D-17-0019.1 2017
Because surface wind speeds within tropical cyclones are important for operational and research interests, it is vital to understand surface wind structure in relation to various storm and environmental influences. In this study, global rain-corrected scatterometer winds are used to quantify and evaluate characteristics of tropical cyclone surface wind asymmetries using a modified version of a proven aircraft-based low wavenumber analysis tool. The globally expanded surface wind dataset provides an avenue for a robust statistical analysis of the changes in structure due to tropical cyclone intensity, deep-layer vertical wind shear, and wind shear’s relationship with forward storm motion. A presentation of the quantified asymmetry indicates that wind shear has a significant influence on tropical storms at all radii but only for areas away from the radius of maximum wind in both non-major and major hurricanes. Evaluation of shear’s directional relation to motion indicates that a cyclonic rotation of the surface wind field asymmetry from downshear-left to upshear-left occurs in conjunction with an anticyclonic rotation of the directional relationship (i.e., from shear direction to the left, same, right, or opposite of the motion direction). It was discovered that in tropical cyclones experiencing effects from wind shear, an increase of absolute angular momentum transport occurs downshear and often downshear-right. The surface wind speed low wavenumber maximum in turn forms downwind of this momentum transport.
Leidner, S.M., T. Nehrkorn, J. Henderson, M. Mountain, T. Yunck, and R.N. Hoffman. A severe weather quick observing system simulation experiment (QuickOSSE) of global navigation satellite system (GNSS) radio occultation (RO) super constellations. Monthly Weather Review, 145(2):637-651, https://doi.org/10.1175/MWR-D-16-0212.1 2017
Global navigation satellite system (GNSS) radio occultations (RO) over the last 10 years have proved to be a valuable and essentially unbiased data source for operational global numerical weather prediction. However, the existing sampling coverage is too sparse in both space and time to support forecasting of severe mesoscale weather. In this study, the case study or quick observing system simulation experiment (QuickOSSE) framework is used to quantify the impact of vastly increased numbers of GNSS RO profiles on mesoscale weather analysis and forecasting. Our study focuses on a severe convective weather event that produced both a tornado and flash flooding in Oklahoma on May 31, 2013. The WRF model is used to compute a realistic and faithful depiction of reality. This 2-km “nature run” (NR) serves as the “truth” in our study. The NR is sampled by two proposed constellations of GNSS RO receivers that would produce 250 thousand and 2.5 million profiles/day globally. These data are then assimilated using WRF and a 24-member, 18-km-resolution, physics-based ensemble Kalman filter. The data assimilation is cycled hourly and makes use of a non-local, excess phase observation operator for RO data. The assimilation of greatly increased numbers of RO profiles produces improved analyses, particularly of the lower tropospheric moisture fields. The forecast results suggest positive impacts on convective initiation. Additional experiments should be conducted for different weather scenarios and with improved OSSE systems.
Li, J., Z. Li, P. Wang, T.J. Schmit, W. Bai, and R. Atlas. An efficient radiative transfer model for hyperspectral IR radiance simulation and applications under cloudy-sky conditions. Journal of Geophysical Research-Atmospheres, 122(14):7600-7613, https://doi.org/10.1002/2016JD026273 2017
An efficient radiative transfer model has been developed for hyperspectral infrared radiance simulation under both clear- and cloudy-sky conditions. The hyperspectral IR cloudy radiative transfer model (HIRTM) combines atmospheric transmittances due to molecular absorption and cloud absorption and scattering from cloud hydrometeors. An efficient analytical Jacobian methodology is also developed under both clear- and cloudy-sky conditions, which is needed both to assimilate cloudy radiances directly into numerical weather prediction models and to retrieve atmospheric soundings and cloud properties simultaneously from cloudy radiance measurements. In comparing HIRTM and its analytical Jacobian with the community radiative transfer model (CRTM), our research has shown that HIRTM's Jacobian calculations are similar to those of CRTM. HIRTM and CRTM synthetic observations derived from model output are compared with corresponding real observations from Geostationary Operational Environmental Satellite 13 Imager observations, and both perform similarly under water clouds, while CRTM is colder than HIRTM for thick ice clouds.
Martinez, J., M.M. Bell, J.L. Vigh, and R.F. Rogers. Examining tropical cyclone structure and intensification with the FLIGHT+ dataset from 1999 to 2012. Monthly Weather Review, 145(11):4401-4421, https://doi.org/10.1175/MWR-D-17-0011.1 2017
A comprehensive examination of tropical cyclone (TC) kinematic and thermodynamic structure in the Atlantic basin is created from the Extended Flight Level Dataset (FLIGHT+) for Tropical Cyclones (Version 1.1). In situ data collected at the 700-hPa flight level by NOAA WP-3D and USAF WC-130 aircraft from 1999 to 2012 are analyzed. A total of 233 azimuthal mean profiles comprised of 1498 radial legs are stratified by TC intensity and 12-hour intensity change. A matrix of composite structures is created for minor (Category 1 and 2) and major (Category 3 and above) hurricanes that are intensifying [intensity increase ≥ 10 kt (12 h)−1], steady-state [intensity change between ± 5 kt (12 h)−1], and weakening [intensity decrease ≤ − 10 kt (12 h)−1. Additional considerations to the impacts of age on TC structure are given as well. Axisymmetric radial composites reveal that intensifying TCs have statistically significant structural differences from TCs that are steady-state or weakening, but that these differences also depend on the intensity of the TC. Intensifying TCs (both minor and major hurricanes) are characterized by steep tangential wind gradients radially inward of the radius of maximum tangential wind (RMW) that contributes to a ring-like structure of vorticity and inertial stability. Tangential wind structural differences are more pronounced in the eye of minor hurricanes compared to major hurricanes. Intensifying TCs are found to have higher inner and outer-core moisture compared to steady-state and weakening TCs. Furthermore, intensifying major hurricanes possess drier eyes compared to steady-state and weakening major hurricanes.
McNoldy, B., B. Annane, S. Majumdar, J. Delgado, L. Bucci, and R. Atlas. Impact of assimilating CYGNSS data on tropical cyclone analyses and forecasts in a regional OSSE framework. Marine Technology Society Journal, 51(1):7-15, https://doi.org/10.4031/MTSJ.51.1.1 2017
The impact of assimilating ocean surface wind observations from the Cyclone Global Navigation Satellite System (CYGNSS) is examined in a high-resolution Observing System Simulation Experiment (OSSE) framework for tropical cyclones (TCs). CYGNSS is a planned National Aeronautics and Space Administration constellation of microsatellites that utilizes existing GNSS satellites to retrieve surface wind speed. In the OSSE, CYGNSS wind speed data are simulated using output from a “nature run” as truth. In a case study using the regional Hurricane Weather Research and Forecasting modeling system and the Gridpoint Statistical Interpolation data assimilation scheme, analyses of TC position, structure, and intensity, together with large-scale variables, are improved due to the assimilation of the additional surface wind data. These results indicate the potential importance of CYGNSS ocean surface wind speed data and furthermore that the assimilation of directional information would add further value to TC analyses and forecasts.
Nguyen, L.T., R.F. Rogers, and P.D. Reasor. Thermodynamic and kinematic influences on precipitation symmetry in sheared tropical cyclones: Bertha and Cristobal (2014). Monthly Weather Review, 145(11):4423-4446, https://doi.org/10.1175/MWR-D-17-0073.1 2017
Prior studies have shown an association between symmetrically-distributed precipitation and tropical cyclone (TC) intensification. Although environmental vertical wind shear typically forces an asymmetric precipitation distribution in TCs, the magnitude of this asymmetry can exhibit considerable variability, even among TCs that experience similar shear magnitudes. This observational study examines the thermodynamic and kinematic influences on precipitation symmetry in two such cases, Bertha and Cristobal (2014). Consistent with the impact of the shear, both TCs exhibited a tilted vortex, as well as a pronounced azimuthal asymmetry, with the maximum precipitation occurring in the downshear-left quadrant. However, Bertha was characterized by more symmetrically distributed precipitation and relatively modest vertical motions, while Cristobal was characterized by more azimuthally confined precipitation and much more vigorous vertical motions. Observations showed three potential hindrances to precipitation symmetry that were more prevalent in Cristobal than in Bertha: (i) Convective downdrafts that transported low entropy air downwards into the boundary layer, cooling and stabilizing the lower troposphere downstream in the left of shear and upshear quadrants; (ii) Subsidence in the upshear quadrants, which acted to increase the temperature and decrease the relative humidity of the mid-troposphere, resulting in capping of the boundary layer; and (iii) Lateral advection of mid-tropospheric dry air from the environment, which dried the TC’s upshear quadrants.
Nolan, D.S., and J.A. Zhang. Spiral gravity waves radiating from tropical cyclones. Geophysical Research Letters, 44(8):3924-3931, https://doi.org/10.1002/2017GL073572 2017
Internal gravity waves are continuously generated by deep moist convection around the globe. Satellite images suggest that tropical cyclones produce short-wavelength, high-frequency waves that radiate outward, with the wave fronts wrapped into tight spirals by the large differential advection of the sheared tangential flow. This letter presents new in situ observations of such waves from two sources: flight level data from research aircraft that show radial wavelengths of 2–10 km and vertical velocity magnitudes from 0.1 to 1.0 ms−1 and surface observations from a research buoy in the Pacific that indicate the passage of gravity waves overhead as tropical cyclones pass by at distances of 100 to 300 km. Numerical simulations are used to interpret these observations and to understand the broader horizontal and vertical structures of the radiating waves. The simulations suggest a correlation between wave amplitude and cyclone intensity, which could be used to make remote estimates of peak wind speeds.
Pu, Z., L. Zhang, S. Zhang, B. Gentry, D. Emmitt, B. Demoz, and R. Atlas. The impact of Doppler wind lidar measurements on high-impact weather forecasting: Regional OSSE and data assimilation studies. In Data Assimilation for Atmospheric, Oceanic and Hydrological Applications, Volume 3, S.K. Park and L. Xu (eds.). Springer International, 259-283, https://doi.org/10.1007/978-3-319-43415-5 2017
Wind profiles are essential for operational weather forecasting on all scales and at all latitudes. However, tropospheric winds are the number one unmet measurement objective for improving weather forecasts. In recent years, ground-based and airborne Doppler wind lidar (DWL) wind profiles have been used in field programs and various applications to obtain the necessary wind measurements. These measurements offer the opportunity to examine the impact of wind profiles on numerical weather prediction (NWP) . In addition, satellite-based DWL missions are also being planned. Observing System Simulation Experiments (OSSEs ) have been conducted to evaluate the impact of future space-based satellite global wind measurements on NWP. While many previous studies have emphasized global NWP systems, in this chapter we provide an overview and summary of recent studies with both data assimilation and OSSEs to demonstrate the value of DWL wind measurements in improving severe weather system forecasts in regional NWP, especially for systems with large societal impacts due to the damage they may cause (e.g., high-impact weather systems). Specifically, we give an overview of previous studies that have examined the impacts of ground-based and airborne DWL on the numerical predictions of mesoscale convective systems and hurricanes. The regional OSSE concept is introduced. Recent results with regional OSSEs using the mesoscale community Weather Research and Forecasting (WRF ) model and the NCEP Hurricane WRF (HWRF) model are presented. The potential configuration (e.g., resolution vs. accuracy) for future satellite-based DWL is evaluated. It is found that fairly good forecast impacts can be obtained from high-resolution observations with larger errors compared with accurate observations at a coarser resolution. Finally, the relative impact of ocean-surface wind measurements and 3-dimensional profiles is compared. The advantages of 3-D wind measurements are evident.
Rogers, R.F., P.D. Reasor, and J.A. Zhang. Reply to "Comments on 'Multiscale structure and evolution of Hurricane Earl (2010) during rapid intensification.'” Monthly Weather Review, 145(4):1573-1575, https://doi.org/10.1175/MWR-D-16-0414.1 2017
Rogers, R.F., S. Aberson, M.M. Bell, D.J. Cecil, J.D. Doyle, T.B. Kimberlain, J. Morgerman, L.K. Shay, and C. Velden. Rewriting the tropical record books: The extraordinary intensification of Hurricane Patricia (2015). Bulletin of the American Meteorological Society, 98(10):2019-2112, https://doi.org/10.1175/BAMS-D-16-0039.1 2017
Hurricane Patricia was a historic tropical cyclone that broke many records, such as intensification rate, peak intensity, and overwater weakening rate, during its brief 4-day lifetime in late October 2015 in the eastern Pacific basin. Patricia confounded all of the intensity forecast guidance owing to its rapid intensity changes. Fortunately, the hurricane-penetrating National Oceanic and Atmospheric Administration WP-3D and U.S. Air Force C-130 aircraft and the National Aeronautics and Space Administration WB-57 high-altitude jet, under support of the Office of Naval Research, conducted missions through and over Patricia prior to and during its extreme intensity changes on all 4 days, while an extensive array of pressure sensors sampled Patricia after landfall. The observations collected from these missions include traditional data sources such as airborne Doppler radar and flight-level instruments as well as new data sources like a high-density array of dropsondes released from high-altitude and wide-swath radiometer. The combination of data from these sources and from satellites provides an excellent opportunity to investigate the physical processes responsible for Patricia’s structure and evolution and offers the potential to improve forecasts of tropical cyclone rapid intensity changes. This paper provides an overview of Patricia as well as the data collected during the aircraft missions.
Rydbeck, A.V., E.D. Maloney, and G.J. Alaka. In situ initiation of east Pacific easterly waves in a regional model. Journal of the Atmospheric Sciences, 74(2):333-351, https://doi.org/10.1175/JAS-D-16-0124.1 2017
The in situ generation of easterly waves (EWs) in the east Pacific (EPAC) is investigated using the Weather Research and Forecasting Model (WRF). The sensitivity of the model to the suppression of EW forcing by locally generated convective disturbances is examined. Specifically, local forcing of EWs is removed by reducing the terrain height in portions of Central and South America to suppress robust sources of diurnal convective variability, most notably in the Panama Bight. High terrain contributes to the initiation of mesoscale convective systems in the early morning that propagate westward into the EPAC warm pool. When such mesoscale convective systems are suppressed in the model, EW variance is significantly reduced. This result suggests that EPAC EWs can be generated locally in association with higher frequency convective disturbances, and these disturbances are determined to be an important source of EPAC EW variability. However, EPAC EW variability is not completely eliminated in such sensitivity experiments, indicating the importance for other sources of EW forcing, namely EWs propagating into the EPAC from west Africa. Examination of the EW vorticity budget in the model suggests that nascent waves are zonally elongated and amplified by horizontal advection and vertical stretching of vorticity. Changes in the mean state between the control run and simulation with reduced terrain height also complicate interpretation of the results.
Smith, R.K., J.A. Zhang, and M.T. Montgomery. The dynamics of intensification in an HWRF simulation of Hurricane Earl (2010). Quarterly Journal of the Royal Meteorological Society, 143(702):293-308, https://doi.org/10.1002/qj.2922 2017
We use a high resolution numerical simulation of Atlantic Hurricane Earl (2010) to increase our understanding of Earl's intensification in relatively strong vertical shear in the context of a recent paradigm for tropical cyclone intensification. The integrity of the simulation is judged by comparing analyses thereof with those of the unprecedented observational data gathered in Earl. Consistent with the classical view of spin up, the amplification of the tangential wind field above the boundary layer is found to occur as the absolute angular momentum surfaces are drawn inwards by the aggregate heating of the rotating convective clouds in the interior of the vortex. In addition to this classical pathway, spin up occurs within the inner-core boundary layer, where the maximum tangential winds occur. The latter is another element of the new paradigm. Despite the detrimental influence of the shear on the vortex alignment and on depressing the pseudo-equivalent potential temperature outside the developing eyewall, the combined eddy processes associated with the vortical plume structures in and around the developing eyewall region are shown to contribute to an enhanced overturning circulation and an intensifying storm. These eddy processes are distinctly agradient effects that are not features of the classical spin up mechanism. It remains to be understood how the rotating convective updraughts combine to produce the diagnosed structures of the eddy terms, themselves, and how vortex Rossby waves and other eddies contribute to the alignment of the vortex during intensification.
Soukup, G.A., and F.D. Marks. Evaluation of hurricane wind speed analyses in a simulation of Hurricane Earl (2010) using low order wavenumbers. Monthly Weather Review, 145(8):3223-3245, https://doi.org/10.1175/MWR-D-14-00281.1 2017
In order to determine how well a low-order wavenumber representation describes a hurricane wind speed field, given its natural variability in space and time, low-order wavenumber representations were calculated for hourly “snapshots” of the 10-m wind speed field generated by the current operational hurricane model. Two distinct periods were examined: the first when the storm is in a reasonably steady state over 7-8 hours; and the second where the storm is changing its internal structure over a similar time interval. Observing system sensitivity experiments were also performed using wind speed field time series obtained from interpolation of the model snapshots for each of the two periods. The time series were sampled along the flight legs of a typical “figure 4” aircraft flight pattern to simulate the surface wind data collection process to ascertain the effects of the wind speed field’s temporal and spatial variability upon the low-order wavenumber analyses. The comparison between the model wind speed field at any time and the wavenumber representations during the “steady-state” period shows that the essential features of the wind speed field are captured by wavenumber 0 and 1 and that including up to wavenumber 3 practically reproduces the model field. However, in the “non-steady” period the wavenumber 0 and 1 representation is frequently unable to capture the essential characteristics of the wind speed field. The observing system sensitivity experiments suggest that when the primary circulation is rapidly changing in amplitude and/or structure during the data collection period the low-order wavenumbers analysis of the wind speed field will only represent the temporal mean structure.
Steward, J.L., A. Aksoy, and Z.S. Haddad. Parallel direct solution of the Ensemble Square-Root Kalman Filter equations with observation principal components. Journal of Atmospheric and Oceanic Technology, 34(9):1867-1884, https://doi.org/10.1175/JTECH-D-16-0140.1 2017
The Square-Root Ensemble Kalman Filter (ESRF) is a variant of the Ensemble Kalman Filter used with deterministic observations that includes a matrix square-root to account for the uncertainty of the unperturbed ensemble observations. Due to the difficulties in solving this equation, a serial approach is often used where observations are assimilated sequentially one after another. As previously demonstrated, in implementations to date the serial approach for the ESRF is suboptimal when used in conjunction with covariance localization as the Schur product used in the localization does not commute with assimilation. In this work we present a new algorithm for the direct solution of the ESRF equations based on finding the eigenvalues and eigenvectors of a sparse, square, symmetric positive semi-definite matrix with dimensions of the number of observations to be assimilated. This is amenable to direct computation using dedicated, massively parallel, and mature libraries. These libraries make it relatively simple to assemble and compute the observation principal components and solve the ESRF without using the serial approach. They also provide the eigenspectrum of the forward observation covariance matrix. Our parallel direct approach neglects the near-zero eigenvalues, which regularizes the problem. Numerical results show this approach is a highly scalable parallel method.
Tyner, B., P. Zhu, J.A. Zhang, S. Gopalakrishnan, F. Marks, and V. Tallapragada. A top-down pathway to secondary eyewall formation in simulated tropical cyclones. Journal of Geophysical Research-Atmospheres, 123(1):174-197, https://doi.org/10.1002/2017JD027410 2017
Idealized and real‐case simulations conducted using the Hurricane Weather Research and Forecasting (HWRF) model demonstrate a “top‐down” pathway to secondary eyewall formation (SEF) for tropical cyclones (TCs). For the real‐case simulations of Hurricane Rita (2005) and Hurricane Edouard (2014), a comparison to observations reveals the timing and overall characteristics of the simulated SEF appear realistic. An important control of the top‐down pathway to SEF is the amount and radial‐height distribution of hydrometeors at outer radii. Examination into the simulated hydrometeor particle fall speed distribution reveals that the HWRF operational microphysics scheme is not producing the lightest hydrometeors, which are likely present in observed TCs and are most conducive to being advected from the primary eyewall to the outer rainband region of the TC. Triggering of SEF begins with the fallout of hydrometeors at the outer radii from the TC primary eyewall, where penetrative downdrafts resulting from evaporative cooling of precipitation promote the development of local convection. As the convection‐induced radial convergence that is initially located in the midtroposphere extends downward into the boundary layer, it results in the eruption of high entropy air out of the boundary layer. This leads to the rapid development of rainband convection and subsequent SEF via a positive feedback among precipitation, convection, and boundary layer processes.
Wang, C., X. Wang, R.H. Weisberg, and M.L. Black. Variability of tropical cyclone rapid intensification in the North Atlantic and its relationship with climate variations. Climate Dynamics, 49(11-12):3627-3645, https://doi.org/10.1007/s00382-017-3537-9 2017
The paper uses observational data from 1950 to 2014 to investigate rapid intensification (RI) variability of tropical cyclones (TCs) in the North Atlantic and its relationships with large-scale climate variations. RI is defined as a TC intensity increase of at least 15.4 m/s (30 knots) in 24 h. The seasonal RI distribution follows the seasonal TC distribution, with the highest number in September. Although an RI event can occur anywhere over the tropical North Atlantic (TNA), there are three regions of maximum RI occurrence: (1) the western TNA of 12°N–18°N and 60°W–45°W, (2) the Gulf of Mexico and the western Caribbean Sea, and (3) the open ocean southeast and east of Florida. RI events also show a minimum value in the eastern Caribbean Sea north of South America—a place called a hurricane graveyard due to atmospheric divergence and subsidence. On longer time scales, RI displays both interannual and multidecadal variability, but RI does not show a long-term trend due to global warming. The top three climate indices showing high correlations with RI are the June-November ENSO and Atlantic warm pool indices, and the January-March North Atlantic oscillation index. It is found that variabilities of vertical wind shear and TC heat potential are important for TC RI in the hurricane main development region, whereas relative humidity at 500 hPa is the main factor responsible for TC RI in the eastern TNA. However, the large-scale oceanic and atmospheric variables analyzed in this study do not show an important role in TC RI in the Gulf of Mexico and the open ocean southeast and east of Florida. This suggests that other factors such as small-scale changes of oceanic and atmospheric variables or TC internal processes may be responsible for TC RI in these two regions. Additionally, the analyses indicate that large-scale atmospheric and oceanic variables are not critical to TC genesis and formation; however, once a tropical depression forms, large-scale climate variations play a role in TC intensification.
Wentz, F.J., L. Ricciardulli, E. Rodriguez, B.W. Stiles, M.A. Bourassa, D.G. Long, R.N. Hoffman, A. Stoffelen, A. Verhoef, L.W. O’Neill, J.T. Farrar, D. Vandemark, A.G. Fore, S.M. Hristova-Veleva, F.J. Turk, R. Gaston, and D. Tyler. Evaluating and extending the ocean wind climate data record. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10(5):2165-2185, https://doi.org/10.1109/JSTARS.2016.2643641 2017
Satellite microwave sensors, both active scatterometers and passive radiometers, have been systematically measuring near-surface ocean winds for nearly 40 years, establishing an important legacy in studying and monitoring weather and climate variability. As an aid to such activities, the various wind datasets are being intercalibrated and merged into consistent climate data records (CDRs). The ocean wind CDRs (OW-CDRs) are evaluated by comparisons with ocean buoys and intercomparisons among the different satellite sensors and among the different data providers. Extending the OW-CDR into the future requires exploiting all available datasets, such as OSCAT-2 scheduled to launch in July 2016. Three planned methods of calibrating the OSCAT-2 σo measurements include: 1) direct Ku-band σo intercalibration to QuikSCAT and RapidScat; 2) multisensor wind speed intercalibration; and 3) calibration to stable rainforest targets. Unfortunately, RapidScat failed in August 2016 and cannot be used to directly calibrate OSCAT-2. A particular future continuity concern is the absence of scheduled new or continuation radiometer missions capable of measuring wind speed. Specialized model assimilations provide 30-year long high temporal/spatial resolution wind vector grids that composite the satellite wind information from OW-CDRs of multiple satellites viewing the Earth at different local times.
Worsnop, R.P., G.H. Bryan, J.K. Lundquist, and J.A. Zhang. Using large-eddy simulations to define spectral and coherence characteristics of the hurricane boundary layer for wind-energy applications. Boundary-Layer Meteorology, 165(1):55-86, https://doi.org/10.1007/s10546-017-0266x 2017
Offshore wind-energy development is planned for regions where hurricanes commonly occur, such as the USA Atlantic coast. Even the most robust wind-turbine design (IEC Class I) may be unable to withstand a Category-2 hurricane (hub-height wind speeds > 50 m−1). Characteristics of the hurricane boundary layer that affect the structural integrity of turbines, especially in major hurricanes, are poorly understood, primarily due to a lack of adequate observations that span typical turbine heights (< 200 m above sea level). To provide these data, we use large-eddy simulations to produce wind profiles of an idealized Category-5 hurricane at high spatial (10 m) and temporal (0.1 s) resolution. By comparison with unique flight-level observations from a field project, we find that a relatively simple configuration of the Cloud Model I model accurately represents the properties of Hurricane Isabel (2003) in terms of mean wind speeds, wind-speed variances, and power spectra. Comparisons of power spectra and coherence curves derived from our hurricane simulations to those used in current turbine design standards suggest that adjustments to these standards may be needed to capture characteristics of turbulence seen within the simulated hurricane boundary layer. To enable improved design standards for wind turbines to withstand hurricanes, we suggest modifications to account for shifts in peak power to higher frequencies and greater spectral coherence at large separations.
Zhang, G., W. Perrie, X. Li, and J.A. Zhang. A hurricane morphology and surface wind vector estimation model for C-band cross-polarization SAR. IEEE Transactions on Geoscience and Remote Sensing, 55(3):1743-1751, https://doi.org/10.1109/TGRS.2016.2631663 2017
Over the last decades, data from spaceborne synthetic aperture radar (SAR) have been used in hurricane research. However, some issues remain. When wind is at hurricane strength, the wind speed retrievals from single-polarization SAR may have errors because the backscatter signal may experience saturation and become double valued. By comparison, wind direction retrievals from cross-polarization SAR are not possible until now. In this paper, we develop a 2-D model, the symmetric hurricane estimates for wind (SHEW) model, and combine it with the modified inflow angle model to detect hurricane morphology and estimate the wind vector field imaged by cross-polarization SAR. By fitting SHEW to the SAR derived hurricane wind speed, we find the initial closest elliptical-symmetrical wind speed fields, hurricane center location, major and minor axes, the azimuthal (orientation) angle relative to the reference ellipse, and maximum wind speed. This set of hurricane morphology parameters, along with the speed of hurricane motion, are input to the inflow angle model, modified with an ellipse-shaped eye, to derive the hurricane wind direction. A total of 14 RADARSAT-2 ScanSAR images are employed to tune the combined model. Two SAR images acquired over Hurricane Arthur (2014) and Hurricane Earl (2010) are used to validate this model. Comparisons between the modeled surface wind vector and measurements from airborne stepped-frequency microwave radiometer and dropwindsondes show excellent agreement. The proposed method works well in areas with significant radar attenuation by precipitation.
Zhang, J.A., and X. Li. Tropical cyclone multiscale wind features from spaceborne synthetic aperture radar. In Hurricane Monitoring with Spaceborne Synthetic Aperture Radar, X. Li (ed.). Springer Singapore, 25-39, https://doi.org/10.1007/978-981-10-2893-9_2 2017
This study presents multi-scale wind features observed in spaceborne synthetic aperture radar (SAR) images in tropical cyclones. Examples of eyewall mesovotices, spiral rainbands, fine-scale-band features, arc clouds, and boundary layer rolls are documented. Although these wind features are strongly tied to tropical cyclone dynamics and intensity based on previous numerical studies, they are not well-observed due to high rainfall and cloudiness that limits remote sensing instruments and the severe environment for in-situ observations to survive. Since SAR images view the actual ocean surface responses to storm-forced winds, they provide clear evidence for the presence of these wind features below clouds and their interaction with the sea surface. Analyses of the characteristics of boundary layer rolls based on SAR images show good agreement with in-situ aircraft observations, suggesting that a SAR image has a great potential to be utilized to study tropical cyclone low-level structure.
Zhang, J.A., J J. Cione, E.A. Kalina, E.W. Uhlhorn, T. Hock, and J.A. Smith. Observations of infrared sea surface temperature and air-sea interaction in Hurricane Edouard (2014) using GPS dropsondes. Journal of Oceanic and Atmospheric Technology, 34(6):1333-1349, https://doi.org/10.1175/JTECH-D-16-0211.1 2017
This study highlights infrared sensor technology incorporated into the Global Positioning System (GPS) dropsonde platforms to obtain sea surface temperature (SST) measurements. This modified sonde (IRsonde) is used to improve understanding of air-sea interaction in tropical cyclones (TCs). As part of the Sandy Supplemental project, IRsondes were constructed and then deployed during the 2014 hurricane season. Comparisons between SSTs measured by collocated IRsondes and ocean expendables show good agreement, especially in regions with no rain contamination. Surface fluxes were estimated using measurements from the IRsondes and AXBTs via a bulk method that requires measurements of SST and near-surface (10 m) wind speed, temperature and humidity. The evolution of surface fluxes and their role in the intensification and weakening of Hurricane Edouard (2014) are discussed in the context of boundary-layer recovery. Our result emphasizes the important role of surface-flux induced boundary-layer recovery in regulating the low-level thermodynamic structure that is tied to the asymmetry of convection and TC intensity change.
Zhang, J.A., R.F. Rogers, and V. Tallapragada. Impact of parameterized boundary layer structure on tropical cyclone rapid intensification forecasts in HWRF. Monthly Weather Review, 145(4):1413-1426, https://doi.org/10.1175/MWR-D-16-0129.1 2017
This study evaluates the impact of the modification of the vertical eddy diffusivity (Km) in the boundary layer parameterization of the Hurricane Weather Research and Forecasting (HWRF) model on forecasts of tropical cyclone (TC) rapid intensification (RI). Composites of HWRF forecasts of Hurricanes Earl (2010) and Karl (2010) were compared for two versions of the planetary boundary layer (PBL) scheme in HWRF. The results show that using a smaller value of Km, in better agreement with observations, improves RI forecasts. The composite-mean, inner-core structures for the two sets of runs at the time of RI onset are compared with observational, theoretical, and modeling studies of RI to determine why the runs with reduced Km are more likely to undergo RI. It is found that the forecasts with reduced Km at the RI onset have a shallower boundary layer with stronger inflow, more unstable near-surface air outside the eyewall, stronger and deeper updrafts in regions farther inward from the radius of maximum wind (RMW), and stronger boundary layer convergence closer to the storm center, although the mean storm intensity (as measured by the 10-m winds) is similar for the two groups. Finally, it is found that the departure of the maximum tangential wind from the gradient wind at the eyewall, and the inward advection of angular momentum outside the eyewall, is much larger in the forecasts with reduced Km. This study emphasizes the important role of the boundary-layer structure and dynamics in TC intensity change, supporting recent studies emphasizing the boundary-layer spin-up mechanism, and recommends further improvement to the HWRF PBL physics.
Zhang, S., Z. Pu, D.J. Posselt, and R. Atlas. Impact of CYGNSS ocean surface wind speeds on numerical simulations of a hurricane in observing system simulation experiments. Journal of Atmospheric and Oceanic Technology, 34(2):375-383, https://doi.org/10.1175/JTECH-D-16-0144.1 2017
The NASA Cyclone Global Navigation Satellite System (CYGNSS) is planned for launch in late 2016. It will make available frequent ocean surface wind speed observations throughout the life cycle of tropical storms and hurricanes. In this study, the impact of CYGNSS ocean surface winds on numerical simulations of a hurricane case is assessed with a research version of the Hurricane Weather Research and Forecasting model and a Gridpoint Statistical Interpolation data assimilation system in a regional observing system simulation experiment framework. Two different methods of reducing the CYGNSS data volume were tested; one in which the winds were thinned and one in which the winds were super-obbed. Results suggest that assimilation of the CYGNSS winds has great potential to improve hurricane track and intensity simulations through improved representations of the surface wind fields, hurricane inner-core structures, and surface fluxes. The assimilation of the super-obbed CYGNSS data seems to be more effective in improving hurricane track forecasts than thinning the data.
Zou, Z., D. Zhao, B. Liu, J.A. Zhang, and J. Huang. Observation-based parameterization of air-sea fluxes in terms of the wind speed and atmospheric stability under low-to-moderate wind conditions. Journal of Geophysical Research-Oceans, 122(5):4123-4142, https://doi.org/10.1002/2016JC012399 2017
This study explores the behavior of the exchange coefficients for wind stress (CD), sensible heat flux (CH), and water vapor flux (CE) as functions of surface wind speed (U10) and atmospheric stability using direct turbulent flux measurements obtained from a platform equipped with fast-response turbulence sensors in a low-to-moderate wind region. Turbulent fluxes are calculated using the eddy-correlation method with extensive observations. The total numbers of quality-controlled 30 min flux runs are 12,240, 5813, and 5637 for estimation of CD, CH, and CE, respectively. When adjusted to neutral stability using the Monin-Obukhov similarity theory (MOST), we found that CDN, CHN, and CEN decrease with neutral-adjusted wind speed when wind speed is less than 5 m/s. CDN is constant over the range 5 m/s < U10N < 12 m/s, then increases with U10N when U10N > 12 m/s. In contrast, CHN and CEN exhibit no clear dependence on wind speed and are generally constant, with mean values of 0.96 × 10−3 and 1.2 × 10−3, respectively. This behavior of neutral exchange coefficients is consistent with the findings of previous studies. We also found that CDN under offshore winds is generally greater than that under onshore wind conditions, which is ascribed to the younger wind waves present due to the shorter fetch in the former case. However, this behavior is not exhibited by CHN or CEN. The original CD, CH, and CE values without MOST adjustment are also investigated to develop a new parameterization based on wind speed and stability. Three stability parameters are tested, including the bulk Richardson number, stability as defined in COARE 3.0, and a simplified Richardson number using the Charnock parameter. This new parameterization is free of MOST and the associated self-correlation. Compared with previous studies and COARE 3.0 results, the new parameterization using the simplified Richardson number performs well, with an increased correlation coefficient and reduction of root-mean-square error and bias.
2016
Abarca, S.F., M.T. Montgomery, S.A. Braun, and J. Dunion. On the secondary eyewall formation of Hurricane Edouard (2014). Monthly Weather Review, 144(9):3321-3331, https://doi.org/10.1175/MWR-D-15-0421.1 2016
A first observationally-based estimation of departures from gradient wind balance during secondary eyewall formation is presented. The study is based on the Atlantic Hurricane Edouard (2014). This storm was observed during the National Aeronautics and Space Administration’s (NASA) Hurricane and Severe Storm Sentinel (HS3) experiment, a field campaign conducted in collaboration with the National Oceanic and Atmospheric Administration (NOAA). A total of 135 dropsondes are analyzed in two separate time periods: one named the secondary eyewall formation period and the other one referred to as the decaying-double eyewalled storm period. During the secondary eyewall formation period, a time when the storm was observed to have only one eyewall, the diagnosed agradient force has a secondary maxima that coincides with the radial location of the secondary eyewall observed in the second period of study. The maximum spin up tendency of the radial influx of absolute vertical vorticity is within the boundary layer in the region of the eyewall of the storm and the spin up tendency structure elongates radially outward into the secondary region of supergradient wind, where the secondary wind maxima is observed in the second period of study. An analysis of the boundary-layer averaged vertical structure of equivalent potential temperature reveals a conditionally unstable environment in the secondary eyewall formation region. These findings support the hypothesis that deep convective activity in this region contributed to spin up of the boundary layer tangential winds and the formation of a secondary eyewall that is observed during the decaying-double eyewalled storm period.
Androulidakis, Y., V. Kourafalou, G.R. Halliwell, M. Le Henaff, H.S. Kang, M. Mehari, and R. Atlas. Hurricane interaction with the upper ocean in the Amazon-Orinoco plume region. Ocean Dynamics, 66(12):1559-1588, https://doi.org/10.1007/s10236-016-0997-0 2016
The evolution of three successive hurricanes (Katia, Maria, and Ophelia) is investigated over the river plume area formed by the Amazon and Orinoco river outflows during September of 2011. The study focuses on hurricane impacts on the ocean structure and the ocean feedback influencing hurricane intensification. High-resolution (1/25° × 1/25° horizontal grid) numerical simulations of the circulation in the extended Atlantic Hurricane Region (Caribbean Sea, Gulf of Mexico, and Northwest Atlantic Ocean) were used to investigate the upper ocean response during the three hurricane-plume interaction cases. The three hurricanes revealed different evolution and intensification characteristics over an area covered by brackish surface waters. The upper ocean response to the hurricane passages over the plume affected region showed high variability due to the interaction of oceanic and atmospheric processes. The existence of a barrier layer (BL), formed by the offshore spreading of brackish waters, probably facilitated intensification of the first storm (Hurricane Katia) because the river-induced BL enhanced the resistance of the upper ocean to cooling. This effect was missing in the subsequent two hurricanes (Maria and Ophelia) as the eroded BL (due to Katia passage) allowed the upper ocean cooling to be increased. As a consequence, the amount of ocean thermal energy provided to these storms was greatly reduced, which acted to limit intensification. Numerical experiments and analyses, in tandem with observational support, lead to the conclusion that the presence of a river plume-induced BL is a strong factor in the ocean conditions influencing hurricane intensification.
Atlas, R., G.D. Emmitt, L. Bucci, K. Ryan, and J.A. Zhang. Impact of Doppler wind lidar data on hurricane prediction. Proceedings, 18th Coherent Laser Radar Conference, Boulder, CO, June 27-July 1, 2016. Cooperative Institute for Research in Environmental Sciences, 4 pp., 2016
One of the most important applications of a space-based Doppler Wind Lidar (DWL) would be to improve atmospheric analyses and weather forecasting. Since the mid-1980s, Observing System Simulation Experiments (OSSEs) have been conducted to evaluate the potential impact of space-based DWL data on numerical weather prediction (NWP). All of these OSSEs have shown significant beneficial impact on global analyses and forecasts. In more recent years, a limited number of OSSEs have been conducted to evaluate the potential impact of DWL data on hurricane forecasting and to also evaluate the impact of real airborne DWL observations. These latest studies suggest that DWL can complement existing hurricane observations effectively and should contribute to improved hurricane track and intensity forecasting.
Bell, G.D., C.W. Landsea, E.S. Blake, J. Schemm, S.B. Goldenberg, T.B. Kimberlain, and R.J. Pasch. Atlantic basin. In State of the Climate in 2015, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 97(8):S105-S108, https://doi.org/10.1175/2016BAMSStateoftheClimate.1 2016
Boukabara, S.A., I. Moradi, R. Atlas, S.P.F. Casey, L. Cucurull, R.N. Hoffman, K. Ide, V.K. Kumar, R. Li, Z. Li, M. Masutani, N. Shahroudi, J. Woollen, and Y. Zhou. Community global Observing System Simulation Experiment (OSSE) package (CGOP): Description and usage. Journal of Atmospheric and Oceanic Technology, 33(8):1759-1777, https://doi.org/10.1175/JTECH-D-16-0012.1 2016
A modular extensible framework for conducting Observing System Simulation Experiments (OSSEs) has been developed with the goals of (1) supporting decision-makers with quantitative assessments of proposed observing systems investments, (2) supporting readiness for new sensors, (3) enhancing collaboration across the community by making the most up-to-date OSSE components accessible, and (4) advancing the theory and practical application of OSSEs. This first implementation, the Community Global OSSE Package (CGOP), is for short- to medium-range global numerical weather prediction applications. The CGOP is based on a new mesoscale global nature run produced by NASA using the 7-km cubed sphere version of the Goddard Earth Observing System Model, version 5 (GEOS-5) Atmospheric General Circulation Model and the latest (January 2015) operational version of the NOAA global data assimilation (DA) system. CGOP includes procedures to simulate the full suite of observing systems used operationally in the global DA system, including conventional in situ, satellite-based radiance, and radio occultation observations. The methodology of adding a new proposed observation type is documented and illustrated with examples of current interest. The CGOP is designed to evolve, both to improve its realism and to keep pace with the advance of operational systems.
Boukabara, S.A., T. Zhu, H.L. Tolman, S. Lord, S. Goodman, R. Atlas, M. Goldberg, T. Auligne, B. Pierce, L. Cucurull, M. Zupanski, M. Zhang, I. Moradi, J. Otkin, D. Santek, B. Hoover, Z. Pu, X. Zhan, C. Hain, E. Kalnay, D. Hotta, S. Nolin, E. Bayler, A. Mehra, S.P.F. Casey, D. Lindsey, L. Grasso, V.K. Kumar, A. Powell, J. Xu, T. Greenwald, J. Zajic, J. Li, J. Li, B. Li, J. Liu, L. Fang, P. Wang, and T.-C. Chen. S4: An O2R/R2O infrastructure for optimizing satellite data utilization in NOAA numerical modeling systems: A step toward bridging the gap between research and operations. Bulletin of the American Meteorological Society, 97(12):2359-2378, https://doi.org/10.1175/BAMS-D-14-00188.1 2016
In 2011, the National Oceanic and Atmospheric Administration (NOAA) began a cooperative initiative with the academic community to address a vexing issue that has long been known as a disconnection between the operational and research realms for weather forecasting and data assimilation. The issue is the gap, or more exotically referred to as the "valley of death," between efforts within the broader research community and NOAA’s activities, which are heavily driven by operational constraints. With the stated goals of leveraging research community efforts to benefit NOAA’s mission and offering a path to operations for the latest research activities which support the NOAA mission, satellite data assimilation, in particular, this initiative aims to enhance the linkage between NOAA’s operational systems and the research efforts. A critical component is the establishment of an efficient Operations-To-Research (O2R) environment on the Supercomputer for Satellite Simulations and data assimilation Studies (S4). This O2R environment is critical for successful Research-To-Operations (R2O) transitions because it allows rigorous tracking, implementation, and merging of any changes necessary (to operational software codes, scripts, libraries, etc.) to achieve the scientific enhancement. So far, the S4 O2R environment, with close to 4700 computing cores (60 TFLOPs) and 1700 TB disk storage capacity, has been a great success and, consequently, was recently expanded to significantly increase its computing capacity. The objective of this article is to highlight some of the major achievements and benefits of this O2R approach, and some lessons learned, with the ultimate goal of inspiring other O2R/R2O initiatives in other areas and for other applications.
Cione, J.J., E.A. Kalina, E.W. Uhlhorn, A.M. Farber, and A.B. Damiano Coyote unmanned aircraft system observations in Hurricane Edouard (2014). Earth and Space Science, 3(9):370-380, https://doi.org/10.1002/2016EA000187 2016
Horizontal wind, temperature, and moisture observations are presented from two Coyote Unmanned Aircraft System (UAS) flights in the boundary layer of Hurricane Edouard (2014). The first flight sampled the meteorological conditions in the eye and eyewall at altitudes from 900-1500 m while Edouard was a major hurricane (105 kt) on 16 September 2014. The following day, a second Coyote sampled the inflow layer outside of the storm core at ~760 m altitude, when Edouard had weakened to an 80-kt hurricane. These flights represent the first deployments of a UAS from an airborne manned aircraft into a tropical cyclone. Comparisons between the Coyote data and the Lockheed WP-3D Orion (WP-3D) flight-level measurements and analyses constructed from dropsonde data are also provided. On 16 September 2014, the Coyote-measured horizontal wind speeds agree, on average, to within ~1 m s-1 of the wind speeds observed by the WP-3D, and reproduce the shape of the radial wind profile from the WP-3D measurements. For the inflow layer experiment on 17 September, the mean wind speeds from the Coyote and the dropsonde analysis differ by only 0.5 m s-1, while the Coyote captured increased variability (σ = 3.4 m s-1) in the horizontal wind field compared to the dropsonde analysis (σ = 2.2 m s-1). Thermodynamic data from the Coyote and dropsondes agree well for both flights, with average discrepancies of 0.4°C and 0.0°C for temperature and 0.7°C and 1.3°C for dew point temperature on 16 and 17 September, respectively.
Dorst, N. Book review: Inventing atmospheric science: Bjerknes, Rossby, Wexler, and the foundations of modern meteorology. Physics Today, 69(9):54, https://doi.org/10.1063/PT.3.3301 2016
Folmer, M.J., R.W. Pasken, S. Chiao, J. Dunion, and J. Halverson. Modeling studies on the formation of Hurricane Helene: The impact of GPS dropwindsondes from the NAMMA 2006 field campaign. Meteorology and Atmospheric Physics, 128(6):733-750, https://doi.org/10.1007/s00703-016-0452-2 2016
Numerical simulations, using the Weather Research and Forecasting (WRF) model in concert with GPS dropwindsondes released during the NASA African Monsoon Multidisciplinary Analyses 2006 Field Campaign, were conducted to provide additional insight on SAL-TC interaction. Using NCEP final analysis datasets to initialize the WRF, a sensitivity test was performed on the assimilated (i.e., observation nudging) GPS dropwindsondes to understand the effects of individual variables (i.e., moisture, temperature, and winds) on the simulation and determine the extent of improvement when compared to available observations. The results suggested that GPS dropwindsonde temperature data provided the most significant difference in the simulated storm organization, storm strength, and synoptic environment, but all of the variables assimilated at the same time give a more representative mesoscale and synoptic picture.
Gopalakrishnan, S., C.V. Srinavas, and K. Bhatia. The hurricane boundary layer. In Advanced Numerical Modeling and Data Assimilation Techniques for Tropical Cyclone Predictions, U.C. Mohanty and S.G. Gopalakrishnan (eds.). Springer Netherlands, 589-626, https://doi.org/10.1007/978-94-024-0896-6 2016
Guimond, S.R., G.M. Heymsfield, P.D. Reasor, and A.C. Didlake. The rapid intensification of Hurricane Karl (2010): New remote sensing observations of convective bursts from the Global Hawk platform. Journal of the Atmospheric Sciences, 73(9):3617-3639, https://doi.org/10.1175/JAS-D-16-0026.1 2016
The evolution of rapidly intensifying Hurricane Karl (2010) is examined from a suite of remote sensing observations during the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment. The novelties of this study are in the analysis of data from the airborne Doppler radar HIWRAP and the new Global Hawk airborne platform that allows long endurance sampling of hurricanes. Supporting data from the HAMSR microwave sounder coincident with HIWRAP and coordinated flights with the NOAA WP-3D aircraft help to provide a comprehensive understanding of the storm. The focus of the analysis is on documenting and understanding the structure, evolution, and role of small scale, deep convective forcing in the storm intensification process. Deep convective bursts are sporadically initiated in the downshear quadrants of the storm and rotate into the upshear quadrants for a period of ~12 h during the rapid intensification. The aircraft data analysis indicates that the bursts are being formed and maintained through a combination of two main processes: (1) convergence generated from counter-rotating mesovortex circulations and the larger vortex-scale flow; and (2) the turbulent (scales of ~25 km) transport of anomalously warm, buoyant air from the eye to the eyewall at low levels. The turbulent mixing across the eyewall interface and forced convective descent adjacent to the bursts assists in carving out the eye of Karl, which leads to an asymmetric enhancement of the warm core. The mesovortices play a key role in the evolution of the features described above. The Global Hawk aircraft allowed an examination of the vortex response and axisymmetrization period in addition to the burst pulsing phase. A pronounced axisymmetric development of the vortex is observed following the pulsing phase that includes a sloped eyewall structure and formation of a clear, wide eye.
Hoffmann, R.N., and R. Atlas. Future observing system simulation experiments. Bulletin of the American Meteorological Society, 97(9):1601-1616, https://doi.org/10.1175/BAMS-D-15-00200.1 2016
As operational forecast and data assimilation (DA) systems evolve, observing system simulation experiment (OSSE) systems must evolve in parallel. Expected development of operational systems—especially the use of data that are currently not used or are just beginning to be used, such as all-sky and surface affected microwave radiances—will greatly challenge our ability to construct realistic OSSE systems. An additional set of challenges will arise when future DA systems strongly couple the different earth system components. In response, future OSSE systems will require coupled models to simulate nature and coupled observation simulators. The requirements for future evolving OSSE systems and potential solutions to satisfy these requirements are discussed. It is anticipated that in the future the OSSE technique will be applied to diverse and coupled domains with the use of increasingly advanced and sophisticated simulations of nature and observations.
Kellner, O., D. Niyogi, and F.D. Marks. Contribution of landfalling tropical system rainfall to the hydroclimate of the eastern U.S. Corn belt, 1981-2012. Weather and Climate Extremes, 13:54-67, https://doi.org/10.1016/j.wace.2016.06.001 2016
This study provides a climatology (1981-2012) of landfalling tropical systems in the eastern U.S. Corn Belt and investigates the total contribution of these storms to the monthly climatological rainfall in the Midwestern United States. The primary focus is on rainfall impacts from landfalling tropical systems on historic corn yields at the climate division and crop reporting district level. Climatologically dry to drought conditions for historic monthly observed rainfall are identified using the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI). It was found that without landfalling tropical system rainfall, the percentage increase in climatologically dry (or drier) conditions across the domain at state climate division resolution increased from 16% up to over 200%. The study also considers the effects of climatologically wet conditions on crop yields. Landfalling tropical system rainfall accounts for approximately 20% of the observed monthly rainfall during the tropical storm season (June-November) across the eastern U.S. Corn Belt (1981-2012). Correlation between the annual number of landfalling tropical systems and annual yield by state results in no relationship, but correlation of August monthly observed rainfall by climate division to crop reporting district annual yields has a weak-to-moderate, statistically-significant correlation in Ohio districts 30-60 and Indiana CRD 90. ANOVA analysis suggests that landfalling tropical rainfall may actually reduce yields in some states' climate divisions/crop reporting districts while increasing yield in others. Results suggest that there is a balance between landfalling tropical storms providing sufficient rainfall or too much rainfall to be of benefit to crops. Findings aim to provide information to producers, crop advisers, risk managers, and commodity groups so that seasonal hurricane forecasts can potentially be utilized in planning for above or below normal precipitation during phenologically important portions of the growing season.
Klotz, B.W., and H. Jiang. Global composites of surface wind speeds in tropical cyclones based on a 12-year scatterometer database. Geophysical Research Letters, 43(19):10,480-10,488, https://doi.org/10.1002/2016GL071066 2016
A 12-year global database of rain-corrected satellite scatterometer surface winds for tropical cyclones (TCs) is used to produce composites of TC surface wind speed distributions relative to vertical wind shear and storm motion directions in each TC-prone basin and various TC intensity stages. These composites corroborate ideas presented in earlier studies, where maxima are located right of motion in the Earth-relative framework. The entire TC surface wind asymmetry is down motion left for all basins and for lower strength TCs after removing the motion vector. Relative to the shear direction, the motion-removed composites indicate that the surface wind asymmetry is located down shear left for the outer region of all TCs, but for the inner-core region it varies from left of shear to down shear right for different basin and TC intensity groups. Quantification of the surface wind asymmetric structure in further stratifications is a necessary next step for this scatterometer data set.
Lee, P., R. Atlas, G. Carmichael, Y. Tang, B. Pierce, A.P. Biazar, L. Pan, H. Kim, D. Tong, and W. Chen. Observing System Simulation Experiments (OSSEs) using a regional air quality application for evaluation. In Air Pollution Modeling and its Application XXIV, D.G. Steyn and N. Chaumerliac (eds.). Springer International Publishing, 599-605, https://doi.org/10.1007/978-3-319-24478-5_97 2016
Satellite-based and high-altitude airborne remotely sensed air quality data complement land-based and routinely commercial-flight and other measurement-campaign acquired remotely sensed and in situ observations. It is important to optimize the combination and placement of these wide ranges of measurements and data acquisition options for cost-effectiveness. Under this initiative, we attempt to quantify the gain by a regional state-of-the-science chemical data assimilation and chemical transport modeling system when incremental sets of observation are acquired into the system. This study represents a first step in a series of steps to ingest such proposed incremental additions of observation. The efficacy of such proposals is quantified systematically by Observation Simulation System Experiments (OSSEs). We compared two end-to-end regional air quality forecasting simulations using: (a) the Weather Forecasting and Research (WRF) regional application initialized by the U.S. National Weather Service (NWS) Global Forecasting System (GFS) coupled with the U.S. Environmental Protection Agency Community Multi-scale Air Quality (CMAQ) chemical model (Byun and Schere, 2006), and (b) the same as above but with a new GFS enhanced by assimilating a fictitious addition of Atmospheric Infrared Sounder (AIRS) retrieved radiances at 13 km spatial resolution at nadir from a proposed geostationary satellite positioned over 75oW staring over the U.S. Both sensitivity runs were performed in 12 km horizontal grid resolution and with daily initialization for 12 days between July 29 and August 9 2005. Noticeable forecast skill improvement in surface concentration for O3 and particulate matter smaller than 2.5 µm in diameter (PM2.5) was achieved.
Mai, M., B. Zhang, X. Li, P.A. Hwang, and J.A. Zhang. Application of AMSR-E and AMSR2 low frequency channel brightness temperature data for hurricane wind retrievals. IEEE Transactions on Geoscience and Remote Sensing, 54(8):4501-4512, https://doi.org/10.1109/TGRS.2016.2543502 2016
We present a method to retrieve wind speeds in hurricanes from spaceborne passive microwave radiometer data. Brightness temperature TBobservations acquired at the 6.9-GHz horizontal polarization channel by the AMSR-E and AMSR2 onboard the Earth Observing System Aqua and Global Change Observation Mission-Water 1 satellites are selected for wind retrieval due to the fact that the signal at this frequency is sensitive to high wind speeds but less sensitive to rain scatter than those acquired at other higher frequency channels. The AMSR-E and AMSR2 observations of 53 hurricanes between 2002 and 2014 are collected and collocated with stepped-frequency microwave radiometer (SFMR) measurements. Based on the small slope approximation/small perturbation method model and an ocean surface roughness spectrum, the wind speeds are retrieved from the TB data and validated against the SFMR measurements. The statistical comparison of the entire data set shows that the bias and root-mean-square error (RMSE) of the retrieved wind speeds are 1.11 and 4.34 m/s, respectively, which suggests that the proposed method can obtain high wind speeds under hurricane conditions. Two case studies show that the wind speed retrieval bias and RMSE are 1.08 and 3.93 m/s for Hurricane Earl and 0.09 and 3.23 m/s for Hurricane Edouard, respectively. The retrieved wind speeds from the AMSR-E and AMSR2 continuous three-day observations clearly show the process of hurricane intensification and weakening.
Marks, F.D. Advancing the understanding and prediction of tropical cyclones using aircraft observations. In Advanced Numerical Modeling and Data Assimilation Techniques for Tropical Cyclone Predictions, U.C. Mohanty and S.G. Gopalakrishnan (eds.). Springer Netherlands, 3-34, https://doi.org/10.1007/978-94-024-0896-6 2016
Advances in the study of tropical cyclones using aircraft observations came about through improvements (scientific and technological) in the ability to observe different aspects of the storms. Early studies provided the basis for understanding tropical cyclone structure and evolution (for an excellent overview see Dorst, 2007). Technological advances in aircraft in-situ and remote sensing observing capabilities, particularly the development of airborne Doppler radars, revolutionized our depiction of tropical cyclone structure and dynamics (for a review, see Marks, 2003).
Ming, J., and J.A. Zhang. Effects of surface flux parameterization on numerically simulated intensity and structure of Typhoon Morakot (2009). Advances in Atmospheric Sciences, 33(1):58-72, https://doi.org/10.1007/s00376-015-4202-z 2016
The effects of surface flux parameterizations on tropical cyclone (TC) intensity and structure are investigated using the Advanced Research Weather Research and Forecasting (WRF-ARW) modeling system with high-resolution simulations of Typhoon Morakot (2009). Numerical experiments are designed to simulate Typhoon Morakot (2009) with different formulations of surface exchange coefficients for enthalpy (CK) and momentum (CD) transfers, including those from recent observational studies based on in situ aircraft data collected in Atlantic hurricanes. The results show that the simulated intensity and structure are sensitive to CK and CD, but the simulated track is not. Consistent with previous studies, the simulated storm intensity is found to be more sensitive to the ratio of CK/CD than to CK or CD alone. The pressure–wind relationship is also found to be influenced by the exchange coefficients, consistent with recent numerical studies. This paper emphasizes the importance of CD and CK on TC structure simulations. The results suggest that CD and CK have a large impact on surface wind and flux distributions, boundary layer heights, the warm core, and precipitation. Compared to available observations, the experiment with observed CD and CK generally simulated better intensity and structure than the other experiments, especially over the ocean. The reasons for the structural differences among the experiments with different CD and CK setups are discussed in the context of TC dynamics and thermodynamics.
Mohanty, U.C., and S.G. Gopalakrishnan (eds.). Advanced Numerical Modeling and Data Assimilation Techniques for Tropical Cyclone Predictions. Springer Netherlands, 746 pp., https://doi.org/10.1007/978-94-024-0896-6 2016
This book deals primarily with monitoring, prediction, and understanding of tropical cyclones (TCs). It was envisioned to serve as a teaching and reference resource at universities and academic institutions for researchers and post-graduate students. It has been designed to provide a broad outlook on recent advances in observations, assimilation, and modeling of TCs with detailed and advanced information on genesis, intensification, movement, and storm-surge prediction. Specifically, it focuses on (i) state-of-the-art observations for advancing TC research, (ii) advances in numerical weather prediction for TCs, (iii) advanced assimilation and vortex initialization techniques, (iv) ocean coupling, (v) current capabilities to predict TCs, and (vi) advanced research in physical and dynamical processes in TCs. The chapters in the book are authored by leading international experts from academic, research, and operational environments. The book is also expected to stimulate critical thinking for cyclone forecasters and researchers, managers, policy makers, and graduate and post-graduate students to carry out future research in the field of TCs.
Quirino, T., and S.G. Gopalakrishnan. Advanced diagnostics for the HWRF hurricane modeling system. In Advanced Numerical Modeling and Data Assimilation Techniques for Tropical Cyclone Predictions, U.C. Mohanty and S.G. Gopalakrishnan (eds.). Springer Netherlands, 517-534, https://doi.org/10.1007/978-94-024-0896-6 2016
Rogers, R.F., J.A. Zhang, J. Zawislak, H. Jiang, G.R. Alvey, E.J. Zipser, and S.N. Stevenson. Observations of the structure and evolution of Hurricane Edouard (2014) during intensity change, Part II: Kinematic structure and the distribution of deep convection. Monthly Weather Review, 144(9):3355-3376, https://doi.org/10.1175/MWR-D-16-0017.1 2016
The structural evolution of the inner core and near-environment throughout the lifecycle of Hurricane Edouard (2014) is examined using a synthesis of airborne and satellite measurements. This study specifically focuses on differences in the distribution of deep convection during two periods: when Edouard intensified towards hurricane status and when Edouard peaked in intensity and began to weaken. While both periods saw precipitation maximized in the downshear left and upshear left quadrants, deep convection was only seen from the aircraft during the intensifying period. Deep convection was located farther inside the radius of maximum winds (RMW) during the intensifying period than the weakening period. This convection is traced to strong updrafts inside the RMW in the downshear right quadrant, tied to strong low-level convergence, and high convective available potential energy (CAPE) as the storm remained over warm water in a moist environment. Strong updrafts persisted upshear left and were collocated with high inertial stability in the inner core. During weakening, no deep convection was present, and the precipitation that was observed was associated with weaker convergence downshear right at larger radii, as CAPE was reduced from lower sea surface temperatures, reduced humidity from subsidence, and a stronger warm core. Weak updrafts were seen upshear left, with little coincidence with the high inertial stability of the inner core. These results highlight the importance of the azimuthal coverage of precipitation and the radial location of deep convection for intensification. A more symmetrical coverage can occur despite the presence of shear-driven azimuthal asymmetries in both the forcing and the local environment of the precipitation.
Ruf, C.S., R. Atlas, P.S. Chang, M.P. Clarizia, J.L. Garrison, S. Gleason, S.J. Katzberg, Z. Jelenak, J.T. Johnson, S.J. Majumdar, A. O’Brien, D.J. Posselt, A.J. Ridley, R.J. Rose, and V.U. Zavorotny. New ocean winds satellite mission to probe hurricanes and tropical convection. Bulletin of the American Meteorological Society, 97(3):385-395, https://doi.org/10.1175/BAMS-D-14-00218.1 2016
The Cyclone Global Navigation Satellite System (CYGNSS) is a new NASA Earth science mission scheduled to be launched in 2016 that focuses on tropical cyclones (TC) and tropical convection. The mission’s two primary objectives are the measurement of ocean surface wind speed with sufficient temporal resolution to resolve short time scale processes such as the rapid intensification phase of TC development, and the ability of its surface observations to penetrate through the extremely high precipitation rates typically encountered in the TC inner core. The mission’s goal is to support significant improvements in our ability to forecast TC track, intensity and storm surge through better observations and, ultimately, better understanding of inner core processes. CYGNSS meets its temporal sampling objective by deploying a constellation of eight satellites. Its ability to see through heavy precipitation is enabled by its operation as a bistatic radar using low frequency GPS signals. The mission will deploy an eight spacecraft constellation in a low inclination (35°) circular orbit to maximize coverage and sampling in the tropics. Each CYGNSS spacecraft carries a 4-channel radar receiver that measures GPS navigation signals scattered by the ocean surface. The mission will measure inner core surface winds with high temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of TC wind speeds.
Stern, D.P., G.H. Bryan, and S.D. Aberson. Extreme low-level updrafts and wind speeds measured by dropsondes in tropical cyclones. Monthly Weather Review, 144(6):2177-2204 , https://doi.org/10.1175/MWR-D-15-0313.1 2016
Previous studies have found surprisingly strong vertical motions in low levels of some tropical cyclones. In this study, all available dropsondes (~12,000) within tropical cyclones from 1997-2013 are examined, in order to create a dataset of the most extreme updrafts (≥ 10 m s−1; 169 sondes) and wind speeds (≥ 90 m s−1; 64 sondes). It is shown that extreme low-level (0-3 km) updrafts are ubiquitous within intense (Category 4 and 5) tropical cyclones, and that few such updrafts have been observed within weaker storms. These extreme updrafts, which are almost exclusively found within the eyewall just inwards of the radius of maximum winds, sometimes occur in close association with extreme horizontal wind speeds. Consistent with previous studies, it is suggested that both the extremes in vertical velocity and wind speed are associated with small-scale (~1 km) vortices that exist along the eye/eyewall interface. As a substantial number of updrafts are found within a kilometer of the surface, it can be shown that it is implausible for buoyancy to be the primary mechanism for vertical acceleration. Additionally, the azimuthal distribution of both the extreme updrafts and wind speeds is strongly associated with the orientation of the environmental vertical wind shear.
Zawislak, J., H. Jiang, G.R. Alvey, E.J. Zipser, R.F. Rogers, J.A. Zhang, and S.N. Stevenson. Observations of the structure and evolution of Hurricane Edouard (2014) during intensity change, Part 1: Relationship between the thermodynamic structure and precipitation. Monthly Weather Review, 144(9):3333-3354, https://doi.org/10.1175/MWR-D-16-0018-1 2016
The structural evolution of the inner core and near-environment throughout the lifecycle of Hurricane Edouard (2014) is examined using a synthesis of airborne and satellite measurements. This study specifically focuses on the precipitation evolution and thermodynamic changes that occur on the vortex-scale during four periods: when Edouard was a slowly intensifying tropical storm, another while a rapidly intensifying hurricane, during the initial stages of weakening after reaching peak intensity, and later while experiencing moderate weakening in the midlatitudes. Results suggest that, in a shear-relative framework, a wavenumber-1 asymmetry exists whereby the downshear quadrants consistently exhibit the greatest precipitation coverage and highest relative humidity, while the upshear quadrants (particularly upshear right) exhibit relatively less precipitation coverage and lower humidity, particularly in the midtroposphere. Whether dynamically- or precipitation-driven, the relatively dry layers upshear appear to be ubiquitously caused by subsidence. The precipitation and thermodynamic asymmetry is observed throughout the intensification and later weakening stages, while a consistently more symmetric distribution is only observed when Edouard reaches peak intensity. The precipitation distribution, which is also discussed in the context of the boundary layer thermodynamic properties, is intimately linked to the thermodynamic symmetry, which becomes greater as the frequency, areal coverage, and, in particular, rainfall rate increases upshear. Although shear is generally believed to be detrimental to intensification, observations in Edouard also indicate that subsidence warming from mesoscale downdrafts in the low- to mid-troposphere very near the center may have contributed favorably to organization early in the intensification stage.
Zhang, R., J. Huang, X. Wang, J.A. Zhang, and F. Huang. Effects of precipitation on sonic anemometer measurements of turbulent fluxes in the atmospheric surface layer. Journal of Ocean University of China, 15(3):389-398, https://doi.org/10.1007/s11802-016-2804-4 2016
Effects caused by precipitation on the measurements of a three-dimensional sonic anemometer are analyzed based on a field observational experiment conducted in Maoming, Guangdong Province, China. Obvious fluctuations induced by precipitation are observed for the outputs of sonic anemometer-derived temperature and wind velocity components. A technique of turbulence spectra and cospectra normalized in the framework of similarity theory is utilized to validate the measured variables and calculated fluxes. It is found that the sensitivity of the sonic anemometer-derived temperature to precipitation is significant when compared with that of the wind velocity components. The spectra of wind velocity and cospectra of momentum flux resemble the standard universal shape, with the slopes of the spectra and cospectra at the inertial subrange following the −2/3 and −4/3 power law, respectively, even under the condition of heavy rain. Contaminated by precipitation, however, the spectra of temperature and cospectra of sensible heat flux do not exhibit a universal shape and have obvious frequency loss at the inertial subrange. From the physical structure and working principle of the sonic anemometer, a possible explanation is proposed to describe this difference, which is found to be related to the variations of precipitation particles. Corrections for errors of sonic anemometer-derived temperature under precipitation is needed, which is still under exploration.
Zhang, X., S.G. Gopalakrishnan, S. Trahan, T.S. Quirino, Q. Liu, Z. Zhang, G. Alaka, and V. Tallapragada. Representing multiple scales in the Hurricane Weather Research and Forecasting modeling system: Design of multiple sets of movable multi-level nesting and the basin-scale HWRF forecast verification. Weather and Forecasting, 31(6):2019-2034, https://doi.org/10.1175/WAF-D-16-0087.1 2016
In this study, the design of movable multi-level nesting (MMLN) in the Hurricane Weather Research and Forecasting (HWRF) modeling system is documented. The configuration of a new experimental HWRF system with a much larger horizontal outer domain and multiple sets of MMLN, referred to as the “basin-scale” HWRF, is also described. The performance of this new system is applied for various difficult forecast scenarios such as: (1) simulating multiple storms, i.e., Hurricanes Earl (2010), Danielle (2010), and Frank (2010); and (2) forecasting tropical cyclone (TC) to extratropical cyclone transitions, specifically Hurricane Sandy (2012). Verification of track forecasts for the 2011-2014 Atlantic and East Pacific hurricane seasons demonstrates that the basin-scale HWRF produces similar overall results to the 2014 operational HWRF, the best operational HWRF at the same resolution. In the Atlantic, intensity forecasts for the basin-scale HWRF were notably worse than for the 2014 operational HWRF, but this deficiency was shown to be from poor intensity forecasts for Hurricane Leslie (2012) associated with the lack of ocean coupling in the basin-scale HWRF. With Leslie removed, the intensity forecast errors were equivalent. The basin-scale HWRF is capable of predicting multiple TCs simultaneously, allowing more realistic storm-to-storm interactions. Even though the basin-scale HWRF produced results only comparable to the regular operational HWRF at this stage, this configuration paves a promising pathway toward operations.
2015
Aberson, S.D., A. Aksoy, K.J. Sellwood, T. Vukicevic, and X. Zhang. Assimilation of high-resolution tropical cyclone observations with an ensemble Kalman filter using HEDAS: Evaluation of 2008-2011 HWRF forecasts. Monthly Weather Review, 143(2):511-523, https://doi.org/10.1175/MWR-D-14-00138.1 2015
NOAA has been gathering high-resolution flight-level, dropwindsonde and airborne Doppler radar data in tropical cyclones for almost three decades; the U.S. Air Force routinely obtained the same type and quality of data, excepting Doppler radar, for most of that time. The data have been used for operational diagnosis and for research, and, starting in 2013, have been assimilated into operational regional tropical cyclone models. This study is an effort to quantify the impact of assimilating these data into a version of the operational Hurricane Weather Research and Forecast model using an ensemble Kalman filter. Eighty-three cases from 2008-2011 were investigated. The aircraft whose data were used in the study all provide high-density flight-level wind and thermodynamic observations as well as surface wind speed data. Forecasts initialized with these data assimilated are compared to those using the model standard initialization. Since only NOAA aircraft provide airborne Doppler radar data, these data are also tested to see their impact above the standard aircraft data. The aircraft data alone are shown to provide some statistically significant improvement to track and intensity forecasts during the critical watch and warning period before projected landfall (through 60 h), with the Doppler radar data providing some further improvement. This study shows the potential for improved forecasts with regular tropical cyclone aircraft reconnaissance and the assimilation of data obtained from them, especially airborne Doppler radar data, into the numerical guidance.
Atlas, R., L. Bucci, B. Annane, R. Hoffman, and S. Murillo. Observing System Simulation Experiments to assess the potential impact of new observing systems on hurricane forecasting. Marine Technology Society Journal, 49(6):140-148, https://doi.org/10.4031/MTSJ.49.6.3 2015
Observing System Simulation Experiments (OSSEs) are an important tool for evaluating the potential impact of new or proposed observing systems, as well as for evaluating trade-offs in observing system design, and in developing and assessing improved methodology for assimilating new observations. Extensive OSSEs have been conducted at the National Aeronautical and Space Administration (NASA) Goddard Space Flight Center (GSFC) and the National Oceanic and Atmospheric Administration (NOAA) Atlantic Oceanographic and Meteorological Laboratory (AOML) over the last three decades. These OSSEs determined correctly the quantitative potential for several proposed satellite observing systems to improve weather analysis and prediction prior to their launch; evaluated trade-offs in orbits, coverage, and accuracy for space-based wind lidars; and were used in the development of the methodology that led to the first beneficial impacts of satellite surface winds on numerical weather prediction. This paper summarizes early applications of global OSSEs to hurricane track forecasting and new experiments using both global and regional models. These latter experiments are aimed at assessing potential impact on hurricane track and intensity prediction over the oceans and at landfall.
Atlas, R., R.N. Hoffman, Z. Ma, G.D. Emmitt, S.A. Wood, S. Greco, S. Tucker, L. Bucci, B. Annane, R.M. Hardesty, and S. Murillo. Observing system simulation experiments (OSSEs) to evaluate the potential impact of an optical autocovariance wind lidar (OAWL) on numerical weather prediction. Journal of Atmospheric and Oceanic Technology, 32(9):1593-1613, https://doi.org/10.1175/JTECH-D-15-0038.1 2015
The potential impact of Doppler wind lidar (DWL) observations from a proposed optical autocovariance wind lidar (OAWL) instrument is quantified in observing system simulation experiments (OSSEs). The OAWL design would provide profiles of useful wind vectors along a ground track to the left of the International Space Station (ISS), which is in a 51.6° inclination low earth orbit (LEO). These observations are simulated realistically, accounting for cloud and aerosol distributions inferred from the OSSE nature runs (NRs), and measurement and sampling error sources. The impact of the simulated observations is determined in both global and regional OSSE frameworks. The global OSSE uses the ECMWF T511 NR and the NCEP operational global data assimilation (DA) system at T382 resolution. The regional OSSE uses an embedded hurricane NR and the NCEP operational HWRF DA system with outer and inner domains of 9 and 3 km resolution. The global OSSE results show improved analyses and forecasts of tropical winds and extratropical geopotential heights. The tropical wind RMSEs are significantly reduced in the analyses and in short term forecasts. The tropical wind improvement decays as the forecasts lengthen. The regional OSSEs are limited but show some improvements in hurricane track and intensity forecasts.
Atlas, R., V. Tallapragada, and S. Gopalakrishnan. Advances in tropical cyclone intensity forecasts. Marine Technology Society Journal, 49(6):149-160, https://doi.org/10.4031/MTSJ.49.6.2 2015
NOAA established the 10 year Hurricane Forecast Improvement Project (HFIP) to accelerate the improvement of forecasts and warnings of tropical cyclones and to enhance mitigation and preparedness by increased confidence in those forecasts. Specific goals include reducing track and intensity errors by 20% in 5 years and 50% in ten years and extending the useful range of hurricane forecasts to 7 days. Under HFIP, there have been significant improvements to NOAA’s operational hurricane prediction model resulting in increased accuracy in the numerical guidance for tropical cyclone intensity predictions. This paper documents many of the improvements that have been accomplished over the last 5 years, as well as some future research directions that are being pursued.
Bell, G.D., E.S. Blake, C.W. Landsea, S.B. Goldenberg, T.B. Kimberlain, R.J. Pasch, and J. Schemm. Tropical cyclones: Atlantic basin. In State of the Climate in 2014, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 96(7):S101-S107, https://doi.org/10.1175/2015BAMSStateoftheClimate.1 2015
Bernardet, L., V. Tallapragada, S. Bao, S. Trahan, Y. Kwon, Q. Liu, M. Tong, M. Biswas, T. Brown, D. Stark, L. Carson, R. Yablonsky, E. Uhlhorn, S. Gopalakrishnan, X. Zhang, T. Marchok, B. Kuo, and R. Gall. Community support and transition of research to operations for the Hurricane Weather Research and Forecasting model. Bulletin of the American Meteorological Society, 96(6):953-960, https://doi.org/10.1175/BAMS-D-13-00093.1 2015
The Hurricane Weather Research and Forecasting (HWRF) model is an operational model used to provide numerical guidance in support of tropical cyclone forecasting at the National Hurricane Center. HWRF is a complex multi-component system, consisting of the Weather Research and Forecasting (WRF) atmospheric model coupled to the Princeton Ocean Model for Tropical Cyclones (POM-TC), a sophisticated initialization package including a data assimilation system, and a set of postprocessing and vortex tracking tools. HWRF's development is centralized at the Environmental Modeling Center of NOAA's National Weather Service, but it incorporates contributions from a variety of scientists spread out over several governmental laboratories and academic institutions. This distributed development scenario poses significant challenges: a large number of scientists need to learn how to use the model, operational and research codes need to stay synchronized to avoid divergence, and promising new capabilities need to be tested for operational consideration. This article describes how the Developmental Testbed Center has engaged in the HWRF developmental cycle in the last three years and the services it provides to the community in using and developing HWRF.
Chen, H., and S.G. Gopalakrishnan. A study on the asymmetric rapid intensification of Hurricane Earl (2010) using the HWRF system. Journal of the Atmospheric Sciences, 72(2):531-550, https://doi.org/10.1175/JAS-D-14-0097.1 2015
In this study, the results of a forecast from the operational Hurricane Weather Research and Forecasting (HWRF) system for Hurricane Earl (2010) are verified against observations and analyzed to understand the asymmetric rapid intensification of a storm in a sheared environment. The forecast verification shows that the HWRF model captured well Earl’s observed evolution of intensity, convection asymmetry, wind field asymmetry, and vortex tilt in terms of magnitude and direction in the pre-rapid and rapid intensification (RI) stages. Examination of the high-resolution forecast data reveals that the tilt was large at the RI onset and decreased quickly once RI commenced, suggesting that vertical alignment is the result instead of the trigger for RI. The RI onset is associated with the development of upper-level warming in the eye, which results from upper-level storm-relative flow advecting the warm air caused by subsidence warming in the upshear-left region towards the low-level storm center. This scenario does not occur until persistent convective bursts (CB) are concentrated in the downshear-left quadrant. The temperature budget calculation indicates that horizontal advection plays an important role in the development of upper-level warming in the early RI stage. The upper-level warming associated with the asymmetric intensification process occurs by means of the cooperative interaction of the convective-scale subsidence, resulting from CBs in favored regions and the shear-induced mesoscale subsidence. When CBs are concentrated in the downshear-left and upshear-left quadrants, the subsidence warming is maximized upshear and then advected towards the low-level storm center by the storm-relative flow at the upper level. Subsequently, the surface pressure falls and RI occurs.
Cione, J.J. The relative roles of the ocean and atmosphere as revealed by buoy air-sea observations in hurricanes. Monthly Weather Review, 143(3):904-913, https://doi.org/10.1175/MWR-D-13-00380.1 2015
Results from this multi-hurricane study suggest that the criticality of the oft-cited 26°C hurricane threshold linked to hurricane maintenance may be more closely associated with atmospheric thermodynamic conditions within the inner core than previously believed. In all cases, a positive sea-air contrast was observed within the storm inner core (i.e., surface ocean temperature greater than surface air temperature), despite the fact that 6% of the hurricanes exhibited sea surface temperatures (SST) less than the 26°C. For the storms sampled in this study, inner core surface dewpoint temperatures never exceeded 26.5°C. This finding may provide an alternate explanation as to the criticality of the 26°C threshold since SSTs above 26°C would, in almost all instances, be associated with a positive enthalpy flux condition. Analyses from this study also illustrate that high wind SSTs fluctuate as a function of storm latitude, while inner core near surface dewpoint temperatures are much less sensitive to this parameter. As a result, and assuming all other factors to be equal, low latitude hurricanes would, on average, be expected to experience surface moisture fluxes ~1/3 greater than storms located farther to the north. For systems sampled within the deep tropics, inner core SST was found to fluctuate much less than surface dewpoint temperature, suggesting that the atmosphere, not the ocean, is more likely to influence the key thermodynamic parameter controlling surface moisture flux for this subset of hurricanes.
Goldenberg, S.B., S.G. Gopalakrishnan, V. Tallapragada, T. Quirino, F. Marks, S. Trahan, X. Zhang, and R. Atlas. The 2012 triply-nested, high-resolution operational version of the Hurricane Weather Research and Forecasting System (HWRF): Track and intensity forecast verifications. Weather and Forecasting, 30(3):710-729, https://doi.org/10.1175/WAF-D-14-00098.1 2015
The Hurricane Weather Research and Forecast system (HWRF) was operationally implemented with a 27-km outer domain and 9-km moving nest in 2007 (H007) as a tropical cyclone forecast model for the North Atlantic and Eastern Pacific hurricane basins. During the 2012 hurricane season, a modified version of the model (H212) that increased horizontal resolution by adding a third (3 km) nest within the 9-km nest replaced H007. H212 thus became the first operational model running at convection-permitting resolution. In addition, there were modifications to initialization, model physics, tracking algorithm, etc. This paper compares H212 model hindcast forecasts for the 2010-2011 Atlantic hurricane seasons with forecasts from H007 and H3GP, a triply-nested research HWRF version. H212 reduced track forecast errors for almost all forecast times versus H007 and H3GP. H3GP was superior for intensity forecasts, although H212 showed some improvement over H007. Stratifying the cases by initial vertical wind shear revealed that the main weakness for H212 intensity forecasts was for cases with initially high shear. In these cases, H212 over- and under-intensified storms that were initially stronger and weaker, respectively. These results suggest the primary deficiency negatively impacting H212 intensity forecasts, especially in cases of rapid intensification, was that physics calls were too infrequent for the 3-km inner mesh. Correcting this deficiency along with additional modifications in the 2013 operational version yielded improved track and intensity forecasts. These intensity forecasts were comparable to statistical/dynamical models, showing that dynamical models can contribute to a decrease in operational forecast errors.
Haddad, Z.S., J.L. Steward, H.-C. Tseng, T. Vukicevic, S.-H. Chen, and S. Hristova-Veleva. A data assimilation technique to account for the nonlinear dependence of scattering microwave observations of precipitation. Journal of Geophysical Research-Atmospheres, 120(11):5548-5563, https://doi.org/10.1002/2015JD023107 2015
Satellite microwave observations of rain, whether from radar or passive radiometers, depend in a very crucial way on the vertical distribution of the condensed water mass and on the types and sizes of the hydrometeors in the volume resolved by the instrument. This crucial dependence is nonlinear, with different types and orders of nonlinearity that are due to differences in the absorption/emission and scattering signatures at the different instrument frequencies. Because it is not monotone as a function of the underlying condensed water mass, the nonlinearity requires great care in its representation in the observation operator, as the inevitable uncertainties in the numerous precipitation variables are not directly convertible into an additive white uncertainty in the forward calculated observations. In particular, when attempting to assimilate such data into a cloud-permitting model, special care needs to be applied to describe and quantify the expected uncertainty in the observations operator in order not to turn the implicit white additive uncertainty on the input values into complicated biases in the calculated radiances. One approach would be to calculate the means and covariances of the nonlinearly calculated radiances given an a priori joint distribution for the input variables. This would be a very resource-intensive proposal if performed in real time. We propose a representation of the observation operator based on performing this moment calculation off line, with a dimensionality reduction step to allow for the effective calculation of the observation operator and the associated covariance in real time during the assimilation. The approach is applicable to other remotely sensed observations that depend nonlinearly on model variables, including wind vector fields. The approach has been successfully applied to the case of tropical cyclones, where the organization of the system helps in identifying the dimensionality-reducing variables.
Halliwell, G.R., S. Gopalakrishnan, F. Marks, and D. Willey. Idealized study of ocean impacts on tropical cyclone intensity forecasts. Monthly Weather Review, 143(4):1142-1165, https://doi.org/10.1175/MWR-D-14-00022.1 2015
Idealized coupled tropical cyclone (TC) simulations are conducted to isolate ocean impacts on intensity forecasts. A one-dimensional ocean model is embedded into the Hurricane Weather Research and Forecasting (HWRF) mesoscale atmospheric forecast model. By inserting an initial vortex into a horizontally uniform atmosphere above a horizontally uniform ocean, SST cooling rate becomes the dominant large-scale process controlling intensity evolution. Westward storm translation is introduced by bodily advecting ocean fields toward the east. The ocean model produces a realistic cold wake structure allowing the sensitivity of quasi-equilibrium intensity to storm (translation speed, size) and ocean (heat potential) parameters to be quantified. The atmosphere provides feedback through adjustments in 10-m temperature and humidity that reduce SST cooling impact on quasi-equilibrium intensity by up to 40%. When storms encounter an oceanic region with different heat potential, enthalpy flux adjustment is governed primarily by changes in air-sea temperature and humidity differences that respond within 2-4 h in the inner-core region, and secondarily by wind speed changes occurring over a time interval up to 18 h after the transition. Atmospheric feedback always acts to limit the change in enthalpy flux and intensity through adjustments in 10-m temperature and humidity. Intensity change is asymmetric, with a substantially smaller increase for storms encountering larger heat potential compared to the decrease for storms encountering smaller potential. The smaller increase results initially from the smaller wind speed present at the transition time plus stronger limiting atmospheric feedback. The smaller wind speed increase resulting from these two factors further enhances the asymmetry.
Hazelton, A.T., R. Rogers, and R.E. Hart. Shear-relative asymmetries in tropical cyclone eyewall slope. Monthly Weather Review, 143(3):883-903, https://doi.org/10.1175/MWR-D-14-00122.1 2015
Recent studies have analyzed azimuthal mean slope of the tropical cyclone (TC) eyewall. This study looks at the shear-relative azimuthal variability of different metrics of eyewall slope: the 20 dBZ surface, the radius of maximum wind (RMW), and an angular momentum (M) surface passing through the RMW. The data used are Doppler radar composites from NOAA HRD. This study examines 34 TCs, with intensities ranging from 35-75 m/s and shear magnitudes ranging from 0-10 m/s. Calculation of the mean slope in each quadrant for all cases shows that RMW slope has the strongest asymmetry, with downshear slope larger than upshear in 62% of cases. Slopes of momentum surfaces and dBZ surfaces are also greater downshear in some cases (65% for M and 47% for dBZ), but there is more variance than in RMW slope. The azimuthal phase of maximum slope occurs most often downshear, particularly downshear left, consistent with the depiction of a mean vortex tilt approximately 10 degrees left of shear. Filtering the cases into high and low shear illustrates that the tendency for greater slope downshear is magnified for high-shear cases. In addition, although the dBZ slope shows less shear-relative signal overall, the difference between dBZ slope and momentum slope is an important factor in distinguishing between strengthening and weakening or steady TCs. Intensifying TCs tend to have dBZ surfaces that are more upright than M surfaces. Further investigation of these results will help to illustrate the ways in which vertical shear can play a role in altering the structure of the TC core region.
Jaimes, B., L.K. Shay, and E.W. Uhlhorn. Enthalpy and momentum fluxes during Hurricane Earl relative to underlying ocean features. Monthly Weather Review, 143(1):111-131, https://doi.org/10.1175/MWR-D-13-00277.1 2015
Using dropsondes from 27 aircraft flights, in-situ, and satellite data acquired during tropical cyclone Earl (category 4 hurricane), bulk air-sea fluxes of enthalpy and momentum are investigated in relation to intensity change and underlying upper-ocean thermal structure. During Earl’s rapid intensification (RI) period, ocean heat content (OHC) variability relative to the 26°C isotherm exceeded 90 kJ cm-2, and sea surface cooling was less than 0.5°C. Enthalpy fluxes of ~1.1 kW m-2 were estimated for Earl’s peak intensity. Daily sea surface heat losses of −6.5±0.8, −7.8±1.1, and +2.3±0.7 kJ cm-2 were estimated for RI, mature, and weakening stages, respectively. A ratio CK/CD of the exchange coefficients of enthalpy (Ck) and momentum (CD) between 0.54 and 0.7 produced reliable estimates for the fluxes relative to OHC changes, even during RI; a ratio CK/CD = 1 overestimated the fluxes. The most important result is that bulk enthalpy fluxes were controlled by the thermodynamic disequilibrium between the sea surface and the near-surface air, independently of wind speed. This disequilibrium was strongly influenced by underlying warm oceanic features; localized maxima in enthalpy fluxes developed over tight horizontal gradients of moisture disequilibrium over these eddy features. These regions of local buoyant forcing preferentially developed during RI. The overall magnitude of the moisture disequilibrium (Δq=qs-qa) was determined by the saturation specific humidity at sea surface temperature (qs) rather than by the specific humidity of the atmospheric environment (qa). These results support the hypothesis that intense local buoyant forcing by the ocean could be an important intensification mechanism in tropical cyclones over warm oceanic features.
Kaplan, J., C.M. Rozoff, M. DeMaria, C.R. Sampson, J.P. Kossin, C.S. Velden, J.J. Cione, J.P. Dunion, J.A. Knaff, J.A. Zhang, J.F. Dostalek, J.D. Hawkins, T.F. Lee, and J.E. Solbrig. Evaluating environmental impacts on tropical cyclone rapid intensification predictability utilizing statistical models. Weather and Forecasting, 30(5):1374-1396, https://doi.org/10.1175/WAF-D-15-0032.1 2015
New multi-lead time versions of three statistical probabilistic tropical cyclone rapid intensification (RI) prediction models are developed for the Atlantic and eastern North Pacific basins. These models are the linear-discriminant, analysis-based Statistical Hurricane Intensity Prediction Scheme Rapid Intensification Index (SHIPS-RII) and logistic regression and Bayesian statistical RI models. Consensus RI models derived by averaging the three individual RI model probability forecasts are also generated. A verification of the cross-validated forecasts of the above RI models conducted for the 12, 24, 36, and 48 h lead times indicates that these models generally exhibit skill relative to climatological forecasts with the eastern Pacific models, providing somewhat more skill than the Atlantic ones and the consensus versions providing more skill than the individual models. A verification of the deterministic RI model forecasts indicates that the operational intensity guidance exhibits some limited RI predictive skill, with the National Hurricane Center (NHC) official forecasts possessing the most skill within the first 24 h and the numerical models providing somewhat more skill at longer lead times. The Hurricane Weather Research and Forecasting (HWRF) model generally provides the most skillful RI forecasts of any of the conventional intensity models, while the new consensus RI model shows potential for providing increased skill over the existing operational intensity guidance. Finally, newly developed versions of the deterministic Rapid Intensification Aid guidance that employs the new probabilistic consensus RI model forecasts along with the existing operational intensity model consensus produces lower mean errors and biases than the intensity consensus model alone.
Li, X., X. Yang, W. Zheng, J.A. Zhang, L.J. Pietrafesa, and W.G. Pichel. Synergistic use of satellite observations and numerical weather model to study atmospheric occluded fronts. IEEE Transactions on Geoscience and Remote Sensing, 53(9):5269-5279, https://doi.org/10.1109/TGRS.2015.2420312 2015
Synthetic aperture radar (SAR) images reveal the surface imprints of atmospheric occluded fronts. An occluded front is characterized as a low-wind zone located between and within two zones of higher winds blowing in the opposite directions on the left and right sides of the occluded front. A group of four SAR images reveal that the width of an individual occluded frontal zone and the wind magnitudes outside fronts vary greatly from case to case. In this paper, we performed a case study to analyze an occluded front observed by an Environmental Satellite (Envisat) Advanced SAR and ASCAT scatterometer along the west coast of Canada on November 24, 2011. The two-way interactive, triply nested grid (9-3-1 km) Weather Research and Forecasting (WRF) model was utilized to simulate the evolution of the occluded front. The occluded front moved toward the east during a 24-h model simulation, and the movement between 18:00 and 21:00 UTC matched the occluded front positions derived from the concurrently collected surface weather maps; from the National Oceanic and Atmospheric National Weather Service archives. The WRF-simulated low-wind zone associated with the occluded front and ocean surface wind speed match well with the SAR and scatterometer wind retrievals. High wind outside the front zone became weaker during the front evolution, whereas the width of the occluded frontal zone was contracted laterally. Analysis of the WRF model derived potential temperature field suggests that the occlusion process occurred below the 800-mb level. The structure of the occluded front studied here not only follows the conventional conceptual model and also supports the findings of a novel wrap-up conceptual model for an atmospheric frontal occlusion process.
Ming, J., J.A. Zhang, and R.F. Rogers. Typhoon kinematic and thermodynamic boundary layer structure from dropsonde composites. Journal of Geophysical Research-Atmospheres, 120(8):3158-3172, https://doi.org/10.1002/2014JD022640 2015
The data from 438 Global Positioning System dropsondes in six typhoons are analyzed to investigate the mean atmospheric boundary layer structure in a composite framework. Following a recent study on boundary layer height in Atlantic hurricanes, we aim to quantify characteristics of boundary layer height scales in Western Pacific typhoons including the inflow layer depth (hinflow), height of the maximum tangential wind speed (hvtmax), and thermodynamic mixed layer depth. In addition, the kinematic and thermodynamic boundary layer structures are compared between the dropsonde composites using data in typhoons and hurricanes. Our results show that similar to the hurricane composite, there is a separation between the kinematic and thermodynamic boundary layer heights in typhoons, with the thermodynamic boundary layer depth being much smaller than hinflow and hvtmax in the typhoon boundary layer. All three boundary layer height scales tend to decrease toward the storm center. Our results confirm that the conceptual model of Zhang et al. (2011a) for boundary layer height variation is applicable to typhoon conditions. The kinematic boundary layer structure is generally similar between the typhoon and hurricane composites, but the typhoon composite shows a deeper inflow layer outside the eyewall than the hurricane composite. The thermodynamic structure of the typhoon boundary layer composite is warmer and moister outside the radius of maximum wind speed than the hurricane composite. This difference is attributed to different environmental conditions associated with typhoons compared to the hurricanes studied here.
Mohanty, U.C., K.K. Osuri, V. Tallapragada, F.D. Marks, S. Pattanayak, M. Mohapatra, L.S. Rathore, S.G. Gopalakrishnan, and D. Niyogi. A great escape from the Bay of Bengal "Super Sapphire-Phailin" tropical cyclone: A case of improved weather forecast and societal response for disaster mitigation. Earth Interactions, 19(17):1-11, https://doi.org/10.1175/EI-D-14-0032.1 2015
The very severe cyclonic storm (VSCS) “Phailin (2013)” was the strongest cyclone that hit the eastern coast of India-Odisha state since the super cyclone of 1999. But the same story of casualties was not repeated as that of 1999 where approximately 10,000 fatalities were reported. In the case of Phailin, a record 1 million people were evacuated across 18,000 villages in both the Odisha and Andhra Pradesh states to coastal shelters following the improved operational forecast guidance, which benefited from highly skillful and accurate numerical model guidance for the movement, intensity, rainfall, and storm surge. Thus, the property damage and death toll were minimized through the proactive involvement of three tier disaster management agencies at central, state, and district levels.
Quirino, T.S., J. Delgado, and X. Zhang. Improving the scalability of a hurricane forecast system in mixed-parallel environments. Proceedings, 16th IEEE International Conference on High Performance Computing and Communications, Paris, France, August 20-22, 2014. IEEE Computer Society, 276-281, 2015
The Hurricane Weather Research and Forecasting (HWRF) model is one of the premier models in NOAA’s operational suite of severe weather forecasting systems. An axiom in numerical weather prediction suggests that modeling the environment at high resolution optimizes forecast accuracy. However, due to operational time constraints, only the region immediately surrounding a single hurricane can be modeled in high resolution. Currently, this is achieved by embedding a relatively small high resolution, storm-following pair of grids within a larger and coarser grid. In a previous work, we extended HWRF to support multiple such independent storm-following pair of grids. The result was improved forecast accuracy by virtue of modeling storm-to-storm interactions in high resolution. However, some shortcomings in the underlying WRF framework cause these independent pairs of grids to be simulated sequentially. This limits the model’s scalability and makes it impossible to harness this novel capability within the operational time constraints. In this paper, we address this issue by modifying the underlying WRF framework to simulate these independent pairs of storm-following grids in parallel. This is the first approach to be successfully implemented in the history of the WRF framework.
Reasor, P.D., and M.T. Montgomery. Evaluation of a heuristic model for tropical cyclone resilience. Journal of the Atmospheric Sciences, 72(5):1765-1782, https://doi.org/10.1175/JAS-D-14-0318.1 2015
This work examines the applicability of a previously-postulated heuristic model for the temporal evolution of the small-amplitude tilt of a tropical cyclone-like vortex under vertical-shear forcing for both a dry and cloudy atmosphere. The heuristic model hinges on the existence of a quasi-discrete vortex-Rossby wave and its ability to represent the coherent precession and tilt-decay of a stable vortex in the free-alignment problem. Linearized numerical solutions for a dry and cloudy vortex confirm the model predictions that an increase in the magnitude of the radial potential vorticity (PV) gradient within the vortex skirt surrounding the core yields a more rapid evolution of a sheared vortex towards the equilibrium, left-of-shear tilt configuration. However, in the moist-neutral limit, in which the effective static stability vanishes in rising and sinking regions, the heuristic model yields a poor approximation to the simulated vortex core evolution, but a left-of-shear tilt of the near-core vortex, radially beyond the heating region, remains the preferred long-time solution. Within the near-core skirt the PV perturbation generated by vertical shearing exhibits continuous-spectrum type vortex-Rossby waves, features that are not captured by the heuristic model. Nevertheless, the heuristic model continues to predict the rapid vertical alignment and equilibrium, left-of-shear tilt configuration of the simulated near-core vortex in the moist-neutral limit.
Rogers, R.F., P.D. Reasor, and J.A. Zhang. Multiscale structure and evolution of Earl (2010) during rapid intensification. Monthly Weather Review, 143(2):536-562, https://doi.org/10.1175/MWR-D-14-00175.1 2015
The structure and evolution of Hurricane Earl (2010) during its rapid intensification as sampled by aircraft is studied here. Rapid intensification occurs in two stages. During the early stage, covering ~24 h, Earl was a tropical storm experiencing moderate northeasterly shear with an asymmetric distribution of convection, and the symmetric structure was shallow, broad, and diffuse. The upper-level circulation center was significantly displaced from the lower-level circulation at the beginning of this stage. Deep, vigorous convection, termed convective bursts, was located on the east side of the storm and appeared to play a role in positioning the upper-level cyclonic circulation center above the low-level center. By the end of this stage the vortex was aligned and extended over a deep layer, and rapid intensification began. During the late stage rapid intensification continued, as Earl intensified ~20 m s-1 during the next 24 h. The vortex remained aligned in the presence of weaker vertical shear, though azimuthal asymmetries persisted that were characteristic of vortices in shear. Convective bursts concentrated near the radius of maximum winds, with the majority located inside the radius of maximum winds. Each of the two stages described here raises questions about the role of convective- and vortex-scale processes in rapid intensification. During the early stage the focus is on the role of convective bursts, and their associated mesoscale convective system, on vortex alignment and the onset of rapid intensification. During the late stage the focus is on the processes that explain the observed radial distribution of convective bursts which peak inside the radius of maximum winds.
Rozoff, C.M., C.S. Velden, J. Kaplan, J.P. Kossin, and A.J. Wimmers. Improvements in the probabilistic prediction of tropical cyclone rapid intensification with passive microwave observations. Weather and Forecasting, 30(4):1016-1038, https://doi.org/10.1175/WAF-D-14-00109.1 2015
The probabilistic prediction of tropical cyclone (TC) rapid intensification (RI) in the Atlantic and eastern Pacific Ocean basins is examined here using a series of logistic regression models trained on environmental and infrared satellite-derived features. The environmental predictors are based on averaged values over a 24-h period following the forecast time. These models are compared against equivalent models enhanced with additional TC predictors created from passive satellite microwave imagery (MI). Leave-one-year-out cross validation on the developmental dataset shows that the inclusion of MI-based predictors yields more skillful RI models for a variety of RI and intensity thresholds. Compared with the baseline forecast skill of the non-MI-based RI models, the relative skill improvements from including MI-based predictors range from 10.6–44.9%. Using archived real-time data in the period 2004-2013, evaluation of simulated real-time models is also carried out. Unlike in the model development stage, the simulated real-time setting involves using Global Forecast System forecasts for the non-satellite-based predictors instead of “perfect” observational-based predictors in the developmental data. In this case, the MI-based RI models still generate superior skill to the baseline RI models lacking MI-based predictors. The relative improvements gained in adding MI-based predictors are most notable in the Atlantic, where the non-MI versions of the models suffer acutely from the use of imperfect real-time data. In the Atlantic, relative skill improvements provided from the inclusion of MI-based predictors range from 53.5-103.0%. The eastern Pacific relative improvements are less impressive but are still uniformly positive.
Simpson, R.H., and N.M. Dorst. Hurricane Pioneer: Memoirs of Bob Simpson. American Meteorological Society (ISBN 9781935704751), 272 pp., 2015
In 1947, Robert H. Simpson lifted off in a specially equipped plane, flying directly into the path of a storm that would send most people running for cover. For more than four hours he observed Typhoon Martha from its eerily calm eye, later describing it in Scientific American as a coliseum of clouds whose walls on one side rose vertically and on the other were banked like the galleries in a great opera house. For Simpson this was just one of his many pioneering explorations of hurricanes and extreme storms. Over his decades-long career his research led to great leaps in our understanding of tropical meteorology and our approach to hurricane safety. He was the first director of the National Hurricane Research Project and a director of the National Hurricane Center, though he may be best known as co-creator of the widely used Saffir-Simpson Hurricane Scale, familiar to anyone who has heard a reporter use the words “category five.” Simpson's memoirs take readers from his experience with the Mississippi Flood of 1927 to his travels to study weather across the globe. Along the way he crosses paths with other weather greats, including his trailblazing wife, meteorologist Joanne Simpson. Hurricane Pioneer is a riveting first-hand account of a revolutionary time in meteorology.
Susca-Lopata, G., J. Zawislak, E.J. Zipser, and R.F. Rogers. The role of observed environmental conditions and precipitation evolution in the rapid intensification of Hurricane Earl (2010). Monthly Weather Review, 143(6):2207-2223, https://doi.org/10.1175/MWR-D-14-00283.1 2015
An investigation into the possible causes of the rapid intensification (RI) of Hurricane Earl (2010) is carried out using a combination of global analyses, aircraft Doppler radar data, and observations from passive microwave satellites and a long-range lightning network. Results point to an important series of events leading to, and just after, the onset of RI, all of which occur despite moderate (7-12 m s-1) vertical wind shear present. Beginning with an initially vertically misaligned vortex, observations indicate that asymmetric deep convection, initially left of shear but not distinctly up- or down-shear, rotates into more decisively upshear regions. Following this convective rotation, the vortex becomes aligned and precipitation symmetry increases. The potential contributions to intensification from each of these structural changes are discussed. The radial distribution of intense convection relative to the radius of maximum wind (RMW; determined from Doppler wind retrievals) is estimated from microwave and lightning data. Results indicate that intense convection is preferentially located within the upper-level (8-km) RMW during RI, lending further support to the notion that intense convection within the RMW promotes tropical cyclone intensification. The distribution relative to the low-level RMW is more ambiguous, with intense convection preferentially located just outside of the low-level RMW at times when the upper-level RMW is much greater than the low-level RMW.
Tang, J., D. Byrne, J.A. Zhang, Y. Wang, X. Lei, D. Wu, P. Fang, and B. Zhao. Horizontal transition of turbulent cascade in the near-surface layer of tropical cyclones. Journal of the Atmospheric Sciences, 72(12):4915-4925, https://doi.org/10.1175/JAS-D-14-0373.1 2015
Tropical cyclones (TC) consist of a large range of interacting scales from hundreds of kilometers to a few meters. The energy transportation amongst these different scales, i.e., from smaller to larger scales (upscale) or vice versa (downscale), may have profound impacts on TC energy dynamics due to the associated changes in available energy sources and sinks. From multi-layer tower measurements in the low-level (less than 120 m) boundary layer of several landing TCs, we found there are two distinct regions where the energy flux changes from upscale to downscale as a function of distance to the storm center. The boundary between these two regions is approximately 1.5 times the radius of maximum wind. Two-dimensional turbulence (upscale cascade) occurs more typically at regions close to the inner core region of TCs, while 3-D turbulence (downscale cascade) mostly occurs in the outer core region in the surface layer.
Wang, J., K. Young, T. Hock, D. Lauritsen, D. Behringer, M. Black, P.G. Black, J. Franklin, J. Halverson, J. Molinari, L. Nguyen, T. Reale, J. Smith, B. Sun, Q. Wang, and J.A. Zhang. A long-term, high-quality, high vertical resolution GPS dropsonde dataset for hurricane and other studies. Bulletin of the American Meteorological Society, 96(6):961-973, https://doi.org/10.1175/BAMS-D-13.00203.1 2015
A GPS dropsonde is a scientific instrument deployed from research and operational aircraft that descends through the atmosphere by a parachute. The dropsonde provides high-quality, high vertical resolution profiles of atmospheric pressure, temperature, relative humidity, wind speed and direction from the aircraft flight level to the surface over oceans and remote areas. Since 1996, GPS dropsondes have been routinely dropped during hurricane reconnaissance and surveillance flights to help predict hurricane track and intensity. From 1996 to 2012, NOAA has dropped 13,681 dropsondes inside hurricane eye walls or in the surrounding environment for 120 tropical cyclones (TCs). All NOAA dropsonde data have been collected, reformatted to one format, and consistently and carefully quality-controlled using state-of-art quality-control (QC) tools. Three value-added products, the vertical air velocity and the radius and azimuth angle of each dropsonde location, are generated and added to the dataset. As a result, a long-term (1996–2012), high-quality, high-vertical resolution (∼5–15 m) GPS dropsonde dataset is created and made readily available for public access. The dropsonde data collected during hurricane reconnaissance and surveillance flights have improved TC track and intensity forecasts significantly. The milestones of dropsonde data's impact on hurricane studies are summarized. The scientific applications of this long-term dropsonde dataset are highlighted, including characterizing TC structures, studying TC environmental interactions, identifying surface-based ducts in hurricane environment which affect electromagnetic wave propagation, and validating satellite temperature and humidity profiling products.
Zhang, D.-L., L. Zhu, X. Zhang, and V. Tallapragada. Sensitivity of idealized hurricane intensity and structures under varying background flows and initial vortex intensities to different vertical resolutions in HWRF. Monthly Weather Review, 143(3):914-932, https://doi.org/10.1175/MWR-D-14-00102.1 2015
A series of 5-day numerical simulations of idealized hurricane vortices under the influence of different background flows is performed by varying vertical grid resolution (VGR) in different portions of the atmosphere with the operational version of the Hurricane Weather Research and Forecasting model in order to study the sensitivity of hurricane intensity forecasts to different distributions of VGR. Increasing VGR from 21 to 43 levels produces stronger hurricanes, whereas increasing it further to 64 levels does not intensify the storms further, but with much reduced intensity fluctuations. Moreover, increasing the lower-level VGRs generates stronger storms, but the opposite is true for increased upper-level VGRs. On average, adding mean flow increases intensity fluctuations and variability (between the strongest and weakest hurricanes), whereas adding vertical wind shear (VWS) delays hurricane intensification and then causes more rapid growth in intensity variability. The stronger the VWS, the larger intensity variability and bifurcation rate occur at later stages. These intensity differences are found to be closely related to inner-core structural changes, and they are attributable to how much latent heat could be released in higher-VGR layers, followed by how much moisture content in nearby layers is converged. Hurricane intensity with higher VGRs is shown to be much less sensitive to varying background flows, and stronger hurricane vortices at the model initial time are less sensitive to the vertical distribution of VGR; the opposite is true for relatively uniform VGRs or weaker hurricane vortices. Results reveal that higher VGRs with a near-parabolic or Ω shape tends to produce smoother intensity variations and more typical inner-core structures.
Zhang, J.A., and F.D. Marks. Effects of horizontal diffusion on tropical cyclone intensity change and structure in idealized three-dimensional numerical simulations. Monthly Weather Review, 143(10):3981-3995, https://doi.org/10.1175/MWR-D-14-00341.1 2015
This study examines the effects of horizontal diffusion on tropical cyclone (TC) intensity change and structure using idealized simulations of the Hurricane Weather and Research Forecast (HWRF) model. We conducted a series of sensitivity experiments with varying horizontal mixing lengths (Lh), but kept the vertical diffusion coefficient and other physical parameterizations unchanged. The results show that both simulated maximum intensity and intensity change are sensitive to the Lh used in the parameterization of the horizontal turbulent flux, in particular, for Lh less than the model’s horizontal resolution. The results also show that simulated storm structures such as storm size, kinematic boundary layer height, and eyewall slope are sensitive to Lh as well. However, Lh has little impact on the magnitude of the surface inflow angle and thermodynamic mixed layer height. Angular momentum budget analyses indicate that the effect of Lh is to mainly spin down a TC vortex. Both mean and eddy advection terms in the angular momentum budget are affected by the magnitude of Lh. For smaller Lh, the convergence of angular momentum is larger in the boundary layer, which leads to a faster spin-up of the vortex. The resolved-eddy advection of angular momentum plays an important role in the spin-up of the low-level vortex inward from the radius of the maximum wind speed when Lh is small.
Zhang, J.A., D.S. Nolan, R.F. Rogers, and V. Tallapragada. Evaluating the impact of improvements in the boundary layer parameterization on hurricane intensity and structure forecasts in HWRF. Monthly Weather Review, 143(8):3136-3155, https://doi.org/10.1175/MWR-D-14-00339.1 2015
As part of the Hurricane Forecast Improvement Project (HFIP), recent boundary-layer physics upgrades in the operational Hurricane Weather Research and Forecasting (HWRF) model have benefited from analyses of in-situ aircraft observations in the low-level eyewall region of major hurricanes. This study evaluates the impact of these improvements to the vertical diffusion in the boundary layer on the simulated track, intensity, and structure of four hurricanes using retrospective HWRF forecasts. Structural metrics developed from observational composites are used in the model evaluation process. The results show improvements in track and intensity forecasts in response to the improvement of the vertical diffusion. The results also demonstrate substantial improvements in the simulated storm size, surface inflow angle, near-surface wind profile and kinematic boundary layer heights in simulations with the improved physics, while only minor improvements are found in the thermodynamic boundary layer height, eyewall slope, and the distributions of vertical velocities in the eyewall. Other structural metrics such as warm core anomaly and warm core height are also explored. Reasons for the structural differences between the two sets of forecasts with different physics are discussed. This work further emphasizes the importance of aircraft observations in model diagnostics and development, endorsing a developmental framework for improving physical parameterizations in hurricane models.
Zhu, P., Z. Zhu, S. Gopalakrishnan, R. Black, F.D. Marks, V. Tallapragada, J.A. Zhang, X. Zhang, and C. Gao. Impact of subgrid-scale processes on eyewall replacement cycle of tropical cyclones in HWRF system. Geophysical Research Letters, 42(22):10027-10036, https://doi.org/10.1002/2015GL066436 2015
Two idealized simulations by the Hurricane Weather Research and Forecast (HWRF) model are presented to examine the impact of model physics on the simulated eyewall replacement cycle (ERC). While no ERC is produced in the control simulation that uses the operational HWRF physics, the sensitivity experiment with different model physics generates an ERC that possesses key features of observed ERCs in real tropical cyclones. Likely reasons for the control simulation not producing ERC include lack of outer rainband convection at the far radii from the eyewall, excessive ice hydrometeors in the eyewall, and enhanced moat shallow convection, which all tend to prevent the formation of a persistent moat between the eyewall and outer rainband. Less evaporative cooling from precipitation in the outer rainband region in the control simulation produces a more stable and dryer environment that inhibits the development of systematic convection at the far radii from the eyewall.
2014
Aberson, S.D. A climatological baseline for assessing the skill of tropical cyclone phase forecasts. Weather and Forecasting, 29(1):122-129, https://doi.org/10.1175/WAF-D-12-00130.1 2014
A simple linear discriminant analysis scheme using climatological predictors is derived for the Atlantic basin as a no-skill baseline for operational phase forecasts from the National Hurricane Center. The model with independent data correctly classifies 80% of the cases at 12 h, and this value decreases to about 45% by 60 h, remaining steady thereafter. Using the same cases, NHC-issued phase predictions were more frequency accurate than the baseline, so their forecasts are said to have skill.
Aksoy, A. Parameter estimation. In Encylopedia of Atmospheric Sciences (2nd edition), G.R. North, J. Pyle, and F. Zhang (eds.). Academic Press, Volume 4, 181-186, https://doi.org/10.1016/B978-0-12-382225-3.00494-1 2014
Atlas, R.M. Observing system simulation experiments to assess the potential impact of proposed satellite instruments on hurricane prediction. Proceedings, SPIE Symposium on Imaging Spectrometry XIX, San Diego, CA, August 17-21, 2014. International Society for Optics and Photonics, SPIE Vol. 9222, 9 pp., 2014
Observing System Simulation Experiments (OSSEs) are an important tool for evaluating the potential impact of proposed new observing systems, as well as for evaluating trade-offs in observing system design, and in developing and assessing improved methodology for assimilating new observations. Extensive OSSEs have been conducted at NASA/ GSFC and NOAA/AOML over the last three decades. These OSSEs determined correctly the quantitative potential for several proposed satellite observing systems to improve weather analysis and prediction prior to their launch, evaluated trade-offs in orbits, coverage and accuracy for space-based wind lidars, and were used in the development of the methodology that led to the first beneficial impacts of satellite surface winds on numerical weather prediction. In this paper, we summarize early applications of global OSSEs to hurricane track forecasting and new experiments using both global and regional models. These experiments are aimed at determining (1) the potential impact of unmanned aerial systems, (2) the relative impact of alternative concepts for space-based lidar winds, and (3) the relative impact of alternative concepts for polar and geostationary hyperspectral sounders.
Baker, W.E., R. Atlas, C. Cardinali, A. Clement, G.D. Emmitt, B.M. Gentry, R.M. Hardesty, E. Kallen, M.J. Kavaya, R. Langland, Z. Ma, M. Masutani, W. McCarty, R.B. Pierce, Z. Pu, L.P. Riishojgaard, J. Ryan, S. Tucker, M. Weissmann, and J.G. Yoe. Lidar-measured wind profiles: The missing link in the global observing system. Bulletin of the American Meteorological Society, 95(4):543-564, https://doi.org/10.1175/BAMS-D-12-00164.1 2014
The three-dimensional global wind field is the most important remaining measurement needed to accurately assess the dynamics of the atmosphere. Wind information in the tropics, high latitudes, and stratosphere, is particularly deficient. Furthermore, only a small fraction of the atmosphere is sampled in terms of wind profiles. This limits our ability to optimally specify initial conditions for numerical weather prediction (NWP) models and our understanding of several key climate change issues. Because of its extensive wind-measurement heritage (since 1968) and especially the rapid recent technology advances, Doppler lidar has reached a level of maturity required for a space-based mission. ESA’s Atmospheric Dynamics Mission (ADM)-Aeolus Doppler Wind Lidar (DWL), now scheduled for launch in 2015, will be a major milestone. This paper reviews the expected impact of DWL measurements on NWP and climate research, measurement concepts, and the recent advances in technology that will set the stage for space-based deployment. Forecast impact experiments with actual airborne DWL measurements collected over the North Atlantic in 2003 and assimilated into the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model are a clear indication of the value of lidar-measured wind profiles. Airborne DWL measurements collected over the Western Pacific in 2008 and assimilated into both the ECMWF and U.S. Navy operational models support the earlier findings. These forecast impact experiments confirm Observing System Simulation Experiments (OSSEs) conducted over the past 25-30 years. The addition of simulated DWL wind observations in recent OSSEs performed at the Joint Center for Satellite Data Assimilation (JCSDA) leads to a statistically significant increase in forecast skill.
Bell, G.D., C.W. Landsea, S.B. Goldenberg, R.J. Pasch, E.S. Blake, J. Schemm, and T.B. Kimberlain. The tropics: Atlantic basin. In State of the Climate in 2013, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 95(7):S86-S90, https://doi.org/10.1175/2014BAMSStateoftheClimate.1 2014
DeHart, J.C., R.A. Houze, and R.F. Rogers. Quadrant distribution of tropical cyclone inner-core kinematics in relation to environmental shear. Journal of the Atmospheric Sciences, 71(7):2713-2732, https://doi.org/10.1175/JAS-D-13-0298.1 2014
Airborne Doppler radar data collected in tropical cyclones by National Oceanic and Atmospheric Administration WP-3D aircraft over an eight-year period (2003-2010) is used to statistically analyze the vertical structure of tropical cyclone eyewalls with reference to the deep layer shear. Convective evolution within the inner core conforms to patterns shown by previous studies: convection initiates downshear-right, intensifies downshear-left and weakens upshear. Analysis of the vertical distribution of radar reflectivity and vertical air motion indicates the development of upper-level downdrafts in conjunction with strong convection downshear-left and a maximum in frequency upshear-left. Intense updrafts and downdrafts both conform to the shear asymmetry pattern. While strong updrafts occur within the eyewall, intense downdrafts show far more radial variability, particularly in the upshear-left quadrant, though they concentrate along the eyewall edges. Strong updrafts are collocated with low-level inflow and upper-level outflow superimposed on the background flow. In contrast, strong downdrafts occur in association with low-level outflow and upper-level inflow.
Dunion, J.P., C.D. Thorncroft, and C.S. Velden. The tropical cyclone diurnal cycle of mature hurricanes. Monthly Weather Review, 142(10):3900-3919, https://doi.org/10.1175/MWR-D-13-00191.1 2014
The diurnal cycle of tropical convection and the tropical cyclone (TC) cirrus canopy have been described extensively in previous studies. However, a complete understanding of the TC diurnal cycle remains elusive and is an area of ongoing research. This work describes a new technique that uses infrared satellite image differencing to examine the evolution of the TC diurnal cycle for all North Atlantic major hurricanes from 2001-2010. The imagery reveals cyclical pulses in the infrared cloud field that regularly propagate radially outward from the storm. These diurnal pulses begin forming in the storm’s inner core near the time of sunset each day and continue to move away from the storm overnight, reaching areas several hundred kilometers from the circulation center by the following afternoon. A marked warming of the cloud tops occurs behind this propagating feature and there can be pronounced structural changes to a storm as it moves away from the inner core. This suggests that the TC diurnal cycle may be an important element of TC dynamics and may have relevance to TC structure and intensity change. Evidence is also presented showing the existence of statistically significant diurnal signals in TC wind radii and objective Dvorak satellite-based intensity estimates for the 10-yr hurricane dataset that was examined. Findings indicate that TC diurnal pulses are a distinguishing characteristic of the TC diurnal cycle and the repeatability of TC diurnal pulsing in time and space suggests that it may be an unrealized, yet fundamental TC process.
Gall, R., F. Toepfer, F. Marks, E.N. Rappaport, A. Aksoy, S. Aberson, J.W. Bao, M. Bender, S. Benjamin, L. Bernardet, M. Biswas, B. Brown, J. Cangialosi, C. Davis, M. DeMaria, J. Doyle, M. Fiorino, J. Franklin, I. Ginis, S. Gopalakrishnan, T. Hamill, R. Hodur, H.S. Kim, J. Knaff, T. Krishnamurti, P. Kucera, Y. Kwon, W. Lapenta, N. Lett, S. Lord, T. Marchok, E. Mifflin, M. Morin, K. Musgrave, L. Nance, C. Reynolds, V. Tallapragada, H. Tolman, R. Torn, G. Vandenberghe, T. Vukicevic, X. Wang, Y. Weng, J. Whittaker, R. Yablonsky, D.-L. Zhang, F. Zhang, J. Zhang, X. Zhang, and D.A. Delinsky. Hurricane Forecast Improvement Project: 2013 HFIP R&D activities summary—Recent results and operational implementation. HFIP Technical Report, HFIP2014-2, 50 pp., 2014
This report describes the activities and results of the Hurricane Forecast Improvement Program (HFIP) in 2013. Since this is the fourth year of the first five years of the project, we, like last year, focus on the improvements in the operational Global Forecast System (GFS) global model and the Hurricane Weather Research and Forecasting (HWRF) regional model. HFIP is organized around the three “streams”: Stream 1 or the operational model development; Stream 1.5 which comprises a group of experimental models that have been evaluated by the National Hurricane Center (NHC) pre-season and then made available to NHC forecasters during their forecast cycle; and Stream 2 representing HFIP experimental models which test and evaluate new techniques and strategies for model forecast guidance before testing is begun for possible operational implementation. Stream 2 also tests techniques that cannot be tested on current operational computers because of their size and time requirements, but can be tested on HFIP computer facilities in Boulder, Colorado. Those studies are looking ahead to possible future operational computational capability. The report outlines the HFIP program, how it is organized, its goals, its models, and then results from each of the three streams.
Halliwell, G.R., A. Srinivasan, V. Kourafalou, H. Yang, D. Willey, M. Le Henaff, and R. Atlas. Rigorous evaluation of a fraternal twin ocean OSSE system for the open Gulf of Mexico. Journal of Oceanic and Atmospheric Technology, 31(1):105-130, https://doi.org/10.1175/JTECH-D-13-00011.1 2014
A new fraternal twin ocean Observing System Simulation Experiment (OSSE) system is validated in a Gulf of Mexico domain. It is the first ocean system that takes full advantage of design criteria and rigorous evaluation procedures developed to validate atmosphere OSSE systems that have not been fully implemented for the ocean. These procedures are necessary to determine a-priori that the OSSE system does not overestimate or underestimate observing system impacts. The new system consists of (1) a nature run (NR) stipulated to represent the true ocean; (2) a data assimilation system consisting of a second ocean model (the “forecast model”) coupled to a new ocean data assimilation system; and (3) software to simulate observations from the NR and add realistic errors. System design is described to illustrate the requirements of a validated OSSE system. The chosen NR reproduces the climatology and variability of ocean phenomena with sufficient realism. Although the same ocean model type is used (the “fraternal twin” approach), the forecast model is configured differently so that it satisfies the requirement that differences (errors) with respect to the NR grow at the same rate as errors that develop between state-of-the-art ocean models and the true ocean. Rigorous evaluation procedures developed for atmospheric OSSEs are then applied by first performing Observing System Experiments (OSEs) to evaluate one or more existing observing systems. OSSEs are then performed that are identical except for the assimilation of synthetic observations simulated from the NR. Very similar impact assessments were realized between each OSE-OSSE pair, thus validating the system without the need for calibration.
Heymsfield, A., and P. Willis. Cloud conditions favoring secondary ice particle production in tropical maritime convection. Journal of the Atmospheric Sciences, 71(12):4500-4526, https://doi.org/10.1175/JAS-D-14-0093.1 2014
Progress in understanding the formation of ice in lower tropospheric clouds is slowed by the difficulties in characterizing the many complex interactions that lead to ice initiation and to the dynamic, non-steady state nature of the clouds. The present study characterizes the conditions where secondary ice particles, specifically identified as needle or thin columnar types, are observed in tropical maritime convection with modest liquid water contents during the Ice in Clouds Experiment-Tropical (ICE-T), based out of St. Croix, V. I., and the NASA African Monsoon Multidisciplinary Analysis experiment (NAMMA) in 2006 sampling from Cape Verde, Africa. The properties of the cloud droplet populations relevant to the secondary ice production process, and the ice particle populations, are characterized as a function of temperature and vertical velocity. These secondary ice particles are observed primarily in regions of low liquid water content and weak vertical velocities. Two situations are examined in detail. First, ice formation is examined by following the tops of a group of ICE-T chimney clouds as they ascend and cool from a temperature of +7°C to −8°C, examining the production of the first ice. Then, using the data from a cloud system sampled during NAMMA, we elucidate a process that promotes ice multiplication. The intention is that this study will lead both to a better understanding of how secondary ice production proceeds in natural clouds as well as to more realistic laboratory studies of the processes involved.
Klotz, B.W., and E.W. Uhlhorn. Improved stepped frequency microwave radiometer tropical cyclone surface winds in heavy precipitation. Journal of Atmospheric and Oceanic Technology, 31(11):2392-2408, https://doi.org/10.1175/JTECH-D-14-00028.1 2014
Surface wind speeds retrieved from airborne stepped frequency microwave radiometer (SFMR) brightness temperature measurements are important for estimating hurricane intensity. The SFMR performance is highly reliable at hurricane force wind speeds, but accuracy is found to degrade at weaker wind speeds, particularly in heavy precipitation. Specifically, a significant over-estimation of surface wind speeds is found in these conditions, suggesting inaccurate accounting for the impact of rain on the measured microwave brightness temperature. In this study, the wind speed bias is quantified over a broad range of operationally-computed wind speeds and rain rates, based on a large sample of co-located SFMR wind retrievals and Global Positioning System dropwindsonde surface-adjusted wind speeds. The retrieval bias is addressed by developing a new SFMR C-band microwave absorption—rain rate (κ-R) relationship from National Oceanic and Atmospheric Administration WP-3D aircraft tail Doppler radar reflectivity and in situ Droplet Measurement Technologies Precipitation Imaging Probe measurements to more accurately model precipitation impacts. Absorption is found to be a factor of two weaker than is estimated by the currently-operational algorithm. With this new κ-R relationship, surface wind retrieval bias is significantly reduced in the presence of rain at wind speeds weaker than hurricane force. At wind speeds greater than hurricane force where little bias exists, no significant change is found. Furthermore, maximum rain rates computed using the revised algorithm are around 50% greater than operational measurements, which is more consistent with maximum reflectivity-estimated rain rates in hurricanes.
Marks, F.D. Advancing tropical cyclone forecasts using aircraft observations. In Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change, U.C. Mohanty, M. Mohapatra, O.P. Singh, B.K. Bandyopadhyay, and L.S. Rathore (eds.). Springer Publishing, 169-191, https://doi.org/10.1007/978-94-007-7720-0 2014
As part of NOAA’s Hurricane Forecast Improvement Program (HFIP), this paper addresses the important role of aircraft observations in hurricane model physics validation and improvement. A model developmental framework for improving the physical parameterizations using quality-controlled and post-processed aircraft observations is presented, with steps that include model diagnostics, physics development, physics implementation and further evaluation. Model deficiencies are first identified through model diagnostics by comparing the simulated axisymmetric multi-scale structures to observational composites. New physical parameterizations are developed in parallel based on in-situ observational data from specially designed hurricane field programs. The new physics package is then implemented in the model, which is followed by further evaluation. The developmental framework presented here is found to be successful in improving the surface layer and boundary layer parameterization schemes in the operational Hurricane Weather Research and Forecast (HWRF) model. Observations for improving physics packages other than boundary layer scheme are also discussed.
Marks, F.D. Hurricanes: Observations. In Encylopedia of Atmospheric Sciences (2nd edition), G.R. North, J. Pyle, and F. Zhang (eds.). Academic Press, Volume 6, 35-56, 2014
Ming, J., J.A. Zhang, R.F. Rogers, F.D. Marks, Y. Wang, and N. Cai. Multiplatform observations of boundary layer structure in the outer rainbands of landfalling typhoons. Journal of the Geophysical Research-Atmospheres, 119(13):7799-7814, https://doi.org/10.1002/2014JD021637 2014
This paper analyzes data collected from a new set of observational platforms in the coastal area of China, which consist of a mobile observation system, meteorological tower, automatic weather station, and Doppler radars, to investigate the mean and turbulent boundary layer structure and evolution during the landfall of typhoons. An example of these data is provided from Typhoon Morakot (2009). Vertical profiles of wind velocities and thermodynamic parameters from the observed data allow us to identify different boundary layer structures during and after landfall. These structures, sampled in regions of the outer core, are stratified into periods where convection is occurring (termed “convective”) and periods where convection has recently (‹2 h) occurred (termed “postconvective”). Data analyses show that the thermodynamic mixed-layer depth and inflow layer depth are higher during the convective period than the postconvective period. The mixed-layer depth is found to be within the strong inflow layer, but the height of the maximum tangential wind speed is above the inflow layer during both periods, contrary to recent observational studies of the boundary-layer structure of tropical cyclones over water. High-frequency wind data show that momentum flux, turbulent kinetic energy (TKE), and integral length scales of wind velocities are all much larger during the convective period than the postconvective period. The results suggest that convective downdrafts may play an important role in modulating turbulent flux, TKE, vertical mixing, and boundary layer recovery processes.
Montgomery, M.T., J.A. Zhang, and R.K. Smith. An analysis of the observed low-level structure of rapidly intensifying and mature Hurricane Earl (2010). Quarterly Journal of the Royal Meteorological Society, 140(684):2132-2146, https://doi.org/10.1002/qj.2283 2014
We examine dynamic and thermodynamic aspects of Atlantic Hurricane Earl (2010) during its intensification and mature phases over four days of intensive measurements. During this period, Earl underwent an episode of rapid intensification, maturity, secondary eyewall replacement, re-intensification, and the early part of the decline. The observations are used to appraise elements of a new model for tropical-cyclone intensification. The results affirm the conventional (vortex interior) and boundary layer spin up mechanisms that form dynamical elements of the azimuthally-averaged view of the new intensification model. The average maximum tangential winds beneath the eyewall are found to exceed the gradient wind by between 20% and 60%. The results suggest also that the gradient wind balance approximation in the low-level vortex interior above the boundary layer may not be as accurate as has been widely held in the inner-core region of a tropical cyclone during its intensification. An analysis of the low-level thermodynamic structure affirms the radial increase of moist equivalent potential temperature, θe, with decreasing radius during the intensification process, a necessary ingredient of the new model for maintaining convective instability in the presence of a warming upper-troposphere. An unanticipated finding is the discovery of an unmixed boundary layer in terms of θe over several hundred kilometers of the vortex. In the inner-core region, this finding is not consistent with the axisymmetric eruption of the boundary layer into the eyewall unless there are non-conservative (eddy) processes acting to modify the entropy of ascending air.
Negron-Juarez, R.I., J.Q. Chambers, G.C. Hurtt, B. Annane, S. Cocke, M. Powell, M. Stott, S. Goosem, D.J. Metcalfe, and S.S. Saatchi. Remote sensing assessment of forest disturbance across complex mountainous terrain: The pattern and severity of impacts of tropical cyclone Yasi on Australian rainforests. Remote Sensing, 6(6):5633-5649, https://doi.org/10.3390/rs6065633 2014
Topography affects the patterns of forest disturbance produced by tropical cyclones. It determines the degree of exposure of a surface and can alter wind characteristics. Whether multispectral remote sensing data can sense the effect of topography on disturbance is a question that deserves attention given the multi-scale spatial coverage of these data and the projected increase in intensity of the strongest cyclones. Here, multispectral satellite data, topographic maps, and cyclone surface wind data were used to study the patterns of disturbance in an Australian rainforest with complex mountainous terrain produced by tropical cyclone Yasi (2011). The cyclone surface wind data (H*wind) was produced by the Hurricane Research Division of the National Oceanic and Atmospheric Administration (HRD/NOAA), and this was the first time that this data was produced for a cyclone outside of United States territory. A disturbance map was obtained by applying spectral mixture analyses on satellite data and presented a significant correlation with field-measured tree mortality. Our results showed that, consistent with cyclones in the southern hemisphere, multispectral data revealed that forest disturbance was higher on the left side of the cyclone track. The highest level of forest disturbance occurred in forests along the path of the cyclone track (±30°). Levels of forest disturbance decreased with decreasing slope and with an aspect facing off the track of the cyclone or away from the dominant surface winds. An increase in disturbance with surface elevation was also observed. However, areas affected by the same wind intensity presented increased levels of disturbance with increasing elevation, suggesting that complex terrain interactions act to speed up wind at higher elevations. Yasi produced an important offset to Australia’s forest carbon sink in 2010. We concluded that multispectral data were sensitive to the main effects of complex topography on disturbance patterns. High resolution cyclone wind surface data are needed in order to quantify the effects of topographic accelerations on cyclone related forest disturbances.
Nolan, D.S., J.A. Zhang, and E.W. Uhlhorn. On the limits of estimating the maximum wind speeds in hurricanes. Monthly Weather Review, 142(8):2814-2837, https://doi.org/10.1175/MWR-D-13-00337.1 2014
This study uses an observing system simulation experiment (OSSE) approach to test the limitations of even nearly ideal observing systems to capture the peak wind speed occurring within a tropical storm or hurricane. The data set is provided by a 1-km resolution simulation of an Atlantic hurricane with surface wind speeds saved every 10 seconds. An optimal observing system consisting of a dense field of anemometers provides perfect measurements of the peak 1-minute wind speed as well as the average peak wind speed. Suboptimal observing systems consisting of a small number of anemometers are sampled and compared to the truth provided by the optimal observing system. Results show that a single, perfect anemometer experiencing a direct hit by the right side of the eyewall will underestimate the actual peak intensity by 10-20%. Even an unusually large number of anemometers (e.g., 3-5) experiencing direct hits by the storm together will underestimate the peak wind speeds by 5-10%. However, the peak winds of just one or two anemometers will provide on average a good estimate of the average peak intensity over several hours. Enhancing the variability of the simulated winds to better match observed winds does not change the results. Adding observational errors generally increases the reported peak winds, thus reducing the underestimates. If the average underestimate (negative bias) were known perfectly for each case, it could be used to correct the wind speeds, leaving only mean absolute errors of 3-5%.
Pattanayak, S., U.C. Mohanty, and S.G. Gopalakrishnan. Improvement in track and intensity prediction of Indian seas tropical cyclones with vortex assimilation. In Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change, U.C. Mohanty, M. Mohapatra, O.P. Singh, B.K. Bandyopadhyay, and L.S. Rathore (eds.). Springer Publishing, 219-229, https://doi.org/10.1007/978-94-007-7720-0 2014
A tropical cyclone is one of the most hazardous weather events over the data sparse warm tropical ocean. It is the most deadly weather system and causes destructive winds, heavy rainfall, high storm surges, and coastal inundation, usually resulting in serious property damage and loss of life in coastal belts of India and, hence, strong impact on the socio-economic conditions of the countries surrounding the Bay of Bengal, especially India, Bangladesh, and Myanmar. The Bay of Bengal contributes about 5% of the global annual total number of tropical storms (Mohanty, 1994). Moreover, the Bay of Bengal storms are exceptionally devastating, especially when they cross the land (De Angelis, 1976). Therefore, the Bay of Bengal tropical cyclone disaster is the costliest and deadliest natural hazard on the Indian subcontinent.
Rio-Berrios, R., T. Vukicevic, and B. Tang. Adopting model uncertainties for tropical cyclone intensity prediction. Monthly Weather Review, 142(1):72-78, https://doi.org/10.1175/MWR-D-13-00186.1 2014
Quantifying and reducing the uncertainty of model parameterizations using observations is evaluated for tropical cyclone (TC) intensity prediction. This is accomplished using a nonlinear inverse modeling technique that produces a joint probability density function (PDF) for a set of parameters. The dependence of estimated parameter values and associated uncertainty on two types of observable quantities is analyzed using an axisymmetric hurricane model. When the observation is only the maximum tangential wind speed, the joint PDF of parameter estimates has large variance and is multimodal. When the full kinematic field within the inner core of the TC is used for the observations, however, the joint parameter estimates are well constrained. These results suggest that model parameterizations may not be optimized using the maximum wind speed. Instead, the optimization should be based on observations of the TC structure to improve the intensity forecasts.
Shpund, J., J.A. Zhang, M. Pinsky, and A. Khain. Microphysical structure of the marine boundary layer under strong wind and sea spray formation as seen from a 2-D explicit microphysical model. Part III: Parameterization of height-dependent droplet size distribution. Journal of the Atmospheric Sciences, 71(6):1914-1934, https://doi.org/10.1175/JAS-D-12-0201.1 2014
This paper completes a series of studies using a 2-D hybrid Lagrangian-Eulerian model to investigate the effect of sea spray on the thermodynamics and microphysics of the hurricane mixed layer. The evolution of the mixed layer was simulated by mimicking the motion of an air-volume (in a Lagrangian sense) toward a tropical cyclone eyewall along a background air-flow. During the radial motion, sea surface temperature, as well as pressure, background wind speed, sea spray production rate and turbulence intensity were altered according to the available observations. Analysis of the interaction between the hurricane mixed layer and the upper layers in terms of entrainment heat and moisture fluxes gives a new insight into the role of sea spray in the thermodynamics and microphysics of the mixed layer. The evaporation of sea spray leads to an increase in the relative humidity by 10-15% and to a decrease in temperature by ~1-1.5 K, as compared to cases where sea spray is excluded. Sea spray leads to formation of drizzling clouds with the cloud base at the height of about 250 m. Taking sea spray effect into account provides a good agreement between the thermodynamics of a simulated mixed layer and the observation data. A parameterization of droplet mass and size distributions as functions of height and wind speed is proposed. The horizontally averaged size distributions are approximated by a sum of lognormal distributions. The moments of size distributions and other integral properties are parameterized as functions of 10-m wind speed by means of simple polynomial expressions.
Smith, R.K., M.T. Montgomery, and G.L. Thomsen. Sensitivity of tropical-cyclone models to the surface drag coefficient in different boundary-layer schemes. Quarterly Journal of the Royal Meteorological Society, 140(680):792-804, https://doi.org/10.1002/qj.2057 2014
The recent study of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al. is extended to include a wind-speed-dependent drag coefficient and one of four boundary-layer parametrization schemes: the bulk, Blackadar, MRF, and Gayno-Seaman schemes. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. Interest is focused on the change in intensity of the azimuthally-averaged tangential wind speed and change in the low-level vortex structure when the standard value of the drag coefficient is halved or doubled. Changing the drag coefficient provides insight into unbalanced effects in the boundary layer and their impact on the vortex evolution and structure. The changes in vortex behavior with changing drag coefficient are qualitatively similar for all schemes, the maximum intensification occurring for a value somewhere near the standard value of the drag coefficient. The interpretation given to explain this behavior underlines the intrinsically unbalanced nature of the boundary-layer dynamics although, for reasons discussed, a complete theory for the behavior does not exist. The behavior found is at odds with the predictions of Emanuel’s (balance) theory for the maximum intensity of a tropical cyclone, which predicts a monotonic decrease in intensity with the drag coefficient if the enthalpy exchange coefficient is held fixed. It is at odds also with a recent numerical study of the maximum intensity by Bryan and Rotunno. The study underscores the importance of boundary-layer dynamics in models for forecasting tropical-cyclone intensity and the need for care in choosing a boundary-layer scheme. However, it is not yet known which boundary-layer formulation is the most appropriate for this purpose, highlighting the need for a concerted research effort in this direction.
Thomsen, G.L., M.T. Montgomery, and R.K. Smith. Sensitivity of tropical-cyclone intensification to perturbations in the surface drag coefficient. Quarterly Journal of the Royal Meteorological Society, 140(679):407-415, https://doi.org/10.1002/qj.2048 2014
The recent studies of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al. and Smith et al. are extended to include perturbations of the surface drag coefficient in one of four boundary layer parameterization schemes: the Bulk scheme, the Blackadar scheme, the MRF scheme, and the Gayno-Seaman scheme. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. We find that the intensification rate and mature intensity when the drag coefficient is perturbed randomly by variations of up to 60 percent are essentially unaltered. The results, in conjunction with an analysis of coherent drag coefficient variations for a moving vortex, challenge the notion that coupled wind-wave models are necessary to accurately forecast tropical cyclone intensification and mature intensity.
Uhlhorn, E.W., B.W. Klotz, T. Vukicevic, P.D. Reasor, and R.F. Rogers. Observed hurricane wind speed asymmetries and relationships to motion and environmental shear. Monthly Weather Review, 142(3):1290-1311, https://doi.org/10.1175/MWR-D-13-00249.1 2014
Wavenumber-1 wind speed asymmetries in 35 hurricanes are quantified in terms of amplitude and phase, based on aircraft observations from 128 individual flights between 1998 and 2011. The impacts of motion and 850-200 mb environmental vertical shear are examined separately to estimate the resulting asymmetric structures at the sea surface and standard 700 mb reconnaissance flight level. The surface asymmetry amplitude is on average around 50% smaller than found at flight level, and while the asymmetry amplitude grows in proportion to storm translation speed at the flight level, no significant growth at the surface is observed, contrary to conventional assumption. However, a significant upwind storm motion-relative phase rotation is found at the surface as translation speed increases, while the flight-level phase remains fairly constant. After removing the estimated impact of storm motion on the asymmetry, a significant residual shear direction-relative asymmetry is found, particularly at the surface, and on average is located down-shear to left-of-shear. Furthermore, the shear-relative phase has a significant downwind rotation as shear magnitude increases, such that the maximum rotates from the downshear to left-of-shear azimuthal location. By stratifying observations according to shear-relative motion, this general pattern of a left-of-shear residual wind speed maximum is found regardless of the orientation between the storm’s heading and shear direction. These results are quite consistent with recent observational studies relating Western Pacific typhoon wind asymmetries to environmental shear. Finally, changes in wind asymmetry over a five-day period in Hurricane Earl (2010) are analyzed to understand combined impacts of motion and the evolving shear.
van Lier-Walqui, M., T. Vukicevic, and D.J. Posselt Linearization of microphysical parameterization uncertainty using multiplicative process perturbation parameters. Monthly Weather Review, 142(1):401-413, https://doi.org/10.1175/MWR-D-13-00076.1 2014
Recent studies have shown the importance of accounting for model physics uncertainty within probabilistic forecasts. Attempts have been made at quantifying this uncertainty in terms of microphysical parameters such as fall speed coefficients, moments of hydrometeor particle size distributions and hydrometeor densities. It has been found that uncertainty in terms of these “traditional” microphysical parameters is highly non-Gaussian, calling into question the possibility of estimating and propagating this error using Gaussian statistical techniques such as as ensemble Kalman methods. Here, a new choice of uncertain control variables is proposed which instead considers uncertainty in individual modeled microphysical processes. These “process parameters” are multiplicative perturbations on contributions of individual modeled microphysical processes to hydrometeor time tendency. The new process parameters provide a natural and appealing choice for the quantification of aleatory microphysical parameterization uncertainty. Results of a non-linear Monte Carlo parameter estimation experiment for these new process parameters are presented and compared with the results using traditional microphysical parameters as uncertain control variables. Both experiments occur within the context of an idealized one-dimensional simulation of moist convection, under the observational constraint of simulated radar reflectivity. Results indicate that the new process parameters have a more Gaussian character compared with traditional microphysical parameters, likely due to a more linear control on observable model evolution. In addition, posterior forecast distributions using the new control variables (process parameters) are shown to have less bias and variance. These results strongly recommend the use of the new process parameters for an Ensemble Kalman-based estimation of microphysical parameterization uncertainty.
Vukicevic, T., E. Uhlhorn, P. Reasor, and B. Klotz. A novel multiscale intensity metric for evaluation of tropical cyclone intensity forecasts. Journal of the Atmospheric Sciences, 71(4):1292-1304, https://doi.org/10.1175/JAS-D-13-0153.1 2014
In this study, a new Multi-Scale Intensity (MSI) metric for evaluating Tropical Cyclone (TC) intensity forecasts is presented. The metric consists of the resolvable and observable, low wavenumber intensity represented by the sum of amplitudes of azimuthal wavenumbers 0 and 1 for wind speed within the TC vortex at the radius of maximum wind, and a stochastic residual, all determined at 10 meter elevation. The residual wind speed is defined as the difference between an estimate of maximum speed and the low wavenumber intensity. The MSI metric is compared to the standard metric that includes only the maximum speed. Using Stepped Frequency Microwave Radiometer wind speed observations from TC aircraft reconnaissance to estimate the low wavenumber intensity and the National Hurricane Center’s Best Track (BT) intensity for the maximum wind speed estimate, it is shown that the residual intensity is well represented as a stochastic quantity with small mean, standard deviation and absolute norm values that are within the expected uncertainty of the BT estimates. The result strongly suggests that the practical predictability of TC intensity is determined by the observable and resolvable low wavenumber intensity within the vortex. Verification of a set of high resolution numerical forecasts using the MSI metric demonstrates that this metric provides more informative and more realistic estimates of the intensity forecast errors. It is also shown that the maximum speed metric allows for error compensation between the low wavenumber and residual intensities, which could lead to forecast skill over-estimation and inaccurate assessment of the impact of forecast system change on the skill.
Walsh, E.J., I. PopStefanija, S.Y. Matrosov, J. Zhang, E. Uhlhorn, and B. Klotz. Airborne rain-rate measurement with a wide swath radar altimeter. Journal of Atmospheric and Oceanic Technology, 31(4):860-875, https://doi.org/10.1175/JTECH-D-13-00111.1 2014
The NOAA Wide Swath Radar Altimeter (WSRA) uses 80 narrow beams spread over ±30° in the cross-track direction to generate raster lines of sea surface topography at a 10 Hz rate from which sea surface directional wave spectra are produced. A ±14° subset of the backscattered power data associated with the topography measurements is used to produce independent measurements of rain rate and sea surface mean square slope at 10 s intervals. Theoretical calculations of rain attenuation at the WSRA 16.15 GHz operating frequency using measured drop size distributions for both mostly convective and mostly stratiform rainfall demonstrate that the WSRA absorption technique for rain determination is relatively insensitive to both ambient temperature and the characteristics of the drop size distribution, in contrast to reflectivity techniques. The variation of the sea surface radar reflectivity in the vicinity of a hurricane is reviewed. Fluctuations in the sea surface scattering characteristics caused by changes in wind speed or the rain impinging on the surface cannot contaminate the rain measurement because they are calibrated out using the WSRA measurement of mean square slope. WSRA rain measurements from a NOAA WP-3D hurricane research aircraft off the North Carolina coast in Hurricane Irene on 26 August 2011 are compared with those from the Stepped Frequency Microwave Radiometer (SFMR) on the aircraft and the NEXRAD National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE) System.
Zhang, B., W. Perrie, J.A. Zhang, E.W. Uhlhorn, and Y. He High-resolution hurricane vector winds from C-band dual-polarization SAR observations. Journal of Oceanic and Atmospheric Technology, 31(2):272-286, https://doi.org/10.1175/JTECH-D-13-00006.1 2014
This study presents a new approach for retrieving hurricane surface wind vectors utilizing C-band dual-polarization (VV, VH) synthetic aperture radar (SAR) observations. The co-polarized geophysical model function (CMOD5.N) and a new cross-polarized wind speed retrieval model for dual-polarization (C-2POD) are employed to construct a cost function. Minimization of the cost function allows optimum estimates for the wind speeds and directions. The wind direction ambiguities are removed using a parametric two dimensional sea-surface inflow angle model. To evaluate the accuracy of the proposed method, two RADARSAT-2 SAR images of Hurricanes Bill and Bertha are analyzed. The retrieved wind speeds and directions are compared with collocated QuikSCAT scatterometer winds, showing good consistency. Results suggest that the proposed method has good potential to retrieve hurricane surface wind vectors from dual-polarization SAR observations.
Zhang, J.A., M.T. Montgomery, F.D. Marks, and R.K. Smith. Comments on “Symmetric and asymmetric structures of hurricane boundary layer in coupled atmosphere-wave-ocean models and observations.” Journal of the Atmospheric Sciences, 71(7):2782-2785, https://doi.org/10.1175/JAS-D-13-0207.1 2014
Aksoy, A. Storm-relative observations in tropical cyclone data assimilation with an ensemble Kalman filter. Monthly Weather Review, 141(2):506-522, https://doi.org/10.1175/MWR-D-12-00094.1 2013
A storm-relative data assimilation method for tropical cyclones is introduced for the ensemble Kalman filter, using the Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) developed at the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory at the National Oceanic and Atmospheric Administration. The method entails translating tropical cyclone observations to storm-relative coordinates and requires the assumption of simultaneity of all observations. The observations are then randomly re-distributed to assimilation cycles to achieve a more homogeneous spatial distribution. A proof-of-concept study is carried out in an observing system simulation experiment in which airborne Doppler radar radial wind observations are simulated from a higher-resolution (4.5/1.5 km) version of the same model. The results here are compared to the Earth-relative version of HEDAS. When storm-relative observations are assimilated using the original HEDAS configuration, improvements are observed in the kinematic representation of the tropical cyclone vortex in analyses. The use of the storm-relative observations with a more homogeneous spatial distribution also reveals that a reduction of the covariance localization horizontal length scale by 1/2 to ~120 km provides the greatest incremental improvements. Potential positive impact is also seen in the slower cycle-to-cycle error growth. Spatially smoother analyses are obtained in the horizontal and the evolution of the azimuthally averaged wind structure during short-range forecasts demonstrates better consistency with the nature run.
Aksoy, A., S.D. Aberson, T. Vukicevic, K.J. Sellwood, S. Lorsolo, and X. Zhang. Assimilation of high-resolution tropical cyclone observations with an ensemble Kalman filter using NOAA/AOML/HRD’s HEDAS: Evaluation of the 2008-2011 vortex-scale analyses. Monthly Weather Review, 141(6):1842-1865, https://doi.org/10.1175/MWR-D-12-00194.1 2013
The Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) is developed to assimilate tropical cyclone inner-core observations for high-resolution vortex initialization. It is based on a serial implementation of the square-root ensemble Kalman filter (EnKF). In this study, HWRF is used in an experimental configuration with horizontal grid spacing of 9/3 km on the outer/inner domains. HEDAS is applied to 83 cases from years 2008-2011. With the exception of two Hurricane Hilary (2011) cases in the eastern North Pacific basin, all cases are observed in the Atlantic basin. Observed storm intensity for these cases ranges from tropical depression to category-4 hurricane. Overall, it is found that high-resolution tropical cyclone observations, when assimilated with an advanced data assimilation technique such as the EnKF, result in analyses of the primary circulation that are realistic in terms of intensity, wavenumber-0 radial structure, as well as wavenumber-1 azimuthal structure. Representing the secondary circulation in the analyses is found to be more challenging with systematic errors in the magnitude and depth of the low-level radial inflow. This is believed to result from a model bias in the experimental HWRF due to the over-diffusive nature of the planetary boundary layer parameterization utilized. Thermodynamic deviations from the observed structure are believed to be due to both an imbalance between the number of the kinematic and thermodynamic observations in general and the sub-optimal ensemble covariances between kinematic and thermodynamic fields. Future plans are discussed to address these challenges.
Bell, G.D., S.B. Goldenberg, C.W. Landsea, E.S. Blake, T.B. Kimberlain, J. Schemm, and R.J. Pasch. The tropics: Atlantic basin. In State of the Climate in 2012, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 94(8):S85-S89, https://doi.org/10.1175/2013BAMSStateoftheClimate.1 2013
Byrne, D., and J.A. Zhang. Height-dependent transition from 3-D to 2-D turbulence in the hurricane boundary layer. Geophysical Research Letters, 40(7):1439-1442, https://doi.org/10.1002/grl.50335 2013
Here we show, from in situ aircraft measurements in the hurricane boundary layer, a height-dependent transition of the flow from 3-D to 2-D turbulence. This marks a fundamental change in the energy dynamics of the hurricane boundary layer due to the fact that in 3-D, energy flows downscale from larger to smaller scales, whereas in 2-D, it flows upscale, from smaller to larger scales. These results represent the first measurement of the 2-D upscale energy flux in the atmosphere and also the first to characterize the transition from 3-D to 2-D. It is shown that the large-scale parent vortex may gain energy directly from small scales in tropical cyclones.
Cione, J.J., E.A. Kalina, J.A. Zhang, and E.W. Uhlhorn. Observations of air-sea interaction and intensity change in hurricanes. Monthly Weather Review, 141(7):2368-2382, https://doi.org/10.1175/MWR-D-12-00070.1 2013
Recent enhancements to the tropical cyclone-buoy database (TCBD) have incorporated data from the Extended Best Track (EBT) and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) archive for tropical cyclones between 1975 and 2007. This information is used to analyze the relationships between large-scale atmospheric parameters, radial and shear-relative air-sea structure, and intensity change in strengthening and weakening hurricanes. Observations from this research illustrate that the direction of the large-scale vertical wind shear at mid-to-low levels can impact atmospheric moisture conditions found near the surface. Drier low-level environments were associated with northerly shear conditions. In a separate analysis comparing strengthening and weakening hurricanes, drier surface conditions were also found for the intensifying sample. Since SST conditions were similar for both groups of storms, it is likely that the atmosphere was primarily responsible for modifying the near-surface thermodynamic environment (and ultimately surface moisture flux conditions) for this particular analysis.
Coddington, O., P. Pilewskie, K.S. Schmidt, P.J. McBride, and T. Vukicevic. Characterizing a new surface-based shortwave cloud retrieval technique, based on transmitted radiance for soil and vegetated surface types. Atmosphere, 4(1):48-71, https://doi.org/10.3390/atmos4010048 2013
This paper presents an approach using the GEneralized Nonlinear Retrieval Analysis (GENRA) tool and general inverse theory diagnostics including the maximum likelihood solution and the Shannon information content to investigate the performance of a new spectral technique for the retrieval of cloud optical properties from surface based transmittance measurements. The cumulative retrieval information over broad ranges in cloud optical thickness (τ), droplet effective radius (re), and overhead sun angles is quantified under two conditions known to impact transmitted radiation; the variability in land surface albedo and atmospheric water vapor content. Our conclusions are: (1) the retrieved cloud properties are more sensitive to the natural variability in land surface albedo than to water vapor content; (2) the new spectral technique is more accurate (but still imprecise) than a standard approach, in particular for τ between 5 and 60 and re less than approximately 20 μm; and (3) the retrieved cloud properties are dependent on sun angle for clouds of from 5 to 10 and re < 10 μm, with maximum sensitivity obtained for an overhead sun.
Elsberry, R.L., L. Chen, J. Davidson, R. Rogers, Y. Wang, and L. Wu. Advances in understanding and forecasting rapidly changing phenomena in tropical cyclones. Tropical Cyclone Research and Review, 2(1):13-24, https://doi.org/10.6057/2013TCRR01.02 2013
This review of new understanding and forecasting of tropical cyclones (TCs) is based on presentations at the International Top-level Forum on Rapid Change Phenomena in Tropical Cyclones in Haikou, China. The major topics are the sudden changes in tracks, rapid changes in structure and intensity, rapid changes in rainfall, and advances in forecasting and forecaster requirements. Although improved track forecast guidance has been achieved with the Australian ACCESS-TC model and in track forecasts to 120 h by the China Meteorological Administration, there is a continuing need for better understanding and improved track forecast guidance. Advances in understanding of processes related to rapid intensification (RI), secondary eyewall formation, mechanisms controlling inner-core structure and size changes, and structure and intensity changes at landfall have been achieved, but progress in prediction of rapid changes in structure and intensity has been slow. Taking into account complex interactions involved in TC-related rainfall, a prioritized list of physical processes that govern rainfall from landfalling TCs in China has been developed. While forecaster participants were generally encouraged by the progress being made, they expressed a strong desire for a transition of that new knowledge to timely and reliable forecast guidance products.
Gall, R., J. Franklin, F.D. Marks E.N. Rappaport, and F. Toepfer The Hurricane Forecast Improvement Project. Bulletin of the American Meteorological Society, 94(3):329-343, https://doi.org/10.1175/BAMS-D-12-00071.1 2013
Over the decade prior to 2007, the increasing vulnerability of the US to damage and economic disruption from tropical storms/hurricanes was dramatically demonstrated by the impacts of a number of land-falling storms. In 2008, the National Oceanic and Atmospheric Administration established the Hurricane Forecast Improvement Project (HFIP) to significantly increase the Agency's capability to address this vulnerability and begin to mitigate the impacts. In fiscal year 2009, The White House amended the President's Budget and Congress appropriated funding to achieve a 20% reduction in forecast error (track and intensity) in 5 years with 50% reduction in 10 years. Over the past 3 years, HFIP has built computational infrastructure and implemented a focused set of cross-organizational R&D activities to develop, demonstrate, and implement enhanced operational modeling capabilities to improve the numerical forecast guidance made available to the National Hurricane Center (NHC). HFIP collaborators, including federal laboratories and academic partners, have demonstrated potential for dramatic improvements in both hurricane track and intensity (up to 40%) prediction through the application of new techniques including improved data assimilation, higher resolution models (global and regional), enhanced model physics, better use of existing data sources to initialize regional hurricane models, and new post processing techniques. During each Hurricane Season, HFIP will run an experimental forecast system on NOAA's R&D high performance computing to provide experimental improved guidance to NHC forecasters. Prior to each season, NHC will review and select a set of enhanced guidance products to evaluate operationally during the season (mid July-October).
Giammanco, I.M., J.L. Schroeder, and M.D. Powell. GPS dropwindsonde and WSR-88D observations of tropical cyclone vertical wind profiles and their characteristics. Weather and Forecasting, 28(1):77-99, https://doi.org/10.1175/WAF-D-11-00155.1 2013
The characteristics of tropical cyclone vertical wind profiles and their associated wind speed peaks below 1.5 km were examined through the use of a large number of GPS dropwindsondes (GPS sondes) and radar-derived velocity azimuth display (VAD) profiles. Composite wind profiles were generated to document the mean structure of tropical cyclone vertical wind profiles and their changes with storm-relative position. Composite profiles were observed to change as radius decreased inward toward the radius of maximum winds. Profiles also varied between three azimuthal sectors. At landfall, wind profiles exhibited changes with radial distance and differences were observed between those within offshore and onshore flow regimes. The observations support a general reduction in boundary layer depth with decreasing radial distance. Wind profiles with peaks at low altitudes were typically confined to radii less than 60 km, near and radially inward from the radius of maximum winds. Wind speed maxima, when scaled by a layer mean wind, decreased in magnitude as radius decreased. At landfall, composite profiles showed a distinct low-level wind speed maximum in the eyewall region with significant differences between the onshore and offshore flow regimes.
Gopalakrishnan, S.G., F. Marks, J.A. Zhang, X. Zhang, J.-W. Bao, and V. Tallapragada. A study of the impacts of vertical diffusion on the structure and intensity of tropical cyclones using the high resolution HWRF system. Journal of the Atmospheric Sciences, 70(2):524-541, https://doi.org/10.1175/JAS-D-11-0340.1 2013
The Hurricane Weather Research and Forecasting (HWRF) system was used in an idealized framework to gain a fundamental understanding of the variability in TC structure and intensity prediction that may arise due to vertical diffusion. The modeling system uses the Medium-Range Forecast (MRF) parameterization scheme. Flight-level data collected by a NOAA WP-3D research aircraft during the eyewall penetration of category 5 Hurricane Hugo (1989) at an altitude of about 450-500 m and Hurricane Allen (1980) were used as the basis to best match the modeled eddy diffusivities with wind speed. While reduction of the eddy diffusivity to a quarter of its original value produced the best match with the observations, such a reduction revealed a significant decrease in the height of the inflow layer as well which, in turn, drastically impacted the size and intensity changes in the modeled TC. The cross-isobaric flow (inflow) was observed to be stronger with the decrease in the inflow depth. Stronger inflow not only increased the spin of the storm, enhancing the generalized Coriolis term in the equations of motion for tangential velocity, but also resulted in enhanced equivalent potential temperature in the boundary layer, a stronger and warmer core and, subsequently, a stronger storm. More importantly, rapid acceleration of the inflow not only produced a stronger outflow at the top of the inflow layer, more consistent with observations, but also a smaller inner core that was less than half the size of the original.
Hoffman, R.N., J.V. Ardizzone, S.M. Leidner, D.K. Smith, and R.M. Atlas. Error estimates for ocean surface winds: Applying Desroziers diagnostics to the cross-calibrated, multiplatform analysis of wind speed. Journal of Oceanic and Atmospheric Technology, 30(11):2596-2603, https://doi.org/10.1175/JTECH-D-13-00018.1 2013
The Desroziers diagnostics (DD) are applied to the cross-calibrated, multi-platform (CCMP) ocean surface wind data sets to estimate wind speed errors of the ECMWF background, the microwave satellite observations and the resulting CCMP analysis. The DD confirm that the ECMWF operational surface wind speed error standard deviations vary with latitude in the range 0.8–1.3 m s−1 and that the cross-calibrated Remote Sensing Systems (RSS) wind speed retrievals standard deviations are in the range 0.5–0.7 m s−1. Further the estimated CCMP analysis wind speed standard deviations are in the range 0.2–0.3 m s−1. The results suggest the need to revise the parameterization of the errors due to the FGAT (first guess at the appropriate time) procedure. Errors for wind speeds −1 are homogeneous, but for the relatively rare, but critical higher wind speed situations, errors are much larger.
Hope, M.E., J.J. Westerink, A.B. Kennedy, P.C. Kerr, J.C. Dietrich, C. Dawson, C.J. Bender, J.M. Smith, R.E. Jensen, M. Zijlema, L.H. Holthuijsen, R.A. Luettich, M.D. Powell, V.J. Cardone, A.T. Cox, H. Pourtaheri, H.J. Roberts, J.H. Atkinson, S. Tanaka, H.J. Westerink, and L.G. Westerink. Hindcast and validation of Hurricane Ike (2008): Waves, forerunner, and storm surge. Journal of Geophysical Research, 118(9):4424-4460, https://doi.org/10.1002/jgrc.20314 2013
Hurricane Ike (2008) made landfall near Galveston, Texas, as a moderate intensity storm. Its large wind field in conjunction with the Louisiana-Texas coastline's broad shelf and large scale concave geometry generated waves and surge that impacted over 1000 km of coastline. Ike's complex and varied wave and surge response physics included: the capture of surge by the protruding Mississippi River Delta; the strong influence of wave radiation stress gradients on the Delta adjacent to the shelf break; the development of strong wind driven shore-parallel currents and the associated geostrophic setup; the forced early rise of water in coastal bays and lakes facilitating inland surge penetration; the propagation of a free wave along the southern Texas shelf; shore-normal peak wind-driven surge; and resonant and reflected long waves across a wide continental shelf. Preexisting and rapidly deployed instrumentation provided the most comprehensive hurricane response data of any previous hurricane. More than 94 wave parameter time histories, 523 water level time histories, and 206 high water marks were collected throughout the Gulf in deep water, along the nearshore, and up to 65 km inland. Ike's highly varied physics were simulated using SWAN + ADCIRC, a tightly coupled wave and circulation model, on SL18TX33, a new unstructured mesh of the Gulf of Mexico, Caribbean Sea, and western Atlantic Ocean with high resolution of the Gulf's coastal floodplain from Alabama to the Texas-Mexico border. A comprehensive validation was made of the model's ability to capture the varied physics in the system.
Katzberg, S.J., J.P. Dunion, and G.G. Ganoe. The use of reflected GPS signals to retrieve ocean surface wind speeds in tropical cyclones. Radio Science, 48(4):371-387, https://doi.org/10.1002/rds.20042 2013
Since the first intentional acquisition of GPS signals reflected from water bodies, one of the objectives which has driven the research is to determine whether the acquired signal can provide useful geophysical information about the reflecting surface. One obvious condition of considerable interest is ocean surface wind speed. Theory suggested that the reflection technique, a form of bistatic RADAR, would be sensitive to surface roughness which in turn is driven by wind speed. This paper reports the results derived from data acquired over the past decade of applying the GPS reflection technique to ocean surface winds, particularly ocean surface winds in tropical cyclones. Examples of wind speed retrievals will be given for some illustrative cases of hurricanes and tropical storms. The results from several hurricanes and tropical storms on how the signal was calibrated will be presented. In addition, a quantitative comparison will be given between dropsondes deployed by NOAA during the storms and GPS reflection derived wind speeds taken at the same time. Conditions in which the GPS technique offers excellent comparisons as well as examples where the comparison is not so good will be presented. Suggestions will be given as to when the GPS technique can be used with confidence and when it is likely to be at variance with other methods.
Kerr, P.C., A.S. Donahue, J.J. Westerink, R.A. Luettich, L.Y. Zheng, R.H. Weisberg, Y. Huang, H.V. Wang, Y. Teng, D.R. Forrest, A. Roland, A.T. Haase, A.W. Kramer, A.A. Taylor, J.R. Rhome, J.C. Feyen, R.P. Signell, J.L. Hanson, M.E. Hope, R.M. Estes, R.A. Dominguez, R.P. Dunbar, L.N. Semeraro, H.J. Westerink, A.B. Kennedy, J.M. Smith, M.D. Powell, V.J. Cardone, and A.T. Cox. U.S. IOOS coastal and ocean modeling testbed: Inter-model evaluation of tides, waves, and hurricane surge in the Gulf of Mexico. Journal of Geophysical Research, 118(C10):5129-5172, https://doi.org/10.1002/jgrc.20376 2013
A Gulf of Mexico performance evaluation and comparison of coastal circulation and wave models was executed through harmonic analyses of tidal simulations, hindcasts of Hurricane Ike (2008) and Rita (2005), and a benchmarking study. Three unstructured coastal circulation models (ADCIRC, FVCOM, and SELFE) validated with similar skill on a new common Gulf scale mesh (ULLR) with identical frictional parameterization and forcing for the tidal validation and hurricane hindcasts. Coupled circulation and wave models, SWAN+ADCIRC and WWMII+SELFE, along with FVCOM loosely coupled with SWAN, also validated with similar skill. NOAA's official operational forecast storm surge model (SLOSH) was implemented on local and Gulf scale meshes with the same wind stress and pressure forcing used by the unstructured models for hindcasts of Ike and Rita. SLOSH's local meshes failed to capture regional processes such as Ike's forerunner and the results from the Gulf scale mesh further suggest shortcomings may be due to a combination of poor mesh resolution, missing internal physics such as tides and nonlinear advection, and SLOSH's internal frictional parameterization. In addition, these models were benchmarked to assess and compare execution speed and scalability for a prototypical operational simulation. It was apparent that a higher number of computational cores are needed for the unstructured models to meet similar operational implementation requirements to SLOSH, and that some of them could benefit from improved parallelization and faster execution speed.
Li, X., J.A. Zhang, X. Yang, W.G. Pichel, M. DeMaria, D. Long, and Z. Li. Tropical cyclone morphology from spaceborne synthetic aperture radar. Bulletin of the American Meteorological Society, 94(2):215-230, https://doi.org/10.1175/BAMS-D-11-00211.1 2013
In 2008, the Canadian Space Agency sponsored the RADARSAT Hurricane Applications Project (RHAP), for researching new developments in the application of RADARSAT-1 synthetic aperture radar (SAR) data and innovative mapping approaches to better understand the dynamics of tropical cyclone genesis, morphology, and movement. Although tropical cyclones can be detected by many remote sensors, SAR can yield high-resolution (sub kilometer) and low-level storm information that cannot be seen below the clouds by other sensors. In addition to the wind field and tropical cyclone eye information, structures associated with atmospheric processes can also be detected by SAR. We have acquired 161 RADARSAT-1 SAR images through RHAP between 2001 and 2007. Among these, 73 images show clear tropical cyclone eye structure. In addition, we also acquired 10 images from the European Space Agency's ENVISAT SAR between 2004 and 2010. Both Atlantic hurricanes and Pacific typhoons are included. In this study, we analyze these 83 (73 RADARSAT-1 and 10 ENVISAT) images with tropical cyclone eye information along with ancillary tropical cyclone intensity information from the archive to generate tropical cyclone morphology statistics. Histograms of wave number asymmetry and intensity are presented. The statistics show that when the storm has higher intensity, the tropical cyclone eye tends to become more symmetric, and the area of the tropical cyclone eye, defined by the minimum wind area, tends to be smaller. Examples of fine-scale structures within the tropical cyclone, i.e., eye-eyewall meso-vortices, arc clouds, double eyewalls, and abnormally high wind or rain within eyes, are presented and discussed.
Li, X., W. Zheng, X. Yang, J.A. Zhang, W.G. Pichel, and Z. Li. Coexistence of atmospheric gravity waves and boundary layer rolls observed by SAR. Journal of the Atmospheric Sciences, 70(11):3448-3459, https://doi.org/10.1175/JAS-D-12-0347.1 2013
Both atmospheric gravity waves (AGW) and marine atmospheric boundary layer (MABL) rolls are simultaneously observed on an Envisat Advanced Synthetic Aperture Radar (ASAR) image acquired along the China coast on May 22, 2005. The Synthetic Aperture Radar (SAR) image covers about 400 by 400 km of a coastal area of the Yellow Sea. The sea surface imprints of AGW show the patterns of both a transverse wave along the coastal plain and a diverging wave in the lee of Mountain Laoshan (1133 m peak), which indicates that terrain forcing affects the formation of AGW. The AGW have a wavelength of 8-10 km and extend about 100 km offshore. Model simulation shows that these waves have amplitude over 3 km. Finer scale (~ 2 km) brush-like roughness features perpendicular to the coast are also observed and they are interpreted as MABL rolls. The FFT analysis shows that the roll wavelengths vary spatially. The two-way interactive, triply nested grid (9/3/1 km) Weather Research and Forecasting (WRF) model simulation reproduces AGW-generated wind perturbations which are in phase at all levels reaching up to the 700 mb level for the diverging AGW and the 900 mb level for the transverse AGW. The WRF simulation also reveals that dynamic instability, rather than thermodynamic instability, is the cause for the MABL roll generation. Differences in atmospheric inflection-point level and instability at different locations are reasons why the roll wavelengths vary spatially.
Lorsolo, S., J. Gamache, and A. Aksoy. Evaluation of the Hurricane Research Division Doppler radar analysis software using synthetic data. Journal of Atmospheric and Oceanic Technology, 30(6):1055-1071, https://doi.org/10.1175/JTECH-D-12-00161.1 2013
The Hurricane Research Division Doppler radar analysis software provides three-dimensional analyses of the three wind components in tropical cyclones. Although this software has been used for over a decade, there has never been a complete and in-depth evaluation of the resulting analyses. The goal here is to provide an evaluation that will permit the best use of the analyses, but also to improve the software. To evaluate the software, analyses are produced from simulated radar data acquired from an output of a HWRF nature run and are compared against the model “truth” wind fields. Comparisons of the three components of the wind show that the software provides analyses of good quality. The tangential wind is best retrieved, exhibiting an overall small mean error of 0.5 m s−1 at most levels and root-mean-squared error less than 2 m s−1. The retrieval of the radial wind is also quite accurate, exhibiting comparable errors, although the accuracy of the tangential wind is generally better. Some degradation of the retrieval quality is observed at higher altitude, mainly due to sparser distribution of data in the model. The vertical component of the wind appears to be the most challenging to retrieve, but the software still provides acceptable results. The tropical cyclone mean azimuthal structure as well as wavenumber structure is found to be very well captured. Sources of errors inherent to airborne Doppler measurements as well as the effects of some the simplications used in the simulation methodology are also discussed.
Misra, V., S. DiNapoli, and M.D. Powell. The track integrated kinetic energy of Atlantic tropical cyclones. Monthly Weather Review, 141(7):2383-2389, https://doi.org/10.1175/MWR-D-12-00349.1 2013
In this paper we introduce the concept of Track Integrated Kinetic Energy (TIKE) as a measure of seasonal Atlantic tropical cyclone activity and applied to seasonal variability in the Atlantic. It is similar in concept to the more commonly used Accumulated Cyclone Energy (ACE) with an important difference that in TIKE we accumulate the Integrated Kinetic Energy (IKE) for the lifespan of the Atlantic tropical cyclone. The IKE is however computed by volume integrating the 10m level sustained winds of tropical strength or higher quadrant-by quadrant, while ACE uses the maximum sustained winds only without accounting for the structure of the storm. In effect TIKE accounts for the intensity, duration, and size of the tropical cyclones. In this research we have examined the seasonality and the interannual variations of the seasonal Atlantic TIKE over a period of 22 years from 1990-2011. We find that the Atlantic TIKE climatologically peaks in the month of September and the frequency of storms with the largest TIKE are highest in the eastern tropical Atlantic. The interannual variations of the Atlantic TIKE reveal that it is likely influenced by SST variations in the equatorial Pacific and in the Atlantic Oceans. The SST variations in the central equatorial Pacific are negatively correlated with the contemporaneous seasonal (June-November) TIKE. The size of the Atlantic Warm Pool (AWP) is positively correlated with seasonal TIKE.
Nicholls, M.E., and M.T. Montgomery. An examination of two pathways to tropical cyclogenesis occurring in idealized simulations with a cloud-resolving numerical model. Atmospheric Chemistry and Physics, 13(12):5999-6022, https://doi.org/10.5194/acp-13-5999-2013 2013
Simulations are conducted with a cloud-resolving numerical model to examine the transformation of a weak incipient mid-level cyclonic vortex into a tropical cyclone. Results demonstrate that two distinct pathways are possible and that development along a particular pathway is sensitive to model physics and initial conditions. One pathway involves a steady increase of the surface winds to tropical cyclone strength as the radius of maximum winds gradually decreases. A notable feature of this evolution is the creation of small-scale lower tropospheric cyclonic vorticity anomalies by deep convective towers and subsequent merger and convergence by the low-level secondary circulation. The second pathway also begins with a strengthening low-level circulation, but eventually a significantly stronger mid-level circulation develops. Cyclogenesis occurs subsequently when a small-scale surface concentrated vortex forms abruptly near the center of the larger-scale circulation. The small-scale vortex is warm core throughout the troposphere and results in a fall in local surface pressure of a few millibars. It usually develops rapidly, undergoing a modest growth to form a small tropical cyclone. Many of the simulated systems approach or reach tropical cyclone strength prior to development of a prominent mid-level vortex so that the subsequent formation of a strong small-scale surface concentrated vortex in these cases could be considered intensification rather than genesis. Experiments are performed to investigate the dependence on the inclusion of the ice phase, radiation, the size and strength of the incipient mid-level vortex, the amount of moisture present in the initial vortex, and the sea surface temperature. Notably, as the sea surface temperature is raised, the likelihood of development along the second pathway is increased. This appears to be related to an increased production of ice. The sensitivity of the pathway taken to model physics and initial conditions revealed by these experiments raise the possibility that the solution to this initial value problem is near a bifurcation point. Future improvements to model parameterizations and more accurate observations of the transformation of disturbances to tropical cyclones should clarify the conditions that favor a particular pathway when starting from a mid-level vortex
Nolan, D.S., R. Atlas, K.T. Bhatia, and L.R. Bucci. Development and validation of a hurricane nature run using the Joint OSSE nature run and the WRF model. Journal of Advances in Modeling Earth Systems, 5(2):382-405, https://doi.org/10.1002/jame.20031 2013
A nature run is a critical component of an observing system simulation experiment (OSSE), which is a framework for evaluating the potential impact of additional observations, enhanced observing systems, or alternative data assimilation schemes toward improving numerical weather forecasts. The nature run is a period of simulated weather generated by a research-quality numerical model, from which synthetic observations are sampled and provided to the data assimilation system and forecast model. This paper describes the development and validation of a nature run that depicts the life cycle of a strong hurricane over the North Atlantic Ocean. For compatibility with related research projects, the hurricane nature run is generated by a regional model, the weather research and forecasting model (WRF), embedded within the Joint OSSE global nature run previously generated by the European Center for Medium-Range Weather Forecasting. The domain sizes, resolution, and physical parameterizations used in the WRF simulation are discussed, and the evolution of the storm from tropical wave to recurving hurricane is described. The realism of the simulated hurricane is evaluated by comparing the model output to composited data from real hurricanes obtained from both in situ and remotely sensed observations. These include the pressure-wind relationship, the kinematic and thermodynamic structure of the boundary layer, the size and outward slope of the radius of maximum winds, and contours of frequency by altitude diagrams of reflectivity and vertical velocity. The strengths and weaknesses of the nature run hurricane are discussed.
Ralph, F.M., J. Intrieri, D. Andra, R. Atlas, S. Boukabara, D. Bright, P. Davidson, B. Entwistle, J. Gaynor, S. Goodman, J.-G. Jiing, A. Harless, J. Huang, G. Jedlovec, J. Kain, S. Koch, B. Kuo, J. Levit, S. Murillo, L.P. Riishojgaard, T. Schneider, R. Schneider, T. Smith, and S. Weiss. The emergence of weather-related testbeds linking research and forecasting operations. Bulletin of the American Meteorological Society, 94(8):1187-1211, https://doi.org/10.1175/BAMS-D-12-00080 2013
Testbeds have emerged as a critical mechanism linking weather research with forecasting operations. The U.S. Weather Research Program (USWRP) was formed in the 1990s to help identify key gaps in research related to major weather prediction problems and the role of observations and numerical models. This planning effort ultimately revealed the need for greater capacity and new approaches to improve the connectivity between the research and forecasting enterprise. Out of this developed the seeds for what is now termed “testbeds.” While many individual projects, and even more broadly the NOAA National Weather Service (NWS) Modernization, were successful in advancing weather prediction services, it was recognized that specific forecast problems warranted a more focused and elevated level of effort. The USWRP helped develop these concepts with science teams and provided seed funding for several of the testbeds described. Based on the varying NOAA mission requirements for forecasting, on differences in the organizational structure and methods used to provide those services, and on differences in the state of the science related to those forecast challenges, testbeds have taken on differing characteristics, strategies, and priorities. Current testbed efforts described have all emerged between 2000-2011 and focus on hurricanes (Joint Hurricane Testbed), precipitation (Hydrometeorology Testbed), satellite data assimilation (Joint Center for Satellite Data Assimilation), severe weather (Hazardous Weather Testbed), satellite data support for severe weather prediction (Short-term Prediction Research and Transition Center), mesoscale modeling (Developmental Testbed Center), climate forecast products (Climate Testbed), testing and evaluation of satellite capabilities (GOES-R Proving Ground), aviation applications (Aviation Weather Testbed), and observing system experiments (OSSE Testbed).
Reasor, P., R. Rogers, and S. Lorsolo. Environmental flow impacts on tropical cyclone structure diagnosed from airborne Doppler radar composites. Monthly Weather Review, 141(9):2949-2969, https://doi.org/10.1175/MWR-D-12-00334.1 2013
Following a recent demonstration of multi-case compositing of axisymmetric tropical cyclone (TC) structure derived from airborne Doppler radar measurements, we extend the analysis to the asymmetric structure using an unprecedented database from 75 TC flights. In particular, we examine the precipitation and kinematic asymmetry forced by the TC’s motion and interaction with vertical wind shear. We quantify for the first time the average magnitude and phase of the three-dimensional shear-relative kinematic asymmetry of observed TCs through a composite approach. The composite analysis confirms principal features of the shear-relative TC asymmetry documented in prior numerical and observational studies (e.g., downshear tilt, downshear-right convective initiation, and a downshear-left precipitation maximum). The statistical significance of the composite shear-relative structure is demonstrated through a stratification of cases by shear magnitude. The impact of storm motion on eyewall convective asymmetry appears to be subdominant to the much greater constraint placed by vertical wind shear on the organization of convection, in agreement with prior studies using lightning and precipitation data.
Renno, N.O., E. Williams, D. Rosenfeld, D.G. Fischer, J. Fischer, T. Kremic, A. Agrawal, M.O. Andreae, R. Bierbaum, R. Blakeslee, A. Boerner, N. Bowles, H. Christian, A. Cox, J. Dunion, A. Horvath, X. Huang, A. Khain, S. Kinne, M.C. Lemos, J.E. Penner, U. Poschl, J. Quaas, E. Seran, B. Stevens, T. Walati, and T. Wagner. CHASER: An innovative satellite mission concept to measure the effects of aerosols on clouds and climate. Bulletin of the American Meteorological Society, 94(5):685-694, https://doi.org/10.1175/BAMS-D-11-00239 2013
The formation of cloud droplets on aerosol particles, technically known as the activation of cloud condensation nuclei (CCN), is the fundamental process driving the interactions of aerosols with clouds and precipitation. The Intergovernmental Panel on Climate Change (IPCC) and the Decadal Survey indicate that the uncertainty in how clouds adjust to aerosol perturbations dominates the uncertainty in the overall quantification of the radiative forcing attributable to human activities. Measurements by current satellites allow the determination of crude profiles of cloud particle size, but not of the activated CCN that seed them. The Clouds, Hazards, and Aerosols Survey for Earth Researchers (CHASER) mission concept responds to the IPCC and Decadal Survey concerns, utilizing a new technique and high-heritage instruments to measure all the quantities necessary to produce the first global survey maps of activated CCN and the properties of the clouds associated with them. CHASER also determines the activated CCN concentration and cloud thermodynamic forcing simultaneously, allowing the effects of each to be distinguished.
Riemer, M., M.T. Montgomery, and M.E. Nicholls. Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear. Atmospheric Chemistry and Physics, 13(1):327-346, https://doi.org/10.5194/acp-13-327-2013 2013
Recent work has developed a new framework for the impact of vertical wind shear on the intensity evolution of tropical cyclones. A focus of this framework is on the frustration of the tropical cyclone's power machine by shear-induced, persistent downdrafts that flush relatively cool and dry (lower equivalent potential temperature, thetae) air into the storm's inflow layer. These previous results have been based on idealized numerical experiments for which we have deliberately chosen a simple set of physical parameterizations. Before efforts are undertaken to test the proposed framework with real atmospheric data, we assess here the robustness of our previous results in a more realistic and representative experimental setup by surveying and diagnosing five additional numerical experiments. The modifications of the experimental setup comprise the values of the exchange coefficients of surface heat and momentum fluxes, the inclusion of experiments with ice microphysics, and the consideration of weaker, but still mature tropical cyclones. In all experiments, the depression of the inflow layer thetae values is significant and all tropical cyclones exhibit the same general structural changes when interacting with the imposed vertical wind shear. Tropical cyclones in which strong downdrafts occur more frequently exhibit a more pronounced depression of inflow layer thetae outside of the eyewall in our experiments. The magnitude of the thetae depression underneath the eyewall early after shear is imposed in our experiments correlates well with the magnitude of the ensuing weakening of the respective tropical cyclone. Based on the evidence presented, it is concluded that the newly proposed framework is a robust description of intensity modification in our suite of experiments.
Rogers, R., P. Reasor, and S. Lorsolo. Airborne Doppler observations of the inner-core structural differences between intensifying and steady-state tropical cyclones. Monthly Weather Review, 141(9):2970-2991, https://doi.org/10.1175/MWR-D-12-00357.1 2013
Differences in the inner-core structure of intensifying (IN; intensity increase of at least 20 kt /24 h) and steady-state (SS; intensity remaining between ± 10 kt/24 h) tropical cyclones (TC’s) are examined using composites of airborne Doppler observations collected from NOAA P-3 aircraft missions. The IN dataset contains 40 eyewall passes from 14 separate missions, while the SS dataset contains 53 eyewall passes from 14 separate missions. Intensifying TC’s have a ring-like vorticity structure inside the radius of maximum wind (RMW), lower vorticity in the outer core, a deeper, stronger inflow layer, and stronger axisymmetric eyewall upward motion compared with steady-state TC’s. There is little difference in the vortex tilt between 2 and 7 km, and both IN and SS TC’s show an eyewall precipitation and updraft asymmetry whose maxima are located in the downshear and downshear left region. The azimuthal coverage of eyewall and outer-core precipitation is greater for IN TC’s. There is little difference in the distribution of downdrafts and weak to moderate updrafts in the eyewall. The primary difference is seen at the high end of the vertical velocity spectrum, where IN TC’s have a larger number of convective bursts. These bursts accomplish more vertical mass flux, but they comprise such a small portion of the total vertical velocity distribution that there is little difference in the shape of the net mass flux profile. The radial location of convective bursts for IN TC’s is preferentially located inside the RMW, where the axisymmetric vorticity is generally higher, whereas for SS TC’s the bursts are located outside the RMW.
Rogers, R.F., S.D. Aberson, A. Aksoy, B. Annane, M. Black, J.J. Cione, N. Dorst, J. Dunion, J.F. Gamache, S.B. Goldenberg, S.G. Gopalakrishnan, J. Kaplan, B.W. Klotz, S. Lorsolo, F.D. Marks, S.T. Murillo, M.D. Powell, P.D. Reasor, K.J. Sellwood, E.W. Uhlhorn, T. Vukicevic, J.A. Zhang, and X. Zhang. NOAA’s Hurricane Intensity Forecasting Experiment (IFEX): A progress report. Bulletin of the American Meteorological Society, 94(6):859-882, https://doi.org/10.1175/BAMS-D-12-00089 2013
An update of the progress achieved as part of the NOAA Intensity Forecasting Experiment (IFEX) is provided. Included is a brief summary of the noteworthy aircraft missions flown in the years since 2005, the first year IFEX flights occurred, as well as a description of the research and development activities that directly address the three primary IFEX goals: (1) Collect observations that span the tropical cyclone (TC) life cycle in a variety of environments for model initialization and evaluation; (2) Develop and refine measurement strategies and technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and (3) Improve the understanding of physical processes important in intensity change for a TC at all stages of its life cycle. Such activities include the real-time analysis and transmission of Doppler radar measurements; numerical model and data assimilation advancements; characterization of tropical cyclone composite structure across multiple scales, from vortex-scale to turbulence-scale; improvements in statistical prediction of rapid intensification; and studies specifically targeting tropical cyclogenesis, extratropical transition, and the impact of environmental humidity on TC structure and evolution. While progress in TC intensity forecasting remains challenging, the activities described here provide some hope for improvement.
Terwey, W., S.F. Abarca, and M.T. Montgomery. Comments on "Convectively generated potential vorticity in rainbands and formation of the secondary eyewall in Hurricane Rita of 2005." Journal of the Atmospheric Sciences, 70(3):984-988, https://doi.org/10.1175/JAS-D-12-030.1 2013
In a previous paper, Judt and Chen proposed that secondary eyewall formation can be the result of the accumulation of convectively generated potential vorticity in the rainbands. They argue that secondary potential vorticity maxima precede the development of the secondary wind maximum and conclude that vortex Rossby waves do not contribute to the formation of the secondary eyewall. Amidst examination of their thought-provoking study, some questions arose regarding their methodology, interpretation, and portrayal of previous literature. Here the authors inquire about aspects of the methodology for diagnosing vortex Rossby waves and assessing their impact on their simulation. Inaccuracies in the literature review are noted and further analysis of existing, three-dimensional, full-physics, numerical hurricane integrations that exhibit canonical secondary eyewalls are encouraged.
Uhlhorn, E.W., and L.K. Shay. Loop Current mixed layer energy response to Hurricane Lili (2002): Part II: Idealized numerical simulations. Journal of Physical Oceanography, 43(6):1173-1192, https://doi.org/10.1175/JPO-D-12-0203.1 2013
In this second part of a two-part study, details of the upper-ocean response within an idealized baroclinic current to a translating tropical cyclone are examined in a series of non-linear, reduced-gravity numerical simulations. Based on observations obtained as part of joint NOAA-NSF experiment in Hurricane Lili (2002), the pre-existing ocean mass and momentum fields are initialized with a Gulf of Mexico Loop Current-like jet, which is subsequently forced by a vortex whose wind stress field approximates that observed in the Lili experiments. Due to (1) favorable coupling between the wind stress and pre-existing current vectors and (2) wind-driven currents flowing across the large horizontal pressure gradient, wind energy transfer to the mixed layer can be more efficient in such a regime as compared to the case of an initially horizontally homogeneous ocean. However, nearly all energy is removed by advection and wave flux by two local inertial periods after storm passage, consistent with the observational results. Experiments are performed to quantify differences in one-dimensional and three-dimensional linearized approximations to the full model equations. In addition, sensitivity experiments to variations in the initial geostrophic current structure are performed to develop a parameter space over which a significant energy response could optimally be observed.
Vukicevic, T., A. Aksoy, P. Reasor, S. Aberson, K. Sellwood, and F. Marks. Joint impact of forecast tendency and state error biases in Ensemble Kalman Filter data assimilation of inner-core tropical cyclone observations. Monthly Weather Review, 141(9):2992-3006, https://doi.org/10.1175/MWR-D-12-00211.1 2013
In this study the properties and causes of systematic errors in high-resolution data assimilation of inner core tropical cyclone (TC) observations were investigated using the HWRF Ensemble Data Assimilation System (HEDAS). Although Aksoy et al. (2012b) demonstrated overall good performance of HEDAS for 83 cases from years 2008-2011 using airborne observations from research and operational aircraft, some systematic errors were identified in the analyses with respect to independent observation-based estimates. The axisymmetric primary circulation intensity was underestimated for hurricane cases and the secondary circulation was systematically weaker for all cases. The diagnostic analysis in this study shows that the underestimate of primary circulation was caused by the systematic spin down of the vortex core in the short term forecasts during the cycling with observations. This tendency bias was associated with the systematic errors in the secondary circulation, temperature and humidity. The biases were reoccurring in each cycle during the assimilation due to inconsistency between the strength of primary and secondary circulation during the short-term forecasts, the impact of model error in planetary boundary layer dynamics, and the effect of forecast tendency bias on the background error correlations. Although limited to the current analysis the findings in this study point to a generic problem of mutual dependence of short-term forecast tendency and state estimate errors in the data assimilation of TC core observations. The results indicate that such coupling of errors in the assimilation would also lead to short term intensity forecast bias after the assimilation for the same reasons.
Xu, H., X. Zhang, and X. Xu. Impact of Tropical Storm Bopha on the intensity change of Supertyphoon Saomai in the 2006 typhoon season. Advances in Meteorology, 2013:487010, 13 pp., https://doi.org/10.1155/2013/487010 2013
Super Typhoon Saomai (2006, 08W), which caused historical disaster in the landfall region, is the most powerful typhoon ever making landfall in Mainland China since 1949. The impact of Tropical Storm Bopha (2006, 10W) on Saomai is regarded as a binary tropical cyclone (TC) interaction. In order to quantify the influence of Bopha on the intensity of Saomai, a set of numerical experiments are performed by artificially modifying the intensity of Bopha in its initial conditions. It is shown that changing the intensity of Bopha has significant effects on simulating Saomai’s intensities, structures, and tracks. We find that moisture transport is a pivotal process of binary TC interaction. It is interesting that there are opposite effects by Bopha at different development stages of Saomai. The existence of Bopha and increasing its intensity would weaken Saomai at its intensifying stage while intensifying Saomai at its weakening stage. A possible explanation of these effects is the direction change of moisture transport from/to Saomai at its intensifying/weakening stages through the channel. It may suggest a significant relevance for operational intensity forecasts under active binary TC interaction.
Zhang, J.A., R.F. Rogers, P.D. Reasor, E.W. Uhlhorn, and F.D. Marks. Asymmetric hurricane boundary layer structure from dropsonde composites in relation to the environmental vertical wind shear. Monthly Weather Review, 141(11):3968-3984, https://doi.org/10.1175/MWF-D-12-00335.1 2013
This study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmental vertical wind shear in the inner core region. Data from 1878 GPS dropsondes deployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 75 flights are compared with the dropsonde composites. Shear-relative quadrant-mean composite analyses show that both the kinematic and thermodynamic boundary-layer height scales tend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary-layer heights. Both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants down shear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results also suggest that air parcels acquire equivalent potential temperature (θe) from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in θe. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower θe again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.
2012
Aksoy, A., S. Lorsolo, T. Vukicevic, K.J. Sellwood, S.D. Aberson, and F. Zhang. The HWRF Hurricane Ensemble Data Assimilation System (HEDAS) for high-resolution data: The impact of airborne Doppler radar observations in an OSSE. Monthly Weather Review, 140(6):1843-1862, https://doi.org/10.1175/MWR-D-11-00212.1 2012
Within the National Oceanic and Atmospheric Administration, the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory has developed the Hurricane Weather Research and Forecasting (HWRF) Ensemble Data Assimilation System (HEDAS) to assimilate hurricane inner-core observations for high-resolution vortex initialization. HEDAS is based on a serial implementation of the square root ensemble Kalman filter. HWRF is configured with a horizontal grid spacing of 9/3 km on the outer/inner domains. In this preliminary study, airborne Doppler radar radial wind observations are simulated from a higher-resolution (4.5/1.5 km) version of the same model with other modifications that resulted in appreciable model error. A 24-h nature run simulation of Hurricane Paloma is initialized at 7 November 2008 12Z and produced a realistic, category-2-strength hurricane vortex. The impact of assimilating Doppler wind observations is assessed in observation space as well as in model space. It is observed that while the assimilation of Doppler wind observations results in significant improvements in the overall vortex structure, a general bias in the average error statistics persists due to the under-estimation of overall intensity. A general deficiency in ensemble spread is also evident. While covariance inflation/relaxation and observation thinning result in improved ensemble spread, these do not translate into improvements in overall error statistics. These results strongly suggest a need to include in the ensemble a representation of forecast error growth from other sources such as model error.
Amarin, R.A., W.L. Jones, S.F. El-Nimri, J.W. Johnson, C.S. Ruf, T.L. Miller, and E. Uhlhorn. Hurricane wind speed measurements in rainy conditions using the airborne Hurricane Imaging Radiometer (HIRAD). IEEE Geoscience and Remote Sensing, 50(1):180-192, https://doi.org/10.1109/TGRS.2011.2161637 2012
This paper describes a realistic computer simulation of airborne hurricane surveillance using the recently developed microwave remote sensor, the hurricane imaging radiometer (HIRAD). An end-to-end simulation is described of HIRAD wind speed and rain rate measurements during two hurricanes while flying on a high-altitude aircraft. This simulation addresses the particular challenge which is accurate hurricane wind speed measurements in the presence of intense rain rates. The objective of this research is to develop baseline retrieval algorithms and provide a wind speed measurement accuracy assessment for future hurricane flights including the NASA GRIP hurricane field program that was conducted in summer of 2010. Examples of retrieved hurricane wind speed and rain rate images are presented, and comparisons of the retrieved parameters with two different numerical hurricane models data are made. Special emphasis is provided on the wind speed measurement error, and statistical results are presented over a broad range of wind and rain conditions over the full measurement swath (earth incidence angle).
Bao, J.-W., S.G. Gopalakrishnan, S.A. Michelson, F.D. Marks, and M.T. Montgomery. Impact of physics representations in the HWRF model on simulated hurricane structure and wind-pressure relationships. Monthly Weather Review, 140(10):3278-3299, https://doi.org/10.1175/MWR-D-11-00332.1 2012
A series of idealized experiments with the NOAA Experimental Hurricane Research and Forecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer, subgrid convection, cloud microphysics and radiation. Results from all the experiments are compared in terms of the maximum surface 10-m wind (VMAX) and minimum sea level pressure (PMIN), operational metrics of TC intensity, as well as the azimuthally-averaged temporal and spatial structure of the tangential wind and its material acceleration. The conventional metrics of TC intensity (VMAX and PMIN) are found to be insufficient to reveal the sensitivity of the simulated TC to variations in model physics. Comparisons of the sensitivity runs indicate that (1) different boundary layer (BL) physics parameterization schemes for vertical sub-grid turbulence mixing lead to differences not only in the intensity evolution in terms of the VMAX and PMIN, but also in the structural characteristics of the simulated tropical cyclone; (2) the surface drag coefficient is a key parameter that controls the VMAX-PMIN relationship near the surface; and (3) different microphysics and subgrid convection parameterization schemes, due to their different realizations of diabatic heating distribution, lead to significant variations in the vortex structure. The quantitative aspects of these results indicate that the current uncertainties in the BL mixing, surface drag, microphysics parameterization schemes have comparable impacts on the intensity and structure of simulated TCs. The results indicate also that there is a need to include structural parameters in the HWRFX model evaluation.
Bell, G.D., E.S. Blake, C.W. Landsea, T.B. Kimberlain, S.B. Goldenberg, J. Schemm, and R.J. Pasch. The tropics: Atlantic basin. In State of the Climate in 2011, J. Blunden and D.S. Arndt (eds.). Bulletin of the American Meteorological Society, 93(7):S99-S105, https://doi.org/10.1175/2012BAMSStateoftheClimate.1 2012
Bell, M.M., M.T. Montgomery, and K.A. Emanuel. Air-sea enthalpy and momentum exchange at major hurricane wind speeds observed during CBLAST. Journal of the Atmospheric Sciences, 69(11):3197-3222, https://doi.org/10.1175/JAS-D-11-0276.1 2012
Quantifying air-sea exchanges of enthalpy and momentum is important for understanding and skillfully predicting tropical cyclone intensity, but the magnitude of the corresponding wind-speed-dependent bulk exchange coefficients is largely unknown at major hurricane wind speeds greater than 50 m s-1. Since direct turbulent flux measurements in these conditions are extremely difficult, the momentum and enthalpy fluxes were respectively deduced via absolute angular momentum and total energy budgets. An error analysis of the methodology was performed to quantify and mitigate potentially significant uncertainties resulting from unresolved budget terms and observational errors. An analysis of six missions from the 2003 CBLAST field program in major hurricanes Fabian and Isabel was conducted using a new variational technique. The analysis indicates a near-surface mean drag coefficient (CD) of 2.4 x 10-3 with a 46% standard deviation and a mean enthalpy coefficient (CK) of 1.0 x 10-3 with a 40% standard deviation for wind speeds between 52 and 72 m s-1. These are the first known estimates of CK and the ratio of enthalpy to drag coefficient (CK/CD) in major hurricanes. The results suggest that there is no significant change in the magnitude of the bulk exchange coefficients estimated at minimal hurricane wind speeds, and that the ratio CK/CD does not significantly increase for wind speeds greater than 50 m s-1.
Bell, M.M., M.T. Montgomery, and W.-C. Lee. An axisymmetric view of concentric eyewall evolution in Hurricane Rita (2005). Journal of the Atmospheric Sciences, 69(8):2414-2432, https://doi.org/10.1175/JAS-D-11-0167.1 2012
Multi-platform observations of Hurricane Rita (2005) were collected as part of the Hurricane Rainband and Intensity Change (RAINEX) field campaign during a concentric eyewall stage of the storm's lifecycle that occurred from 21-22 September. Satellite, aircraft, dropwindsonde, and Doppler radar data are used here to examine the symmetric evolution of the hurricane as it underwent eyewall replacement. During the approximately one day observation period, developing convection associated with the secondary eyewall became more symmetric and contracted inwards. Latent heating in the emergent secondary eyewall led to the development of a distinct toroidal (overturning) circulation with inertially-constrained radial inflow above the boundary layer and compensating subsidence in the moat region, properties that are consistent broadly with the balanced vortex response to an imposed ring of diabatic heating outside the primary eyewall. The primary eyewall's convection became more asymmetric during the observation period, but the primary eyewall was still the dominant swirling wind and vorticity structure throughout the period. The observed structure and evolution of Rita's secondary eyewall suggest that spin up of the tangential winds occurred both within and above the boundary layer, and that both balanced and unbalanced dynamical processes played an important role. Although Rita's core intensity decreased during the observation period, the observations indicate a 125% increase in areal extent of hurricane force winds and a 19% increase in integrated kinetic energy resulting from the eyewall replacement.
Black, R.A., and J. Hallett. Rain rate and water content in hurricanes compared with summer rain in Miami, Florida. Journal of Applied Meteorology and Climatology, 51(12):2218-2235, https://doi.org/10.1175/JAMC-D-11-0144.1 2012
Liquid water content (g m-3), precipitation rate (mm hr-1) and radar reflectivity (dBZ) are inferred from cross-sections of particle images obtained by aircraft. Each data set is presented in a probability format to display changing functional relationships for the selected intervals. The probability of intercepting a given quantity during a flight provides guidance in required instrument sensitivity together with the frequency of precipitation and liquid water content events for given rainfall totals. These data are compared with surface rain rate obtained over two years in the May-October warm seasons in Miami, Florida with a hotplate rain gauge. The warm season Miami surface rain rate probability distribution is similar to the 2005 hurricane rain rate distribution. Rain rates > ~120 mm hr-1 were responsible for over half of the accumulation, even though lighter rain dominated by time. Hurricane rainfall is somewhat more intense than the normal surface convective rainfall in that 10% of the 1977-2001 (old) hurricane rain rates exceeded 20 mm hr-1, whereas only 10% of the surface rain rates exceeded only ~10 mm hr-1. The shape of the rain rate probability distributions from the 2005 (recent) hurricane data were nearly identical to the probability distribution of rain rates in the Miami, FL data. The radar reflectivity distributions were similar, whose 90% level was about 45 dBZ for the old storms, and about 35 dBZ for the 2005 storms. These data clearly show the low bias of the 2005 hurricane data caused by the systematic avoidance of heavy precipitation.
Bourassa, M.A., A. Stoffelen, P. Chang, D.B. Chelton, R. Edson, Z. Jelenak, T. Lee, W.T. Liu, D.G. Long, M. Powell, E. Rodriguez, D.K. Smith, and F.J. Wentz. Remotely-sensed winds and wind stresses for marine forecasting and ocean modeling. Proceedings, U.S. Integrated Ocean Observing System (IOOS) Summit, Interagency Ocean Observation Committee (IOOC), Herndon, VA, November 13-16, 2012. Community White Paper, 6 pp., 2012
Coddington, O., P. Pilewskie, and T. Vukicevic. The Shannon information content of hyperspectral shortwave cloud albedo measurements: Quantification and practical applications. Journal of Geophysical Research, 117:D04205, 12 pp., https://doi.org/10.1029/2011JD016771 2012
The Shannon information content provides an objective measure of the information in a data set. In this paper, we quantify the information content of hyperspectral liquid water cloud measurements over a spectral range (300-2500 nm) representing approximately 95% of the total energy in the solar spectrum. We also use the Shannon information content to analyze the cloud retrieval wavelengths and weightings used by the Solar Spectral Flux Radiometer (SSFR) and to determine the cumulative information in the SSFR retrieval. These applications illustrate the utility of the Shannon information content in guiding the effective processing of hyperspectral data. Such efficiency is of growing importance considering the push toward spectrally resolved satellite measurements of reflected solar irradiance used to study climate.
Di Napoli, S.M., M.A. Bourassa, and M.D. Powell. Uncertainty and intercalibration analysis of H*Wind. Atmospheric and Oceanic Technology, 29(6):822-833, https://doi.org/10.1175/JTECH-D-11-00165.1 2012
The HRD Real-time Hurricane Wind Analysis System (H*Wind) is a software application used by NOAA's Hurricane Research Division to create a gridded tropical cyclone wind analysis based on a wide range of observations. These analyses are used in both forecasting and research applications. Although mean bias and RMS errors are listed, H*Wind lacks robust uncertainty information that considers the contributions of random observation errors, relative biases between observation types, temporal drift resulting from combining non-simultaneous measurements into a single analysis, and smoothing and interpolation errors introduced by the H*Wind analysis. This investigation seeks to estimate the total contributions of these sources, and thereby provide an overall uncertainty estimate for the H*Wind product. A series of statistical analyses show that in general, the total uncertainty in the H*Wind product in hurricanes is approximately 6% near the storm center, increasing to nearly 13% near the tropical storm force wind radius. The H*Wind analysis algorithm is found to introduce a positive bias to the wind speeds near the storm center, where the analyzed wind speeds are enhanced to match the highest observations. In addition, spectral analyses are performed to ensure that the filter wavelength of the final analysis product matches user specifications. With increased knowledge of bias and uncertainty sources and their effects, researchers will have a better understanding of the uncertainty in the H*Wind product, and can then judge the suitability of H*Wind for various research applications.
Giammanco, I.M., J.L. Schroeder, and M.D. Powell. Observed characteristics of tropical cyclone vertical wind profiles. Wind and Structures, 15(1):65-86, 2012
Over the last decade substantial improvements have been made in our ability to observe the tropical cyclone boundary layer. Low-level wind speed maxima have been frequently observed in Global Positioning System dropwindsonde (GPS sonde) profiles. Data from GPS sondes and coastal Doppler radars were employed to evaluate the characteristics of tropical cyclone vertical wind profiles in open ocean conditions and at landfall. Changes to the mean vertical wind profile were observed azimuthally and with decreasing radial distance toward the cyclone center. Wind profiles within the hurricane boundary layer exhibited a logarithmic increase with height up to the depth of the wind maximum.
Gopalakrishnan, S.G., S. Goldenberg, T. Quirino, F. Marks, X. Zhang, K.-S. Yeh, R. Atlas, and V. Tallapragada. Towards improving high-resolution numerical hurricane forecasting: Influence of model horizontal grid resolution, initialization, and physics. Weather and Forecasting, 27(3):647-666, https://doi.org/10.1175/WAF-D-11-00055.1 2012
This paper provides an account of the performance of an experimental version of the Hurricane Weather Research and Forecasting system (HWRFX) for 87 cases of Atlantic tropical cyclones during the 2005, 2007, and 2009 hurricane seasons. The HWRFX system was used to study the influence of model grid resolution, initial conditions, and physics. The model was run with two versions of horizontal resolution; (i) a parent domain at a resolution of about 27 km with a 9 km moving nest (27:9) (consistent with the current operational resolution) and (ii) a parent domain at a resolution of 9 km with a 3 km moving nest (9:3). The 9:3 configuration is the first step in testing the impact of finer resolutions for future versions of the operational model. The two configurations were run with initial conditions for tropical cyclones obtained from the operational Geophysical Fluid Dynamics Laboratory (GFDL) and HWRF models. Sensitivity experiments were also conducted with the physical parameterization scheme. The study shows that the 9:3 HWRFX system using GFDL initial conditions and a system of physics similar to the operational version (HWRF) provides the best results in terms of both track and intensity prediction. Use of the HWRF initial conditions in HWRFX model provides reasonable skill, when used in cases with initially strong storms (hurricane strength). However, initially weak storms (below hurricane strength) posed special challenges for the models. For the weaker storm cases, none of the predictions from the HWRFX runs or the operational GFDL forecasts provided any consistent improvement when compared to the statistical-dynamical intensity model (Decay-SHIPS).
Hagen, A.B., D. Strahan-Sakoskie, and C. Luckett. A reanalysis of the 1944-53 Atlantic hurricane seasons: The first decade of aircraft reconnaissance. Journal of Climate, 25(13):4441-4460, https://doi.org/10.1175/JCLI-D-11-00419.1 2012
The main historical archive of all tropical storms, subtropical storms, and hurricanes in the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico from 1851 to the present is known as the Atlantic hurricane database (HURDAT), which is the fundamental database for meteorological, engineering, and financial studies of these cyclones. Previous work has demonstrated that a reanalysis of HURDAT is necessary because it contains many random errors and systematic biases. The Atlantic Hurricane Reanalysis Project is an ongoing effort to correct the errors in HURDAT and to make HURDAT as accurate a database as possible with utilization of all available data. For this study, HURDAT is reanalyzed for the period 1944-53, the first decade of the "aircraft reconnaissance era." The track and intensity of each existing tropical cyclone in HURDAT are reassessed, and previously unrecognized tropical cyclones are discovered, analyzed, and recommended to the HURDAT Best Track Change Committee for inclusion into HURDAT (existing tropical cyclones may be removed from the database as well if analyses indicate evidence that no tropical storm existed). Changes to the number of tropical storms, hurricanes, major hurricanes, accumulated cyclone energy, and U.S. landfalling hurricanes are recommended for most years of the decade. Estimates of uncertainty in the reanalyzed database for the decade are also provided.
Holthuijsen, L.H., M.D. Powell, and J.D. Pietrzak Wind and waves in extreme hurricanes. Journal of Geophysical Research, 117:C09003, 15 pp., https://doi.org/10.1029/2012JC007983 2012
Waves breaking at the ocean surface are important to the dynamical, chemical, and biological processes at the air-sea interface. The traditional view is that the white capping and aero-dynamical surface roughness increase with wind speed up to a limiting value. This view is fundamental to hurricane forecasting and climate research but it has never been verified at extreme winds. Here we show with observations that at high wind speeds white caps remain constant and at still higher wind speeds are joined, and increasingly dominated, by streaks of foam and spray. At surface wind speeds of ~40 m/s the streaks merge into a white out, the roughness begins to decrease and a high-velocity surface jet begins to develop. The roughness reduces to virtually zero by ~80 m/s wind speed, rendering the surface aero-dynamically extremely smooth in the most intense part of extreme (or major) hurricanes (wind speed > 50 m/s). A preliminary assessment shows that cross swell, dominant in large regions of hurricanes, allows the roughness under high wind conditions to increase considerably before it reduces to the same low values.
Huang, Y.-H., M.T. Montgomery, and C.-C. Wu. Concentric eyewall formation in Typhoon Sinlaku (2008), Part II: Axisymmetric dynamical processes. Journal of the Atmospheric Sciences, 69(2):662-674, https://doi.org/10.1175/JAS-D-11-0114.1 2012
In Part I of this study, the association between the secondary eyewall formation (SEF) and the broadening of the outer swirling wind in Typhoon Sinlaku (2008) was documented. The findings from Part I help lay the groundwork for the application of a newly proposed intensification paradigm to SEF. Part II presents a new model for SEF that utilizes this new paradigm and its axisymmetric view of the dynamics. The findings point to a sequence of structure changes that occur in the vortex's outer-core region, culminating in SEF. The sequence begins with a broadening of the tangential winds, followed by an increase of the corresponding boundary layer (BL) inflow and an enhancement of convergence in the BL where the secondary eyewall forms. The narrow region of strong BL convergence is associated with the generation of supergradient winds in and just above the BL that acts to rapidly decelerate inflow there. The progressive strengthening of BL inflow and the generation of an effective adverse radial force therein leads to an eruption of air from the BL to support convection outside the primary eyewall in a favorable thermodynamic/kinematic environment. The results suggest that the unbalanced response in the BL serves as an important mechanism for initiating and sustaining a ring of deep convection in a narrow supergradient wind zone outside the primary eyewall. This progressive BL control on SEF suggests that the BL scheme and its coupling to the interior flow need to be adequately represented in numerical models to improve the prediction of SEF timing and preferred location.
Klotz, B.W., and P. Kucera. Observations of coastally transitioning west African mesoscale convective systems during NAMMA. International Journal of Geophysics, 2012:438706, 25 pp., https://doi.org/10.1155/2012/438706 2012
Observations from the NASA 10 cm polarimetric Doppler weather radar (NPOL) were used to examine structure, development, and oceanic transition of West African Mesoscale Convective Systems (MCSs) during the NASA African Monsoon Multidisciplinary Analysis (NAMMA) to determine possible indicators leading to downstream tropical cyclogenesis. Characteristics examined from the NPOL data include echo-top heights, maximum radar reflectivity, height of maximum radar reflectivity, and convective and stratiform coverage areas. Atmospheric radiosondes launched during NAMMA were used to investigate environmental stability characteristics that the MCSs encountered while over land and ocean, respectively. Strengths of African Easterly Waves (AEWs) were examined along with the MCSs in order to improve the analysis of MCS characteristics. Mean structural and environmental characteristics were calculated for systems that produced TCs and for those that did not in order to determine differences between the two types. Echo-top heights were similar between the two types, but maximum reflectivity and height and coverage of intense convection (>50 dBZ) are all larger than for the TC producing cases. Striking differences in environmental conditions related to future TC formation include stronger African Easterly Jet, increased moisture especially at middle and upper levels, and increased stability as the MCSs coastally transition.
Kruczynski, W.L., P.J. Fletcher, and N. Dorst. Hurricanes and tropical storms are regular features in south Florida. In Tropical Connections: South Florida's Marine Environment, W.L. Kruczynski and P.J. Fletcher (eds.). IAN Press, University of Maryland Center for Environmental Science, Cambridge, MD, 44-45, 2012
Laureano-Bozeman, M., D. Niyogi, S. Gopalakrishnan, F.D. Marks, X. Zhang, and V. Tallapragada. An HWRF-based ensemble assessment of the land surface feedback on the post-landfall intensification of Tropical Storm Fay (2008). Natural Hazards, 63(3):1543-1571, https://doi.org/10.1007/s11069-011-9841-5 2012
While tropical cyclones (TCs) usually decay after landfall, Tropical Storm Fay (2008) initially developed a storm central eye over South Florida by anomalous intensification over land. Unique to the Florida peninsula are Lake Okeechobee and the Everglades, which may have provided a surface feedback as the TC tracked near these features around the time of peak intensity. Analysis is done with the use of an ensemble model-based approach with the Developmental Testbed Center (DTC) version of the Hurricane WRF (HWRF) model using an outer domain and a storm-centered moving nest with 27- and 9-km grid spacing, respectively. Choice of land surface parameterization and small-scale surface features may influence TC structure, dictate the rate of TC decay, and even the anomalous intensification after landfall in model experiments. Results indicate that the HWRF model track and intensity forecasts are sensitive to three features in the model framework: land surface parameterization, initial boundary conditions, and the choice of planetary boundary layer (PBL) scheme. Land surface parameterizations such as the Geophysical Fluid Dynamics Laboratory (GFDL) Slab and Noah land surface models (LSMs) dominate the changes in storm track, while initial conditions and PBL schemes cause the largest changes in the TC intensity over land. Land surface heterogeneity in Florida from removing surface features in model simulations shows a small role in the forecast intensity change with no substantial alterations to TC track.
Lorsolo, S., and A. Aksoy. Wavenumber analysis of azimuthally-distributed data: Assessing maximum allowable gap size. Monthly Weather Review, 140(6):1945-1956, https://doi.org/10.1175/MWR-D-11-00219.1 2012
Performing wavenumber decomposition on azimuthally-distributed data such as those in tropical cyclones can be challenging when data gaps exist in the signal. In the literature, one usually finds ad hoc approaches to determine maximum gap size beyond which not to perform Fourier decomposition. The goal of the present study is to provide a more objective and systematic method to choose the maximum gap size allowed to perform a Fourier analysis on observational data. A Monte-Carlo-type experiment is conducted where signals of various wavenumber configurations are generated with gaps of varying size, then a simple interpolation scheme is applied and Fourier decomposition is performed. The wavenumber decomposition is evaluated in a way that requires retrieval of at least 80% of the original amplitude with less than 20° phase shift. Maximum allowable gap size is then retrieved for wavenumbers 0-2. When prior assessment of signal configuration is available, we believe that the present study can provide valuable guidance for gap size beyond which Fourier decomposition is not advisable.
Mohanty, U.C., D. Niyogi, S. Tripathy, F.D. Marks, G.S. Gopalakrishnan, and V. Tallapragada. Modeling and data assimilation for tropical predictions: Predicting landfalls. Connect, 4(2):4-11, 2012
Tropical cyclones are one of the deadliest and costliest weather phenomena worldwide. As a killer, tropical cyclones are far ahead of many other natural disasters. The word "cyclone" was coined in 1848 by Henry Piddington, British meteorologist, and is derived from the Greek word "kuklos," i.e., the coil of a snake as the air flow of the storm resembles it. The nomenclature of tropical cyclones is different in different parts of the world. In the Atlantic and eastern Pacific, they are known as hurricanes and in the western Pacific as typhoons. In the Indian region, they are simply known as tropical cyclones. Almost all of these storms form within 25° latitude on both sides of the equator except over the 5°N to 5°S equatorial region.
Montgomery, M.T., and R.K. Smith. The genesis of Typhoon Nuri as observed during the Tropical Cyclone Structure 2008 (TCS08) field experiment, Part 2: Observations of the convective environment. Atmospheric Chemistry and Physics, 12(9):4001-4009, https://doi.org/10.5194/acp-12-4001-2012 2012
Analyses of thermodynamic data gathered from airborne dropwindsondes during the Tropical Cyclone Structure (2008) experiment are presented for the disturbance that became Typhoon Nuri. Although previous work has suggested that Nuri formed within the protective recirculating "pouch" region of a westward propagating wave-like disturbance and implicated rotating deep convective clouds in driving the inflow to spin up the tangential circulation of the system-scale flow, the nature of the thermodynamic environment that supported the genesis remains a topic of debate. During the genesis phase, vertical profiles of virtual potential temperature show little variability between soundings on a particular day and the system-average soundings likewise show a negligible change. There is a tendency also for the lower and middle troposphere to moisten. However, the data show that, on the scale of the recirculating region of the disturbance, there was no noticeable reduction of virtual temperature in the lower troposphere, but a small warming (less than 1 K) in the upper troposphere. Vertical profiles of pseudo-equivalent potential temperature, thetae, during the genesis show a modestly decreasing deficit of thetae in the vertical between the surface and the height of minimum θe (between 3 and 4 km), from 17.5 K to 15.2 K. The findings reported here are consistent with those found for developing disturbances observed in the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment in 2010. Some implications of the findings are discussed.
Montgomery, M.T., C. Davis, T. Dunkerton, Z. Wang, C. Velden, R. Torn, S.J. Majumdar, F. Zhang, R.K. Smith, L. Bosart, M.M. Bell, J.S. Haase, A. Heymsfield, J. Jensen, T. Campos, and M.A. Boothe. The Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment: Scientific basis, new analysis tools, and some first results. Bulletin of the American Meteorological Society, 93(2):153-172, https://doi.org/10.1175/BAMS-D-11-00046.1 2012
The principal hypotheses of a new model of tropical cyclogenesis, known as the marsupial paradigm, were tested in the context of Atlantic tropical disturbances during the National Science Foundation (NSF)-sponsored Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment in 2010. PREDICT was part of a tri-agency collaboration, along with the National Aeronautics and Space Administration's Genesis and Rapid Intensification Processes (NASA GRIP) experiment and the National Oceanic and Atmospheric Administration's Intensity Forecasting Experiment (NOAA IFEX), intended to examine both developing and nondeveloping tropical disturbances. During PREDICT, a total of 26 missions were flown with the NSF/NCAR Gulfstream V (GV) aircraft sampling eight tropical disturbances. Among these were four cases (Fiona, ex-Gaston, Karl, and Matthew) for which three or more missions were conducted, many on consecutive days. Because of the scientific focus on the Lagrangian nature of the tropical cyclogenesis process, a wave-relative frame of reference was adopted throughout the experiment in which various model- and satellite-based products were examined to guide aircraft planning and real-time operations. Here, the scientific products and examples of data collected are highlighted for several of the disturbances. The suite of cases observed represents arguably the most comprehensive, self-consistent dataset ever collected on the environment and mesoscale structure of developing and nondeveloping predepression disturbances.
Pattanayak, S., U.C. Mohanty, and S.G. Gopalakrishnan. Simulation of very severe cyclone Mala over Bay of Bengal with HWRF modeling system. Natural Hazards, 63(3):1413-1437, https://doi.org/10.1007/s11069-011-9863-z 2012
Tropical cyclone is one of the most devastating weather phenomena all over the world. The Environmental Modeling Center (EMC) of the National Center for Environmental Prediction (NCEP) has developed a sophisticated mesoscale model known as Hurricane Weather Research and Forecasting (HWRF) system for tropical cyclone studies. The state-of-the-art HWRF model (atmospheric component) has been used in simulating most of the features our present study of a very severe tropical cyclone "Mala", which developed on April 26 over the Bay of Bengal and crossed the Arakan coast of Myanmar on April 29, 2006. The initial and lateral boundary conditions are obtained from Global Forecast System (GFS) analysis and forecast fields of the NCEP, respectively. The performance of the model is evaluated with simulation of cyclone Mala with six different initial conditions at an interval of 12 h each from 00 UTC 25 April 2006 to 12 UTC 27 April 2006. The best result in terms of track and intensity forecast as obtained from different initial conditions is further investigated for large-scale fields and structure of the cyclone. For this purpose, a number of important predicted fields’ viz. central pressure/pressure drop, winds, precipitation, etc. are verified against observations/verification analysis. Also, some of the simulated diagnostic fields such as relative vorticity, pressure vertical velocity, heat fluxes, precipitation rate, and moisture convergences are investigated for understanding of the characteristics of the cyclone in more detail. The vector displacement errors in track forecasts are calculated with the estimated best track provided by the India Meteorological Department (IMD). The results indicate that the model is able to capture most of the features of cyclone Mala with reasonable accuracy.
Powell, M.D., and S. Cocke. Hurricane wind fields needed to assess risk to offshore wind farms. Comment on "Quantifying the hurricane risk to offshore wind turbines. Proceedings of the National Academy of Sciences, USA, 109(33):E2192, https://doi.org/10.1073/pnas.1206189109 2012
Rappaport, E.N., J.-G. Jiing, C.W. Landsea, S.T. Murillo, and J.L. Franklin. The Joint Hurricane Testbed: Its first decade of tropical cyclone research-to-operations activities reviewed. Bulletin of the American Meteorological Society, 93(3):371-380, https://doi.org/10.1175/BAMS-D-11-00037.1 2012
The Joint Hurricane Testbed (JHT) is evaluated after its first decade. The JHT's impact on National Hurricane Center forecast operations, the testbed's highest rated research-to-operations projects, and its most significant challenges are described. The Joint Hurricane Testbed (JHT) is reviewed at the completion of its first decade. Views of the program by hurricane forecasters at the National Hurricane Center, the testbed's impact on forecast accuracy, and highlights of the top rated projects are presented. Key concerns encountered by the testbed are identified as possible "lessons learned" for future research-to-operations efforts. The paper concludes with thoughts on the potential changing role of the JHT.
Reasor, P.D., and M. Eastin. Rapidly intensifying Hurricane Guillermo (1997), Part II: Resiliency in shear. Monthly Weather Review, 140(2):425-444, https://doi.org/10.1175/MWR-D-11-00080.1 2012
This paper examines the structure and evolution of a mature tropical cyclone in vertical wind shear (VWS) using airborne Doppler radar observations of Hurricane Guillermo (1997). In Part I, the modulation of eyewall convection via the rotation of vorticity asymmetries through the downshear-left quadrant was documented during rapid intensification. Here, the focus is on the relationship between VWS, vortex tilt, and associated asymmetry within the tropical cyclone core region during two separate observation periods. A method for estimating local VWS and vortex tilt from radar datasets is further developed, and the resulting vertical structure and its evolution are subjected to statistical confidence tests. Guillermo was a highly resilient vortex, evidenced by its small tilt magnitude relative to the horizontal scale of the vortex core. The deep-layer tilt was statistically significant, oriented on average ~60° left of shear. Large-scale vorticity and thermal asymmetries oriented along the tilt direction support a response of Guillermo to shear forcing that is consistent with balanced dynamics. The time-averaged vertical motion asymmetry within the eyewall exhibited maximum ascent values ~40° left of the deep-layer shear, or in this case, right of the deep-layer tilt. The observation-based analysis of Guillermo's interaction with VWS confirms findings of recent theoretical and numerical studies, and serves as the basis for a more comprehensive investigation of VWS and tropical cyclone intensity change using a recently constructed multistorm database of Doppler radar analyses.
Rogers, R., S. Lorsolo, P. Reasor, J. Gamache, and F.D. Marks. Multiscale analysis of tropical cyclone kinematic structure from airborne Doppler radar composites. Monthly Weather Review, 140(1):77-99, https://doi.org/10.1175/MWR-D-10-05075.1 2012
The multiscale inner-core structure of mature tropical cyclones is presented via the use of composites of airborne Doppler radar analyses. The structure of the axisymmetric vortex and the convective and turbulent-scale properties within this axisymmetric framework are shown to be consistent with many previous studies focusing on individual cases or using different airborne data sources. On the vortex scale, these structures include the primary and secondary circulations, eyewall slope, decay of the tangential wind with height, low-level inflow layer and region of enhanced outflow, radial variation of convective and stratiform reflectivity, eyewall vorticity and divergence fields, and rainband signatures in the radial wind, vertical velocity, vorticity, and divergence composite mean and variance fields. Statistics of convective-scale fields and how they vary as a function of proximity to the radius of maximum wind show that the inner eyewall edge is associated with stronger updrafts and higher reflectivity and vorticity in the mean and have broader distributions for these fields compared with the outer radii. In addition, the reflectivity shows a clear characteristic of stratiform precipitation in the outer radii and the vorticity distribution is much more positively skewed along the inner eyewall than it is in the outer radii. Composites of turbulent kinetic energy (TKE) show large values along the inner eyewall, in the hurricane boundary layer, and in a secondary region located at about 2-3 times the radius of maximum wind. This secondary peak in TKE is also consistent with a peak in divergence and in the variability of vorticity, and they suggest the presence of rainbands at this radial band.
Shpund, J., J.A. Zhang, M. Pinsky, and A. Khain. Microphysical structure of the marine boundary layer under strong wind and spray formation as seen from simulations using a two-dimensional explicit microphysical model, Part II: The role of sea spray. Journal of the Atmospheric Sciences, 69(12):3501-3514, https://doi.org/10.1175/JAS-D-11-0281.1 2012
The effect of sea spray on thermodynamics and microphysical structure of the hurricane boundary layer (HBL) under strong wind speed is investigated using a 2-D hybrid Lagrangian-Eulerian model with spectral bin microphysics. A large number of adjacent and interacting Lagrangian parcels move within a turbulent-like flow with largest vortices are being interpreted as large eddies (LE) with characteristic velocity of a few meters per seconds. It is shown that sea spray effect strongly depends on the environmental conditions, largely on relative humidity RH. In case the RH < ~ 90%, spray evaporates and contributes to moistening and cooling of the HBL, as well as to increase in surface fluxes. In case RH > ~ 90 RH% the effects of spray on the BL thermodynamics substantially decreases. Super-saturation at the upper levels leads to the formation of cloud drops on background aerosols. This high sensitivity is related to high salinity of spray drops. It is shown that LE transport about 20% of large spray drops with radius exceeding 150 μm to the upper levels in the HBL. It is hypothesized that this effect is of high importance as regards to the spray effect on microphysics and dynamics of deep convective clouds typical of hurricane eyewall.
Smith, J.W., A.E. Reynolds, A.S. Pratt, S. Salack, B. Klotz, T.L. Battle, D. Grant, A. Diop, T. Fall, A.T. Gaye, D. Robertson, M.S. DeLonge, and S. Chan. Observations of an 11 September Sahelian squall line and Saharan Air Layer outbreak during NAMMA-06. International Journal of Geophysics, 2012:153256, 14 pp., https://doi.org/10.1155/2012/153256 2012
The 2006 NASA-African Monsoon Multidisciplinary Analyses (NAMMA-06) field campaign examined a compact, low-level vortex embedded in the trough of an AEW between 9-12 September. The vortex triggered a squall line (SL) in southeastern Senegal in the early morning of 11 September and became Tropical Depression 8 on 12 September. During this period, there was a Saharan Air Layer (SAL) outbreak in northwestern Senegal and adjacent Atlantic Ocean waters in the proximity of the SL. Increases in aerosol optical thicknesses in Mbour, Senegal, high dewpoint depressions observed in the Kawsara and Dakar rawinsondes, and model back-trajectories suggest the SAL exists. The close proximity of this and SL suggests interaction through dust entrainment and precipitation invigoration.
Smith, R.K., and M.T. Montgomery. Observations of the convective environment in developing and non-developing tropical disturbances. Quarterly Journal of the Royal Meteorological Society, 138(668):1721-1739, https://doi.org/10.1002/qj.1910 2012
Analyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. We focus on two systems that finally became hurricanes Karl and Matthew, and one system (Gaston) that attained tropical storm status, but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo-equivalent potential temperature, θe, show much more variability between soundings on a particular day on account of the variability in moisture. In all systems, there was is a tendency for the lower troposphere to moisten, but in the non-developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non-developing system and the two developing systems was the much larger reduction of θe between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the developing systems. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non-developing system and would thereby act to suppress the development. Here we propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development.
Sullivan, K., F. Marks, W. Browning, V. Brown, T. Adams-Fuller, S. Jasko, M. Allen, S. Wink, A. Fish, J. Gordon, A. Haynes, J. Brost, W. Hooke, R. Tanabe, S. Lindsey, M. Clay, J.G.W. Kelley, and R. Dittmann. Service assessment: Hurricane Irene, August 21-30, 2011. NOAA Special Report, 91 pp., 2012
Uhlhorn, E.W., and D.S. Nolan. Observational undersampling in tropical cyclones and implications for estimated intensity. Monthly Weather Review, 140(3):825-840, https://doi.org/10.1175/MWR-D-11-00073.1 2012
The maximum surface wind speed is an important parameter for tropical cyclone operational analysis and forecasting, since it defines the intensity of a cyclone. Operational forecast centers typically refer the wind speed to a maximum 1-min or 10-min averaged value. Aircraft reconnaissance provides measurements of surface winds, however due the large variation of winds in the eyewall, it remains unclear to what extent observing the maximum wind is limited by the sampling pattern. Estimating storm intensity as simply the maximum of the observed winds is generally assumed by forecasters to underestimate the true storm intensity. The work presented herein attempts to quantify this difference by applying a methodology borrowed from the observing system simulation experiment concept, in which simulated "observations" are drawn from a numerical model. These "observations" may then be compared to the actual peak wind speed of the simulation. By sampling a high-resolution numerical simulation of Hurricane Isabel (2003) with a virtual aircraft equipped with a stepped frequency microwave radiometer flying a standard "figure-4" pattern, we find the highest wind observed over a flight typically underestimates the 1-min averaged model wind speed by 8.5 ± 1.5%. In contrast, due to its corresponding larger spatial scale, the 10-min averaged maximum wind speed is far less underestimated (1.5 ± 1.7%) using the same sampling method. These results support the National Hurricane Center's practice which typically assumes that the peak 1-min wind is somewhat greater than the highest observed wind speed over a single reconnaissance aircraft mission.
Uhlhorn, E.W., and L.K. Shay. Loop Current mixed-layer energy response to Hurricane Lili (2002): Part I: Observations. Journal of Physical Oceanography, 42(3):409-419, https://doi.org/10.1175/JPO-D-11-096.1 2012
The ocean mixed layer response to a tropical cyclone within, and immediately adjacent to, the Gulf of Mexico Loop Current is examined. In the first of a two-part study, a comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy evolution in response to Hurricane Lili's (2002) passage. Mixed-layer temperature analyses show that the Loop Current cooled <1°C in response to the storm, in contrast to typically observed larger decreases of 3-5°C. Correspondingly, vertical current shear associated with mixed-layer currents, which is responsible for entrainment mixing of cooler water, was found to be up to 50% weaker, on average, than observed in previous studies within the directly-forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical water from the cooler, heavier Gulf Common water, was found to decrease in intensity by -0.18 ± 0.25 m s-1 over an approximately 10-day period within the mixed layer. Contrary to previous ocean response studies which have assumed approximately horizontally homogeneous ocean structure prior to storm passage, a kinetic energy loss of 5.8 ± 6.4 kJ m-2, or approximately -1 wind stress-scaled energy unit, was observed. By examining nearsurface currents derived from satellite altimetry data, the Loop Current is found to vary similarly in magnitude over such time scales, suggesting storm-generated energy is rapidly removed by the pre-exiting Loop Current. In a future study, the simulated mixed-layer evolution to a Hurricane Lili-like storm within an idealized pre-existing baroclinic current is analyzed to help understand the complex air-sea interaction and resulting energetic response.
van Lier-Walqui, M., T. Vukicevic, and D.J. Posselt. Quantification of cloud microphysical parameterization uncertainty using radar reflectivity. Monthly Weather Review, 140(11):3442-3466, https://doi.org/10.1175/MWR-D-11-00216.1 2012
Uncertainty in cloud microphysical parameterization, a leading order contribution to numerical weather prediction error, is estimated using a Markov chain Monte Carlo (MCMC) algorithm. An inversion is performed on ten microphysical parameters using radar reflectivity observations with a vertically covarying error as the likelihood constraint. An idealized 1D atmospheric column model with prescribed forcing is used to simulate the microphysical behavior of a mid-latitude squall line. Novel diagnostics are employed for the probabilistic investigation of individual microphysical process behavior vis-a-vis parameter uncertainty. Uncertainty in the microphysical parameterization is presented via posterior probability density functions (PDFs) of parameters, observations, and microphysical processes. The results of this study show that radar reflectivity observations, as expected, provide a much stronger constraint on microphysical parameters than column-integral observations, in most cases reducing both the variance and bias in the maximum likelihood estimate of parameter values. This highlights the enhanced potential of radar reflectivity observations to provide information about microphysical processes within convective storm systems despite the presence of strongly nonlinear relationships within the microphysics model. The probabilistic analysis of parameterization uncertainty in terms of both parameter and process activity PDFs suggest the prospect of a stochastic representation of microphysical parameterization uncertainty; specifically, the results indicate that error may be more easily represented and estimated by microphysical process uncertainty rather than microphysical parameter uncertainty. In addition, these new methods of analysis allow for a detailed investigation of the full nonlinear and multivariate relationships between microphysical parameters, microphysical processes and radar observations.
Wang, Z., M.T. Montgomery, and C. Fritz. A first look at the structure of the wave pouch during the 2009 PREDICT-GRIP dry runs over the Atlantic. Monthly Weather Review, 140(4):1144-1163, https://doi.org/10.1175/MWR-D-10-05063.1 2012
In support of the NSF-PREDICT and NASA-GRIP Dry Run exercises and NOAA-IFEX during the 2009 hurricane season, a real-time wave tracking algorithm and corresponding diagnostic analyses based on a recently proposed tropical cyclogenesis model were applied to tropical easterly waves over the Atlantic. The model emphasizes the importance of a Lagrangian re-circulation region within a tropical wave critical layer (the so-called "pouch"), where persistent deep convection and vorticity aggregation as well as column moistening are favored for tropical cyclogenesis. Distinct scenarios of hybrid wave-vortex evolution are highlighted. It was found that easterly waves without a pouch or with a shallow pouch did not develop. Although not all waves with a deep pouch developed into a tropical storm, a deep wave pouch had formed prior to genesis for all the sixteen named storms originating from monochromatic easterly waves during the 2008 and 2009 seasons. On the other hand, the diagnosis of two non-developing waves with a deep pouch suggests that strong vertical shear or dry air intrusion at the middle to upper levels (where a wave pouch was absent) can disrupt deep convection and suppress storm development. To sum up, this study suggests that a deep wave pouch extending from the mid-troposphere (600~700 hPa) to near the surface is a necessary condition for tropical cyclone formation within an easterly wave. It is hypothesized also that a deep wave pouch together with other large-scale favorable conditions provides a sufficient condition for sustained convection and tropical cyclone formation. This hypothesized sufficient condition requires further testing and will be pursued in future work.
Winterbottom, H.R., E.W. Uhlhorn, and E.P. Chassignet. Design and an application of a regional coupled atmosphere-ocean model for tropical cyclone prediction. Journal of Advances in Modeling Earth Systems, 4:M10002, 17 pp., https://doi.org/10.1029/2012MS000172 2012
The prediction of tropical cyclone (TC) track has improved greatly in recent decades due in part to the implementation and improvement of numerical weather prediction (NWP) models. However, the prediction of TC intensity using NWP models remains difficult. Several hypotheses have been proposed to explain the factors contributing to the TC intensity prediction errors and one of the leading candidates is the implication of an evolving sea-surface temperature (SST) boundary condition beneath the TC. In this study, a regional scale coupled atmosphere-ocean model is developed using the Advanced Research Weather Research and Forecasting (ARW) model and the HYbrid Coordinate Ocean Model (HYCOM). A coupling algorithm and a methodology to define appropriate ocean initial conditions is provided. Experiments are conducted, during the lifecycle of TC Ike (2008), using both the coupled-model and static (e.g., temporally fixed) SST to illustrate the impacts of the coupled-model for the TC track, intensity, and structure, as well as upon the larger (synoptic) scale. The results from this study suggest that the impact of the evolving SST (e.g., from a coupled atmosphere-ocean model) begin to impact the intensity, size, and thermodynamic structure for TC Ike (2008) at forecast lead-times beyond 48-hours. Further, the forecast trajectories (i.e., tracks) do not illustrate large differences between the non- and coupled-models. Finally, the impact of the SST boundary condition upon TC Ike (2008) appears to be a function of the strength of the atmospheric forcing, in particular the size and intensity of the TC wind field.
Wu, C.-C., S.-G. Chen, C.-C. Yang, P.-H. Lin, and S.D. Aberson. Potential vorticity diagnosis of the factors affecting the track of Typhoon Sinlaku (2008) and the impact from dropwindsonde data during T-PaRC. Monthly Weather Review, 140(8):2670-2688, https://doi.org/10.1175/MWR-D-11-00229.1 2012
In 2008, abundant dropwindsonde data were collected during both reconnaissance and surveillance flights in and around tropical cyclones (TCs) in the western North Pacific basin under the framework of the Observing System Research and Predictability Experiment (THORPEX) - Pacific Asian Regional Campaign (T-PARC). The National Centers for Environmental Prediction Global Forecast System (GFS) showed significant track improvements for Typhoon Sinlaku (2008) after the assimilation of dropwindsonde data. For this particular typhoon, the potential vorticity (PV) diagnosis is adopted to understand the key factors affecting the track. A data denial run initialized at 0000 UTC 10 September is examined to evaluate how the extra data collected during T-PARC improve GFS track forecasts. A quantitative analysis of the steering flow based on the PV diagnosis indicates that the Pacific subtropical high to the east of Sinlaku is a primary factor that advects Sinlaku northwestward, while the monsoon trough plays a secondary role. The assimilation of dropwindsonde data improves the structure and intensity of the initial vortex and maintains the forecast vortex structure in the vertical. The difference in the vertical extent of the vortices could be regarded as a cause for the discrepancy in steering flow between runs with and without the dropwindsonde data. This paper highlights the importance of improved analyses of the vertical TC structure, and thus of a representative steering flow in the deep troposphere during the forecasts.
Yeh, K.-S., X. Zhang, S.G. Gopalakrishnan, S. Aberson, R. Rogers, F.D. Marks, and R. Atlas. Performance of the experimental HWRF in the 2008 hurricane season. Natural Hazards, 63(3):1439-1449, https://doi.org/10.1007/s11069-011-9787-7 2012
In response to the needs of improving hurricane forecasts, we have built an experimental version of the operational Hurricane Weather Research and Forecasting Model (HWRF), which is based on the Weather Research and Forecasting Nonhydrostatic Mesoscale Model of the National Oceanic and Atmospheric Administration (NOAA). The experimental HWRF (HWRFx) is adopted to study the intensity change problem at the highest possible resolutions with the existing computing facility, using moving nests to focus the model resolution in the vicinity of the storms. Although this is at an early stage of development, results from real-time experiments in the 2008 hurricane season show that the HWRFx is generally comparable to the NOAA operational models, in terms of the accuracy of both track and intensity forecasts. The HWRFx, however, has a negative bias in the intensity forecasts as opposed to the positive biases of the NOAA operational models. We present in this article a brief description of the HWRFx and its performance during the 2008 hurricane season in comparison with the NOAA operational models.
Zhang, J.A., and E.W. Uhlhorn. Hurricane sea surface inflow angle and an observation-based parametric model. Monthly Weather Review, 140(11):3587-3605, https://doi.org/10.1175/MWR-D-11-00339.1 2012
This study presents an analysis of near-surface (10-m) inflow angles using wind vector data from over 1600 quality-controlled Global Positioning System dropwindsondes deployed by aircraft on 187 flights into 18 hurricanes. The mean inflow angle in hurricanes is found to be -22.6 ± 2.2° (95% confidence). Composite analysis results indicate little dependence of storm-relative axisymmetric inflow angle on local surface wind speed, and a weak but statistically-significant dependence on the radial distance from the storm center. A small, but statistically-significant dependence of the axisymmetric inflow angle on storm intensity, is also found, especially well outside the eyewall. By compositing observations according to radial and azimuthal location relative to storm motion direction, significant inflow angle asymmetries are found to depend on storm motion speed, although a large amount of unexplained variability remains. Generally, the largest storm-relative inflow angles (<-50°) are found in the fastest moving storms (> 8 m s-1) at large radii (> 8 times the radius of maximum wind) in the right-front storm quadrant, while the smallest inflow angles (>-10°) are found in the fastest moving storms in the left-rear quadrant. Based on these observations, a parametric model of low-wavenumber inflow angle variability as a function of radius, azimuth, storm intensity, and motion speed, is developed. This model can be applied for purposes of ocean surface remote sensing studies when wind direction is either unknown or ambiguous, for forcing storm surge, surface wave, and ocean circulation models which require a parametric surface wind vector field, and evaluating surface wind field structure in numerical models of tropical cyclones.
Zhang, J.A., and M.T. Montgomery. Observational estimates of the horizontal eddy diffusivity and mixing length in the low-level region of intense hurricanes. Journal of the Atmospheric Sciences, 69(4):1306-1316, https://doi.org/100.1175/JAS-D-11-0180.1 2012
This study examines further the characteristics of turbulent flow in the low-level region of intense hurricanes using in-situ aircraft observations. The data analyzed here are the flight-level data collected by research aircraft that penetrated the eyewalls of Category 5 Hurricane Hugo (1989), Category 4 Hurricane Allen (1980) and Category 5 Hurricane David (1979) between 1 km and the sea surface. Estimates of horizontal eddy momentum flux, horizontal eddy diffusivity, and horizontal mixing length are obtained. It is found that the horizontal momentum flux and horizontal diffusivity increase with increasing wind speed. The horizontal mixing length increases slightly with wind speed also, but the mixing length is not significantly dependent on the wind speed. The magnitude of the horizontal momentum flux is found to be comparable to that of the vertical momentum flux, indicating that horizontal mixing by turbulence becomes non-negligible in the hurricane boundary layer, especially in the eyewall region. Within the context of simple K-theory, the results suggest that the average horizontal eddy diffusivity and mixing length are approximately 1500 m2 s-1 and 750 m, respectively, at ~500 m in the eyewall region corresponding to the mean wind speed of approximately 52 m s-1. It is recalled also that the mixing length is a virtual scale in numerical models, and is quantitatively smaller than the energy-containing scale of turbulent eddies. The distinction between these two scales is a useful reminder for the modeling community on the representation of small-scale turbulence in hurricanes.
Zhang, J.A., and W.M. Drennan. An observational study of vertical eddy diffusivity in the hurricane boundary layer. Journal of the Atmospheric Sciences, 69(11):3223-3236, https://doi.org/10.1175/JAS-D-11-0348.1 2012
Although vertical eddy diffusivity or viscosity has been extensively used in theoretical and numerical models simulating tropical cyclones, little observational study has documented the magnitude of the eddy diffusivity in high-wind conditions (>20 m s-1) until now. Through analyzing in-situ aircraft data that were collected in the atmospheric boundary layer of four intense hurricanes, this study provides the first estimates of vertical distributions of the vertical eddy diffusivities for momentum, sensible heat, and latent heat flux in the surface wind speed range between 18-30 m s-1. In this work, eddy diffusivity is determined from directly measured turbulent fluxes and vertical gradients of the mean variable, such as wind speed, temperature and humidity. The analyses show that the magnitudes of vertical eddy diffusivities for momentum and moisture fluxes are comparable to each other, but the eddy diffusivity for sensible heat flux is much smaller than that for the moisture flux. The vertical distributions of the eddy diffusivities are generally alike, increasing from the surface to a maximum value within the thermodynamic mixed layer then deceasing with height. The results indicate also that momentum and moisture are mainly transferred down gradient of the mean flow, while counter-gradient transport of the sensible heat may exist. The observational estimates are compared with the eddy diffusivities derived based on different methods as used in planetary boundary layer (PBL) parameterization schemes in numerical models, as well as ones used in previous observational studies.
Zhang, J.A., S. Gopalakrishnan, F.D. Marks, R.F. Rogers, and V. Tallapragada. A developmental framework for improving hurricane model physical parameterizations using aircraft observations. Tropical Cyclone Research and Review, 1(4):419-429, https://doi.org/10.6057/2012TCRR04.01 2012
As part of NOAA’s Hurricane Forecast Improvement Program (HFIP), this paper addresses the important role of aircraft observations in hurricane model physics validation and improvement. A model developmental framework for improving the physical parameterizations using quality-controlled and post- processed aircraft observations is presented, with steps that include model diagnostics, physics development, physics implementation, and further evaluation. Model deficiencies are first identified through model diagnostics by comparing the simulated axisymmetric multi-scale structures to observational composites. New physical parameterizations are developed in parallel based on in-situ observational data from specially designed hurricane field programs. The new physics package is then implemented in the model, which is followed by further evaluation. The developmental framework presented here is found to be successful in improving the surface layer and boundary layer parameterization schemes in the operational Hurricane Weather Research and Forecast (HWRF) model. Observations for improving physics packages other than boundary layer scheme are also discussed.
2011
Aberson, S.D. The impact of dropwindsonde data from the THORPEX-Pacific Area Regional Campaign and the NOAA Hurricane Field Program on tropical cyclone forecasts in the Global Forecast System. Monthly Weather Review, 139(9):2689-2703, https://doi.org/10.1175/2011MWR3634.1 2011
Four aircraft released dropwindsondes in and around tropical cyclones in the west Pacific during The Observing System Research and Predictability Experiment 2008 Pacific Area Regional Campaign and Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region; multiple aircraft concurrently participated in similar missions in the Atlantic. Previous studies have treated each region separately and have focused on the tropical cyclones whose environments were sampled. The large number of missions and tropical cyclones in both regions, and additional tropical cyclones in the east Pacific and Indian Oceans allows for the global impact of these observations on tropical cyclone track forecasts to be studied. The study shows that there are unintended global consequences to local changes in initial conditions, in this case due to the assimilation of dropwindsonde data in tropical cyclone environments. These global impacts are mainly due to the spectral nature of the model system. These differences should be small and slightly positive, since improved local initial conditions should lead to small global forecast improvements. However, the impacts on tropical cyclones far removed from the data are shown to be as large and positive as those on the tropical cyclones specifically targeted for improved track forecasts. Causes of this unexpected result are hypothesized, potentially providing operational forecasters tools to identify when large remote impacts from surveillance missions might occur.
Aberson, S.D., S.J. Majumdar, C.A. Reynolds, and B.J. Etherton. An observing system experiment for tropical cyclone targeting techniques using the Global Forecast System. Monthly Weather Review, 139(3):895-907, https://doi.org/10.1175/2010MWR33979.1 2011
In 1997, the National Oceanic and Atmospheric Administration's National Hurricane Center and the Hurricane Research Division began operational synoptic surveillance missions with the Gulfstream IV-SP jet aircraft to improve the numerical guidance for hurricanes that threaten the continental United States, Puerto Rico, the U.S. Virgin Islands, and Hawaii. The dropwindsonde observations from these missions were processed and formatted aboard the aircraft and sent to the National Centers for Environmental Prediction and the Global Telecommunications System to be ingested into the Global Forecasting System, which serves as initial and boundary conditions for regional numerical models that also forecast tropical cyclone track and intensity. As a result of limited aircraft resources, optimal observing strategies for these missions are investigated. An Observing System Experiment in which different configurations of the dropwindsonde data based on three targeting techniques (ensemble variance, ensemble transform Kalman filter, and total energy singular vectors) are assimilated into the model system was conducted. All three techniques show some promise in obtaining maximal forecast improvements while limiting flight time and expendables. The data taken within and around the regions specified by the total energy singular vectors provide the largest forecast improvements, though the sample size is too small to make any operational recommendations. Case studies show that the impact of dropwindsonde data obtained either outside of fully sampled, or within nonfully sampled target regions is generally, though not always, small; this suggests that the techniques are able to discern in which regions extra observations will impact the particular forecast.
Bell, G.D., E.S. Blake, T.B. Kimberlain, C.W. Landsea, J. Schemm, R.J. Pasch, and S.B. Goldenberg. The tropics: Atlantic basin. In State of the Climate in 2010, J. Blunden, D.S. Arndt, and M.O. Baringer (eds.). Bulletin of the American Meteorological Society, 92(6):S115-S121, https://doi.org/10.1175/1520-0477-92.6.S1 2011
Chou, K.-S., C.-C. Wu, P.-H. Lin, S.D. Aberson, M. Weissmann, F. Harnisch, and T. Nakazawa. The impact of dropwindsonde observations on typhoon track forecasts in DOTSTAR and T-PaRC. Monthly Weather Review, 139(6):1728-1743, https://doi.org/10.1175/2010MWR3582.1 2011
The typhoon surveillance program Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) has been conducted since 2003 to obtain dropwindsonde observations around tropical cyclones near Taiwan. In addition, an international field project. The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in which dropwindsonde observations were obtained by both surveillance and reconnaissance flights was conducted in summer 2008 in the same region. In this study, the impact of the dropwindsonde data on track forecasts is investigated for DOTSTAR (2003-09) and T-PARC (2008) experiments. Two operational global models from NCEP and ECMWF are used to evaluate the impact of dropwindsonde data. In addition, the impact on the two-model mean is assessed. The impact of dropwindsonde data on track forecasts is different in the NCEP and ECMWF model systems. Using the NCEP system, the assimilation of dropwindsonde data leads to improvements in 1- to 5-day track forecasts in about 60% of the cases. The differences between track forecasts with and without the dropwindsonde data are generally larger for cases in which the data improved the forecasts than in cases in which the forecasts were degraded. Overall, the mean 1- to 5-day track forecast error is reduced by about 10%-20% for both DOTSTAR and T-PARC cases in the NCEP system. In the ECMWF system, the impact is not as beneficial as in the NCEP system, likely because of more extensive use of satellite data and more complex data assimilation used in the former, leading to better performance even without dropwindsonde data. The stronger impacts of the dropwindsonde data are revealed for the 3- to 5-day forecast in the two-model mean of the NCEP and ECMWF systems than for each individual model.
Dietrich, J.C., J.J. Westerink, A.B. Kennedy, J.M. Smith, R.E. Jensen, M. Zijlema, L.H. Holthuijsen, C. Dawson, R.A. Luettich, M.D. Powell, V.J. Cardone, A.T. Cox, G.W. Stone, H. Pourtaheri, M.E. Hope, S. Tanaka, L.G. Westerink, H.J. Westerink, and Z. Cobell. Hurricane Gustav (2008) waves and storm surge: Hindcast, synoptic analysis, and validation in southern Louisiana. Monthly Weather Review, 139(8):2488-2522, https://doi.org/10.1175/2011MWR3611.1 2011
Hurricane Gustav (2008) made landfall in southern Louisiana on 1 September 2008 with its eye never closer than 75 km to New Orleans, but its waves and storm surge threatened to flood the city. Easterly tropical-storm-strength winds impacted the region east of the Mississippi River for 12-15 h, allowing for early surge to develop up to 3.5 m there and enter the river and the city's navigation canals. During landfall, winds shifted from easterly to southerly, resulting in late surge development and propagation over more than 70 km of marshes on the river's west bank, over more than 40 km of Caernarvon marsh on the east bank, and into Lake Pontchartrain to the north. Wind waves with estimated significant heights of 15 m developed in the deep Gulf of Mexico but were reduced in size once they reached the continental shelf. The barrier islands further dissipated the waves, and locally generated seas existed behind these effective breaking zones. The hardening and innovative deployment of gauges since Hurricane Katrina (2005) resulted in a wealth of measured data for Gustav. A total of 39 wind wave time histories, 362 water level time histories, and 82 high water marks were available to describe the event. Computational models--including a structured-mesh deepwater wave model (WAM) and a nearshore steady-state wave (STWAVE) model, as well as an unstructured-mesh "simulating waves nearshore" (SWAN) wave model and an advanced circulation (ADCIRC) model--resolve the region with unprecedented levels of detail, with an unstructured mesh spacing of 100-200 m in the wave-breaking zones and 20-50 m in the small-scale channels. Data-assimilated winds were applied using NOAA's Hurricane Research Division Wind Analysis System (H*Wind) and Interactive Objective Kinematic Analysis (IOKA) procedures. Wave and surge computations from these models are validated comprehensively at the measurement locations ranging from the deep Gulf of Mexico and along the coast to the rivers and floodplains of southern Louisiana and are described and quantified within the context of the evolution of the storm.
Dunion, J.P. Re-writing the climatology of the tropical North Atlantic and Caribbean Sea atmosphere. Journal of Climate, 24(3):893-908, https://doi.org/10.1175/2010JCLI3496.1 2011
The Jordan mean tropical sounding has provided a benchmark reference for representing the climatology of the tropical North Atlantic and Caribbean Sea atmosphere for over 50 years. However, recent observations and studies have suggested that during the months of the North Atlantic hurricane season, this region of the world is affected by multiple air masses with very distinct thermodynamic and kinematic characteristics. This study examined ~6,000 rawinsonde observations from the Caribbean Sea region taken during the core months (July-October) of the 1995-2002 hurricane seasons. It was found that single mean soundings created from this new dataset were very similar to Jordan's 1958 sounding work. However, recently developed multi-spectral satellite imagery that can track low- to mid-level dry air masses indicated that the 1995-2002 hurricane season dataset (and likely Jordan's dataset as well) was dominated by three distinct air masses: moist tropical (MT), Saharan Air Layer (SAL), and mid-latitude dry air intrusions (MLDAIs). Findings suggest that each sounding is associated with unique thermodynamic, kinematic, stability, and mean sea level pressure characteristics and that none of these soundings is particularly well-represented by a single mean sounding like Jordan's. This work presents three new mean tropical soundings (MT, SAL, and MLDAI) for the tropical North Atlantic Ocean and Caribbean Sea region and includes information on their temporal variability, thermodynamics, winds, wind shear, stability, total precipitable water, and mean sea level pressure attributes. It is concluded that the new MT, SAL, and MLDAI soundings presented here provide a more robust depiction of the tropical North Atlantic and Caribbean Sea atmosphere during the Atlantic hurricane season and should replace the Jordan mean tropical sounding as the new benchmark soundings for this part of the world.
Gopalakrishnan, S.G., F. Marks, X. Zhang, J.-W. Bao, K.-S. Yeh, and R. Atlas. The experimental HWRF system: A study on the influence of horizontal resolution on the structure and intensity changes in tropical cyclones using an idealized framework. Monthly Weather Review, 139(6):1762-1784, https://doi.org/10.1175/2010MWR3535.1 2011
Forecasting intensity changes in tropical cyclones (TCs) is a complex and challenging multiscale problem. While cloud-resolving numerical models using a horizontal grid resolution of 1-3 km are starting to show some skill in predicting the intensity changes in individual cases, it is not clear at this time what may be a reasonable horizontal resolution for forecasting TC intensity changes on a day-to-day-basis. The Experimental Hurricane Weather Research and Forecasting System (HWRFX) was used within an idealized framework to gain a fundamental understanding of the influence of horizontal grid resolution on the dynamics of TC vortex intensification in three dimensions. HWFRX is a version of the National Centers for Environmental Prediction (NCEP) Hurricane Weather Research and Forecasting (HWRF) model specifically adopted and developed jointly at NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) and Earth System Research Laboratory (ESRL) for studying the intensity change problem at a model grid resolution of about 3 km. Based on a series of numerical experiments at the current operating resolution of about 9 km and at a finer resolution of about 3 km, it was found that improved resolution had very little impact on the initial spinup of the vortex. An initial axisymmetric vortex with a maximum wind speed of 20 m s-1 rapidly intensified to 50 m s-1 within about 24 h in either case. During the spinup process, buoyancy appears to have had a pivotal influence on the formation of the warm core and the subsequent rapid intensification of the modeled vortex. The high-resolution simulation at 3 km produced updrafts as large as 48 m s-1. However, these extreme events were rare, and this study indicated that these events may not contribute significantly to rapid deepening. Additionally, although the structure of the buoyant plumes may differ at 9- and 3-km resolution, interestingly, the axisymmetric structure of the simulated TCs exhibited major similarities. Specifically, the similarities included a deep inflow layer extending up to about 2 km in height with a tangentially averaged maximum inflow velocity of about 12-15 m s-1, vertical updrafts with an average velocity of about 2 m s-1, and a very strong outflow produced at both resolutions for a mature storm. It was also found in either case that the spinup of the primary circulation occurred not only due to the weak inflow above the boundary layer but also due to the convergence of vorticity within the boundary layer. Nevertheless, the mature phase of the storm's evolution exhibited significantly different patterns of behavior at 9 and 3 km. While the minimum pressure at the end of 96 h was 934 hPa for the 9-km simulation, it was about 910 hPa for the 3-km run. The maximum tangential wind at that time showed a difference of about 10 m s-1. Several sensitivity experiments related to the initial vortex intensity, initial radius of the maximum wind, and physics were performed. Based on ensembles of simulations, it appears that radial advection of the tangential wind and, consequently, radial flux of vorticity become important forcing terms in the momentum budget of the mature storm. Stronger convergence in the boundary layer leads to a larger transport of moisture fluxes and, subsequently, a stronger storm at higher resolution.
Guimond, S.R., M.A. Bourassa, and P.D. Reasor. A latent heat retrieval and its effects on the intensity and structure change of Hurricane Guillermo (1997). Part I: The algorithm and observations. Journal of the Atmospheric Sciences, 68(8):1549-1567, https://doi.org/10.1175/2011JAS3700.1 2011
Despite the fact that latent heating in cloud systems drives many atmospheric circulations, including tropical cyclones, little is known of its magnitude and structure, largely because of inadequate observations. In this work, a reasonably high-resolution (2 km), four-dimensional airborne Doppler radar retrieval of the latent heat of condensation/evaporation is presented for rapidly intensifying Hurricane Guillermo (1997). Several advancements in the basic retrieval algorithm are shown, including (1) analyzing the scheme within the dynamically consistent framework of a numerical model, (2) identifying algorithm sensitivities through the use of ancillary data sources, and (3) developing a precipitation budget storage term parameterization. The determination of the saturation state is shown to be an important part of the algorithm for updrafts of ~5 m s-1 or less. The uncertainties in the magnitude of the retrieved heating are dominated by errors in the vertical velocity. Using a combination of error propagation and Monte Carlo uncertainty techniques, biases are found to be small, and randomly distributed errors in the heating magnitude are ~16% for updrafts greater than 5 m s-1 and ~156% for updrafts of 1 m s-1. Even though errors in the vertical velocity can lead to large uncertainties in the latent heating field for small updrafts/downdrafts, in an integrated sense the errors are not as drastic. In Part II, the impact of the retrievals is assessed by inserting the heating into realistic numerical simulations at 2-km resolution and comparing the generated wind structure to the Doppler radar observations of Guillermo.
Kennedy, A.B., U. Gravois, B.C. Zachry, J.J. Westerink, M.E. Hope, J.C. Dietrich, M.D. Powell, A.T. Cox, R.A. Luettich, and R.G. Dean. Origin of the Hurricane Ike forerunner surge. Geophysical Research Letters, 38:L08608, 5 pp., https://doi.org/10.1029/2011GL047090 2011
A large, unpredicted, water level increase appeared along a substantial section of the western Louisiana and northern Texas (LATEX) coasts 12-24 hrs in advance of the landfall of Hurricane Ike (2008), with water levels in some areas reaching 3 m above mean sea level. During this time the cyclonic wind field was largely shore parallel throughout the region. A similar early water level rise was reported for both the 1900 and the 1915 Galveston Hurricanes. The Ike forerunner anomaly occurred over a much larger area and prior to the primary coastal surge which was driven by onshore directed winds to the right of the storm track. We diagnose the forerunner surge as being generated by Ekman setup on the wide and shallow LATEX shelf. The longer forerunner time scale additionally served to increase water levels significantly in narrow-entranced coastal bays. The forerunner surge generated a freely propagating continental shelf wave with greater than 1.4 m peak elevation that travelled coherently along the coast to Southern Texas, and was 300 km in advance of the storm track at the time of landfall. This was, at some locations, the largest water level increase seen throughout the storm, and appears to be the largest freely-propagating shelf wave ever reported. Ekman setup-driven forerunners will be most significant on wide, shallow shelves subject to large wind fields, and need to be considered for planning and forecasting in these cases.
Misra, V., E. Carlson, R.K. Craig, D. Enfield, B. Kirkman, W. Landing, S.-K. Lee, D. Letson, F. Marks, J. Obeysekera, M. Powell, and S.-I. Shin. Climate Scenarios: A Florida-Centric View (White Paper on Climate Change Scenarios for Florida). Florida Climate Change Task Force, State University System of Florida Board of Governors, 61 pp. (2011) (available online at http://floridaclimate.org/whitepapers/), 2011
The purpose of this document is to provide an informed opinion on future climate scenarios relevant to Florida. It offers a primer on Florida's vulnerabilities to climate variability and change. The document is an excellent compilation of diverse viewpoints on future climate projection. It implores the readers to be cognizant of the associated uncertainty but not to use that as an excuse for inaction in climate adaptation and mitigation. Experts in diverse fields employed in institutions across Florida have contributed to this document and provided candid and informed assessments of future climate variation and change. The uniqueness of this document is that it broadens the discussion of a rather restrictive sounding title like "climate scenarios" to involve experts in sociology, environmental law, and economics, in addition to oceanography and meteorology. The earth's climate is a very complex system. Climate is intimately interrelated to many components of the earth system. However, climate is not limited to these interactions alone. It also includes the modulation of these interactions by external factors such as anthropogenic influence (or interference), volcanic eruptions, changes in solar activity, and changing planetary factors like orbital eccentricity, obliquity, and precession. Against this backdrop of complexity, this paper has tried to distill the information that is relevant to Florida. It is well understood that climate has no borders, and yet we focus here on Florida because of the huge demand for locally applicable information on climate change and variation. Therefore, time and again throughout this paper the impact of remote climate variations and change on Florida is emphasized. Finally this document provides some initial suggestions to further fortify our understanding of the impact of global climate change on Florida. The caveat however, is that these fledgling suggestions will have to be further molded by a developing synergy between the federal, state, private stakeholders and university researchers.
Murillo, S.T., W.-C. Lee, M.M. Bell, G.M. Barnes, F.D. Marks, and P.P. Dodge. Intercomparison of ground-based velocity track display (GBVTD)-retrieved circulation centers and structures of Hurricane Danny (1997) from two coastal WSR-88Ds. Monthly Weather Review, 139(1):153-174, https://doi.org/10.1175/2010MWR3036.1 2011
A plausible primary circulation and circulation center of a tropical cyclone (TC) can be deduced from a coastal Doppler radar using the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex algorithm. The quality of the retrieved primary circulation is highly sensitive to the accuracy of the circulation center that can only be estimated from the degree of scattering of all possible centers obtained in GBVTD-simplex analyses from a single radar in real TCs. This study extends previous work to examine the uncertainties in the GBVTD-simplex-derived circulation centers and the GBVTD-derived primary circulations in Hurricane Danny (1997) sampled simultaneously from two Doppler radars [Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) in Mobile, Alabama, and Slidell, Louisiana] for 5 h. It is found that the mean difference between the individually computed GBVTD-simplex-derived centers is 2.13 km, similar to the estimates in previous studies. This value can be improved to 1.59 km by imposing time continuity in the radius of maximum wind, maximum mean tangential wind, and the center position in successive volumes. These additional physical criteria, not considered in previous work, stabilized the GBVTD-simplex algorithm and paved the way for automating the center finding and wind retrieval procedures in the future. Using the improved set of centers, Danny's axisymmetric tangential wind structures retrieved from each radar showed general agreement with systematic differences (up to 6 m s-1) in certain periods. The consistency in the wavenumber-1 tangential winds was not as good as their axisymmetric counterparts. It is suspected that the systematic differences in the axisymmetric tangential winds were caused by the unresolved wavenumber-2 sine components rather than from the relatively small cross-beam mean wind components in Danny.
Pandya, R., D. Smith, S.A. Ackerman, P.P. Brahma, D.J. Charlevoix, S.Q. Foster, V.K. Gaertner, T.F. Lee, M.J. Hayes, A. Mostek, S.T. Murillo, K.A. Murphy, L. Olsen, D.M. Stanitski, and T. Whittaker. A summary of the 18th Symposium on Education. Bulletin of the American Meteorological Society, 92(1):61-64, https://doi.org/10.1175/2010BAMS2933.1 2011
Educators at all levels shared effective strategies for increasing the quality and quantity of education in the atmospheric and related sciences. Broad themes included a focus on collaboration, use of technology to enable learning, and strategies to get more atmospheric and related sciences into K-12 schools.
Polkinghorne, R., and T. Vukicevic. Data assimilation of cloud-affected radiances in cloud resolving model. Monthly Weather Review, 139(3):755-773, https://doi.org/10.1175/MWR3360.1 2011
Assimilation of cloud-affected infrared radiances from the Geostationary Operational Environmental Satellite-8 (GOES-8) is performed using a four-dimensional variational data assimilation (4DVAR) system designated as the Regional Atmospheric Modeling Data Assimilation System (RAMDAS). A cloud mask is introduced in order to limit the assimilation to points that have the same type of cloud in the model and observations, increasing the linearity of the minimization problem. A series of experiments is performed to determine the sensitivity of the assimilation to factors such as the maximum-allowed residual in the assimilation, the magnitude of the background error decorrelation length for water variables, the length of the assimilation window, and the inclusion of other data such as ground-based data including data from the Atmospheric Emitted Radiance Interferometer (AERI), a microwave radiometer, radiosonde, and cloud radar. In addition, visible and near-infrared satellite data are included in a separate experiment. The assimilation results are validated using independent ground-based data. The introduction of the cloud mask where large residuals are allowed has the greatest positive impact on the assimilation. Extending the length of the assimilation window in conjunction with the use of the cloud mask results in a better-conditioned minimization, as well as a smoother response of the model state to the assimilation.
Powell, M.D., E.W. Uhlhorn, and J.D. Kepert. Reply. Weather and Forecasting, 26(5):777-779, https://doi.org/10.1175/WAF-D-10-05054.1 2011
Riemer, M., and M.T. Montgomery. Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear. Atmospheric Chemistry and Physics, 11(17):9395-9414, https://doi.org/10.5194/acp-11-9395-2011 2011
A major impediment to the intensity forecast of tropical cyclones (TCs) is believed to be associated with the interaction of TCs with dry environmental air. However, the conditions under which pronounced TC-environment interaction takes place are not well understood. As a step towards improving our understanding of this problem, we analyze here the flow topology of a TC immersed in an environment of vertical wind shear in an idealized, three-dimensional, convection-permitting numerical experiment. A set of distinct streamlines, the so-called manifolds, can be identified under the assumptions of steady and layer-wise horizontal flow. The manifolds are shown to divide the flow around the TC into distinct regions. The manifold structure in our numerical experiment is more complex than the well-known manifold structure of a non-divergent point vortex in uniform background flow. In particular, one manifold spirals inwards and ends in a limit cycle, a meso-scale dividing streamline encompassing the eyewall above the layer of strong inflow associated with surface friction and below the outflow layer in the upper troposphere. From the perspective of a steady and layer-wise horizontal flow model, the eyewall is well protected from the intrusion of environmental air. In order for the environmental air to intrude into the inner-core convection, time-dependent and/or vertical motions, which are prevalent in the TC inner-core, are necessary. Air with the highest values of moist-entropy resides within the limit cycle. This "moist envelope" is distorted considerably by the imposed vertical wind shear, and the shape of the moist envelope is closely related to the shape of the limit cycle. In a first approximation, the distribution of high- and low-thetae air around the TC at low to mid-levels is governed by the stirring of convectively modified air by the steady, horizontal flow. Motivated by the results from the idealized numerical experiment, an analogue model based on a weakly divergent point vortex in background flow is formulated. The simple kinematic model captures the essence of many salient features of the manifold structure in the numerical experiment. A regime diagram representing realistic values of TC intensity and vertical wind shear can be constructed for the point-vortex model. The results indicate distinct scenarios of environmental interaction depending on the ratio of storm intensity and vertical-shear magnitude. Further implications of the new results derived from the manifold analysis for TCs in the real atmosphere are discussed.
Sampson, C.R., J. Kaplan, J.A. Knaff, M. DeMaria, and C.A. Sisko. A deterministic rapid intensification aid. Weather and Forecasting, 26(4):579-585, https://doi.org/10.1175/WAF-D-10-05010.1 2011
Rapid intensification (RI) is difficult to forecast, but some progress has been made in developing probabilistic guidance for predicting these events. One such method is the RI index. The RI index is a probabilistic text product available to National Hurricane Center (NHC) forecasters in real time. The RI index gives the probabilities of three intensification rates [25, 30, and 35 kt (24 h)-1; or 12.9, 15.4, and 18.0 m s-1 (24 h)-1] for the 24-h period commencing at the initial forecast time. In this study the authors attempt to develop a deterministic intensity forecast aid from the RI index and, then, implement it as part of a consensus intensity forecast (arithmetic mean of several deterministic intensity forecasts used in operations) that has been shown to generally have lower mean forecast errors than any of its members. The RI aid is constructed using the highest available RI index intensification rate available for probabilities at or above a given probability (i.e., a probability threshold). Results indicate that the higher the probability threshold is, the better the RI aid performs. The RI aid appears to outperform the consensus aids at about the 50% probability threshold. The RI aid also improves forecast errors of operational consensus aids starting with a probability threshold of 30% and reduces negative biases in the forecasts. The authors suggest a 40% threshold for producing the RI aid initially. The 40% threshold is available for approximately 8% of all verifying forecasts, produces approximately 4% reduction in mean forecast errors for the intensity consensus aids, and corrects the negative biases by approximately 15%-20%. In operations, the threshold could be moved up to maximize gains in skill (reducing availability) or moved down to maximize availability (reducing gains in skill).
Shay, L.K., B. Jaimes, J.K. Brewster, P. Meyers, E.C. McCaskill, E. Uhlhorn, F. Marks, G.R. Halliwell, O.M. Smedstad, and P. Hogan. Airborne ocean surveys of the Loop Current complex from NOAA WP-3D in support of the Deepwater Horizon oil spill. In Monitoring and Modeling the Deepwater Horizon Oil Spill: A Record-Breaking Enterprise, Y. Liu, A. MacFadyen, Z.-G. Ji, and R. Weisberg (eds.). AGU Geophysical Monograph Book Series, 195:131-151, https://doi.org/10.1029/2011GM001101 2011
At the time of the Deepwater Horizon oil rig explosion, the Loop Current (LC), a warm ocean current in the Gulf of Mexico (GoM), extended to 27.5°N just south of the rig. To measure the regional scale variability of the LC, oceanographic missions were flown on a NOAA WP-3D research aircraft to obtain ocean structural data during the spill and provide thermal structure profiles to ocean forecasters aiding in the oil spill disaster at 7 to 10 day intervals. The aircraft flew nine grid patterns over the eastern GoM between May and July 2010 deploying profilers to measure atmospheric and oceanic properties such as wind, humidity, temperature, salinity, and current. Ocean current profilers sampled as deep as 1500 m, conductivity, temperature, and depth profilers sampled to 1000 m, and bathythermographs sampled to either 350 or 800 m providing deep structural measurements. Profiler data were provided to modeling centers to predict possible trajectories of the oil and vector ships to regions of anomalous signals. In hindcast mode, assimilation of temperature profiles into the Hybrid Coordinate Ocean Model improved the fidelity of the simulations by reducing RMS errors by as much as 30% and decreasing model biases by half relative to the simulated thermal structure from models that assimilated only satellite data. The synoptic snapshots also provided insight into the evolving LC variability, captured the shedding of the warm core eddy Franklin, and measured the small-scale cyclones along the LC periphery.
Smith, R.K., C.W. Schmidt, and M.T. Montgomery. An investigation of rotational influences on tropical-cyclone size and intensity. Quarterly Journal of the Royal Meteorological Society, 137(660):1841-1855, https://doi.org/10.1002/qj.862 2011
We investigate the rotational constraint on the intensity and size of a tropical cyclones using a minimal, three-layer, axisymmetric tropical-cyclone model. In the first of two sets of experiments, the same initial baroclinic vortex is spun up in a quiescent environment with different levels of background rotation, characterized by the Coriolis parameter, f. It is found that the strongest vortices, as characterized by their final intensity, develop in environments with intermediate background rotation. It is found also that there exists a similar optimum background rotation strength to obtain the largest storm as measured by the radius of gale-force winds. These results appear to be in line with those of classical laboratory experiments by Turner and Lilly, an analogy that we explore in the present article. While the analogy is found to have certain limitations, including the fact that spin-up of the maximum tangential winds in the inner-core in the model takes place in the boundary layer, the study raises aspects of tropical-cyclone dynamics that we believe to be of fundamental importance and require further investigation. As an aid to understanding the foregoing results, a second set of calculations is carried out with the vortex forced by a prescribed radial profile of diabatic heating rate typical of that in the first set and with other moist processes excluded. For this distribution of heating rate, there is no optimum background rotation rate for intensity within a realistic range of values for f, implying that the relationship between the forcing strength and rotation strength is an important additional constraint in tropical cyclones. However, in these experiments, there is an optimum latitude for size, comparable with that in the first set of experiments. An interpretation is offered for these findings.
Speer, M.S., L.M. Leslie, and A.O. Fierro. Australian east coast rainfall decline related to large scale climate drivers. Climate Dynamics, 36(7-8):1419-1429, https://doi.org/10.1007/s00382-009-0726-1 2011
Rainfall on the subtropical east coast of Australia has declined at up to 50 mm per decade since 1970. Wavelet analysis is used to investigate eight station and four station-averaged rainfall distributions along Australia's subtropical east coast with respect to the El Niño-Southern Oscillation (ENSO), the inter-decadal Pacific oscillation (IPO), and the southern annular mode (SAM). The relationships are examined further using composite atmospheric circulation anomalies. Here we show that the greatest rainfall variability occurs in the 15-30 year periodicity of the 1948-1975 or "cool" phase of the IPO when the subtropical ridge is located sufficiently poleward for anomalous moist onshore airflow to occur together with high ENSO rainfall variability and high, negative phase, SAM variability. Thus, the mid-latitude westerlies are located at their most equatorward position in the Australian region. This maximizes tropospheric interaction of warm, moist tropical air with enhanced local baroclinicity over the east coast, and hence rainfall.
Weissmann, M., F. Harnisch, C.-C. Wu, P.-H. Lin, Y. Ohta, K. Yamashita, Y.-H. Kim, E-H. Jeon, T. Nakazawa, and S.D. Aberson. The influence of assimilating dropsonde data on typhoon track and mid-latitude forecasts. Monthly Weather Review, 139(3):908-920, https://doi.org/10.1175/2010MWR3377.1 2011
A unique data set of targeted dropsonde observations was collected during the THORPEX Pacific Asian Regional Campaign (T-PARC) in autumn 2008. The campaign was supplemented by an enhancement of the operational Dropsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) program. For the first time, up to four different aircraft were available for typhoon observations and over 1500 additional soundings were collected. This study investigates the influence of assimilating additional observations during the two major typhoon events of T-PARC on the typhoon track forecast by the global models of the European Centre for Medium-range Weather Forecasts (ECMWF), the Japan Meteorological Agency (JMA), the National Center for Environmental Prediction (NCEP) and the limited area Weather Research and Forecasting (WRF) model. Additionally, the influence of T-PARC observations on ECMWF mid-latitude forecasts is investigated. All models show an improving tendency of typhoon track forecasts, but the degree of improvement varied from about 20-40% in NCEP and WRF to a comparably low influence in ECMWF and JMA. This is likely related to lower track forecast errors without dropsondes in the latter two models, presumably caused by a more extensive use of satellite data and 4D-Var assimilation at ECMWF and JMA compared to 3D-Var of NCEP and WRF. The different behavior of the models emphasizes that the benefit gained strongly depends on the quality of the first-guess field and the assimilation system.
Zhang, F., Y. Weng, J.F. Gamache, and F.D. Marks. Performance of convection-permitting hurricane initialization and prediction during 2008-2010 with ensemble data assimilation of inner-core airborne Doppler radar observations. Geophysical Research Letters, 38:L15810, 6 pp., https://doi.org/10.1029/2011GL048469 2011
This study examines a hurricane prediction system that uses an ensemble Kalman filter (EnKF) to assimilate high-resolution airborne radar observations for convection-permitting hurricane initialization and forecasting. This system demonstrated very promising performance, especially on hurricane intensity forecasts, through experiments over all 61 applicable NOAA P-3 airborne Doppler missions during the 2008-2010 Atlantic hurricane seasons. The mean absolute intensity forecast errors initialized with the EnKF-analysis of the airborne Doppler observations at the 24- to 120-h lead forecast times were 20-40% lower than the National Hurricane Center's official forecasts issued at similar times. This prototype system was first implemented in real-time for Hurricane Ike (2008). It represents the first time that airborne Doppler radar observations were successfully assimilated in real-time into a hurricane prediction model. It also represents the first time that the convection-permitting ensemble analyses and forecasts for hurricanes were performed in real-time. Also unprecedented was the on-demand usage of more than 23,000 computer cluster processors simultaneously in real-time.
Zhang, J.A., F.D. Marks, M.T. Montgomery, and S. Lorsolo. An estimation of turbulent characteristics in the low-level region of intense Hurricanes Allen (1980) and Hugo (1989). Monthly Weather Review, 139(5):1447-1462, https://doi.org/10.1175/2010MWR3435.1 2011
This study analyzes the flight-level data collected by research aircraft that penetrated the eyewalls of category 5 Hurricane Hugo (1989) and category 4 Hurricane Allen (1980) between 1 km and the sea surface. Estimates of turbulent momentum flux, turbulent kinetic energy (TKE), and vertical eddy diffusivity are obtained before and during the eyewall penetrations. Spatial scales of turbulent eddies are determined through a spectral analysis. The turbulence parameters estimated for the eyewall penetration leg are found to be nearly an order of magnitude larger than those for the leg outside the eyewall at similar altitudes. In the low-level intense eyewall region, the horizontal length scale of the dominant turbulent eddies is found to be between 500 and 3000 m, and the corresponding vertical length scale is approximately 100 m. The results suggest also that it is unwise to include eyewall vorticity maxima (EVM) in the turbulence parameter estimation because the EVMs are likely to be quasi-two-dimensional vortex structures that are embedded within the three-dimensional turbulence on the inside edge of the eyewall. This study is a first attempt at estimating the characteristics of turbulent flow in the low-level troposphere of an intense eyewall using in situ aircraft observations. The authors believe that the results can offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity. Because of the small sample size analyzed in this study, further analyses of the turbulent characteristics in the high-wind region of hurricanes are imperative.
Zhang, J.A., P. Zhu, F.J. Masters, R.F. Rogers, and F.D. Marks. On momentum transport and dissipative heating during hurricane landfalls. Journal of the Atmospheric Sciences, 68(6):1397-1404, https://doi.org/10.1175/JAS-D-10-05018.1 2011
Momentum transport and dissipative heating are investigated using the high-resolution (10 Hz) wind data collected by Florida Coastal Monitoring Program portable weather stations in the surface layer of three landfalling hurricanes. The momentum flux is calculated using the eddy correlation method. The drag coefficient is determined from the momentum flux and surface wind speed. The values of the momentum flux and drag coefficient are found to be generally larger than those observed over the ocean at similar wind speeds up to near hurricane strength. The rate of dissipation is determined from the wind velocity spectra. The dissipative heating is estimated using two different methods: (1) integrating the rate of dissipation in the surface layer; and (2) multiplying the drag coefficient by the cubic of the surface wind speed. It is found that the second method, which has been widely used in previous theoretical and numerical studies, significantly overestimates the magnitude of dissipative heating. This finding is consistent with a recent study on estimation of the dissipative heating over the ocean using in situ aircraft observations. This study is a first attempt at estimating the magnitude of dissipative heating in landfalling hurricanes using in situ observations. The results are believed to offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity.
Zhang, J.A., R.F. Rogers, D.S. Nolan, and F.D. Marks. On the characteristic height scales of the hurricane boundary layer. Monthly Weather Review, 139(8):2523-2535, https://doi.org/10.1175/MWR-D-10-05017.1 2011
In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.
Zhang, X., T. Quirino, K.-S. Yeh, S. Gopalakrishnan, F. Marks, S. Goldenberg, and S. Aberson. HWRFx: Improving hurricane forecasts with high-resolution modeling. Computing in Science and Engineering, 13(1):13-21, https://doi.org/10.1109/MCSE.2010.121 2011
Using the hurricane weather research and forecasting experimental modeling system (HWRFx), researchers examined the impact of increased model resolution on system performance in forecasting a select sample of tropical cyclones from the 2005 and 2007 hurricane seasons.
2010
Aberson, S.D. Ten years of hurricane synoptic surveillance (1997-2006). Monthly Weather Review, 138(5):1536-1549, https://doi.org/10.1175/2009MWR3090.1 2010
In 1997, the National Hurricane Center and the Hurricane Research Division began operational synoptic surveillance missions with the Gulfstream IV-SP jet aircraft to improve the numerical guidance for hurricanes that threaten the continental United States, Puerto Rico, the U. S. Virgin Islands, and Hawaii. During the first ten years, 176 such missions were conducted. Global Positioning System dropwindsondes were released from the aircraft at 150-200 km intervals along the flight track in the environment of each tropical cyclone to obtain wind, temperature, and humidity profiles from flight level (about 150 hPa) to the surface. The observations were processed and formatted aboard the aircraft and sent to the National Centers for Environmental Prediction and the Global Telecommunications System to be ingested into the Global Forecast System, which serves as initial and boundary conditions for regional numerical models that also forecast tropical cyclone track and intensity. The results of an observing system experiment using these data are presented.
Aberson, S.D., J. Cione, C.-C. Wu, M.M. Bell, J. Halverson, C. Fogarty, and M. Weissmann. Aircraft observations of tropical cyclones. In Global Perspectives on Tropical Cyclones: From Science to Mitigation, J.C.L. Chan and J.D. Kepert (eds.). World Scientific Publishing Company, 2nd edition, 227-240, 2010
Nine different types of aircraft are currently in use to observe tropical cyclones and their environments for operations and research. The following is a description of those aircraft, their instrumentation, and the field programs with which they have been involved.
Aksoy, A., D.C. Dowell, and C. Snyder. A multicase comparative assessment of the ensemble Kalman filter for assimilation of radar observations, Part II: Short-range ensemble forecasts. Monthly Weather Review, 138(4):1273-1292, https://doi.org/10.1175/2009MWR3086.1 2010
The quality of convective-scale ensemble forecasts, initialized from analysis ensembles obtained through the assimilation of radar observations using an ensemble Kalman filter (EnKF), is investigated for cases whose behaviors span supercellular, linear, and multicellular organization. This work is the companion to Part I, which focused on the quality of analyses during the 60-min analysis period. Here, the focus is on 30-min ensemble forecasts initialized at the end of that period. As in Part I, the Weather Research and Forecasting (WRF) model is employed as a simplified cloud model at 2-km horizontal grid spacing. Various observation-space and state-space verification metrics, computed both for ensemble means and individual ensemble members, are employed to assess the quality of ensemble forecasts comparatively across cases. While the cases exhibit noticeable differences in predictability, the forecast skill in each case, as measured by various metrics, decays on a time scale of tens of minutes. The ensemble spread also increases rapidly but significant outlier members or clustering among members are not encountered. Forecast quality is seen to be influenced to varying degrees by the respective initial soundings. While radar data assimilation is able to partially mitigate some of the negative effects in some situations, the supercell case, in particular, remains difficult to predict even after 60 min of data assimilation.
Bell, G.D., E.S. Blake, T.B. Kimberlain, C.W. Landsea, R.J. Pasch, J. Schemm, and S.B. Goldenberg. Atlantic basin. In State of the Climate in 2009, D.S. Arndt, M.O. Baringer, and M.R. Johnson (eds.). Bulletin of the American Meteorological Society, 91(7):84-88, https://doi.org/10.1175/BAMS-91-7-StateoftheClimate 2010
Bell, M.M., and M.T. Montgomery. Sheared deep vortical convection in pre-depression Hagupit during TCS08. Geophysical Research Letters, 37(6):L06802, 5 pp., https://doi.org/10.1029/2009GL042313 2010
Airborne Doppler radar observations from the recent Tropical Cyclone Structure 2008 (TCS08) field campaign in the western North Pacific reveal the presence of deep, buoyant and vortical convective features within a vertically-sheared, westward-moving pre-depression disturbance that later developed into Typhoon Hagupit. On two consecutive days, the observations document tilted, vertically coherent precipitation, vorticity, and updraft structures in response to the complex shearing flows impinging on and occurring within the disturbance near 18 north latitude. The observations and analyses herein suggest that the low-level circulation of the pre-depression disturbance was enhanced by the coupling of the low-level vorticity and convergence in these deep convective structures on the meso-gamma scale, consistent with recent idealized studies using cloud-representing numerical weather prediction models. Further examination of these new observations is needed to quantify the relative role of these vortical convection features in the tropical cyclone spin up process.
Bourassa, M.A., A. Stoffelen, H. Bonekamp, P. Chang, D.B. Chelton, J. Courtney, R. Edson, J. Figa, Y. He, H. Hersbach, K. Hilburn, Z. Jelenak, K.A. Kelly, R. Knabb, T. Lee, E.J. Lindstrom, W.T. Liu, D.G. Long, W. Perrie, M. Portabella, M.D. Powell, E. Rodriguez, D.K. Smith, V. Swail, and F.J. Wentz. Remotely sensed winds and wind stresses for marine forecasting and ocean modeling. In Proceedings, OceanObs09: Sustained Ocean Observations and Information for Society (Volume 2), Venice, Italy, September 21-25, 2009, J. Hall, D.E. Harrison, and D. Stammer (eds.). ESA Publication, WPP-306, 17 pp., https://doi.org/10.5270/OceanObs09.cwp.08 2010
Braun, S.A., M.T. Montgomery, K.J. Mallen, and P.D. Reasor. Simulation and interpretation of the genesis of Tropical Storm Gert (2005) as part of the NASA Tropical Cloud Systems and Processes Experiment. Journal of the Atmospheric Sciences, 67(4):999-1025, https://doi.org/10.1175/2009JAS3140.1 2010
Several hypotheses have been put forward for the mechanisms of generation of surface circulation associated with tropical cyclones. This paper examines high-resolution simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during NASAs Tropical Cloud Systems and Processes Experiment, to investigate the development of low-level circulation and its relationship to the precipitation evolution. Two simulations are examined: one that better matches available observations but underpredicts the storms minimum sea level pressure and a second one that somewhat overintensifies the storm but provides a set of simulations that encapsulates the overall genesis and development characteristics of the observed storm. The roles of convective and stratiform precipitation processes within the mesoscale precipitation systems that formed Gert are discussed. During 21-25 July, two episodes of convective system development occurred. In each, precipitation system evolution was characterized by intense and deep convective upward motions followed by increasing stratiform-type vertical motions (upper-level ascent, low-level descent). Potential vorticity (PV) in convective regions was strongest at low levels while stratiform-region PV was strongest at midlevels, suggesting that convective processes acted to spin up lower levels prior to the spinup of middle levels by stratiform processes. Intense vortical hot towers (VHTs) were prominent features of the low-level cyclonic vorticity field. The most prominent PV anomalies persisted more than 6 h and were often associated with localized minima in the sea level pressure field. A gradual aggregation of the cyclonic PV occurred as existing VHTs near the center continually merged with new VHTs, gradually increasing the mean vorticity near the center. Nearly concurrently with this VHT-induced development, stratiform precipitation processes strongly enhanced the mean inflow and convergence at middle levels, rapidly increasing the midlevel vorticity. However, the stratiform vertical motion profile is such that while it increases midlevel vorticity, it decreases vorticity near the surface as a result of low-level divergence. Consequently, the results suggest that while stratiform precipitation regions may significantly increase cyclonic circulation at midlevels, convective vortex enhancement at low to mid levels is likely necessary for genesis.
Bunya, S., J.C. Dietrich, J.J. Westerink, B.A. Ebersole, J.M. Smith, J.H. Atkinson, R. Jensen, D.T. Resio, R.A. Luettich, C. Dawson, V.J. Cardone, A.T. Cox, M.D. Powell, H.J. Westerink, and H.J. Roberts. A high-resolution coupled riverine flow, tide, wind, wind wave, and storm surge model for southern Louisiana and Mississippi, Part I: Model development and validation. Monthly Weather Review, 138(2):345-377, https://doi.org/10.1175/2009MWR2906.1 2010
A coupled system of wind, wind wave, and coastal circulation models has been implemented for southern Louisiana and Mississippi to simulate riverine flows, tides, wind waves, and hurricane storm surge in the region. The system combines the NOAA Hurricane Research Division Wind Analysis System (H*WIND) and the Interactive Objective Kinematic Analysis (IOKA) kinematic wind analyses, the Wave Model (WAM) offshore and Steady-State Irregular Wave (STWAVE) nearshore wind wave models, and the Advanced Circulation (ADCIRC) basin to channel-scale unstructured grid circulation model. The system emphasizes a high-resolution (down to 50 m) representation of the geometry, bathymetry, and topography; nonlinear coupling of all processes including wind wave radiation stress-induced set up; and objective specification of frictional parameters based on land-cover databases and commonly used parameters. Riverine flows and tides are validated for no storm conditions, while winds, wind waves, hydrographs, and high water marks are validated for Hurricanes Katrina and Rita.
Coddington, O.M., P. Pilewskie, J. Redemann, S. Platnick, P.B. Russell, K.S. Schmidt, W.J. Gore, J. Livingston, G. Wind, and T. Vukicevic. Examining the impact of overlying aerosols on the retrieval of cloud optical properties from passive remote sensing. Journal of Geophysical Research, 115:D10211, 13 pp., https://doi.org/10.1029/2009JD012829 2010
Haywood et al. (2004) showed that an aerosol layer above a cloud can cause a bias in the retrieved cloud optical thickness and effective radius. Monitoring for this potential bias is difficult because space-based passive remote sensing cannot unambiguously detect or characterize aerosol above cloud. We show that cloud retrievals from aircraft measurements above cloud and below an overlying aerosol layer are a means to test this bias. The data were collected during the Intercontinental Chemical Transport Experiment (INTEX-A) study based out of Portsmouth, New Hampshire, United States, above extensive, marine stratus cloud banks affected by industrial outflow. Solar Spectral Flux Radiometer (SSFR) irradiance measurements taken along a lower level flight leg above cloud and below aerosol were unaffected by the overlying aerosol. Along upper level flight legs, the irradiance reflected from cloud top was transmitted through an aerosol layer. We compare SSFR cloud retrievals from below-aerosol legs to satellite retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) in order to detect an aerosol-induced bias. In regions of small variation in cloud properties, we find that SSFR and MODIS-retrieved cloud optical thickness compares within the uncertainty range for each instrument while SSFR effective radius tend to be smaller than MODIS values (by 1-2 µm) and at the low end of MODIS uncertainty estimates. In regions of large variation in cloud properties, differences in SSFR and MODIS-retrieved cloud optical thickness and effective radius can reach values of 10 and 10 µm, respectively. We include aerosols in forward modeling to test the sensitivity of SSFR cloud retrievals to overlying aerosol layers. We find an overlying absorbing aerosol layer biases SSFR cloud retrievals to smaller effective radii and optical thickness while nonabsorbing aerosols had no impact.
Conzemius, R.J., and M.T. Montgomery. Mesoscale convective vortices in multiscale, idealized simulations: Dependence on background state, interdependency with moist baroclinic cyclones, and comparison with BAMEX observations. Monthly Weather Review, 138(4):1119-1139, https://doi.org/10.1175/2009MWR2981.1 2010
A set of multiscale, nested, idealized numerical simulations of mesoscale convective systems (MCSs) and mesoscale convective vortices (MCVs) was conducted. The purpose of these simulations was to investigate the dependence of MCV development and evolution on background conditions and to explore the relationship between MCVs and larger, moist baroclinic cyclones. In all experiments, no mesoscale convective system (MCS) developed until a larger-scale, moist baroclinic system with surface pressure amplitude of at least 2 hPa was present. The convective system then enhanced the development of the moist baroclinic system by its diabatic production of eddy available potential energy (APE), which led to the enhanced baroclinic conversion of basic-state APE to eddy APE. The most rapid potential vorticity (PV) development occurred in and just behind the leading convective line. The entire system grew upscale with time as the newly created PV rotated cyclonically around a common center as the leading convective line continued to expand outward. Ten hours after the initiation of deep moist convection, the simulated MCV radii, heights of maximum winds, tangential velocity, and shear corresponded reasonably well to their counterparts in BAMEX. The increasing strength of the simulated MCVs with respect to larger values of background CAPE and shear supports the hypothesis that as long as convection is present, CAPE and shear both add to the strength of the MCV.
Dietrich, J.C., S. Bunya, J.J. Westerink, B.A. Ebersole, J.M. Smith, J.H. Atkinson, R. Jensen, D.T. Resio, R.A. Luettich, C. Dawson, V.J. Cardone, A.T. Cox, M.D. Powell, H.J. Westerink, and H.J. Roberts. A high-resolution coupled riverine flow, tide, wind, wind wave, and storm surge model for southern Louisiana and Mississippi, Part II: Synoptic description and analysis of Hurricanes Katrina and Rita. Monthly Weather Review, 138(2):378-404, https://doi.org/10.1175/2009MWR2907.1 2010
Hurricanes Katrina and Rita were powerful storms that impacted southern Louisiana and Mississippi during the 2005 hurricane season. In Part I, the authors describe and validate a high-resolution coupled riverine flow, tide, wind, wave, and storm surge model for this region. Herein, the model is used to examine the evolution of these hurricanes in more detail. Synoptic histories show how storm tracks, winds, and waves interacted with the topography, the protruding Mississippi River delta, eastwest shorelines, manmade structures, and low-lying marshes to develop and propagate storm surge. Perturbations of the model, in which the waves are not included, show the proportional importance of the wave radiation stress gradient induced setup.
Goni, G., M. DeMaria, J. Knaff, C. Sampson, J. Price, A. Mehra, I. Ginis, I.-I. Lin, P. Sandery, S. Ramos-Buarque, M.M. Ali, F. Bringas, S. Aberson, R. Lumpkin, G. Halliwell, C. Lauer, E. Chassignet, A. Mavume, and K. Kang. The ocean observing system for tropical cyclone intensification forecasts and studies. In Proceedings, OceanObs09: Sustained Ocean Observations and Information for Society (Volume 2), Venice, Italy, September 21-25, 2009, J. Hall, D.E. Harrison, and D. Stammer (eds.). ESA Publication, WPP-306, 13 pp., https://doi.org/10.5270/OceanObs09.cwp.36 2010
Gruskin, Z. Reply. Monthly Weather Review, 138(12):4583-4584, https://doi.org/10.1175/2010MWR3559.1 2010
Gruskin, Z. Structure and evolution of a possible U.S. landfalling tropical cyclone in 2006. Monthly Weather Review, 138(1):265-278, https://doi.org/10.1175/2009MWR3000.1 2010
A tropical disturbance made landfall near Morehead City, North Carolina, on 27 June 2006. Surface observations, Air Force reconnaissance, and Doppler velocity data suggest that the disturbance had a closed surface circulation at landfall, with maximum 1-min surface winds >18 m s-1, the threshold of tropical storm strength. A cyclostrophic wind calculation using Doppler velocity data and surface observations indicates that the circulation of the disturbance likely caused the tropical storm force winds observed, rather than an environmental pressure gradient or short-lived convective process. Doppler velocity cross sections of the disturbance further suggest that the disturbance was warm core, and an analysis of the disturbances environment reveals that latent heat of condensation was likely a large source of energy for the disturbance, though there was some baroclinic forcing. These observations and analyses make a compelling case for the upgrade of the disturbance to a tropical storm in the best-track database.
Hamid, S., B.M. Golam Kibria, S. Gulati, M. Powell, B. Annane, S. Cocke, J.-P. Pinelli, K. Gurley, and S.-C. Chen. Predicting losses of residential structures in the state of Florida by the public hurricane loss evaluation model. Statistical Methodology, 7(5):552-573, https://doi.org/10.1016/j.stamet.2010.02.004 2010
As an environmental phenomenon, hurricanes cause significant property damage and loss of life in coastal areas almost every year. Although a number of commercial loss projection models have been developed to predict the property losses, only a handful of studies are available in the public domain to predict damage for hurricane prone areas. The state of Florida has developed an open, public model for the purpose of probabilistic assessment of risk to insured residential property associated with wind damage from hurricanes. The model comprises three components; viz. the atmospheric science component, the engineering component and the actuarial science component. The atmospheric component includes modeling the track and intensity life cycle of each simulated hurricane within the Florida threat area. Based on historical hurricane statistics, thousands of storms are simulated allowing determination of the wind risk for all residential zip code locations in Florida. The wind risk information is then provided to the engineering and actuarial components to model damage and average annual loss, respectively. The actuarial team finds the county-wise loss and the total loss for the entire state of Florida. The computer team then compiles all information from atmospheric science, engineering and actuarial components, processes all hurricane related data and completes the project. The model was submitted to the Florida Commission on Hurricane Loss Projection Methodology for approval and went through a rigorous review and was revised as per the suggestions of the commission. The final model was approved for use by the insurance companies in Florida by the commission. At every stage of the process, statistical procedures were used to model various parameters and validate the model. This paper presents a brief summary of the main components of the model (meteorology, vulnerability and actuarial) and then focuses on the statistical validation of the same.
Haus, B.K., D. Jeong, M.A. Donelan, J.A. Zhang, and I. Savelyev. Relative rates of sea-air heat transfer and frictional drag in very high winds. Geophysical Research Letters, 37(7):L07802, 5 pp., https://doi.org/10.1029/2009GL042206 2010
Hurricanes are fueled by evaporation and convection from the ocean and they lose energy through the frictional drag of the atmosphere on the ocean surface. The relative rates of these processes have been thought to provide a limit on the maximum potential hurricane intensity. Here we report laboratory observations of these transfers for scaled winds equivalent to a strong Category 1 hurricane (38 ms-1). We show that the transfer coefficient ratio holds closely to a level of ~0.5 even in the highest observed winds, where previous studies have suggested there is a distinct regime change at the air-sea interface. This value is well below the expected threshold value for intense hurricanes of 0.75. Recent three-dimensional model studies also find that the coefficient ratio can be much lower than 0.75, which suggests that other factors such as eyewall and/or vortex dynamics are responsible for the formation of very strong hurricanes.
Ismail, S., R.A. Ferrare, E.V. Browell, S.A. Kooi, J.P. Dunion, G. Heymsfield, A. Notari, C.F. Butler, S. Burton, M. Fenn, T.N. Krishnamurti, M.K. Biswas, G. Chen, and B. Anderson. LASE measurements of water vapor, aerosol, and cloud distributions in Saharan air layers and tropical disturbances. Journal of the Atmospheric Sciences, 67(4):1026-1047, https://doi.org/10.1175/2009JAS3136.1 2010
The Lidar Atmospheric Sensing Experiment (LASE) on board the NASA DC-8 measured high-resolution profiles of water vapor and aerosols, and cloud distributions in 14 flights over the eastern North Atlantic during the NASA African Monsoon Multidisciplinary Analyses (NAMMA) field experiment. These measurements were used to study African easterly waves (AEWs), tropical cyclones (TCs), and the Saharan air layer (SAL). These LASE measurements represent the first simultaneous water vapor and aerosol lidar measurements to study the SAL and its interactions with AEWs and TCs. Three case studies were selected for detailed analysis: (i) a stratified SAL, with fine structure and layering (unlike a well-mixed SAL), (ii) a SAL with high relative humidity (RH), and (iii) an AEW surrounded by SAL dry air intrusions. Profile measurements of aerosol scattering ratios, aerosol extinction coefficients, aerosol optical thickness, water vapor mixing ratios, RH, and temperature are presented to illustrate their characteristics in the SAL, convection, and clear air regions. LASE extinction-to-backscatter ratios for the dust layers varied from 35 ± 5 to 45 ± 5 sr, well within the range of values determined by other lidar systems. LASE aerosol extinction and water vapor profiles are validated by comparison with onboard in situ aerosol measurements and GPS dropsonde water vapor soundings, respectively. An analysis of LASE data suggests that the SAL suppresses low-altitude convection. Midlevel convection associated with the AEW and transport are likely responsible for high water vapor content observed in the southern regions of the SAL on 20 August 2008. This interaction is responsible for the transfer of about 7 x 1015 J (or 8 x 103 J m-2) latent heat energy within a day to the SAL. Initial modeling studies that used LASE water vapor profiles show sensitivity to and improvements in model forecasts of an AEW.
Kaplan, J., M. DeMaria, and J.A. Knaff. A revised tropical cyclone rapid intensification index for the Atlantic and eastern North Pacific basins. Weather and Forecasting, 25(1):220-241, https://doi.org/10.1175/2009WAF2222280.1 2010
A revised rapid intensity index (RII) is developed for the Atlantic and eastern North Pacific basins. The RII uses large-scale predictors from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) to estimate the probability of rapid intensification (RI) over the succeeding 24 h utilizing linear discriminant analysis. Separate versions of the RII are developed for the 25-, 30-, and 35-kt RI thresholds, which represent the 90th (88th), 94th (92nd), and 97th (94th) percentiles of 24-h over water intensity changes of tropical and subtropical cyclones in the Atlantic (eastern North Pacific) basins from 1989 to 2006, respectively. The revised RII became operational at the NHC prior to the 2008 hurricane season. The relative importance of the individual RI predictors is shown to differ between the two basins. Specifically, the previous 12-h intensity change, upper-level divergence, and vertical shear have the highest weights for the Atlantic basin, while the previous 12-h intensity change, symmetry of inner-core convection, and the difference in a systems current and maximum potential intensity are weighted highest in the eastern North Pacific basin. A verification of independent forecasts from the 2006 and 2007 hurricane seasons shows that the probabilistic RII forecasts are generally skillful in both basins when compared to climatology. Moreover, when employed in a deterministic manner, the RII forecasts were superior to all other available operational intensity guidance in terms of the probability of detection (POD) and false alarm ratio (FAR). Specifically, the POD for the RII ranged from 15% to 59% (53% to 73%) while the FAR ranged from 71% to 85% (53% to 79%) in the Atlantic (eastern North Pacific) basins, respectively, for the three RI thresholds studied. Nevertheless, the modest POD and relatively high FAR of the RII and other intensity guidance demonstrate the difficulty of predicting RI, particularly in the Atlantic basin.
Levina, G.V., and M.T. Montgomery. A first examination of the helical nature of tropical cyclogenesis. Doklady Earth Sciences, 434(1):1285-1289, https://doi.org/10.1134/S1028334X1009031X 2010
Lorsolo, S., F.D. Marks, J.F. Gamache, and J.A. Zhang. Estimation and mapping of hurricane turbulent energy using airborne Doppler measurements. Monthly Weather Review, 138(9):3656-3670, https://doi.org/10.1175/2010MWR3183.1 2010
Hurricane turbulent kinetic energy (TKE) was computed using airborne Doppler measurements from the NOAA WP-3D tail radars and TKE data were retrieved for a variety of storms at different stages of their lifecycle. The geometry of the radar analysis coupled with the relatively small beam resolution at ranges <8 km allowed for the estimation of sub-kilometer turbulent processes. Two dimensional profiles of TKE were constructed and revealed that the strongest turbulence was in general located in convective regions such as the eyewall with magnitude often exceeding 15 m2 s-2, and in the boundary layer with values of 5-10 m2 s-2 in the lowest km. A correlation analysis showed that the strong turbulence was in general associated with strong horizontal shear of vertical and radial wind components in the eyewall and strong vertical shear of horizontal wind in the boundary layer. Mean vertical profiles of TKE decrease sharply above the hurricane boundary layer and level off at low magnitude for all regions outside the radius of maximum wind. The quality of the retrieval method was evaluated and showed very good agreement with TKE values directly calculated from the three-dimensional wind components of in-situ measurements. The method presented here provides a unique opportunity to assess hurricane turbulence throughout the storm, especially in high wind regions, and can be applied on extensive data sets of past and future airborne hurricane penetrations.
Majumdar, S.J., K.J. Sellwood, D. Hodyss, Z. Toth, and Y. Song. Characteristics of target areas selected by the Ensemble Transform Kalman Filter for medium-range forecasts of high-impact winter weather. Monthly Weather Review, 138(7):2803-2824, https://doi.org/10.1175/2010MWR3106.1 2010
The characteristics of target locations of tropospheric wind and temperature identified by a modified version of the ensemble transform Kalman filter (ETKF), in order to reduce 0-7-day forecast errors over North America, are explored from the perspective of a field program planner. Twenty cases of potential high-impact weather over the continent were investigated, using a 145-member ensemble comprising perturbations from NCEP, ECMWF, and the Canadian Meteorological Centre (CMC). Multiple targets were found to exist in the midlatitude storm track. In half of the cases, distinctive targets could be traced upstream near Japan at lead times of 4-7 days. In these cases, the flow was predominantly zonal and a coherent Rossby wave packet was present over the northern Pacific Ocean. The targets at the longest lead times were often located within propagating areas of baroclinic energy conversion far upstream. As the lead time was reduced, these targets were found to diminish in importance, with downstream targets corresponding to a separate synoptic system gaining in prominence. This shift in optimal targets is sometimes consistent with the radiation of ageostrophic geopotential fluxes and transfer of eddy kinetic energy downstream, associated with downstream baroclinic development. Concurrently, multiple targets arise due to spurious long-distance correlations in the ETKF. The targets were least coherent in blocked flows, in which the ETKF is known to be least reliable. The effectiveness of targeting in the medium range requires evaluation, using data such as those collected during the winter phase of The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Field Campaign (T-PARC) in 2009.
Montgomery, M.T., and R.K. Smith. On an analytical model for the rapid intensification of tropical cyclones. Quarterly Journal of the Royal Meteorological Society, 136(647):549-551, https://doi.org/10.1002/qj.573 2010
Stimulated by recent developments in understanding tropical cyclones, we offer an evaluation of an analytical model that has been proposed to explain the rapid intensification of these storms. We articulate a number of concerns with this model, including the neglect of both the vertical momentum equation and the thermodynamic equation, and conclude that it falls a little short of achieving its stated aims.
Montgomery, M.T., R.K. Smith, and V.S. Nguyen. Sensitivity of tropical-cyclone models to the surface drag coefficient. Quarterly Journal of the Royal Meteorological Society, 136(653):1945-1953, https://doi.org/10.1002/qj.702 2010
Motivated by recent developments in tropical-cyclone dynamics, this paper re-examines a basic aspect of tropical-cyclone behavior, namely, the sensitivity of tropical-cyclone models to the surface drag coefficient. Previous theoretical and numerical studies of the sensitivity in axisymmetric models have found that the intensity decreases markedly with increasing drag coefficient. Here we present a series of three-dimensional convection-permitting numerical experiments in which the intensification rate and intensity of the vortex increase with increasing surface drag coefficient until a certain threshold value is attained and then decrease. In particular, tropical depression-strength vortices intensify to major hurricane intensity for values of CK/CD as small as 0.1, significantly smaller than the critical threshold value of about 0.75 for major hurricane development predicted by Emanuel using an axisymmetric balance model. Moreover, when the drag coefficient is set to zero, no system-scale intensification occurs, despite persistent sea-to-air fluxes of moisture that maintain deep convective activity. This result is opposite to that found in a prior axisymmetric study by Craig and Gray. The findings are interpreted using recent insights obtained on tropical-cyclone intensification, which highlight the intrinsically unbalanced dynamics of the tropical-cyclone boundary layer. The reasons for the differences from earlier axisymmetric studies and some potential ramifications of our findings are discussed. The relative insensitivity of the intensification rate and intensity found for drag coefficients typical of high wind speeds over the ocean calls into question the need for coupled ocean wave-atmospheric models to accurately forecast tropical-cyclone intensity.
Posselt, D.J., and T. Vukicevic. Robust characterization of model physics uncertainty for simulations of deep moist convection. Monthly Weather Review, 138(5):1513-1535, https://doi.org/10.1175/2009MWR3094.1 2010
This study explores the functional relationship between model physics parameters and model output variables for the purpose of (1) characterizing the sensitivity of the simulation output to the model formulation and (2) understanding model uncertainty so that it can be properly accounted for in a data assimilation framework. A Markov chain Monte Carlo algorithm is employed to examine how changes in cloud microphysical parameters map to changes in output precipitation, liquid and ice water path, and radiative fluxes for an idealized deep convective squall line. Exploration of the joint probability density function (PDF) of parameters and model output state variables reveals a complex relationship between parameters and model output that changes dramatically as the system transitions from convective to stratiform. Persistent nonuniqueness in the parameter-state relationships is shown to be inherent in the construction of the cloud microphysical and radiation schemes and cannot be mitigated by reducing observation uncertainty. The results reinforce the importance of including uncertainty in model configuration in ensemble prediction and data assimilation, and they indicate that data assimilation efforts that include parameter estimation would benefit from including additional constraints based on known physical relationships between model physics parameters to render a unique solution.
Powell, M.D. Observing and analyzing the near-surface wind field in tropical cyclones. In Global Perspectives on Tropical Cyclones: From Science to Mitigation, J.C.L. Chan and J.D. Kepert (eds.). World Scientific Publishing Company, 2nd edition, 177-199, 2010
This paper describes the current state of the art in measuring and analyzing surface winds in tropical cyclones. Observing platforms and strategies will be reviewed, along with their advantages and limitations.
Powell, M.D., S.T. Murillo, P.P. Dodge, E.W. Uhlhorn, J.F. Gamache, V. Cardone, A. Cox, S. Otero, N. Carrasco, B. Annane, and R. St. Fleur. Reconstruction of Hurricane Katrina's wind fields for storm surge and wave hindcasting. Ocean Engineering, 37(1):26-36, https://doi.org/10.1016/j.oceaneng.2009.08.014 2010
As the most costly U.S. natural disaster in history, Hurricane Katrina fostered the IPET forensic study to better understand the event. All available observations from several hundred space-, land-, sea-, and aircraft-based measurement platforms were gathered and processed to a common framework for height, exposure, and averaging time, to produce a series of wind field snapshots at 3 h intervals to depict the wind structure of Katrina when in the Gulf of Mexico. The stepped-frequency microwave radiometer was calibrated against GPS sondes to establish the upper range of the instrument and then used to determine the wind field in the storm's core region in concert with airborne Doppler radar winds adjusted to the surface from near the top of the PBL (500 m). The SFMR data were used to develop a method to estimate surface winds from 3 km level reconnaissance aircraft observations, taking into consideration the observed azimuthal variation of the reduction factor. The SFMR method was used to adjust reconnaissance flight-level measurements to the surface in the core region when SFMR and Doppler winds were not available. A variety of coastal and inland mesonet data were employed, including portable towers deployed by Texas Tech University, University of Louisiana at Monroe, and the Florida Coastal Monitoring Program, as well as fixed mesonet stations from Louisiana State Universities Marine Consortium, University of Southern Mississippi, and Agricultural Networks from Louisiana, Mississippi, and Alabama, and the Coastal Estuarine Network of Alabama and Mississippi. Also included were land- (WSR-88D VAD and GBVTD, ASOS, Metar, LLWAS, HANDAR), space- (QuikScat, GOES cloud drift winds, WindSat), and marine- (GPS sondes, Buoys, C-MAN, ships) platforms. The wind fields serve as an analysis of record and were used to provide forcing for wave and storm surge models to produce hindcasts of water levels in the vicinity of flood control structures.
Reed, D.A., M.D. Powell, and J.M. Westerman. Energy infrastructure damage analysis for Hurricane Rita. Natural Hazards Review, 11(3):102-109, https://doi.org/10.1061/(ASCE)NH.1527-6996.0000012 2010
In 2005, Hurricane Rita caused significant damage to the energy infrastructure in the Gulf of Mexico region. In the context of this investigation, the energy infrastructure refers to the offshore oil platforms, refineries, and gasoline supply stations in the region, often referred to as the petroleum infrastructure, the natural gas supply lines, and the delivery of electric power. In this paper, we examine the structural damage to the networks as defined by restoration, resilience, and fragility with a focus on the analysis of the electric power delivery disruptions. Our concern is not on the evaluation of risk, but rather to provide those who assess hurricane risk with relevant structural damage prediction models. We provide correlations of hurricane wind speed data with outages. We conclude that high winds alone can create significant damage to the energy infrastructure system.
Riemer, M., M.T. Montgomery, and M.E. Nicholls. A new paradigm for intensity modification of tropical cyclones: Thermodynamic impact of vertical wind shear on the inflow layer. Atmospheric Chemistry and Physics, 10(7):3163-3188, https://doi.org/10.5194/acp-10-3163-2010 2010
An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. In this paper, we re-visit the canonical problem of a TC in vertical wind shear on an f-plane. A suite of numerical experiments is performed with intense TCs in moderate to strong vertical shear. We employ a set of simplified model physics, a simple bulk aerodynamic boundary layer scheme and warm rain microphysics, to foster better understanding of the dynamics and thermodynamics that govern the modification of TC intensity. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. The ventilation of the TC core with dry environmental air at mid-levels and the dilution of the upper-level warm core are two prevailing hypotheses for the adverse effect of vertical shear on storm intensity. Here we propose an alternative and arguably more effective mechanism for how cooler and drier (lower θe) air, anti-fuel for the TC power machine, can enter the core region of the TC. Strong and persistent, shear-induced downdrafts flux low θe air into the boundary layer from above, significantly depressing the θe values in the storm's inflow layer. Air with lower θe values enters the eyewall updrafts, considerably reducing eyewall θe values in the azimuthal mean. When viewed from the perspective of an idealized Carnot-cycle heat engine, a decrease of storm intensity can thus be expected. Although the Carnot cycle model is, if at all, only valid for stationary and axisymmetric TCs, a close association of the downward transport of low θe into the boundary layer and the intensity evolution offers further evidence in support of our hypothesis. The downdrafts that flush the boundary layer with low θe air are tied to a quasi-stationary, azimuthal wave number 1 convective asymmetry outside of the eyewall. This convective asymmetry and the associated downdraft pattern extends outwards to approximately 150 km. Downdrafts occur on the vortex scale and form when precipitation falls out from sloping updrafts and evaporates in the unsaturated air below. It is argued that, to zero order, the formation of the convective asymmetry is forced by frictional convergence associated with the azimuthal wave number 1 vortex Rossby wave structure of the outer-vortex tilt. This work points to an important connection between the thermodynamic impact in the near-core boundary layer and the asymmetric balanced dynamics governing the TC vortex evolution.
Rogers, R.F. Convective-scale structure and evolution during a high-resolution simulation of tropical cyclone rapid intensification. Journal of the Atmospheric Sciences, 67(1):44-70, https://doi.org/10.1175/2009JAS3122.1 2010
The role of convective-scale processes in a 1.67-km mesoscale model simulation of the rapid intensification (RI) of Hurricane Dennis (2005) is presented. The structure and evolution of inner-core precipitating areas during RI, the statistical properties of precipitation during times experiencing vigorous convection (termed convective bursts here) and how they differ from nonburst times, possible differences in convective bursts associated with RI and those not associated with RI, and the impacts of precipitation morphology on the vortex-scale structure and evolution during RI are all examined. The onset of RI is linked to an increase in the areal extent of convective precipitation in the inner core, while the inner-core stratiform precipitating area remains unchanged and the intensity increases only after RI has begun. RI is not tied to a dramatic increase in the number of convective bursts nor in the characteristics of the bursts, such as burst intensity. Rather, the immediate cause of RI is a significant increase in updraft mass flux, particularly in the lowest 1.5 km. This increase in updraft mass flux is accomplished primarily by updrafts on the order of 1-2 m s-1, representing the bulk of the vertical motion distribution. However, a period of enhanced updraft mass flux in the midlevels by moderate to strong (>5 m s-1) updrafts located inside the radius of maximum winds occurs ~6 h prior to RI, indicating a synergistic relationship between convective bursts and the background secondary circulation prior to RI. This result supports the assertion that both buoyantly driven updrafts and slantwise near-neutral ascent are important features in eyewall structure, evolution, and intensification, including RI.
Rutherford, B., G. Dangelmayr, J. Persing, M. Kirby, and M.T. Montgomery. Lagrangian mixing in an axisymmetric hurricane model. Atmospheric Chemistry and Physics, 10(14):6777-6791, https://doi.org/10.5194/acp-10-6777-2010 2010
This paper discusses the extension of established Lagrangian mixing measures to make them applicable to data extracted from a 2-D axisymmetric hurricane simulation. Because of the non-steady and unbounded characteristics of the simulation, the previous measures are extended to a moving frame approach to create time-dependent mixing rates that are dependent upon the initial time of particle integration, and are computed for nonlocal regions. The global measures of mixing derived from finite-time Lyapunov exponents, relative dispersion, and a measured mixing rate are applied to distinct regions representing different characteristic feautures within the model. It is shown that these time-dependent mixing rates exhibit correlations with maximal tangential winds during a quasi-steady state, establishing a connection between mixing and hurricane intensity.
Rutherford, B., G. Dangelmayr, J. Persing, W.H. Schubert, and M.T. Montgomery. Advective mixing in a nondivergent barotropic hurricane model. Atmospheric Chemistry and Physics, 10(2):475-497, https://doi.org/10.5194/acp-10-475-2010 2010
This paper studies Lagrangian mixing in a two-dimensional barotropic model for hurricane-like vortices. Since such flows show high shearing in the radial direction, particle separation across shear-lines is diagnosed through a Lagrangian field, referred to as R-field, that measures trajectory separation orthogonal to the Lagrangian velocity. The shear-lines are identified with the level-contours of another Lagrangian field, referred to as S-field, that measures the average shear-strength along a trajectory. Other fields used for model diagnostics are the Lagrangian field of finite-time Lyapunov exponents (FTLE-field), the Eulerian Q-field, and the angular velocity field. Because of the high shearing, the FTLE-field is not a suitable indicator for advective mixing, and in particular does not exhibit ridges marking the location of finite-time stable and unstable manifolds. The FTLE-field is similar in structure to the radial derivative of the angular velocity. In contrast, persisting ridges and valleys can be clearly recognized in the R-field, and their propagation speed indicates that transport across shear-lines is caused by Rossby waves. A radial mixing rate derived from the R-field gives a time-dependent measure of flux across the shear-lines. On the other hand, a measured mixing rate across the shear-lines, which counts trajectory crossings, confirms the results from the R-field mixing rate, and shows high mixing in the eyewall region after the formation of a polygonal eyewall, which continues until the vortex breaks down. The location of the R-field ridges elucidates the role of radial mixing for the interaction and breakdown of the mesovortices shown by the model.
Smith, R.K., and M.T Montgomery. Hurricane boundary-layer theory. Quarterly Journal of the Royal Meteorological Society, 136(652):1665-1670, https://doi.org/10.1002/qj.679 2010
In the light of the plethora of definitions for the hurricane boundary layer, we advocate a dynamical definition based on the distribution of a gradient flow. We seek also to clarify the fundamental role of the boundary layer in the hurricane intensification process. In particular, we contrast the differences between unsteady boundary layers that are able to facilitate the spin-up of the vortex above and steady boundary layers that cannot. If slaved to the time-dependent vortex aloft, the latter can spin up the interior vortex only indirectly by changing its thermodynamic properties through vertical advection of these from below and adjustment to thermal wind balance. These differences are highlighted by an analytical demonstration that the application of a zero-vertical-gradient condition on velocity above a steady boundary layer does not provide a direct means of allowing the boundary layer to determine the flow in the interior vortex. This result assumes that frictional forces are negligible at this boundary. Finally, echoing a few previous insights, we question the applicability of conventional boundary-layer theory at radii of strong ascent into the eyewall, where the flow is akin to that of separation in aerodynamic boundary layers.
Vukicevic, T., O. Coddington, and P. Pilewskie. Characterizing the retrieval of cloud properties from optical remote sensing. Journal of Geophysical Research, 115:D20211, 14 pp., https://doi.org/10.1029/2009JD012830 2010
This paper presents a new approach to the formal characterization of the optical retrieval of cloud optical thickness and effective droplet radius based on a nonlinear methodology that is derived from a general stochastic inverse problem formulation similar to standard Bayesian estimation theory. The methodology includes efficient use of the precomputed radiative transfer model simulations which are already available in standard retrieval algorithms. Another important property of the methodology is that it does not require performing the retrieval with actual measurements in order to characterize the retrieval results. One utility of this analysis is the quantification of information content in the standard retrieval problem, and the increase of information through adding channels (radiances at different wavelengths) to the inversion. This was demonstrated for the five-wavelength retrieval using airborne hyperspectral shortwave irradiance measurements. The ability of the method to evaluate the impact of observation and radiative transfer model uncertainties on the retrieved cloud properties is also demonstrated. Further benefits from this study will be in its application to the cloud retrieval algorithms to be developed for future space- and airborne instruments. The present study puts forth the framework necessary to quantify that increase in information and to optimize new retrieval algorithms that efficiently accommodate the enhanced measurement space.
Wang, Z., M.T. Montgomery, and T.J. Dunkerton. Genesis of pre-Hurricane Felix (2007), Part 1: The role of the easterly wave critical layer. Journal of the Atmospheric Sciences, 67(6):1711-1729, https://doi.org/10.1175/2009JAS3420.1 2010
The formation of pre-Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in Part II of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities. In this first part, the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the wave pouch from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.
Wang, Z., M.T. Montgomery, and T.J. Dunkerton. Genesis of pre-Hurricane Felix (2007), Part 2: Warm core formation, precipitation evolution, and predictability. Journal of the Atmospheric Sciences, 67(6):1730-1744, https://doi.org/10.1175/2010JAS3435.1 2010
This is the second of a two-part study examining the simulated formation of Atlantic Hurricane Felix (2007) in a cloud-representing framework. Here several open issues are addressed concerning the formation of the storm's warm core, the evolution and respective contribution of stratiform versus convective precipitation within the parent waves pouch, and the sensitivity of the development pathway reported in Part I to different model physics options and initial conditions. All but one of the experiments include ice microphysics as represented by one of several parameterizations, and the partition of convective versus stratiform precipitation is accomplished using a standard numerical technique based on the high-resolution control experiment. The transition to a warm-core tropical cyclone from an initially cold-core, lower tropospheric wave disturbance is analyzed first. As part of this transformation process, it is shown that deep moist convection is sustained near the pouch center. Both convective and stratiform precipitation rates increase with time. While stratiform precipitation occupies a larger area even at the tropical storm stage, deep moist convection makes a comparable contribution to the total rain rate at the pregenesis stage, and a larger contribution than stratiform processes at the storm stage. The convergence profile averaged near the pouch center is found to become dominantly convective with increasing deep moist convective activity there. Low-level convergence forced by interior diabatic heating plays a key role in forming and intensifying the near-surface closed circulation, while the midlevel convergence associated with stratiform precipitation helps to increase the midlevel circulation and thereby contributes to the formation and upward extension of a tropospheric-deep cyclonic vortex. Sensitivity tests with different model physics options and initial conditions demonstrate a similar pregenesis evolution. These tests suggest that the genesis location of a tropical storm is largely controlled by the parent waves critical layer, whereas the genesis time and intensity of the protovortex depend on the details of the mesoscale organization, which is less predictable. Some implications of the findings are discussed.
Zhang, J.A. Estimation of dissipative heating using low-level in-situ aircraft observations in the hurricane boundary layer. Journal of the Atmospheric Sciences, 67(6):1853-1862, https://doi.org/10.1175/2010JAS3397.1 2010
Data collected in the low-level atmospheric boundary layer in five hurricanes by NOAA research aircraft are analyzed to measure turbulence with scales small enough to retrieve the rate of dissipation. A total of 49 flux runs suitable for analysis are identified in the atmospheric boundary layer within 200 m above the sea surface. Momentum fluxes are directly determined using the eddy correlation method, and drag coefficients are also calculated. The dissipative heating is estimated using two different methods: (1) integrating the rate of dissipation in the surface layer; and (2) multiplying the drag coefficient by the cube of surface wind speed. While the latter method has been widely used in theoretical models as well as several numerical models simulating hurricanes, these analyses show that using this method would significantly overestimate the magnitude of dissipative heating. Although the dataset used in this study is limited by the surface wind speed range < 30 m s-1, this work highlights that it is crucial to understand the physical processes related to dissipative heating in the hurricane boundary layer for implementing it into hurricane models.
Zhang, J.A. Spectral characteristics of turbulence in the hurricane boundary layer over ocean between the outer rainbands. Quarterly Journal of the Royal Meteorological Society, 136(649):918-926, https://doi.org/10.1002/qj.610 2010
Spectra and cospectra of wind velocity, potential temperature, and humidity have been analyzed using data collected in the atmospheric boundary layer in Hurricanes Fabian (2003) and Isabel (2003) during the Coupled Boundary Layer Air- Sea Transfer (CBLAST) hurricane experiment. The spectra and cospectra are normalized following the surface layer scaling methods according to similarity theory. It is found that the CBLAST data gathered in the mixed layer between the outer rain bands below 400 m can be grouped into well-defined curves for spectra of wind velocity, potential temperature and humidity, and for cospectra of momentum and humidity flux. However, the cospectra of sensible heat flux do not exhibit well-defined universal shape. The CBLAST universal shape spectra and cospectra generally resemble the shapes of those from previous studies, but shift to higher frequencies that correspond to smaller wavelengths of turbulent eddies that contain most of the energy. This work highlights the structural difference between the hurricane boundary layer and the standard atmospheric boundary layer over land and ocean.
Zhu, P., J.A. Zhang, and F.J. Masters. Wavelet analyses of turbulence in the hurricane surface layer during landfalls. Journal of the Atmospheric Sciences, 67(12):3793-3805, https://doi.org/10.1175/2010JAS3437.1 2010
Using wavelet transform (WT), this study analyzes the surface wind data collected by the portable wind towers during the landfalls of six hurricanes and one tropical storm in the 2002-04 seasons. The WT, which decomposes a time series onto the scale-time domain, provides a means to investigate the role of turbulent eddies in the vertical transport in the unsteady, inhomogeneous hurricane surface layer. The normalized WT power spectra (NWPS) show that the hurricane boundary layer roll vortices tend to suppress the eddy circulations immediately adjacent to rolls, but they do not appear to have a substantial effect on eddies smaller than 100 m. For low-wind conditions with surface wind speeds less than 10 m s-1, the contributions of small eddies (<236 m) to the surface wind stress and turbulent kinetic energy (TKE) decrease with the increase of wind speed. The opposite variation trend is found for eddies greater than 236 m. However, for wind speeds greater than 10 m s-1, contributions of both small and large eddies tend to level off as wind speeds keep increasing. It is also found that the scale of the peak NWPS of the surface wind stress is nearly constant with a mean value of approximately 86 m, whereas the scale of the peak NWPS of TKE generally increases with the increase of wind speed, suggesting the different roles of eddies in generating fluxes and TKE. This study illustrates the unique characteristics of the surface layer turbulent structures during hurricane landfalls. It is hoped that the findings of this study could enlighten the development and improvement of turbulent mixing schemes so that the vertical transport processes in the hurricane surface layer can be appropriately parameterized in forecasting models.
2009
Aberson, S.D. Regimes or cycles in tropical cyclone activity in the North Atlantic. Bulletin of the American Meteorological Society, 90(1):39-43, https://doi.org/10.1175/2008BAMS2549.1 2009
The important role of the correct use of statistics in the atmospheric sciences literature is once again emphasized. Despite previous work on this topic, statistical techniques, even very simple ones, continue to be misused or altogether neglected, with the inevitable result of misleading or erroneous conclusions. An example concerning the impact of global climate change and hurricane activity is presented.
Aksoy, A., D.C. Dowell, and C. Snyder. A multicase comparative assessment of the ensemble Kalman filter for assimilation of radar observations, Part I: Storm-scale analyses. Monthly Weather Review, 137(6):1805-1824, https://doi.org/10.1175/2009MWR3086.1 2009
The effectiveness of the ensemble Kalman filter (EnKF) for assimilating radar observations at convective scales is investigated for cases whose behaviors span supercellular, linear, and multicellular organization. The parallel EnKF algorithm of the Data Assimilation Research Testbed (DART) is used for data assimilation, while the Weather Research and Forecasting (WRF) Model is employed as a simplified cloud model at 2-km horizontal grid spacing. In each case, reflectivity and radial velocity measurements are utilized from a single Weather Surveillance Radar-1988 Doppler (WSR-88D) within the U.S. operational network. Observations are assimilated every 2 min for a duration of 60 min and correction of folded radial velocities occurs within the EnKF. Initial ensemble uncertainty includes random perturbations to the horizontal wind components of the initial environmental sounding. The EnKF performs effectively and with robust results across all the cases. Over the first 18-30 min of assimilation, the rms and domain-averaged prior fits to observations in each case improve significantly from their initial levels, reaching comparable values of 3-6 m s-1 and 7-10 dBZ. Representation of mesoscale uncertainty, albeit in the simplest form of initial sounding perturbations, is a critical part of the assimilation system, as it increases ensemble spread and improves filter performance. In addition, assimilation of no precipitation observations (i.e., reflectivity observations with values small enough to indicate the absence of precipitation) serves to suppress spurious convection in ensemble members. At the same time, it is clear that the assimilation is far from optimal, as the ensemble spread is consistently smaller than what would be expected from the innovation statistics and the assumed observation-error variance.
Bell, G.D., E. Blake, S.B. Goldenberg, T. Kimberlain, C.W. Landsea, R. Pasch, and J. Schemm. Tropical cyclones: Atlantic basin. In State of the Climate in 2008, T.C. Peterson and M.O. Baringer (eds.). Bulletin of the American Meteorological Society, 90(8):S79-S83, https://doi.org/10.1175/BAMS-90-8-StateoftheClimate 2009
Bui, H.-H., R.K. Smith, M.T. Montgomery, and J. Peng. Balanced and unbalanced aspects of tropical cyclone intensification. Quarterly Journal of the Royal Meteorological Society, 135(644):1715-1731, https://doi.org/10.1002/qj.502 2009
We investigate the extent to which the azimuthally-averaged fields from a three-dimensional, non-hydrostatic, tropical cyclone model can be captured by axisymmetric balance theory. The secondary (overturning) circulation and balanced tendency for the primary circulation are obtained by solving a general form of the Sawyer-Eliassen equation with the diabatic heating, eddy heat fluxes and tangential momentum sources (eddy momentum fluxes, boundary-layer friction and subgrid-scale diffusion) diagnosed from the model. The occurrence of regions of weak symmetric instability at low levels and in the upper-tropospheric outflow layer requires a regularization procedure so that the Sawyer-Eliassen equation remains elliptic. The balanced calculations presented capture a major fraction of the azimuthally-averaged secondary circulation of the three-dimensional simulation except in the boundary layer, where the balanced assumption breaks down and where there is an inward agradient force. In particular, the balance theory is shown to significantly underestimate the low-level radial inflow and therefore the maximum azimuthal-mean tangential wind tendency. In the balance theory, the diabatic forcing associated with the eyewall convection accounts for a large fraction of the secondary circulation. The findings herein underscore both the utility of axisymmetric balance theory and also its limitations in describing the axisymmetric intensification physics of a tropical cyclone vortex.
Burpee, R.W. The Sanders barotropic tropical cyclone track model (SANBAR). In Synoptic-Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, L.F. Bosart and H.B. Bluestein (eds.). Meteorological Monograph, Volume 33, No. 55, American Meteorological Society, 233-240, 2009
Sanders designed a barotropic tropical cyclone (TC) track model for the North Atlantic TC basin that became known as SANBAR. It predicted the stream function of the deep layer mean winds (averaged from 1000-100 hPa) that represented the vertically averaged tropical circulations. Originally, the wind input for the operational objective analysis (OA) consisted of winds measured by radiosondes and 44 bogus winds subjectively estimated by analysts at the National Hurricane Center (NHC) that corresponded to the vertically averaged flow over sparsely observed tropical and subtropical oceanic regions. The model covered a fixed regional area and had a grid size of about 154 km. It estimated the initial storm motion solely on the basis of the prevailing flow from the OA, not taking into account the observed storm motion. During 1970, the SANBAR model became the first dynamical TC track model to be run operationally at NHC. The track forecasts of SANBAR were verified from the 1971 TC season when track model verifications began at NHC until its retirement after the 1989 season. The average annual SANBAR forecast track errors are verified relative to CLIPER, the standard no-skill track forecast. Comparison with CLIPER determines the skill of track forecast methods. Verifications are presented for two different versions of the model system used operationally from 1973-84 and 1985-89. In homogeneous comparisons for the former period, SANBARs track forecasts were slightly better than CLIPER at 24-48 h forecast intervals; however, from 1985-89, the average SANBAR track forecast errors from 24-72 h were ~10% more skillful than homogeneous CLIPER track forecasts.
Conzemius, R.J., and M.T. Montgomery. Clarification on the generation of absolute and potential vorticity in mesoscale convective vortices. Atmospheric Chemistry and Physics, 9(19):7591-7605, https://doi.org/10.5194/acp-9-7591-2009 2009
In this paper, we clarify several outstanding issues concerning the predominant mechanism of vorticity generation in mesoscale convective vortices (MCVs) in weak to modest baroclinic environments with nonzero Coriolis parameter. We examine also the corresponding diabatic heating profiles of the convective and stratiform components of the MCS and their effects on the concentration and dilution of PV substance.
Dunkerton, T.J., M.T. Montgomery, and Z. Wang. Tropical cyclogenesis in a tropical wave critical layer: Easterly waves. Atmospheric Chemistry and Physics, 9(15):5587-5646, https://doi.org/10.5194/acp-9-5587-2009 2009
The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a surface low along the wave as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, resembles the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development within the critical layer is given by the intersection of the waves critical latitude and trough axis, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally this marsupial paradigm one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. This translation requires an appropriate gauge that renders translating streamlines and isopleths of translating stream function approximately equivalent to flow trajectories. In the translating frame, the closed circulation is stationary, and a dividing streamline effectively separates air within the critical layer from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because it provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the marsupial paradigm. A survey of 55 named tropical storms in 1998-2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cats eyes, but are usually less regular, with one or more recirculation regions in the translating frame. It is shown that a wave gauge given by the translation speed of the parent wave is the appropriate choice, as well, for isolated proto-vortices carried by the wave. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.
Fierro, A.O., J. Simpson, M.A. LeMone, J.M. Straka, and B.F. Smull. On how hot towers fuel the Hadley cell: An observational and modeling study of line-organized convection in the equatorial trough from TOGA COARE. Journal of the Atmospheric Sciences, 66(9):2730-2746, https://doi.org/10.1175/2009JAS3017.1 2009
An airflow trajectory analysis was carried out based on an idealized numerical simulation of the nocturnal 9 February 1993 equatorial oceanic squall line observed over the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) ship array. This simulation employed a nonhydrostatic numerical cloud model, which features a sophisticated 12-class bulk microphysics scheme. A second convective system that developed immediately south of the ship array a few hours later under similar environmental conditions was the subject of intensive airborne quad-Doppler radar observations, allowing observed airflow trajectories to be meaningfully compared to those from the model simulation. The results serve to refine the so-called hot tower hypothesis, which postulated the notion of undiluted ascent of boundary layer air to the high troposphere, which has for the first time been tested through coordinated comparisons with both model output and detailed observations. For parcels originating ahead (north) of the system near or below cloud base in the boundary layer (BL), the model showed that a majority (>62%) of these trajectories were able to surmount the 10-km level in their lifetime, with about 5% exceeding 14-km altitude, which was near the modeled cloud top (15.5 km). These trajectories revealed that during ascent, most air parcels first experienced a quick decrease of equivalent potential temperature (θe) below 5-km MSL as a result of entrainment of lower ambient θe air. Above the freezing level, ascending parcels experienced an increase in θe with height attributable to latent heat release from ice processes consistent with previous hypotheses. Analogous trajectories derived from the evolving observed airflow during the mature stage of the airborne radar-observed system identified far fewer (~5%) near-BL parcels reaching heights above 10 km than shown by the corresponding simulation. This is attributed to both the idealized nature of the simulation and to the limitations inherent to the radar observations of near-surface convergence in the subcloud layer. This study shows that latent heat released above the freezing level can compensate for buoyancy reduction by mixing at lower levels, thus enabling air originating in the boundary layer to contribute to the maintenance of both local buoyancy and the large-scale Hadley cell despite acknowledged dilution by mixing along updraft trajectories. A tropical hot tower should thus be redefined as any deep convective cloud with a base in the boundary layer and reaching near the upper-tropospheric outflow layer.
Fierro, A.O., R.F. Rogers, F.D. Marks, and D.S. Nolan. The impact of horizontal grid spacing on the microphysical and kinematic structures of strong tropical cyclones simulated with the WRF-ARW model. Monthly Weather Review, 137(11):3717-3743, https://doi.org/10.1175/2009MWR2946.1 2009
Using the Advanced Weather Research and Forecasting numerical model, the impact of horizontal grid spacing on the microphysical and kinematic structure of a numerically simulated tropical cyclone (TC), and their relationship to storm intensity was investigated with a set of five numerical simulations using input data for the case of Hurricane Rita (2005). The horizontal grid spacing of the parent domain was systematically changed such that the horizontal grid spacing of the inner nest varied from 1 to 5 km by an increment of 1 km, this while keeping geographical dimensions of the domains identical. Within this small range of horizontal grid spacing, the morphology of the simulated storms and the evolution of the kinematic and microphysics field showed noteworthy differences. As grid spacing increased, the model produced a wider, more tilted eyewall, a larger radius of maximum winds, and higher-amplitude, low wavenumber eyewall asymmetries. The coarser-resolution simulations also produced larger volume, areal coverage, and mass flux of updraft speeds >5 m s-1; larger volumes of condensate and ice-phase particles aloft; larger boundary layer kinetic energy; and a stronger secondary circulation. While the contribution of updrafts >5 m s-1 to the total updraft mass flux varied little between the five cases, the contribution of downdrafts <2 m s-1 to the total downdraft mass flux was by far the largest in the finest-resolution simulation. Despite these structural differences, all of the simulations produced storms of similar intensity, as measured by peak 10-m wind speed and minimum surface pressure, suggesting that features in the higher-resolution simulations that tend to weaken TCs (i.e., smaller area of high surface fluxes and weaker total updraft mass flux) compensate for features that favor TC intensity (i.e., smaller-amplitude eyewall asymmetries and larger radial gradients). This raises the possibility that resolution increases in this range may not be as important as other model features (e.g., physical parameterization and initial condition improvements) for improving TC intensity forecasts.
Heymsfield, A.J., A. Bansemer, G. Heymsfield, and A.O. Fierro. Microphysics of maritime tropical convective updrafts at temperatures from -20° to-60°. Journal of the Atmospheric Sciences, 66(12):3530-3562, https://doi.org/10.1175/2009JAS3107.1 2009
Anvils produced by vigorous tropical convection contribute significantly to the earths radiation balance, and their radiative properties depend largely on the concentrations and sizes of the ice particles that form them. These microphysical properties are determined to an important extent by the fate of supercooled droplets, with diameters from 3 to about 20 microns, lofted in the updrafts. The present study addresses the question of whether most or all of these droplets are captured by ice particles or if they remain uncollected until arriving at the -38°C level where they freeze by homogeneous nucleation, producing high concentrations of very small ice particles that can persist and dominate the albedo. Aircraft data of ice particle and water droplet size distributions from seven field campaigns at latitudes from 25°N to 11°S are combined with a numerical model in order to examine the conditions under which significant numbers of supercooled water droplets can be lofted to the homogeneous nucleation level. Microphysical data were collected in pristine to heavily dust-laden maritime environments, isolated convective updrafts, and tropical cyclone updrafts with peak velocities reaching 25 m s-1. The cumulative horizontal distance of in-cloud sampling at temperatures of 20°C and below exceeds 50 000 km. Analysis reveals that most of the condensate in these convective updrafts is removed before reaching the 20°C level, and the total condensate continues to diminish linearly upward. The amount of condensate in small (μm in diameter) droplets and ice particles, however, increases upward, suggesting new droplet activation with an appreciable radiative impact. Conditions promoting the generation of large numbers of small ice particles through homogeneous ice nucleation include high concentrations of cloud condensation nuclei (sometimes from dust), removal of most of the water substance between cloud base and the −38°C levels, and acceleration of the updrafts at mid- and upper levels such that velocities exceed 5–7 m s−1.
Jones, R.W., H.E. Willoughby, and M.T. Montgomery. Alignment of hurricane-like vortices on f- and beta-planes. Journal of the Atmospheric Sciences, 66(6):1779-1792, https://doi.org/10.1175/2008JAS2850.1 2009
A nonlinear, two-layer, vortex-tracking semispectral model (i.e., Fourier transformed in azimuth only) is used to study the evolution of dry, but otherwise hurricane-like, initially tilted vortices in quiescent surroundings on f and beta planes. The tilt projects onto vorticity asymmetries that are dynamically vortex Rossby waves. Since the swirling wind in the principal mean vortex used here decays exponentially outside the eyewall, it has an initial potential vorticity (PV) minimum. The resulting reversal of PV gradient meets the necessary condition for inflectional (i.e., barotropic or baroclinic) instability. Thus, the vortex may be inflectionally stable or unstable. On an f plane, the tilt precesses relatively slowly because the critical radius, where the phase speeds of the waves match the mean swirling flow, is far from the center. An alternative Gaussian-like PV monopole that has a monotonic outward decrease of PV is stable to inflectional instability. It has a smaller critical radius and rapid tilt precession. Generally, vortices with fast tilt precession are more stable, as are stronger vortices in higher latitudes. On a beta plane, the interaction between the symmetric vortex and the planetary PV gradient induces beta gyres that push the vortex poleward and westward. The interaction between the beta gyres and the planetary PV gradient may either create a PV minimum or intensify a minimum inherited from the initial condition. Thus, the nonlinear beta effect reduces the ability of the vortex to recover from initial tilt, relative to the same vortex on an f plane. This result contrasts with previous studies of barotropic vortices on f planes, where the linear and nonlinear solutions were nearly identical.
Katzberg, S.J., and J.P. Dunion. Comparison of reflected GPS wind speed retrievals with dropsondes in tropical cyclones. Geophysical Research Letters, 36(17):L17602, 5 pp., https://doi.org/10.1029/2009GL039512 2009
In an earlier communication, data were presented that demonstrated that quasi-specular, L-Band reflection measurements could be used to infer ocean surface winds. Applying an indirect calibration technique, a mean square slope versus surface wind speed was developed and reported. Retrievals using this calibration showed that the resulting surface wind speeds were comparable with other measurements. This report extends the previous results by presenting direct comparisons between GPS dropwindsonde (dropsonde)-reported wind speeds and the Bi-static GPS wind speed retrievals for data sets acquired in 2008. Editing of the Bi-static GPS data will be discussed that takes into effect overland and inside-the-eye winnowing. Data will be presented with a regression line to determine the comparative relationship. It will be shown that good agreement exists between the reflected Bi-static GPS retrieved winds and those reported by the dropsondes when certain well-defined types of data are excluded.
Montgomery, M.T., V.S. Nguyen, R.K. Smith, and J. Persing. Do tropical cyclones intensity by WISHE? Quarterly Journal of the Royal Meteorological Society, 135(644):1697-1714, https://doi.org/10.1002/qj.459 2009
In this paper we seek and obtain a basic understanding of tropical cyclone intensification in three dimensions when precipitation and evaporative-cooling (warm rain) processes are included. Intensification with warm rain physics included is found to be dominated by highly localized deep convective structures possessing strong cyclonic vorticity in their cores, dubbed "Vortical Hot Towers" (VHTs). Unlike previous studies, the findings herein suggest an intensification pathway that is distinct from the "evaporation-wind" feedback mechanism known as wind-induced surface heat exchange (WISHE), which requires a positive feedback between the azimuthal-mean boundary-layer equivalent potential temperature and the azimuthal-mean surface wind speed underneath the eyewall of the storm. Intensification from a finite-amplitude initial vortex is shown to not require this evaporation-wind feedback process. Indeed, when the surface wind speed in the sea-to-air vapour fluxes is capped at a nominal (trade-wind) value, the vortex still intensifies by the same pathway identified in the main experiments via the generation of locally buoyant VHTs and the near-surface convergence that the VHTs induce within the boundary layer. The present findings and interpretations challenge the prevailing view that tropical cyclones are premier examples of vortical systems arising from WISHE. Given the potential significance on our understanding of the dynamics of hurricanes, and given the limitations of the present modelling framework, further tests of these predictions are advocated.
Nolan, D.S., J.A. Zhang, and D.P. Stern. Evaluation of planetary boundary layer parameterizations in tropical cyclones by comparison of in-situ observations and high-resolution simulations of Hurricane Isabel (2003), Part I: Initialization, maximum winds, and the outer core boundary layer. Monthly Weather Review, 137(11):3651-3674, https://doi.org/10.1175/2009MWR2785.1 2009
In this study, the first of two parts, the planetary boundary layer (PBL) depicted in high-resolution Weather Research and Forecast Model (WRF) simulations of Hurricane Isabel (2003) is studied and evaluated by direct comparisons with in situ data obtained during the Coupled Boundary Layer and Air-Sea Transfer Experiment (CBLAST). In particular, two boundary layer schemes are evaluated: the Yonsei University (YSU) parameterization and the Mellor-Yamada-Janjic (MYJ) parameterization. Investigation of these schemes is useful since they are available for use with WRF, are both widely used, and are based on entirely different methods for simulating the PBL. In this first part, the model domains and initialization are described. For additional realism of the low-level thermodynamic environment, a simple mixed layer ocean model is used to simulate ocean cooling. The YSU and MYJ schemes are discussed, along with some modifications. Standard measures of the accuracy of the hurricane simulations, such as track, maximum surface wind speed, and minimum surface pressure are described for a variety of parameter choices and for the two parameterizations. The effects on track and intensity of increased horizontal and vertical resolutions are also shown. A modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes. Instantaneous wind maxima on the innermost grid with 1.33-km resolution are shown to be an accurate representation of the simulated 1-min sustained winds. The simulated boundary layers are evaluated by direct comparison of the PBL as simulated and as observed by in situ data from the CBLAST experiment in the outer core region of the storm. The two PBL schemes and their modified counterparts reproduce the observed PBL remarkably well. Comparisons are also made to the observed vertical fluxes of momentum, heat, and moisture. In Part II, the detailed comparisons of the intensities and structures of the simulated and observed inner-core boundary layers are presented, and the reasons for the differences are discussed.
Nolan, S.D., D.P. Stern, and J.A. Zhang. Evaluation of planetary boundary layer parameterizations in tropical cyclones by comparison of in-situ observations and high-resolution simulations of Hurricane Isabel (2003), Part II: Inner-core boundary layer and eyewall structure. Monthly Weather Review, 137(11):3675-3698, https://doi.org/10.1175/2009MWR2786.1 2009
This is the second of a two-part study of the representation of the planetary boundary layer (PBL) in high-resolution Weather Research and Forecast Model (WRF) simulations of Hurricane Isabel (2003). The Yonsei University (YSU) PBL parameterization and the Mellor-Yamada-Janjic (MYJ) PBL parameterization are evaluated by direct comparison to in situ data obtained by research aircraft. The numerical model, simulation design, details of the PBL schemes, and the representation of the boundary layer in the outer-core were presented in Part I. This part presents a detailed study of the inner-core PBL, including its axisymmetric and asymmetric structures, and comparisons to analyses of dropsonde data from previous studies. Although neither PBL scheme was designed specifically for hurricane conditions, their simulated boundary layers are reasonably good representations of the observed boundary layer. Both schemes reproduce certain unique features of the hurricane boundary layer, such as the separate depths of the well-mixed layer and the inflow layer, and the pronounced wind speed maxima near the top of the inflow layer. Modification of the original YSU and MYJ schemes to have ocean roughness lengths more in agreement with recent studies considerably improves the results of both schemes. Even with these improvements, the MYJ consistently produces larger frictional tendencies in the boundary layer than the YSU scheme, leading to a stronger low-level inflow and a stronger azimuthal wind maximum at the top of the boundary layer. For both schemes, differences in the low-level asymmetries between the simulated and observed wind fields appear to be related to eyewall asymmetries forced by environmental wind shear. The effects of varying horizontal and vertical resolutions are also considered. Increasing the vertical resolution in the PBL results in minor improvements in the inner-core structures. Increasing the horizontal resolution around the eyewall also leads to improved boundary layers, as well as an improvement of the vertical structure of the inner-core wind field. A summary and discussion of the results of both Parts I and II is provided.
Panda, J., M. Sharan, and S.G. Gopalakrishnan. Study of regional-scale boundary layer characteristics over northern India with a special reference to the role of the Thar Desert in regional-scale transport. Journal of Applied Meteorology and Climatology, 48(11):2377-2402, https://doi.org/10.1175/2009JAMC1926.1 2009
Extensive contrasts of land surface heterogeneities have a pivotal role in modulating boundary layer processes and, consequently, the regional-scale dispersion of air pollutants. The Weather Research and Forecasting (WRF) modeling system has been used to analyze the regional-scale boundary layer features over northern India. Two cases, 9-11 December 2004 and 20-22 May 2005, representing the winter and summer seasons, respectively, are chosen for the simulations. The model results have been compared with the observations from the India Meteorological Department (IMD) and Wyoming Weather Web data archive over three cities: Delhi, Ahmedabad, and Jodhpur. The simulations show that the thermal stratifications and the associated wind patterns are very well supported by land surface characteristics over the region. The results signify that the underlying land surface along with the prevailing hemispheric-scale meteorological processes (synoptic conditions) is the driver of the simulated patterns. The study implies that thermally-driven regional circulations play a major role in the transport of particulate matter from the Thar Desert to Delhi and its neighboring regions during summer.
Pandya, R.E., D.R. Smith, D.J. Charlevoix, S.Q. Foster, R. Hart, M.J. Hayes, M. McGuirk, S.T. Murillo, K.A. Murphy, D.M. Stanitski, and T.M. Whittaker. A summary of the 17th AMS Education Symposium. Bulletin of the American Meteorological Society, 90(10):1545-1548, https://doi.org/10.1175/2009BAMS2862.1 2009
Pandya, R.E., D.R. Smith, D.J. Charlevoix, W. Hart, M.J. Hayes, S.T. Murillo, K.A. Murphy, D.M. Stanitski, and T.M. Whittaker. A summary of the 16th Symposium on Education. Bulletin of the American Meteorological Society, 90(6):861-865, https://doi.org/10.1175/2009BAMS2510.1 2009
Powell, M.D., E.W. Uhlhorn, and J.D. Kepert. Estimating maximum surface winds from hurricane reconnaissance measurements. Weather and Forecasting, 24(3):868-883, https://doi.org/10.1175/2008WAF2007087.1 2009
Radial profiles of surface winds measured by the Stepped Frequency Microwave Radiometer (SFMR) are compared to radial profiles of flight-level winds to determine the slant ratio of the maximum surface wind speed to the maximum flight-level wind speed, for flight altitude ranges of 2-4 km. The radius of maximum surface wind is found on average to be 0.875 of the radius of the maximum flight-level wind, and very few cases have a surface wind maximum at greater radius than the flight-level maximum. The mean slant reduction factor is 0.84 with a standard deviation of 0.09 and varies with storm-relative azimuth from a maximum of 0.89 on the left side of the storm to a minimum of 0.79 on the right side. Larger slant reduction factors are found in small storms with large values of inertial stability and small values of relative angular momentum at the flight-level radius of maximum wind, which is consistent with Keperts recent boundary layer theories. The global positioning system (GPS) dropwindsonde-based reduction factors that are assessed using this new dataset have a high bias and substantially larger RMS errors than the new technique. A new regression model for the slant reduction factor based upon SFMR data is presented, and used to make retrospective estimates of maximum surface wind speeds for significant Atlantic basin storms, including Hurricanes Allen (1980), Gilbert (1988), Hugo (1989), Andrew (1992), and Mitch (1998).
Reasor, P.D., M.D. Eastin, and J.F. Gamache. Rapidly intensifying Hurricane Guillermo (1997), Part I: Low-wavenumber structure and evolution. Monthly Weather Review, 137(2):603-631, https://doi.org/10.1175/2008MWR2487.1 2009
The structure and evolution of rapidly intensifying Hurricane Guillermo (1997) is examined using airborne Doppler radar observations. In this first part, the low-azimuthal-wavenumber component of the vortex is presented. Guillermo's intensification occurred in an environmental flow with 7-8 m s-1 of deep-layer vertical shear. As a consequence of the persistent vertical shear forcing of the vortex, convection was observed primarily in the downshear left quadrant of the storm. The greatest intensification during the ~6-h Doppler observation period coincided with the formation and cyclonic rotation of several particularly strong convective bursts through the left-of-shear semicircle of the eyewall. Some of the strongest convective bursts were triggered by azimuthally propagating low-wavenumber vorticity asymmetries. Mesoscale budget analyses of axisymmetric angular momentum and relative vorticity within the eyewall are presented to elucidate the mechanisms contributing to Guillermo's structural evolution during this period. The observations support a developing conceptual model of the rapidly intensifying, vertically sheared hurricane in which shear-forced mesoscale ascent in the downshear eyewall is modulated by internally generated vorticity asymmetries yielding episodes of anomalous intensification.
Rogers, R.F., F.D. Marks, and T. Marchok. Tropical cyclone rainfall. In Encyclopedia of Hydrological Sciences, M.G. Anderson (ed.). John Wiley and Sons, Chicester, UK, https://doi.org/10.1002/0470848944.hsa030 2009
A brief survey of the relevant research of tropical cyclone (TC) rainfall is presented here. The importance of TC rainfall in global and regional rainfall budgets is discussed, as is its mean characteristics as derived from airborne and satellite observational studies. Discussion is also presented on the physical processes that can modulate TC rainfall distributions, including topography, storm motion, vertical shear, and extratropical transition. Some tools that have been developed to predict and evaluate forecasts of TC rainfall are discussed. Finally, a summary and outlook for the future is presented, including a discussion of opportunities for improving TC rainfall forecasts and conducting research into the role of TC rainfall in intensity and structure changes in TCs.
Smith, R.K., M.T. Montgomery, and V.S. Nguyen. Tropical cyclone spin-up revisited. Quarterly Journal of the Royal Meteorological Society, 135(642):1321-1335, https://doi.org/10.1002/qj.428 2009
We present numerical experiments to investigate axisymmetric interpretations of tropical cyclone spin-up in a three-dimensional model. Two mechanisms are identified for the spin-up of the mean tangential circulation. The first involves the convergence of absolute angular momentum above the boundary layer and is a mechanism to spin up the outer circulation, i.e., to increase the vortex size. The second involves the convergence of absolute angular momentum within the boundary layer and is a mechanism to spin up the inner core. It is associated with the development of supergradient wind speeds in the boundary layer. The existence of these two mechanisms provides a plausible physical explanation for certain long-standing observations of typhoons by Weatherford and Gray, which indicate that inner-core changes in the azimuthal-mean tangential wind speed often occur independently from those in the outer core. The unbalanced dynamics in the inner-core region are important in determining the maximum radial and tangential flow speeds that can be attained and are, therefore, important in determining the azimuthal-mean intensity of the vortex. We illustrate the importance of unbalanced flow in the boundary layer with a simple thought experiment. The analyses and interpretations presented are novel and support a recent hypothesis of the boundary layer in the inner-core region.
Vickery, P.J., D. Wadhera, M.D. Powell, and Y. Chen. A hurricane boundary layer and wind field model for use in engineering applications. Journal of Applied Meteorology and Climatology, 48(2):381-405, https://doi.org/10.1175/2008JAMC1841.1 2009
This article examines the radial dependence of the height of the maximum wind speed in a hurricane, which is found to lower with increasing inertial stability (which in turn depends on increasing wind speed and decreasing radius) near the eyewall. The leveling off, or limiting value, of the marine drag coefficient in high winds is also examined. The drag coefficient, given similar wind speeds, is smaller for smaller-radii storms; enhanced sea spray by short or breaking waves is speculated as a cause. A fitting technique of dropsonde wind profiles is used to model the shape of the vertical profile of mean horizontal wind speeds in the hurricane boundary layer, using only the magnitude and radius of the gradient wind. The method slightly underestimates the surface winds in small but intense storms, but errors are less than 5% near the surface. The fit is then applied to a slab layer hurricane wind field model, and combined with a boundary layer transition model to estimate surface winds over both marine and land surfaces.
Vickery, P.J., F.J. Masters, M.D. Powell, and D. Wadhera. Hurricane hazard modeling: The past, present, and future. Journal of Wind Engineering and Industrial Aerodynamics, 97(7-8):392-405, https://doi.org/10.1016/j.jweia.2009.05.005 2009
Hurricane hazard modeling has become a commonly used tool for assessing hurricane risk. The type of hurricane risk considered varies with the user and can be an economic risk, as in the case of the insurance and banking industries, a wind exceedance risk, a flood risk, etc. The most common uses for hurricane hazard models today include: (i) Simulation of wind speed and direction for use with wind tunnel test data to estimate wind loads versus return period for design of structural systems and cladding. (ii) Estimation of design wind speeds for use in buildings codes and standards. (iii) Coastal hazard risk modeling (e.g., storm surge elevations and wave heights vs. return period). (iv) Insurance loss estimation (e.g., probable maximum losses, average annual losses). This paper presents an overview of the past and present work in hurricane modeling. The wind model is the key input to each of the examples presented above and is the focus herein. We discuss the evolution and current state of wind field modeling, modeling uncertainties, and possible future directions of the hurricane risk modeling process.
Wang, Z., M.T. Montgomery, and T.J. Dunkerton. A dynamically-based method for forecasting tropical cyclogenesis location in the Atlantic sector using global model products. Geophysical Research Letters, 36(2):L03801, 7 pp., https://doi.org/10.1029/2008GL035586 2009
A real-time forecast method is developed for prediction of the tropical cyclogenesis location over the Atlantic using global model operational products. The method is based on the marsupial theory for tropical cyclogenesis proposed in a recent observational study. A moisture front is usually found ahead of the precursor wave trough, which separates the relatively dry air outside of the wave pouch (a region of closed circulation) from the relatively moist air inside the wave pouch. The propagation speed of the pouch can be determined by tracking the propagation of this moisture front, and the intersection of the critical surface and the trough axis pinpoints the predicted genesis location. Using the global model operational products, the genesis location can be predicted up to three days in advance with an error less than 200 km, which can provide useful guidance for forecasters and flight planning.
Wright, C.W., E.J. Walsh, W.B. Krabill, W.A. Shaffer, S.R. Baig, M. Peng, L.J. Pietrafesa, A.W. Garcia, F.D. Marks, P.G. Black, J. Sonntag, and B.D. Beckley. Measuring storm surge with an airborne wide-swath radar altimeter. Journal of Atmospheric and Oceanic Technology, 26(10):2200-2215, https://doi.org/10.1175/2009JTECHO627.1 2009
Over the years, hurricane track forecasts and storm surge models, as well the digital terrain and bathymetry data they depend on, have improved significantly. Strides have also been made in the knowledge of the detailed variation of the surface wind field driving the surge. The area of least improvement has been in obtaining data on the temporal/spatial evolution of the mound of water that the hurricane wind and waves push against the shore to evaluate the performance of the numerical models. Tide gauges in the vicinity of the landfall are frequently destroyed by the surge. Survey crews dispatched after the event provide no temporal information and only indirect indications of the maximum water level over land. The landfall of Hurricane Bonnie on 26 August 1998, with a surge less than 2 m, provided an excellent opportunity to demonstrate the potential benefits of direct airborne measurement of the temporal/spatial evolution of the water level over a large area. Despite a 160-m variation in aircraft altitude, an 11.5-m variation in the elevation of the mean sea surface relative to the ellipsoid over the flight track, and the tidal variation over the 5-h data acquisition interval, a survey-quality global positioning system (GPS) aircraft trajectory allowed the NASA scanning radar altimeter carried by a NOAA hurricane research aircraft to demonstrate that an airborne wide-swath radar altimeter could produce targeted measurements of storm surge that would provide an absolute standard for assessing the accuracy of numerical storm surge models.
Wu, C.-C., J.-H. Chen, S.J. Majumdar, M.S. Peng, C.A. Reynolds, S.D. Aberson, R. Buizza, M. Yamaguchi, S.-G. Chen, T. Nakazawa, and K.-H. Chou. Intercomparison of targeted observation guidance for tropical cyclones in the northwestern Pacific. Monthly Weather Review, 137(8):2471-2492, https://doi.org/10.1175/2009MWR2762.1 2009
This study compares six different guidance products for targeted observations over the northwest Pacific Ocean for 84 cases of 2-day forecasts in 2006 and highlights the unique dynamical features affecting the tropical cyclone (TC) tracks in this basin. The six products include three types of guidance based on total-energy singular vectors (TESVs) from different global models, the ensemble transform Kalman filter (ETKF) based on a multimodel ensemble, the deep-layer mean (DLM) wind variance, and the adjoint-derived sensitivity steering vector (ADSSV). The similarities among the six products are evaluated using two objective statistical techniques to show the diversity of the sensitivity regions in large, synoptic-scale domains and in smaller domains local to the TC. It is shown that the three TESVs are relatively similar to one another in both the large and the small domains while the comparisons of the DLM wind variance with other methods show rather low similarities. The ETKF and the ADSSV usually show high similarity because their optimal sensitivity usually lies close to the TC. The ADSSV, relative to the ETKF, reveals more similar sensitivity patterns to those associated with TESVs. Three special cases are also selected to highlight the similarities and differences among the six guidance products and to interpret the dynamical systems affecting the TC motion in the northwestern Pacific. Among the three storms studied, Typhoon Chanchu was associated with the subtropical high, Typhoon Shanshan was associated with the midlatitude trough, and Typhoon Durian was associated with the subtropical jet. The adjoint methods are found to be more capable of capturing the signal of the dynamic system that may affect the TC movement or evolution than are the ensemble methods.
Zeng, H., J.Q. Chambers, R.I. Negron-Juarez, G.C. Hurtt, D.B. Baker, and M.D. Powell. Impacts of tropical cyclones on U.S. forest tree mortality and carbon flux from 1851 to 2000. Proceedings of the National Academy of Sciences, 106(19):7888-7892, https://doi.org/10.1073/pnas.0808914106 2009
Tropical cyclones cause extensive tree mortality and damage to forested ecosystems. A number of patterns in tropical cyclone frequency and intensity have been identified. There exist, however, few studies on the dynamic impacts of historical tropical cyclones at a continental scale. Here, we synthesized field measurements, satellite image analyses, and empirical models to evaluate forest and carbon cycle impacts for historical tropical cyclones from 1851 to 2000 over the continental U.S. Results demonstrated an average of 97 million trees affected each year over the entire United States, with a 53-Tg annual biomass loss, and an average carbon release of 25 Tg y-1. Over the period 1980-1990, released CO2 potentially offset the carbon sink in forest trees by 9-18% over the entire United States. U.S. forests also experienced twice the impact before 1900 than after 1900 because of more active tropical cyclones and a larger extent of forested areas. Forest impacts were primarily located in Gulf Coast areas, particularly southern Texas and Louisiana and south Florida, while significant impacts also occurred in eastern North Carolina. Results serve as an important baseline for evaluating how potential future changes in hurricane frequency and intensity will impact forest tree mortality and carbon balance.
Zhang, J.A., W.M. Drennan, P.G. Black, and J.R. French. Turbulence structure of the hurricane boundary layer between the outer rain bands. Journal of the Atmospheric Sciences, 66(8):2455-2467, https://doi.org/10.1175/2009JAS2954.1 2009
As part of the Coupled Boundary Layers Air-Sea Transfer (CBLAST)-Hurricane program, flights were conducted to directly measure turbulent fluxes and turbulence properties in the high-wind boundary layer of hurricanes between the outer rainbands. For the first time, vertical profiles of normalized momentum fluxes, sensible heat and humidity fluxes, and variances of three-dimensional wind velocities and specific humidity are presented for the hurricane boundary layer with surface wind speeds ranging from 20 to 30 m s-1. The turbulent kinetic energy budget is estimated, indicating that the shear production and dissipation are the major source and sink terms, respectively. The imbalance in the turbulent kinetic energy budget indicates that the unmeasured terms, such as horizontal advection, may be important in hurricane boundary layer structure and dynamics. Finally, the thermodynamic boundary layer height, estimated based on the virtual potential temperature profiles, is roughly half of the boundary layer height estimated from the momentum flux profiles. The latter height where momentum and humidity fluxes tend to vanish is close to that of the inflow layer and also of the maximum in the tangential velocity profiles.
Zipser, E.J., C.H. Twohy, S.-C. Tsay, K.L. Thornhill, S. Tanelli, R. Ross, T.N. Krishnamurti, Q. Ji, G. Jenkins, S. Ismail, N.C. Hsu, R. Hood, G.M. Heymsfield, A. Heymsfield, J. Halverson, H.M. Goodman, R. Ferrare, J.P. Dunion, M. Douglas, R. Cifelli, G. Chen, E.V. Browell, and B. Anderson. The Saharan air layer and the fate of African easterly waves: NASA's AMMA field study of tropical cyclogenesis. Bulletin of the American Meteorological Society, 90(8):1137-1156, https://doi.org/10.1175/2009BAMS2728.1 2009
In 2006, NASA led a field campaign to investigate the factors that control the fate of African easterly waves (AEWs) moving westward into the tropical Atlantic Ocean. Aircraft and surface-based equipment were based on Cape Verdes islands, helping to fill some of the data void between Africa and the Caribbean. Taking advantage of the international African Monsoon Multidisciplinary Analysis (AMMA) program over the continent, the NASA-AMMA (NAMMA) program used enhanced upstream data, whereas NOAA aircraft farther west in the Atlantic studied several of the storms downstream. Seven AEWs were studied during AMMA, with at least two becoming tropical cyclones. Some of the waves that did not develop while being sampled near Cape Verde likely intensified in the central Atlantic instead. NAMMA observations were able to distinguish between the large-scale wave structure and the smaller-scale vorticity maxima that often form within the waves. A special complication of the east Atlantic environment is the Saharan air layer (SAL), which frequently accompanies the AEWs and may introduce dry air and heavy aerosol loading into the convective storm systems in the AEWs. One of the main achievements of NAMMA was the acquisition of a database of remote sensing and in-situ observations of the properties of the SAL, enabling dynamic models and satellite retrieval algorithms to be evaluated against high-quality real data. Ongoing research with this database will help determine how the SAL influences cloud micro-physics and perhaps also tropical cyclogenesis, as well as the more general question of recognizing the properties of small-scale vorticity maxima within tropical waves that are more likely to become tropical cyclones.
2008
Aberson, S.D. An alternative tropical cyclone intensity forecast verification technique. Weather and Forecasting, 23(6):1304-1310, https://doi.org/10.1175/2008WAF2222123.1 2008
The National Hurricane Center (NHC) does not verify official or model forecasts if those forecasts call for a tropical cyclone to dissipate or if the real tropical cyclone dissipates. A new technique in which these forecasts are included in a contingency table with all other forecasts is presented. Skill scores and probabilities are calculated. Forecast verifications with the currently used technique have shown a slight improvement in intensity forecasts. The new technique, taking into account all forecasts, suggests that the probability of a forecast having a large (>30 kt) error is decreasing, and the likelihood of the error being less than about 10 kt is increasing in time, at all forecast lead times except 12 h when the forecasts are already quite good.
Aberson, S.D. Large forecast degradations due to synoptic surveillance during the 2004 and 2005 hurricane seasons. Monthly Weather Review, 136(8):3138-3150, https://doi.org/10.1175/2007MWR2192.1 2008
Though operational tropical cyclone synoptic surveillance generally leads to smaller track forecast errors in the National Oceanic and Atmospheric Administration Global Forecasting System (GFS) than would occur otherwise, not every case is improved. Very large GFS forecast degradations due to surveillance are investigated. Small perturbations to model initial conditions may have a large impact locally or downstream in a short time. In these cases, the perturbations are due either to erroneous data assimilated into the models or to issues with the complex data assimilation system itself, and may have caused the forecast degradations. Investigation of forecast and observing system failures can lead to procedural changes that may eliminate some causes of future large forecast errors.
Bell, G.D., E. Blake, C.W. Landsea, S.B. Goldenberg, R. Pasch, and T. Kimberlain. The tropics: Atlantic basin. In State of the Climate in 2007, D.H. Levinson and J.H. Lawrimore (eds.). Bulletin of the American Meteorological Society, 89(7):S68-S71, https://doi.org/10.1175/BAMS-89-7-StateoftheClimate 2008
Bell, M.M., and M.T. Montgomery. Observed structure, evolution, and potential intensity of category 5 Hurricane Isabel (2003) from 12-14 September. Monthly Weather Review, 136(6):2023-2046, https://doi.org/10.1175/2007MWR1858.1 2008
Unprecedented observations of Hurricane Isabel (2003) at category 5 intensity were collected from 12 to 14 September. This study presents a detailed analysis of the inner-core structure, atmospheric boundary layer, sea surface temperature, and outflow layer of a superintense tropical cyclone using high-resolution in situ flight-level, NCAR GPS dropwindsonde, Doppler radar, and satellite measurements. The analysis of the dropwindsonde and in situ data includes a comprehensive discussion of the uncertainties associated with this observational dataset and provides an estimate of the storm-relative axisymmetric inner-core structure using Barnes objective analysis. An assessment of gradient and thermal wind balance in the inner core is also presented. The axisymmetric data composites presented in this study suggest that Isabel built a reservoir of high moist entropy air by sea-to-air latent heat flux inside the low-level eye that was utilized as an additional energy source to nearly maintain its extreme intensity even after crossing the cool wake of Hurricane Fabian. It is argued here that the combined mean and asymmetric eddy flux of high moist entropy air from the low-level eye into the eyewall represents an additional power source or "turbo boost" to the hurricane heat engine. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound for the given environmental conditions on all three days. This discrepancy between a priori potential intensity theory and observations may be as high as 35 m s-1 on 13 September.
DeMaria, M., J.D. Hawkins, J.P. Dunion, and D.K. Smith. Tropical intensity forecasting using a satellite-based total precipitable water product. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 5 pp., 2008
Dunion, J.P., and C.S. Marron. A reexamination of the Jordan mean tropical sounding based on awareness of the Saharan air layer: Results from 2002. Journal of Climate, 21(20):5242-5253, https://doi.org/10.1175/2008JCLI1868.1 2008
The Jordan mean tropical sounding has provided a benchmark for representing the climatology of the tropical North Atlantic and Caribbean Sea since 1958. However, recent studies of the Saharan air layer (SAL) have suggested that the tropical atmosphere in these oceanic regions may contain two distinct soundings (SAL and non-SAL) with differing thermodynamic and kinematic structures and that a single mean sounding like Jordans does not effectively represent these differences. This work addresses this possibility by examining over 750 rawinsondes from the tropical North Atlantic Ocean and Caribbean Sea during the 2002 hurricane season. It was found that a two-peak bimodal moisture distribution (dry SAL and moist non-SAL) exists in this region and that the Jordan sounding does not represent either distribution particularly well. Additionally, SAL soundings exhibited higher values of geopotential height, unique temperature profiles, and stronger winds (with an enhanced easterly component) compared to the moist tropical non-SAL soundings. The results of this work suggest that the Jordan mean tropical sounding may need to be updated to provide a more robust depiction of the thermodynamics and kinematics that exist in the tropical North Atlantic Ocean and Caribbean Sea during the hurricane season.
Giammanco, I.M., J.L. Schroeder, M.D. Powell, and D.A. Smith. GPS dropwindsonde observations of tropical cyclone low-level wind maxima. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 8 pp., 2008
Halliwell, G.R., L.K. Shay, S.D. Jacob, O.M. Smedstad, and E.W. Uhlhorn. Improving ocean model initialization for coupled tropical cyclone forecast models using GODAE nowcasts. Monthly Weather Review, 136(7):2576-2591, https://doi.org/10.1175/2007MWR2154.1 2008
To simulate tropical cyclone (TC) intensification, coupled ocean-atmosphere prediction models must realistically reproduce the magnitude and pattern of storm-forced sea surface temperature (SST) cooling. The potential for the ocean to support intensification depends on the thermal energy available to the storm, which in turn depends on both the temperature and thickness of the upper-ocean warm layer. The ocean heat content (OHC) is used as an index of this potential. Large differences in available thermal energy associated with energetic boundary currents and ocean eddies require their accurate initialization in ocean models. Two generations of the experimental U.S. Navy ocean nowcast-forecast system based on the Hybrid Coordinate Ocean Model (HYCOM) are evaluated for this purpose in the northwest Caribbean Sea and Gulf of Mexico prior to Hurricanes Isidore and Lili (2002), Ivan (2004), and Katrina (2005). Evaluations are conducted by comparison to in situ measurements, the Navy's three-dimensional Modular Ocean Data Assimilation System (MODAS) temperature and salinity analyses, microwave satellite SST, and fields of OHC and 26°C isotherm depth derived from satellite altimetry. Both nowcast-forecast systems represent the position of important oceanographic features with reasonable accuracy. Initial fields provided by the first-generation product had a large upper-ocean cold bias because the nowcast was initialized from a biased older-model run. SST response in a free-running Isidore simulation is improved by using initial and boundary fields with reduced cold bias generated from a HYCOM nowcast that relaxed model fields to MODAS analyses. A new climatological initialization procedure used for the second-generation nowcast system tended to reduce the cold bias, but the nowcast still could not adequately reproduce anomalously warm conditions present before all storms within the first few months following nowcast initialization. The initial cold biases in both nowcast products tended to decrease with time. A realistic free-running HYCOM simulation of the ocean response to Ivan illustrates the critical importance of correctly initializing both warm-core rings and cold-core eddies to correctly simulate the magnitude and pattern of SST cooling.
Jones, L., P.G. Black, S.S. Chen, R.E. Hood, J.W. Johnson, C.S. Ruf, A. Mims, and C.C. Hennon. Next generation airborne Hurricane Imaging Radiometer (HIRAD): Improved forecast skill with wide field imagery. In Tropical Meteorology Special Symposium, New Orleans, LA, January 20-24, 2008. American Meteorological Society, Boston, 8 pp., 2008
Landsea, C.W., D.A. Glenn, W. Bredemeyer, M. Chenoweth, R. Ellis, J.F. Gamache, L. Hufstetler, C.J. Mock, R. Perez, R. Prieto, J. Sanchez-Sesma, D. Thomas, and L. Woolcock. A reanalysis of the 1911-1920 Atlantic hurricane database. Journal of Climate, 21(10):2138-2168, https://doi.org/10.1175/2007JCLI1119.1 2008
A reanalysis of the Atlantic basin tropical storm and hurricane database ("best track") for the period of 1911-20 has been completed. This reassessment of the main archive for tropical cyclones of the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico was necessary to correct systematic biases and random errors in the data as well as to search for previously unrecognized systems. A methodology for the reanalysis process for revising the track and intensity of tropical cyclone data is provided in detail. The dataset now includes several new tropical cyclones, excludes one system previously considered a tropical storm, makes generally large alterations in the intensity estimates of most tropical cyclones (both toward stronger and weaker intensities), and typically adjusts existing tracks with minor corrections. Average errors in intensity and track values are estimated for both open ocean conditions as well as for landfalling systems. Finally, highlights are given for changes to the more significant hurricanes to impact the United States, Central America, and the Caribbean for this decade.
Lorsolo, S., J. Gamache, F. Marks, P. Dodge, and J.A. Zhang. Characterization of hurricane turbulence using airborne Doppler measurements. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 4 pp., 2008
Lorsolo, S., J.L. Schroeder, P.P. Dodge, and F.D. Marks. An observational study of hurricane boundary layer small-scale coherent structures. Monthly Weather Review, 136(8):2871-2893, https://doi.org/10.1175/2008MWR2273.1 2008
Data with high temporal and spatial resolution from Hurricanes Isabel (2003) and Frances (2004) were analyzed to provide a detailed study of near-surface linear structures with subkilometer wavelengths of the hurricane boundary layer (HBL). The analysis showed that the features were omnipresent throughout the data collection, displayed a horizontal and vertical coherency, and maintained an average orientation of 7° left of the low-level wind. A unique objective wavelength analysis was conducted, where wavelength was defined as the distance between two wind maxima or minima perpendicular to the features' long axis, and revealed that although wavelengths as large as 1400 m were observed, the majority of the features had wavelengths between 200 and 650 m. The assessed wavelengths differ from those documented in a recent observational study. To evaluate the correlation between the features and the underlying near-surface wind field, time and spectral analyses were completed and ground-relative frequency distributions of the features were retrieved. High-energy regions of the power spectral density (PSD) determined from near-surface data were collocated with the features' ground-relative frequency, illustrating that the features have an influence on the near-surface wind field. The additional energy found in the low-frequency range of the PSDs was previously identified as characteristic of the hurricane surface flow, suggesting that the features are an integral component of the HBL flow.
Lowag, A., M.L. Black, and M.D. Eastin. External and internal influences on structural and intensity changes of Hurricane Bret (1999), Part I: Atmospheric and oceanic influences. Monthly Weather Review, 136(11):4320-4333, https://doi.org/10.1175/2008MWR2438.1 2008
Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC 21 August and 1200 UTC 22 August 1999 before it made landfall on the Texas coast 12 h later. Its minimum sea level pressure fell 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These datasets are combined with gridded data from the National Centers for Environmental Prediction (NCEP) Global Model and the NCEP-National Center for Atmospheric Research (NCAR) reanalyses to document Brets atmospheric and oceanic environment as well as their relation to the observed structural and intensity changes. Brets RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a concurrent decrease in vertical wind shear. SSTs at the beginning of the storms RI were approximately 29°C and steadily increased to 30°C as it moved to the north. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20% higher at the beginning of the RI period than 6 h prior. The subsequent weakening was linked to the cooling of near-coastal shelf waters (to between 25° and 26°C) by prestorm mixing combined with an increase in vertical wind shear. The available observations suggest no intrusion of dry air into the circulation core contributed to the intensity evolution. Sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted to quantitatively describe the influence of environmental conditions on the intensity forecast. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between the four cases were relatively small because of the model design, but the greatest intensity changes resulted for much cooler prescribed SSTs. The results of this study underscore the importance of OHC and vertical wind shear as significant factors during RIs; however, internal dynamical processes appear to play a more critical role when a favorable environment is present.
Marks, F.D., P.G. Black, M.T. Montgomery, and R.W. Burpee. Structure of the eye and eyewall of Hurricane Hugo (1989). Monthly Weather Review, 136(4):1237-1259, https://doi.org/10.1175/2007MWR2073.1 2008
On 15 September 1989, one of NOAA's WP-3D research aircraft, N42RF [lower aircraft (LA)], penetrated the eyewall of Hurricane Hugo. The aircraft had an engine fail in severe turbulence while passing the radius of maximum wind and before entering the eye at 450-m altitude. After the aircraft returned to controlled flight within the 7-km radius eye, it gained altitude gradually as it orbited in the eye. Observations taken during this period provide an updated model of the inner-core structure of an intense hurricane and suggest that LA penetrated an intense cyclonic vorticity maximum adjacent to the strongest convection in the eyewall [eyewall vorticity maximum (EVM)]. This EVM was distinct from the vortex-scale cyclonic circulation observed to orbit within the eye three times during the 1 h that LA circled in the eye. At the time, Hugo had been deepening rapidly for 12 h. The maximum flight-level tangential wind was 89 m s-1 at a radius of 12.5 km; however, the primary vortex peak tangential wind, derived from a 100-s filter of the flight-level data, was estimated to be 70 m s-1, also at 12.5-km radius. The primary vortex tangential wind was in approximate gradient wind balance, was characterized by a peak in angular velocity just inside the radius of maximum wind, and had an annular vorticity structure slightly interior to the angular velocity maximum. The EVM along the aircraft's track was roughly 1 km in diameter with a peak cyclonic vorticity of 1.25 x 10-1 s-1. The larger circulation center, with a diameter >15 km, was observed within the eye and exhibited an average orbital period of 19 min. This period is about the same as that of the angular velocity maximum of the axisymmetric mean vortex and is in reasonable agreement with recent theoretical and model predictions of a persistent trochoidal "wobble" of circulation centers in mature hurricane-like vortices. This study is the first with in situ documentation of these vortical entities, which were recently hypothesized to be elements of a lower-tropospheric eye/eyewall mixing mechanism that supports strong storms.
Miller, T.L., R. Atlas, P.G. Black, C.C. Hennon, S.S. Chen, R.E. Hood, J.W. Johnson, L. Jones, C.S. Ruf, and E.W. Uhlhorn. Simulation of the impact of new ocean surface wind measurements on H*Wind analyses. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 7 pp., 2008
Miller, T.L., R.M. Atlas, P.G. Black, J.L. Case, S.S. Chen, R.E. Hood, J.W. Johnson, J. Jones, C.S. Ruf, and E.W. Uhlhorn. Simulation of the impact of new aircraft and satellite-based ocean surface wind measurements on H*Wind analyses. Proceedings, 12th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface (IOAS-AOLS), New Orleans, LA, January 20-24, 2008. American Meteorological Society, Boston, 7 pp., 2008
Moore, R.W., M.T. Montgomery, and H.C. Davies. The integral role of a diabatic Rossby vortex in a heavy snowfall event. Monthly Weather Review, 136(6):1878-1897, https://doi.org/10.1175/2007MWR2257.1 2008
On 24-25 February 2005, a significant east coast cyclone deposited from 4 to nearly 12 in. (~10-30 cm) of snow on parts of the northeastern United States. The heaviest snowfall and most rapid deepening of the cyclone coincided with the favorable positioning of an upper-level, short-wave trough immediately upstream of a preexisting surface cyclone. The surface cyclone in question formed approximately 15 h before the heaviest snowfall along a coastal front in a region of frontogenesis and heavy precipitation. The incipient surface cyclone subsequently intensified as it moved to the northeast, consistently generating the strongest convection to the east-northeast of the low-level circulation center. The use of potential vorticity (PV) inversion techniques and a suite of mesoscale model simulations illustrates that the early intensification of the incipient surface cyclone was primarily driven by diabatic effects and was not critically dependent on the upper-level wave. These facts, taken in conjunction with the observed structure, energetics, and Lagrangian evolution of the incipient surface disturbance, identify it as a diabatic Rossby vortex (DRV). The antecedent surface vorticity spinup associated with the DRV phase of development is found to be integral to the subsequent rapid growth. The qualitative similarity with a number of observed cases of explosive cyclogenesis leaves open the possibility that a DRV-like feature comprises the preexisting positive low-level PV anomaly in a number of cyclogenetic events that exhibit a two-stage evolution.
Murillo, S.T. Determination of the circulation center and inner core evolution of Hurricane Danny (1997) using the GBVTD-simplex algorithm. M.S. thesis, University of Hawaii at Manoa, 57 pp., 2008
The evolution and structure of Hurricane Danny (1997) is examined via a single-Doppler radar tropical cyclone (TC) wind retrieval technique, the ground-based velocity track display (GBVTD) algorithm. The GBVTD technique is applied to 5-hours of data gathered simultaneously by two WSR-88D radars in Mobile, AL (KMOB) and Slidell, LA (KLIX) at ~6 minute intervals. The circulation centers and the primary circulations of Danny derived from these two independent radar observations (~60 volumes from each radar) are used to evaluate the accuracy of the GBVTD algorithm and the GBVTD-simplex center finding algorithm. Other observations, such as dual-Doppler analysis, dropwindsonde, and in-situ measurements from the NOAA WP-3D and the U.S. Air Force Reserve Command WC-130 (AFRC), aircraft provide independent assessments of the TC center and structure. It is found that the GBVTD-simplex derived centers, which used only the maximum mean tangential wind as the sole criterion, were unsatisfactory and unstable. An improved algorithm is proposed to seek time continuity in RMW, maximum mean tangential wind, and the center position in order to reduce the large fluctuations experienced in this study and the results are used to quantify the accuracy of the derived circulation centers. The quality of the GBVTD-derived circulation from these new centers is assessed. Danny's kinematic structure retrieved from KLIX and KMOB data using the improved sets of centers are consistent with the structures retrieved from the dual-Doppler analyses. Danny evolved from a mostly axisymmetric TC into a wave number one asymmetric TC then returned to an axisymmetric TC during this five-hour period.
Murillo, S.T., R.E. Pandya, R.Y. Chu, J.A. Winkler, R. Czujko, and E.M.C. Cutrim. AMS membership survey results: An overview and longitudinal analysis of the demographics of the American Meteorological Society. Bulletin of the American Meteorological Society, 89(5):727-733, https://doi.org/10.1175/BAMS-89-5-727 2008
The 2005 membership survey is the fifth in a series of surveys that has monitored the composition of the AMS since 1975. The responses of the 2005 survey reveal several interesting changes in the educational level, employment characteristics, and personal status of Society members. The proportion of members with Ph.D. degrees has increased with time to 46% of the regular (nonstudent) and retired members in 2005. Universities/colleges, the federal government, and radio/TV remain the three most important employers of AMS members, although their relative importance has changed with time, with universities/colleges now employing more members than the federal government. Most AMS members continue to report that they became interested in the atmospheric sciences in either elementary school or as undergraduates, although the importance of early (K-6) experiences has increased with time. The age distribution of AMS members in 2005 suggests that the gradual aging of the AMS membership reported earlier (based on the responses to the 1993 and 1999 surveys) is no longer evident. The 2005 survey results also suggest that the percentage of women in the AMS, although still small, has nearly doubled since 1999. However, there has not been comparable progress in increasing the ethnic diversity of the AMS membership. This paper is the first of a series, each focusing on a particular aspect of the survey results.
Murillo, S.T., W.-C. Lee, G.M. Barnes, M.M. Bell, and F.D. Marks. Determination of the circulation center and inner core evolution of Hurricane Danny (1997) using the GBVTD-simplex algorithm. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, 5 pp., 2008
Musgrave, K.D., C.A. Davis, and M.T. Montgomery. Numerical simulations of the formation of Hurricane Gabrielle (2001). Monthly Weather Review, 136(8):3151-3167, https://doi.org/10.1175/2007MWR2110.1 2008
This study examines the formation of Hurricane Gabrielle (2001), focusing on whether an initial disturbance and vertical wind shear were favorable for development. This examination is performed by running numerical experiments using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). Gabrielle is chosen as an interesting case to study since it formed in the subtropics only a few days before making landfall in Florida. Three simulations are run: a control run and two sensitivity experiments. The control run is compared with observations to establish the closeness of the model output to Gabrielle's observed formation. The two sensitivity experiments are designed to test the response of the developing tropical cyclone to alterations in the initial conditions. The first sensitivity experiment removes the initial (or precursor) disturbance, a midtropospheric vortex located over Florida. The second sensitivity experiment reduces the vertical wind shear over the area of formation. The control run produces a system comparable to Gabrielle. The convection in the control run is consistently located downshear of the center of circulation. In the first sensitivity experiment, with the removal of the initial disturbance, no organized system develops. This indicates the importance of the midtropospheric vortex in Gabrielle's formation. The second sensitivity experiment, which reduces the vertical wind shear over the area of Gabrielle's formation, produces a system that can be identified as Gabrielle. This system, however, is weaker than both the control run and the observations of Gabrielle. This study provides direct evidence of a favorable influence of modest vertical wind shear on the formation of a tropical cyclone in this case.
Nguyen, V.S., R.K. Smith, and M.T. Montgomery. Tropical cyclone intensification and predictability in three dimensions. Quarterly Journal of the Royal Meteorological Society, 134(632):563-582, https://doi.org/10.1002/qj.235 2008
We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundary-layer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximum-wind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes. Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a β-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind evaporation feedback process for tropical-cyclone intensification is offered also. The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and β-gyres, are found to be less variable than their mesoscale counterparts.
Persing, J., and M.T. Montgomery. Isolating surface flux influences on simulated hurricane intensity. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 3 pp., 2008
Powell, M.D., and T.A. Reinhold. Reply to Hsu and Blanchards comments on "Tropical cyclone destructive potential by integrated kinetic energy." Bulletin of the American Meteorological Society, 89(10):1577, https://doi.org/10.1175/2008BAMS2688.1 2008
Powell, M.D., and T.A. Reinhold. Reply. Bulletin of the American Meteorological Society, 89(2):221-223, https://doi.org/10.1175/BAMS-89-2-221 2008
Pu, Z., X. Li, C.S. Velden, S.D. Aberson, and W.T. Liu. The impact of aircraft dropsonde and satellite wind data on numerical simulations of two landfalling tropical storms during Tropical Cloud Systems and Processes Experiment. Weather and Forecasting, 23(1):62-79, https://doi.org/10.1175/2007WAF2007006.1 2008
Dropwindsonde, Geostationary Operational Environmental Satellite-11 (GOES-11) rapid-scan atmospheric motion vectors, and NASA Quick Scatterometer (QuikSCAT) near-surface wind data collected during NASA's Tropical Cloud Systems and Processes (TCSP) field experiment in July 2005 were assimilated into an advanced research version of the Weather Research and Forecasting (WRF) model using its three-dimensional variational data assimilation (3DVAR) system. The impacts of the mesoscale data assimilation on WRF numerical simulation of Tropical Storms Cindy and Gert (2005) near landfall are examined. Sensitivity of the forecasts to the assimilation of each single data type is investigated. Specifically, different 3DVAR strategies with different analysis update cycles and resolutions are compared in order to identify the better methodology for assimilating the data from research aircraft and satellite for tropical cyclone study. The results presented herein indicate the following. (1) Assimilation of dropwindsonde and satellite wind data into the WRF model improves the forecasts of the two tropical storms up to the landfall time. The QuikSCAT wind information is very important for improving the storm track forecast, whereas the dropwindsonde and GOES-11 wind data are also necessary for improved forecasts of intensity and precipitation. (2) Data assimilation also improves the quantitative precipitation forecasts (QPFs) near landfall of the tropical storms. (3) A 1-h rapid-update analysis cycle at high resolution (9 km) provides more accurate tropical cyclone forecasts than a regular 6-h analysis cycle at coarse (27 km) resolution. The high-resolution rapidly updated 3DVAR analysis cycle might be a practical way to assimilate the data collected from tropical cyclone field experiments.
Rogers, R.F., and E.W. Uhlhorn. Observations of the structure and evolution of surface and flight-level wind asymmetries in Hurricane Rita (2005). Geophysical Research Letters, 35(21):L22811, 6 pp., https://doi.org/10.1029/2008GL034774 2008
Knowledge of the magnitude and distribution of surface winds, including the structure of azimuthal asymmetries in the wind field, are important factors for tropical cyclone forecasting. With its ability to remotely measure surface wind speeds, the stepped frequency microwave radiometer (SFMR) has assumed a prominent role for the operational tropical cyclone forecasting community. An example of this instruments utility is presented here, where concurrent measurements of aircraft flight-level and SFMR surface winds are used to document the wind field evolution over three days in Hurricane Rita (2005). The amplitude and azimuthal location (phase) of the wavenumber-1 asymmetry in the storm-relative winds varied at both levels over time. The peak was found to the right of storm track at both levels on the first day. By the third day, the peak in flight-level storm-relative winds remained to the right of storm track, but it shifted to left of storm track at the surface, resulting in a 60-degree shift between the surface and flight-level and azimuthal variations in the ratio of surface to flight-level winds. The asymmetric differences between the surface and flight-level maximum wind radii also varied, indicating a vortex whose tilt was increasing.
Sellwood, K.J., S. Majumdar, I. Szunyogh, and B. Mapes. Predicting the influence of observations on medium-range forecasts of atmospheric flow. Quarterly Journal of the Royal Meteorological Society, 134(637):2011-2027, https://doi.org/10.1002/qj.341 2008
In recent years, the Ensemble Transform Kalman Filter (ETKF) has been demonstrated to be useful for identifying a priori dynamically important locations for the placement of supplementary dropwindsonde observations aimed at improving short-range (1-3 day) forecasts of high-impact winter weather. In this paper, the ability of this strategy to predict the influence (or signal) of assimilating observations into the NCEP Global Forecast System for forecasts of 200 hPa wind up to 6 days is evaluated. Using a 50-member ECMWF ensemble, the ETKF was found to exhibit significantly higher skill than a seasonal climatology of the important locations in predicting both (1) the spatial structure of signals within the storm track on a case-by-case basis; and (2) the variance of these signals over a 2-month period, within objectively chosen verification regions on synoptic scales. The verification region was selected by extracting the zonal envelope of the Rossby wave packet associated with the propagating ETKF signal variance. It is recommended that larger verification regions be used for longer lead times, due to the eastward expansion of the wave packet. The capability of the ETKF to predict signal variance out to 6 days was found to be dependent on the flow regime. The ETKF was most capable when the background flow was predominantly zonal, and least capable in instances where the observations were placed upstream of a blocking high over the north-eastern Pacific. Therefore, the ETKF is sometimes (but not always) able to predict when there exists significant potential for a particular group of observations to improve medium-range forecasts.
Shay, L.K., and E.W. Uhlhorn. Loop Current response to Hurricanes Isidore and Lili. Monthly Weather Review, 136(9):3248-3274, https://doi.org/10.1175/2007MWR2169.1 2008
Recent hurricane activity over the Gulf of Mexico basin has underscored the importance of the Loop Current (LC) and its deep, warm thermal structure on hurricane intensity. During Hurricanes Isidore and Lili in 2002, research flights were conducted from both National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft to observe pre-, in- and poststorm ocean conditions using airborne expendable ocean profilers to measure temperature, salinity, and current structure. Atmospheric thermodynamic and wind profiles and remotely sensed surface winds were concurrently acquired as each storm moved over the LC. Observed upper-ocean cooling was about 1°C as Isidore moved across the Yucatan Straits at a speed of 4 m s-1. Given prestorm ocean heat content (OHC) levels exceeding 100 kJ cm-2 in the LC (current velocities >1 m s-1), significant cooling and deepening of the ocean mixed layer (OML) did not occur in the straits. Estimated surface enthalpy flux at Isidores eyewall was 1.8 kW m-2, where the maximum observed wind was 49 m s-1. Spatially integrating these surface enthalpy fluxes suggested a maximum surface heat loss of 9.5 kJ cm-2 at the eyewall. Over the Yucatan Shelf, observed ocean cooling of 4.5°C was caused by upwelling processes induced by wind stress and an offshore wind-driven transport. During Hurricane Lili, ocean cooling in the LC was ~1°C but more than 2°C in the Gulf Common Water, where the maximum estimated surface enthalpy flux was 1.4 kW m-2, associated with peak surface winds of 51 m s-1. Because of Lili's asymmetric structure and rapid translational speed of 7 m s-1, the maximum surface heat loss resulting from the surface enthalpy flux was less than 5 kJ cm-2. In both hurricanes, the weak ocean thermal response in the LC was primarily due to the lack of energetic near-inertial current shears that develop across the thin OML observed in quiescent regimes. Bulk Richardson numbers remained well above criticality because of the strength of the upper-ocean horizontal pressure gradient that forces northward current and thermal advection of warm water distributed over deep layers. As these oceanic regimes are resistive to shear-induced mixing, hurricanes experience a more sustained surface enthalpy flux compared to storms moving over shallow quiescent mixed layers. Because ocean cooling levels induced by hurricane force winds depend on the underlying oceanic regimes, features must be accurately initialized in coupled forecast models.
Smith, R.K., and M.T. Montgomery. Balanced boundary layers used in hurricane models. Quarterly Journal of the Royal Meteorological Society, 134(635):1385-1395, https://doi.org/10.1002/qj.296 2008
We examine the formulation and accuracy of various approximations made in representing the boundary layer in simple axisymmetric hurricane models, especially those that assume strict gradient wind balance in the radial direction. Approximate solutions for a steady axisymmetric slab boundary-layer model are compared with a full model solution. It is shown that the approximate solutions are generally poor in the inner core region of the vortex, where the radial advection term in the radial momentum equation is important and cannot be neglected. These results affirm some prior work and have implications for a range of theoretical studies of hurricane dynamics, including theories of potential intensity, that employ balanced boundary-layer formulations.
Smith, R.K., M.T. Montgomery, and S. Vogl. A critique of Emanuel's hurricane model and potential intensity theory. Quarterly Journal of the Royal Meteorological Society, 134(632):551-561, https://doi.org/10.1002/qj.241 2008
We present a critique of Emanuel's steady-state hurricane model, which is a precursor to his theory for hurricane potential intensity (PI). We show that a major deficiency of the theory is the tacit assumption of gradient wind balance in the boundary layer, a layer that owes its existence to gradient wind imbalance in the radial momentum equation. If a more complete boundary-layer formulation is included using the gradient wind profiles obtained from Emanuel's theory, the tangential wind speed in the boundary layer becomes supergradient, invalidating the assumption of gradient wind balance. We show that the degree to which the tangential wind is supergradient depends on the assumed boundary-layer depth. The full boundary-layer solutions require a knowledge of the tangential wind profile above the boundary layer in the outer region where there is subsidence into the layer and they depend on the breadth of this profile. This effect is not considered in Emanuel's theory. We argue that a more complete theory for the steady-state hurricane would require the radial pressure gradient above the boundary layer to be prescribed or determined independently of the boundary layer. The issues raised herein highlight a fundamental problem with Emanuel's theory for PI, since that theory makes the same assumptions as in the steady-state hurricane model. Our current findings together with recent studies examining intense hurricanes suggest a way forward towards a more consistent theory for hurricane PI.
Stern, D.P., D.S. Nolan, and S.D. Aberson. Simulations and observations of extreme low-level updrafts in Hurricane Isabel. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 6 pp., 2008
Terwey, W.D., and M.T. Montgomery. Secondary eyewall formation in two idealized, full-physics modeled hurricanes. Journal of Geophysical Research, 113(D12):D12112, 18 pp., https://doi.org/10.1029/2007JD0088979 2008
Prevailing hypotheses for secondary eyewall formation are examined using data sets from two high-resolution mesoscale numerical model simulations of the long-time evolution of an idealized hurricane vortex in a quiescent tropical environment with constant background rotation. The modeled hurricanes each undergo a secondary eyewall cycle, casting doubt on a number of other authors' hypotheses for secondary eyewall formation due to idealizations present in the simulation formulations. A new hypothesis for secondary eyewall formation is proposed here and is shown to be supported by these high-resolution numerical simulations. The hypothesis requires the existence of a region with moderate horizontal strain deformation and a sufficient low-level radial potential vorticity gradient associated with the primary swirling flow, moist convective potential, and a wind-moisture feedback process at the air-sea interface to form the secondary eyewall. The crux of the formation process is the generation of a finite-amplitude lower-tropospheric cyclonic jet outside the primary eyewall with a jet width on the order of a local effective beta scale determined by the mean low-level radial potential vorticity gradient and the root-mean square eddy velocity. This jet is hypothesized to be generated by the anisotropic upscale cascade and axisymmetrization of convectively generated vorticity anomalies through horizontal shear turbulence and sheared vortex Rossby waves as well as by the convergence of system-scale cyclonic vorticity by the low-level radial inflow associated with the increased convection. Possible application to the problem of forecasting secondary eyewall events is briefly considered.
Tory, K.J., and M.T. Montgomery. Tropical cyclone formation: A synopsis of the system-scale development. Extended Abstracts, 28th Conference on Hurricanes and Tropical Meteorology, Orlando, FL, April 28-May 2, 2008. American Meteorological Society, Boston, 6 pp., 2008
Uhlhorn, E.W. Gulf of Mexico Loop Current mechanical energy and vorticity response to a tropical cyclone. Ph.D. thesis, University of Miami, Rosenstiel School of Marine and Atmospheric Science, 148 pp., 2008
The ocean mixed layer response to a tropical cyclone within, and immediately adjacent to, the Gulf of Mexico Loop Current is examined using a combination of ocean profiles and a numerical model. A comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy and circulation evolution in response to Hurricane Lili's (2002) passage. Mixed-layer temperature analyses show that the Loop Current cooled <1°C in response to the storm, in contrast to typically observed larger decreases of 3-5°C. Correspondingly, vertical current shears, which are partly responsible for entrainment mixing, were found to be up to 50% weaker, on average, than observed in previous studies within the directly-forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical water from the cooler, heavier Gulf Common water, was found to decrease in intensity by -0.18 ± 0.25 m s-1 over an approximately 10-day period within the mixed layer. Contrary to previous tropical cyclone ocean response studies which have assumed an approximately horizontally homogeneous ocean structure prior to storm passage, a kinetic energy loss of 5.8 ± 6.3 k Jm-2, or approximately 1 wind stress-scaled energy unit, was observed. Using near-surface currents derived from satellite altimetry data, the Loop Current is found to vary similarly in magnitude, suggesting storm-generated energy is rapidly removed by the pre-existing Loop Current. Further examination of the energy response using an idealized numerical model reveals that due to: (1) favorable coupling between the wind stress and pre-existing current vectors; and (2) wind-driven currents flowing across the large horizontal pressure gradient; wind energy transfer to mixed-layer kinetic energy can be more efficient in these regimes as compared to the case of an initially horizontally homogeneous ocean. However, nearly all of this energy is removed by advection by 2 local inertial periods after storm passage, and little evidence of the storms impact remains. Mixed-layer vorticity within the idealized current also shows a strong direct response, but little evidence of a near-inertial wave wake results.
Westerink, J.J., R.A. Luettich, J.C. Feyen, J.H. Atkinson, C. Dawson, H.J. Roberts, M.D. Powell, J.P. Dunion, E.J. Kubatko, and H. Pourtaheri. A basin- to channel-scale unstructured grid hurricane storm surge model applied to southern Louisiana. Monthly Weather Review, 136(3):833-864, https://doi.org/10.1175/2007MWR1946.1 2008
Southern Louisiana is characterized by low-lying topography and an extensive network of sounds, bays, marshes, lakes, rivers, and inlets that permit widespread inundation during hurricanes. A basin- to channel-scale implementation of the Advanced Circulation (ADCIRC) unstructured grid hydrodynamic model has been developed that accurately simulates hurricane storm surge, tides, and river flow in this complex region. This is accomplished by defining a domain and computational resolution appropriate for the relevant processes, specifying realistic boundary conditions, and implementing accurate, robust, and highly parallel unstructured grid numerical algorithms. The model domain incorporates the western North Atlantic, the Gulf of Mexico, and the Caribbean Sea so that interactions between basins and the shelf are explicitly modeled and the boundary condition specification of tidal and hurricane processes can be readily defined at the deep water open boundary. The unstructured grid enables highly refined resolution of the complex overland region for modeling localized scales of flow while minimizing computational cost. Kinematic data assimilative or validated dynamic-modeled wind fields provide the hurricane wind and pressure field forcing. Wind fields are modified to incorporate directional boundary layer changes due to overland increases in surface roughness, reduction in effective land roughness due to inundation, and sheltering due to forested canopies. Validation of the model is achieved through hindcasts of Hurricanes Betsy and Andrew. A model skill assessment indicates that the computed peak storm surge height has a mean absolute error of 0.30 m.
Zhang, J.A., K.B. Katsaros, P.G. Black, S. Lehner, J.R. French, and W.M. Drennan. Effects of roll vortices on turbulent fluxes in the hurricane boundary layer. Boundary-Layer Meteorology, 128(2):173-189, https://doi.org/10.1007/s10546-008-9281-2 2008
Boundary-layer secondary circulations or "roll vortices" can have a significant influence on the turbulent exchange of momentum, sensible heat and moisture throughout the hurricane boundary layer. In this study, analyses of data from a WP-3D aircraft of the National Oceanic and Atmospheric Administration (NOAA) are presented. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-hurricane experiment sponsored through the Office of Naval Research and NOAA's annual hurricane research program, flights were conducted to investigate energy exchange across the air-sea interface. We present the first in-situ aircraft-based observations of rolls in the hurricane boundary layer and investigate their influence on energy and momentum exchange. The rolls detected in Hurricane Isidore (year 2002) have a characteristic wavelength of about 900 m, in good agreement with analyses of data from a synthetic aperture radar image captured by the Canadian Space Agency's RADARSAT satellite in the same storm. Our analyses of the airborne data suggest that roll vortices may be a significant factor modulating the air-sea momentum exchange.
Zhang, J.A., P.G. Black, J.R. French, and W.M. Drennan. First direct measurements of enthalpy flux in the hurricane boundary layer: The CBLAST results. Geophysical Research Letters, 35(11):L14813, 4 pp., https://doi.org/10.1029/2008GL034374 2008
Hurricanes extract energy from the warm ocean through enthalpy fluxes. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST) experiment, flights were conducted to measure turbulent fluxes in the high-wind boundary layer of hurricanes. Here we present the first field observations of sensible heat and enthalpy flux for 10m wind speeds to 30 m s-1. The analyses indicate no statistically significant dependence of these bulk exchange coefficients on wind speed. As a measure of hurricane development potential, we compute the mean ratio of the exchange coefficient for enthalpy to that for momentum and find it to be significantly below the lowest threshold estimated by previous investigators. This suggests that the enthalpy flux required for hurricane development may come from sources other than turbulent fluxes, such as lateral fluxes from the vortex warm core, or sea spray. Alternatively, it demands a re-evaluation of the theoretical models used to derive the threshold.
2007
Bell, G.D., E. Blake, C.W. Landsea, M. Chelliah, R. Pasch, K.C. Mo, and S.B. Goldenberg. The tropics: Tropical cyclones-Atlantic basin. In State of the Climate in 2006, A. Arguez, A.M. Waple, and A.M.Sanchez-Lugo (eds.). Bulletin of the American Meteorological Society, 88(6):S48-S51, https://doi.org/10.1175/BAMS-88-6-929 2007
Black, P.G., E.A. D'Asaro, W.M. Drennan, J.R. French, P.P. Niiler, T.B. Sanford, E.J. Terrill, E.J. Walsh, and J.A. Zhang. Air-sea exchange in hurricanes: Synthesis of observations from the Coupled Boundary Layer Air-Sea Transfer Experiment. Bulletin of the American Meteorological Society, 88(3):357-374, https://doi.org/10.1175/BAMS-88-3-357 2007
The Coupled Boundary Layer Air-Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air-sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (CD) values derived from CBLAST momentum flux measurements show CD becoming invariant with wind speed near a 23 m s-1 threshold rather than a hurricane-force threshold near 33 m s-1. Values above 23 m s-1 are lower than previous open-ocean measurements. The Dalton number estimates (CE) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s-1, which is in approximate agreement with previous measurements at lower winds. These observations imply a CE/CD ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be "roll vortex" or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9-1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery. As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and sub-surface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s-1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.
Carrasco, H.N. Data mining assisted automated quality control of tropical cyclone wind data. M.S. thesis, University of Miami, Rosenstiel School of Marine and Atmospheric Science, 82 pp., 2007
Meteorological observations are collected around the world in real-time to help meteorologists understand severe weather events, such as tropical cyclones. Tropical cyclones affect life on much of the world's coastlines. Data from meteorological observations are utilized by many models and analysis systems to help predict tropical cyclones and increase meteorologists' understanding of them. Data mining techniques such as clustering are useful tools that help find patterns within data. Particularly, a key aspect of most spatial clustering algorithms is their ability to detect noise. In the area of tropical cyclone quality control, this noise is the data that needs to be removed before scheduling an analysis. DBSCAN is an existing clustering algorithm that can locate patterns based on the density of observations to find complex shapes and eliminate noise, and thus is adopted in this study. In this thesis, the H*Wind-DBSCAN framework was designed and implemented to create a data mining tool that assists automated quality control within H*Wind for the various observations collected in and around tropical cyclones. H*Wind is a tool used to monitor and analyze tropical cyclone data around the globe. Combining many platforms of different observing systems, H*Wind allows meteorologists to interact with observation data. However, the quality control process is a slow and difficult task performed manually. Depending on the number of available observations the process can take from only a few minutes to sometimes over half an hour. Therefore, the H*Wind-DBSCAN framework was designed and developed using knowledge about the structure of a tropical cyclone in an attempt to efficiently and effectively cluster observations using DBSCAN. The H*Wind-DBSCAN framework flags those observations considered noise by DBSCAN and prevents them from being used in the analysis process. Typical spatial clustering routines cluster data points using the distance between two data points on a single coordinate plane. The proposed H*Wind-DBSCAN framework integrates the Cartesian Coordinate System and Cylindrical Coordinate System based on the circulation found within tropical cyclones in order to more accurately construct clusters. Several case studies were conducted, and the results demonstrate that the proposed H*Wind-DBSCAN framework improves the overall automated quality control performance as compared to using either coordinate system alone. Using the framework, the resulting flagged observations are helpful to determine areas of interest for a closer examination and to construct a decent quality controlled data set for the tropical cyclone wind analysis system.
Cerveny, R.S., J. Lawrimore, R. Edwards, and C.W. Landsea. Extreme weather records: Compilation, adjudication, and publication. Bulletin of the American Meteorological Society, 88(6):853-860, https://doi.org/10.1175/BAMS-88-6-85 2007
Conzemius, R.J., R.W. Moore, M.T. Montgomery, and C.A. Davis. Mesoscale convective vortex formation in a weakly sheared moist neutral environment. Journal of the Atmospheric Sciences, 64(5):1443-1466, https://doi.org/10.1175/JAS3898.1 2007
Idealized simulations of a diabatic Rossby vortex (DRV) in an initially moist neutral baroclinic environment are performed using the fifth-generation National Center for Atmospheric Research-Pennsylvania State University (NCAR-PSU) Mesoscale Model (MM5). The primary objective is to test the hypothesis that the formation and maintenance of midlatitude warm-season mesoscale convective vortices (MCVs) are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear. As a part of this objective, the simulated DRV is placed within the context of the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) field campaign by comparing its tangential velocity, radius of maximum winds, CAPE, and shear with the MCVs observed in BAMEX. The simulations reveal two distinct scales of development. At the larger scale, the most rapidly growing moist baroclinic mode is excited, and exponential growth of this mode occurs during the simulation. Embedded within the large-scale baroclinic wave is a convective system exhibiting the characteristic DRV development, with a positive potential vorticity (PV) anomaly in the lower troposphere and a negative PV anomaly in the upper troposphere, and the positive/negative PV doublet tilted downshear with height. The DRV warm-air advection mechanism is active, and the resulting deep convection helps to reinforce the DRV against the deleterious effects of environmental shear, causing an eastward motion of the convective system as a whole. The initial comparisons between the simulated DRVs and the BAMEX MCVs show that the simulated DRVs grew within background conditions of CAPE and shear similar to those observed for BAMEX MCVs and suggest that the same dynamical mechanisms are active. Because the BAMEX field campaign sampled MCVs in different backgrounds of CAPE and shear, the comparison also demonstrates the need to perform additional simulations to explore these different CAPE and shear regimes and to understand their impacts on the intensity and longevity of MCVs. Such a study has the additional benefit of placing MCV dynamics in an appropriate context for exploring their relevance to tropical cyclone formation.
Cram, T.A., J. Persing, M.T. Montgomery, and S.A. Braun. A Lagrangian trajectory view on transport and mixing processes between the eye, eyewall, and environment using a high-resolution simulation of Hurricane Bonnie (1998). Journal of the Atmospheric Sciences, 64(6):1835-1856, https://doi.org/10.1175/JAS3921.1 2007
The transport and mixing characteristics of a large sample of air parcels within a mature and vertically sheared hurricane vortex are examined. Data from a high-resolution (2-km horizontal grid spacing) numerical simulation of real-case Hurricane Bonnie (1998) are used to calculate Lagrangian trajectories of air parcels in various subdomains of the hurricane (namely, the eye, eyewall, and near environment) to study the degree of interaction (transport and mixing) between these subdomains. It is found that: (1) there is transport and mixing from the low-level eye to the eyewall that carries air possessing relatively high values of equivalent potential temperature (thetae), which can enhance the efficiency of the hurricane heat engine; (2) a portion of the low-level inflow of the hurricane bypasses the eyewall to enter the eye, and this air both replaces the mass of the low-level eye and lingers for a sufficient time (order 1 h) to acquire enhanced entropy characteristics through interaction with the ocean beneath the eye; (3) air in the mid- to upper-level eye is exchanged with the eyewall such that more than half the air of the eye is exchanged in 5 h in this case of a sheared hurricane; and (4) that one-fifth of the mass in the eyewall at a height of 5 km has an origin in the mid- to upper-level environment where thetae is much less than in the eyewall, which ventilates the ensemble average eyewall thetae by about 1 K. Implications of these findings for the problem of hurricane intensity forecasting are briefly discussed.
Dorst, N.M. The National Hurricane Research Project: 50 years of research, rough rides, and name changes. Bulletin of the American Meteorological Society, 88(10):1566-1588, https://doi.org/10.1175/BAMS-88-10-1566 2007
After the disastrous Atlantic hurricane season of 1954, the Weather Bureau created the National Hurricane Research Project (NHRP) to advance tropical cyclone science and improve forecasts. In the late 1950s, NHRP pioneered quantitative observations with instrumented aircraft and shaped the modern understanding of tropical cyclones. By the early 1960s, it was intimately involved in Project STORMFURY, the U.S. Government's hurricane modification program. During this time, it was collocated with the Miami, Florida, hurricane forecast office, and became a permanent laboratory. Its scientists became involved in international experiments and collaborated with researchers from around the world. In the 1970s, its theoretical and computer modeling work advanced, supporting STORMFURY. The project required the acquisition of new aircraft. Ironically, the improved instrumentation led to the dissolution of STORMFURY in 1983. Researchers found new avenues of investigation, including hurricane climatology, synoptic flow interaction, tropical cyclone dynamics, and improving intensity forecasts.
Dorst, N.M., and E. Rule. Fifty years of NOAA hurricane research. Mariners Weather Log, 51(1):10-13, 2007
For over 50 years, NOAA scientists have applied theoretical studies and computer models and have flown aircraft into hurricanes, all to better understand what makes these storms tick. This research has resulted in a much deeper scientific understanding of hurricanes and improved NOAA hurricane forecasts.
Drennen, W.M., J.A. Zhang, J.F. French, C. McCormick, and P.G. Black. Turbulent fluxes in the hurricane boundary layer, Part II: Latent heat flux. Journal of the Atmospheric Sciences, 64(4):1103-1115, https://doi.org/10.1175/JAS3889.1 2007
As part of the recent ONR-sponsored Coupled Boundary Layer Air-Sea Transfer (CBLAST) Departmental Research Initiative, an aircraft was instrumented to carry out direct turbulent flux measurements in the high wind boundary layer of a hurricane. During the 2003 field season flux measurements were made during Hurricanes Fabian and Isabel. Here the first direct measurements of latent heat fluxes measured in the hurricane boundary layer are reported. The previous wind speed range for humidity fluxes and Dalton numbers has been extended by over 50%. Up to 30 m s-1, the highest 10-m winds measured, the Dalton number is not significantly different from the Humidity Exchange over the Sea (HEXOS) result, with no evidence of an increase with wind speed.
Dye, J.E., M.G. Bateman, H.J. Christian, E. Defer, C.A. Grainger, W.D. Hall, E.P. Krider, S.A. Lewis, D.M. Mach, F.J. Merceret, J.C. Willett, and P.T. Willis. Electric fields, cloud microphysics, and reflectivity in anvils of Florida thunderstorms. Journal of Geophysical Research, 112(D11):D11215, 18 pp., https://doi.org/10.1029/2006JD007550 2007
A coordinated aircraft-radar project that investigated the electric fields, cloud microphysics, and radar reflectivity of thunderstorm anvils near Kennedy Space Center is described. Measurements from two cases illustrate the extensive nature of the microphysics and electric field observations. As the aircraft flew from the edges of anvils into the interior, electric fields very frequently increased abruptly from ~1 to >10 kV m1 even though the particle concentrations and radar reflectivity increased smoothly. The abrupt increase in field usually occurred when the aircraft entered regions with a reflectivity of 10-15 dBZ. We suggest that the abrupt increase in electric field was because the charge advection from the convective core did not occur across the entire breadth of the anvil and because the advection of charge was not constant in time. Also, some long-lived anvils showed enhancement of electric field and reflectivity far downwind of the convective core. Screening layers were not detected near the edges of the anvils. Comparisons of electric field magnitude with particle concentration or reflectivity for a combined data set that included all anvil measurements showed a threshold behavior. When the average reflectivity, such as in a 3-km cube, was less than approximately 5 dBZ, the electric field magnitude was 1. Based on these findings, the Volume Averaged Height Integrated Radar Reflectivity (VAHIRR) is now being used by the NASA, the Air Force, the and Federal Aviation Administration in new Lightning Launch Commit Criteria as a diagnostic for high electric fields in anvils.
French, J.F., W.M. Drennan, J.A. Zhang, and P.G. Black. Turbulent fluxes in the hurricane boundary layer, Part I: Momentum flux. Journal of the Atmospheric Sciences, 64(4):1089-1102, https://doi.org/10.1175/JAS3887.1 2007
An important outcome from the ONR-sponsored Coupled Boundary Layer Air-Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s-1. Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.
Halverson, J., M.L. Black, S. Braun, D. Cecil, M. Goodman, A. Heymsfield, G. Heymsfield, R. Hood, T. Krishnamurti, G. McFarquhar, M.J. Mahoney, J. Molinari, R.F. Rogers, J. Turk, C. Velden, D.-L. Zhang, E. Zipser, and R. Kakar. NASA's Tropical Cloud Systems and Processes (TCSP) Experiment: Investigating tropical cyclogenesis and hurricane intensity change. Bulletin of the American Meteorological Society, 88(6):867-882, https://doi.org/10.1175/BAMS-88-6-867 2007
In July 2005, the National Aeronautics and Space Administration investigated tropical cyclogenesis, hurricane structure, and intensity change in the eastern North Pacific and western Atlantic using its ER-2 high-altitude research aircraft. The campaign, called the Tropical Cloud Systems and Processes (TCSP) experiment, was conducted in conjunction with the National Oceanic and Atmospheric Administration/ Hurricane Research Division's Intensity Forecasting Experiment. A number of in situ and remote sensor datasets were collected inside and above four tropical cyclones representing a broad spectrum of tropical cyclone intensity and development in diverse environments. While the TCSP datasets directly address several key hypotheses governing tropical cyclone formation, including the role of vertical wind shear, dynamics of convective bursts, and upscale growth of the initial vortex, two of the storms sampled were also unusually strong, early season storms. Highlights from the genesis missions are described in this article, along with some of the unexpected results from the campaign. Interesting observations include an extremely intense, highly electrified convective tower in the eyewall of Hurricane Emily and a broad region of mesoscale subsidence detected in the lower stratosphere over landfalling Tropical Storm Gert.
Jones, T.A., D.J. Cecil, and J.P. Dunion. The environmental and inner-core conditions governing the intensity of Hurricane Erin (2001). Weather and Forecasting, 22(4):708-725, https://doi.org/10.1175/WAF1017.1 2007
The evolution of Hurricane Erin (2001) is presented from the perspective of its environmental and inner-core conditions, particularly as they are characterized in the Statistical Hurricane Intensity Prediction Scheme with Microwave Imagery (SHIPS-MI). Erin can be described as having two very distinct periods. The first, which occurred between 1 and 6 September 2001, was characterized by a struggling tropical storm failing to intensify as the result of unfavorable environmental and inner-core conditions. The surrounding environment during this period was dominated by moderate shear and mid- to upper-level dry air, both caused in some part by the presence of a Saharan air layer (SAL). Further intensification was inhibited by the lack of sustained deep convection and latent heating near the low-level center. The authors attribute this in part to negative effects from the SAL. The thermodynamic conditions associated with the SAL were not well sampled by the SHIPS parameters, resulting in substantial overforecasting by both SHIPS and SHIPS-MI. Instead, the hostile conditions surrounding Erin caused its dissipation on 6 September. The second period began on 7 September when Erin re-formed north of the original center. Erin began to pull away from the SAL and moved over 29°C sea surface temperatures, beginning a rapid intensification phase and reaching 105 kt by 1800 UTC 9 September. SHIPS-MI forecasts called for substantial intensification as in the previous period, but this time the model underestimated the rate of intensification. The addition of inner-core characteristics from passive microwave data improved the skill somewhat compared to SHIPS, but still left much room for improvement. For this period, it appears that the increasingly favorable atmospheric conditions caused by Erin moving away from the SAL were not well sampled by SHIPS or SHIPS-MI. As a result, the intensity change forecasts were not able to take into account the more favorable environment.
Li, T., C.-Y. She, H.-L. Liu, and M.T. Montgomery. Evidence of a gravity wave breaking event and the estimation of the wave characteristics from sodium lidar observation over Fort Collins, Colorado (41°N, 105°W). Geophysical Research Letters, 34(5):L05815, 5 pp., https://doi.org/10.1029/2006GL028988 2007
On the night of December 3rd, 2004 (UT day 338), we observed a significant acceleration of horizontal wind near 100 km between 0900 and 0915 UT accompanied by a temperature cooling at the same altitude and warming below it. The Lomb spectrum analysis of the raw dataset revealed that a gravity wave with 1.5 hr period was significant between 0500 and 0900 UT, but blurred after 0900 UT, suggesting the transfer of wave energy and momentum from wave field to mean flow. Most likely, this observed phenomenon is due to the breaking of an upward propagating gravity wave with an apparent period of ~1.5 hr. Using linear saturation theory and assuming a monochromatic wave packet, we estimated the characteristics of breaking gravity wave, eddy diffusion coefficient, and a simple relation between Prandtl number and turbulence localization measure when the wave is breaking, from the experimentally determined heating rate, horizontal wind acceleration, and background temperature and winds.
Lonfat, M., R.F. Rogers, T. Marchok, and F.D. Marks. A parametric model for predicting hurricane landfall. Monthly Weather Review, 135(9):3086-3097, https://doi.org/10.1175/MWR3433.1 2007
This study documents a new parametric hurricane rainfall prediction scheme, based on the rainfall climatology and persistence model (R-CLIPER) used operationally in the Atlantic Ocean basin to forecast rainfall accumulations. Although R-CLIPER has shown skill at estimating the mean amplitude of rainfall across the storm track, one underlying limitation is that it assumes that hurricanes produce rain fields that are azimuthally symmetric. The new implementations described here take into account the effect of shear and topography on the rainfall distribution through the use of parametric representations of these processes. Shear affects the hurricane rainfall by introducing spatial asymmetries, which can be reasonably well modeled to first order using a Fourier decomposition. The effect of topography is modeled by evaluating changes in elevation of flow parcels within the storm circulation between time steps and correcting the rainfall field in proportion to those changes. Effects modeled in R-CLIPER and those from shear and topography are combined in a new model called the Parametric Hurricane Rainfall Model (PHRaM). Comparisons of rainfall accumulations predicted from the operational R-CLIPER model, PHRaM, and radar-derived observations show some improvement in the spatial distribution and amplitude of rainfall when shear is accounted for and significant improvements when both shear and topography are modeled.
Marchok, T.P., R.F. Rogers, and R.E. Tuleya. Validation schemes for tropical cyclone quantitative precipitation forecasts: Evaluation of operational models for U.S. landfalling cases. Weather and Forecasting, 22(4):726-746, https://doi.org/10.1175/WAF1024.1 2007
A scheme for validating quantitative precipitation forecasts (QPFs) for landfalling tropical cyclones is developed and presented here. This scheme takes advantage of the unique characteristics of tropical cyclone rainfall by evaluating the skill of rainfall forecasts in three attributes: the ability to match observed rainfall patterns, the ability to match the mean value and volume of observed rainfall, and the ability to produce the extreme amounts often observed in tropical cyclones. For some of these characteristics, track-relative analyses are employed that help to reduce the impact of model track forecast error on QPF skill. These characteristics are evaluated for storm-total rainfall forecasts of all U.S. landfalling tropical cyclones from 1998 to 2004 by the NCEP operational models, that is, the Global Forecast System (GFS), the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, and the North American Mesoscale (NAM) model, as well as the benchmark Rainfall Climatology and Persistence (R-CLIPER) model. Compared to R-CLIPER, all of the numerical models showed comparable or greater skill for all of the attributes. The GFS performed the best of all of the models for each of the categories. The GFDL had a bias of predicting too much heavy rain, especially in the core of the tropical cyclones, while the NAM predicted too little of the heavy rain. The R-CLIPER performed well near the track of the core, but it predicted much too little rain at large distances from the track. Whereas a primary determinant of tropical cyclone QPF errors is track forecast error, possible physical causes of track-relative differences lie with the physical parameterizations and initialization schemes for each of the models. This validation scheme can be used to identify model limitations and biases and guide future efforts toward model development and improvement.
Marks, F.D. Recent results from NOAA's hurricane Intensity Forecast Experiment (IFEX). Preprints, 11th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface (IOAS-AOLS), San Antonio, TX, January 14-18, 2007. American Meteorological Society, Boston, CD-ROM, 6 pp., 2007
Moyer, A.C., J.L. Evans, and M.D. Powell. Comparison of observed gale radius statistics. Meteorology and Atmospheric Physics, 97(1-4):41-55, https://doi.org/10.1007/s00703-006-0243-2 2007
Forecasts of tropical cyclone track and intensity have long been used to characterize the evolution and expected threat from a tropical storm. However, in recent years, recognition of the contributions of subtropical cyclogenesis to tropical storm formation and the process of extratropical transition to latter stages of the once-tropical storm's lifecycle have raised awareness about the importance of storm structure. Indeed, the structure of a cyclone determines the distribution and intensity of the significant weather associated with that storm. In this study, storm structure is characterized in terms of significant wind radii. The radii of tropical storm, damaging, and hurricane-force winds, as well as the radius of maximum winds are all analyzed. These wind radii are objectively derived from the H*Wind surface wind analysis system. Initially, six years of these data are examined for consistency with previous studies. Having ascertained that the H*Wind radii are realistic, detailed comparisons are performed between the H*Wind and NHC Best Track wind radii for two years (2004 and 2005) of North Atlantic tropical storm and hurricane cases. This intercomparison reveals an unexpected bias: the H*Wind radii are consistently larger than the NHC Best Track for all but the smallest and least intense storms. Further examination of the objectively-determined H*Wind tropical storm force wind radius data compared to subjectively-determined radii for the same storm times demonstrates that the objective wind radii are underestimating the extent of the tropical storm force wind area. Since the objective H*Wind radii are large compared to the NHC Best Track - and yet underestimate the area of tropical storm force winds - this argues for further examination of the methods used to ascertain these significant wind radii.
Pandya, R.E., D.R. Smith, D.J. Charlevoix, G.M. Fisher, S.T. Murillo, K.A. Murphy, D.M. Stanitski, and T.M. Whittaker. The 15th AMS Education Symposium. Bulletin of the American Meteorological Society, 88(1):83-85, https://doi.org/10.1175/BAMS-88-1-83 2007
Powell, M.D., and T.A. Reinhold. Reply to comments on "Tropical cyclone destructive potential by integrated kinetic energy." Bulletin of the American Meteorological Society, 88(11):1800-1801, https://doi.org/10.1175/BAMS-88-11-1800 2007
Powell, M.D., and T.A. Reinhold. Tropical cyclone destructive potential by integrated kinetic energy. Bulletin of the American Meteorological Society, 88(4):513-526, https://doi.org/10.1175/BAMS-88-4-513 2007
Tropical cyclone damage potential, as currently defined by the Saffir-Simpson scale and the maximum sustained surface wind speed in the storm, fails to consider the area impact of winds likely to force surge and waves or cause particular levels of damage. Integrated kinetic energy represents a framework that captures the physical process of ocean surface stress forcing waves and surge while also taking into account structural wind loading and the spatial coverage of the wind. Integrated kinetic energy was computed from gridded, objectively analyzed surface wind fields of 23 hurricanes representing large and small storms. A wind destructive potential rating was constructed by weighting wind speed threshold contributions to the integrated kinetic energy, based on observed damage in Hurricanes Andrew, Hugo, and Opal. A combined storm surge and wave destructive potential rating was assigned according to the integrated kinetic energy contributed by winds greater than tropical storm force. The ratings are based on the familiar 1-5 range, with continuous fits to allow for storms as weak as 0.1 or as strong as 5.99.
Reynolds, C.A., M.S. Peng, S.J. Majumdar, S.D. Aberson, C.H. Bishop, and R. Buizza. Interpretation of adaptive observing guidance for Atlantic tropical cyclones. Monthly Weather Review, 135(12):4006-4029, https://doi.org/10.1175/2007MWR2027.1 2007
Adaptive observing guidance products for Atlantic tropical cyclones are compared using composite techniques that allow one to quantitatively examine differences in the spatial structures of the guidance maps and relate these differences to the constraints and approximations of the respective techniques. The guidance maps are produced using the ensemble transform Kalman filter (ETKF) based on ensembles from the National Centers for Environmental Prediction and the European Centre for Medium-Range Weather Forecasts (ECMWF), and total-energy singular vectors (TESVs) produced by ECMWF and the Naval Research Laboratory. Systematic structural differences in the guidance products are linked to the fact that TESVs consider the dynamics of perturbation growth only, while the ETKF combines information on perturbation evolution with error statistics from an ensemble-based data assimilation scheme. The impact of constraining the SVs using different estimates of analysis error variance instead of a total-energy norm, in effect bringing the two methods closer together, is also assessed. When the targets are close to the storm, the TESV products are a maximum in an annulus around the storm, whereas the ETKF products are a maximum at the storm location itself. When the targets are remote from the storm, the TESVs almost always indicate targets northwest of the storm, whereas the ETKF targets are more scattered relative to the storm location and often occur over the northern North Atlantic. The ETKF guidance often coincides with locations in which the ensemble-based analysis error variance is large. As the TESV method is not designed to consider spatial differences in the likely analysis errors, it will produce targets over well-observed regions, such as the continental United States. Constraining the SV calculation using analysis error variance values from an operational 3D variational data assimilation system (with stationary, quasi-isotropic background error statistics) results in a modest modulation of the target areas away from the well-observed regions, and a modest reduction of perturbation growth. Constraining the SVs using the ETKF estimate of analysis error variance produces SV targets similar to ETKF targets and results in a significant reduction in perturbation growth, due to the highly localized nature of the analysis error variance estimates. These results illustrate the strong sensitivity of SVs to the norm (and to the analysis error variance estimate used to define it) and confirm that discrepancies between target areas computed using different methods reflect the mathematical and physical differences between the methods themselves.
Rogers, R.F., M.L. Black, S.S. Chen, and R.A. Black. An evaluation of microphysics fields from mesoscale model simulations of tropical cyclones, Part I: Comparisons with observations. Journal of the Atmospheric Sciences, 64(6):1811-1834, https://doi.org/10.1175/JAS3932.1 2007
This study presents a framework for comparing hydrometeor and vertical velocity fields from mesoscale model simulations of tropical cyclones with observations of these fields from a variety of platforms. The framework is based on the Yuter and Houze constant frequency by altitude diagram (CFAD) technique, along with a new hurricane partitioning technique, to compare the statistics of vertical motion and reflectivity fields and hydrometeor concentrations from two datasets: one consisting of airborne radar retrievals and microphysical probe measurements collected from tropical cyclone aircraft flights over many years, and another consisting of cloud-scale (1.67-km grid length) tropical cyclone simulations using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). Such comparisons of the microphysics fields can identify biases in the simulations that may lead to an identification of deficiencies in the modeling system, such as the formulation of various physical parameterization schemes used in the model. Improvements in these schemes may potentially lead to better forecasts of tropical cyclone intensity and rainfall. In Part I of this study, the evaluation framework is demonstrated by comparing the radar retrievals and probe measurements to MM5 simulations of Hurricanes Bonnie (1998) and Floyd (1999). Comparisons of the statistics from the two datasets show that the model reproduces many of the gross features seen in the observations, though notable differences are evident. The general distribution of vertical motion is similar between the observations and simulations, with the strongest up- and downdrafts making up a small percentage of the overall population in both datasets, but the magnitudes of vertical motion are weaker in the simulations. The model-derived reflectivities are much higher than observed, and correlations between vertical motion and hydrometeor concentration and reflectivity show a much stronger relationship in the model than what is observed. Possible errors in the data processing are discussed as potential sources of differences between the observed and simulated datasets in Part I. In Part II, attention will be focused on using the evaluation framework to investigate the role that different model configurations (i.e., different resolutions and physical parameterizations) play in producing different microphysics fields in the simulation of Hurricane Bonnie. The microphysical and planetary boundary layer parameterization schemes, as well as higher horizontal and vertical resolutions, will be tested in the simulation to identify the extent to which changes in these schemes are reflected in improvements of the statistical comparisons with the observations.
Schecter, D.A., and M.T. Montgomery. Waves in a cloudy vortex. Journal of the Atmospheric Sciences, 64(2):314-337, https://doi.org/10.1175/JAS3849.1 2007
This paper derives a system of equations that approximately govern small-amplitude perturbations in a nonprecipitating cloudy vortex. The cloud coverage can be partial or complete. The model is used to examine moist vortex Rossby wave dynamics analytically and computationally. One example shows that clouds can slow the growth of phase-locked counter-propagating vortex Rossby waves in the eyewall of a hurricane-like vortex. Another example shows that clouds can (indirectly) damp discrete vortex Rossby waves that would otherwise grow and excite spiral inertia-gravity wave radiation from a monotonic cyclone at high Rossby number.
Tory, K.J., N.E. Davidson, and M.T. Montgomery. Prediction and diagnosis of tropical cyclone formation in an NWP system, Part III: Diagnosis of developing and nondeveloping storms. Journal of the Atmospheric Sciences, 64(9):3195-3213, https://doi.org/10.1175/JAS4023.1 2007
This is the third of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. In Parts I and II, a primary and two secondary vortex enhancement mechanisms were illustrated, and shown to be responsible for TC genesis in a simulation of TC Chris. In this paper, five more TC-LAPS simulations are investigated: three developing and two nondeveloping. In each developing simulation the pathway to genesis was essentially the same as that reported in Part II. Potential vorticity (PV) cores developed through low- to middle-tropospheric vortex enhancement in model-resolved updraft cores (primary mechanism) and interacted to form larger cores through diabatic upscale vortex cascade (secondary mechanism). On the system scale, vortex intensification resulted from the large-scale mass redistribution forced by the upward mass flux, driven by diabatic heating, in the updraft cores (secondary mechanism). The nondeveloping cases illustrated that genesis can be hampered by (i) vertical wind shear, which may tilt and tear apart the PV cores as they develop, and (ii) an insufficient large-scale cyclonic environment, which may fail to sufficiently confine the warming and enhanced cyclonic winds, associated with the atmospheric adjustment to the convective updrafts. The exact detail of the vortex interactions was found to be unimportant for qualitative genesis forecast success. Instead the critical ingredients were found to be sufficient net deep convection in a sufficiently cyclonic environment in which vertical shear was less than some destructive limit. The often-observed TC genesis pattern of convection convergence, where the active convective regions converge into a 100-km-diameter center, prior to an intense convective burst and development to tropical storm intensity is evident in the developing TC-LAPS simulations. The simulations presented in this study and numerous other simulations not yet reported on have shown good qualitative forecast success. Assuming such success continues in a more rigorous study (currently under way) it could be argued that TC genesis is largely predictable provided the large-scale environment (vorticity, vertical shear, and convective forcing) is sufficiently resolved and initialized.
Uhlhorn, E.W., P.G. Black, J.L. Franklin, M. Goodberlet, J. Carswell, and A.S. Goldstein. Hurricane surface wind mesurements from an operational Stepped Frequency Microwave Radiometer. Monthly Weather Review, 135(9):3070-3085, https://doi.org/10.1175/MWR3454.1 2007
For the first time, the NOAA/Aircraft Operations Center (AOC) flew stepped frequency microwave radiometers (SFMRs) on both WP-3D research aircraft for operational hurricane surface wind speed measurement in 2005. An unprecedented number of major hurricanes provided ample data to evaluate both instrument performance and surface wind speed retrieval quality up to 70 m s-1 (Saffir-Simpson category 5). To this end, a new microwave emissivity-wind speed model function based on estimates of near-surface winds in hurricanes by global positioning system (GPS) dropwindsondes is proposed. For practical purposes, utilizing this function removes a previously documented high bias in moderate SFMR-measured wind speeds (10-50 m s-1), and additionally corrects an extreme wind speed (>60 m s-1) underestimate. The AOC operational SFMRs yield retrievals that are precise to within ~2% at 30 m s-1, which is a factor of 2 improvement over the NOAA Hurricane Research Division's SFMR, and comparable to the precision found here for GPS dropwindsonde near-surface wind speeds. A small (1.6 m s-1), but statistically significant, overall high bias was found for independent SFMR measurements utilizing emissivity data not used for model function development. Across the range of measured wind speeds (10-70 m s-1), SFMR 10-s averaged wind speeds are within 4 m s-1 (rms) of the dropwindsonde near-surface estimate, or 5%-25% depending on speed. However, an analysis of eyewall peak wind speeds indicates an overall 2.6 m s-1 GPS low bias relative to the peak SFMR estimate on the same flight leg, suggesting a real increase in the maximum wind speed estimate due to SFMR's high-density sampling. Through a series of statistical tests, the SFMR is shown to reduce the overall bias in the peak surface wind speed estimate by ~50% over the current flight-level wind reduction method and is comparable at extreme wind speeds. The updated model function is demonstrated to behave differently below and above the hurricane wind speed threshold (~32 m s-1), which may have implications for air-sea momentum and kinetic energy exchange. The change in behavior is at least qualitatively consistent with recent laboratory and field results concerning the drag coefficient in high wind speed conditions, which show a fairly clear "leveling off" of the drag coefficient with increased wind speed above ~30 m s-1. Finally, a composite analysis of historical data indicates that the earth-relative SFMR peak wind speed is typically located in the hurricane's right-front quadrant, which is consistent with previous observational and theoretical studies of surface wind structure.
Walsh, K.J.E., M. Fiorino, C.W. Landsea, and K. McInnes. Objectively determined resolution-dependent threshold criteria for the detection of tropical cyclones in climate models and reanalyses. Journal of Climate, 20(10):2307-2314, https://doi.org/10.1175/JCLI4074.1 2007
Objectively derived resolution-dependent criteria are defined for the detection of tropical cyclones in model simulations and observationally based analyses. These criteria are derived from the wind profiles of observed tropical cyclones, averaged at various resolutions. Both an analytical wind profile model and two-dimensional observed wind analyses are used. The results show that the threshold wind speed of an observed tropical cyclone varies roughly linearly with resolution. The criteria derived here are compared to the numerous different criteria previously employed in climate model simulations. The resulting method provides a simple means of comparing climate model simulations and reanalyses.
Willoughby, H.E., E.N. Rappaport, and F.D. Marks. Hurricane forecasting: The state of the art. Natural Hazards Review, 8(3):45-49, https://doi.org/10.1061/(ASCE)1527-6988(2007)8:3(45) 2007
In this article, we summarize current forecasting practice, the performance of the forecasting enterprise, and the impacts of tropical cyclones from a meteorological perspective. In the past, a forecast was considered successful if it predicted the hurricane's position and intensity 12-72 h into the future. By the 1990s, forecast users came to expect more specific details such as spatial distributions of rainfall, winds, flooding, and high seas. In the early 21st century, forecasters extended their time horizons to 120 h. Meteorologists have maintained homogeneous statistics on forecast accuracy for more than 50 years. These verification statistics are reliable metrics of meteorological performance. In terms of outcomes, forecasting in the late 20th century prevented 66-90% of the hurricane-related deaths in the United States that would have resulted from techniques used in the 1950s, but it is difficult to demonstrate an effect on property damage. The economic and human consequences of the response to forecasts and warnings are also poorly known. A final key concern is how to frame forecasts to address users' needs and to elicit optimum responses.
Wu, C.-C., K.-H. Chou, P.-H. Lin, S.D. Aberson, M.S. Peng, and T. Nakazawa. The impact of dropsonde data on typhoon track forecasting in DOTSTAR. Weather and Forecasting, 22(6):1157-1176, https://doi.org/10.1175/2007WAF2006062.1 2007
Starting from 2003, a new typhoon surveillance program, Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR), was launched. During 2004, 10 missions for eight typhoons were conducted successfully with 155 dropwindsondes deployed. In this study, the impact of these dropwindsonde data on tropical cyclone track forecasts has been evaluated with five models (four operational and one research models). All models, except the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, show the positive impact that the dropwindsonde data have on tropical cyclone track forecasts. During the first 72 h, the mean track error reductions in the National Centers for Environmental Predictions (NCEP) Global Forecast System (GFS), the Navy Operational Global Atmospheric Prediction System (NOGAPS) of the Fleet Numerical Meteorology and Oceanography Center (FNMOC), and the Japanese Meteorological Agency (JMA) Global Spectral Model (GSM) are 14%, 14%, and 19%, respectively. The track error reduction in the Weather Research and Forecasting (WRF) model, in which the initial conditions are directly interpolated from the operational GFS forecast, is 16%. However, the mean track improvement in the GFDL model is a statistically insignificant 3%. The 72-h average track error reduction from the ensemble mean of the above three global models is 22%, which is consistent with the track forecast improvement in Atlantic tropical cyclones from surveillance missions. In all, despite the fact that the impact of the dropwindsonde data is not statistically significant due to the limited number of DOTSTAR cases in 2004, the overall added value of the dropwindsonde data in improving typhoon track forecasts over the western North Pacific is encouraging. Further progress in the targeted observations of the dropwindsonde surveillances and satellite data, and in the modeling and data assimilation system, is expected to lead to even greater improvement in tropical cyclone track forecasts.
2006
Aberson, S.D., and B.J. Etherton. Targeting and data assimilation studies during Hurricane Humberto (2001). Journal of the Atmospheric Sciences, 63(1):175-186, https://doi.org/10.1175/JAS3594.1 2006
Two operational synoptic surveillance missions were conducted by the National Oceanic and Atmospheric Administration into Hurricane Humberto (2001). Forecasts from two leading dynamical hurricane track forecast models were improved substantially during the watch and warning period before a projected landfall by the assimilation of the additional dropwindsonde data. Feasibility tests with a barotropic model suggest that further improvements may be obtained by the use of the ensemble transform Kalman filter for assimilating these additional data into the model. This is the first effort to assimilate data into a hurricane model using the ensemble transform Kalman filter.
Aberson, S.D., and D.P. Stern. Extreme horizontal winds measured by dropwindsondes in hurricanes. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Aberson, S.D., and J.B. Halverson. Kelvin-Helmholtz billows in the eyewall of Hurricane Erin. Monthly Weather Review, 134(3):1036-1038, https://doi.org/10.1175/MWR3094.1 2006
A photograph of vertically aligned KelvinHelmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.
Aberson, S.D., J.P. Dunion, and F.D. Marks. A photograph of a wavenumber-2 asymmetry in the eye of Hurricane Erin. Journal of the Atmospheric Sciences, 63(1):387-391, https://doi.org/10.1175/JAS3593.1 2006
A photograph of a wavenumber-2 asymmetry in the eye of Hurricane Erin taken during a NOAA WP-3D research flight during the Fourth Convection and Moisture Experiment (CAMEX-4) field program on 10 September 2001 is described. The photograph of the cloud structure within the eye is evaluated using airborne and satellite remote sensing observations, and a possible explanation for the asymmetry is presented.
Aberson, S.D., M.L. Black, R.A. Black, R.W. Burpee, J.J. Cione, C.W. Landsea, and F.D. Marks. Thirty years of tropical cyclone research with the NOAA P-3 aircraft. Bulletin of the American Meteorological Society, 87(8):1039-1055, https://doi.org/10.1175/BAMS-87-8-1039 2006
In 1976 and 1977, the National Oceanic and Atmospheric Administration purchased two customized WP-3D (P-3) aircraft to conduct tropical cyclone (TC) research. During their first 30 years, the P-3s have proved to be invaluable research platforms, obtaining data at the micro- to synoptic scale, with missions conducted in 134 TCs in the Atlantic and eastern Pacific Oceans and near Australia. Analyses of the observations led to many new insights about TC structure, dynamics, thermodynamics, and environmental interactions. The real-time use of the information by the National Hurricane and Environmental Modeling Centers of the National Centers for Environmental Prediction (NCEP), as well as later research, has helped to increase the accuracy of wind, flood, and storm surge forecasts and severe weather warnings and has resulted in significant improvements to operational numerical model guidance for TC-track forecasts. In commemoration of the first 30 years of research with these aircraft, this manuscript presents a brief overview of the instrumentation aboard the aircraft and the major research findings during this period.
Aberson, S.D., M.T. Montgomery, M.M. Bell, and M.L. Black. Hurricane Isabel (2003): New insights into the physics of intense storms, Part II: Extreme localized wind. Bulletin of the American Meteorological Society, 87(10):1349-1354, https://doi.org/10.1175/BAMS-87-10-1335 2006
An unprecedented dataset of category-5 Hurricane Isabel was collected on 12-14 September 2003. This two-part series focuses on novel dynamical and thermodynamical aspects of Isabel's inner-core structure on 13 September. In Part I, using a composite of dropwindsonde and in situ aircraft data, the authors suggested that the axisymmetric structure of Isabel showed that the storm was superintense. Mesocyclones seen clearly in satellite imagery within the eye of Hurricane Isabel are hypothesized to mix high-entropy air at low levels in the eye into the eyewall, stimulating explosive convective development and a concomitant local horizontal wind acceleration. Part II focuses on a unique set of observations into an extraordinary small- (miso) scale cyclonic feature inside of the inner edge of the eyewall of Hurricane Isabel. A dropwindsonde released into this feature measured the strongest known horizontal wind in a tropical cyclone. This particular observation is discussed in the context of concurrent observations from airborne Doppler radar and other airborne instruments. These observations show wind even stronger than the system-scale superintense wind suggested in Part I. Speculation on the frequency of occurrence of these "little whirls" and their potentially catastrophic impacts are presented.
Bell, G.D., E. Blake, K.C. Mo, C.W. Landsea, R. Pasch, M. Chelliah, and S.B. Goldenberg. Tropical cyclones: Atlantic basin. In State of the Climate in 2005, K.A. Shein, A.M. Waple, H.J. Diamond, and J.M. Levy (eds.). Bulletin of the American Meteorological Society, 87(6):S33-S37, https://doi.org/10.1175/BAMS-87-6-801 2006
Bell, G.D., E. Blake, K.C. Mo, C.W. Landsea, R. Pasch, M. Chelliah, S.B. Goldenberg, and H.J. Diamond. The record-breaking 2005 Atlantic hurricane season. In State of the Climate in 2005, K.A. Shein, A.M. Waple, H.J. Diamond, and J.M. Levy (eds.). Bulletin of the American Meteorological Society, 87(6):S44-S45, https://doi.org/10.1175/BAMS-87-6-801 2006
Black, P.G., E.A. D'Asaro, J.R. French, and W.M. Drennan. Synthesis of major results from the Coupled Boundary Layer Air-Sea Transfer Experiment (CBLAST) in hurricanes (2003-2004). Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 8 pp. (CD-ROM), 2006
Chen, S.S., J.A. Knaff, and F.D. Marks. Effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries deduced from TRMM. Monthly Weather Review, 134(11):3190-3208, https://doi.org/10.1175/MWR3245.1 2006
Vertical wind shear and storm motion are two of the most important factors contributing to rainfall asymmetries in tropical cyclones (TCs). Global TC rainfall structure, in terms of azimuthal distribution and asymmetries relative to storm motion, has been previously described using the Tropical Rainfall Measuring Mission Microwave Imager rainfall estimates. The mean TC rainfall distribution and the wavenumber-1 asymmetry vary with storm intensity and geographical location among the six oceanic basins. This study uses a similar approach to investigate the relationship between the structure of TC rainfall and the environmental flow by computing the rainfall asymmetry relative to the vertical wind shear. The environmental vertical wind shear is defined as the difference between the mean wind vectors of the 200- and 850-hPa levels over an outer region extending from the radius of 200-800 km around the storm center. The wavenumber-1 maximum rainfall asymmetry is downshear left (right) in the Northern (Southern) Hemisphere. The rainfall asymmetry decreases (increases) with storm intensity (shear strength). The rainfall asymmetry maximum is predominantly downshear left for shear values > 7.5 m s-1. Large asymmetries are usually observed away from the TC centers. As TC intensity increases, the asymmetry maximum shifts upwind to the left. The analysis is further extended to examine the storm motion and the vertical wind shear and their collective effects on TC rainfall asymmetries. It is found that the vertical wind shear is a dominant factor for the rainfall asymmetry when shear is >5 m s-1. The storm motion-relative rainfall asymmetry in the outer rainband region is comparable to that of shear relative when the shear is -1, suggesting that TC translation speed becomes an important factor in the low shear environment. The overall TC rainfall asymmetry depends on the juxtaposition and relative magnitude of the storm motion and environmental shear vectors in all oceanic basins.
Chou, K.-H., C.-C. Wu, P.-H. Lin, S.D. Aberson, M. Peng, and T. Nakazawa. The impact of dropsonde data from DOTSTAR on tropical cyclone track forecasting. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 12 pp. (CD-ROM), 2006
Contreras, R.F., D. Esteban Fernandez, P.S. Chang, and P.G. Black. High resolution airborne radar measurements of Hurricane Isabel. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
The Imaging Wind and Rain Airborne Profiler (IWRAP) is a dual-frequency, conically-scanning Doppler radar that measures high resolution profiles of rain's effective reflectivity Ze and Doppler velocity, as well as surface wind vectors via scatterometry. IWRAP was flown aboard a NOAA WP-3D aircraft during the 2002, 2003, 2004, and 2005 hurricane seasons as part of the ONR's Coupled Boundary Layers Air-Sea Transfer (CBLAST) experiment, NASA's Ocean Vector Winds research, and the NOAA/NESDIS Ocean Winds and Rain experiments. We will start with a description of IWRAP and its capabilities. Following this we will introduce a new dataset available to the CBLAST community. We will finish with high resolution radar observations of Hurricane Isabel with an emphasis on the 3-D structure of the storm, especially in the atmospheric boundary layer (ABL).
Corbosiero, K.L., J. Molinari, A.R. Aiyyer, and M.L. Black. The structure and evolution of Hurricane Elena (1985), Part II: Convective asymmetries and evidence for vortex Rossby waves. Monthly Weather Review, 134(11):3073-3091, https://doi.org/10.1175/MWR3250.1 2006
A portable data recorder attached to the Weather Surveillance Radar-1957 (WSR-57) in Apalachicola, Florida, collected 313 radar scans of the reflectivity structure within 150 km of the center of Hurricane Elena (in 1985) between 1310 and 2130 UTC 1 September. This high temporal and spatial (750 m) resolution dataset was used to examine the evolution of the symmetric and asymmetric precipitation structure in Elena as the storm rapidly strengthened and attained maximum intensity. Fourier decomposition of the reflectivity data into azimuthal wavenumbers revealed that the power in the symmetric (wavenumber 0) component dominated the reflectivity pattern at all times and all radii by at least a factor of 2. The wavenumber 1 asymmetry accounted for less than 20% of the power in the reflectivity field on average and was found to be forced by the environmental vertical wind shear. The small-amplitude wavenumber 2 asymmetry in the core was associated with the appearance and rotation of an elliptical eyewall. This structure was visible for nearly 2 h and was noted to rotate cyclonically at a speed equal to half of the local tangential wind. Outside of the eyewall, individual peaks in the power in wavenumber 2 were associated with repeated instances of cyclonically rotating, outward-propagating inner spiral rainbands. Four separate convective bands were identified with an average azimuthal velocity of 25 m s-1, or ~68% of the local tangential wind speed, and an outward radial velocity of 5.2 m s-1. The azimuthal propagation speeds of the elliptical eyewall and inner spiral rainbands were consistent with vortex Rossby wave theory. The elliptical eyewall and inner spiral rainbands were seen only in the 6-h period prior to peak intensity, when rapid spinup of the vortex had produced an annular vorticity profile, similar to those that have been shown to support barotropic instability. The appearance of an elliptical eyewall was consistent with the breakdown of eyewall vorticity into mesovortices, asymmetric mixing between the eye and eyewall, and a slowing of the intensification rate. The inner spiral rainbands might have arisen from high eyewall vorticity ejected from the core during the mixing process. Alternatively, because the bands were noted to emanate from the vertical shear-forced deep convection in the northern eyewall, they could have formed through the axisymmetrization of the asymmetric diabatically generated eyewall vorticity.
DeMaria, M., J.A. Knaff, and J. Kaplan. On the decay of tropical cyclone winds crossing narrow landmasses. Journal of Applied Meteorology and Climatology, 45(3):491-499, https://doi.org/10.1175/JAM2351.1 2006
A method is developed to adjust the Kaplan and DeMaria tropical cyclone inland wind decay model for storms that move over narrow landmasses. The basic assumption that the wind speed decay rate after landfall is proportional to the wind speed is modified to include a factor equal to the fraction of the storm circulation that is over land. The storm circulation is defined as a circular area with a fixed radius. Application of the modified model to Atlantic Ocean cases from 1967 to 2003 showed that a circulation radius of 110 km minimizes the bias in the total sample of landfalling cases and reduces the mean absolute error of the predicted maximum winds by about 12%. This radius is about 2 times the radius of maximum wind of a typical Atlantic tropical cyclone. The modified decay model was applied to the Statistical Hurricane Intensity Prediction Scheme (SHIPS), which uses the Kaplan and DeMaria decay model to adjust the intensity for the portion of the predicted track that is over land. The modified decay model reduced the intensity forecast errors by up to 8% relative to the original decay model for cases from 2001 to 2004 in which the storm was within 500 km from land.
Dunion, J.P., J.D. Hawkins, and C.S. Velden. Hunting for Saharan air with the NOAA G-IV jet. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Eastin, M.D., P.D. Reasor, D.S. Nolan, F.D. Marks, and J.F. Gamache. Evolution of low-wavenumber vorticity during rapid intensification: A dual-Doppler analysis. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 6 pp. (CD-ROM), 2006
Esteban-Fernandez, D., J.R. Carswell, S. Frasier, P.S. Chang, P.G. Black, and F.D. Marks. Dual-polarized C- and Ku-band ocean backscatter response to hurricane-force winds. Journal of Geophysical Research, 111(C8):C08013, 17 pp., https://doi.org/10.1029/2005JC003048 2006
Airborne ocean backscatter measurements at C- and Ku-band wavelengths and H and V polarizations at multiple incidence angles obtained in moderate to very high wind speed conditions (25-65 m s-1) during missions through several tropical cyclones are presented. These measurements clearly show that the normalized radar cross sections (NRCS) response stops increasing at hurricane-force winds for both frequency bands and polarizations except for high incidence angles at C-band and H polarization. The results also show the mean NRCS departing from a power law behavior for all the presented frequency bands, polarizations, and incidence angles, suggesting a reduction in the drag coefficient. The overall flattening of the azimuthal response of the NRCS is also very apparent in all cases. A new set of geophysical model functions (GMFs) at C- and Ku-band are developed from these direct ocean backscatter observations for ocean surface winds ranging from 25 to 65 m s-1. The developed GMFs provide a much more accurate characterization of the NRCS versus wind speed and direction, and their implementation in operational retrieval algorithms from satellite-based scatterometer observations would result in better wind fields. The differences between these measurements and other currently available GMFs, such as QuikSCAT, NSCAT2, CMOD4, and CMOD5, are reported. The implementation of these GMFs in retrieval algorithms will result in better wind fields from satellite-based scatterometers measurements.
Etherton, B.J., C.-C. Wu, S.J. Majumdar, and S.D. Aberson. A comparison of targeting techniques for 2005 Atlantic tropical cyclones. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Evan, A.T., J.P. Dunion, J.A. Foley, A.K. Heidinger, and C.S. Velden. New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks. Geophysical Research Letters, 33(19):L19813, 5 pp., https://doi.org/10.1029/2006GL026408 2006
It is well known that Atlantic tropical cyclone activity varies strongly over time, and that summertime dust transport over the North Atlantic also varies from year to year, but any connection between tropical cyclone activity and atmospheric dust has been limited to a few case studies. Here we report new results that demonstrate a strong relationship between interannual variations in North Atlantic tropical cyclone activity and atmospheric dust cover as measured by satellite, for the years 1982-2005. While we cannot conclusively demonstrate a direct causal relationship, there appears to be robust link between tropical cyclone activity and dust transport over the Tropical Atlantic.
French, J.R., W.M. Drennan, J.A. Zhang, and P.G. Black. Direct airborne measurements of momentum flux in hurricanes. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 6 pp. (CD-ROM), 2006
Graber, H.C., V.J. Cardone, R.E. Jensen, D.N. Slinn, S.C. Hagen, A.T. Cox, M.D. Powell, and C. Grassl. Coastal forecasts and storm surge predictions for tropical cyclones: A timely partnership program. Oceanography, 19(1):130-141, https://doi.org/10.5670/oceanog.2006.96 2006
As more people and associated infrastructure concentrate along coastal areas, the United States is becoming more vulnerable to the impact of tropical cyclones. It is not surprising, especially after the past two hurricane seasons, that hurricanes are the costliest natural disasters because of the migration of the population towards the coast and the resulting changes in the national wealth density or revenue. A better understanding of both hurricane frequencies and intensities as they vary from year to year and their relation to changes in damages is of great interest to scientists, public and private-decision makers, and the general public. The estimation of tropical-cyclone-generated waves and surge in coastal waters and the nearshore zone is of critical importance to the timely evacuation of coastal residents, and the assessment of damage to coastal property in the event that a storm makes landfall. The model predictions of waves and storm surge in coastal waters are functionally related and both depend on the reliability of the atmospheric forcing. Hurricane Georges (1998), Ivan (2004), and Katrina and Wilma (2005) are excellent examples of intense tropical cyclones with numerous landfalls and unexpected changes in intensity and movement. Although there are no perfect predictions of the time and location of landfall and the intensity and size of the storm, we are able to forecast wind strength, storm-wave height, and surge levels that are expected along the official track from the National Hurricane Center (NHC) as well as from an ensemble of about a dozen track forecasts that would bracket the results from the least to worst conditions. The variability of these parameters, if known for different forecast tracks, could positively impact the advisories. To be effective and useful, a critical component of any forecast system is its timeliness.
Graves, L.P., J.C. McWilliams, and M.T. Montgomery. Vortex evolution due to straining: A mechanism for dominance of strong, interior anticyclones. Geophysical and Astrophysical Fluid Dynamics, 100(3):151-183, https://doi.org/0.1080/03091920600792041 2006
In this article we address two questions: Why do freely evolving vortices weaken on average, even when the viscosity is very small? Why, in the fluid's interior, away from vertical boundaries and under the influence of Earth's rotation and stable density stratification, do anticyclonic vortices become dominant over cyclonic ones when the Rossby number and deformation radius are finite? The context for answering these questions is a rotating, conservative, Shallow-water model with Asymmetric and Gradient-wind Balance approximations. The controlling mechanisms are vortex weakening under straining deformation (with a weakening that is substantially greater for strong cyclones than strong anticyclones) followed by a partially compensating vortex strengthening during a relaxation phase dominated by Vortex Rossby Waves (VRWs) and their eddy-mean interaction with the vortex. The outcome is a net, strain-induced vortex weakening that is greater for cyclones than anticyclones when the deformation radius is not large compared to the vortex radius and the Rossby number is not small. Furthermore, when the exterior strain flow is sustained, the vortex changes also are sustained: for small Rossby number (i.e., the quasigeostrophic limit, QG), vortices continue to weaken at a relatively modest rate, but for larger Rossby number, cyclones weaken strongly and anticyclones actually strengthen systematically when the deformation radius is comparable to the vortex radius. The sustained vortex changes are associated with strain-induced VRWs on the periphery of the mean vortex. It therefore seems likely that, in a complex flow with many vortices, anticyclonic dominance develops over a sequence of transient mutual straining events due to the greater robustness of anticyclones (and occasionally their net strengthening).
Halliwell, G.R., L.K. Shay, E.W. Uhlhorn, S.D. Jacob, and O.M. Smedstad. Improving ocean state initialization in coupled tropical cyclone forecast models. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Hausman, S.A., K.V. Ooyama, and W.H. Schubert. Potential vorticity structure of simulated hurricanes. Journal of the Atmospheric Sciences, 63(1):87-108, https://doi.org/10.1175/JAS3601.1 2006
To better understand the processes involved in tropical cyclone development, the authors simulate an axisymmetric tropical-cyclone-like vortex using a two-dimensional model based on nonhydrostatic dynamics, equilibrium thermodynamics, and bulk microphysics. Sensitivity experiments reveal that the simulated cyclones are sensitive to the effects of ice, primarily through the reduced fall velocity of precipitation above the freezing level rather than through the latent heat of fusion, and to the effects of vertical entropy transport by precipitation.
Hendricks, E.A., and M.T. Montgomery. Rapid scan views of convectively generated mesovortices in sheared Tropical Cyclone Gustav. Weather and Forecasting, 21(6):1041-1050, https://doi.org/10.1175/WAF950.1 2006
On 9-10 September 2002, multiple mesovortices were captured in great detail by rapid scan visible satellite imagery in subtropical, then later, Tropical Storm Gustav. These mesovortices were observed as low-level cloud swirls while the low-level structure of the storm was exposed due to vertical shearing. They are shown to form most plausibly via vortex tube stretching associated with deep convection; they become decoupled from the convective towers by vertical shear; they are advected with the low-level circulation; finally they initiate new hot towers on their boundaries. Partial evidence of an axisymmetrizing mesovortex and its hypothesized role in the parent vortex spinup is presented. Observations from the mesoscale and synoptic scale are synthesized to provide a multiscale perspective of the intensification of Gustav that occurred on 10 September. The most important large-scale factors were the concurrent relaxation of the 850-200 hPa-deep layer vertical wind shear from 10-15 to 5-10 m s-1 and movement over pockets of very warm sea surface temperatures (approximately 29.5°-30.5°C). The mesoscale observations are not sufficient alone to determine the precise role of the deep convection and mesovortices in the intensification. However, qualitative comparisons are made between the mesoscale processes observed in Gustav and recent full-physics and idealized numerical simulations to obtain additional insight.
Hood, R.E., D.J. Cecil, F.J. LaFontaine, R.J. Blakeslee, D.M. Mach, G.M. Heymsfield, F.D. Marks, E.J. Zipser, and M. Goodman. Classification of tropical oceanic precipitation using high altitude aircraft microwave and electric field measurements. Journal of the Atmospheric Sciences, 63(1):218-233, https://doi.org/10.1175/JAS3606.1 2006
During the 1998 and 2001 hurricane seasons of the western Atlantic Ocean and Gulf of Mexico, the Advanced Microwave Precipitation Radiometer (AMPR), the ER-2 Doppler (EDOP) radar, and the Lightning Instrument Package (LIP) were flown aboard the NASA ER-2 high-altitude aircraft as part of the Third Convection and Moisture Experiment (CAMEX-3) and the Fourth Convection and Moisture Experiment (CAMEX-4). Several hurricanes, tropical storms, and other precipitation systems were sampled during these experiments. An oceanic rainfall screening technique has been developed using AMPR passive microwave observations of these systems collected at frequencies of 10.7, 19.35, 37.1, and 85.5 GHz. This technique combines the information content of the four AMPR frequencies regarding the gross vertical structure of hydrometeors into an intuitive and easily executable precipitation mapping format. The results have been verified using vertical profiles of EDOP reflectivity and lower-altitude horizontal reflectivity scans collected by the NOAA WP-3D Orion radar. Matching the rainfall classification results with coincident electric field information collected by the LIP readily identifies convective rain regions within the precipitation fields. This technique shows promise as a real-time research and analysis tool for monitoring vertical updraft strength and convective intensity from airborne platforms such as remotely operated or uninhabited aerial vehicles. The technique is analyzed and discussed for a wide variety of precipitation types using the 26 August 1998 observations of Hurricane Bonnie near landfall.
Hood, R.E., E. Zipser, G.M. Heymsfield, R. Kakar, J. Halverson, R.F. Rogers, and M.L. Black. Overview of the field phase of the NASA Tropical Cloud Systems and Processes (TCSP) Experiment. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 3 pp. (CD-ROM), 2006
Houze, R.A., S.S. Chen, W.-C. Lee, R.F. Rogers, J.A. Moore, G.J. Stossmeister, M.M. Bell, J.L. Cetrone, W. Zhao, and S.R. Brodzik. The Hurricane Rainband and Intensity Experiment: Observations and modeling of Hurricanes Katrina, Ophelia, and Rita (2005). Bulletin of the American Meteorological Society, 87(11):1503-1521, https://doi.org/10.1175/BAMS-87-11-1503 2006
The Hurricane Rainband and Intensity Change Experiment (RAINEX) used three P3 aircraft aided by high-resolution numerical modeling and satellite communications to investigate the 2005 Hurricanes Katrina, Ophelia, and Rita. The aim was to increase the understanding of tropical cyclone intensity change by interactions between a tropical cyclone's inner core and rainbands. All three aircraft had dual-Doppler radars, with the Electra Doppler Radar (ELDORA) on board the Naval Research Laboratory's P3 aircraft, providing particularly detailed Doppler radar data. Numerical model forecasts helped plan the aircraft missions, and innovative communications and data transfer in real time allowed the flights to be coordinated from a ground-based operations center. The P3 aircraft released approximately 600 dropsondes in locations targeted for optimal coordination with the Doppler radar data, as guided by the operations center. The storms were observed in all stages of development, from tropical depression to category 5 hurricane. The data from RAINEX are readily available through an online Field Catalog and RAINEX Data Archive. The RAINEX data-set is illustrated in this article by a preliminary analysis of Hurricane Rita, which was documented by multiaircraft flights on five days (1) while a tropical storm, (2) while rapidly intensifying to a category 5 hurricane, (3) during an eye-wall replacement, (4) when the hurricane became asymmetric upon encountering environmental shear, and (5) just prior to landfall.
Jaimes, B., L.K. Shay, E.W. Uhlhorn, T.M. Cook, J. Brewster, G.R. Halliwell, and P.G. Black. Influence of Loop Current ocean heat content on Hurricanes Katrina, Rita, and Wilma. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Jiang, H., P.G. Black, E.J. Zipser, F.D. Marks, and E.W. Uhlhorn. Validation of rain-rate estimation in hurricanes from the Stepped Frequency Microwave Radiometer: Algorithm correction and error analysis. Journal of the Atmospheric Sciences, 63(1):252-267, https://doi.org/10.1175/JAS3605.1 2006
Simultaneous observations by the lower fuselage (LF) radar, the tail (TA) radar, and the Stepped Frequency Microwave Radiometer (SFMR) on board the NOAA WP-3D aircraft are used to validate the rainfall rate estimates from microwave emission measurements of SFMR in tropical cyclones. Data collected in Hurricane Bonnie (1998) and Hurricane Humberto (2001) with a total of 820 paired samples are used in the comparisons. The SFMR 10-s path-integrated rain rates are found to have an overestimate in light rain and an underestimate in heavy rain relative to radar rainfall estimates. Examination of the existing SFMR algorithm shows that the coefficient should be changed in the attenuation-rain-rate relationship used in the inversion algorithm. After this correction, a linear regression result with a correlation coefficient of 0.8 and a slope close to 1 is obtained. But an overall high bias of 5 mm h-1 of the SFMR rainfall estimate relative to radar is also found. The error analysis shows that the bias is nearly independent of rain type, a result confirming Jorgensen and Willis' conclusion that the drop size distributions between convective and stratiform rain in hurricanes are similar. It is also shown that the bias is a weak function of wind speed, as well as a weak inverse function of radial distance to the hurricane center. Temperature dependence has been ruled out as the main explanation. After doing sensitivity tests, the authors conclude that the bias results from a combination of two factors: an underestimate of the freezing-level height and a downward increase of radar reflectivity in the high wind regions. If the true downward increase is 1-2 dBZ km-1, a 0.5-km underestimate of the freezing-level height could account for up to a 3-5 mm h-1 bias.
Kakar, R., F.D. Marks, G. McFarquhar, and R. Hood. Preface. Journal of the Atmospheric Sciences, 63(1):3-4, https://doi.org/10.1175/JAS9012.1 2006
Kaplan, J., and M. DeMaria. Estimating the likelihood of rapid intensification in the Atlantic and east Pacific basins using SHIPS model data. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Liu, Q., M. Surgi, S. Lord, W.-S. Wu, D. Parrish, S. Gopalakrishnan, J. Waldrop, and J.F. Gamache. Hurricane initialization in HWRF model. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Lonfat, M., R.F. Rogers, F.D. Marks, T. Marchok, and A. Boissonnade. The effect of shear and topography on rainfall forecasting with R-CLIPER. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 2 pp. (CD-ROM), 2006
Majumdar, S.J., S.D. Aberson, C.H. Bishop, R. Buizza, M.S. Peng, and C.A. Reynolds. A comparison of adaptive observing guidance for Atlantic tropical cyclones. Monthly Weather Review, 134(9):2354-2372, https://doi.org/10.1175/MWR3193.1 2006
Airborne adaptive observations have been collected for more than two decades in the neighborhood of tropical cyclones, to attempt to improve short-range forecasts of cyclone track. However, only simple subjective strategies for adaptive observations have been used, and the utility of objective strategies to improve tropical cyclone forecasts remains unexplored. Two objective techniques that have been used extensively for midlatitude adaptive observing programs, and the current strategy based on the ensemble deep-layer mean (DLM) wind variance, are compared quantitatively using two metrics. The ensemble transform Kalman filter (ETKF) uses ensembles from NCEP and the ECMWF. Total-energy singular vectors (TESVs) are computed by the ECMWF and the Naval Research Laboratory, using their respective global models. Comparisons of 78 guidance products for 2-day forecasts during the 2004 Atlantic hurricane season are made, on both continental and localized scales relevant to synoptic surveillance missions. The ECMWF and NRL TESV guidance identifies similar large-scale target regions in 90% of the cases, but are less similar to each other in the local tropical cyclone environment (56% of the cases) with a more stringent criterion for similarity. For major hurricanes, all techniques usually indicate targets close to the storm center. For weaker tropical cyclones, the TESV guidance selects similar targets to those from the ETKF (DLM wind variance) in only 30% (20%) of the cases. ETKF guidance using the ECMWF ensemble is more like that provided by the NCEP ensemble (and DLM wind variance) for major hurricanes than for weaker tropical cyclones. Minor differences in these results occur when a different metric based on the ranking of fixed storm-relative regions is used.
Marchok, T., R.F. Rogers, and R. Tuleya. New methods for evaluating rainfall forecasts from operational models for landfalling tropical cyclones. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 5 pp. (CD-ROM), 2006
McFarquhar, G.M., H. Zhang, G.M. Heymsfield, J.B. Halverson, R.E. Hood, J. Dudhia, and F.D. Marks. Factors affecting the evolution of Hurricane Erin (2001) and the distributions of hydrometeors: Role of microphysical processes. Journal of the Atmospheric Sciences, 63(1):127-150, https://doi.org/10.1175/JAS3590.1 2006
Fine-resolution simulations of Hurricane Erin are conducted using the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) to investigate roles of thermodynamic, boundary layer, and microphysical processes on Erin's structure and evolution. Choice of boundary layer scheme has the biggest impact on simulations, with the minimum surface pressure (Pmin) averaged over the last 18 h (when Erin is relatively mature) varying by over 20 hPa. Over the same period, coefficients used to describe graupel fall speeds (Vg) affect Pmin by up to 7 hPa, almost equivalent to the maximum 9-hPa difference between microphysical parameterization schemes; faster Vg and schemes with more hydrometeor categories generally give lower Pmin. Compared to radar reflectivity factor (Z) observed by the NOAA P-3 lower fuselage radar and the NASA ER-2 Doppler radar (EDOP) in Erin, all simulations overpredict the normalized frequency of occurrence of Z larger than 40 dBZ and underpredict that between 20 and 40 dBZ near the surface; simulations overpredict Z larger than 25 to 30 dBZ and underpredict that between 15 and 25 or 30 dBZ near the melting layer, the upper limit depending on altitude. Brightness temperatures (Tb) computed from modeled fields at 37.1- and 85.5-GHz channels that respond to scattering by graupel-size ice show enhanced scattering, mainly due to graupel, compared to observations. Simulated graupel mixing ratios are about 10 times larger than values observed in other hurricanes. For the control run at 6.5 km averaged over the last 18 simulated hours, Doppler velocities computed from modeled fields (Vdop) greater than 5 m s-1 make up 12% of Erin's simulated area for the base simulation but less than 2% of the observed area. In the eyewall, 5% of model updrafts above 9 km are stronger than 10 m s-1, whereas statistics from other hurricanes show that 5% of updrafts are stronger than only 5 m s-1. Variations in distributions of Z, vertical motion, and graupel mixing ratios between schemes are not sufficient to explain systematic offsets between observations and models. A new iterative condensation scheme, used with the Reisner mixed-phase microphysics scheme, limits unphysical increases of equivalent potential temperature associated with many condensation schemes and reduces the frequency of Z larger than 50 dBZ, but has minimal effect on Z below 50 dBZ, which represent 95% of the modeled hurricane rain area. However, the new scheme changes the Erin simulations in that 95% of the updrafts are weaker than 5 m s-1 and Pmin is up to 12 hPa higher over the last 18 simulated hours.
Molinari, J., P.P. Dodge, D. Vollaro, K.L. Corbosiero, and F.D. Marks. Mesoscale aspects of the downshear reformation of a tropical cyclone. Journal of the Atmospheric Sciences, 63(1):341-354, https://doi.org/10.1175/JAS3591.1 2006
The downshear reformation of Tropical Storm Gabrielle (2001) was investigated using radar reflectivity and lightning data that were nearly continuous in time, as well as frequent aircraft reconnaissance flights. Initially the storm was a marginal tropical storm in an environment with strong 850-200-hPa vertical wind shear of 12-13 m s-1 and an approaching upper tropospheric trough. Both the observed outflow and an adiabatic balance model calculation showed that the radial-vertical circulation increased with time as the trough approached. Convection was highly asymmetric, with almost all radar return located in one quadrant left of downshear in the storm. Reconnaissance data show that an intense mesovortex formed downshear of the original center. This vortex was located just south of, rather than within, a strong downshear-left lightning outbreak, consistent with tilting of the horizontal vorticity associated with the vertical wind shear. The downshear mesovortex contained a 972-hPa minimum central pressure, 20 hPa lower than minimum pressure in the original vortex just 3 h earlier. The mesovortex became the new center of the storm, but weakened somewhat prior to landfall. It is argued that dry air carried around the storm from the region of upshear subsidence, as well as the direct effects of the shear, prevented the reformed vortex from continuing to intensify. Despite the subsequent weakening of the reformed center, it reached land with greater intensity than the original center. It is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough. In addition, the new center formed much closer to the coast and made landfall much earlier than predicted. Such vertical-shear-induced intensity and track fluctuations are important to understand, especially in storms approaching the coast.
Montgomery, M.T., M.M. Bell, S.D. Aberson, and M.L. Black. Hurricane Isabel (2003): New insights into the physics of intense storms, Part I: Mean vortex structure and maximum intensity estimates. Bulletin of the American Meteorological Society, 87(10):1335-1347, https://doi.org/10.1175/BAMS-87-10-1335 2006
This study is an observational analysis of the inner-core structure, sea surface temperature, outflow layer, and atmospheric boundary layer of an intense tropical cyclone whose intensity and structure is consistent with recent numerical and theoretical predictions of superintense storms. The findings suggest new scientific challenges for the current understanding of hurricanes. Unprecedented observations of the category-5 Hurricane Isabel (2003) were collected during 12-14 September. This two-part article reports novel dynamic and thermodynamic aspects of the inner-core structure of Isabel on 13 September that was made possible by analysis of these data. Here, a composite of the axisymmetric structure of the inner core and environment of Isabel is estimated using global positioning system dropwindsondes and in situ aircraft data. In Part II, an extreme wind speed observation on the same day is discussed in the context of this work. The axisymmetric data composite suggests a reservoir of high-entropy air inside the low-level eye and significant penetration of inflowing near-surface air from outside. The analysis suggests that the low-level air penetrating the eye is enhanced thermodynamically by acquiring additional entropy through interaction with the ocean and replaces air mixed out of the eye. The results support the hypothesis that this high-entropy eye air "turboboosts" the hurricane engine upon its injection into the eyewall clouds. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound by 10-35 m s-1 for the given environmental conditions. Additional study with other datasets is encouraged to further test the superintensity hypothesis.
Piekle, R.A., C.W. Landsea, M. Mayfield, J. Laver, and R. Pasch. Reply to "Hurricanes and global warming potential linkages and consequences." Bulletin of the American Meteorological Society, 87(5):628-631, https://doi.org/10.1175/BAMS-87-5-622 2006
Reynolds, C.A., M.S. Peng, S.J. Majumdar, S.D. Aberson, C.H. Bishop, and R. Buizza. Interpretation of tropical cyclone targeting guidance. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 10 pp. (CD-ROM), 2006
Rogers, R.F., M.L. Black, F.D. Marks, K.M. Valde, and S.S. Chen. A comparison of tropical cyclone hydrometeor profiles from TRMM, airborne radar, and high-resolution simulations. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 3 pp. (CD-ROM), 2006
Rogers, R.F., M.L. Black, P.T. Willis, R.A. Black, A. Heymsfield, A. Bansemer, and G. Heymsfield. An evaluation of the microphysics fields of Hurricane Dennis (2005) at different stages of its lifecycle. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Rogers, R.F., M.L. Black, R. Hood, J. Halverson, E. Zipser, and G. Heymsfield. The Intensity Forecasting Experiment (IFEX): A NOAA multi-year field program for improving tropical cyclone intensity forecasting. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 5 pp. (CD-ROM), 2006
Rogers, R.F., S.D. Aberson, M.L. Black, P.G. Black, J.J. Cione, P.P. Dodge, J.P. Dunion, J.F. Gamache, J. Kaplan, M.D. Powell, L.N. Shay, N. Surgi, and E.W. Uhlhorn. The Intensity Forecasting Experiment: A NOAA multi-year field program for improving tropical cyclone intensity forecasts. Bulletin of the American Meteorological Society, 87(11):1523-1537, https://doi.org/10.1175/BAMS-87-11-1523 2006
In 2005, NOAA's Hurricane Research Division (HRD), part of the Atlantic Oceanographic and Meteorological Laboratory, began a multi-year experiment called the Intensity Forecasting Experiment (IFEX). By emphasizing a partnership among NOAA's HRD, Environmental Modeling Center (EMC), National Hurricane Center (NHC), Aircraft Operations Center (AOC), and National Environmental Satellite Data Information Service (NESDIS), IFEX represents a new approach for conducting hurricane field program operations. IFEX is intended to improve the prediction of tropical cyclone (TC) intensity change by: (1) collecting observations that span the TC life cycle in a variety of environments; (2) developing and refining measurement technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and (3) improving the understanding of the physical processes important in intensity change for a TC at all stages of its life cycle. This paper presents a summary of the accomplishments of IFEX during the 2005 hurricane season. New and refined technologies for measuring such fields as surface and three-dimensional wind fields, and the use of unmanned aerial vehicles, were achieved in a variety of field experiments that spanned the life cycle of several tropical cyclones, from formation and early organization to peak intensity and subsequent landfall or extratropical transition. Partnerships with other experiments during 2005 also expanded the spatial and temporal coverage of the data collected in 2005. A brief discussion of the plans for IFEX in 2006 is also provided.
Shay, L.K., and E.W. Uhlhorn. Loop Current interactions during Hurricanes Isidore and Lili. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 4 pp. (CD-ROM), 2006
Stern, D., and S.D. Aberson. Extreme vertical winds measured by dropsondes in hurricanes. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 5 pp. (CD-ROM), 2006
Tory, K.J., M.T. Montgomery, and N.E. Davidson. Prediction and diagnosis of tropical cyclone formation in an NWP system, Part I: The critical role of vortex enhancement in deep convection. Journal of the Atmospheric Sciences, 63(12):3077-3090, https://doi.org/10.1175/JAS3764.1 2006
This is the first of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. The primary TC-LAPS vortex enhancement mechanism is presented in Part I, the entire genesis process is illustrated in Part II using a single TC-LAPS simulation, and in Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. The primary vortex enhancement mechanism in TC-LAPS is found to be convergence/stretching and vertical advection of absolute vorticity in deep intense updrafts, which result in deep vortex cores of 60-100 km in diameter (the minimum resolvable scale is limited by the 0.15° horizontal grid spacing). On the basis of the results presented, it is hypothesized that updrafts of this scale adequately represent mean vertical motions in real TC genesis convective regions, and perhaps that explicitly resolving the individual convective processes may not be necessary for qualitative TC genesis forecasts. Although observations of sufficient spatial and temporal resolution do not currently exist to support or refute this proposition, relatively large-scale (30 km and greater), lower- to midlevel tropospheric convergent regions have been observed in tropical oceanic environments during the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE), the Equatorial Mesoscale Experiment (EMEX), and the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and regions of extreme convection of the order of 50 km are often (remotely) observed in TC genesis environments. These vortex cores are fundamental for genesis in TC-LAPS. They interact to form larger cores, and provide net heating that drives the system-scale secondary circulation, which enhances vorticity on the system scale akin to the classical Eliassen problem of a balanced vortex driven by heat sources. These secondary vortex enhancement mechanisms are documented in Part II. In some recent TC genesis theories featured in the literature, vortex enhancement in deep convective regions of mesoscale convective systems (MCSs) has largely been ignored. Instead, they focus on the stratiform regions. While it is recognized that vortex enhancement through midlevel convergence into the stratiform precipitation deck can greatly enhance midtropospheric cyclonic vorticity, it is suggested here that this mechanism only increases the potential for genesis, whereas vortex enhancement through low- to midlevel convergence into deep convective regions is necessary for genesis.
Tory, K.J., M.T. Montgomery, N.E. Davidson, and J.D. Kepert. Prediction and diagnosis of tropical cyclone formation in an NWP system, Part II: A diagnosis of Tropical Cyclone Chris formation. Journal of the Atmospheric Sciences, 63(12):3091-3113, https://doi.org/10.1175/JAS3765.1 2006
This is the second of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS). The primary TC-LAPS vortex enhancement mechanism (convergence/stretching and vertical advection of absolute vorticity in convective updraft regions) was presented in Part I. In this paper (Part II) results from a numerical simulation of TC Chris (western Australia, February 2002) are used to illustrate the primary and two secondary vortex enhancement mechanisms that led to TC genesis. In Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. During the first 18 h of the simulation, a mature vortex of TC intensity developed in a monsoon low from a relatively benign initial state. Deep upright vortex cores developed from convergence/stretching and vertical advection of absolute vorticity within the updrafts of intense bursts of cumulus convection. Individual convective bursts lasted for 6-12 h, with a new burst developing as the previous one weakened. The modeled bursts appear as single updrafts, and represent the mean vertical motion in convective regions because the 0.15° grid spacing imposes a minimum updraft scale of about 60 km. This relatively large scale may be unrealistic in the earlier genesis period when multiple smaller-scale, shorter-lived convective regions are often observed, but observational evidence suggests that such scales can be expected later in the process. The large scale may limit the convection to only one or two active bursts at a time, and may have contributed to a more rapid model intensification than that observed. The monsoon low was tilted to the northwest, with convection initiating about 100-200 km west of the low-level center. The convective bursts and associated upright potential vorticity (PV) anomalies were advected cyclonically around the low, weakening as they passed to the north of the circulation center, leaving remnant cyclonic PV anomalies. Strong convergence into the updrafts led to rapid ingestion of nearby cyclonic PV anomalies, including remnant PV cores from decaying convective bursts. Thus convective intensity, rather than the initial vortex size and intensity, determined dominance in vortex interactions. This scavenging of PV by the active convective region, termed diabatic upscale vortex cascade, ensured that PV cores grew successively and contributed to the construction of an upright central monolithic PV core. The system-scale intensification (SSI) process active on the broader scale (300-500-km radius) also contributed. Latent heating slightly dominated adiabatic cooling within the bursts, which enhanced the system-scale secondary circulation. Convergence of low- to midlevel tropospheric absolute vorticity by this enhanced circulation intensified the system-scale vortex. The diabatic upscale vortex cascade and SSI are secondary processes dependent on the locally enhanced vorticity and heat respectively, generated by the primary mechanism.
Uhlhorn, E.W., and L.K. Shay. Mechanical energy and vorticity balances within the ocean mixed layer under tropical cyclones. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 3 pp. (CD-ROM), 2006
Walsh, K.J.E., M. Fiorina, C.W. Landsea, and K. McInnes. Objective detection of tropical cyclones in climate models. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 2 pp. (CD-ROM), 2006
Wang, C., D.B. Enfield, S.-K. Lee, and C.W. Landsea. Influences of the Atlantic warm pool on western hemisphere summer rainfall and Atlantic hurricanes. Journal of Climate, 19(12):3011-3028, https://doi.org/10.1175/JCLI3770.1 2006
The Atlantic warm pool (AWP) of water warmer than 28.5°C comprises the Gulf of Mexico, the Caribbean Sea, and the western tropical North Atlantic (TNA). The AWP reaches its maximum size around September, with large AWPs being almost three times larger than small ones. Although ENSO teleconnections are influential on the AWP, about two-thirds of the large and small AWP variability appears unrelated to ENSO. The AWP is usually geographically different from the TNA; however, the AWP size is correlated with the TNA SST anomalies. During August to October, large AWPs and warm TNA are associated with increased rainfall over the Caribbean, Mexico, the eastern subtropical Atlantic, and the southeast Pacific, and decreased rainfall in the northwest United States, Great Plains, and eastern South America. In particular, rainfall in the Caribbean, Central America, and eastern South America from August to October is mainly related to the size of the AWP. Large (small) AWPs and warm (cold) TNA correspond to a weakening (strengthening) of the northward surface winds from the AWP to the Great Plains that disfavors (favors) moisture transport for rainfall over the Great Plains. On the other hand, large (small) AWPs and warm (cold) TNA strengthen (weaken) the summer regional Atlantic Hadley circulation that emanates from the warm pool region into the southeast Pacific, changing the subsidence over the southeast Pacific and thus the stratus cloud and drizzle there. The large AWP, associated with a decrease in sea level pressure and an increase in atmospheric convection and cloudiness, corresponds to a weak tropospheric vertical wind shear and a deep warm upper ocean, and thus increases Atlantic hurricane activity.
Willoughby, H.E., R.W. Darling, and M.E. Rahn. Parametric representation of the primary hurricane vortex, Part II: A new family of sectionally continuous profiles. Monthly Weather Review, 134(4):1102-1120, https://doi.org/10.1175/MWR3106.1 2006
For applications such as windstorm underwriting or storm-surge forecasting, hurricane wind profiles are often approximated by continuous functions that are zero at the vortex center, increase to a maximum in the eyewall, and then decrease asymptotically to zero far from the center. Comparisons between the most commonly used functions and aircraft observations reveal systematic errors. Although winds near the peak are too strong, they decrease too rapidly with distance away from the peak. Pressure-wind relations for these profiles typically overestimate maximum winds. A promising alternative is a family of sectionally continuous profiles in which the wind increases as a power of radius inside the eye and decays exponentially outside the eye after a smooth polynomial transition across the eyewall. Based upon a sample of 493 observed profiles, the mean exponent for the power law is 0.79 and the mean decay length is 243 km. The database actually contains 606 aircraft sorties, but 113 of these failed quality-control screening. Hurricanes stronger than Saffir-Simpson category 2 often require two exponentials to match the observed rapid decrease of wind with radius just outside the eye and slower decrease farther away. Experimentation showed that a fixed value of 25 km was satisfactory for the faster decay length. The mean value of the slower decay length was 295 km. The mean contribution of the faster exponential to the outer profile was 0.10, but for the most intense hurricanes it sometimes exceeded 0.5. The power-law exponent and proportion of the faster decay length increased with maximum wind speed and decreased with latitude, whereas the slower decay length decreased with intensity and increased with latitude, consistent with the qualitative observation that more intense hurricanes in lower latitudes usually have more sharply peaked wind profiles.
Yablonsky, R.M., I. Ginis, E.W. Uhlhorn, and A. Falkovich. Using AXBTs to improve the performance of coupled hurricane-ocean models. Preprints, 27th Conference on Hurricanes and Tropical Meteorology, Monterey, CA, April 24-28, 2006. American Meteorological Society, Boston, 7 pp. (CD-ROM), 2006
2005
Bateman, M., D. Mach, S. Lewis, J. Dye, E. Defer, C.A. Grainger, and P.T. Willis. Comparison of in-situ electric field and radar derived parameters for stratiform clouds in central Florida. Preprints, Conference on Meteorological Applications of Lightning Data, San Diego, CA, January 9-13, 2005. American Meteorological Society, Boston, 8 pp., 2005
Bell, G.D., S.B. Goldenberg, C.W. Landsea, E.S. Blake, R. Pasch, M. Chelliah, and K. Mo. Atlantic hurricane season. In State of the Climate in 2004, D.H. Levinson (ed.). Bulletin of the American Meteorological Society, 86(6):S26-S29, https://doi.org/10.1175/BAMS-86-6-Levinson 2005
Blake, E.S., E.N. Rappaport, J.D. Jarrell, and C.W. Landsea. The deadliest, costliest, and most intense United States tropical cyclones from 1851 to 2004 (and other frequently requested hurricane facts). NOAA Technical Memorandum, NWS-TPC-4, 48 pp., 2005
This technical memorandum lists the deadliest and costliest tropical cyclones in the United States during 1851-2004. The compilation ranks damage, as expressed by monetary losses, in three ways: (1) contemporary estimates; (2) contemporary estimates adjusted by inflation to 2004 dollars; and (3) contemporary estimates adjusted for inflation and the growth of population and personal wealth (Pielke and Landsea, 1998) to 2004. In addition, the most intense (i.e., major) hurricanes to make landfall in the United States during the period are listed. Some additional statistics on United States hurricanes of this and previous centuries, and tropical cyclones in general, are also presented.
Corbosiero, K.L., J. Molinari, and M.L. Black. The structure and evolution of Hurricane Elena (1985), Part 1: Symmetric intensification. Monthly Weather Review, 133(10):2905-2921, https://doi.org/10.1175/MWR3010.1 2005
One of the most complete aircraft reconnaissance and ground-based radar datasets of a single tropical cyclone was recorded in Hurricane Elena (1985) as it made a slow, three-day anticyclonic loop in the Gulf of Mexico. Eighty-eight radial legs and 47 vertical incidence scans were collected aboard NOAA WP-3D aircraft, and 1,142 ground-based radar scans were made of Elena's eyewall and inner rainbands as the storm intensified from a disorganized category 2 to an intense category 3 hurricane. This large amount of continuously collected data made it possible to examine changes that occurred in Elena's inner-core symmetric structure as the storm intensified. On the first day of study, Elena was under the influence of vertical wind shear from an upper-tropospheric trough to the west. The storm was disorganized, with no discernable eyewall and nearly steady values of tangential wind and relative vorticity. Early on the second day of study, a near superposition and constructive interference occurred between the trough and Elena, coincident with upward vertical velocities and the radial gradient of reflectivity becoming concentrated around the 30-km radius. Once an inner wind maximum and eyewall developed, the radius of maximum winds contracted and a sharp localized vorticity maximum emerged, with much lower values on either side. This potentially unstable vorticity profile was accompanied by a maximum in equivalent potential temperature in the eyewall, deeper and stronger inflow out to 24 km from the eyewall, and mean outflow toward the eyewall from the eye. Within 6-12 h, intensification came to an end and Elena began to slowly weaken. Vorticity and equivalent potential temperature at 850 hPa showed indications of prior mixing between the eye and eyewall. During the weakening stage, an outflow jet developed at the eyewall radius. A strong 850-hPa updraft accompanied the outflow jet, yet convection was less active aloft than before. This feature appeared to represent a shallow, outward-sloping updraft channel associated with the spindown of the storm.
DeMaria, M., M. Mainelli, L.K. Shay, J.A. Knaff, and J. Kaplan. Further improvements to the Statistical Hurricane Intensity Prediction Scheme (SHIPS). Weather and Forecasting, 20(4):531-543, https://doi.org/10.1175/WAF862.1 2005
Modifications to the Atlantic and east Pacific versions of the operational Statistical Hurricane Intensity Prediction Scheme (SHIPS) for each year from 1997 to 2003 are described. Major changes include the addition of a method to account for the storm decay over land in 2000, the extension of the forecasts from three to five days in 2001, and the use of an operational global model for the evaluation of the atmospheric predictors instead of a simple dry-adiabatic model beginning in 2001. A verification of the SHIPS operational intensity forecasts is presented. Results show that the 1997-2003 SHIPS forecasts had statistically significant skill (relative to climatology and persistence) out to 72 h in the Atlantic, and at 48 and 72 h in the east Pacific. The inclusion of the land effects reduced the intensity errors by up to 15% in the Atlantic, and up to 3% in the east Pacific, primarily for the shorter-range forecasts. The inclusion of land effects did not significantly degrade the forecasts at any time period. Results also showed that the four to five-day forecasts that began in 2001 did not have skill in the Atlantic, but had some skill in the east Pacific. An experimental version of SHIPS that included satellite observations was tested during the 2002 and 2003 seasons. New predictors included brightness temperature information from Geostationary Operational Environmental Satellite (GOES) channel 4 (10.7 µm) imagery, and oceanic heat content (OHC) estimates inferred from satellite altimetry observations. The OHC estimates were only available for the Atlantic basin. The GOES data significantly improved the east Pacific forecasts by up to 7% at 12-72 h. The combination of GOES and satellite altimetry improved the Atlantic forecasts by up to 3.5% through 72 h for those storms west of 50°W.
Eastin, M.D., W.M. Gray, and P.G. Black. Buoyancy of convective vertical motions in the inner core of intense hurricanes, Part I: General statistics. Monthly Weather Review, 133(1):188-208, https://doi.org/10.1175/MWR-2848.1 2005
The buoyancy of hurricane convective vertical motions is studied using aircraft data from 175 radial legs collected in 14 intense hurricanes at four altitudes ranging from 1.5 to 5.5 km. The data of each leg are initially filtered to separate convective-scale features from background mesoscale structure. Convective vertical motion events, called cores, are identified using the criteria that the convective-scale vertical velocity must exceed 1.0 m s-1 for at least 0.5 km. A total of 620 updraft cores and 570 downdraft cores are included in the dataset. Total buoyancy is calculated from convective-scale virtual potential temperature, pressure, and liquid water content using the mesoscale structure as the reference state. Core properties are summarized for the eyewall and rainband regions at each altitude. Characteristics of core average convective vertical velocity, maximum convective vertical velocity, and diameter are consistent with previous studies of hurricane convection. Most cores are superimposed upon relatively weak mesoscale ascent. The mean eyewall (rainband) updraft core exhibits small, but statistically significant, positive total buoyancy below 4 km (between 2 and 5 km) and a modest increase in vertical velocity with altitude. The mean downdraft core not superimposed upon stronger mesoscale ascent also exhibits positive total buoyancy and a slight decrease in downward vertical velocity with decreasing altitude. Buoyant updraft cores cover less than 5% of the total area in each region but accomplish ~40% of the total upward transport. A one-dimensional updraft model is used to elucidate the relative roles played by buoyancy, vertical perturbation pressure gradient forces, water loading, and entrainment in the vertical acceleration of ordinary updraft cores. Small positive total buoyancy values are found to be more than adequate to explain the vertical accelerations observed in updraft core strength, which implies that ordinary vertical perturbation pressure gradient forces are directed downward, opposing the positive buoyancy forces. Entrainment and water loading are also found to limit updraft magnitudes. The observations support some aspects of both the hot tower hypothesis and symmetric moist neutral ascent, but neither concept appears dominant. Buoyant convective updrafts, however, are integral components of the hurricane's transverse circulation.
Eastin, M.D., W.M. Gray, and P.G. Black. Buoyancy of convective vertical motions in the inner core of intense hurricanes, Part II: Case studies. Monthly Weather Review, 133(1):209-227, https://doi.org/10.1175/MWR-2849.1 2005
This is the second of two papers on the buoyancy of convective vertical motions in the inner core of intense hurricanes. This paper uses extensive airborne radar, dropwindsonde, and flight-level observations in Hurricanes Guillermo (1997) and Georges (1998) to illustrate typical azimuthal distribution of buoyant convection and demonstrate that the low-level eye can be an important source region for buoyant eyewall convection. In both hurricanes, eyewall vertical velocity and radar reflectivity are asymmetric and exhibit persistent relationships with the direction of the environmental vertical wind shear. Mesoscale vertical motions exhibit a wavenumber-1 structure with maximum ascent downshear and weak descent upshear. The mesoscale reflectivity maxima are located left-of-shear. Buoyant eyewall updraft cores and transient convective-scale reflectivity cells are predominantly downshear and left-of-shear. Most eyewall downdraft cores that transport significant mass downward are located upshear. Negative buoyancy was most common in left-of-shear downdrafts, with positive buoyancy dominant in upshear downdrafts. Inward-spiraling rainbands located outside the eyewall exhibit upband/downband asymmetries. Upband segments contain more convective reflectivity cells and buoyant updraft cores than the more stratiform downband segments. Equal numbers of downdraft cores are found upband and downband, but the majority exhibit negative buoyancy. Several buoyant updraft cores encountered in the midlevel eyewall exhibit equivalent potential temperatures (θe) much higher than the θe observed in the low-level eyewall, but equivalent to the θe observed in the low-level eye. Asymmetric low-wavenumber circulations appear responsible for exporting the high-θe eye air into the relatively low-θe eyewall and generating the locally buoyant updraft cores. Implications of these results upon conceptual models of hurricane structure are discussed. Three mechanisms, whereby an ensemble of asymmetric buoyant convection could contribute to hurricane evolution, are also discussed.
Etherton, B.J., and S.D. Aberson. Ensemble based data assimilation of observations of Hurricane Humberto. Preprints, 9th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface, San Diego, CA, January 9-13, 2005. American Meteorological Society, Boston, 5 pp., 2005
Knabb, R.D., J.-G. Jiing, C.W. Landsea, and W.R. Seguin. The Joint Hurricane Testbed (JHT): Progress and future plans. Preprints, 9th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface, San Diego, CA, January 9-13, 2005. American Meteorological Society, Boston, 3 pp., 2005
Landsea, C.W. Meteorology: Hurricanes and global warming. Nature, 438(7071):E11-E12, https://doi.org/10.1038/nature04477 2005
Majumdar, S.J., S.D. Aberson, C.H. Bishop, R. Buizza, M.S. Peng, and C.A. Reynolds. A comparison of adaptive observing guidance for Atlantic tropical cyclones. European Centre for Medium-Range Weather Forecasts, Technical Memorandum No. 482, 24 pp., 2005
Airborne adaptive observations have been collected for more than two decades in the neighborhood of tropical cyclones, to attempt to improve short-range forecasts of cyclone track. However, only simple subjective strategies for adaptive observations have been used, and the utility of objective strategies to improve tropical cyclone forecasts remains unexplored. Two objective techniques that have been used extensively for mid-latitude adaptive observing programs, and the current strategy based on the ensemble deep-layer mean (DLM) wind variance, are compared quantitatively using two metrics. The ensemble transform Kalman filter (ETKF) uses ensembles from NCEP and ECMWF. Total-energy singular vectors (TESVs) are computed by ECMWF and the Naval Research Laboratory, using their respective global models. Comparisons of 78 guidance products for two-day forecasts during the 2004 Atlantic hurricane season are made, on both continental and localized scales relevant to synoptic surveillance missions. The ECMWF and NRL TESV guidance identifies similar large-scale target regions in 90% of the cases, but are less similar to each other in the local tropical cyclone environment (56% of the cases) with a more stringent criterion for similarity. For major hurricanes, all techniques usually indicate targets close to the storm center. For weaker tropical cyclones, the TESV guidance selects similar targets to those from the ETKF (DLM wind Variance) in only 30% (20%) of the cases. ETKF guidance using the ECMWF ensemble is more like that provided by the NCEP ensemble (and DLM wind variance) for major hurricanes than for weaker tropical cyclones. Minor differences in these results occur when a different metric based on the ranking of fixed storm-relative regions is used.
Michaels, P.J., P.C. Knappenberger, and C.W. Landsea. Comments on "Impacts of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective scheme." Journal of Climate, 18(23):5179-5182, https://doi.org/10.1175/JCLI3592.1 2005
In a simulation of enhanced tropical cyclones in a warmer world, Knutson and Tuleya make several assumptions that are not borne out in the real world. They include an unrealistically large carbon dioxide growth rate, an overly strong relationship between sea surface temperature and hurricane intensity, and the use of a mesoscale model that has shown little to no useful skill in predicting current-day hurricane intensity. After accounting for these inaccuracies, a detectable increase in Atlantic hurricane intensity in response to growing atmospheric greenhouse gas levels during this century becomes unlikely.
Morrison, I., S. Businger, F.D. Marks, P.P. Dodge, and J.A. Businger. An observational case for the prevalence of roll vortices in the hurricane boundary layer. Journal of the Atmospheric Sciences, 62(8):2662-2673, https://doi.org/10.1175/JAS3508.1 2005
Doppler velocity data from the WSR-88D radar during four hurricane landfalls are analyzed to investigate the presence of organized vortices in the hurricane boundary layer (HBL). The wavelength, depth, magnitude, and track of velocity anomalies were compiled through analysis of Doppler velocity data. The analysis reveals alternating bands of enhanced and reduced azimuthal winds that are closely aligned with the mean wind direction. Resulting statistics provide compelling evidence for the presence of organized secondary circulations or boundary layer rolls across significant areas during four hurricane landfalls. The results confirm previous observations of the presence of rolls in the HBL. A potential limitation of the study presented here is the resolution of the WSR-88D data. In particular, analysis of higher resolution data (e.g., from the Doppler on Wheels) is needed to confirm that data aliasing has not unduly impacted the statistics reported here. Momentum fluxes associated with the secondary circulations are estimated using the covariance between the horizontal and vertical components of the wind fluctuations in rolls, with resulting fluxes 2-3 times greater than estimated by parameterizations in numerical weather prediction models. The observational analysis presented here, showing a prevalence of roll vortices in the HBL, has significant implications for the vertical transport of energy in hurricanes, for the character of wind damage, and for improvements in numerical simulations of hurricanes.
Nuissier, O., R.F. Rogers, and F. Roux. A numerical simulation of Hurricane Bret on 22-23 August 1999 initialized with airborne Doppler radar and dropsonde data. Quarterly Journal of the Royal Meteorological Society, 131(605):155-194, https://doi.org/10.1256/qj.02.233 2005
This study concerns the simulation of Hurricane Bret on 22-23 August 1999 with the MesoNH non-hydrostatic, two-way interactive, quadruply nested grid mesoscale model. A 30-h integration, from 0000 UTC 22 August to 0600 23 August, covers the period of maximum intensity over the Gulf of Mexico and landfall over Texas. Special attention is paid to the initial conditions from which the model is integrated. A balanced vortex, derived from airborne Doppler radar data, is used to replace the ill-defined cyclone in the large-scale analysis. In addition, the analyzed humidity field over the Gulf of Mexico is modified in accordance with specific dropsonde observations. A comparison between the simulated storm track and intensity for three different numerical experiments shows that the inclusion of the radar-derived vortex and high spatial resolution are necessary to obtain a realistic simulation. After an initial period of adjustment, the simulation with the inserted radar-derived vortex and high resolution produces a storm only 10 hPa weaker than the observation after 24 h, compared to the control run that was nearly 50 hPa weaker at the same time. The characteristics of this simulated storm at mature stage are then presented, with particular emphasis placed on the processes that modulate the intensity of the inner core region.
Piekle, R.A., Jr., C.W. Landsea, M. Mayfield, J. Laver, and R. Pasch. Hurricanes and global warming. Bulletin of the American Meteorological Society, 86(11):1571-1575, https://doi.org/10.1175/BAMS-86-11-1571 2005
This paper reviews recent research on tropical cyclones and climate change from the perspective of event riskthe physical behavior of storms; vulnerabilitythe characteristics of a system that creates the potential for impacts, but are independent of event risk; and also outcome riskthe integration of considerations of vulnerability with event risk to characterize an event that causes losses. The paper concludes that with no trend identified in various metrics of hurricane damage over the 20th century, it is exceedingly unlikely that scientists will identify large changes in historical storm behavior that have significant societal implications, though scientists may identify discernible changes in storm behavior. Looking to the future, until scientists conclude (a) that there will be changes to storms that are significantly larger than observed in the past, (b) that such changes are correlated to measures of societal impact, and (c) that the effects of such changes are significant in the context of inexorable growth in population and property at risk, then it is reasonable to conclude that the significance of any connection of human-caused climate change to hurricane impacts necessarily has been and will continue to be exceedingly small.
Powell, M.D., G.A. Soukup, S. Cocke, S. Gulati, N. Morisseau-Leroy, S. Hamid, N.M. Dorst, and L. Axe. State of Florida hurricane loss projection model: Atmospheric science component. Journal of Wind Engineering and Industrial Aerodynamics, 93(8):651-674, https://doi.org/10.1016/j.jweia.2005.05.008 2005
The State of Florida has developed an open, public model for the purpose of probabilistic assessment of risk to insured residential property associated with wind damage from hurricanes. The model comprises atmospheric science, engineering, and financial/actuarial components and is planned for submission to the Florida Commission on Hurricane Loss Projection Methodology. The atmospheric component includes modeling the track and intensity life cycle of each simulated hurricane within the Florida threat area. When a model storm approaches within a damage threshold distance of a Florida zip code location, the wind field is computed by a slab model of the hurricane boundary layer coupled with a surface layer model based on the results of recent GPS sonde research. A time series of open terrain surface winds is then computed for each zip code in the threatened area. Depending on wind direction, an effective roughness length is assigned to each zip code based on the upstream fetch roughness as determined from remotely sensed land cover/land use products. Based on historical hurricane statistics, thousands of storms are simulated allowing determination of the wind risk for all residential zip code locations in Florida. The wind risk information is then provided to the engineering and loss models to assess damage and average annual loss, respectively.
Velden, C., J. Daniels, D. Stettner, D. Santek, J. Key, J.P. Dunion, K. Holmlund, G. Dengel, W. Bresky, and P. Menzel. Recent innovations in deriving tropospheric winds from meteorological satellites. Bulletin of the American Meteorological Society, 86(2):205-223, https://doi.org/10.1175/BAMS-86-2-205 2005
The evolving constellation of environmental/meteorological satellites and their associated sensor technology is rapidly advancing. This is providing opportunities for creatively improving satellite-derived products used in weather analysis and forecasting. For example, the retrieval methods for deriving atmospheric motion vectors (AMVs) from satellites have been expanding and evolving since the early 1970s. Contemporary AMV processing methods are continuously being updated and advanced through the exploitation of new sensor technologies and innovative new approaches. It is incumbent upon the research community working in AMV extraction techniques to ensure that the quality of the current operational products meets or exceeds the needs of the user community. In particular, the advances in data assimilation and numerical weather prediction in recent years have placed an increasing demand on data quality. To keep pace with these demands, innovative research toward improving methods of deriving winds from satellites has been a focus of the World Meteorological Organization and Coordination Group for Meteorological Satellites (CGMS) cosponsored International Winds Workshops (IWWs). The IWWs are held every tw years, and bring together AMV researchers from around the world to present new ideas on AMV extraction techniques, interpretation, and applications. The NWP community is always well represented at these workshops, which provide an important exchange of information on the latest in data assimilation issues. This article draws from recent IWWs, and describes several new advances in satellite-produced wind technologies, derivation methodologies, and products. Examples include AMVs derived from Geostationary Operational Environmental Satellite (GOES) rapid scans and the short-wave IR channel, AMVs over the polar regions from the Moderate Resolution Imaging Spectroradiometer (MODIS), improved AMV products from the new Meteosat Second Generation satellite, and new processing approaches for deriving AMVs. The article also provides a glimpse into the pending opportunities that will be afforded with emerging/anticipated new sensor technologies.
Wu, C.-C., P.-H. Lin, and S.D. Aberson. Typhoon surveillance in northwestern Pacific. Vaisala News, 167:24-25, 2005
Wu, C.-C., P.-H. Lin, S.D. Aberson, T.-C. Yeh, W.-P. Huang, K.-H. Chou, J.-S. Hong, G.-C. Lu, C.-T. Fong, K.-C. Hsu, I.-I. Lin, P.-L. Lin, and C.-H. Liu. Dropsonde observations for typhoon surveillance near the Taiwan region (DOTSTAR): An overview. Bulletin of the American Meteorological Society, 86(6):787-790, https://doi.org/10.1175/BAMS-86-6-787 2005
2004
Aberson, S.D. The G-IV surveillance era, targeting, and ensemble forecasts (1997-present). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 236-237, 2004
Aberson, S.D., M.L. Black, M.T. Montgomery, and M. Bell. A record wind measurement in Hurricane Isabel: Direct evidence of an eyewall mesocyclone? Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 168-169, 2004
Bell, G.D., S.B. Goldenberg, C.W. Landsea, E. Blake, R. Pasch, M. Chelliah, and K. Mo. Atlantic hurricane season. In State of the Climate in 2003, D.H. Levinson and A.M. Waple (eds.). Bulletin of the American Meteorological Society, 85(6):S20-S24, https://doi.org/10.1175/BAMS-85-6-Levinson 2004
Bell, M.M., M.T. Montgomery, M.L. Black, and S.D. Aberson. Observed vortex and thermodynamic structure of Hurricane Isabel at maximum intensity. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 196-197, 2004
Black, M.L., F.D. Marks, R.F. Rogers, L.K. Shay, B.A. Albrecht, and H.E. Willoughby. The mean structure of vertical velocities and radar reflectivities in the hurricane eyewall as they relate to environmental wind shear. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society,Boston, 94-95, 2004
Black, M.L., J.P. Dunion, J.P. Kossin, W.H. Schubert, C.S. Velden, P.G. Black, R. Zehr, and S.D. Aberson. Mesovortices in Hurricane Isabel (2003): A comparison of satellite, radar, and photographic observations. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 128-129, 2004
Black, P.G. An overview of CBLAST flights into Hurricanes Fabian and Isabel (2003). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society,Boston, 1-2, 2004
Black, P.G., E.W. Uhlhorn, J.F. Gamache, W.D. Ramstrom, K. Emanuel, D. Esteban-Fernandez, J. Carswell, and P.S. Chang. Eyewall boundary layer structure in Hurricanes Fabian and Isabel. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 352-353, 2004
Chen, S.-C., S. Gulati, S. Hamid, X. Huang, L. Luo, N. Morriseau-Leroy, M.D. Powell, C. Zhan, and C. Zhang. A web-based distributed system for hurricane occurrence projection. Software: Practice and Experience, 34(6):549-571, https://doi.org/10.1002/spe.580 2004
As an environmental phenomenon, hurricanes cause significant property damage and loss of life in coastal areas almost every year. Research concerning hurricanes and their aftermath is gaining more and more attention nowadays. This paper presents our work in designing and building a Web-based distributed software system that can be used for the statistical analysis and projection of hurricane occurrences. Firstly, our system is a large-scale system and can handle the huge amount of hurricane data and intensive computations in hurricane data analysis and projection. Secondly, it is a distributed system, which allows multiple users at different locations to access the system simultaneously and to share and exchange the data and data model. Thirdly, our system is a database-centered system where the Oracle database is employed to store and manage the large amount of hurricane data, the hurricane model and the projection results. Finally, a three-tier architecture has been adopted to make our system robust and resistant to the potential change in the lifetime of the system. This paper focuses on the three-tier system architecture, describing the design and implementation of the components at each layer.
Chenoweth, M., and C.W. Landsea. The San Diego hurricane of 2 October 1858. Bulletin of the American Meteorological Society, 85(11):1689-1697, https://doi.org/10.1175/BAMS-85-11-168 2004
On 2 October 1858, estimated sustained hurricane-force winds produced by a tropical cyclone located a short distance offshore were felt in San Diego, California. Unprecedented damage was done in the city and was described as the severest gale ever felt to that date, and it has not been matched or exceeded in severity since. A "southeaster" and high seas from the diminishing tropical cyclone were also felt in the night of 2-3 October at San Pedro (the port serving Los Angeles), California, with shipping interests lightly damaged. The hurricane-force winds at San Diego are the first and only documented instance of winds of this strength from a tropical cyclone in the recorded history of the state. Available evidence suggests that the hurricane tracked just offshore from San Diego, without the eye coming inland, but close enough to produce damaging winds along the entire coast from San Diego to Long Beach, California. The rediscovery of this storm is relevant to climate change issues and the insurance-emergency management communities risk assessment of rare and extreme events in the region.
Dodge, P.P., J.F. Gamache, E.W. Uhlhorn, D. Esteban-Fernandez, J. Carswell, and P.S. Chang. Onshore and offshore wind flow regimes at the landfall of Hurricane Isabel (2003). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 564-565, 2004
Dunion, J.P., and C.S. Velden. The impact of the Saharan air layer on Atlantic tropical cyclone activity. Bulletin of the American Meteorological Society, 85(3):353-365, https://doi.org/10.1175/BAMS-85-3-353 2004
A deep well-mixed, dry adiabatic layer forms over the Sahara Desert and Shale regions of North Africa during the late spring, summer, and early fall. As this air mass advances westward and emerges from the northwest African coast, it is undercut by cool, moist low-level air and becomes the Saharan air layer (SAL). The SAL contains very dry air and substantial mineral dust lifted from the arid desert surface over North Africa, and is often associated with a midlevel easterly jet. A temperature inversion occurs at the base of the SAL where very warm Saharan air overlies relatively cooler air above the ocean surface. Recently developed multispectral Geostationary Operational Environmental Satellite (GOES) infrared imagery detects the SAL's entrained dust and dry air as it moves westward over the tropical Atlantic. This imagery reveals that when the SAL engulfs tropical waves, tropical disturbances, or preexisting tropical cyclones (TCs), its dry air, temperature inversion, and strong vertical wind shear (associated with the midlevel easterly jet) can inhibit their ability to strengthen. The SAL's influence on TCs may be a factor in the TC intensity forecast problem in the Atlantic and may also contribute to this ocean basin's relatively reduced level of TC activity.
Dunion, J.P., C.S. Velden, J.D. Hawkins, and J.R. Parrish. The Saharan air layer: Insights from the 2002 and 2003 Atlantic hurricane seasons. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 495-496, 2004
Eastin, M.D., P.D. Reasor, J.F. Gamache, F.D. Marks, and M.L. Black. Observed evolution of eyewall convection and low-wavenumber flow in Hurricane Guillermo (1997). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 445-446, 2004
Esteban-Fernandez, D., S. Frasier, J. Carswell, P.S. Chang, P.G. Black, and F.D. Marks. Three-dimensional atmospheric boundary layer wind fields from Hurricanes Fabian and Isabel. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 3-4, 2004
Feuer, S.E., C.W. Landsea, L. Woolcock, and J.O. Berkeley. The reanalysis of Atlantic basin tropical cyclones from the 1920s: A reexamination of three catastrophic hurricanes that impacted Florida. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 669-670, 2004
French, J.R., and P.G. Black. Turbulent flux measurements within a hurricane boundary layer from an instrumented aircraft. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 9-10, 2004
Gamache, J.F., J.S. Griffin, P.P. Dodge, and N.F. Griffin. Automatic Doppler analysis of three-dimensional wind fields in hurricane eyewalls. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 164-165, 2004
Goni, G.J., M. DeMaria, J.A. Trinanes, and P.G. Black. Testing global estimates of the tropical cyclone heat potential fields to improve hurricane intensification prediction. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 7-8, 2004
Harasti, P.R., C.J. McAdie, P.P. Dodge, W.-C. Lee, J. Tuttle, S.T. Murillo, and F.D. Marks. Real-time implementation of single-Doppler radar analysis methods for tropical cyclones: Algorithm improvements and use with WSR-88D display data. Weather and Forecasting, 19(2):219-239, doi:10.1175/1520-0434(2004)019<0219:RIOSRA>2.0.CO; 2004
The NOAA/NWS/NCEP/Tropical Prediction Center/National Hurricane Center has sought techniques that use single-Doppler radar data to estimate the tropical cyclone wind field. A cooperative effort with NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division and NCAR has resulted in significant progress in developing a method whereby radar display data are used as a proxy for a full-resolution base data and in improving and implementing existing wind retrieval and center-finding techniques. These techniques include the ground-based velocity track display (GBVTD), tracking radar echoes by correlation (TREC), GBVTD-simplex, and the principal component analysis (PCA) methods. The GBVTD and TREC algorithms are successfully applied to the Weather Surveillance Radar-1988 Doppler (WSR-88D) display data of Hurricane Bret (1999) and Tropical Storm Barry (2001). GBVTD analyses utilized circulation center estimates provided by the GBVTD-simplex and PCA methods, whereas TREC analyses utilized wind center estimates provided by radar imagery and aircraft measurements. GBVTD results demonstrate that the use of the storm motion as a proxy for the mean wind is not always appropriate and that results are sensitive to the accuracy of the circulation center estimate. TREC results support a previous conjecture that the use of polar coordinates would produce improved wind retrievals for intense tropical cyclones. However, there is a notable effect in the results when different wind center estimates are used as the origin of coordinates. The overall conclusion is that GBVTD and TREC have the ability to retrieve the intensity of a tropical cyclone with an accuracy of ~2 m s-1 or better if the wind intensity estimates from individual analyses are averaged together.
Hood, R.E., D. Cecil, F.J. LaFontaine, R. Blakeslee, D. Mach, G.M. Heymsfield, and F.D. Marks. Classification of tropical oceanic precipitation using high altitude aircraft microwave and electric field measurements. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 172-173, 2004
Houston, S.H., and P.P. Dodge. Comparison of GPS-sonde observations and surface data: An update. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 376, 2004
Howard, J.R., A.L. Doggett, R.E. Peterson, P.G. Black, J.L. Schroeder, D.A. Smith, and J.P. Dunyak. Transition in onshore hurricane boundary layer winds during the landfall of Hurricane Lili (2002). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 526-527, 2004
Huang, W.-P., C.-C. Wu, P.-H. Lin, S.D. Aberson, and K.-C. Hsu. The preliminary analysis of the dropsonde data from DOTSTAR and their impact on the typhoon track forecasts. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 188-189, 2004
Jacob, S.D., L.K. Shay, G.R. Halliwell, C.J. Koblinsky, and M.D. Powell. Surface fluxes and upper ocean budgets during 2002 Hurricanes Isidore and Lili. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 617-618, 2004
Jones, R.W., and H.E. Willoughby. Vortex alignment on the f and beta plane. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society,Boston, 625-626, 2004
Jones, S.C., C.S. Velden, S.D. Aberson, J. Abraham, and P.A. Harr. Extratropical transition and THORPEX. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 643-644, 2004
Kaplan, J., and M. DeMaria. Estimating the probability of rapid intensification of tropical cyclones in the Atlantic and eastern Pacific basins. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 538-539, 2004
Kossin, J.P., W.H. Schubert, C.S. Velden, M.L. Black, P.G. Black, R.M. Zehr, S.D. Aberson, and J.P. Dunion. Mesovortices in Hurricane Isabel (2003). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 447-448, 2004
Landsea, C.W., C. Anderson, N. Charles, G. Clark, J.P. Dunion, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer. The Atlantic hurricane database re-analysis project: Documentation for the 1851-1910 alterations and additions to the HURDAT database. In Hurricanes and Typhoons: Past, Present, and Future, R.J. Murnane and K.-B. Liu (eds.). Columbia University Press (ISBN 0-231-12388-4), 177-221, 2004
Landsea, C.W., J.L. Franklin, C.J. McAdie, J.L. Beven, J.M. Gross, B.R. Jarvinen, R.J. Pasch, E.N. Rappaport, J.P. Dunion, and P.P. Dodge. A reanalysis of Hurricane Andrew's intensity. Bulletin of the American Meteorological Society, 85(11):1699-1712, https://doi.org/10.1175/BAMS-85-11-169 2004
Hurricane Andrew of 1992 caused unprecedented economic devastation along its path through the Bahamas, southeastern Florida, and Louisiana. Damage in the United States was estimated to be $26 billion (in 1992 dollars), making Andrew one of the most expensive natural disasters in U.S. history. This hurricane struck southeastern Florida with maximum 1-min surface winds estimated in a 1992 post-storm analysis at 125 kt (64 m s-1). This original assessment was primarily based on an adjustment of aircraft reconnaissance flight-level winds to the surface. Based on recent advancements in the understanding of the eyewall wind structure of major hurricanes, the official intensity of Andrew was adjusted upward for five days during its track across the Atlantic Ocean and Gulf of Mexico by the National Hurricane Center Best Track Change Committee. In particular, Andrew is now assessed by the National Hurricane Center to be a Saffir-Simpson Hurricane Scale category-5 hurricane (the highest intensity category possible) at its landfall in southeastern Florida, with maximum 1-min winds of 145 kt (75 m s-1). This makes Andrew only the third category-5 hurricane to strike the United States since at least 1900. Implications for how this change impacts society's planning for such extreme events are discussed.
Lonfat, M., F.D. Marks, and S.S. Chen. Precipitation distribution in tropical cyclones using the Tropical Rainfall Measuring Mission (TRMM) microwave imager: A global perspective. Monthly Weather Review, 132(7):1645-1660, doi:10.1175/1520-0493(2004)132<1645:PDITCU>2.0.CO; 2004
TRMM microwave imager rain estimates are used to quantify the spatial distribution of rainfall in tropical cyclones (TCs) over the global oceans. A total of 260 TCs were observed worldwide from 1 January 1998-31 December 2000, providing 2,121 instantaneous TC precipitation observations. To examine the relationship between the storm intensity, its geographical location, and the rainfall distribution, the data set is stratified into three intensity groups and six oceanic basins. The three intensity classes used in this study are tropical storms (TSs) with winds -1, category 1-2 hurricane-strength systems (CAT12) with winds from 34-48 m s-1, and category 3-5 systems (CAT35) with winds >49 m s-1. The axisymmetric component of the TC rainfall is represented by the radial distribution of the azimuthal mean rainfall rates (R). The mean rainfall distribution is computed using 10-km annuli from the storm center to a 500-km radius. The azimuthal mean rain rates vary with storm intensity and from basin to basin. The maximum R is about 12 mm h-1 for CAT35, but decreases to 7 mm h-1 for CAT12, and to 3 mm h-1 for TS. The radius from the storm center of the maximum rainfall decreases with increasing storm intensity, from 50 km for TS to 35 km for CAT35 systems. The asymmetric component is determined by the first-order Fourier decomposition in a coordinate system relative to the storm motion. The asymmetry in TC rainfall varies significantly with both storm intensity and geographic locations. For the global average of all TCs, the maximum rainfall is located in the front quadrants. The location of the maximum rainfall shifts from the front-left quadrant for TS to the front-right for CAT35. The amplitude of the asymmetry varies with intensity as well; TS shows a larger asymmetry than CAT12 and CAT35. These global TC rainfall distributions and variability observed in various ocean basins should help to improve TC rainfall forecasting worldwide.
Majumdar, S.J., S.D. Aberson, B.J. Etherton, L.D. Holland, Z. Toth, and C.H. Bishop. Targeting strategies to improve hurricane track forecasts. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 190-191, 2004
McFarquhar, G.M., and R.A. Black. Hurricane observations of particle size and phase and their implications for mesoscale modeling of microphysical processes. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 348-349, 2004
McFarquhar, G.M., and R.A. Black. Observations of particle size and phase in tropical cyclones: Implications for mesoscale modeling of microphysical processes. Journal of the Atmospheric Sciences, 61(4):422-439, doi:10.1175/1520-0469(2004)061<0422:OOPSAP>2.0.CO; 2004
Mesoscale model simulations of tropical cyclones are sensitive to representations of microphysical processes, such as fall velocities of frozen hydrometeors. The majority of microphysical parameterizations are based on observations obtained in clouds not associated with tropical cyclones, and hence their suitability for use in simulations of tropical cyclones is not known. Here, representations of mass-weighted fall speed Vm for snow and graupel are examined to show that parameters describing the exponential size distributions and fall speeds of individual hydrometeors [through use of relations such as V(D) = aDb are identically important for determining Vm. The a and b coefficients are determined by the composition and shape of snow and graupel particles; past modeling studies have not adequately considered the possible spread of a and b values. Step variations in these coefficients, associated with different fall velocity regimes, however, do not have a large impact on Vm for observed size distributions in tropical cyclones and the values of a and b used here, provided that coefficients are chosen in accordance with the sizes where the majority of mass occurs. New parameterizations for Vm are developed such that there are no inconsistencies between the diameters used to define the mass, number concentration, and fall speeds of individual hydrometeors. Effects due to previous inconsistencies in defined diameters on mass conversion rates between different hydrometeor classes (e.g., snow, graupel, cloud ice) are shown to be significant. In situ microphysical data obtained in Hurricane Norbert (1984) and Hurricane Emily (1987) with two-dimensional cloud and precipitation probes are examined to determine typical size distributions of snow and graupel particles near the melting layer. Although well represented by exponential functions, there are substantial differences in how the intercept and slope of these distributions vary with mass content when compared to observations obtained in other locations; most notably, the intercepts of the size distributions associated with tropical cyclones increase with mass content, whereas some observations outside tropical cyclones show a decrease. Differences in the characteristics of the size distributions in updraft and downdraft regions, when compared to stratiform regions, exist, especially for graupel. A new representation for size distributions associated with tropical cyclones is derived and has significant impacts on the calculation of Vm.
McFarquhar, G.M., H. Zhang, G.M. Heymsfield, J. Dudhia, J.B. Halverson, R.E. Hood, and F.D. Marks. A modeling and observational study of the impacts of microphysical processes on the evolution of Hurricane Erin (2001). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 520-521, 2004
McNoldy, B.D., M.D. Eastin, C.M. Rozoff, and W.H. Schubert. Multiple eyewall structure of Hurricane Juliette (2001). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 126-127, 2004
Murillo, S.T., P.P. Dodge, W.-C. Lee, and G.M. Barnes. Implementing a single-Doppler radar technique for tropical cyclones and integrating radar-derived wind fields into H*Wind surface analyses. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 606-607, 2004
Pandya, R.E., D.R. Smith, M.K. Ramamurthy, P.J. Croft, M.J. Hayes, K.A. Murphy, J.D. Mcdonnell, R.M. Johnson, and H.A. Friedman. 11th American Meteorological Society Education Symposium. Bulletin of the American Meteorological Society, 85(3):425-430, https://doi.org/10.1175/BAMS-85-3-425 2004
The 11th American Meteorological Society (AMS) Education Symposium was held from 13 to 15 January 2002 in Orlando, Florida, as part of the 82nd Annual Meeting of the AMS. The theme of the symposium was "creating opportunities in educational outreach in the atmospheric and related sciences." Drawing from traditional strengths in meteorology and numerous national recommendations, the presentations and posters of the symposium highlighted three opportunities for reform. These opportunities build on partnerships between diverse educational stakeholders, efforts to make science education more like scientific practice, and strategies that place the atmospheric sciences within a larger, multi-disciplinary context that includes oceanography, hydrology, and earth-system science.
Powell, M.D., D. Bowman, D. Gilhousen, S.T. Murillo, N. Carrasco, and R. St. Fleur. Tropical cyclone winds at landfall: The ASOS-C-MAN Wind Exposure Documentation Project. Bulletin of the American Meteorological Society, 85(6):845-851, https://doi.org/10.1175/BAMS-85-6-845 2004
Photographs describing the wind exposure at automatic weather stations susceptible to tropical cyclones are now available on web pages at the National Climatic Data Center and the National Data Buoy Center. Given the exposure for one of eight wind direction sectors, a user may estimate the aerodynamic roughness and correct mean wind measurements to an open-terrain exposure. The open-terrain exposure is consistent with the tropical cyclone advisories and forecasts issued by the National Weather Service, as well as building design wind load standards published by the American Society of Civil Engineers.
Powell, M.D., J.D. Kepert, A. Boissonade, and P.J. Vickery. Uncertainty in hurricane winds: What do new measurements and simulations tell us about Hurricane Andrew? Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 522-523, 2004
Rogers, R.F., F.D. Marks, T.P. Marchok, and R. Tuleya. The development of a new validation technique for tropical cyclone rainfall. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 292-293, 2004
Rogers, R.F., M.L. Black, S.S. Chen, and R.A. Black. Evaluating microphysical parameterization schemes for use in hurricane environments. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 518-519, 2004
Schneider, R., G.M. Barnes, and P.P. Dodge. Low-level kinematic, thermodynamic, and reflectivity fields of Hurricane Bonnie (1998) near landfall. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society,Boston, 572-573, 2004
Uhlhorn, E.W., and L.K. Shay. Analysis of upper-ocean thermodynamic observations forced by Hurricane Lili (2002). Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 619-620, 2004
Uhlhorn, E.W., and P.G. Black. Comparison of boundary layer profiles in Hurricanes Fabian and Isabel observed by GPS dropwindsonde and aircraft during CBLAST "stepped descents." Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 391-392, 2004
White, S.R., and L.K. Shay. Modulations of surface wave energy over near-inertial time scales. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 621-622, 2004
Willoughby, H.E., and M.E. Rahn. Parameteric representation of the primary hurricane vortex, Part I: Observations and evaluation of the Holland (1980) model. Monthly Weather Review, 132(12):3033-3048, https://doi.org/10.1175/MWR2831.1 2004
Although numerical models are essential to hurricane forecasting, many other applications require only statistical depiction of the wind distribution. In Holland's 1980 parametric profile, radius of maximum wind, maximum wind, and a measure of the profile width describe the radial variation of the axisymmetric wind. Variants of the Holland profile are used to predict insurance underwriting risk, ocean response, and storm-surge inundation. Since these calculations guide high-stakes financial and emergency management decisions, it is logical to test them against observations. The Hurricane Research Division's flight-level database archives observations were obtained by NOAA and U.S. Air Force Reserve aircraft. The data considered here are winds and geopotential heights observed during 606 lower- and midtropospheric flights into Atlantic and eastern Pacific tropical cyclones during 1977-2000. The 493 profiles that meet quality control criteria are seasonally geographically representative. Least squares fits of the Holland model to these data provide evaluation of the parameters' distributions and critical examination of the profile's realism. Individual fitted profiles differ from the observations in a consistent pattern. The areas of strong winds in the eyewall and of nearly calm winds at the vortex center are too wide. Beyond 2 or 3 times the eye radius, the wind decreases too rapidly with distance from the center. Although the average bias in fitted profiles is -1, the root-mean-square error is 4.2 m s-1 (5.2 m s-1 for independent data). Maximum winds estimated from the fitted Holland-profile height-wind relation average 2.5 m s-1 too strong with an rms error of 6.5 m s-1. The pattern of too strong wind spread over too much real estate exaggerates the occurrence of winds stronger than 50 m s-1 by ~50%.
Willoughby, H.E., and M.E. Rahn. The climatology of hurricane wind profiles. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 264-265, 2004
Wu, C.-C., P.-H. Lin, T.-C. Yeh, and S.D. Aberson. Dropsonde observations for typhoon surveillance near the Taiwan region (DOTSTAR): An overview. Preprints, 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 3-7, 2004. American Meteorological Society, Boston, 186-187, 2004
Since 1997, the Tropical Prediction Center and the Hurricane Research Division have conducted operational synoptic surveillance missions with a Gulfstream IV-SP jet aircraft to improve numerical forecast guidance. Due to limited aircraft resources, optimal observing strategies for these missions must be developed. In the current study, the most rapidly growing modes are represented by areas of large forecast spread in the NCEP bred-vector ensemble forecasting system. The sampling strategy requires sampling of the entire target region with regularly spaced dropwindsonde observations. Three dynamical models were employed in testing the targeting and sampling strategies. With the assimilation into the numerical guidance of all the observations gathered during the surveillance missions, only the 12-h Geophysical Fluid Dynamics Laboratory Hurricane Model forecast showed statistically significant improvement. Assimilation of only the subset of data from the subjectively found fully sampled target regions produced a statistically significant reduction of the track forecast errors of up to 25% within the critical first two days of the forecast. This is comparable with the cumulative business-as-usual improvement expected over 18 yr.
Aberson, S.D., and C.R. Sampson. On the predictability of tropical cyclone tracks in the northwest Pacific basin. Monthly Weather Review, 131(7):1491-1497, doi:10.1175/1520-0493(2003)131<1491:OTPOTC>2.0.CO; 2003
A new northwest Pacific climatology and persistence (CLIPER) model is derived with historical tropical cyclone tracks during the satellite and aircraft reconnaissance era (1970-1995). The new CLIPER extends the forecasts from three to five days and exhibits smaller forecast biases than the previous CLIPER, although forecast errors are comparable. The new model is based on more accurate historical tropical cyclone track data, and a simpler derivation of the regression equations, than is the old model. Nonlinear systems analysis shows that the predictability timescale in which the average errors increase by a factor e is just over 15 h, which is about the same as that calculated by similar methods near Australia and in the North Atlantic. This suggests that five-day tropical cyclone track forecasts may be beneficial, assuming small initial errors; therefore, a CLIPER model extended to five days is needed as a baseline to measure the forecast skill.
Black, M.L., F.D. Marks, R.F. Rogers, L.K. Shay, B.A. Albrecht, and H.E. Willoughby. The relationship between environmental wind shear and the distribution of vertical velocities and precipitation in the hurricane eyewall. Preprints, 31st Conference on Radar Meteorology, Seattle, WA, August 6-12, 2003. American Meteorological Society, Boston, 1016-1019, 2003
Black, R.A., G.M. Heymsfield, and J. Hallett. Extra large particle images at 12 km in a hurricane eyewall: Evidence of high-altitude supercooled water? Geophysical Research Letters, 30(21):2124, 4 pp., https://doi.org/10.1029/2003GL017864 2003
The conventional wisdom about hurricanes suggests that updrafts are weak and supercooled water is scarce in the eyewall, and almost non-existent at temperatures colder than about -5°C (Black and Hallett, 1986). However, there is evidence that some hurricanes are different. Questions about the existence of high-altitude supercooled cloud water cannot be answered with only the instruments aboard the typical propeller-driven aircraft. During the summer of 1998, the NASA DC-8 aircraft made penetrations of the intensifying eyewall of Hurricane Bonnie at 12 km MSL, collecting the first truly high-altitude two-dimensional particle imagery in a hurricane. The similarity of the splash images in Hurricane Bonnie to those from raindrops obtained at higher temperatures in other hurricanes suggests that the large images obtained by the DC-8 were soft, low density graupel, rather than hard, high-density graupel particles or frozen raindrops. This implies that these particles grew to several millimeters in diameter at altitude, rather than simply advecting from lower, warmer altitudes. This growth in turn requires the presence of deeply supercooled cloud droplets. Thermal emission from supercooled water aloft increases the microwave brightness temperatures, giving a misleading impression that there is much less ice aloft than actually exists. The extra attenuation from the occasional presence of large graupel at these altitudes reduces the ability of microwave sensors to see precipitation at lower altitudes. Both of these effects impede efforts to accurately quantify condensate mass remotely from radiometric data such as that provided by the TRMM
Burpee, R.W. Characteristics of African easterly waves. In A Half Century of Progress in Meteorology: A Tribute to Richard Reed, R.H. Johnson and R.A. Houze, Jr. (eds.). American Meteorological Society, Meteorological Monograph, 31(53):91-108, https://doi.org/10.1175/0065-9401-31.53.91 2003
Cione, J.J., and E.W. Uhlhorn. Sea surface temperature variability in hurricanes: Implications with respect to intensity change. Monthly Weather Review, 131(8):1783-1796, https://doi.org/10.1175//2562.1 2003
Scientists at NOAA's Hurricane Research Division recently analyzed the inner-core upper-ocean environment for 23 Atlantic, Gulf of Mexico, and Caribbean hurricanes between 1975 and 2002. The interstorm variability of sea surface temperature (SST) change between the hurricane inner-core environment and the ambient ocean environment ahead of the storm is documented using airborne expendable bathythermograph (AXBT) observations and buoy-derived archived SST data. The authors demonstrate that differences between inner-core and ambient SST are much less than poststorm, "cold wake" SST reductions typically observed (i.e., 0-2°C versus 4-5°C). These findings help define a realistic parameter space for storm-induced SST change within the important high-wind, inner-core hurricane environment. Results from a recent observational study yielded estimates of upper-ocean heat content, upper-ocean energy extracted by the storm, and upper-ocean energy utilization for a wide range of tropical systems. Results from this analysis show that, under most circumstances, the energy available to the tropical cyclone is at least an order of magnitude greater than the energy extracted by the storm. This study also highlights the significant impact that changes in inner-core SST have on the magnitude of air-sea fluxes under high-wind conditions. Results from this study illustrate that relatively modest changes in inner-core SST (order 1°C) can effectively alter maximum total enthalpy (sensible plus latent heat) flux by 40% or more. The magnitude of SST change (ambient minus inner core) was statistically linked to subsequent changes in storm intensity for the 23 hurricanes included in this research. These findings suggest a relationship between reduced inner-core SST cooling (i.e., increased inner-core surface enthalpy flux) and tropical cyclone intensification. Similar results were not found when changes in storm intensity were compared with ambient SST or upper-ocean heat content conditions ahead of the storm. Under certain circumstances, the variability associated with inner-core SST change appears to be an important factor directly linked to the intensity change process.
Dunion, J.P., C.W. Landsea, S.H. Houston, and M.D. Powell. A reanalysis of the surface winds for Hurricane Donna of 1960. Monthly Weather Review, 131(9):1992-2011, doi:10.1175/1520-0493(2003)131<1992:AROTSW>2.0.CO; 2003
Hurricane Donna, the only major hurricane to strike the United States during the 1960 Atlantic hurricane season, passed over the middle Florida Keys near Sombrero Key before making landfall southeast of Naples, near Goodland, Florida, on 10 September at approximately 1600 UTC. This study makes detailed retrospective surface wind analyses of Hurricane Donna utilizing the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division's (HRD) H*Wind surface wind analysis system. Analyses were produced at intervals of 6 h between 1800 UTC 9 September and 1200 UTC 11 September 1960 while the hurricane was close to and over Florida. These analyses depict the storm track as well as the distribution and extent of tropical storm force, 50 kt (25.7 m s-1), and the hurricane-force wind radii throughout this time period and include new methodologies for adjusting aircraft flight-level data to the surface in the tropical cyclone core environment. Algorithms were developed to account for the effects of eyewall tilt and the warm core structure typical of tropical cyclones. Additional methods were developed using global positioning system (GPS) dropwindsondes (sondes) to more accurately adjust boundary layer winds to equivalent surface winds. The Kaplan-DeMaria Inland Wind Decay Model was also used for the first time to adjust landfall data being input into the H*Wind system. These data were used to generate low-weighted background fields that helped generate postlandfall wind field analyses of Hurricane Donna. Finally, swaths of peak winds, duration of hurricane- and major hurricane-force winds, and wind steadiness were produced to facilitate damage assessment. The information provided by these objective analyses is significantly more detailed than the more limited descriptions of peak winds, storm position, and minimum central pressure available in the National Hurricane Center's (NHC) hurricane database archive (HURDAT).
Franklin, J.L., M.L. Black, and K. Valde. GPS dropwindsonde wind profiles in hurricanes and their operational implications. Weather and Forecasting, 18(1):32-44, doi:10.1175/1520-0434(2003)018<0032:GDWPIH>2.0.CO; 2003
The recent development of the global positioning system (GPS) dropwindsonde has allowed the wind and thermodynamic structure of the hurricane eyewall to be documented with unprecedented accuracy and resolution. In an attempt to assist operational hurricane forecasters in their duties, dropwindsonde data have been used in this study to document, for the first time, the mean vertical profile of wind speed in the hurricane inner core from the surface to the 700-hPa level, the level typically flown by reconnaissance aircraft. The dropwindsonde-derived mean eyewall wind profile is characterized by a broad maximum centered 500 m above the surface. In the frictional boundary layer below this broad maximum, the wind decreases nearly linearly with the logarithm of the altitude. Above the maximum, the winds decrease because of the hurricane's warm core. These two effects combine to give a surface wind that is, on average, about 90% of the 700-hPa value. The dropwindsonde observations largely confirm recent operational practices at the National Hurricane Center for the interpretation of flight-level data. Hurricane wind profiles outside of the eyewall region are characterized by a higher level of maximum wind, near 1 km, and a more constant wind speed between 700 hPa and the top of the boundary layer. Two factors that likely affect the eyewall profile structure are wind speed and vertical motion. A minimum in surface wind adjustment factor (i.e., relatively low surface wind speeds) was found when the wind near the top of the boundary layer was between 40 and 60 m s-1. At higher wind speeds, the fraction of the boundary layer wind speed found at the surface increased, contrary to expectation. Low-level downdrafts and enhanced vertical motion generally were also associated with higher relative surface winds. These results may be of interest to engineers concerned with building codes, to emergency managers who may be tempted to use high-rise buildings as a "refuge of last resort" in coastal areas, and to those people on locally elevated terrain. The top of a 25-story coastal high-rise in the hurricane eyewall will experience a mean wind that is about 17% higher (or one SaffirSimpson hurricane-scale category) than the surface or advisory value. For this reason, residents who must take refuge in coastal high-rises should generally do so at the lowest levels necessary to avoid storm surge.
Gedzelman, S., J. Lawrence, J.F. Gamache, M.L. Black, E. Hindman, R.A. Black, J.P. Dunion, H.E. Willoughby, and X. Zhang. Probing hurricanes with stable isotopes of rain and water vapor. Monthly Weather Review, 131(6):1112-1127, doi:10.1175/1520-0493(2003)131<1112:PHWSIO>2.0.CO; 2003
Rain and water vapor were collected during flights in Hurricanes Olivia (1994), Opal (1995), Marilyn (1995), and Hortense (1995) and analyzed for their stable isotopic concentrations, or ratios, H218O:H2O and HDO:H2O. The spatial patterns and temporal changes of isotope ratios reflect details of a hurricane's structure, evolution, microphysics, and water budget. At all flight levels over the sea (850-475 hPa) the lowest isotope ratios occur in or near regions of stratiform rains between about 50 and 250 km from the eye. Isotope ratios are higher in the eyewall and were particularly high in the crescent-shaped eyewall of Hurricane Opal at a time when no rain was falling over a large area near the storm center. In Hurricane Olivia, isotope ratios decreased from 24 to 25 September after vertical and radial circulation weakened. A two-layer isotope model of a radially symmetric hurricane simulates these features. The low isotope ratios are caused by fractionation in extensive, thick, precipitating clouds with predominantly convergent low-level flow accompanied by removal of heavy isotopes by falling raindrops. Evaporation and isotope equilibration of sea spray increase isotope ratios of the ambient vapor and produce a deuterium excess or enrichment of D relative to 18O that increases with decreasing relative humidity and increasing wind speed. Model results show that sea spray supplies the eyewall with up to 50% of its water vapor and is largely responsible for its high isotope ratios.
Goni, G.J., P.G. Black, and J.A. Trinanes. Using satellite altimetry to identify regions of hurricane intensification. AVISO Newsletter, 9:19-20, 2003
Houston, S.H., and M.D. Powell. Surface wind fields for Florida Bay hurricanes. Journal of Coastal Research, 19(3):503-513, 2003
The surface wind fields of several tropical cyclones which impacted Florida Bay and the surrounding coastal areas were reconstructed by the Hurricane Research Division (HRD) of the national Oceanographic and Atmospheric Administration. These cyclones provided the forcing for significant changes in water levels, waves, and currents, resulting in sediment transport, deposition, and other physical processes affecting the Bay. In addition, tropical cyclones had direct and indirect effects on plant and animal life in the Bay and the surrounding coastal areas, such as the Florida Keys and Everglades. The HRD wind fields are being made available in gridded form for use in hindcasts, which may help researchers to estimate the potential impacts of future tropical cyclones on the south Florida ecosystem, especially in relation to Florida Bay. The tropical cyclones investigated represent vastly different scenarios for the type of events that might be expected over extreme south Florida. The reconstructed storms range in intensity from Tropical Storm Gordon of 1994 to the Labor Day Hurricane of 1935 (the United States' most intense hurricane at landfall). This paper summarizes the methods used to reconstruct tropical cyclone surface wind fields and provides examples of their circulation features and wind swaths. Comparisons of winds to observed damage are also presented for three major hurricanes. The wind fields for all of these tropical cyclones are being made available to researchers as graphical products and gridded data sets on a Web site maintained by HRD (www.aoml.noaa.gov/hrd).
Kaplan, J., and M. DeMaria. Large-scale characteristics of rapidly intensifying tropical cyclones in the North Atlantic basin. Weather and Forecasting, 18(6):1093-1108, doi:10.1175/1520-0434(2003)018<1093:LCORIT>2.0.CO; 2003
The National Hurricane Center (NHC) and Statistical Hurricane Intensity Prediction Scheme (SHIPS) databases are employed to examine the large-scale characteristics of rapidly intensifying Atlantic basin tropical cyclones. In this study, rapid intensification (RI) is defined as approximately the 95th percentile of over-water 24-h intensity changes of Atlantic basin tropical cyclones that developed from 1989 to 2000. This equates to a maximum sustained surface wind speed increase of 15.4 m s-1 (30 kt) over a 24-h period. It is shown that 31% of all tropical cyclones, 60% of all hurricanes, 83% of all major hurricanes, and all category 4 and 5 hurricanes underwent RI at least once during their lifetimes. The mean initial (t = 0 h) conditions of cases that undergo RI are compared to those of the non-RI cases. These comparisons show that the RI cases form farther south and west and have a more westward component of motion than the non-RI cases. In addition, the RI cases are typically intensifying at a faster rate during the previous 12 h than the non-RI cases. The statistical analysis also shows that the RI cases are further from their maximum potential intensity and form in regions with warmer SSTs and higher lower-tropospheric relative humidity than the non-RI cases. The RI cases are also embedded in regions where the upper-level flow is more easterly and the vertical shear and upper-level forcing from troughs or cold lows is weaker than is observed for the non-RI cases. Finally, the RI cases tend to move with the flow within a higher layer of the atmosphere than the non-RI cases. A simple technique for estimating the probability of RI is described. Estimates of the probability of RI are determined using the predictors for which statistically significant differences are found between the RI and non-RI cases. Estimates of the probability of RI are also determined by combining the five predictors that had the highest individual probabilities of RI. The probability of RI increases from 1% to 41% when the total number of thresholds satisfied increases from zero to five. This simple technique was used in real time for the first time during the 2001 Atlantic hurricane season as part of the Joint Hurricane Testbed (JHT).
Knaff, J.A., N. Wang, M. DeMaria, M. Zehr, J.S. Griffin, and F.D. Marks. A demonstration of real-time transmission and display of GOES imagery aboard the NOAA P-3 aircraft during the 2002 hurricane season. Preprints, 12th Conference on Satellite Meteorology and Oceanography and 3rd Conference on Artificial Intelligence Applications to Environmental Science, Long Beach, CA, February 8-13, 2003. American Meteorological Society, Boston, 5 pp. (CD-ROM), 2003
Kollias, P., B.A. Albrecht, and F.D. Marks. Cloud radar observations of vertical drafts and microphysics in convective rain. Journal of Geophysical Research, 108(D2):4053, 12 pp., https://doi.org/10.1029/2001JD002033 2003
Observations of convective precipitation using a 94-GHz cloud radar are presented. Due to Mie scattering, the Doppler power spectra collected at vertical incidence contains characteristics of the scatterers (hydrometeors). These characteristics are used for the retrieval of the vertical air motion and the associated raindrop size distribution in an attempt to accurately map the time-height structure of the vertical air motion and raindrop fields within intense convective precipitation. The data provide strong evidence of the interaction between draft intensity and raindrop size distribution and highlight the variability of convective precipitation at small scales. Horizontal sorting of the raindrops caused by the air motion is documented. Signal attenuation measured at 94 GHz is shown to be well correlated to rainfall rates. The observations demonstrate the capability of 94-GHz cloud radars for studies of precipitation processes at low altitudes even under intense convective conditions.
Landsea, C.W., C. Anderson, N. Charles, G. Clark, J.P. Dunion, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer. The Atlantic hurricane database re-analysis project: Results for the first 60 years, 1851-1910. Preprints, 14th Symposium on Global Change and Climate Variations, Long Beach, CA, February 9-13, 2003. American Meteorological Society, Boston, 36 pp. (CD reprint), 2003
Lee, W.-C., F.D. Marks, and C. Walther. Airborne Doppler radar data analysis workshop. Bulletin of the American Meteorological Society, 84(8):1063-1075, https://doi.org/10.1175/BAMS-84-8-1063 2003
The Airborne Doppler Radar Data Analysis Workshop, sponsored by the Atmospheric Technology Division (ATD) of the National Center for Atmospheric Research (NCAR), was the first to focus on analyzing airborne Doppler radar data. The workshop (held 13-16 March 2000 at NCAR) aimed to (1) summarize the current airborne Doppler radar data analysis techniques, and (2) promote the use of airborne Doppler radar data in the atmospheric sciences community. The workshop also intended to encourage new users to analyze this Doppler data and to provide a forum for experienced users to exchange ideas and discuss problems related to analyzing the data. It also provided a forum to train the users in planning future airborne Doppler radar programs. Graduate students, recent PhDs, faculty and researchers participantsthe leading experts in the fieldcovered the theory of airborne Doppler radar, experiment design, standard data analysis procedures and software, and recently developed analysis techniques. Eight working groups were organized among the participants to analyze preselected airborne Doppler radar datasets collected in past experiments using the standard software available from NCAR. Each working group used standard data analysis procedures to obtain dual-Doppler radar winds from raw airborne Doppler radar data.
Marks, F.D. Hurricanes. In Encyclopedia of Atmospheric Sciences, J. Holton, J. Pyle, and J. Curry (eds.). Elsevier Science Ltd., London, UK, 942-966, 2003
Marks, F.D. Hurricanes. In Handbook of Weather, Climate and Water: Dynamics, Climate, Physical Meteorology, Weather Systems, and Measurements, T.D. Potter and B.R. Colman (eds.). John Wiley and Sons, 641-676, 2003
Marks, F.D. State of the science: Radar view of tropical cyclones. In Radar and Atmospheric Science: A Collection of Essays in Honor of David Atlas, R.M. Wakimoto and R.C. Srivastava (eds.). American Meteorological Society, Meteorological Monograph, 30(52):33-74, 2003
Mo, Q., A.G. Detwiler, J. Hallett, and R.A. Black. Horizontal structure of the electric field in the stratiform region of an Oklahoma mesoscale convective system. Journal of Geophysical Research, 108(D7):4225, 15 pp., https://doi.org/10.1029/2001JD001140 2003
This analysis combines vertical electric field components Ez observed by two research aircraft flying horizontally at two levels, with vertical soundings of thermodynamic parameters and Ez made by five balloons, to produce a quasi-three-dimensional view of the space charge distribution in the trailing stratiform cloud region behind a mesoscale convective system (MCS) that developed in central Oklahoma late in the afternoon of 2 June 1991. The balloons were launched serially at one-hour intervals from two sites separated by 80 km along a north-south line as the MCS moved eastward, yielding two east-west time-height cross-sections of the Ez structure within the quasi-steady state trailing stratiform region behind the MCS. The balloon measurements are consistent with a vertical stack of five rearward- and downward-sloping horizontal sheets of charge of alternating polarity, beginning at the bottom with a negative charge layer below the 0°C level and a positive layer near the 0°C level. This structure persisted for more than 2 hours. The two aircraft flew back and forth along a north-south line through the balloon launch sites during the balloon launch period. Aircraft measurements demonstrated that the vertical electric field (Ez) at constant altitude varied in the north-south direction. The peak magnitudes of Ez deduced from the airborne instrument systems agreed with the magnitudes deduced from the balloon measurements at the aircraft altitudes of 4.5 km and 5.8 km AGL. Rapid reversals in polarity of Ez with peak magnitude >50 kV m-1 observed by the aircraft at 4.5 km, just above the 0°C level, confirms the thin concentrated positive charge layer observed there by balloons and suggests that this charge layer is undulating above and below 4.5 km altitude, at least in the north-south direction. Microphysically, this layer contained large aggregates and pockets of low cloud liquid water concentration. At the 5.8 km level, the polarity of Ez was always positive but the magnitude varied from zero to 25 kV m-1. Aircraft-observed Ez at both altitudes varied on horizontal scales of ~10 km or greater at both levels, suggesting that the charge density derived using the one-dimensional infinite-layer Gauss's law approximation applied to the balloon soundings of Ez is valid in this study. These observations show that layers of charge can persist for hours as they advect rearward in a storm-relative sense, possibly due to continuing in situ charge separation, and/or due to weak dispersion, slow recombination, and slow settling of charge attached to low mobility low terminal velocity ice hydrometeors.
Owens, B.F., and C.W. Landsea. Assessing the skill of operational Atlantic seasonal tropical cyclone forecasts. Weather and Forecasting, 18(1):45-54, doi:10.1175/1520-0434(2003)018<0045:ATSOOA>2.0.CO; 2003
Since 1984, William Gray of Colorado State University and a team of researchers have been issuing seasonal tropical cyclone forecasts for the North Atlantic Ocean. Prior to this, little work had been done in the area of long-term tropical cyclone forecasting because researchers saw minimal potential skill in any prediction models and no obvious benefits to be gained. However, seasonal forecasts have been attracting more attention as economic and insured losses from hurricane-related catastrophes rose sharply during recent decades. Initially, the forecasts issued by Gray consisted of output from simple statistical prediction models. Over time, the models became increasingly more complex and sophisticated, with new versions being introduced in 1992, 1993, 1994, 1996, and 1997. In addition, based on a combination of experience with the statistical models and other qualitative considerations such as examinations of analog years, the statistical forecasts were modified to create adjusted seasonal forecasts. This analysis assessed the skill demonstrated, if any, of both the statistical and adjusted forecasts over the benchmarks of climatology and persistence and examined whether the adjusted forecasts were more accurate than the statistical forecasts. The analysis indicates that, over the past 18 years, both the statistical and adjusted forecasts demonstrated some skill over climatology and persistence. There is also evidence to suggest that the adjusted forecast was more skillful than the statistical model forecast.
Pielke, R.A., J. Rubiera, C. Landsea, M.L. Fernandez, and R. Klein. Hurricane vulnerability in Latin America and the Caribbean: Normalized damage and loss potentials. Natural Hazards Review, 4(3):101-114, https://doi.org/10.1061/(ASCE)1527-6988(2003)4:3(101) 2003
In late October 1998, the remnants of Hurricane Mitch stalled over Honduras and Nicaragua, killing more than 10,000 people and causing as much as $8.5 billion in damage. While Central America and the Caribbean have a history of natural disasters, the fatalities and destruction caused by Mitch were the greatest in at least several decades, prompting many questions including: What accounts for the extent of these losses? Is Mitch a harbinger of future disasters in the region? and What might be done in response? This paper seeks to shed light on these questions by examining the historical and geographic context of hurricane vulnerability in Latin America and the Caribbean. The paper examines trends in economic and other societal factors that increase vulnerability to hurricanes in Central America and the Caribbean and includes a case study of normalized hurricane losses in Cuba made possible by newly collected damage data published herein. The paper places its findings into the context of policies related to climate change and natural hazards.
Powell, M.D., P.J. Vickery, and T.A. Reinhold. Reduced drag coefficient for high wind speeds in tropical cyclones. Nature, 422:279-283, https://doi.org/10.1038/nature01481 2003
The transfer of momentum between the atmosphere and the ocean is described in terms of the variation of wind speed with height and a drag coefficient that increases with sea surface roughness and wind speed. But direct measurements have only been available for weak winds; momentum transfer under extreme wind conditions has therefore been extrapolated from these field measurements. Global Positioning System sondes have been used since 1997 to measure the profiles of the strong winds in the marine boundary layer associated with tropical cyclones. Here we present an analysis of these data, which show a logarithmic increase in mean wind speed with height in the lowest 200 m, maximum wind speed at 500 and a gradual weakening up to a height of 3 km. By determining surface stress, roughness length, and neutral stability drag coefficient, we find that surface momentum flux levels off as the wind speeds increase above hurricane force. This behavior is contrary to surface flux parameterizations that are currently used in a variety of modeling applications, including hurricane risk assessment and prediction of storm motion, intensity, waves, and storm surges.
Rogers, R.F., S. Chen, J. Tenerelli, and H.E. Willoughby. A numerical study of the impact of vertical shear on the distribution of rainfall in Hurricane Bonnie (1998). Monthly Weather Review, 131(8):1577-1599, https://doi.org/10.1175//2546.1 2003
Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem. A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm. These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow. The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm. This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall. The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) to simulate the storm between 0000 UTC 22 August and 0000 UTC 27 August 1998. During this period significant changes in the vertical shear occurred in the simulation. It changed from strong west-southwesterly, and across track, to much weaker south-southwesterly, and along track. Nearly concurrently, the azimuthal distribution of convection changed from a distinct wavenumber-1 pattern to almost azimuthally symmetric by the end of the time period. The strongest convection in the core was generally located on the downshear left side of the shear vector when the shear was strong. The azimuthal distributions and magnitudes of low-level radial inflow, reflectivity, boundary layer divergence, and low-level vertical motion all varied consistently with the evolution of the vertical shear. Additionally, the vortex showed a generally downshear tilt from the vertical. The magnitude of the tilt correlated well with changes in magnitude of the environmental shear. The accumulated rainfall was distributed symmetrically across the track of the storm when the shear was strong and across track, and it was distributed asymmetrically across the track of the storm when the shear was weak and along track.
Sandrik, A., and C.W. Landsea. Chronological listing of tropical cyclones affecting north Florida and coastal Georgia, 1565-1899. NOAA Technical Memorandum, NWS-SR-244 (PB2003-104513), 74 pp., 2003
This chronology is a portion of an ongoing re-analysis project for tropical cyclone events along the Georgia and northeast Florida coasts, including inland north Florida and southeast Georgia. The domain for this study ranges from Savannah, Georgia in the north to Flagler Beach, Florida in the south, the adjacent coastal waters, the inland cities (and their surrounding areas) of Palatka, Gainesville, and Lake City in Florida and Waycross, Georgia. The number of hurricanes and principle areas affected after 1900 are considered to be fairly accurate, but are the subject of a reevaluation by the Hurricane Research Division (HRD) in Miami, Florida (Landsea et al., 1999, 2003). The intention of this study is to accurately extend the historical hurricane landfall data base for the study area back as far as possible, but at a minimum to 1800.
Uhlhorn, E.W., and P.G. Black. Verification of remotely sensed sea surface winds in hurricanes. Journal of Atmospheric and Oceanic Technology, 20(1):99-116, doi:10.1175/1520-0426(2003)020<0099:VORSSS>2.0.CO; 2003
Surface winds in hurricanes have been estimated remotely using the Stepped-Frequency Microwave Radiometer (SFMR) from the NOAA WP-3D aircraft for the past 15 years. Since the use of the GPS dropwindsonde system in hurricanes was first initiated in 1997, routine collocated SFMR and GPS surface wind estimates have been made. During the 1998, 1999, and 2001 hurricane seasons, a total of 249 paired samples were acquired and compared. The SFMR equivalent 1-min mean, 10-m level neutral stability winds were found to be biased high by 2.3 m s-1 relative to the 10-m GPS winds computed from an estimate of the mean boundary layer wind. Across the range of wind speeds from 10 to 60 m s-1, the rms was 3.3 m s-1. The bias was found to be dependent on storm quadrant and independent of wind speed, a result that suggests a possible relationship between microwave brightness temperatures and surface wave properties. Tests of retrieved winds' sensitivities to sea surface temperature, salinity, atmospheric thermodynamic variability, and surface wind direction indicate wind speed errors of less than 1 m s-1 above 15 m s-1.
Willoughby, H.E. A century of progress in tracking and warning: Improvements in observations, models, and forecasts. In Hurricane! Coping with Disaster: Progress and Challenges since Galveston, 1900, R.H. Simpson, R.A. Anthes, M. Garstang, and J. Simpson (eds.). American Geophysical Union, Special Publication Series 55, 205-216, https://doi.org/10.1029/SP055p0205 2003
2002
Aberson, S.D. Operational targeting of hurricane tracks in the Atlantic: Processes and procedures. Proceedings, Second Workshop on Landfalling Typhoons in the Taiwan Area, Taipei, Taiwan, April 25-26, 2002. National Science Council, 53-67, 2002
NOAA has been conducting operational targeting of dropwindsonde observations to improve tropical cyclone track forecasts since 1997. During the first two years, however, the impact of the observations was minimal, with only a slight improvement in track forecasts. However, with improvements to models, data assimilation, and targeting techniques, the forecasts for Hurricane Michelle in late 2001 were improved by 45 to 60% in the NCEP global model. This talk will present the basic premise behind targeting and the various targeting techniques available, and the process used in the U.S. to accomplish the targeting missions.
Aberson, S.D. Tropical cyclone track predictability limits. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 220-221, 2002
Aberson, S.D. Two years of operational hurricane synoptic surveillance. Weather and Forecasting, 17(5):1101-1110, doi:10.1175/1520-0434(2002)017<1101:TYOOHS>2.0.CO; 2002
In 1997, the National Hurricane Center and the Hurricane Research Division began operational synoptic surveillance missions with the Gulfstream IV-SP jet aircraft to improve the numerical guidance for hurricanes that threaten the continental United States, Puerto Rico, the Virgin Islands, and Hawaii. During the first two years, 24 missions were conducted. Global positioning system dropwindsondes were released from the aircraft at 150-200 km intervals along the flight track in the environment of each tropical cyclone to obtain profiles of wind, temperature, and humidity from flight level (nearly 150 hPa) to the surface. The observations were processed and formatted aboard the aircraft and sent to NCEP to be ingested into the Global Data Assimilation System, which subsequently served as initial and boundary conditions for a number of numerical models that forecast the track and intensity of tropical cyclones. The current study is an attempt to mimic this process to assess the impact of these operational missions on the numerical guidance. Although the small number of missions flown in 1997 showed error reductions of as much as 32%, the improvements seen in the two-year sample are not promising. The additional dropwindsonde data from the synoptic surveillance missions provided statistically significant improvements in the GFDL forecasts only at 12 h. The "VBAR" and Global Forecast System (AVN) forecasts were not significantly improved at any forecast time. Further examination suggests that the AVN synthetic vortex procedure, combined with difficulty in the quantification of the current storm-motion vector operationally, may have caused the mediocre improvements. Forecast improvements of 14-24% in GFDL forecasts are shown in the subset of cases in which the synthetic vortex data do not seem to be a problem. Improvements in the landfall forecasts are also seen in this subset of cases. A reassessment of tropical cyclone vortex initialization schemes used by forecast centers and numerical modelers may be necessary.
Atlas, D., C.W. Ulbrich, and F.D. Marks. Reply to comment by S.E. Yuter and R.A. Houze, Jr. on "Partitioning tropical oceanic convective and stratiform rains by draft strength." Journal of Geophysical Research, 107(D1):4006, 2 pp., https://doi.org/10.1029/2001JD000658 2002
Black, M.L., E.W. Uhlhorn, S.E. Feuer, W.P. Barry, and L.K. Shay. The relationship between GPS dropsonde wind profiles and sea-surface temperature in Hurricane Bret (1999). Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 551-552, 2002
Black, M.L., J.F. Gamache, F.D. Marks, C.E. Samsury, and H.E. Willoughby. Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The effect of vertical shear on structure and intensity. Monthly Weather Review, 130(9):2291-2312, 2002
Shear is a key inhibitor of tropical cyclone intensification. Although its signature is readily recognized in satellite imagery and theoretical or modeling studies provide some insight, detailed observations have been limited. Airborne radar and in-situ observations in Hurricanes Jimena of 1991 and Olivia of 1994 are a step toward better understanding. Each storm was observed on two consecutive days. Initially, both had small eyes, 16-18 km radius, and maximum winds of 57 m s-1 over sea surface temperatures (SST) >28°C in easterly environmental shear. Jimena maintained constant intensity or weakened gradually for 2 days in 13-20 m s-1 easterly shear. Olivia intensified in 8 m s-1 shear on the first day. Overnight, the shear diminished to reverse and became westerly. On the second day, Olivia weakened as the shear increased to >15 m s-1 from the west, the storm moved over cooler SST, and became surrounded by dryer air. As convection weakened and the outer rainbands ceased to be effective barriers, relative flow due to the environmental shear penetrated more deeply into the vortex core. In both storms, shear controlled the convective structure. Convection organized itself into axisymmetric rings as Olivia intensified in weak shear. When both storms encountered stronger shear, radar reflectivity and vertical motion had strong wavenumber-1 components. Highest reflectivity lay generally to the left of the shear. Most radar echoes and updrafts formed in the downshear quadrant of the storm and advected around the eye with 60-80% of the swirling wind, consistent with vortex Rossby wave propagation. The buoyant updrafts accelerated and reflectivity increased as they passed through the left-of-shear semicircle. On the upshear side, the updrafts rose through the 0°C isotherm, and hydrometeors fell out or froze. Reflectivity declined as the echoes transformed into lower-tropospheric downdrafts overlain by glaciated upper-tropospheric updrafts in the right-of-shear semicircle. In relatively weak shear, clusters of echoes could be tracked completely around the eye. Each time the clusters passed through the downshear and left-of-shear quadrants, new echoes would form. In strong shear, all echoes were short lived, and none could be tracked around the eye. Echoes appeared downshear of the center and completed their life cycles on the left side of the shear vector where the composite reflectivities were greatest.
Black, R.A., and G.M. Heymsfield. Extra large particle images at 40,000 ft in a hurricane eyewall: Evidence of partially frozen raindrops? Preprints, 11th Conference on Cloud Physics, Ogden, UT, June 3-7, 2002. American Meteorological Society, Boston, 3 pp., 2002
Bosart, L.F., P.G. Black, J.L. Evans, J.E. Molinari, C.S. Velden, and M.J. Dickinson. The double transition of Hurricane Michael (2000): Baroclinic to tropical to baroclinic. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 503-504, 2002
Burpee, R.W., and P.G. Black. Ocean mixed layer thermal changes induced by moving tropical cyclones, Part I: Analyses of inner core observations obtained by research aircraft. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 571-572, 2002
Cecil, D.J., G.M. Heymsfield, F.J. LaFontaine, M.G. Bateman, E.J. Zipser, and F.D. Marks. Precipitation structures observed in CAMEX hurricanes. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 63-65, 2002
Cione, J.J., and E.W. Uhlhorn. Upper ocean heat content and energy extracted by the storm: Analytical look. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 631-632, 2002
Cook, T.M., L.K. Shay, S.D. Jacob, C.W. Wright, P.G. Black, and E.W. Uhlhorn. Surface wave effects on the ocean mixed layer response to Hurricane Bonnie. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 633-634, 2002
Dodge, P.P., M.L. Black, J.L. Franklin, J.F. Gamache, and F.D. Marks. High-resolution observations of the eyewall in an intense hurricane: Bret on 21-22 August 1999. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 607-608, 2002
Dunion, J.P., and C.S. Velden. Application of surface-adjusted GOES low-level cloud-drift winds in the environment of Atlantic tropical cyclones. Part I: Methodology and validation. Monthly Weather Review, 130(5):1333-1346, doi:10.1175/1520-0493(2002)130<1333:AOSAGL>2.0.CO; 2002
Beginning with the 1997 hurricane season, the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison began demonstrating the derivation of real-time Geostationary Operational Environmental Satellite (GOES) low-level cloud-drift winds in the vicinity of Atlantic tropical cyclones. The winds are derived from tracking low-level clouds in sequential, high-resolution GOES visible channel imagery. Since then, these data have been provided to the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD) for evaluation in their real-time tropical cyclone surface wind objective analyses (H*Wind) that are disseminated to forecasters at the NOAA National Hurricane Center on an experimental basis. These wind analyses are proving useful as guidance to support forecasters' tropical cyclone advisories and warnings. The GOES satellite wind observations often provide essential near-surface coverage in the outer radii of the tropical cyclone circulation where conventional in-situ observations (e.g., ships and buoys) are frequently widely spaced or nonexistent and reconnaissance aircraft do not normally fly. The GOES low-level cloud-tracked winds are extrapolated to the surface using a planetary boundary layer model developed at HRD for hurricane environments. In this study, the unadjusted GOES winds are validated against wind profiles from the newly deployed global positioning system dropwindsondes, and the surface-adjusted winds are compared with collocated in-situ surface measurements. The results show the ability of the GOES winds to provide valuable quantitative data in the periphery of tropical cyclones. It is also shown that the current scheme employed to extrapolate the winds to the surface results in small biases in both speed and direction. Nonlinear adjustments to account for these biases are presented.
Dunion, J.P., and C.S. Velden. Satellite applications for tropical wave/tropical cyclone tracking. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 132-133, 2002
Dunion, J.P., and C.S. Velden. Satellite applications for tropical wave/tropical cyclone tracking. Preprints, 11th Conference on Satellite Meteorology and Oceanography, Madison, WI, April 29-May 3, 2002. American Meteorological Society, Boston, 314-317, 2002
Dunion, J.P., and M.D. Powell. Improvements to the NOAA Hurricane Research Division's surface reduction algorithm for inner core aircraft flight-level winds. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 581-582, 2002
Dunion, J.P., S.H. Houston, C.S. Velden, and M.D. Powell. Application of surface adjusted GOES low-level cloud-drift winds in the environment of Atlantic tropical cyclones. Part II: Integration into surface wind analyses. Monthly Weather Review, 130(5):1347-1355, doi:10.1175/1520-0493(2002)130<1347:AOSAGL>2.0.CO; 2002
The Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison recently (1997 season) began providing real-time Geostationary Operational Environmental Satellite (GOES) low-level cloud-drift winds in the vicinity of tropical cyclones on an experimental basis to the National Oceanic and Atmospheric Administration's (NOAA) Hurricane Research Division (HRD). The cloud-drift winds are derived from sequential high-resolution GOES visible channel imagery. These data were included in many of HRD's real-time tropical cyclone surface wind objective analyses, which were sent to NOAA's National Hurricane Center and the Central Pacific Hurricane Center on an experimental basis during the 1997-2001 hurricane seasons. These wind analyses were used to support the forecasters' tropical cyclone advisories and warnings. The satellite wind observations provide essential low-level coverage in the periphery of the tropical cyclone circulation where conventional in-situ observations (e.g., ships, buoys, and Coastal-Marine Automated Network stations) are often widely spaced or nonexistent and reconnaissance aircraft do not normally fly. Though winds derived from microwave channels on polar-orbiting satellites provide valuable surface wind data for HRD surface wind analyses, their swath coverage and orbital passes are limited spatially and temporally. GOES low-level visible (GLLV) winds offer nearly continuous spatial and temporal coverage in the western Atlantic and eastern Pacific basins. The GLLV winds were extrapolated to the surface using a planetary boundary layer model developed at HRD. These surface-adjusted satellite data were used in real-time surface wind analyses of 1998 Hurricane Georges, as well as in post-storm analyses of 1996 Hurricane Lili and 1997 Tropical Storm Claudette. The satellite observations often helped to define the spatial extent of the 17.5 m s-1 (34 kt) surface wind radii and also redefined the 25.7 m s-1 (50 kt) wind radius for one case. Examples of the impact of these data on real-time hurricane surface wind fields provided to the NHC will be discussed.
Eastin, M.D., P.G. Black, and W.M. Gray. Flight-level thermodynamic instrument wetting errors in hurricanes. Part I: Observations. Monthly Weather Review, 130(4):825-841, doi:10.1175/1520-0493(2002)130<0825:FLTIWE>2.0.CO; 2002
Flight-level thermodynamic errors caused by the wetting of temperature and moisture sensors immersed within the airstream are studied using data from 666 radial legs collected in 31 hurricanes at pressure levels ranging from 850 to 500 mb. Concurrent measurements from a modified Barnes radiometer and a Rosemount 102 immersion thermometer are compared to identify regions, called instrument wetting events (IWE), in which Rosemount temperatures are significantly cooler than radiometer-derived temperatures by a specified amount. A total of 420 IWE are identified in the data set. Roughly 50% of the radial legs contain at least one instrument wetting event. More than 90% of IWE are associated with updrafts containing cloud water and are confined to scales less than 10 km. IWE are also found to be more frequent in eyewalls and intense hurricanes. Thermodynamic errors within IWE and convective updrafts and downdrafts are summarized as distributions of average temperature, specific humidity, virtual potential temperature, and equivalent potential temperature error. Distributions are skewed toward larger error values at all levels. Median average errors within IWE indicate that the thermodynamic quantities are typically too low by ~1°C, 1 g kg-1, ~1.5 K, and ~5 K, respectively. The largest errors (>90% of the distribution) are nearly twice the median values. Error magnitudes tend to increase with height, but rarely achieve theoretical predictions. In addition, more than 65% of updrafts and 35% of downdrafts are found to contain significant thermodynamic errors. A correction method used in earlier studies was found to be inadequate at removing the majority of errors, but reduced the errors by 30%V50% on average.
Eastin, M.D., P.G. Black, and W.M. Gray. Flight-level thermodynamic instrument wetting errors in hurricanes. Part II: Implications. Monthly Weather Review, 130(4):842-851, doi:10.1175/1520-0493(2002)130<0842:FLTIWE>2.0.CO; 2002
The implications of flight-level instrument wetting error removal upon the mean thermodynamic structure across the eyewall, buoyancy of rainband vertical motions, and vertical energy fluxes near the top of the inflow layer, are studied. Thermodynamic quantities across the mean eyewall are found to increase at all levels. As a result, maximum radial gradients of each quantity are shifted from the center of the eyewall cloud toward the outer edge. The increase in equivalent potential temperature lifts eyewall values to comparable magnitudes observed in the eye. The mean virtual potential temperature deviation of rainband updrafts increases from slightly negative to slightly positive. This increase and shift in sign are more pronounced in stronger updrafts. The mean deviation in rainband downdrafts decreases slightly toward neutral conditions. Vertical sensible heat fluxes near the top of the inflow layer are found to shift from downward to upward. Upward latent heat fluxes increase. Implications of these results upon hurricane structure and evolution are discussed.
Etherton, B.J., and S.D. Aberson. Assimilation of GPS dropwindsonde data using a VICBAR ensemble. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 218-219, 2002
Evans, J.L., C.S. Velden, L.F. Bosart, J.E. Molinari, and P.G. Black. Hurricane Michael: The "two-way TC." Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 505-506, 2002
Feuer, S.E., J.F. Gamache, M.L. Black, F.D. Marks, and J.B. Halverson. A multiple aircraft experiment in Hurricane Humberto (2001), Part I: Wind fields. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 206-207, 2002
Gamache, J.F., P.D. Reasor, H.E. Willoughby, M.L. Black, and F.D. Marks. Observations of the evolution of precipitation and kinematic structure in a hurricane as it encountered strong westerly shear. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 547-548, 2002
Goni, G.J., P.G. Black, J.J. Cione, and L.K. Shay. Use of satellite altimetry to identify regions of hurricane intensification. Proceedings, 7th International Conference on Remote Sensing for Marine and Coastal Environments, Miami, FL, May 20-22, 2002. Veridian, Ann Arbor, MI, CD-ROM, 4 pp., 2002
Harasti, P.R., W.-C. Lee, J.D. Tuttle, C.J. McAdie, P.P. Dodge, S.T. Murillo, and F.D. Marks. Operational implementation of single-Doppler radar algorithms for tropical cyclones. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 487-488, 2002
Heymsfield, G.M., J.B.Halverson, M.L. Black, F.D. Marks, E.J. Zipser, L. Tian, L. Belcher, P. Bui, and E. Im. Structure of the highly sheared Tropical Storm Chantal during CAMEX-4. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 202-203, 2002
Houston, S.H., and M.D. Powell. Sensitivity study of HRD's H*WIND surface wind analyses for tropical cyclones. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 583-584, 2002
Jiang, H., P.G. Black, E.W. Uhlhorn, P.A. Leighton, E.J. Zipser, and F.D. Marks. Optimal rain rate estimation in tropical cyclones: Validation of SFMR remote sensing rain rates. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 475-476, 2002
Jones, R.W., and H.E. Willoughby. Nonlinear motion of a two-layer baroclinic hurricane in shear. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 134-135, 2002
Kaplan, J., and M. DeMaria. Estimating the probability of rapid intensification using the SHIPS model output: Some preliminary results. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 124-125, 2002
Katsaros, K.B., P.W. Vachon, W.T. Liu, and P.G. Black. Microwave remote sensing of tropical cyclones from space. Journal of Oceanography, 58:137-151, 2002
This article reviews several microwave instruments employed in research and analysis of tropical cyclones (TCs), typhoons, and hurricanes. The instruments discussed include scatterometers, microwave radiometers, synthetic aperture radars (SAR), and rain radar from space. Examples of the particular contribution by one or more of these instruments in analysis of several storms illustrate the comprehensive new views provided by the SeaWinds scatterometers, the detailed high-resolution wind field provided by RADARSAT SAR, particularly inside and in the vicinity of hurricane "eyes," and the presence of secondary flows in the region between rainbands in TCs. The high spatial resolution of precipitation data from the Tropical Rainfall Measuring Mission's rain radar, combined with scatterometer or SAR data, give a significant improvement in the details that can be seen from space, at the surface, and in the precipitating areas of TCs. The microwave instruments provide the penetrating view below the upper level cirrus clouds.
Kollias, P., B.A. Albrecht, and F.D. Marks. Why Mie? Bulletin of the American Meteorological Society, 83(10):1471-1483, https://doi.org/10.1175/BAMS-83-10-1471 2002
This article demonstrates an innovative method for the observation of vertical air motion and raindrop size distribution in precipitation using a 94-GHz Doppler radar. The method is particularly appealing since it is based on fundamental physics, the scattering of microwave radiation by large particles (Mie scattering). The technique was originally proposed in 1988 by Dr. Roger Lhermitte, who ironically pioneered the development of 94-GHz Doppler radars for the study of nonprecipitating clouds. Since then, no real effort for the evaluation and demonstration of the technique was undertaken. In this article, observations from stratiform rain are presented to illustrate the potential and accuracy of the method. The retrievals from this technique provide vertical air motion to an accuracy of 5-10 cm s-1. Despite attenuation, the Doppler velocity measurements remain unbiased and the data revealed high-resolution kinematical and microphysical structures within the stratiform precipitation for the first time. This article will hopefully expose the potential of this technique to the meteorological community and will serve as another example of the visionary contributions that Dr. Lhermitte has made to radar meteorology.
Landsea, C.W., C. Anderson, N. Charles, G. Clark, J.P. Dunion, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer. The Atlantic hurricane database re-analysis project documentation for the 1851-1910 alterations and additions to the HURDAT database. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 460-461, 2002
The oxygen and hydrogen isotopic compositions of rains from Hurricane Olivia (1994) in the eastern Pacific were measured. The rains were collected on 24 and 25 September during airplane flights conducted at an elevation of 3 km. Hurricane Olivia peaked in intensity to a category-4 storm between the two dates. Isotope ratios of rains from Hurricane Olivia were markedly lower (delta18O = -13.9 parts per thousand to -28.8 parts per thousand) than that of rain collected from a thunderstorm at an elevation of 2.3 km outside the influence of Olivia (delta18O = -3.8 parts per thousand). A distinct decrease in isotope ratios from the first day to the next (delta18O = -18.4 parts per thousand to V21.9 parts per thousand) in Hurricane Olivia was attributed to decreased updraft velocities and outflow aloft. This shifted the isotopic water mass balance so that fewer hydrometeors were lifted and more ice descended to flight level. A decrease in the average deuterium excess from the first day to the next (delta = 15.5 to 7.1 parts per thousand) was attributed to an increase in the relative humidity of the water vapor "source" area. We hypothesize that the "source" region for the rain was in the boundary layer near the storm center and that because the hurricane was at peak intensity prior to the second day the relative humidity was higher.
Liu, Q., S.J. Lord, N. Surgi, H.L. Pan, and F.D. Marks. Hurricane initialization using reconnaissance data in GFDL hurricane forecast model. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 267-268, 2002
Marks, F.D., G. Kappler, and M. DeMaria. Development of a tropical cyclone rainfall climatology and persistence (R-CLIPER) model. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 327-328, 2002
Mayrinck, C.E., P.P. Dodge, F.D. Marks, S.H. Houston, and J.F. Gamache. Evolution of the coastal windfield during the landfall of Hurricane Floyd (1999). Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 337-338, 2002
Morrison, I.J., F.D. Marks, and S. Businger. WSR-88D observations of boundary layer rolls during hurricane landfall. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 341-342, 2002
Murillo, S.T., W.-C. Lee, F.D. Marks, and P.P. Dodge. Examining structural changes and circulation center of Hurricane Danny (1997) using a single-Doppler radar wind retrieval technique. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 485-486, 2002
Nuissier, O., R.F. Rogers, and F. Roux. An initialization technique using airborne Doppler radar observations for numerical simulations of Hurricane Bret (21-23 August 1999). Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 403-404, 2002
Ooyama, K.V. The cubic-spline transform method: Basic definitions and tests in a 1D single domain. Monthly Weather Review, 130(10):2392-2415, doi:10.1175/1520-0493(2002)130<2392:TCSTMB>2.0.CO; 2002
The purpose of the paper is to describe the technical details of a numerical method that combines the cubic-spline representation of spatial variables in a finite domain with the logistics of the spectral transform method for the time integration of nonlinear meteorological equations. The reason for developing the method lies in its application to two-way interacting nested models of the atmosphere. When compared with the gridpoint representation, the cubic-spline representation allows direct evaluation of derivatives in the model equations, and leads to a substantial reduction of shortwave dispersion of advecting and propagating waves. When compared with the Fourier spectral representation, the cubic B-splines as basis functions provide simple but exact means of implementing a variety of boundary conditions that are needed at the domain interfaces, as well as at natural boundaries. A sharp (sixth order) low-pass filter, which is built into the cubic-spline transform, effectively eliminates adverse nonlinear accumulation of small-scale errors near the resolution limit. These features, critically important to noise-free nesting, are defined and analyzed in this paper in the simpler context of a single 1D domain. The actual procedures for two-way interactive nesting will be presented in a subsequent paper.
Peterson, R.E., P.G. Black, and V. Pudov. Russian/FSU tropical cyclone research: The last 25 years. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 513-514, 2002
Powell, M.D., and S.D. Aberson. Accuracy of United States tropical cyclone landfall forecasts in the Atlantic basin, 1976-2001. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 301-302, 2002
Reasor, P.D., and M.T. Montgomery. Understanding the dynamics of vertically sheared hurricanes. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 317-318, 2002
Rogers, R.F., R.A. Black, and D.-L. Zhang. A preliminary investigation of a common microphysical parameterization and its applicability to tropical cyclone simulations. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 436-437, 2002
Rogers, R.F., S. Chen, J.E. Tenerelli, and H.E. Willoughby. The role of vertical shear in determining the distribution of accumulated rainfall in high-resolution numerical simulations of tropical cyclones. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 319-320, 2002
Rogers, R.F., S.D. Aberson, J. Kaplan, and S.B. Goldenberg. A pronounced upper-tropospheric warm anomaly encountered by the NOAA Gulfstream-IV aircraft in the vicinity of deep convection. Monthly Weather Review, 130(1):180-187, 2002
Recent flights near deep convection by the National Oceanic and Atmospheric Administration's Gulfstream-IV surveillance aircraft have occasionally experienced significant positive temperature anomalies that sometimes impact the aircraft performance. One such event occurred over the Bahamas on 23 August 1999. During a 20-s time period, when the plane was cruising at an altitude of 175 hPa, the flight-level ambient temperature rose 15°C and returned to ambient values, concurrent with significant fluctuations in the horizontal and vertical winds. Large temperature anomalies such as that reported here can cause the avionics on the aircraft to compensate with a sudden decrease in air speed and a loss of altitude. Possible explanations for this anomaly include instrument error and convectively forced gravity waves or upper-level subsidence.
Schecter, D.A., M.T. Montgomery, and P.D. Reasor. A theory for the vertical alignment of a quasigeostrophic vortex. Journal of the Atmospheric Sciences, 59(2):150-168, 2002
This article presents a new theory for the rate at which a quasigeostrophic vortex realigns, under conservative dynamics, after being tilted by an episode of external vertical shear. The initial tilt is viewed as the excitation of a three-dimensional "vortex Rossby mode." This mode, that is, the tilt, decays exponentially with time during its early evolution. The decay rate, gamma, is proportional to the potential vorticity gradient at a critical radius, where the fluid rotation is resonant with the mode. The decay rate gamma also depends on the internal Rossby deformation radius lR, which is proportional to the stratification strength of the atmospheric or oceanic layer containing the vortex. The change of gammawith lR is sensitive to the form of the vortex. For the case of a "Rankine-with-skirt" vortex, the magnitude of gamma increases (initially) with increasing lR. On the other hand, for the case of a "Gaussian" vortex, the magnitude of gamma decreases with increasing lR. The relevance of this theory to tropical cyclogenesis is discussed.
Schecter, D.A., M.T. Montgomery, and P.D. Reasor. The vertical alignment of an incipient tropical cyclone. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 399-400, 2002
Shay, L.K., S.D. Jacob, T.M. Cook, M.M. Mainelli, S.R. White, P.G. Black, G.J. Goni, and R.E. Cheney. Hurricane heat potential variability from in-situ and radar altimetry measurements. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 575-576, 2002
Uhlhorn, E.W., and J.J. Cione. Real-time simulation of hurricane inner-core ocean cooling as a gauge for intensity change. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 658-659, 2002
Walsh, E.J., C.W. Wright, D. Vandemark, L.F. Bliven, E.W. Uhlhorn, P.G. Black, and F.D. Marks. Rain rate measurements in Hurricane Humberto using the airborne NASA scanning radar altimeter. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 208-209, 2002
Walsh, E.J., C.W. Wright, D. Vandemark, W.B. Krabill, A.W. Garcia, S.H. Houston, S.T. Murillo, M.D. Powell, P.G. Black, and F.D. Marks. Hurricane directional wave spectrum spatial variation at landfall. Journal of Physical Oceanography, 32(6):1667-1684, 2002
The NASA Scanning Radar Altimeter (SRA) flew aboard one of the NOAA WP-3D hurricane research aircraft to document the sea surface directional wave spectrum in the region between Charleston, South Carolina, and Cape Hatteras, North Carolina, as Hurricane Bonnie was making landfall near Wilmington, North Carolina, on 26 August 1998. Two days earlier, the SRA had documented the hurricane wave field spatial variation in open water when Bonnie was 400 km east of Abaco Island, Bahamas. Bonnie was similar in size during the two flights. The maximum wind speed was lower during the landfall flight (39 m s-1) than it had been during the first flight (46 m s-1). Also, Bonnie was moving faster prior to landfall (9.5 m s-1) than when it was encountered in the open ocean (5 m s-1). The open ocean wave height spatial variation indicated that Hurricane Bonnie would have produced waves of 10 m height on the shore northeast of Wilmington had it not been for the continental shelf. The gradual shoaling distributed the wave energy dissipation process across the shelf so that the wavelength and wave height were reduced gradually as the shore was approached. The wave height 5 km from shore was about 4 m. Despite the dramatic differences in wave height caused by shoaling and the differences in the wind field and forward speed of the hurricane, there was a remarkable agreement in the wave propagation directions for the various wave components on the two days. This suggests that, in spite of its complexity, the directional wave field in the vicinity of a hurricane may be well behaved and lend itself to be modeled by a few parameters, such as the maximum wind speed, the radii of the maximum and gale force winds, and the recent movement of the storm.
White, S.R., M.M. Mainelli, S.D. Jacob, and L.K. Shay. Hurricane heat potential estimates from monthly versus seasonal temperature and salinity data. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 128-129, 2002
Willoughby, H.E. Aircraft observations of Hurricane Floyd. Proceedings, Second Workshop on Landfalling Typhoons in the Taiwan Area, Taipei, Taiwan, April 25-26, 2002. National Science Council, 35-51, 2002
The 1995 through 2001 hurricane seasons produced 27 "major" hurricanes, in categories 3, 4, or 5 on the Saffir-Simpson scale. Only three of the major hurricanes that formed during the last six seasons reached U.S. shores with category 3 or greater intensity. This experience contrasts with a long-term expectation that about a third of Atlantic major hurricanes (i.e.,9 of the 27) would make U.S. landfall. Hurricane Floyd of 1999 is representative of the anticlimatic late 20th century major hurricanes. Like most of these storms, it formed from an African Wave. It intensified rapidly east of the Bahamas, reaching a minimum central pressure of 921 hPa on 13 September 1999. This pressure was nearly in equilibrium with the actual ocean surface temperature under the storm at that time. Subsequently, Floyd weakened through a concentric eyewall replacement, reintensified somewhat, and then weakened as a result of large-scale shear and less favorable thermodynamic conditions to category 2 before landfall in eastern North Carolina. Floyd's most serious impact was torrential rainfall that claimed 75 lives through drowning in the northeastern U.S., the largest mortality in a hurricane since Agnes in 1972. Intensive observations from instrumented aircraft, including flight-level data, radar, dropsondes, and air-expendable bathythermographs are the key to understanding of the factors that caused Floyd's rapid intensification and more gradual weakening.
Willoughby, H.E., and M.E. Rahn. A new parametric model of hurricane wind profiles. Preprints, 25th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, April 29-May 3, 2002. American Meteorological Society, Boston, 553-554, 2002
2001
Aberson, S.D. The ensemble of tropical cyclone track forecasting models in the North Atlantic Basin (1976-2000). Bulletin of the American Meteorological Society, 82(9):1895-1904, doi:10.1175/1520-0477(2001)082<0000:TEOTCT> 2001
The suite of tropical cyclone track forecast models in the Atlantic basin from the 1976 to 2000 hurricane seasons are treated as a forecast ensemble. The 12-h ensemble mean forecast, adjusted for forecast difficulty, has improved at a rate of just under 1% per year, and the improvement rate increases to almost 2.4% per year for the 72-h forecasts. The average size of the 72-h (48-h) error in 1976 is less than the average size of the 48-h (36-h) error in 2000. The average 36-h forecast error in 2000 is comparable to the 24-h forecast error in 1976. The ensemble currently spans the true path of the tropical cyclone in the cross-track direction more than 90% of the time and in the alongtrack direction between 60% and 90% of the time depending on the forecast lead time. The ensemble spread is unable to provide estimates of individual forecast reliability, likely making probabilistic landfall forecasts from this ensemble unreliable. The reliability of the spread in the cross-track direction suggests the possibility of limiting hurricane watch and warning regions depending upon the ensemble spread at landfall.
Aberson, S.D., S.J. Majumdar, and C.H. Bishop. A real-time ensemble for the prediction of hurricane tracks in the Atlantic basin. Preprints, 18th Conference on Weather Analysis and Forecasting and 14th Conference on Numerical Weather Prediction, Fort Lauderdale, FL, July 30-August 2, 2001. American Meteorological Society, Boston, 456-457, 2001
Dunion, J.P., C.S. Velden, and J.R. Rhome. Satellite applications for tropical wave/tropical cyclone tracking. Preprints, 18th Conference on Weather Analysis and Forecasting, Ft. Lauderdale, FL, July 30-August 2, 2001. American Meteorological Society, Boston, 436-438, 2001
Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nunez, and W.M. Gray. The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293(5529):474-479, https://doi.org/10.1126/science.1060040 2001
The years 1995 to 2000 experienced the highest level of North Atlantic hurricane activity in the reliable record. Compared with the generally low activity of the previous 24 years (1971 to 1994), the last six years have seen a doubling of overall activity for the whole basin, a 2.5-fold increase in major hurricanes (>50 m/s), and a fivefold increase in hurricanes affecting the Caribbean. The greater activity is caused by simultaneous increases in North Atlantic sea-surface temperatures and decreases in vertical wind shear, both of which are known to favor hurricane formation. Because these changes exhibit a multidecadal time scale, the present high level of hurricane activity is likely to persist for an additional 10 to 40 years. The shift in climate calls for a reevaluation of preparedness and mitigation strategies.
Jarrell, J.D., M. Mayfield, E.N. Rappaport, and C.W. Landsea. Deadliest, costliest, and most intense United States hurricanes from 1900 to 2000 (and other frequently requested hurricane facts), updated October 2001. NOAA Technical Memorandum, NWS-TPC-3 (PB2002-100134), 44 pp., 2001
This version of the "Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000" extends the work of Herber et al. (1997) through the 2000 season. It also includes an estimate of the monetary loss that historical hurricanes could exact on the current property-at-risk in the same location.
Kaplan, J., and M. DeMaria. On the decay of tropical winds after landfall in the New England area. Journal of Applied Meteorology, 40(2):280-286, 2001
A version of the Kaplan and DeMaria empirical model for predicting the decay of tropical cyclone 1-min maximum sustained surface winds after landfall is developed for the New England region. The original model was developed from the National Hurricane Center (NHC) best-track wind estimates for storms that made landfall in the United States south of 37°N from 1967 to 1993. In this note, a similar model is developed for U.S. storms north of 37°N,which primarily made landfall in New York or Rhode Island and then moved across New England. Because of the less frequent occurrence of New England tropical cyclones, it was necessary to include cases back to 1938 to obtain a reasonable sample size. In addition, because of the faster translational speed and the fairly rapid extratropical transition of the higher-latitude cases, it was necessary to estimate the wind speeds at 2-h intervals after landfall, rather than every 6 h, as in the NHC best track. For the model development, the estimates of the maximum sustained surface winds of nine landfalling storms (seven hurricanes and two tropical storms) at 2-h intervals were determined by an analysis of all available surface data. The wind observations were adjusted to account for variations in anemometer heights, averaging times, and exposures. Results show that the winds in the northern model decayed more (less) rapidly than those of the southern model, when the winds just after landfall are greater (less) than 33 knots. It is hypothesized that this faster rate of decay is due to the higher terrain near the coast for the northern sample and to the more hostile environmental conditions (e.g., higher vertical wind shear). The slower decay rate when the winds fall below 33 knots in the northern model might be due to the availability of a baroclinic energy source as the storms undergo extratropical transition.
Katzberg, S.J., R.A. Walker, J.H. Roles, T. Lynch, and P.G. Black. First GPS signals reflected from the interior of a tropical storm: Preliminary reults from Hurricane Michael. Geophysical Research Letters, 28(10):1981-1984, https://doi.org/10.1029/2000GL012823 2001
Using GPS signals reflected from the ocean surface is developing into a simple technique for measuring sea-state and inferring surface wind speeds. Theoretical models have been developed which are considered valid to approximately 24 m/s. The GPS reflection technique has an obvious extension to extremely high sea states, cyclones, and extra-tropical storms. In October 2000, a GPS system mounted in a NOAA hurricane hunter research aircraft was flown into Hurricane Michael off the South Carolina coast. The first acquisition of GPS signals reflected from the sea surface inside tropical cyclones was accomplished. This paper presents some examples of the data sets, as well as early wind speed retrieval results using direct extensions of current models. Data from the GPS wind speed retrievals, as well as from direct aircraft measurements, are compared and discussed.
Knaff, J.A., and C.W. Landsea. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 10(2):31-34, 2001
Knaff, J.A., and C.W. Landsea. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 10(3):40-42, 2001
Kollias, P., B.A. Albrecht, and F.D. Marks. Raindrop sorting induced by vertical drafts in convective clouds. Geophysical Research Letters, 28(14):2787-2790, https://doi.org/10.1029/2001GL013131 2001
Evidence of raindrop sorting by a convective updraft is presented. Using a vertically pointing 94-GHz Doppler radar (lambda = 3.2 mm) and capitalizing on the resonant nature of the backscattering cross-section as a function of the raindrop size (Mie scattering), the vertical air motions to an accuracy of 0.1 m s-1, and the shape of the raindrop size distribution are retrieved from the Doppler spectra. The interaction of vertical drafts and raindrops is documented for the first time by high resolution radar data. The updraft structure clearly causes horizontal and vertical sorting of the raindrops. In the updraft core, small raindrops (D < 1.7 mm) that have terminal velocities less than the updraft velocities (6-7 m s-1) and a clear absence of drops > 3 mm are observed. Towards the updraft periphery, a gradual increase in the raindrop sizes is documented where large raindrops (D > 3 mm) are observed. The observations demonstrate the importance of updrafts in distributing the raindrops in space.
Landsea, C.W. Comment on "Changes in the rates of North Atlantic major hurricane activity during the 20th century." Geophysical Research Letters, 28(14):2871-2872, https://doi.org/10.1029/2000GL012832 2001
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 10(4):41-43, 2001
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 10(1):31-33, 2001
Marks, F.D. Quantitative precipitation forecasting in hurricanes: Issues and opportunities. Preprints, Symposium on Precipitation Extremes: Prediction, Impacts, and Responses, Albuquerque, NM, January 14-19, 2001. American Meteorological Society, Boston, 293-296, 2001
McAdie, C.J., P.R. Harasti, P.P. Dodge, W.-C. Lee, S.T. Murillo, and F.D. Marks. Real-time implementation of tropical cyclone-specific radar data processing algorithms. Preprints, 30th International Conference on Radar Meteorology, Munich, Germany, July 19-24, 2001. American Meteorological Society, Boston, 466-468, 2001
Michaels, M., M. Shepard, S.D. Aberson, H.A. Friedman, and K. Murphy. Survey results of Society membership: The face of our profession at the threshold of the new millennium. Bulletin of the American Meteorological Society, 82(7):1331-1352, 2001
In the spring of 1999, the American Meteorological Society surveyed its membership in order to update demographic information on the Society and to gain a more detailed perspective on the workplace. The survey was sent out with the dues statement and was solicited on a separate form returned independently to protect privacy and maintain anonymity. The responses were captured in a newly employed, machine-readable format to provide an ease of statistical analysis and data compilation not available in prior survey analysis. This data collection and subsequent demographic analysis represents the first attempt to update information regarding the membership since the 1993 survey results were published by Zevin and Seitter. The format of the 1999 survey was designed to logically follow and expand upon the historical data of the membership collected at varying intervals since 1975. The 1999 survey was broken into six parts. The sections on demographics, education, and current employment closely followed the previous surveys from 1993 and 1990 to facilitate direct comparisons between historical datasets whenever possible. The last three sections were reworked to elicit more declarative responses regarding personal circumstances, workplace circumstances, and additional issues concerning career choice and AMS membership, respectively. An additional space was provided for narrative comments regarding opportunities for women and minorities in the AMS-related sciences. Some 10,000 members were sent the 1999 dues statement and enclosed survey questionnaire. A total of 4,669 members responded. The following is a detailed analysis of the data collected from the 1999 membership survey.
Murillo, S.T., W.-C. Lee, F.D. Marks, and P.P. Dodge. Using a single-Doppler radar wind retrieval technique to examine structural changes in Hurricane Danny (1997). Preprints, 30th International Conference on Radar Meteorology, Munich, Germany, July 19-24, 2001. American Meteorological Society, Boston, 148-149, 2001
Ooyama, K.V. A dynamic and thermodynamic foundation for modeling the moist atmosphere with parameterized microphyics. Journal of the Atmospheric Sciences, 58(15):2073-2102, 2001
Moist convection is an exquisite yet powerful participant in the creation of weather on our planet. To facilitate numerical modeling of weather systems in a moist atmosphere, a direct and consistent application of dynamic and thermodynamic principles, in conjunction with parameterized microphysics, is proposed. An earlier formulation of reversible thermodynamics, in terms of the mass of air and water substance and the total entropy, is now extended to include the irreversible process of precipitation through parameterized microphysics. The dynamic equations are also formulated to account consistently for the mass and momentum of precipitation. The theoretical proposal is tested with a two-dimensional model that utilizes a versatile and accurate spectral method based on a cubic-spline representation of the spatial fields. In order to allow a wide range of scale interactions, the model is configured on multiply-nested domains of outwardly decreasing resolution, with noise-free, two-way interfaces. The semi-implicit method provides efficient time integration for the nested spectral model. The tests performed are the simulation of the growth of single-cell clouds and also the generation of self-sustaining multicell squall lines, and the effects of various resolutions on the simulations are examined. The results favorably compare with similar results found in the literature, but also offer new insights into the interplay between dynamics and precipitation.
Phoebus, P.A., D.R. Smith, P.J. Croft, H.A. Friedman, M.C. Hayes, K.A. Murphy, M.K. Ramamurthy, B. Watkins, and J.W. Zeitler. Meeting summary: Ninth AMS symposium of education. Bulletin of the American Meteorological Society, 82(2):295-303, 2001
The American Meteorological Society held its Ninth Symposium on Education in conjunction with the 80th Annual Meeting in Long Beach, California. The theme of this year's symposium was "Atmospheric and Oceanographic Education-Expanding our Vision for the New Millennium." Thirty-five oral presentations and 53 poster presentations summarized a variety of educational programs or examined educational issues for both the precollege and university levels. There was a special session reporting on a recent survey conducted by the Board on Women and Minorities, as well as a special session on the educational applications of satellite meteorology and oceanography. Over 200 people representing a wide spectrum of the Society attended one or more of the sessions in this two-day conference. The program for the Ninth Symposium on Education can be viewed in the October 1999 issue of the Bulletin.
Powell, M.D., and S.D. Aberson. Accuracy of United States tropical cyclone landfall forecasts in the Atlantic Basin (1976-2000). Bulletin of the American Meteorological Society, 82(12):2749-2768, 2001
About 13% of all Atlantic basin tropical cyclone forecasts issued from 1976 to 2000 are for landfalls along the United States coastline, and 2% more are for storms forecast to make landfall in the United States but that remain at sea. Landfall position and time forecasts are skillful at all forecast time periods and are more skillful than Atlantic basin track forecasts as a whole, but within 30 h of predicted landfall, timing errors demonstrate an early bias of 1.5-2.5 h. Landfall forecasts are most accurate for storms moving at oblique or normal angles to the coastline and slow-moving storms. During the last quarter century, after adjustment for forecast difficulty, no statistically significant improvement or degradation is noted for landfall position forecasts. Time of landfall forecasts indicate no degradation at any period and significant improvement for the 19-30 h period. The early bias and lack of improvement are consistent with a conservative or "least regret" forecast and warning strategy to account for possible storm accelerations. Landfall timing uncertainty is ~11 h at 24 and 36 h, which suggests that hurricane warnings could be disseminated about 12 h earlier (at 36 h, rather than 24 h, before predicted landfall) without substantial loss of lead time accuracy (although warning areas necessarily would be larger). Reconsideration of the National Weather Service Strategic Plan and United States Weather Research Program track forecast goals is recommended in light of these results.
Reasor, P.D., and M.T. Montgomery. Three-dimensional alignment and corotation of weak, TC-like vortices via linear vortex Rossby waves. Journal of the Atmospheric Sciences, 58(16):2306-2330, 2001
The vertical alignment of an initially tilted geostrophic vortex is shown here to be captured by linear vortex Rossby wave dynamics when the vortex cores at upper and lower levels overlap. The vortex beta Rossby number, defined as the ratio of nonlinear advection in the potential vorticity equation to linear radial advection, is less than unity in this case. A useful means of characterizing a tilted vortex flow in this parameter regime is through a wave-mean flow decomposition. From this perspective, the alignment mechanism is elucidated using a quasigeostrophic model in both its complete and linear equivalent barotropic forms. Attention is focused on basic-state vortices with continuous and monotonically decreasing potential vorticity profiles. For internal Rossby deformation radii larger than the horizontal scale of the tilted vortex, an azimuthal wavenumber 1 quasi mode exists. The quasi mode is characterized by its steady cyclonic propagation, long lifetime, and resistance to differential rotation, behaving much like a discrete vortex Rossby wave. The quasi mode traps disturbance energy, causing the vortex to precess, or corotate, and thus prevents alignment. For internal deformation radii smaller than the horizontal vortex scale, the quasi mode disappears into the continuous spectrum of vortex Rossby waves. Alignment then proceeds through the irreversible redistribution of potential vorticity by the sheared vortex Rossby waves. Further decreases in the internal deformation radius result in a decreased dependence of vortex evolution on initial tilt magnitude, consistent with a reduction of the vortex beta Rossby number. These results are believed to have relevance to the problem of tropical cyclone (TC) genesis. Cyclogenesis initiated through the merger and alignment of low-level convectively generated positive potential vorticity within a weak incipient vortex is captured by quasi-linear dynamics. A potential dynamical barrier to TC development in which the quasi mode frustrates vertical alignment can be identified using the linear alignment theory in this case.
Reasor, P.D., M.T. Montgomery, F.D. Marks, and J.G. Gamache. Studies of tropical cyclone vorticity dynamics using airborne Doppler-derived wind fields. Preprints, 30th International Conference on Radar Meteorology, Munich, Germany, July 19-24, 2001. American Meteorological Society, Boston, 142-144, 2001
Rogers, R.F., and J.M. Fritsch. Surface cyclogenesis from convectively-driven amplification of mid-level mesoscale convective vortices. Monthly Weather Review, 129(4):605-637, 2001
Mesoscale convective vortices (MCVs) are mid-tropospheric warm-core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems. Typically, divergent, anticyclonically-circulating, mesoscale cold anomalies appear both above and below the MCV. The upper level cold anomaly is usually found near the tropopause while the low-level anomaly is surface-based and exhibits locally higher pressure. One aspect of MCVs that has received much attention recently is the role that they may play in tropical cyclogenesis. Of special interest is how an MCV amplifies when deep convection redevelops within the borders of its mid-level cyclonic circulation and how the amplified MCV transforms the divergent surface-based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure. The Pennsylvania State University/National Center for Atmospheric Research mesoscale model MM5 is used to simulate an MCV that was instrumental in initiating, within the borders of the mid-level vortex's circulation, several successive cycles of convective development and decay over a two-day period. After each cycle of convection, both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase. The simulation reproduces the development and evolution of the MCV and associated convective cycles. Mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated. A conceptual model is presented to explain how convective redevelopment within the MCV causes low-level heights to fall and cyclonic vorticity to grow downward to the surface. Applying this conceptual model to a tropical marine environment is also considered.
Rogers, R.F., S.S. Chen, J.E. Tenerelli, and H.E. Willoughby. A numerical study of the impact of vertical shear on the distribution of rainfall in Hurricane Bonnie (1998). Preprints, Ninth Conference on Mesoscale Processes, Ft. Lauderdale, FL, July 30-August 2, 2001. American Meteorological Society, Boston, 2001
Schubert, W.H., S.A. Hausman, M. Garcia, K.V. Ooyama and H.-C. Kuo. Potential vorticity in a moist atmosphere. Journal of the Atmospheric Sciences, 58(21):3148-3157, 2001
The potential vorticity principle for a nonhydrostatic, moist, precipitating atmosphere is derived. An appropriate generalization of the well-known (dry) Ertel potential vorticity is found to be P = ρ-1 (2Ω + gradient x u) • gradient θρ, where ρ is the total density, consisting of the sum of the densities of dry air, airborne moisture (vapor and cloud condensate), and precipitation; u is the velocity of the dry air and airborne moisture; and θρ = Tρ (p0/p)Ra/CPa is the virtual potential temperature, with Tρ = p/(ρ Ra) the virtual temperature, p the total pressure (the sum of the partial pressures of dry air and water vapor), p0 the constant reference pressure, Ra the gas constant for dry air, and CPa the specific heat at constant pressure for dry air. Since θρ is a function of total density and total pressure only, its use as the thermodynamic variable in P leads to the annihilation of the solenoidal term, that is, gradient θρ • (gradientρ × gradientp) = 0. In the special case of an absolutely dry atmosphere, P reduces to the usual (dry) Ertel potential vorticity. For balanced flows, there exists an invertibility principle that determines the balanced mass and wind fields from the spatial distribution of P. It is the existence of this invertibility principle that makes P such a fundamentally important dynamical variable. In other words, P (in conjunction with the boundary conditions associated with the invertibility principle) carries all the essential dynamical information about the slowly evolving balanced part of the flow.
Smith, D.R., M.C. Hayes, M.K. Ramamurthy, J.W. Zeitler, K.A. Murphy, P.J. Croft, J.M. Nese, H.A. Friedman, H.W. Robinson, C.D. Thormeyer, P.A. Ruscher, and R.E. Pandya. Meeting summary: 10th AMS symposium on education. Bulletin of the American Meteorological Society, 82(12):2817-2824, 2001
The American Meteorological Society held its 10th Symposium on Education in conjunction with the 82nd Annual Meeting in Albuquerque, New Mexico. The theme of 2001's symposium was "enhancing public awareness of the atmospheric and oceanic environments." Thirty-six oral presentations and 38 poster presentations summarized a variety of educational programs or examined educational issues at both the precollege and university levels. There was a special session on increasing awareness of meteorology and oceanography through popular and informal educational activities, as well as a joint session with the 17th International Conference on Interactive Information and Processing Systems (IIPS) for Meteorology, Oceanography, and Hydrology on using the World Wide Web to deliver information pertaining to the atmosphere, oceans, and coastal zone. Over 200 people representing a wide spectrum of the Society attended one or more of the sessions in this two-day conference. The program for the 10th Symposium on Education can be viewed in the November 2000 issue of the Bulletin.
Testud, J., S. Oury, R.A. Black, P. Amayenc, and X. Dou. The concept of "normalized" distribution to describe raindrop spectra: A tool for cloud physics and cloud remote sensing. Journal of Applied Meteorology, 40(6):1118-1140, 2001
The shape of the drop size distribution (DSD) reflects the physics of rain. The DSD is the result of the microphysical processes that transform the condensed water into rain. The question of the DSD is also central in radar meteorology, because it rules the relationships between the radar reflectivity and the rainfall rate R. Normalizing raindrop spectra is the only way to identify the shape of the distribution. The concept of normalization of DSD developed in this paper is founded upon two reference variables, the liquid water content LWC and the mean volume diameter Dm. It is shown mathematically that it is appropriate to normalize by N0* proportional to LWC/Dm4 with respect to particle concentration and by Dm with respect to drop diameter. Also, N0* may be defined as the intercept parameter that would have an exponential DSD with the same LWC and Dm as the real one. The major point of the authors' approach is that it is totally free of any assumption about the shape of the DSD. This new normalization has been applied to the airborne microphysical data of the Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) collected by the National Center for Atmospheric Research Electra aircraft. The classification of the TOGA-COARE raindrop spectra into four categories (one stratiform, and three convective [0-10, 10-30, and 30-100 mm h-1]) allowed the following features to be identified. (1) There is a distinct behavior of N0* between stratiform and convective rains; typical values are 2.2 × 106 m4 for stratiform and 2 × 107 m4 for convective. (2) In convective rain, there is a clear trend for Dm to increase with R, but there is no correlation between N0* and R. (3) The "average" normalized shape of the DSD is remarkably stable among the four rain categories. This normalized shape departs from the exponential, but also from all the analytical shapes considered up to now (e.g., gamma, lognormal, modified gamma). The stability of the normalized DSD shape and the physical variability of N0* and Dm are discussed in respect to the equilibrium theory of List et al. The stability of the shape implies that two parameters (and only two) are needed to describe the DSD. This stability supports the robustness of rain relations parameterized by N0*. The same TOGA-COARE dataset is used to check that the rain relations parameterized by N0* are much less dispersed than the classical ones, even after rain-type classification.
Willoughby, H.E., and R.W. Jones. Nonlinear motion of a barotropic vortex in still air and in an environmental zonal flow. Journal of the Atmospheric Sciences, 58(14):1907-1923, 2001
This study employs a Vortex Tracking Semispectral (VTSS) model cast in cylindrical coordinates that move with the vortex. Variables are represented spectrally in azimuth only, so that the model becomes a set of linear equations for each azimuthal wavenumber component, forced by the environmental flow and coupled by wave-wave interactions that account for all of the nonlinearity. The vortex is advected by the surrounding wind and propagates when potential vorticity (PV) gradients due to the surrounding flow or the beta effect force wavenumber one (WN1) asymmetries. Nonlinearity generally plays a dissipative role. Although propagation is faster in stronger PV gradients, nonlinear interactions cause the motions due to superposed PV gradients to be slower than the sum of their individual motions. In still air or uniform wind on a beta plane, the wave energy spectrum falls off rapidly with wavenumber. For most situations, the calculations converge for truncation at WN6 on a 4000-km domain. In an anticyclonically sheared environmental zonal flow, the spectrum of asymmetric energy narrows because the WN2 asymmetry is forced directly by the environmental deformation. The deformation-induced asymmetry interferes destructively with WN2 due to internal wave-wave interaction. In a cyclonically sheared zonal flow, the deformation-induced and nonlinearly-induced asymmetries interfere constructively, resulting in a broader spectrum. Energy cascades from WN2 to wavenumbers >2. A reverse cascade also carries energy to WN1, changing the beta gyres and the motion. Consequent perturbation of WN1 leads to slow convergence of the predicted vortex position after 10 simulated days with increasing spectral resolution. When imposed mass sources and sinks are used to supply energy directly to the asymmetries in the middle of the spectrum, similar wave-wave interactions force WN1, leading to a trochoidal vortex track.
Wright, C.W., E.J. Walsh, D. Vandemark, W.B. Krabill, A.W. Garcia, S.H. Houston, M.D. Powell, P.G. Black, and F.D. Marks. Hurricane directional wave spectrum spatial variation in the open ocean. Journal of Physical Oceanography, 31(8):2472-2488, 2001
The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane's inner core over open water. The NASA airborne Scanning Radar Altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane research aircraft at 1.5-km height acquired the open-ocean data on 24 August 1998 when Bonnie, a large hurricane with 1-min sustained surface winds of nearly 50 m s-1, was about 400 km east of Abaco Island, Bahamas. The NOAA aircraft spent more than five hours within 180 km of the eye and made five eye penetrations. Grayscale coded images of Hurricane Bonnie wave topography include individual waves as high as 19 m peak to trough. The dominant waves generally propagated at significant angles to the downwind direction. At some positions, three different wave fields of comparable energy crossed each other. Partitioning the SRA directional wave spectra enabled determination of the characteristics of the various components of the hurricane wave field and mapping of their spatial variation. A simple model was developed to predict the dominant wave propagation direction.
2000
Aberson, S.D. The first three years of operational targeting with the NOAA Gulfstream-IV. Preprints, 4th Symposium on Integrated Observing Systems, Long Beach, CA, January 9-14, 2000. American Meteorological Society, Boston, 198-199, 2000
Aberson, S.D. Three years of tropical cyclone synoptic surveillance in the Atlantic basin. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 108-109, 2000
Since 1997, NOAA has performed more than 50 synoptic surveillance missions in the core and environments of tropical cyclones threatening the United States mainland, Puerto Rico, and the Virgin Islands with their G-IV and P3 aircraft. GPS dropwindsonde observations are taken approximately every 250 km along the flight tracks and sent to the National Centers for Environmental Prediction and the National Hurricane Center for incorporation in numerical guidance and for subjective evaluation. The impact of these data on both track and intensity forecasts will be presented. Since small differences in initial conditions are known to grow in the numerical models at different rates, targeting the fastest growing modes has been studied. Results of such targeting, including methods to find target locations and sampling strategies, will be presented.
Aberson, S.D. Woman and minorities in meteorology since 1950. 80th Annual Meeting and Exhibition, Long Beach, CA, January 9-14, 2000. American Meteorological Society, Boston, 70-71, 2000
Aberson, S.D. Women's trends: The changing status of women in the profession/society. Preprints, 9th Symposium on Education, Long Beach, CA, January 9-14, 2000. American Meteorological Society, Boston, 70-71, 2000
Aberson, S.D., and K. Bedka. The operational ensemble of tropical cyclone track guidance at the National Hurricane Center (1976-1998). Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 177-178, 2000
A suite of operational track forecast models has been run at NHC in support of NHC s task to provide tropical cyclone track forecasts. Official NHC forecasts have improved at a rate faster than 1% during the 1990s, suggesting substantial improvements to the numerical guidance. This operational ensemble since 1976 has been analyzed as a set to mark the improvements of the guidance with time. The improvements in the ability of the guidance to span the actual track of tropical cyclones, the performance of the ensemble mean with time, and changes in individual model performance are to be presented.
Atlas, D., C.W. Ulbrich, F.D. Marks, R.A. Black, E. Amitai, P.T. Willis, and C.E. Samsury. Partitioning tropical oceanic convective and stratiform rains by draft strength. Journal of Geophysical Research, 105(D2):2259-2267, https://doi.org/10.1029/1999JD901009 2000
The discrimination of convective from stratiform tropical oceanic rains by conventional radar-based textural methods is problematic because of the small size and modest horizontal reflectivity gradients of the oceanic convective cells. In this work, the vertical air motion measured by an aircraft gust probe is used as a discriminator which is independent of the textural methods. A threshold draft magnitude approximately equal to 1 m s-1 separates the two rain types. Simultaneous airborne in-situ observations of drop size distributions (DSD) made during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) were used to compute Z, R, and other integral parameters. The data were quality controlled to minimize misclassifications. The convective and stratiform rains, observed just below the melting level but adjusted to surface air density, are characterized by power law Z-R relations (Z = 129R1.38 [convective]) and 224R1.28 [stratiform]). However, at R < 10 mm h-1, the convective population is essentially coincident with the small-drop size, small-Z portion of the stratiform population. Tokay and Short (1996) found a similar result when their algorithm did not separate the rain types unambiguously at R < 10 mm h-1. The physical reasons for the wide variability of the drop size spectra and Z-R points in stratiform rain and their overlap with that of convective rain are proposed. The subtle distinctions in the microphysical properties and the Z-R relations by rain type could not be found by Yuter and Houze using the same airborne DSD data set as that in this work and a radar-based textural classification algorithm.
Black, M.L., A.B. Damiano, and S.R. White. The first eyewall penetration by the NOAA G-IV aircraft. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 175-176, 2000
On August 9, 1999, NOAA's Aircraft Operations Center (AOC) was tasked by the Tropical Prediction Center/National Hurricane Center and the Central Pacific Hurricane Center (CPHC) to deploy the NOAA Gulfstream G-IV jet aircraft and crew to Honolulu, Hawaii for synoptic surveillance missions around Hurricanes Eugene and Dora. Both of these storms had tracked westward across the eastern Pacific basin into the area of responsibility of CPHC (west of 140°W)and posed potential threats to Hawaii. After a successful G-IV mission around Hurricane Eugene on 12 August, a similar flight-track was designed to collect synoptic data from GPS dropsondes around Hurricane Dora on 14 August. At the time, Dora was steadily weakening from a peak intensity of 120 kts on 13 August with maximum sustained surface winds forecast to be 70 kts during the mission. Dora was a compact hurricane with a circular, well-defined eye and had only a couple of weak rainbands outside of the central dense overcast. A deviation from the proposed flight track was planned to fly the G-IV on a heading towards the eye during the closest approach to Hurricane Dora. The maneuver's purpose was to observe the structure of a hurricane at altitudes >40,000 feet with the aircraft's nose radar system. During the flight, the G-IV crew observed that Hurricane Dora was closer to the flight track than was forecast, so that when the aircraft turned toward the south side of the storm, the eyewall was approximately 80 nmi away. After a brief discussion of the structure of Dora and safety considerations, the flight director and aircraft commander decided to fly into the eye before heading back to the original track. This represented the first time that the G-IV would penetrate the eyewall of a hurricane, and would do so at an altitude of 45,000 feet (~145 mb). The aircraft flew through a thick cirrus cloud cover in the eyewall and that thinned while in the eye. Two GPS dropsondes were released while in (above) the eye of Hurricane Dora, and a third sonde was dropped just outside of the southwest eyewall while the G-IV was exiting the storm. Both of the eye drops drifted near or into the eyewall as they descended and one of them showed winds in excess of 80 kts at altitudes below 3000 ft. During the penetration, wind speeds at a flight level of 45,000 feet were approximately 5 kts and the wind direction showed anticyclonic flow.
Black, M.L., and J.L. Franklin. GPS dropsonde observations of the wind structure in convective and non-convective regions of the hurricane eyewall. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 448-449, 2000
GPS dropsonde observations in the inner core regions of tropical cyclones have shown remarkable vertical variation in the wind structure. Vertical profiles from sondes released in the convective portions of the hurricane eyewall frequently exhibit multiple low to mid-level wind maxima. These maxima may contain peak winds significantly higher than those measured at typical reconnaissance altitudes (3 km). Convective mixing is thought to be a mechanism that may bring this high momentum air to altitudes at or near the sea-surface. In non-convective regions, both outside and within the eyewall, however, the wind profiles typically do not have the large low-level wind maxima and the wind speed frequently decreases rapidly toward the surface in the boundary layer. Preliminary analyses of dropsonde wind profiles have suggested systematic differences in the shape of these soundings. An important result from these analyses is that the surface wind speed is a substantially higher fraction of the wind at altitude in convective regions than in non-convective or stratiform regions. We plan on classifying several hundred dropsonde observations according to the convective environment they fall through. The classifications will be based upon simultaneous radar observations from NOAA P-3 research flights into tropical cyclones in various stages of development. Individual profiles from convective and non-convective regions of the storms will be presented to highlight some of the observed differences in wind structure. A brief statistical analyses is planned to describe the variance in the mean structure derived from these classifications. A discussion of some of the possible physical mechanisms for the difference in the observed wind profiles will be discussed.
Black, P.G., E.W. Uhlhorn, J.J. Cione, G.J. Goni, L.K. Shay, S.D. Jacob, E.J. Walsh, and E.A. D'Asaro. Hurricane intensity change modulated by air-sea interaction effects based on unique airborne measurements during the 1998-1999 hurricane seasons. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J7-J8, 2000
Black, P.G., E.W. Uhlhorn, M.D. Powell, and J. Carswell. A new era in hurricane reconnaissance: Real-time measurement of surface wind structure and intensity via microwave remote sensing. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 199-200, 2000
Bosart, L.F., W.E. Bracken, J. Molinari, C.S. Velden, and P.G. Black. Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Monthly Weather Review, 128(2):322-352, doi:10.1175/1520-0493(2000)1282.0.CO;2 2000
Hurricane Opal intensified rapidly and unexpectedly over the Gulf of Mexico between 1800 UTC 3 October 1995 and 1000 UTC 4 October 1995. During this period, the storm central pressure decreased from 963 to 916 hPa and sustained winds reached 68 m s-1. Analyses that include high-resolution GOES-8 water vapor winds and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) gridded datasets are employed to examine the rapid intensification phase of Opal. Opal first reached tropical storm strength on 29V30 September 1995 as it interacted with a trough while situated over the Yucatan Peninsula. Opal deepened moderately (20 hPa) in the 24 h ending 1200 UTC 2 October as it achieved minimal hurricane strength and as it turned northeastward. The deepening occurred in conjunction with an environmental flow interaction as determined by an Eliassen balanced vortex outflow calculation. As Opal accelerated toward the Gulf coast by 1200 UTC 3 October, it approached the equatorward jet-entrance region of a progressive synoptic-scale trough. The trough tail extended southwestward toward the lower Texas coast. As the poleward portion of the trough moved eastward, the equatorward end of the trough lagged behind, stretched meridionally, and partially fractured as it encountered a deformation region over the northwest Gulf. Enhanced outflow and increased divergence in the upper troposphere poleward of Opal was associated with the deformation zone and the partially fractured trough tail. An analysis of the 300-200-hPa layer-averaged divergence and 6-h divergence change based on an analysis of the water vapor winds shows a significant increase in the magnitude and equatorward extension of the divergence core toward Opal that begins at 1200 UTC 3 October and is most apparent by 1800 UTC 3 October and 0000 UTC 4 October. This divergence increase is shown to precede convective growth in the eyewall and the onset of rapid intensification and is attributed to a jet-trough-hurricane interaction in a low-shear environment. Calculations of balanced vortex outflow based on the ECMWF and NCEP gridded datasets confirms this interpretation. A crucial finding of this work is that the jet-trough-hurricane interaction and explosive intensification of Opal begins near 0000 UTC 4 October when the storm is far from its maximum potential intensity (MPI), and the 850-200-hPa shear within 500 km of the center is weak (2-3 m s-1). In this first stage of rapid intensification, the winds increase by almost 15 m s-1 to 52 m s-1 prior to the storm reaching an oceanic warm-core eddy. The second stage of rapid intensification occurs between 0600 and 1000 UTC 4 October when Opal is over the warm-core eddy and sustained winds increase to 68 m s-1. During this second stage, conditions are still favorable for a jet-trough-hurricane interaction as demonstrated by the balanced vortex outflow calculation. Opal weakens rapidly after 1200 UTC 4 October when the storm is near its MPI, the shear is increasing, and the eye is leaving the warm-core eddy. This weakening occurs as Opal moves closer to the trough. It is suggested that an important factor in determining whether a storm-trough interaction is favorable or unfavorable for intensification is how far a storm is from its MPI. The results suggest that a favorable storm-trough interaction ("good trough") can occur when a storm is far from its MPI. It is suggested that although the ECMWF (and to lesser extent NCEP) analyses reveal the trough-jet-hurricane interaction through the balanced vortex outflow calculation, that the failure of the same models to predict the rapid intensification of Opal can be attributed to the inability of the model to resolve the eye and internal storm structure and the associated influence of the trough-jet-hurricane interaction on the diabatically driven storm secondary circulation. The analyses also indicate that the high spatial and temporal resolution of the GOES-8 water vapor winds reveal important mesoscale details of the trough-jet-hurricane interaction that would otherwise be hidden.
Cione, J.J., E.W. Uhlhorn, and P.G. Black. Atmospheric boundary layer and upper ocean structure observed in Hurricane Erika (1997). Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J70-J71, 2000
Cione, J.J., P. Molina, J. Kaplan, and P.G. Black. SST time series directly under tropical cyclones: Observations and implications. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 1-2, 2000
Cione, J.J., P.G. Black, and S.H. Houston. Surface observations in the hurricane environment. Monthly Weather Review, 128(5):1550-1561, doi:10.1175/1520-0493(2000)1282.0.CO;2 2000
Composite analyses of marine surface observations from 37 hurricanes between 1975 and 1998 show that the difference between the sea surface temperature and the surface air temperature significantly increases just outside the hurricane inner core. This increase in the sea-air contrast is primarily due to a reduction in surface air temperature and is more likely to occur when sea temperatures are at least 27°C. Results show that 90% of the observed cooling occurs 3.25°-1.25° latitude from the hurricane center, well outside the region of strongest surface winds. Since surface pressure only decreases 3 mb over this interval, the 2°C drop in air temperature is not a result of adiabatic expansion. For the subset of observations that contained moisture measurements, surface specific humidity decreased 1.2 g kg-1 4.5°-1.75° latitude from the storm center. This finding suggests that the observed reduction in surface air temperature is not simply a result of near-surface evaporation from sea spray or precipitation. An alternate explanation may be that outside the hurricane inner core, unsaturated convective downdrafts act to dry and evaporatively cool the near-surface environment. Between 3.25° and 1.25° radius, composite analyses show that low-level inflow is not isothermal, surface moisture is not constant, and the near-surface environment is not in thermodynamic equilibrium with the sea. Calculations based on these observations show that thetae decreases between 4.0° and 1.25° radius and then quickly rises near the inner core as surface pressures fall and specific humidity increases. Surface fluxes of heat and moisture are also observed to significantly increase near the inner core. The largest increase in surface sensible heat flux occurs radially inward of 1.5°, where surface winds are strong and sea-air temperature contrasts are greatest. As a result, the average Bowen ratio is 0.20°-0.5° radius from the composite storm center. This increase in sensible heat flux (in conjunction with near-saturated conditions at low to midlevels) may help explain why average surface air temperatures inside 1.25° radius remain relatively constant, despite the potential for additional cooling from evaporation and adiabatic expansion within the high wind inner core.
Cook, T.M., L.K. Shay, P.G. Black, G.J. Goni, M.M. Huber, S.D. Jacob, and J.J. Cione. Coupled air-sea interactions during Hurricane Bonnie. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J68-J69, 2000
D'Asaro, E.A., and P.G. Black. Turbulence in the ocean boundary layer below Hurricane Dennis. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J62-J63, 2000
Dodge, P.P., S.M. Spratt, F.D. Marks, D.W. Sharp, and J.F. Gamache. Dual-Doppler analyses of mesovortices in a hurricane rainband. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 302-303, 2000
The U.S. Weather Research Program identified landfalling tropical cyclones as a major focus for research in the coming years. In 1998, the Hurricane Research Division (HRD) of NOAA's Atlantic Oceanographic and Meteorological Laboratory coordinated experiments with other agencies and university groups in Hurricanes Bonnie, Earl, and Georges. On these flights, airborne Doppler radar data were collected to combine with WSR-88D radar data in three-dimensional analyses to document evolution of tropical cyclones as they make landfall, and to provide data for testing WSR-88D tropical cyclone algorithms. Hurricane Bonnie made landfall in near Wilmington, North Carolina as a Category 2 hurricane on 26 August. There were two HRD missions near the time of landfall. The first flight concentrated on examining the structure of the spiral rainbands and the second flight surveyed the hurricane as it interacted with the coast. During the flights, there was a vigourous rainband ~180 km northeast of the center with several mesocyclones (as identified on the Morehead City WSR-88D) that later produced confirmed tornadoes on land. Both NOAA aircraft had to deviate around strong cells in this band, between 1540 and 1830 UTC, and those deviations resulted in small Doppler analysis boxes enclosing some of the mesocyclones. A companion paper (Spratt et al.) uses dropsondes and adjacent radiosondes to describe the local environment in which the Bonnie mesocylones were embedded, and in this paper we will present windfield analyses, from combining WSR-88D and airborne Doppler radar data, that provide the three dimensional structure of the mesocyclones. The Doppler data are too coarse to resolve actual tornadoes, but the parent mesoscale circulations are clearly resolved.
Dunion, J.P., S.H. Houston, M.D. Powell, C.S. Velden, and P.G. Black. Using surface adjusted GOES low-level cloud-drift winds to improve the estimation of tropical cyclone outer wind radii. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 488-489, 2000
Feuer, S.E., M.L. Black, and J.L. Franklin. The asymmetric wind structure of tropical cyclones in various shear environments. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 450-451, 2000
Franklin, J.L., M.L. Black, and K. Valde. Eyewall wind profiles in hurricanes determined by GPS dropwindsondes. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 446-447, 2000
Gamache, J.F., M.L. Black, and H.E. Willoughby. Radial variation of azimuthally-averaged flow across the hurricane core as observed with airborne Doppler radar. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 416-417, 2000
Goldenberg, S.B. Intraseasonal predictability of Atlantic basin hurricane activity. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 59-60, 2000
Goldenberg, S.B., C.W. Landsea, and G.D. Bell. Summary of the 1999 Atlantic hurricane season: A climatic perspective. Proceedings, 24th Annual Climate Diagnostics and Prediction Workshop, Tucson, AZ, November 1-5, 1999. National Weather Service, 1-4, 2000
Goni, G.J., L.K. Shay, P.G. Black, S.D. Jacob, T.M. Cook, J.J. Cione, and E.W. Uhlhorn. Role of the upper ocean structure on the intensification of Hurricane Bret from satellite altimetry. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J5-J6, 2000
Jacob, S.D., L.K. Shay, P.G. Black, and S.H. Houston. Upper ocean response to hurricane wind asymmetries. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J66-J67, 2000
Jacobs, S.D., L.K. Shay, A.J. Mariano, and P.G. Black. The 3D oceanic mixed layer response to Hurricane Gilbert. Journal of Physical Oceanography, 30(6):1407-1429, doi:10.1175/1520-0485(2000)030<1407:TOMLRT>2.0.CO; 2000
Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core eddy in the region with velocities of O(1) m s-1. Based upon conservation of heat and mass, the three-dimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m-2 and a heat flux of 1200 W m-2 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m-2 corresponding to a less than 7 m s-1 wind speed. Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, the shear-induced entrainment scheme predicts a narrow region of cooling compared to the stress-induced mixing scheme and observed SST decreases. After storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity, and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.
Jones, R.W., and H.E. Willoughby. Linear motion of a two-layer baroclinic hurricane in shear. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 83-84, 2000
Kaplan, J., and M. DeMaria. Large-scale characteristics of rapidly intensifying tropical cyclones in the North Atlantic basin. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 232-233, 2000
Katsaros, K.B., P. Vachon, P.G. Black, P.P. Dodge, and E.W. Uhlhorn. Wind fields from SAR: Could they improve our understanding of storm dynamics? John Hopkins APL Technical Digest, 21(1):86-93, 2000
Four hurricane images obtained by RADARSAT are examined for what they reveal about the storms. Features seen include strong variations in backscatter from the surface in and around convective cells associated with rain cells and rainbands, coupled with increased backscatter in regions of high wind outflow. Long linear features of scale 3-6 km are observed in three of the four hurricanes, probably from secondary circulations in the atmospheric boundary layer (roll vortices). They occur between convective rainbands, where the descending motion could produce a well-defined boundary layer. Although the origins of and the mechanisms producing the features are still not clear, the high resolution, wide-swath coverage modes of synthetic aperture radar provide new observations and present important questions for further research.
Knaff, J.A., and Landsea, C.W. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 9(3):48-50, 2000
Landsea C.W., and J.A. Knaff. How much skill was there in forecasting the very strong 1997-1998 El Niño? Bulletin of the American Meteorological Society, 81(9):2107-2120, doi10.1175/1520-0477(2000)081<2107:HMSWTI>2.3.CO;2 2000
The very strong 1997-1998 El Niño was the first major event in which numerous forecasting groups participated in its real-time prediction. A previously developed simple statistical tool, the El Niño-Southern Oscillation Climatology and Persistence (ENSO-CLIPER) model, is utilized as a baseline for determination of skill in forecasting this event. Twelve statistical and dynamical models were available in real time for evaluation. Some of the models were able to outperform ENSO-CLIPER in predicting either the onset or the decay of the 1997-1998 El Niño, but none were successful at both for a medium-range two season (6-8 months) lead time. There were no models, including ENSO-CLIPER, able to anticipate even one-half of the actual amplitude of the El Niño's peak at medium-range (6-11 months) lead. In addition, none of the models showed skill (i.e., lower root-mean-square error than ENSO-CLIPER) at the zero season (0-2 months) through the two season (6-8 months) lead times. No dynamical model and only two of the statistical models (the canonical correlation analysis [CCA] and the constructed analog [ANALOG]) outperformed ENSO-CLIPER by more than 5% of the root-mean-square error at the three season (9-11 months) and four season (12-14 months) lead time. El Niño impacts were correctly anticipated by national meteorological centers one-half year in advance, because of the tendency for El Niño events to persist into and peak during the boreal winter. Despite this, the zero to two season (0-8 month) forecasts of the El Niño event itself were no better than ENSO-CLIPER and were in that sense, not skillful, a conclusion that remains unclear to the general meteorological and oceanographic communities.
Landsea, C.W. Climate variability of tropical cyclones: Past, present, and future. In Storms (Volume 1), R.A. Pielke, Sr. and R.A. Pielke, Jr. (eds.). Routledge, New York (ISBN 041517239X), 220-241, 2000
Landsea, C.W. El Niño-Southern Oscillation and the seasonal predictability of tropical cyclones. In El Niño and the Southern Oscillation: Multiscale Variability and Global and Regional Impacts, H.F. Diaz and V. Markgraf (eds.). Cambridge University Press (ISBN 0521621380), 149-181, 2000
Perhaps the most dramatic effect that El Niño has upon the climate system is in changing tropical cyclone characteristics around the world. This chapter reviews how tropical cyclone frequency, intensity, and areas of occurrence are altered in all of the cyclone basins by the phases of El Niño-Southern Oscillation (ENSO). In addition to ENSO, other global (such as the stratospheric Quasi-Biennial Oscillation) and local factors (such as sea surface temperature, monsoon intensity and rainfall, sea level pressures, and tropospheric vertical shear) can also help modulate tropical cyclone ariability. Understanding how these various factors relate to tropical cyclone activity can be challenging due to the fairly short (on the scale of only tens of years) record of reliable data. Despite this limitation, many of the factors that have been linked to tropical cyclones, the foremost of which being ENSO, have substantial lead relationships and can be utilized to provide seasonal forecasts of tropical cyclones. Details of methodologies that have been developed for the North Atlantic, northwest Pacific, south Pacific and Australian basin tropical cyclones are presented, as well as the real-time forecasting performance of Atlantic hurricanes as issued by Professor William Gray.
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 9(1):32-34, 2000
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 9(2):31-33, 2000
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 9(4):48-50, 2000
Landsea, C.W., and J.A. Knaff. How much "skill" is there in forecasting El Niño? Weatherzine, 23:2-4, 2000
Landsea, C.W., and J.A. Knaff. How much "skill" was there in forecasting the strong 1997-1998 El Niño and 1998-2000 La Niña events? Proceedings, 25th Annual Climate Diagnostics and Prediction Workshop, Palisades, NY, October 23-27, 2000. NOAA/Climate Prediction Center and the International Research Institute for Climate Prediction, 4-7, 2000
Landsea, C.W., C. Anderson, N. Charles, G. Clark, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer. The Atlantic hurricane database re-analysis project: Results for 1851-1885. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 542-543, 2000
Lawrence, J.R., S.D. Gedzelman, and J.F. Gamache. Tropical cyclogenesis and stable isotope ratios of water. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 260-261, 2000
Lee, W.-C., and F.D. Marks. An objective method to determine tropical cyclone center near landfall from WSR-88D data: The GBVTD-simplex algorithm. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 310-311, 2000
Lee, W.-C., and F.D. Marks. Tropical cyclone kinematic structure retrieved from single Doppler radar observations, Part II: The GBVTD-simplex center finding algorithm. Monthly Weather Review, 128(6):1925-1936, doi:10.1175/1520-0493(2000)128<1925:TCKSRF>2.0.CO; 2000
This paper is the second of a series and focuses on developing an algorithm to objectively identify tropical cyclone (TC) vorticity centers using single-Doppler radar data. The first paper dealt with the formulation of a single-Doppler radar TC wind retrieval technique, the ground-based velocity-track-display (GBVTD), and the results are verified using analytical TCs. It has been acknowledged that the quality of the GBVTD-retrieved TC circulation strongly depends on accurately knowing its center position. However, existing single-Doppler radar center finding algorithms are limited to estimate centers for axisymmetric TCs. The proposed algorithm uses a simplex method to objectively estimate the TC vorticity center by maximizing GBVTD-retrieved mean tangential wind. When tested with a number of axisymmetric and asymmetric analytical TCs, the accuracy of the TC centers estimated by the GBVTD-simplex algorithm is approximately equal to 340 m from the true center. When adding 5 m s-1 random noise to the Doppler velocities, the accuracy of the TC centers is nearly unchanged at 350 m. When applying the GBVTD-simplex algorithm to Typhoon Alex (1987), the estimated uncertainty varies between 0.1 and 2 km. When the overall velocity gradient is weak, the uncertainties in the retrieved TC centers are usually large. The GBVTD-simplex algorithm sometimes has problems finding a solution when a large sector of Doppler radar data is missing in conjunction with weak velocity gradients. The GBVTD-simplex algorithm significantly reduces the uncertainties in estimating TC center position compared with existing methods and improves the quality of the GBVTD-retrieved TC circulation. The GBVTD-simplex algorithm is computationally efficient and can be easily adapted for real-time applications.
Lee, W.-C., B. J.-D. Jou, P.-L. Chang, and F.D. Marks. Tropical cyclone kinematic structure retrieved from single-Doppler radar observations. Part III: Evolution and structures of Typhoon Alex (1987). Monthly Weather Review, 128(12):3982-4001, 2000
This paper is the third of a series that focuses on the applications of the ground-based velocity track display (GBVTD) technique and the GBVTD-simplex center finding algorithm developed in the previous two papers to a real tropical cyclone (TC). The evolution and structure of Typhoon Alex (1987), including full tangential winds, mean radial winds, one component of the mean flow, and their derived axisymmetric angular momentum and perturbation pressure fields are reconstructed from 16 volume scans (6.5 h of data with a 2-h gap) from the Civil Aeronautic Administration (CAA) Doppler radar while Typhoon Alex moved across the mountainous area in northern Taiwan. This analysis retrieves a plausible and physically consistent three-dimensional primary circulation of a landfalling TC using a single ground-based Doppler radar. Highly asymmetric wind structures were resolved by the GBVTD technique where the maximum relative tangential wind at z = 2 km evolved from 52 m s-1 (before landfall), to less than 40 m s-1 (after landfall), to less than 35 m s-1 (entering the East China Sea). Alex's eye began to fill withprecipitation while its intensity decreased rapidly after landfall, a characteristic of circulations disrupted by terrain. The mean radial wind field revealed a layer of low-level inflow in agreement with past TC observations. The outward slope of the eyewall reflectivity maximum was consistent with the constant angular momentum contours within the eyewall. After Alex entered the East China Sea, its circulation became more axisymmetric. The axisymmetric perturbation pressure field was retrieved using the gradient wind approximation which, when used in conjunction with one or more surface pressure measurements within the analysis domain, can estimate the central pressure. The retrieved perturbation pressure fields at two time periods were compared with surface pressures reported in northern Taiwan. Considering the assumptions involved and the influence of terrain, good agreement (only 1V2-mb deviation) was found between them. This agreement indicates the relative quality of the GBVTD-retrieved axisymmetric circulation and suggests GBVTD-retrieved quantities can be useful in operational and research applications.
Lonfat, M., F.D. Marks, and S. Chen. A study of the rain distribution in tropical cyclones using TRMM/TMI. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 480-481, 2000
Marks, F.D., L. Selevan, and J.F. Gamache. WSR-88D derived rainfall distributions in Hurricane Danny (1997). Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 298-299, 2000
McAdie, C.J., and P.P. Dodge. Maximum sustained winds in Hurricane Irene as measured by the Miami WSR-88D. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 212-213, 2000
Morisseau-Leroy, N., M.K. Solomon, and J. Basu. Oracle 8i: Java Component Programming. Osborne McGraw-Hill (ISBN 0072127376), 697 pp., 2000
Murillo, S.T., W.-C. Lee, and F.D. Marks. Evaluating the GBVTD-tropical center finding simplex algorithm. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 312-313, 2000
The GBVTD simplex algorithm has been tested using axisymmetric and asymmetric analytic tropical cyclones by Lee et al. (1999). The algorithm objectively identifies the tropical cyclone center by maximizing the GBVTD-derived mean tangential wind field. Lee and Marks (1999) applied the GBVTD simplex algorithm to Typhoon Alex (1987). However, a true center was not available to verify the accuracy of the algorithm. This study applies the GBVTD simplex algorithm to Hurricane Danny (1997). The estimated storm track derived by the algorithm is compared to radar and aircraft storm fixes. The derived track is in good agreement with the true storm track within 2 km. Results will be presented that show how the GBVTD simplex algorithm improves the quality of the GBVTD retrieved wind analysis.
Murnane, R.J., C. Barton, E. Collins, J. Donnelly, J. Elsner, K. Emanuel, I. Ginis. S. Howard, C.W. Landsea, K. Liu, D. Malmquist, M. McKay, A. Michaels, N. Nelson, J. O'Brien, D. Scott, and T. Webb. Model estimates of hurricane wind speed probabilities. EOS, Transactions, American Geophysical Union, 81(38):433, 438, https://doi.org/10.1029/00EO00319 2000
Nolan, D.S., M.T. Montgomery, and P.D. Reasor. Studies of the wavenumber one instability in hurricane-like vortices. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 29-30, 2000
Ooyama, K.V. A dynamic and thermodynamic foundation for modeling the moist atmosphere with classical thermodynamics and parameterized microphysics. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 561-562, 2000
Otero, S., N. Morisseau-Leroy, N. Carrasco, and M.D. Powell. A distributed real-time hurricane wind analysis system. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 197-198, 2000
Parrish, J.R., M.L. Black, S.H. Houston, P.P. Dodge, and J.J. Cione. The structure of Hurricane Irene over South Florida. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 456-457, 2000
Powell, M.D. Tropical cyclones during and after landfall. In Storms (Volume 1), R. Pielke, Sr. and R. Pielke, Jr. (eds.). Routledge, New York (ISBN 041517239X), 196-219, 2000
Protat, A., Y. Lemaitre, D. Bouniol, and R.A. Black. Microphysical observations during FASTEX from airborne Doppler radar and in-situ measurements. Physics and Chemistry of the Earth, Part B: Hydrology, Oceans, and Atmosphere, 25(10-12):1097-1102, https://doi.org/10.1016/S1464-1909(00)00159-3 2000
A major objective of FASTEX is to document the three-dimensional dynamic and microphysical structure of the North-Atlantic frontal cyclones in their mature stage at different scales of motion. In this paper, we combine the airborne Doppler radar and microphysical 2D-C and 2D-P probes data to recover the 3D microphysical and radiative properties of the IOP16 and IOP12 frontal cyclones (terminal fall velocity, cloud and precipitation water contents, precipitation fall rate, effective radius). The first step is to derive statistical relationships between the microphysical quantities and reflectivity from the 2D-P and 2D-C probes. Then, the Doppler-derived 3D reflectivity field is combined with these statistical relationships to access the 3D microphysical fields.
Reasor, P.D., and M.T. Montgomery. 3D alignment and co-rotation of weak, TC-like vortices via linear vortex Rossby waves. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 268-269, 2000
Reasor, P.D., M.T. Montgomery, F.D. Marks, and J.F. Gamache. Low-wavenumber structure and evolution of the hurricane inner core observed by airborne dual-Doppler radar. Monthly Weather Review, 128(6):1653-1680, doi:10.1175/1520-0493(2000)1282.0.CO;2 2000
The asymmetric dynamics of the hurricane inner-core region is examined through a novel analysis of high temporal resolution, three-dimensional wind fields derived from airborne dual-Doppler radar. Seven consecutive composites of Hurricane Olivia's (1994) wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes. The symmetric and asymmetric mechanisms involved in this transformation are considered separately. To zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of Eliassen and Lystad for a neutrally-stratified atmosphere. Vertical shear, however, increased dramatically during the observation period, leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector. Recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry. The role of asymmetric vorticity dynamics in explaining some of the physics of hurricane intensity change motivates a special focus on Olivia's vorticity structure. It is found that an azimuthal wavenumber 2 feature dominates the asymmetry in relative vorticity below 3-km height. The characteristics of this asymmetry deduced from reflectivity and wind composites during a portion of the observation period show some consistency with a wavenumber 2 discrete vortex Rossby edge wave. Barotropic instability is suggested as a source for the wavenumber 2 asymmetry through a series of barotropic numerical simulations. Trailing bands of vorticity with radial wavelengths of 5-10 km are observed in the inner core approximately 20 km from the storm center, and may be symmetrizing vortex Rossby waves. Elevated reflectivity bands with radial scales comparable to those of the vorticity bands, also near 20-25-km radius, may be associated with these vorticity features.
Rogers, R.F. Surface-based modification of convectively-generated mesovortices and its implications for tropical cyclogenesis. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 151-152, 2000
Rogers, R.F., J.M. Fritsch, and W.C. Lambert. A simple technique for using radar data in the dynamic initialization of a mesoscale model. Monthly Weather Review, 128(7):2560-2574, doi:10.1175/1520-0493(2000)128<2560:ASTFUR>2.0.CO; 2000
A simple technique for using radar reflectivity to improve model initialization is presented. Unlike previous techniques, the scheme described here does not infer rain rates and heating profiles from assumed relationships between remotely-sensed variables and precipitation rates. Rather, the radar data are only used to tell the model when and where deep moist convection is occurring. This information is then used to activate the model's convective parameterization scheme in the grid elements where convection is observed. This approach has the advantage that the convective precipitation rates and heating profiles generated by the convective parameterization are compatible with the local (grid element) environment. The premise is that if convection is forced to develop when and where it is observed during a data assimilation period, convectively-forced modifications to the environment will be in the correct locations at the model initial forecast time and the resulting forecast will be more accurate. Three experiments illustrating how the technique is applied in the simulation of deep convection in a warm-season environment are presented: a control run in which no radar data are assimilated, and two additional runs where radar data are assimilated for 12 h in one run and 24 h in the other. The results indicate that assimilating radar data can improve a model's description of the mesoscale environment during the pre-forecast time period, thereby resulting in an improved forecast of precipitation and the mesoscale environment.
Rogers, R.F., S.S. Chen, J.E. Tenerelli, and M. Lonfat. A numerical study of the distribution of precipitation in Hurricane Bonnie (1998). Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 408-409, 2000
Sandrik, A., C.W. Landsea, and B. Jarvinen. The North Florida hurricane of 29 September 1896: A historical case of extreme inland high winds. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 547-548, 2000
Schubert, W.H., S.A. Hausman, M. Garcia, K.V. Ooyama, and H.-C. Kuo. Potential vorticity in a moist atmosphere. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 563-564, 2000
Shay, L., G.J. Goni, and P.G. Black. Effects of a warm oceanic feature on Hurricane Opal. Monthly Weather Review, 128(5):1366-1383, doi:10.1175/1520-0493(2000)128<1366:EOAWOF>2.0.CO; 2000
On 4 October 1995, Hurricane Opal deepened from 965 to 916 hPa in the Gulf of Mexico over a 14-h period upon encountering a warm core ring (WCR) in the ocean shed by the Loop Current during an upper-level atmospheric trough interaction. Based on historical hydrographic measurements placed within the context of a two-layer model and surface height anomalies (SHA) from the radar altimeter on the TOPEX mission, upper-layer thickness fields indicated the presence of two warm core rings during September and October 1995. As Hurricane Opal passed directly over one of these WCRs, the 1-min surface winds increased from 35 to more than 60 m s-1, and the radius of maximum wind decreased from 40 to 25 km. Pre-Opal SHAs in the WCR exceeded 30 cm where the estimated depth of the 20°C isotherm was located between 175 and 200 m. Subsequent to Opal's passage, this depth decreased approximately 50 m, which suggests upwelling underneath the storm track due to Ekman divergence. The maximum heat loss of approximately 24 Kcal cm-2 relative to depth of the 26°C isotherm was a factor of 6 times the threshold value required to sustain a hurricane. Since most of this loss occurred over a period of 14 h, the heat content loss of 24 Kcal cm-2 equates to approximately 20 kW m-2. Previous observational findings suggest that about 10%-15% of upper-ocean cooling is due to surface heat fluxes. Estimated surface heat fluxes based upon heat content changes range from 2000 to 3000 W m-2 in accord with numerically simulated surface heat fluxes during Opal's encounter with the WCR. Composited AVHRR-derived SSTs indicated a 2°-3°C cooling associated with vertical mixing in the along-track direction of Opal except over the WCR where AVHRR-derived and buoy-derived SSTs decreased only by about 0.5°-1°C. Thus, the WCR's effect was to provide a regime of positive feedback to the hurricane rather than negative feedback induced by cooler waters due to upwelling and vertical mixing as observed over the Bay of Campeche and north of the WCR.
Shay, L.K., G.J. Goni, P.G. Black, S.D. Jacob, J.J. Cione, and E.W. Uhlhorn. Global analogues of deep warm upper ocean layers: Hurricane heat potential estimates. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J3-J4, 2000
Spratt, S.M., F.D. Marks, P.P. Dodge, and D.W. Sharp. Examining the pre-landfall environment of mesovortices within a Hurricane Bonnie (1998) outer rainband. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 300-301, 2000
Tenerelli, J.E., S.S. Chen, M. Lonfat, R. Foster, and R.F. Rogers. Surface winds in Hurricane Floyd: A comparison between numerical simulations, aircraft data, and QuikScat satellite data. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 418-419, 2000
Uhlhorn, E.W., K.B. Katsaros, and M.D. Powell. Assimilation of scatterometer-derived winds into real-time tropical cyclone surface wind analyses. Preprints, 10th Conference on Satellite Meteorology and Oceanography, Long Beach, CA, January 9-14, 2000. American Meteorological Society, Boston, 214-215, 2000
Uhlhorn, E.W., P.G. Black, L.K. Shay, J.J. Cione, S.D. Jacob, and G.J. Goni. Warm core ocean features in the central and eastern Gulf of Mexico. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 147-148, 2000
Walsh, E.J., C.W. Wright, D.C. Vandemark, W.B. Krabill, A.W. Garcia, S.H. Houston, M.D. Powell, P.G. Black, and F.D. Marks. Hurricane directional wave spectrum spatial variation at landfall. Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 327-328, 2000
Willoughby, H.E. and R.W. Jones. Are the beta gyres really normal modes? Preprints, 24th Conference on Hurricanes and Tropical Meteorology, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, 187-188, 2000
Wright, C.W., E.J. Walsh, D.C. Vandemark, W.B. Krabill, A.W. Garcia, S.H. Houston, M.D. Powell, P.G. Black, and F.D. Marks. Hurricane directional wave spectrum spatial variation in the open ocean. Proceedings, 10th Conference on Interaction of the Sea and Atmosphere, Ft. Lauderdale, FL, May 29-June 2, 2000. American Meteorological Society, Boston, J1-J2, 2000
1999
Aberson, S.D. Ensemble-based products to improve tropical cyclone forecasting. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 843-844, 1999
Aberson, S.D. Targeting and sampling strategies to improve hurricane forecasts. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 105-107, 1999
Aberson, S.D., and J.L. Franklin. Impact on hurricane track and intensity forecasts of GPS dropwindsonde observations from the first-season flights of the NOAA Gulfstream-IV jet aircraft. Bulletin of the American Meteorological Society, 80(3):421-428, doi:10.1175/1520-0477(1999)080<0421:IOHTAI>2.0.CO; 1999
In 1997, the Tropical Prediction Center (TPC) began operational Gulfstream-IV jet aircraft missions to improve the numerical guidance for hurricanes threatening the continental United States, Puerto Rico, and the Virgin Islands. During these missions, the new generation of Global Positioning System dropwindsondes were released from the aircraft at 150-200-km intervals along the flight track in the environment of the tropical cyclone to obtain profiles of wind, temperature, and humidity from flight level to the surface. The observations were ingested into the global model at the National Centers for Environmental Prediction, which subsequently served as initial and boundary conditions to other numerical tropical cyclone models. Because of a lack of tropical cyclone activity in the Atlantic basin, only five such missions were conducted during the inaugural 1997 hurricane season. Due to logistical constraints, sampling in all quadrants of the storm environment was accomplished in only one of the five cases during 1997. Nonetheless, the dropwindsonde observations improved mean track forecasts from the Geophysical Fluid Dynamics Laboratory hurricane model by as much as 32%, and the intensity forecasts by as much as 20% during the hurricane watch period (within 48 h of projected landfall). Forecasts from another dynamical tropical cyclone model (VICBAR) also showed modest improvements with the dropwindsonde observations. These improvements, if confirmed by a larger sample, represent a large step toward the forecast accuracy goals of TPC. The forecast track improvements are as large as those accumulated over the past 20-25 years, and those for forecast intensity provide further evidence that better synoptic-scale data can lead to more skillful dynamical tropical cyclone intensity forecasts.
Albrecht, B., T. Faber, A. Savtchenko, D. Churchill, F.D. Marks, and P.G. Black. Surface-based remote sensing of a landfalling tropical storm. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 489-492, 1999
Amat, L.R., M.D. Powell, and S.H. Houston. A real-time, Internet-based application for the archival, quality control, and analysis of hurricane surface wind observations. Preprints, 15 International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 573-576, 1999
Atlas, D., C.W. Ulbrich, F.D. Marks, E. Amitai, and C.R. Williams. Systematic variation of drop size and radar-rainfall relations. Journal of Geophysical Research, 104(D6):6155-6169, https://doi.org/10.1029/1998JD200098 1999
Time histories of the characteristics of the drop size distribution of surface disdrometer measurements collected at Kapingamarangi Atoll were partitioned for several storms using rain rate, R, reflectivity factor Z, and median diameter of the distribution of water content D0. This partitioning produced physically based systematic variations of the drop size distribution (DSD) and Z-R relations in accord with the precipitation types viewed simultaneously by a collocated radar wind profiler. These variations encompass the complete range of scatter around the mean Z-R relations previously reported by Tokay and Short (1996) for convective and stratiform rain and demonstrate that the scatter is not random. The systematic time or space variations are also consistent with the structure of mesoscale convective complexes with a sequence of convective, transition, and stratiform rain described by various authors. There is a distinct inverse relation between the coefficient A and the exponent of the Z-R relations which has been obscured in prior work because of the lack of proper discrimination of the rain types. Contrary to previous practice, it is evident that there is also a distinct difference in the DSD and the Z-R relations between the initial convective and the trailing transition zones. The previously reported Z-R relation for convective rain is primarily representative of the transition rain that was included in the convective class. The failure of present algorithms to distinguish between the initial convective and the trailing transition rains causes an erroneous apportionment of the diabatic heating and cooling and defeats the primary intent of discriminating stratiform from convective rains.
Bishop, C.H., S. Majumdar, I. Szunyogh, Z. Toth, and S.D. Aberson. Using ensembles to simulate the impact of targeted observations. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 117-118, 1999
Black, M.L. Recent observations of the hurricane eyewall: Unusual and complex structure. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 313-316, 1999
Black, M.L., and J.L. Franklin. Recent observations of the convective structure associated with low-level wind maxima in the hurricane eyewall. Preprints, 29th International Conference on Radar Meteorology, Montreal, Quebec, Canada, July 12-16, 1999. American Meteorological Society, Boston, 370-373, 1999
Black, R.A., and J. Hallett. Electrification of the hurricane. Journal of the Atmospheric Sciences, 56(12):2004-2028, doi:10.1175/1520-0469(1999)056<2004:EOTH>2.0.CO;2 1999
A survey of reports of electrical activity in hurricanes and typhoons from flight notes and personal experience (18 years, >230 eyewall penetrations for R. A. Black; ~20 years for J. Hallett, plus that of others at the Hurricane Research Division), and perusal of flight notes dating from 1980, show that lightning in and within 100 km or so of the eyewall is usually sparse. However, occasionally, significant electrical activity (>one flash per minute) occurs in or near the eyewall. National Oceanic and Atmospheric Administration WP-3D aircraft penetrations through a number of storms relate the lightning occurrence to strong vertical velocity (>10 m s-1) and the presence of supercooled liquid cloud droplets extending to temperatures below 20°C. Specific measurements of cloud properties during eyewall penetrations show that the supercooled cloud water content increases with upward velocities > ~5.0 m s-1, as does the presence of large (>2 mm) supercooled drops. Measurements at temperatures >-13°C show that the transition of supercooled cloud water to ice along an outward radial in all systems is associated with local electric fields (occasionally >20 kV m-1) and negative charge above positive charge. In systems with stronger vertical velocity there is a larger region of supercooled cloud extending to lower temperatures where charge separation may occur, as judged by the presence of regions containing graupel, small ice, and cloud droplets. The ratio of ice to supercooled water increases radially outward from the eyewall and depends upon altitude (temperature). The spatial distribution of charge is further influenced by the relation of vertical velocity to the radial flow, with the upper charge regions tending to be advected outward. In symmetrical, mature hurricanes, supercooled water usually occurs only in regions at temperatures above about -5°C. The upward transport of supercooled cloud water is limited by a balance between water condensed in the eyewall updraft and its erosion by ice in downdrafts descending in the outward regions of the eyewall. This ice originates from both primary and secondary ice nucleation in the updraft. This is consistent with an exponential increase in ice concentration, as the rate at which the ice particle concentrations increase depends on the production of secondary particles by preexisting graupel, some of which ultimately grow into new graupel, and its outward transport in the anvil flow aloft. Penetrations at temperatures as low as -15°C show the presence of electric fields consistent with specific laboratory-derived criteria for charge separated during ice-graupel collisions, given that a liquid water-dependent sign reversal temperature may occur. Such a reversal may result from either a changing temperature in the vertical, a changing cloud liquid water content in the horizontal, or a combination of the two. Since cloud-to-ground (CG) lightning can be observed with remote detection networks that provide the polarity and frequency of CG lightning, there is potential that hurricane evolution may be detected remotely and that lightning may be usable as an indicator of a change in the storm intensity and/or track.
Bosart, L.F., W.E. Bracken, J. Molinari, C.S. Velden, and P.G. Black. Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 983-984, 1999
Bove, M.C., J.B. Elsner, C.W. Landsea, X. Niu, and J.J. O'Brien. Effect of El Niño on U.S. landfalling hurricanes, revisited. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 278-281, 1999
Cione, J.J., P.G. Black, and S.H. Houston. Cooling and drying within the hurricane near-surface environment? Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 1027-1030, 1999
DeMaria, M., and J. Kaplan. An updated Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic and eastern North Pacific basins. Weather and Forecasting, 14(3):326-337, doi:10.1175/1520-0434(1999)014<0326:AUSHIP>2.0.CO; 1999
Updates to the Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic basin are described. SHIPS combines climatological, persistence, and synoptic predictors to forecast intensity changes using a multiple regression technique. The original version of the model was developed for the Atlantic basin and was run in near-real time at the Hurricane Research Division beginning in 1993. In 1996, the model was incorporated into the National Hurricane Center operational forecast cycle, and a version was developed for the eastern North Pacific basin. Analysis of the forecast errors for the period 1993-1996 shows that SHIPS had little skill relative to forecasts based upon climatology and persistence. However, SHIPS had significant skill in both the Atlantic and east Pacific basins during the 1997 hurricane season. The regression coefficients for SHIPS were rederived after each hurricane season since 1993 so that the previous season's forecast cases were included in the sample. Modifications to the model itself were also made after each season. Prior to the 1997 season, the synoptic predictors were determined only from an analysis at the beginning of the forecast period. Thus, SHIPS could be considered a "statistical-synoptic" model. For the 1997 season, methods were developed to remove the tropical cyclone circulation from the global model analyses and to include synoptic predictors from forecast fields, so the current version of SHIPS is a "statistical-dynamical" model. It was only after the modifications for 1997 that the model showed significant intensity forecast skill.
Dodge, P.P., J.F. Gamache, S.H. Houston, and F.D. Marks. Windfields in landfalling hurricanes from multiple Doppler radar data: The 1998 hurricane season. Preprints, 29th International Conference on Radar Meteorology, Montreal, Quebec, Canada, July 12-16, 1999. American Meteorological Society, Boston, 273-275, 1999
Dodge, P.P., R.W. Burpee, and F.D. Marks. The kinematic structure of a hurricane with sea-level pressure less than 900 mb. Monthly Weather Review, 127(6):987-1004, doi:10.1175/1520-0493(1999)127<0987:TKSOAH>2.0.CO; 1999
A National Oceanic and Atmospheric Administration aircraft recorded the first Doppler radar data in a tropical cyclone with a minimum sea level pressure (MSLP) 50 m s-1 extended to 12 km, higher than has been reported in previous hurricanes. The inner eyewall contained weak inflow throughout most of its depth. In contrast, the portion of the outer eyewall described here had shallow inflow and a broad region of outflow. The stratiform region between the two eyewalls had lower reflectivities and was the only region where the vertically incident Doppler radar data seemed to show downward motion below the freezing level. Gilbert's structure is compared with other intense Atlantic and eastern North Pacific hurricanes with MSLP >900 mb. Storms with lower MSLP have higher wind speeds in both inner and outer eyewalls, and wind speeds >50 m s-1 extend higher in storms with lower MSLP. Hurricanes Gilbert and Gloria (1985), the strongest Atlantic hurricanes yet analyzed by the Hurricane Research Division, had different outer eyewall structures. Gloria's outer eyewall had a deep region of inflow, while Gilbert's inflow layer was shallow. This may explain differences in the subsequent evolution of the two storms.
Dodge, P.P., S.H. Houston, W.-C. Lee, J.F. Gamache, and F.D. Marks. Windfields in Hurricane Danny (1997) at landfall from combined WSR-88D and airborne Doppler radar data. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 61-62, 1999
Donnelly, W.J., J.R. Carswell, R.E. McIntosh, P.S. Chang, J.C. Wilkerson, F.D. Marks, and P.G. Black. Revised ocean backscatter models at C and Ku-bands under high wind conditions. Journal of Geophysical Research, 104(C5):11,485-11,498, https://doi.org/10.1029/1998JC900030 1999
A series of airborne scatterometer experiments designed to collect C and Ku-band ocean backscatter data in regions of high ocean surface winds has recently been completed. Over 100 hours of data were collected using the University of Massachusetts C and Ku-band scatterometers, CSCAT and KUSCAT. These instruments measure the full azimuthal normalized radar cross section (NRCS) of a common surface area of the ocean simultaneously at four incidence angles. Our results demonstrate limitations of the current empirical models, CMOD4, SASSII and NSCAT1, that relate ocean backscatter to the near surface wind at high wind speeds. The discussion focuses on winds in excess of 15 m/sec in clear atmospheric conditions. The scatterometer data is collocated with measurements from ocean data buoys and GPS dropsondes, and a Fourier analysis is performed as a function of wind regime. A three-term Fourier series is fit to the backscatter data, and a revised set of coefficients is tabulated. These revised models, CMOD4HW and KUSCAT1, are the basis for a discussion of the NRCS at high wind speeds. Our scatterometer data show a clear over prediction of the derived NRCS response to high winds based on the CMOD4, SASSII and NSCAT1 models. Furthermore, saturation of the NRCS response begins to occur above 15 m/sec. Sensitivity of the upwind and crosswind response is discussed with implications towards high wind speed retrieval. wind speed retrieval.
Ellsberry, R.L., and F.D. Marks. The Hurricane Landfall Workshop summary. Bulletin of the American Meteorological Society, 80(4):683-685, 1999
Franklin, J.F., M.L. Black, and S.E. Feuer. Wind profiles in hurricanes determined by GPS dropwindsondes. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 167-168, 1999
Gamache, J.F. Airborne Doppler observations of intensity change in eastern Pacific Hurricane Guillermo. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 325-328, 1999
Goldenberg, S.B., and C.W. Landsea. Relationships between decadal-scale fluctuations in vertical shear from NCEP/NCAR reanalysis data and Atlantic basin tropical cyclone activity. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 1089-1091, 1999
Hellin, J., M. Haig, and F.D. Marks. Rainfall characteristics of Hurricane Mitch. Nature, 399(6734):316, https://doi.org/10.1038/20577 1999
Hock, T.F., and J.L. Franklin. The NCAR GPS dropwindsonde. Bulletin of the American Meteorological Society, 80(3):407-420, doi:10.1175/1520-0477(1999)080<0407:TNGD>2.0.CO;2 1999
The National Center for Atmospheric Research (NCAR), in a joint effort with the National Oceanic and Atmospheric Administration (NOAA) and the German Aerospace Research Establishment, has developed a dropwindsonde based on the Global Positioning System (GPS) satellite navigation. The NCAR GPS dropwindsonde represents a major advance in both accuracy and resolution for atmospheric measurements over data-sparse oceanic areas of the globe, providing wind accuracies of 0.52 m s-1 with a vertical resolution of ~5 m. One important advance over previous generations of sondes is the ability to measure surface (10 m) winds. The new dropwindsonde has already been used extensively in one major international research field experiment (Fronts and Atlantic Storm Track Experiment), in operational and research hurricane flights from NOAA's National Weather Service and Hurricane Research Division, during NCAR's SNOWBAND experiment, and in recent CALJET and NORPEX El Niño experiments. The sonde has been deployed from a number of different aircraft, including NOAA's WP-3Ds and new Gulfstream IV jet, the Air Force C-130s, NCAR's Electra, and a leased Lear-36. This paper describes the characteristics of the new dropwindsonde and its associated aircraft data system, details the accuracy of its measurements, and presents examples from its initial applications.
Houston, S.H., and M.D. Powell. Hurricanes and tropical storms in Florida Bay. Florida Sea Grant College Program, FLSGP-G-99-016, 2 pp., 1999
Houston, S.H., G. Forbes, A. Chiu, W.-C. Lee, and P.P. Dodge. Super Typhoon Paka's (1997) surface winds. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 1032-1033, 1999
Houston, S.H., W.A. Shaffer, M.D. Powell, and J. Chen. Comparisons of HRD and SLOSH surface wind fields in hurricanes: Implications for storm surge modeling. Weather and Forecasting, 14(5):671-686, doi:10.1175/1520-0434(1999)0142.0.CO;2 1999
Surface wind observations analyzed by the Hurricane Research Division (HRD) were compared to those computed by the parametric wind model used in the National Weather Service Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model's storm surge computations for seven cases in five recent hurricanes. In six cases, the differences between the SLOSH and HRD surface peak wind speeds were 6% or less, but in one case (Hurricane Emily of 1993) the SLOSH computed peak wind speeds were 15% less than the HRD. In all seven cases, statistics for the modeled and analyzed wind fields showed that for the region of strongest winds, the mean SLOSH wind speed was 14% greater than that of the HRD and the mean inflow angle for SLOSH was 19° less than that of the HRD. The radii beyond the region of strongest winds in the seven cases had mean wind speed and inflow angle differences that were very small. The SLOSH computed peak storm surges usually compared closely to the observed values of storm surge in the region of the maximum wind speeds, except Hurricane Emily where SLOSH underestimated the peak surge. HRD's observation-based wind fields were input to SLOSH for storm surge hindcasts of Hurricanes Emily and Opal (1995). In Opal, the HRD input produced nearly the same computed storm surges as those computed from the SLOSH parametric wind model, and the calculated surge was insensitive to perturbations in the HRD wind field. For Emily, observation-based winds produced a computed storm surge that was closer to the peak observed surge, confirming that the computed surge in Pamlico Sound was sensitive to atmospheric forcing. Using real-time, observation-based winds in SLOSH would likely improve storm surge computations in landfalling hurricanes affected by synoptic and mesoscale factors that are not accounted for in parametric models (e.g., a strongly sheared environment, convective asymmetries, and stably stratified boundary layers). An accurate diagnosis of storm surge flooding, based on the actual track and wind fields, could be supplied to emergency management agencies, government officials, and utilities to help with damage assessment and recovery efforts.
Jones, R.W., and H.E. Willoughby. Results of generalizing a semispectral shallow-water barotropic hurricane tracking model into a two-layer baroclinic model. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 747-750, 1999
Jones, R.W., and M. DeMaria. Further studies of the optimization of a hurricane track prediction model using the adjoint equations. Monthly Weather Review, 127(7):1586-1598, doi:10.1175/1520-0493(1999)127<1586:FSOTOO>2.0.CO; 1999
The method of model fitting, or adjoint method, is applied to a barotropic hurricane track forecast model described by DeMaria and Jones using a large sample of forecast cases. The sample includes all Atlantic tropical cyclones that reached hurricane intensity during the 1989-1993 hurricane seasons (141 72-h forecasts of 17 storms). The cases considered by DeMaria and Jones are a subset of the present sample. Model-fitting calculations using strong, weak, strong followed by weak, or weak followed by strong model constraints are discussed for data assimilation periods varying from 6 to 72 h. Generally, the best track forecasts occur for shorter assimilation periods and for weak constraints, although only the 12-h assimilation with the weak constraint has less track error than the control forecast without assimilation, and only for the 12-h forecast. The principle reason for this lack of improvement is that the fit of the model to the observed track is good at the middle of the assimilation period, but not very good at the end where the forecast begins. When a future track position at 6 h is included in the assimilation, in order to improve the track fit at the synoptic data time, the resulting track errors average about 10% smaller than the control forecast. The control forecast may also be improved in the same way. In that case, the best assimilation forecasts have 2.5% smaller track errors than the modified control forecasts.
Kaplan, J., and M. DeMaria. Climatological and synoptic characteristics of rapidly intensifying tropical cyclones in the North Atlantic basin. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 592-595, 1999
Landsea, C.W., and J.A. Knaff. Application of the El Niño-Southern Oscillation CLImatology and PERsistence (CLIPER) forecasting scheme. Experimental Long-Lead Forecast Bulletin, 8(4):34-36, 1999
Landsea, C.W., C.A. Anderson, G. Clark, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer. The Atlantic hurricane database re-analysis project. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 394-397, 1999
Landsea, C.W., R.A. Pielke, A.M. Mestas-Nunez, and J.A. Knaff. Atlantic basin hurricanes: Indices of climatic changes. Climatic Change, 42(1):89-129, 1999
Accurate records of basin-wide Atlantic and U.S. landfalling hurricanes extend back to the mid 1940s and the turn of the century, respectively, as a result of aircraft reconnaissance and instrumented weather stations along the U.S. coasts. Such long-term records are not exceeded elsewhere in the tropics. The Atlantic hurricanes, U.S. landfalling hurricanes, and U.S. normalized damage time series are examined for interannual trends and multidecadal variability. It is found that only weak linear trends can be ascribed to the hurricane activity and that multidecadal variability is more characteristic of the region. Various environmental factors including Caribbean sea level pressures and 200 mb zonal winds, the stratospheric Quasi-Biennial Oscillation, the El Niño-Southern Oscillation, African West Sahel rainfall, and Atlantic sea surface temperatures, are analyzed for interannual links to the Atlantic hurricane activity. All show significant, concurrent relationships to the frequency, intensity, and duration of Atlantic hurricanes. Additionally, variations in the El Niño-Southern Oscillation are significantly linked to changes in U.S. tropical cyclone-caused damages. Finally, much of the multidecadal hurricane activity can be linked to the Atlantic Multidecadal Mode, an empirical orthogonal function pattern derived from a global sea surface temperature record. Such linkages may allow for prediction of Atlantic hurricane activity on a multidecadal basis. These results are placed into the context of climate change and natural hazards policy.
Majumdar, S.J., S.D. Aberson, C.H. Bishop, and Z. Toth. Real time hurricane track targeting using a VICBAR ensemble. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 755-756, 1999
Marks, F.D., P.P. Dodge, and C. Sandin. WSR-88D observations of hurricane atmospheric boundary layer structure at landfall. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 1051-1054, 1999
Marks, F.D., P.P. Dodge, and C. Sandin. WSR-88D observations of hurricane atmospheric boundary layer structure at landfall. Preprints, 29th International Conference on Radar Meteorology, Montreal, Quebec, Canada, July 12-16, 1999. American Meteorological Society, Boston, 374-377, 1999
Morisseau-Leroy, N., M.K. Solomon, G.P. Momplaisir, T. Kurian, and E. Griffin. Oracle 8I SQLJ Programming. Osborne McGraw-Hill (ISBN 0072121602), 557 pp., 1999
Murillo, S.T., and J.J. O'Brien. The influence of ENSO on eastern Pacific tropical cyclones. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 437-438, 1999
Murillo, S.T., P.P. Dodge, W.-C. Lee, and F.D. Marks. Using the GBVTD technique in nowcasting hurricane windfields using the WSR-88D. Preprints, 29th International Conference on Radar Meteorology, Montreal, Quebec, Canada, July 12-16, 1999. American Meteorological Society, Boston, 276-277, 1999
Murillo, S.T., W.-C. Lee, K. Hondl, P.P. Dodge, C. McAdie, and F.D. Marks. Implementation of the GBVTD technique in nowcasting hurricane wind fields using the WSR-88D. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 311-312, 1999
Ooyama, K.V. Boundary-layer parameterization in a cloud-resolving model using the radical thermodynamic formulation. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 150-152, 1999
Pielke, R.A., and C.W. Landsea. La Niña, El Niño, and Atlantic hurricane damages in the United States. Bulletin of the American Meteorological Society, 80(10):2027-2034, doi:10.1175/1520-0477(1999)080<2027:LNAENO>2.0.CO; 1999
Hurricanes result in considerable damage in the United States. Previous work has shown that Atlantic hurricane landfalls in the United States have a strong relationship with the El Niño-Southern Oscillation phenomena. This paper compares the historical record of La Niña and El Niño events defined by eastern Pacific sea surface temperature with a data set of hurricane losses normalized to 1997 values. A significant relationship is found between the ENSO cycle and U.S. hurricane losses, with La Niña years exhibiting much more damage. Used appropriately, this relationship is of potential value to decision makers who are able to manage risk based on probabilistic information.
Pielke, R.A., C.W. Landsea, R.T. Musulin, and M. Downton. Evaluation of catastrophe models using a normalized historical record: Why it is needed and how to do it. Journal of Risk and Insurance, 18(2):177-194, 1999
This article explains the role of catastrophe simulation models in managing exposures to hurricane losses. The credibility and accuracy of these models have been called into question by consumer groups and regulators. This paper suggests a practical method for establishing a baseline to test the skill of these models in estimating catastrophe losses. In establishing a baseline for model evaluation, performance can be measured quantitatively. Given the vast exposure of society to the impacts of catastrophic events, it is imperative that the financial markets of the world develop effective strategies to manage risk. It follows that such strategies will be more effective to the degree that they are based on reliable and publicly understood estimates of future risk. Finally, the transparent nature of the proposed methodology, in contrast to the complex and often proprietary structure of the computer models, highlights the societal factors underlying the sharp increase in catastrophic loss exposure.
Powell, M.D. Hurricanes at landfall. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 107-108, 1999
Powell, M.D., and S.H. Houston. Comments on "A multiscale numerical study of Hurricane Andrew (1992). Part I: Explicit simulation and verification." Monthly Weather Review, 127(7):1706-1710, doi:10.1175/1520-0493(1999)127<1706:COAMNS>2.0.CO; 1999
Powell, M.D., P.G. Black, S.H. Houston, and T.A. Reinhold. GPS sonde insights on boundary layer structure in hurricanes. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 881-884, 1999
Powell, M.D., T.A. Reinhold, and R.D. Marshall. GPS sonde insights on boundary layer wind structure in hurricanes. Proceedings, 10th Conference on Wind Engineering, Copenhagen, Denmark, June 21-24, 1999. ICWE, 307-314, 1999
Reasor, P.D., M.T. Montgomery, and F.D. Marks. The asymmetric structure of Hurricane Olivia's inner core. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 301-304, 1999
Rogers, R.F. Amplification of warm-core vortices by convective redevelopment: A key component of tropical cyclogenesis. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 929-932, 1999
Rogers, R.F., and J.M. Fritsch. Amplification of warm-core vortices by convective redevelopment: A key component of tropical cyclogenesis. Preprints, Eighth Conference on Mesoscale Processes, Boulder, CO, June 28-July 1, 1999. American Meteorological Society, Boston, 55-60, 1999
Shapiro, L.J., and J.L. Franklin. Potential vorticity asymmetries and tropical cyclone motion. Monthly Weather Review, 127(1):124-131, doi:10.1175/1520-0493(1999)127<0124:PVAATC>2.0.CO; 1999
A set of nine synoptic-flow cases, incorporating Omega dropwindsonde observations for six tropical storms and hurricanes, is used to deduce the three-dimensional distribution of potential vorticity (PV) that contributed to the deep-layer mean (DLM) wind that steered the cyclones. A piecewise inversion technique, the same as that previously applied by Shapiro to Hurricane Gloria of 1985, is used to derive the DLM wind induced by pieces of anomalous PV restricted to cylinders of different radii centered on each cyclone. The cylinder of PV that induces a DLM wind that best matches the observed DLM wind near the center of each cyclone is evaluated. It is found that the results can be loosely placed into two categories describing the spatial scale of the PV anomalies that influenced the cyclone's motion. Four of the cases, including Hurricane Gloria, had "local" control, with a good match (to within 40%) between the observed DLM wind near the cyclone center and the DLM wind attributable to a cylinder of PV with a given radius of 1500 km. Further decomposition of the PV anomaly into upper (400 mb and above) and lower levels (500 mb and below) indicates the dominance of upper-level features in steering two of the cyclones (Hurricanes Gloria of 1985 and Andrew of 1992), while Hurricane Debby of 1982 was steered by more barotropic features. These results supplement those found in other studies. Five of the cases, by contrast, had "large-scale" control, with no cylinder of radius 2000 km having a good match between the induced and observed DLM wind. Hurricanes Emily of 1987 and 1993 fell into this category, as did Hurricane Josephine of 1984. Implications of the results for guiding in-situ wind measurements to improve hurricane track forecasts are discussed.
Uhlhorn, E.W., P.G. Black, and A.F. Hasler. Evolution of mesoscale flow in a mature tropical cyclone as determined from satellite imagery. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 200-203, 1999
Vachon, P.W., K.B. Katsaros, P.G. Black, and P.P. Dodge. RADARSAT synthetic aperture radar measurements of some 1998 hurricanes. Proceedings, 1999 International Geoscience and Remote Sensing Symposium (IGARSS '99), Hamburg, Germany, June 28-July 2, 1999. Institute of Electrical and Electronic Engineers, 1631-1633, 1999
The RADARSAT synthetic aperture radar (SAR) acquired C-band and HH polarization images over four 1998 hurricanes: Bonnie, Danielle, Georges, and Mitch. We present the SAR images and discuss their quantitative use in understanding hurricane morphology. The SAR provides a complementary "view from below" that is most beneficial when considered in the context of more conventional hurricane observations.
Watson, A.I., K.M. Stellman, K.J. Gould, and P.P. Dodge. Local applications of the WSR-88D hourly digital precipitation product at the National Weather Service office in Tallahassee, Florida. Preprints, 29th International Conference on Radar Meteorology, Montreal, Quebec, Canada, July 12-16, 1999. American Meteorological Society, Boston, 232-235, 1999
Willis, P.T. The WSR-88D tropical Z-R relationship in south Florida. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 237-240, 1999
Willoughby, H.E. Vortex tracking semispectral hurricane models. Preprints, 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX, January 10-15, 1999. American Meteorological Society, Boston, 662-665, 1999
1998
Aberson, S.D. Five-day tropical cyclone track forecasts in the North Atlantic basin. Weather and Forecasting, 13(4):1005-1015, doi:10.1175/1520-0434(1998)013<1005:FDTCTF>2.0.CO; 1998
Statistical analyses of the most recent 40 years of hurricane tracks (1956-1995) are presented, leading to a version of the North Atlantic climatology and persistence (CLIPER) model that exhibits much smaller forecast biases but similar forecast errors compared to the previously used version. Changes to the model involve the inclusion of more accurate historical tropical cyclone track data and a simpler derivation of the regression equations. Nonlinear systems analysis shows that the predictability timescale in which the average errors increase by a factor e is approximately 2.5 days in the Atlantic basin, which is larger than that found by similar methods near Australia. This suggests that five-day tropical cyclone track forecasts may have some benefit, and, therefore, a version of CLIPER extended to five days to be used as a baseline to measure this skill is needed.
Aberson, S.D., M.A. Bender, and R.E. Tuleya. Ensemble forecasting of tropical cyclone intensity. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 150-153, 1998
Aberson, S.D., M.A. Bender, and R.E. Tuleya. Ensemble forecasting of tropical cyclone tracks. Preprints, 12th Conference on Numerical Weather Prediction, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 290-292, 1998
Amat, L.R., M.D. Powell, and S.H. Houston. WANDA: HRD's real-time tropical cyclone "Wind Analysis Distributed Application." Preprints, 16th Conference on Weather Analysis and Forecasting and Symposium on the Research Foci of the U.S. Weather Research Program, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, J29-J31, 1998
Black, P.G., and L.K. Shay. Observations of tropical cyclone intensity change due to air-sea interaction processes. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 161-168, 1998
Bosart, L.F., W.E. Bracken, J. Molinari, C.S. Velden, and P.G. Black. Environmental influences on the rapid intensification stage of Hurricane Opal (1995) over the Gulf of Mexico. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 105-112, 1998
Bove, M.C., J.J. O'Brien, J.B. Eisner, C.W. Landsea, and X. Niu. Effect of El Niño on U.S. landfalling hurricanes, revisited. Bulletin of the American Meteorological Society, 79(11):2477-2482, doi:10.1175/1520-0477(1998)079<2477:EOENOO>2.0.CO; 1998
Changes in the frequency of U.S. landfalling hurricanes with respect to the El Niño-Southern Oscillation (ENSO) cycle are assessed. Ninety-eight years (1900-1997) of U.S. landfalling hurricanes are classified, using sea surface temperature anomaly data from the equatorial Pacific Ocean, as occurring during an El Niño (anomalously warm tropical Pacific waters), La Niña (anomalously cold tropical Pacificwaters), or neither (neutral). The mean and variance of U.S. landfalling hurricanes are determined for each ENSO phase. Each grouping is then tested for Poisson distribution using a chi-squared test. Resampling using a "bootstrap" technique is then used to determine the 5% and 95% confidencelimits of the results. Last, the frequency of major U.S. landfalling hurricanes (sustained winds of 96 kt or more) with respect to ENSO phase is assessed empirically. The results indicated that El Niño events show a reduction in the probability of a U.S. landfalling hurricane, while La Niña shows an increase in the chance of a U.S. hurricane strike. Quantitatively, the probability of two or more landfalling U.S. hurricanes during an El Niño is 28%, of two or more landfalls during neutral conditions is 48%, and of two or more landfalls during La Niña is 66%. The frequencies of landfalling major hurricanes show similar results. The probability of one or more major hurricane landfall during El Niño is 23% but is 58% during neutral conditions and 63% during La Niña.
Bryan, G.H., R.F. Rogers, and J.M. Fritsch. Cloud-scale resolution simulations in moist absolutely unstable layers. Preprints, Eighth Pennsylvania State University-National Center for Atmospheric Research's Mesoscale Model Users Workshop, Boulder, CO. National Center for Atmospheric Research, 59-62, 1998
Cione, J.J., and P.G. Black. Surface thermodynamic observations within the tropical cyclone inner core. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 141-145, 1998
Cione, J.J., R.A. Neuherz, S. Raman, L.J. Pietrafesa, K. Keeter, and X. Li. The use of pre-storm boundary-layer baroclinicity in determining and operationally implementing the Atlantic Surface Cyclone Intensification Index. Boundary-Layer Meteorology, 89(2):211-224, 1998
The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low level baroclinicity within the coastal zone during periods of offshore cold advection. The relative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period oftentimes precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advective period, and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass, as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24-48 h prior to the initial cyclonic intensification), are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting. The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982- April 1990), 116-storm study conducted by Cione et al. (1993). During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.
Eads, L.J., H.A. Friedman, and D.J. Garcia. From humble beginnings as the Inner City Marine Project to selection as a National School of Excellence. Preprints, Seventh Symposium on Education, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 162-165, 1998
The evolution of the MAST Academy (Maritime and Science Technology High School), a Dade County Magnet School of Choice, from its predecessor, the Inner City Marine Project (ICMP), is described. ICMP originated after Dade County experienced civil unrest in the Black community in 1984. At that time, Dr. Linda J. Eads, currently MAST Academy's principal, was assigned to design a program in maritime education which emphasized career exploration for minorities. The ICMP operated from the District Office of the Dade County Public Schools and targeted elementary and middle schools in the inner city with high minority populations. When the MAST Academy opened its doors in 1991, the ICMP became the MAST Academy Outreach Department which continued to provide programs for the targeted schools. The MAST Academy presently carries on the tradition of the ICMP by providing high school students with specialized marine-theme science and technology courses. In 1996, the MAST Academy was selected as a U.S. Department of Education National Blue Ribbon School of Excellence.
Ellsberry, R.L., and F.D. Marks. U.S. Weather Research Program Hurricane Landfall Workshop Report. National Center for Atmospheric Research, Technical Note, NCAR/TN-442, 40 pp., 1998
Friedman, H.A., and D.J. Garcia. Tropical cyclone public awareness programs: Preparing for the 21st century. Preprints, Seventh Symposium on Education, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 166-168, 1998
Garcia, D.J., H.A. Friedman, and L.J. Eads. MAST Academy outreach: Serving the community with marine theme programs. Preprints, Seventh Symposium on Education, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 169-171, 1998
Hasler, A.F., K. Palaniappan, C. Kambhammetu, P.G. Black, E.W. Uhlhorn, and D. Chesters. High-resolution wind fields within the inner core and eye of a mature tropical cyclone from GOES 1-min images. Bulletin of the American Meteorological Society, 79(11):2483-2496, doi:10.1175/1520-0477(1998)079<2483:HRWFWT>2.0.CO; 1998
Mesoscale wind fields have been determined for a mature hurricane with high spatial and temporal resolution, continuity, and coherency. These wind fields, near the tropopause in the inner core and at low levels inside the eye, allow the evolution of mesoscale storm features to be observed. Previously, satellite-derived winds near hurricanes have been determined only at some distance from the eye over a typical time period of 1V2 h. Hurricane reconnaissance aircraft take 30 min to 1 h to complete an inner-core pattern. With the long observation periods of these previous methods, steady-state conditions must be assumed to give a complete description of the observed region. With the advent of 1-min interval imagery, and fourfold improvement of image dynamic range from NOAA's current generation of GOES satellites, there is a new capability to measure inner-core tropical cyclone wind fields near the tropopause and within the eye, enabling mesoscale dynamical processes to be inferred. These measurements give insights into the general magnitude and structure of the hurricane vortex, along with very detailed measurements of the cloud-top wind's variations in response to convective outbursts. This paper describes the new techniques used to take advantage of the GOES satellite improvements that, in turn, allowed the above innovations to occur. The source of data for this study is a nearly continuous 12-h sequence of 1-min visible images from NOAA GOES-9 on 6 September 1995. These images are centered on Hurricane Luis with maximum winds of 120 kt (CAT4) when it was 250 km northeast of Puerto Rico. A uniform distribution of long-lived cirrus debris with detailed structure is observed in the central dense overcast (CDO), which has been tracked using the 1-min images. The derived wind field near the tropopause at approximately 15 km in the CDO region has a strong closed circulation with speeds up to 25 m s-1, which pulses in response to the convective outbursts in the eyewall. Cloud displacements are computed at every pixel in every image, resulting in a quarter-million uVv winds in each of 488 hurricane images observed at 1- to 4-min intervals over 12 h. For analysis and presentation, these ultradense wind fields are reduced to 8- or 16-km grids using a 7-min time base by smoothing displacement vectors in space and time. Cloud structures were tracked automatically on a massively parallel processing computer, but with manual spot-checking. Manual tracking has been used to follow CDO structure over long time periods, up to 90 min for a small test sample. Cloud tracking for the wind fields presented here is accomplished using a Massively Parallel Semi-Fluid Motion Analysis (MPSMA) automatic technique. This robust deformable surface-matching algorithm has been implemented on the massively parallel Maspar supercomputer. MPSMA automatic tracking typically follows a feature for 7 min. For this time base the error of these winds is estimated to be 1.5 m s-1. However, systematic navigation and height assignment errors in the moderately sheared hurricane environment must still be considered. Spatial and temporal smoothing of the wind field have been performed to reduce systematic navigation errors and small-scale turbulent noise. The synthesis used here to compute the wind fields gives an order of magnitude reduction in the amount of data presented compared to the amount of data processed. Longer tracking could give higher accuracy but would smooth out the smaller-scale spatial and temporal features that appear dynamically significant. The authors believe that the techniques described in this paper have great potential for further research on tropical cyclones and severe weather as well as in operational use for nowcasting and forecasting. United States and foreign policymakers are urged to augment the GOES, GMS, FY2, and Meteosat geostationary satellite systems with dual imaging systems such that 1-min observations are routinely taken.
Henderson-Sellers, A., H. Zhang, G. Berz, K. Emanuel, W. Gray, C.W. Landsea, G. Holland, J. Lighthill, S.-L. Shieh, P. Webster, and K. McGuffie. Tropical cyclones and global climate change: A post-IPCC assessment. Bulletin of the American Meteorological Society, 79(1):19-38, doi:10.1175/1520-0477(1998)079<0019:TCAGCC>2.0.CO; 1998
The very limited instrumental record makes extensive analyses of the natural variability of global tropical cyclone activities difficult in most of the tropical cyclone basins. However, in the two regions where reasonably reliable records exist (the North Atlantic and the western North Pacific), substantial multidecadal variability (particularly for intense Atlantic hurricanes) is found, but there is no clear evidence of long-term trends. Efforts have been initiated to use geological and geomorphological records and analysis of oxygen isotope ratios in rainfall recorded in cave stalactites to establish a paleoclimate of tropical cyclones, but these have not yet produced definitive results. Recent thermodynamical estimation of the maximum potential intensities (MPI) of tropical cyclones shows good agreement with observations. Although there are some uncertainties in these MPI approaches, such as their sensitivity to variations in parameters and failure to include some potentially important interactions such as ocean spray feedbacks, the response of upper-oceanic thermal structure and eye and eyewall dynamics do appear to be an objective tool with which to predict present and future maxima of tropical cyclone intensity. Recent studies indicate the MPI of cyclones will remain the same or undergo a modest increase of up to 10%-20%. These predicted changes are small compared with the observed natural variations and fall within the uncertainty range in current studies. Furthermore, the known omissions (ocean spray, momentum restriction, and possibly also surface to 300-hPa lapse rate changes) could all operate to mitigate the predicted intensification. A strong caveat must be placed on analysis of results from current GCM simulations of the "tropical-cyclone-like" vortices. Their realism, and hence prediction skills (and also that of "embedded" mesoscale models), is greatly limited by the coarse resolution of current GCMs and the failure to capture environmental factors that govern cyclone intensity. Little, therefore, can be said about the potential changes of the distribution of intensities as opposed to maximum achievable intensity. Current knowledge and available techniques are too rudimentary for quantitative indications of potential changes in tropical cyclone frequency. The broad geographic regions of cyclogenesis and, therefore, also the regions affected by tropical cyclones are not expected to change significantly. It is emphasized that the popular belief that the region of cyclogenesis will expand with the 26 C SST isotherm is a fallacy. The very modest available evidence points to an expectation of little or no change in global frequency. Regional and local frequencies could change substantially in either direction, because of the dependence of cyclone genesis and track on other phenomena (e.g., ENSO) that are not yet predictable. Greatly improved skills from coupled global ocean-atmosphere models are required before improved predictions are possible.
Houston, S.H., and M.D. Powell. Reconstruction of surface wind fields for hurricanes affecting Florida Bay. Preprints, Second Conference on Coastal Atmospheric and Oceanic Prediction and Processes, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 241-244, 1998
Houston, S.H., and M.D. Powell. Surface wind fields in hurricanes. Proceedings, Third International Symposium, Waves '97, Virginia Beach, VA, November 3-7, 1997. American Society of Civil Engineers (ASCE), 1391-1399, 1998
Houston, S.H., M. Lawrence, S. Spisak, and S.T. Murillo. A verification of National Hurricane Center forecasts of surface wind speed radii in hurricanes. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 139-140, 1998
Jameson, A.R., A.B. Kostinski, and R.A. Black. The texture of clouds. Journal of Geophysical Research, 103(D6):6211-6220, https://doi.org/10.1029/98JD00081 1998
Using a precise definition of clustering, it is shown that in two tropical cumulus clouds, droplets appear to be bunched over distances ranging from at least a kilometer or more down to several centimeters. A statistical framework is proposed for quantifying clustering in terms of a Poisson probability mixture. While these observations require further substantiation in many different clouds, droplet clustering may play a role in diverse phenomena from the coalescence growth of raindrops to the scattering of radiation by clouds.
Kaplan, J., and M. DeMaria. Climatological and synoptic characteristics of rapidly intensifying tropical cyclones in the North Atlantic basin. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 119-123, 1998
Landsea, C.W., and R.A. Pielke. Trends in U.S. hurricane losses, 1925-1995. Preprints, Ninth Symposium on Global Change Studies, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 210-212, 1998
Landsea, C.W., G.D. Bell, W.M. Gray, and S.B. Goldenberg. The extremely active 1995 Atlantic hurricane season: Environmental conditions and verification of seasonal forecasts. Monthly Weather Review, 126(5):1174-1193, doi: 10.1175/1520-0493(1998)126<1174:TEAAHS>2.0.CO 1998
The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950-1990) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days (average is 46.6) and 60 hurricane days (average is 23.9), respectively. There were five intense (or major) Saffir-Simpson category 3, 4, or 5 hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes and their days present, was 229% of the average. Additionally, 1995 saw the return of hurricane activity to the deep tropical latitudes: seven hurricanes developed south of 25°N (excluding all of the Gulf of Mexico) compared with just one during all of 1991-1994. Interestingly, all seven storms that formed south of 20°N in August and September recurved to the northeast without making landfall in the United States. The sharply increased hurricane activity during 1995 is attributed to the juxtaposition of virtually all of the large-scale features over the tropical North Atlantic that favor tropical cyclogenesis and development. These include extremely low vertical wind shear, below-normal sea level pressure, abnormally warm ocean waters, higher than average amounts of total precipitable water, and a strong west phase of the stratospheric quasi-biennial oscillation. These various environmental factors were in strong contrast to those of the very unfavorable conditions that accompanied the extremely quiet 1994 hurricane season. The favorable conditions for the 1995 hurricane season began to develop as far back as the previous winter. Their onset well ahead of the start of the hurricane season indicates that they are a cause of the increased hurricane activity, and not an effect. The extreme duration of the atmospheric circulation anomalies over the tropical North Atlantic is partly attributed to a transition in the equatorial Pacific from warm episode conditions (El Niño) to cold episode conditions (La Niña) prior to the onset of the hurricane season. Though the season as a whole was extremely active, 1995's Atlantic tropical cyclogenesis showed a strong intraseasonal variability with above-normal storm frequency during August and October and below normal for September. This variability is likely attributed to changes in the upper-tropospheric circulation across the tropical North Atlantic, which resulted in a return to near-normal vertical shear during September. Another contributing factor to the reduction in tropical cyclogenesis during September may have been a temporary return to the near-normal SSTs across the tropical and subtropical North Atlantic, caused by the enhanced tropical cyclone activity during August. Seasonal hurricane forecasts for 1995 issued at Colorado State University on 30 November 1994, 5 June 1995, and 4 August 1995 correctly anticipated an above-average season, but underforecast the extent of the extreme hurricane activity.
Landsea, C.W., J. Kaplan, and M. DeMaria. The differing roles of the large-scale environment in the intensity changes of recent Atlantic hurricanes. Preprints, Symposium on Tropical Cyclone Intensity Change, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 113-114, 1998
Landsea, C.W., N. Nicholls, and J. Gill. Australian region tropical cyclones: Recent trend and interannual predictions. Preprints, Ninth Conference on Interaction of the Sea and Atmosphere, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 1-4, 1998
Marks, F.D., and L.K. Shay (and the Fifth Prospectus Development Team [PDF-5]: G. Barnes, P. Black, M. DeMaria, J. Dungan, B. McCaul, J.Molinari, M. Montgomery, M. Powell, R. Tuleya, G. Tripoli, L. Xie, andR. Zehr). Landfalling tropical cyclones: Forecast problems and associated research opportunities. Report of the Fifth Prospectus Development Team to the U.S. Weather Research Program. Bulletin of the American Meteorological Society, 79(2):305-323, 1998
The Fifth Prospectus Development Team of the U.S. Weather Research Program was charged to identify and delineate emerging research opportunities relevant to the prediction of local weather, flooding, and coastal ocean currents associated with landfalling U.S. hurricanes specifically, and tropical cyclones in general. Central to this theme are basic and applied research topics, including rapid intensity change, initialization of and parameterization in dynamical models, coupling of atmospheric and oceanic models, quantitative use of satellite information, and mobile observing strategies to acquire observations to evaluate and validate predictive models. To improve the necessary understanding of physical processes and provide the initial conditions for realistic predictions, a focused, comprehensive mobile observing system in a translating storm-coordinate system is required. Given the development of proven instrumentation and improvement of existing systems, three-dimensional atmospheric and oceanic data sets need to be acquired whenever major hurricanes threaten the United States. The spatial context of these focused three-dimensional data sets over the storm scales is provided by satellites, aircraft, expendable probes released from aircraft, and coastal (both fixed and mobile), moored, and drifting surface platforms. To take full advantage of these new observations, techniques need to be developed to objectively analyze these observations, and initialize models aimed at improving prediction of hurricane track and intensity from global-scale to mesoscale dynamical models. Multinested models allow prediction of all scales from the global, which determine long-term hurricane motion to the convective scale, which affect intensity. Development of an integrated analysis and model forecast system optimizing the use of three-dimensional observations and providing the necessary forecast skill on all relevant spatial scales is required. Detailed diagnostic analyses of these data sets will lead to improved understanding of the physical processes of hurricane motion, intensity change, the atmospheric and oceanic boundary layers, and the air-sea coupling mechanisms. The ultimate aim of this effort is the construction of real-time analyses of storm surge, winds, and rain, prior to and during landfall, to improve warnings and provide local officials with the comprehensive information required for recovery efforts in the hardest hit areas as quickly as possible.
Marks, F.D., and L.K. Shay. Landfalling tropical cyclones: Forecast problems and associated research opportunities. Preprints, 16th Conference on Weather Analysis and Forecasting and Symposium on the Research Foci of the U.S. Weather Research Program, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 520-523, 1998
Molinari, J., S. Skubis, D. Vollaro, F. Alsheimer, and H.E. Willoughby. Potential vorticity analysis of tropical cyclone intensification. Journal of the Atmospheric Sciences, 55(6):2632-2644, doi:10.1175/1520-0469(1998)055<2632:PVAOTC>2.0.CO; 1998
The interaction of marginal Tropical Storm Danny (1985) with an upper-tropospheric positive potential vorticity anomaly was examined. The intensification mechanism proposed earlier for mature Hurricane Elena appears to be valid for Danny as well, despite significant differences in the synoptic-scale environment and in the stage of the tropical cyclone prior to the interaction. Both storms experienced rapid pressure falls as a relatively small-scale positive upper potential vorticity anomaly began to superpose with the low-level tropical cyclone center. The interaction is described in terms of a complex interplay between vertical wind shear, diabatic heating, and mutual advection among vortices at and below the level of the outflow anticyclone. Despite this complexity, the superposition principle appears to be conceptually useful to describe the intensification of tropical cyclones during such interactions.
Montgomery, M.T., and J.L. Franklin. An assessment of the balance approximation in hurricanes. Journal of the Atmospheric Sciences, 55(12):2193-2200, doi:10.1175/1520-0469(1998)055<2193:AAOTBA>2.0.CO; 1998
The validity of the traditional balance approximation for the asymmetric flow above the boundary layer generally in hurricanes is examined here. Scaling considerations of the divergence equation show that the validity of the balance approximation hinges on the smallness of the nondimensional product. The first term represents the ratio of asymmetric horizontal divergence to asymmetric vertical vorticity for azimuthal wavenumber, n, while the second term represents a Rossby number based upon the azimuthal mean tangential wind and absolute vertical vorticity of the hurricane vortex. Wind observations of Hurricane Gloria (1985) indicate that this product is not at all small in the near-vortex region (several hundred kilometers beyond the radius of maximum tangential winds) where asymmetric convergence forced by surface friction and cumulus convection is typically large. Although the Gloria observations represent only a single case, there are dynamical reasons to expect this product to be 0(1) just above the hurricane boundary layer in steadily translating hurricanes. The meteorological relevance of these results to the problem of balance dynamics in hurricanes is briefly discussed.
Nicholls, N., C.W. Landsea, and J. Gill. Recent trends in Australian region tropical cyclone activity. Meteorology and Atmospheric Physics, 65(3-4):197-205, 1998
The number of tropical cyclones observed in the Australian region (south of equator; 105-160°E) has apparently declined since the start of reliable (satellite) observations in the 1969/1970 season. However, the number of more intense cyclones (with minimum pressures dropping to 970 hPa or lower) has increased slightly. The numbers of weak (minimum pressures not dropping below 990 hPa) and moderate systems (minimum pressures between 970 and 990 hPa) have declined. Possible reasons for these different trends are discussed. The decline in the number of weaker cyclones may at least partly reflect improved understanding of the nature of some weak systems. The decline in the number of cyclones more intense than 990 hPa primarily reflects the downward trend in the Southern Oscillation Index (SOI). Previous work has demonstrated that the number of tropical cyclones observed in the Australian region each cyclone season is related to the value of the SOI prior to the start of the cyclone season. This relationship is clearest with the number of moderate cyclones. The SOI is only weakly related to the number of intense or weak cyclones. The increase in the number of intense cyclones is not attributable to the trend in the SOI. Nor is there clear reason, at present, to suspect that it is artificial (i.e.,due to changes in observing or analysis techniques).
Niyogi, D.S., J.J. Cione, and S. Raman. Gulf Stream influence on the North Carolina mesoclimate. Preprints, 2nd Conference on Coastal Atmospheric and Oceanic Prediction, Phoenix, AZ, January 12-16, 1998. American Meteorological Society, Boston, 421-424, 1998
Pielke, R.A., and C.W. Landsea. Normalized hurricane damages in the United States: 1925-1995. Weather and Forecasting, 13(3):621-631, doi:10.1175/1520-0434(1998)013<0621:NHDITU>2.0.CO; 1998
Hurricanes are the costliest natural disasters in the United States. Understanding how both hurricane frequencies and intensities vary from year to year, as well as how this is manifested in changes in damages that occur, is a topic of great interest to meteorologists, public and private decision makers, and the general public alike. Previous research into long-term trends in hurricane-caused damage along the U.S. coast has suggested that damage has been quickly increasing within the last two decades, even after considering inflation. However, to best capture the year-to-year variability in tropical cyclone damage, consideration must also be given toward two additional factors: coastal population changes and changes in wealth. Both population and wealth have increased dramatically over the last several decades and act to enhance the recent hurricane damages preferentially over those occurring previously. More appropriate trends in the United States hurricane damages can be calculated when a normalization of the damages are done to take into account inflation and changes in coastal population and wealth. With this normalization, the trend of increasing damage amounts in recent decades disappears. Instead, substantial multidecadal variations in normalized damages are observed: the 1970s and 1980s actually incurred less damages than in the preceding few decades. Only during the early 1990s does damage approach the high level of impact seen back in the 1940s through the 1960s, showing that what has been observed recently is not unprecedented. Over the long term, the average annual impact of damages in the continental United States is about $4.8 billion (1995 $), substantially more than previous estimates. Of these damages, over 83% are accounted for by the intense hurricanes (Saffir-Simpson categories 3, 4, and 5), yet these make up only 21% of the U.S.-landfalling tropical cyclones.
Powell, M.D., and S.D. Aberson. How well do we forecast the position and time of hurricane landfall? Preprints, 16th Conference on Weather Analysis and Forecasting and Symposium on the Research Foci of the U.S. Weather Research Program, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 9-12, 1998
Powell, M.D., and S.H. Houston. Surface wind fields of 1995 Hurricanes Erin, Opal, Luis, Marilyn, and Roxanne at landfall. Monthly Weather Review, 126(5):1259-1273, doi:10.1175/1520-0493(1998)126<1259:SWFOHE>2.0.CO; 1998
Hurricanes Erin, Opal, Luis, Marilyn, and Roxanne were the most destructive hurricanes of 1995. At landfall, Luis and Marilyn contained maximum sustained winds (marine exposure) estimated at near 60 and 46 m s-1, respectively. The strongest landfalling storm of the 1995 season, Luis, decreased in intensity from a category 4 to 3 on the Saffir-Simpson scale shortly before the eyewall crossed the Islands of Antigua, Barbuda, St. Kitts-Nevis, St. Barthelemy, St. Martin, and Anguilla. Hurricane Marilyn strengthened as it approached the U.S. Virgin Islands, with St. Thomas bearing the brunt of the north and south eyewall winds of 46 m s-1 (marine exposure) and St. Croix being affected by the relatively weak western eyewall peak winds of 35-40 m s-1 (marine exposure). For Luis and Marilyn, only surface winds with marine exposures were analyzed because of unknown small-scale interactions associated with complex island terrain with 500-1000-m elevations. Wind engineering studies suggest that wind acceleration over blunt ridges can increase or "speed up" winds by 20%-80%. Topographic effects were evident in damage debris analyses and suggest that an operational method of assessing terrain-induced wind gusts (such as a scaled down mesoscale model) is needed. After landfall as a marginal hurricane over central Florida, Hurricane Erin regained strength over the Gulf of Mexico with a well-defined radar reflectivity structure. Erin struck the Florida panhandle near Navarre Beach with maximum sustained surface winds of 35-40 m s-1 affecting the Destin-Ft. Walton area. Hurricane Opal made landfall in nearly the identical area as Erin, with maximum sustained surface winds of 40-45 m s-1, having weakened from an intensity of nearly 60 m s-1 only 10 h earlier. Opal was characterized by an asymmetric structure that was likely related to cold front interaction and an associated midlevel southwesterly jet. Roxanne struck Cozumel, Mexico, with sustained surface winds (marine exposure) of 46 s-1, crossed the Yucatan, and meandered in the southwest Gulf of Mexico for several days. While in the Bay of Campeche, Roxanne's large area of hurricane-force winds disabled a vessel, which led to the drowning deaths of five oil industry workers. High-resolution wind records are critical to preserving an accurate extreme wind climatology required for assessment of realistic building code risks. Unfortunately, power interruptions to Automated Surface Observing Stations (ASOS) on the U.S. Virgin Islands (St. Croix, St. Thomas) and Destin, Florida, prevented complete wind records of the eyewall passages of Marilyn and Opal, respectively.
Powell, M.D., and S.K. Rinard. Marine forecasting at the 1996 centennial Olympic games. Weather and Forecasting, 13(3):764-782, doi:10.1175/1520-0434(1998)013<0764:MFATCO>2.0.CO; 1998
A team of meteorologists from the United States, Canada, and Australia provided marine weather support to the sailing events of the 1996 Centennial Olympic Games, held in Wassaw Sound near Savannah, Georgia. The team conducted research on the weather and climate and developed a set of forecast products designed to inform athletes, volunteers, and race managers of the wind, tidal current, wave, and weather behavior expected each day during the pre-Olympic and Olympic periods. The Olympic period proved to be a challenge with thunderstorms delaying, abandoning, or postponing races on half of the days. Thunderstorm development and movement was linked to the timing and strength of the sea breeze as well as the direction and speed of the gradient wind. Numerous thunderstorm warnings were issued with the assistance of the WSR-88D radar and the Warning Decision Support System. Frequent lightning was a legitimate safety concern due to the long distances between race courses and lack of suitable shelter; fortunately no one was injured during the lightning episodes. Forecasters benefited from access to a variety of monitoring tools and models including real-time Olympic buoy wind and current time series displays; satellite and radar imagery animation; 2-, 8-, and 10-km resolution mesoscale models; a live video feed of race coverage; and communications with forecasters aboard patrol craft offshore. Official wind forecasts, mesoscale models, and a simple vector addition model performed better than climatology and persistence as defined by mean vector error and rms wind direction error. Climatology was difficult to beat on the basis of wind speed error.
Powell, M.D., S.H. Houston, L.R. Amat, and N. Morisseau-Leroy. The HRD real-time hurricane wind analysis system. Journal of Wind Engineering and Industrial Aerodynamics, 77&78:53-64, https://doi.org/10.1016/S0167-6105(98)00131-7 1998
The HRD real-time wind analysis system is currently undergoing evaluation in the operational forecasting environment of the National Hurricane Center. The system is an object-oriented, distributed, three-tiered client-server application that assimilates disparate observations and processes the data into a common framework for exposure, height, and averaging time. The data are then examined collectively or by type, quality controlled, and passed on to a scale-controlled objective analysis algorithm. Several products are derived from the analysis wind field and storm track, yielding effective tools for disaster assessment, emergency management, and recovery.
Rogers, S.M., and S.H. Houston. Hurricane surge and wave conditions: Research needs. Proceedings, Third International Symposium, Waves '97, Virginia Beach, VA, November 3-7, 1997. American Society of Civil Engineers, 1414-1424, 1998
For many years, coastal engineers have recognized the importance of reliable wave height and frequency information in the design of major coastal structures. Over time, research by government agencies, such as the U.S. Army Corps of Engineers and NOAA, and private interests such as the offshore oil industry has made a substantial investment in wave gages, wave hindcasting, and wave forecasting. Design needs for breakwaters, jetties, coastal protection, offshore oil facilities, and similar large-scale projects have driven the acquisition of better wave data. Our ability to optimize design wave conditions has improved significantly and with ongoing research is likely to continue to improve in the future. Most designers and researchers would expect wave data is most important for large coastal projects. However, in the United States the most frequent application of design wave conditions is for the design of single family homes and other coastal buildings. Each day hundreds of coastal buildings in communities around the U.S. begin construction in Coastal High Hazard Areas (or V-zones) as identified by the National Flood Insurance Program (NFIP) and the Federal Emergency Management Agency (FEMA). The NFIP prepares flood hazard maps specifying minimum flood elevation standards that include wave height predictions during a 100-year storm surge, generally occurring during a hurricane or other severe coastal storm. The NFIP uses several relatively simple models for shoreline erosion, wave setup, wave runup and depth-limited linear waves. Given the uncertainties inherent in any 100-year surge model, the simple models are not unreasonable, including the use of depth limited, linear waves. For many years we have been measuring water marks along the coast following severe coastal storms with the goal of measuring the storm surge, still water elevation, and/or wave height maximum. This paper will report on case studies of U.S. hurricanes from U.S. hurricanes describing the discrepancies between even the most reliable still water marks and the lower limit of wave damage at the same locations. It is reassuring that our ability to predict offshore wave conditions has improved. However, it is clear that we have little understanding of the water-levels and wave conditions during hurricanes where we need them the most; that is, flooded building sites that are normally dry land. Some of the discrepancies can be explained by measurement errors, wave setup, wave runup, and localized setup in confined spaces and other factors, but no rational theory can explain local variations. In short, we may have a good wave gage record somewhere offshore and many post-storm water marks, but we have little idea what water elevations and wave conditions occurred in flooded building sites near the beach.Two problems result. First, small buildings are usually designed to avoid waves by being elevated on piling foundations. Without a reasonable understanding of the wave conditions, buildings will be improperly elevated for cost-effective designs (i.e., either too high or too low). A second potentially more serious problem is that the high water marks will eventually be used to calibrate the underlying storm surge models on which all design conditions are based. Our lack of understanding increases the risk of improperly calibrating the storm surge models, which are also used to predict flood elevations much further inland than those areas affected by waves. The authors believe that there is a substantial need for wave and water-level measurements near coastal building sites which are flooded during hurricanes and other design-level storms. The frequent application of wave predictions is to design cost-effective, storm-resistant buildings. The deployment of multiple, self contained wave gages at preselected sites near coastal buildings that are expected to be flooded during a landfalling hurricane is now a practical research goal with recent improvements in instrumentation. It is time to stop guessing the wave and water-level conditions and produce some real measurements.
Shapiro, L.J., and S.B. Goldenberg. Atlantic sea surface temperatures and tropical cyclone formation. Journal of Climate, 11(4):578-590, doi:10.1175/1520-0442(1998)0112.0.CO;2 1998
It has long been accepted that interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in seasonal Atlantic basin tropical cyclone frequency. To isolate the physical mechanism responsible for this relationship, a singular value decomposition (SVD) is used to establish the dominant covarying modes of tropospheric wind shear and SST, as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which comprises equatorially confined near-zonal vertical wind shear fluctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Niño) and west African Sahel rainfall. While this mode is strongly related to tropical storms, hurricanes, and major hurricane frequency in the Atlantic, it is not associated with any appreciable Atlantic SST signal. By contrast, the second SVD mode of covarying vertical shear and horizontal SST gradient variability, which is effectively uncorrelated with the dominant mode, is associated with SST fluctuations concentrated in the main tropical cyclone development region between 10°N and 20°N. This mode is significantly correlated with tropical storm and hurricane frequency but not with major hurricane frequency. Statistical tests confirm the robustness of the mode, and lag correlations and physical reasoning demonstrate that the SST anomalies are not due to the developing tropical cyclones themselves. Anomalies of SST and vertical shear during years where the mode has substantial amplitude confirm the resemblance of the individual fields to the modal structure, as well as the association of hurricane development with the warmer SSTs. Although SSTs are of secondary importance to vertical shear in modulating hurricane formation, explaining only 10% of the interannual variability in hurricane frequency over the 50% explained by vertical shear, the results support the conclusion that warmer SSTs directly enhance development. The lack of correlation with major hurricanes implies that the underlying SSTs are not a significant factor in the development of these stronger systems.
Shay, L., G.J. Goni, F.D. Marks, J.J. Cione, and P.G. Black. Role of warm ocean features on intensity change: Hurricane Opal. Preprints, Symposium on Tropical Intensity Change, Phoenix, AZ, January 11-16,1998. American Meteorological Society, Boston, 131-138, 1998
White, S.R., J.D. McFadden, and J.L. Franklin. Atmospheric observations with the NOAA Gulfstream IV-SP. Preprints, 10th Symposium on Meteorological Observations and Instrumentation, Phoenix, AZ, January 11-16, 1998. American Meteorological Society, Boston, 38-41, 1998
In intense tropical cyclones, sea level pressures at the center are 50-100 hPa lower than outside the vortex, but only 10-30 hPa of the total pressure fall occurs inside the eye between the eyewall and the center. Warming by dry subsidence accounts for this fraction of the total hydrostatic pressure fall. Convection in the eyewall causes the warming by doing work on the eye to force the thermally indirect subsidence. Soundings inside hurricane eyes show warm and dry air aloft, separated by an inversion from cloudy air below. Dewpoint depressions at the inversion level, typically 850-500 hPa, are 10-30 K rather than the 100 K that would occur if the air descended from tropopause level without dilution by the surrounding cloud. The observed temperature and dewpoint distribution above the inversion can, however, be derived by 100 hPa of undilute dry subsidence from an initial sounding that is somewhat more stable than a moist adiabat. It is hypothesized that the air above the inversion has remained in the eye since it was enclosed when the eyewall formed and that it has subsided at most a few kilometers. The cause of the subsidence is the enclosed air's being drawn downward toward the inversion level as the air below it flows outward into the eyewall. Shrinkage of the eye's volume is more than adequate to supply the volume lost as dry air is incorporated into the eyewall or converted to moist air by turbulent mixing across the eye boundary. The moist air below the inversion is in thermodynamic contact with the sea surface. Its moisture derives from evaporation of seawater inside the eye, frictional inflow of moist air under the eyewall, and from moist downdrafts induced as condensate mixes into the eye. The moist air's residence time in the eye is much shorter than that of the dry air above the inversion. The height of the inversion is determined by the balance between evaporation, inflow, and inward mixing on one hand and loss to the eyewall updrafts on the other.
1997
Aberson, S.D. Adaptive observations in a hurricane environment. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 308-309, 1997
Aberson, S.D. The prediction of the performance of a nested barotropic hurricane track forecast model. Weather and Forecasting, 12(1):24-30, doi:10.1175/1520-0434(1997)012<0024:TPOTPO>2.0.CO; 1997
Linear multiple regression and discriminant analyses provide estimates of the errors of track forecasts from a nested barotropic hurricane track forecast model (VICBAR), which was run in the North Atlantic Basin during the 1989-94 hurricane seasons. Predictors are determined from the synoptic situation, the magnitude of atmospheric changes in the environment of the tropical cyclone, the consistency between current and past predictions, and the past performance of the model for each particular storm. This technique distinguishes cases in which VICBAR performs well from those for which it performs poorly and can provide skillful operational predictions of model performance to forecasts.
Barnston, A.G., M. Chelliah, and S.B. Goldenberg. Documentation of a highly ENSO-related SST region in the equatorial Pacific. Atmosphere-Ocean, 35(3):367-383, https://doi.org/10.1080/07055900.1997.9649597 1997
A new ENSO SST index is documented that is strongly correlated to the core ENSO phenomenon. The SST anomaly in much of the east-central and eastern tropical Pacific is closely related to ENSO. However, the anomaly from approximately the centre of the eastern half of the equatorial Pacific westward to near the date line is suggested to be most strongly ENSO-related when data spanning the most recent several decades are used. This is the case both with respect to the (1) strength of association with other oceanic/atmospheric ENSO-related anomalies (both simultaneously and as a time-delayed predictand), and (2) impact on remote worldwide climate anomalies. This observational insight was lacking in the early 198Os when the four "Niño" regions were developed. While a firmer dynamical foundation for this regional preference still needs to be established, the region straddling Niño 3 and Niño 4 may be regarded as an appropriate general SST index of the ENSO state by researchers, diagnosticians, and forecasters. A dataset of this index, called "Niño 3.4" (5°N-5°S, 120-170°W) is maintained on the Internet, shown in the Climate Diagnostics Bulletin, and provided in the Appendix ofthis note.
Black, M.L., J.F. Gamache, H.E. Willoughby, C.E. Samsury, F.D. Marks, and R.W. Burpee. Airborne radar observations of shear-induced asymmetries in the convective structure of Hurricane Olivia (1994). Proceedings, 28th Conference on Radar Meteorology, September 9-12, 1997, Austin, TX. American Meteorological Society, Boston, 577-578, 1997
Black, M.L., R.W. Burpee, and F.D. Marks. The asymmetric distribution of vertical motions and precipitation in the hurricane eyewall. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society,Boston, 100-101, 1997
Black, P.G., J.R. Proni, J.C. Wilkerson, and C.E. Samsury. Oceanic rainfall detection and classification in tropical and subtropical mesoscale convective systems using underwater acoustic methods. Monthly Weather Review, 125(9):2014-2042, doi:10.1175/1520-0493(1997)1252.0.CO;2 1997
Measurements of the underwater sound produced by rain were made at three U.S. coastal sites in a study to determine the feasibility and limitations of the acoustic detection and classification of rainfall over water. In the analysis of the rain sound spectra, concurrent radar reflectivity observations were used to identify convective and stratiform regions of the precipitating clouds overhead. It was found that acoustic classifications of rainfall as to type, based on information in the 4-30 kHz frequency band, were in general agreement with radar-derived classifications. The classification technique is based on use of an acoustic discriminant, DR, defined as the difference in average spectral levels between the 10-30 kHz and 4-10 kHz bands. A high correlation was found between sound spectrum levels (in dB) in the 4-10 kHz frequency band and radar reflectivity, dBZ, suggesting the possible use of the 4-10 kHz band sound spectral level as a classification tool in the same way that radar reflectivity is used in classifying precipitation. Our results demonstrate the feasibility of the acoustic method for detecting and classifying rainfall at sea.
Black, R.A. Giant raindrops observed from large aircraft. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 494-495, 1997
Bringi, V.N., K.Knupp, A. Detwiler, L. Liu, I.J. Caylor, and R.A. Black. Evolution of a Florida thunderstorm during the Convection and Precipitation/Electrification Experiment: The case of August 9, 1991. Monthly Weather Review, 125(9):2131-2160, doi:10.1175/1520-0493(1997)125<2131:EOAFTD>2.0.CO; 1997
The relationships among kinematic, microphysical, and electric field properties within a multicell Florida thunderstorm are investigated using observations from three Doppler radar (one with multiple wavelength and polarization diversity capabilities), four instrumented penetrating aircraft, a surface-based electric field mill network, and other observation facilities. The storm was convectively active for about 1 h and at least five primary cells developed within the storm during this time, one of which went through three consecutive development cycles. The updrafts in this storm were 2-4 km wide, exhibited bubble-like evolution, and had lifetimes of 10-20 min. The maximum updraft determined by the multiple Doppler analysis was about 20 m s-1. A differential reflectivity (ZDR) "column," indicating regions containing millimeter-size raindrops, extending above the freezing level, was associated with each cell during its developing stages. This column reached altitudes exceeding 6 km (-8°C) in the stronger updrafts. As the ZDR columns reached maximum altitude, a "cap" of enhanced linear depolarization ratio (LDR) and enhanced 3-cm wavelength attenuation (A3) formed, overlapping the upper regions of the ZDR column. These parameters indicate rapid development of mixed-phase conditions initiated by freezing of supercooled raindrops. Lightning was observed only in the central and strongest convective cell. Electric fields exceeding 10 k V m-1 were noted during aircraft penetrations in this as well as several other cells that did not produce lightning. Fields exceeding 1 k V m-1 were noted by the instrumented aircraft at midcloud levels within a few minutes of development of mixed-phase conditions at these levels or aloft. The first intracloud lightning was detected by the surface field mill network within 5 min of development of mixed-phase conditions aloft in the first cycle of development in the central cell, and the first cloud-to-ground event was noted within 9 min of this development. Lightning continued through two additional cycles of updraft growth in this central region and diminished as the convection subsided after about 30 min. Aircraft-measured electric fields and lightning retrievals from the surface field meter network are consistent with a tendency for negative charge to accumulate above the 6.5 km (-12°C) level within regions of radar reflectivity maxima and for positive charge to accumulate in the anvil region well above 9 km (-30°C).
Cione, J.J., and S. Raman. The impact of SST gradients on propagating low-level mesovortices near the Gulf Stream. Preprints, Conference on Coastal Oceanic and Atmospheric Prediction, Atlanta, GA, January 28-February 2, 1996. American Meteorological Society, Boston, 204-211, 1997
DeMaria, M., and J. Kaplan. An operational evaluation of a statistical intensity prediction scheme (SHIPS). Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 280-281, 1997
Dodge, P.P., S.H. Houston, and J.F. Gamache. Three-dimensional wind fields in Hurricane Fran (1996) at landfall. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 115-116, 1997
Dodge, P.P., S.H. Houston, and J.F. Gamache. Wind fields in Hurricane Fran (1996) at landfall from combined WSR-88D and airborne Doppler radar data. Proceedings, 28th Conference on Radar Meteorology, Austin, TX, September 9-12, 1997. American Meteorological Society, Boston, 575-576, 1997
Faber, T., L.K. Shay, S.D. Jacob, S.H. Houston, and P.G. Black. Observed air-sea interactions during Hurricane Emily. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 433-434, 1997
Franklin, J.L., H.L. Cole, T.F. Hock, D.K. Lauritsen, K.D. Norris, and E.F. Chamberlain. GPS dropwindsondes and the NOAA G-IV jet aircraft: New opportunities for forecasting and research. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 135-136, 1997
Gamache, J.F. Evaluation of a fully three-dimensional variational Doppler analysis technique. Proceedings, 28th Conference on Radar Meteorology, Austin, TX, September 9-12, 1997. American Meteorological Society, Boston, 422-423, 1997
Gamache, J.F., H.E. Willoughby, M.L. Black, and C.E. Samsury. Wind shear, sea surface temperature, and convection in hurricanes observed by airborne Doppler radar. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 121-122, 1997
Goldenberg, S.B., C.W. Landsea, and L.J. Shapiro. Are we seeing the beginning of a long-term upturn in Atlantic basin major hurricane activity? Proceedings, Tropical Cyclone Symposium, Melbourne, Australia, December 9-13 1996. U.S. Office of Naval Research, 10 pp., 1997
Goldenberg, S.B., L.J. Shapiro, and C.W. Landsea. Are we seeing a long-term upturn in Atlantic basin major hurricane activity related to decadal-scale SST fluctuations? Preprints, 7th Conference on Climate Variations, Long Beach, CA, February 2-6, 1997. American Meteorological Society, Boston, 305-310, 1997
Goldenberg, S.B., L.J. Shapiro, and C.W. Landsea. Mounting evidence for a decadal-scale upturn in Atlantic basin tropical cyclone activity. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 507-508, 1997
Goldenberg, S.B., L.J. Shapiro, and C.W. Landsea. The hyperactive 1995 Atlantic hurricane season: A spike or a harbinger of things to come? Workshop Proceedings, Climate Change and Climate Variability in the Atlantic, Halifax, Nova Scotia, December 3-6, 1996. American Meteorological Society, Boston, 113-119, 1997
Goldenberg, S.B., L.J. Shapiro, and C.W. Landsea. The hyperactive 1995 Atlantic hurricane season: Just a spike or a harbinger of things to come? Proceedings, 21st Climate Diagnostics and Prediction Workshop, Hunstville, AL, October 28-November 1, 1996. American Meteorological Society, Boston, 9-12, 1997
Gray, W.M., J.D. Sheaffer, and C.W. Landsea. Climate trends associated with multi-decadal variability of Atlantic hurricane activity. In Hurricanes, Climate, and Socioeconomic Impacts, H.F. Diaz and R.S. Pulwarty (eds.). Springer, Berlin, 15-53, 1997
Haddad, Z.S., D.A. Short, S.L. Durden, E. Im, S. Hensley, M.B. Grable, and R.A. Black. A new parameterization of the rain drop size distribution. IEEE, Transactions of Geoscience and Remote Sensing, 35(3):532-539, https://doi.org/10.1109/36.581961 1997
This paper revisits the problem of finding a parametric form for rain drop size distribution (DSD) which (1) is an appropriate model for tropical rainfall, and (2) involves statistically-independent parameters. Using TOGA/COARE data, we derive a parameterization which meets these criteria. This new parameterization is an improvement on the one that was derived in [3], using TRMM ground truth data from Darwin, Australia. The new COARE data allows us to verify that the spatial variability of the two "shape" parameters is relatively small, thus confirming that this parameterization should be particularly useful for remote sensing applications. We also derive new DSD-based radar-reflectivity-rain-rate power laws, whose coefficients are directly related to the shape parameters of the DSD. Perhaps most important, since the coefficients are independent of the rain-rate itself, and very little spatially, the relations are ideally suited for rain retrieval algorithms. It should also prove straightforward to extend this method to the problems of extimating cloud hydrometeors from remote-sensing measurements.
Hasler, A.F., P.G. Black, V.M. Karyampudy, M. Jentoft-Nilsen, K. Palaniappan, and D. Chesters. Synthesis of eyewall mesovortex and supercell convective structures in Hurricane Luis with GOES-8/9 stereo, concurrent 1-min GOES-9, and NOAA airborne radar observations. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 201-202, 1997
Houston, S.H., M.D. Powell, and P.P. Dodge. Surface wind fields in 1996 Hurricanes Bertha and Fran at landfall. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 92-93, 1997
Jones, R.W., and H.E. Willoughby. Sensitivity of a spectral shallow-water barotropic vortex to variations of domain size and spectral truncation. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 577-578, 1997
Kaplan, J., C.W. Landsea, M. DeMaria, and J.J. Cione. The differing roles of the large-scale environment in the intensity changes of three 1996 Atlantic hurricanes. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 358-359, 1997
Knaff, J.A., and C.W. Landsea. An El Niño-Southern Oscillation Climatology and Persistence (CLIPER) forecasting scheme. Weather and Forecasting, 12(3):633-652, doi:10.1175/1520-0434(1997)012<0633:AENOSO>2.0.CO; 1997
A statistical prediction method is developed for the El Niño-Southern Oscillation (ENSO) phenomena which is based entirely on the optimal combination of persistence, month-to-month trend of initial conditions and climatology. The selection of predictors is by design intended to avoid any pretense of predictive ability based on "model physics" and the like, but rather is to specify the optimal "no-skill" forecast as a baseline comparison for more sophisticated forecast methods. Multiple least squares regression using the method of leaps and bounds is employed to test a total of fourteen possible predictors for the selection of the best predictors, based upon 1950-1994 developmental data. A range of zero to four predictors were chosen in developing twelve separate regression models, developed separately for each initial calendar month. The predictands to be forecast include the Southern Oscillation (pressure) Index (SOI) and the Niño 1+2, Niño 3, Niño 4 and Niño3.4 SST indices for the equatorial eastern and central Pacific at lead times ranging from zero seasons (0-2 months) through seven seasons (18-20 months). Though hindcast ability is strongly seasonally dependent, substantial improvement is achieved over simple persistence wherein largest gains occur for two to seven season (6 to 21 months) lead times. For example, expected maximum forecast ability for the Niño 3.4 SST region, depending on the initial date, reaches 92, 85, 64, 41, 36, 24, 24 and 28 percent of variance for leads of zero to seven seasons. Comparable maxima of persistence only forecasts explain 92, 77, 50, 17, 6, 14, 21 and 17 percent, respectively. More sophisticated statistical and dynamical forecasting models are encouraged to utilize this ENSO-CLIPER model in place of persistence when assessing whether they have achieved forecasting skill; to this end, real-time results for this model are made available via a Web site.
Landsea, C.W. Comments on "Will greenhouse gas-induced warming over the next 50 years lead to higher frequency and greater intensity of hurricanes?" Tellus A, 49(5):622-623, https://doi.org/10.1034/j.1600-0870.1997.t01-3-00008.x 1997
Landsea, C.W., G.D. Bell, W.M. Gray, and S.B. Goldenberg. The hyperactive 1995 Atlantic hurricane season: A juxtaposition of favorable conditions. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 505-506, 1997
Landsea, C.W., N. Nicholls, W.M. Gray, and L.A. Avilia. Reply to comment by R.W. Wilson on "Downward trend in the frequency of intense Atlantic hurricanes during the past five decades." Geophysical Research Letters, 24(17):2205-2206, https://doi.org/10.1034/j.1600-0870.1997.t01-3-00008.x 1997
Marks, F.D., and P.P. Dodge. Hurricane concentric eyewall characteristics as revealed by airborne Doppler radar analyses. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 102-103, 1997
Mielke, P.W., K.J. Berry, C.W. Landsea, and W.M. Gray. A single-sample estimate of shrinkage in meteorological forecasting. Weather and Forecasting, 12(4):847-858, doi:10.1175/1520-0434(1997)012<0847:ASSEOS>2.0.CO; 1997
An estimator of shrinkage based on information contained in a single sample is presented and the results of a simulation study are reported. The effects of sample size, amount, and severity of nonrepresentative data in the population, inclusion of noninformative predictors, and least (sum of) absolute deviations and least (sum of) squared deviations regression models are examined on the estimator. A single-sample estimator of shrinkage based on drop-one cross-validation is shown to be highly accurate under a wide variety of research conditions.
Murillo, S.T., S.H. Houston, and M.D. Powell. Composites of surface marine observations for hurricanes during 1975-1996. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 78-79, 1997
Ooyama, K.V. Footnotes to "conceptual evolution." Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 13-18, 1997
Ooyama, K.V. The semi-implicit integration of a nested spectral model and the result of tests in squall-line simulation. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 531-532, 1997
Parrish, J.R., and M.L. Black. The NOAA G-IV and the tropical cyclone environment. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 123-124, 1997
Powell, M.D., and S.H. Houston. Surface wind fields of 1995 Hurricanes Erin, Opal, Luis, Marilyn, and Roxanne at landfall. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 90-91, 1997
Samsury, C.E., M.L. Black, P.P. Dodge, and R.E. Orville. Utilization of airborne and NEXRAD data in the analysis of cloud-to-ground lightning in 1995 and 1996 tropical cyclones. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 125-126, 1997
Shapiro, L.J., and J.L. Franklin. Potential vorticity and hurricane motion. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 156-157, 1997
Song, C.G., K.M. Lee, S.D. Lee, S.J. Nahm, D.K. Lee, J.M. Lewis, and R.W. Jones. The parallelization of adjoint data assimilation method for the barotropic tropical cyclone forecast model. Journal of the Korean Meteorological Society, 33(1):111-125, 1997
The adjoint assimilation method has been used to improve the initial conditions for forecasting, but the method requires formidable computation time. In this paper, we speed up the adjoint assimilation procedure, developed by DeMaria and Jones, using a parallelized block cyclic reduction method which solves the Poisson equation. Our study indicates that variational assimilation improves the track forecast of Hurricane Elena for the 24-72 h period, but shows little or no improvement in the 12-24 h period. These results are nearly the complement of DeMaria and Jones' result, which indicated improvements out to about 48 h but were detrimental beyond 48 h.
Willis, P.T., P.P. Dodge, F.D. Marks, D. Smith, and D. Churchill. Evaluation of the accuracy of the NEXRAD radar rainfall estimates in tropical summer convective rainfall over the Everglades/Florida Bay. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 679-680, 1997
Willoughby, H.E. More about hurricane eye thermodynamics. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, May 19-23, 1997. American Meteorological Society, Boston, 96-97, 1997
1996
Black, M.L., R.W. Burpee, and F.D. Marks. Vertical motion characteristics of tropical cyclones determined with airborne Doppler radial velocities. Journal of the Atmospheric Sciences, 53(13):1887-1909, doi:10.1175/1520-0469(1996)053<1887:VMCOTC>2.0.CO; 1996
Vertical motions in seven Atlantic hurricanes are determined from data recorded by Doppler radar on research aircraft. The database consists of Doppler velocities and reflectivities from vertically pointing radar rays collected along radial flight legs through hurricane centers. The vertical motions are estimated throughout the depth of the troposphere from the Doppler velocities and bulk estimates of particle fallspeeds. Portions of the flight tracks are subjectively divided into eyewall, rainband, stratiform, and "other" regions. Characteristics of the vertical velocity and radar structure are described as a function of altitude for the entire data set and each of the four regions. In all of the regions, more than 70% of the vertical velocities range from -2 to 2 m s-1. The broadest distribution of vertical motion is in the eyewall region where ~5% of the vertical motions are >5 m s-1. Averaged over the entire data set, the mean vertical velocity is upward at all altitudes. Mean downward motion occurs only in the lower troposphere of the stratiform region. Significant vertical variations in the mean profiles of vertical velocity and reflectivity are discussed and related to microphysical processes. In the lower and middle troposphere, the characteristics of the Doppler-derived vertical motions are similar to those described in an earlier study using flight-level vertical velocities, even though the horizontal resolution of the Doppler data is ~750 m compared to ~125 m from the in-situ flight-level measurements. The Doppler data are available at higher altitudes than those reached by turboprop aircraft and provide information on vertical as well as horizontal variations. In a vertical plane along the radial flight tracks, Doppler up- and downdrafts are defined at each 300-m altitude interval as vertical velocities whose absolute values continuously exceed 1.5 m s-1, with at least one speed having an absolute value greater than 3.0 m s-1. The properties of the Doppler drafts are lognormally distributed. In each of the regions, updrafts outnumber downdrafts by at least a factor of 2 and updrafts are wider and stronger than downdrafts. Updrafts in the eyewall slope radially outward with height and are significantly correlated over larger radial and vertical extents than in the other three regions. If the downwind (tangential) slope with height of updrafts varies little among the regions, updrafts capable of transporting air with relatively large moist static energy from the boundary layer to the upper troposphere are primarily in the eyewall region. Downdrafts affect a smaller vertical and horizontal area than updrafts and have no apparent radial slope. The total upward or downward mass flux is defined as the flux produced by all of the upward or downward Doppler vertical velocities. The maximum upward mass flux in all but the "other" region is near 1-km altitude, an indication that boundary-layer convergence is efficient in producing upward motion. Above the sea surface, the downward mass flux decreases with altitude. At every altitude, the total net mass flux is upward, except for the lower troposphere in the stratiform region where it is downward. Doppler-derived up- and downdrafts are a subset of the vertical velocity field that occupy small fractions of the total area, yet they contribute a substantial fraction to the total mass flux. In the eyewall and rainband regions, for example, the Doppler updrafts cover less than 30% of the area but are responsible for >75% and >50% to the total upward mass flux, respectively. The Doppler downdrafts typically encompass less than 10% of the area yet provide ~50% of the total downward mass flux in the eyewall and ~20% of the total downward flux in the rainband, stratiform, and "other" regions.
Burpee, R.W. Hurricanes. In Macmillan Encyclopedia of Earth Science, Volume 1, E.J. Dasch (ed.). Macmillan Reference USA, New York, 467-471, 1996
Burpee, R.W., J.L. Franklin, S.J. Lord, R.E. Tuleya, and S.D. Aberson. The impact of Omega dropwindsondes on operational hurricane track forecast models. Bulletin of the American Meteorological Society, 77(5):925-933, doi:10.1175/1520-0477(1996)0772.0.CO;2 1996
Since 1982, the Hurricane Research Division (HRD) has conducted a series of experiments with research aircraft to enhance the number of observations in the environment and the core of hurricanes threatening the United States. During these experiments, the National Oceanic and Atmospheric Administration WP-3D aircraft crews release Omega dropwindsondes (ODWs) at 15-20 min intervals along the flight track to obtain profiles of wind, temperature, and humidity between flight level and the sea surface. Data from the ODWs are transmitted back to the aircraft and then sent via satellite to the Tropical Prediction Center and the National Centers for Environmental Prediction (NCEP), where the observations become part of the operational database. This paper tests the hypothesis that additional observations improve the objective track forecast models that provide operational guidance to the hurricane forecasters. The testing evaluates differences in forecast tracks from models run with and without the ODW data in a research mode at HRD, NCEP, and the Geophysical Fluid Dynamics Laboratory. The middle- and lower-tropospheric ODW data produce statistically significant reductions in 12-60 h mean forecast errors. The error reductions, which range from 16% to 30%, are at least as large as the accumulated improvement in operational forecasts achieved over the last 20-25 years. This breakthrough provides strong experimental evidence that more comprehensive observations in the hurricane environment and core will lead to immediate improvements in operational forecast guidance.
DeMaria, M. A history of hurricane forecasting for the Atlantic basin, 1920-1995. In Historical Essays on Meteorology, 1919-1995, J.R. Fleming (ed.). American Meteorological Society, Boston, 263-305, 1996
The history of hurricane forecasting for the Atlantic basin from 1920-1995 is reviewed. The focus is on the forecast problem of estimating storm tracks and intensities, although other aspects of the hurricane warning process are mentioned when appropriate. Technological advances including in the implementation of the upper-air network in the late 1930s, the establishment of routine aircraft reconnaissance in the 1940s, the advent of numerical weather prediction beginning in the 1950s, and the availability of satellite observations starting in the 1960s, and their impact on hurricane forecasting are reviewed. Organizational changes in the hurricane forecast offices and the interaction between the research and forecast communities are also described. Specific forecast cases are presented to illustrate the improvements and limitations of hurricane forecasting. The outlook for the future is also discussed.
DeMaria, M. The effect of vertical shear on tropical cyclone intensity change. Journal of the Atmospheric Sciences, 53(14):2076-2088, doi:10.1175/1520-0469(1996)053<2076:TEOVSO>2.0.CO; 1996
The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of "ventilation" where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-level diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development. Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of vortex circulation. These studies have also shown that there is an adiabatic process which acts to reduce the vertical tilt of a vortex. The effectiveness of the adiabatic process depends on the Rossby penetration depth, which increases with latitude, horizontal scale, and vortex amplitude. Large-scale analyses from the 1989-1994 Atlantic hurricane seasons are used to show that high-latitude, large, and intense tropical cyclones tend to be less sensitive to the effect of vertical shear than low-latitude, small, and weak storms.
Franklin, J.L., S.E. Feuer, J. Kaplan, and S.D. Aberson. Tropical cyclone motion and surrounding flow relationships: Searching for beta gyres in Omega dropwindsonde datasets. Monthly Weather Review, 124(1):64-84, doi:10.1175/1520-0493(1996)124<0064:TCMASF>2.0.CO; 1996
In 1982, the National Oceanic and Atmospheric Admininstration's Hurricane Research Division began a series of experiments to collect Omega dropwindsonde (ODW) observations within about 1000 km of the center of tropical cyclones. By 1992, 16 ODW datasets had been collected in ten Atlantic basin hurricanes and tropical storms. Objective wind analyses for each dataset, at ten levels from 100 mb to the surface, have been produced using a consistent set of analysis parameters. The objective analyses, which resolve synoptic-scale features in the storm environment with an accuracy and confidence unattainable from routine operational analyses, have been used to examine relationships between a tropical cyclone's motion and its surrounding synoptic-scale flow. Tropical cyclone motion is found to be consistent with barotropic steering of the vortex by the surrounding flow within 3° latitude (333 km) of the cyclone center. At this radius, the surrounding deep-layer mean flow explains over 90% of the variance in vortex motion. The analyses show vorticity asymmetries that strongly resemble the beta gyres common to barotropic models, although other synoptic features in the environment make isolation of these gyres from the wind fields difficult. A reasonably strong relationship is found between the motion of the vortex (relative to its large scale surrounding flow) and the absolute vorticity gradient of the vortex environment.
Goldenberg, S.B. Are we seeing the beginning of a long-term upturn in Atlantic basin major activity? Catastrophe Reinsurance: Predictions and Protections, New York City, NY, November 7-8, 1996. American Conference Institute, section 3, 33 pp., 1996
Goldenberg, S.B. Where we are at in forecasting seasonal Atlantic basin tropical cyclone activity. Final Report, Expert Meeting on Public Weather Services and Hurricane Disaster Preparedness, Port-of-Spain, Trinidad and Tobago, December 11-15, 1995. South African Weather Bureau, 5 pp., 1996
Most of the tropical cyclones in the North Atlantic basin (including the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico) form from easterly (African) wave disturbances. Although the number of easterly waves in the tropical Atlantic tends to be fairly steady from year to year, the fraction of these that develop into tropical cyclones exhibits substantial interannual variability. Not only the number, but also the strength and location of the Atlantic basin tropical cyclones vary greatly from year to year. The Atlantic basin, unlike the east and west Pacific basins, in the mean, is not particularly favorable for tropical cyclone development. Year-to-year changes in the large-scale environment have a substantial influence on the overall activity of each Atlantic hurricane season. The search for conditions that result in the development or nondevelopment of the waves into tropical cyclones has been the subject of numerous studies, some of which have attempted to relate the variability to fluctuations in the tropical and global climate. Some of the climatic indicators that have been associated with fluctuations in Atlantic basin hurricane activity include sea-surface temperature (SST) anomalies for the equatorial central and eastern Pacific (i.e., El Niño activity), SSTs for the tropical Atlantic, rainfall variations over west Africa, the stratospheric Quasi-Biennial Oscillation (QBO), and vertical wind shear over the tropical Atlantic. Various studies have examined some of these indicators in an attempt to establish the actual physical mechanisms responsible for the associations with tropical cyclone activity. Other work has used selected parameters to predict Atlantic basin activity as much as 6-11 months in advance. Information will be presented describing the various parameters and their possible physical association with Atlantic hurricane activity. Current methodology used by other investigators to make seasonal predictions of activity and a discussion of their past performance will be presented with a special emphasis on implications for the Caribbean. The extended-range forecast for the 1996 Atlantic seasonal hurricane activity and Wverifications for the 1995 seasonal forecast will be discussed. The strengths and weaknesses of the current state of seasonal forecasting will be presented. The implications of fluctuations in hurricane activity on the multi-decadal scale will also be discussed.
Goldenberg, S.B., and L.J. Shapiro. Physical mechanisms for the association of El Niño and west African rainfall with Atlantic major hurricane activity. Journal of Climate, 9(6):1169-1187, doi:10.1175/1520-0442(1996)009<1169:PMFTAO>2.0.CO; 1996
Physical mechanisms responsible for the contemporaneous association, shown in earlier studies, of North Atlantic basin major hurricane (MH) activity with western Sahelian monsoon rainfall and an equatorial eastern Pacific sea surface temperature index of El Niño are examined, using correlations with 200- and 700-mb level wind data for the period 1968-1992. The use of partial correlations isolates some of the relationships associated with the various parameters. The results support previous suggestions that the upper- and lower-level winds over the region in the basin between ~10°N and 20°N where most MHs begin developing and critical determinants of the MH activity in each hurricane season. In particular, interannual fluctuations in the winds that produce changes in the magnitude of vertical shear are one of the most important factors, with reduced shear being associated with increased activity and stronger shear with decreased activity. The results show that most of these critical wind fluctuations are explained by their relationship to the SST and rainfall fluctuations. Results confirm previous findings that positive (warm) eastern Pacific SST and negative (drought) Sahelian rainfall anomalies are associated with suppressed Atlantic basin tropical cyclone activity through an equatorially confined near-zonal circulation with upper-level westerlies and lower-level easterlies that act to increase the climatological westerly vertical shear in the main development region. SST and rainfall anomalies of the opposite sense are related to MH activity through a zonal circulation with upper-level easterly and lower-level westerly wind anomalies that act to cancel out some of the climatological westerly vertical shear. The results also show that changes in vertical shear to the north of the main development region are unrelated to, or possibly even out of phase with, changes in the development region, providing a possible physical explanation for the observations from recent studies of the out-of-phase relationship of interannual fluctuations in MH activity in the region poleward of ~25°N with fluctuations in activity to the south. The interannual variability of MH activity explained by Sahel rainfall is almost three times that explained by the eastern Pacific SSTs. It is demonstrated that a likely reason for this result is that SST-associated vertical shears are more equatorially confined, so that the changes in shear in the main development region have a stronger association with the rainfall than with the SSTs.
Houston, S.H., W.A. Schaffer, M.D. Powell, and J. Chen. Incorporating HRD surface wind fields into the SLOSH model. Preprints, Conference on Coastal Oceanic and Atmospheric Prediction, Atlanta, GA, January 28-February 2, 1996. American Meteorological Society, Boston, 265-267, 1996
Landsea, C.W., N. Nicholls, W.M. Gray, and L.A. Avila. Downward trends in the frequency of intense Atlantic hurricanes during the past five decades. Geophysical Research Letters, 23(13):1697-1700, https://doi.org/10.1029/96GL01029 1996
There is concern that the enhanced greenhouse effect may be affecting extreme weather events such as tropical cyclones. The North Atlantic basin offers a reliable, long-term record of tropical cyclone activity, though it may not be representative of tropical cyclones throughout the rest of the tropics. The most recent years of 1991 through 1994 have experienced the quietest cyclone activity on record in terms of frequency of tropical storms, hurricanes, and intense hurricanes. This was followed by the 1995 hurricane season, one of the busiest in the past 50 years. Despite 1995's activity, a long-term (five decade) downward trend continues to be evident primarily in the frequency of intense hurricanes. In addition, the mean maximum intensity (i.e., averaged over allcyclones in a season) has decreased, while the maximum intensity attained by the strongest hurricane each year has not shown a significant change.
Mielke, P.W., K.J. Berry, C.W. Landsea, and W.M. Gray. Artificial skill and validation in weather forecasting. Weather and Forecasting, 11(2):153-169, doi:10.1175/1520-0434(1996)011<0153:ASAVIM>2.0.CO; 1996
The results of a simulation study of multiple regression prediction models for meteorological forecasting are reported. The effects of sample size, amount, and severity of nonrepresentative data in the population, inclusion of noninformative predictors, and least (sum of) absolute deviations (LAD) and least (sum of) squared deviations (LSD) regression models are examined on five populations constructed from meteorological data. Artificial skill is shown to be a product of small sample size, LSD regression, and nonrepresentative data. Validation of sample results is examined, and LAD regression is found to be superior to LSD regression when sample size is small and nonrepresentative data are present.
Nystuen, J.A., J.R. Proni, P.G. Black, and J.C. Wilkerson. A comparison of automatic rain gauges. Journal of Atmospheric and Oceanic Technology, 13(1):62-73, doi:10.1175/1520-0426(1996)013<0062:ACOARG>2.0.CO; 1996
Automatic rain gauge systems are required to collect rainfall data at remote locations, especially oceanic sites where logistics prevent regular visits. Rainfall data from six different types of automatic rain gauge systems have been collected for a set of summertime subtropical rain events and for a set of wintertime rain events at Miami, Florida. The rain gauge systems include three types of collection gauges: weighing, capacitance, and tipping bucket; two gauges that inherently measure rainfall rate: optical scintillation and underwater acoustical inversion; and one gauge that detects individual raindrops: the disdrometer. All of these measurement techniques perform well; that is, they produce rainfall estimates that are highly correlated to one another. However, each method has limitations. The collection gauges are affected by flow irregularities between the catchment basin and the measurement chambers. This affects the accuracy of rainfall-rate measurements from these instruments, especially at low rainfall rates. In the case of the capacitance gauge, errors in 1-min rainfall rates can exceed +10 mm h-1. The rainfall rate gauges showed more scatter than the collection gauges for rainfall rates over 5 mm h-1, and the scatter was relatively independent of rainfall rate. Changes in drop size distribution within an event could not be used to explain the scatter observed in the optical rain gauge data. The acoustical inversion method can be used to measure the drop size distribution, allowing rainfall classification and estimation of other rain parameters, for example, reflectivity or liquid water content, in addition to rainfall rate. The acoustical inversion method has the advantage of an extremely large catchment area, resulting in very high time resolution. The disdrometer showed a large scatter relative to the other rain gauge systems for low rainfall rates. This is consistent with the small catchment area for the disdrometer system.
Powell, M.D., and S.H. Houston. Hurricane Andrew's wind field at landfall in south Florida. Part II: Surface wind fields and potential real-time applications. Weather and Forecasting, 11(3):329-349, doi:10.1175/1520-0434(1996)0112.0.CO;2 1996
All available wind data associated with Hurricane Andrew's passage were analyzed for periods corresponding to landfall south of Miami and emergence from southwest Florida. At landfall in southeast Florida, maximum sustained (1 min) surface wind speeds (VM1) reached 62 m s-1 in the northern eyewall over land; by the time Andrew exited the Florida peninsula, the peak value of VM1 over land decreased to 44 m s-1. Radar reflectivity observations from Tampa and Melbourne could not support an obvious correlation of convective cell development with coastal convergence during landfall on the southeast coast. On the southwest coast, however, convective cell development in the southern eyewall was supported by a coastal convergence maximum. Comparison of the wind swath with two independent Fujita-scale damage maps indicated that peak swath speeds compared well with speed equivalents in the worst damaged areas but were higher than equivalents in moderately damaged areas. Comparison of the analysis maximum wind swath with an engineering survey of damaged homes suggests that home sexposed to a wide range of wind directions while subjected to high wind speeds suffered the most damage. Potential real-time applications of wind field products include warning dissemination, emergency management, storm surge and wave forecasting, and wind engineering. Development of damage assessment models for disaster mitigation is addressed from the viewpoint of an electrical utility.
Powell, M.D., S.H. Houston, and T.A. Reinhold. Hurricane Andrew's landfall in south Florida. Part I: Standardizing measurements for documentation of surface wind fields. Weather and Forecasting, 11(3):304-328, doi:10.1175/1520-0434(1996)011<0304:HALISF>2.0.CO; 1996
Hurricane Andrew's landfall in south Florida left a swath of destruction, including many failed anemometer recording systems. Extreme destruction led to exaggerated claims of the range of wind speeds that caused such damage. The authors accumulated all available data from surface platforms at heights ranging from 2 to 60 m and reconnaissance aircraft at altitudes near 3 km. Several procedures were used to represent the various types of wind measurements in a common framework for exposure, measurement height, and averaging period. This set of procedures allowed documentation of Andrew's winds in a manner understandable to both meteorologists and wind engineers. The procedures are accurate to ±10% for marine and land observing platforms, and boundary layer model adjustments of flight-level winds to the surface compare within 20% of the nearest surface measurements. Failure to implement the adjustment procedures may lead to errors of 15%-40%. Quality control of the data is discussed, including treatment of peak wind observations and determination of the radius of maximum winds at the surface.
Roux, F., and F.D. Marks. Extended velocity track display (EVTD): An improved processing method for Doppler radar observations of tropical cyclones. Journal of Atmospheric and Oceanic Technology, 13(4):875-899, doi:10.1175/1520-0426(1996)0132.0.CO;2 1996
We present an improved version of the Velocity Track Display (VTD) method, proposed by Lee et al. (1994) to deduce the primary vortex circulation in hurricanes from airborne Doppler radar data obtained during straight-line legs through the storm center. VTD allows the derivation of one projection of the mean horizontal wind, the wave number 0, 1, and 2 components of the tangential wind and one projection of the radial wind, in a series of concentric rings centered on the storm circulation center. The extended VTD (EVTD) algorithm determines additional information through a combination of data collected during successive legs: the Cartesian components of the mean horizontal wind, the wave number 0, 1, and 2 components of the tangential wind, and the wave number 0 and 1 components of the radial wind. Application of EVTD to airborne Doppler data collected on 17 September 1989 in Hurricane Hugo is discussed. Comparisons between the EVTD-derived winds, the flight-level measurements, and winds deduced from "pseudo-dual Doppler" analyses show qualitatively good agreement. These results reveal the asymmetric structure of the storm and show that it was in a deepening stage, with increasing tangential wind, inflow, and upward velocity. Further applications are finally discussed.
Shapiro, L.J. The motion of Hurricane Gloria: A potential vorticity diagnosis. Monthly Weather Review, 124(11):2497-2508, doi:10.1175/1520-0493(1996)124<2497:TMOHGA>2.0.CO; 1996
Multilevel, multinested analyses of Hurricane Gloria of 1985 are the most comprehensive kinematic data set yet developed for a single hurricane. A piecewise inversion technique is used with these analyses and the nonlinear balance equation to deduce the three-dimensional distribution of potential vorticity (PV) that contributed to the deep-layer mean (DLM) flow that steered Hurricane Gloria toward the northwest. The background state is taken to be the azimuthally averaged winds in balance with a geopotential distribution on an f plane. Advantage is taken of the near-linearity of the weak asymmetries near the hurricane's core and of PV in the environment. Thus, ad hoc aspects of the linearization required by other investigators are effectively eliminated. Removal of the hurricane vortex and the use of a climatological mean background state are avoided as well. The insensitivity of the results to the imposed lateral boundary condition is also demonstrated. Wind anomalies attributable to pieces of anomalous PV restricted to cylinders of different radii centered on the hurricane are evaluated. The DLM wind that steered Gloria to the northwest is primarily attributable to PV anomalies confined within a cylinder of radius 1000 km and levels 500 mb and above, including positive anomalies associated with a cold low over Cuba. The vector difference between the hurricane's observed motion and the DLM wind at Gloria's center attributable to these PV anomalies is 1.0 m s-1, explaining more than five-sixths of the hurricane's 6.2 m s-1 motion. Implications for measurements required to establish short-term changes of the environmental steering flow are considered. Difficulties in the interpretation of results are discussed for PV anomalies that are confined to noncircular regions; the implication for other studies is considered as well.
Willoughby, H.E., and P.G. Black. Hurricane Andrew in Florida: Dynamics of a disaster. Bulletin of the American Meteorological Society, 77(3):543-652, doi:10.1175/1520-0477(1996)077<0543:HAIFDO>2.0.CO; 1996
Four meteorological factors aggravated the devastation when Hurricane Andrew struck south Florida: completed replacement of the original eyewall by an outer, concentric eyewall while Andrew was still at sea; storm translation so fast that the eye crossed the populated coastline before the influence of land could weaken it appreciably; extreme wind speed, 82 m s-1 winds measured by aircraft flying at 2.5 km; and formation of an intense, but nontornadic, convective vortex in the eyewall at the time of landfall. Although Andrew weakened for 12 h during the eyewall replacement, it contained vigorous convection and was reintensifying rapidly as it passed onshore. The Gulf Stream just offshore was warm enough to support a sea level pressure 20-30 hPa lower than the 922 hPa attained, but Andrew hit land before it could reach this potential. The difficult-to-predict mesoscale and vortex-scale phenomena determined the course of events on that windy morning, not a long-term trend toward worse hurricanes.
1995
Aberson, S.D., S.J. Lord, M. DeMaria, and M.S. Tracton. Short-range ensemble forecasting of hurricane tracks. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 494-496, 1995
Atlas, D., P.T. Willis, and F.D. Marks. The effect of convective updrafts and downdrafts on reflectivity-rain rate relations and water budgets. Preprints, 27th Conference on Radar Meteorology, Vail, CO, October 9-13, 1995. American Meteorological Society, Boston, 19-22, 1995
Bakker, C. Development of object-oriented software for real-time presentations and quality control of wind data in hurricanes. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 86-88, 1995
Black, M.L., R.W. Burpee, and F.D. Marks. Two-dimensional spatial structure of hurricane updrafts and downdrafts. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 605-607, 1995
Black, M.L., R.W. Burpee, and F.D. Marks. Vertical motion asymmetries in the hurricane eyewall. Preprints, 27th Conference on Radar Meteorology, Vail, CO, October 9-13, 1995. American Meteorological Society, Boston, 574-576, 1995
Black, P.G., and G.J. Holland. The boundary layer of Tropical Cyclone Kerry (1979). Monthly Weather Review, 123(7):2007-2028, doi:10.1175/1520-0493(1995)123<2007:TBLOTC>2.0.CO; 1995
The boundary layer structure of Tropical Cyclone Kerry (1979) is investigated using composite analysis of research aircraft, surface ship, and automatic weather station observations. The boundary layer was moist, convective, and strongly confluent to the east of the tropical cyclone center but was dry, subsident, and diffluent to the west. The vertical momentum transport in the eastern convective sector of Kerry was around two to three times the surface frictional dissipation. In contrast, the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low-level jet, which underwent a marked diurnal oscillation. Three mechanisms appear to have contributed to the observed asymmetry: (1) a general, zonal distortion arose from cyclonic rotation across a gradient of earth vorticity; (2) a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the lower and midtroposphere; and (3) the western sector boundary layer was modified by an upstream cold tongue generated by the tropical cyclone passage. The authors present evidence that substantial drying also resulted from shear-induced mixing of the subsident environmental air in the region of the low-level jet. Thermal boundary layer budgets are derived using both a general mixing theory approach and direct flux calculations from aircraft reconnaissance data. Use of actual sea surface temperature fields are essential. The surface flux estimates of latent heat are near the average of previous studies, but the sensible heat fluxes are downward into the ocean. Since horizontal advection also cooled the boundary layer, the thermal structure was maintained by downward fluxes of sensible heat from the top of the boundary layer of around 100 W m-2. We conclude that the pattern of oceanic cooling directly determines the pattern of vertical air-sea and advective sensible heat fluxes and indirectly determines the pattern of latent heat fluxes through forcing of PBL drying at the downwind end of the SST cold pool. It further enhances the inward penetration and negative feedback resulting from an easterly trade wind surge associated with a mobile trough in the westerlies.
Black, P.G., and L.K. Shay. Observed sea surface temperature variability in tropical cyclones: Implications for structure and intensity change. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 603-604, 1995
Black, P.G., J.R. Proni, J.C. Wilkerson, and C.E. Samsury. Classification of convective and stratiform regions in oceanic tropical and subtropical mesoscale convective systems using underwater acoustic methods. Preprints, 27th Conference on Radar Meteorology, Vail, CO, October 9-13, 1995. American Meteorological Society, Boston, 237-239, 1995
Black, P.G., R. McIntosh, C. Swift, J. Carswell, K. St. Germain, I. Popstefanija, and M. Goodberlet. Ocean surface wind, stress, and rain rate measurements in tropical cyclones from concurrent airborne microwave scatterometer and radiometer observations. Preprints, 27th Conference on Radar Meteorology, Vail, CO, October 9-13, 1995. American Meteorological Society, Boston, 623-625, 1995
Black, R.A., H.B. Bluestein, and M.L. Black. Unusually strong vertical motions in a Caribbean hurricane. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 260-262, 1995
Burpee, R.W., J.L. Franklin, S.J. Lord, and R.E. Tuleya. The performance of hurricane track guidance models with and without Omega dropwindsondes. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 6-8, 1995
Churchill, D.D., S.H. Houston, and N.A. Bond. The Daytona Beach wave of 3-4 July 1992: A shallow-water gravity wave forced by a propagating squall line. Bulletin of the American Meteorological Society, 76(1):21-32, doi:10.1175/1520-0477(1995)076<0021:TDBWOJ>2.0.CO; 1995
An unexpected runup of the ocean along Daytona Beach, Florida, on 3-4 July 1992 was associated with at least one large ocean wave. The wave, which reached a height of about 3 m above normal tide, injured 75 people and damaged property along Daytona Beach. Analyses of meteorological and oceanographic observations are consistent with the hypothesis that a squall line generated a long water wave. The critical evidence is that the propagation speed of the squall line matched the shallow-water wave speed that prevailed along the direction of motion of the squall line. The squall line exerted force on the ocean for at least 3 h. The issues of recurrence and public safety are discussed.
Darling, R.W.R., S.H. Houston, and H.E. Willoughby. Parametric models of hurricane surface winds for storm surge calculations. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 454-456, 1995
DeMaria, M. Another look at the effect of vertical shear on tropical cyclone intensity change. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 323-325, 1995
DeMaria, M. Evaluation of hydrostatic height-coordinate formulation of the primitive equations for atmospheric modeling. Monthly Weather Review, 123(12):3576-3589, doi:10.1175/1520-0493(1995)123<3576:EOAHHC>2.0.CO; 1995
The hydrostatic form of the primitive equations described by Ooyama is evaluated by comparing nonhydrostatic and hydrostatic integrations of a dry axisymmetric model with a specified entropy (heat) source. In this formulation, pressure is a diagnostic variable, so that the hydrostatic approximation can be included simply by replacing the vertical momentum equation with a diagnostic vertical velocity equation. This diagnostic equation is a one-dimensional (height) second-order elliptic equation that can be solved using a direct method. Results show that hydrostatic solutions are very sensitive to the accuracy of the method used to solve the diagnostic vertical velocity equation. However, this sensitivity can be eliminated by adding an extra term to the diagnostic equation that ensures that the solution does not drift away from hydrostatic balance due to numerical approximation. When the extra term is added, this formulation of the primitive equations allows for the design of a numerical model in height coordinates that can be used in hydrostatic and nonhydrostatic regimes.
Dodge, P.P., F.D. Marks, J.F. Gamache, J.S. Griffin, and N.F. Griffin. EVTD radar analyses of the inner core of Hurricane Olivia (1994). Preprints, 27th Conference on Radar Meteorology, Vail, CO, October 9-13, 1995. American Meteorological Society, Boston, 299-301, 1995
Dodge, P.P., F.D. Marks, J.F. Gamache, J.S. Griffin, and N.F. Griffin. The evolution of the inner core of Hurricane Olivia (1994) from EVTD Doppler radar analyses. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 463-465, 1995
Feuer, S.E., and J. Kaplan. Tropical cyclone intensity change and environmental kinematic structure in Omega dropwindsonde data sets. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston,362-364, 1995
Finley, S.V., P.J. Fitzpatrick, J.A. Knaff, and C.W. Landsea. A systematic bias in the Aviation model's forecast of the Atlantic TUTT: Implications for tropical cyclone forecasting. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 238-240, 1995
Fitzpatrick, P.J., J.A. Knaff, C.W. Landsea, and S.V. Finley. Documentation of a systematic bias in the Aviation model's forecast of the Atlantic tropical upper-tropospheric trough: Implications for tropical cyclone forecasting. Weather and Forecasting, 10(2):433-446, doi:10.1175/1520-0434(1995)010<0433:DOASBI>2.0.CO; 1995
This study uncovers what appears to be a systematic bias in the National Meteorological Center's Aviation (AVN) model at 200 mb over the Caribbean Sea. In general, the 48 h forecast in the vicinity of the Tropical Upper Tropospheric Trough (TUTT) underpredicts the magnitude of the westerly 200-mb winds on the order of 5-10 m s-1. This unrealistic weakening of the TUTT and associated cold lows by the AVN model results in erroneous values of the vertical (850-200 mb) wind shear. These systematic errors are in the same order of magnitude as the minimum shear threshold for tropical cyclone genesis and development. Thus, 48-h tropical cyclone formation and intensity forecasts based upon the AVN model are often incorrect in the vicinity of the TUTT. Knowing the correct future upper wind regime is also crucial for track forecasting of more intense tropical cyclones, especially in cases of recurvature. It is shown that simple persistence or climatology of the 200-mb winds south of a TUTT axis is superior to the AVN model's 48-h forecast. Until this bias in the AVN is successfully removed, the tropical cyclone forecaster for the Atlantic basin should be aware of this systematic error and make subjective changes in his/her forecasts. For 200-mb west winds greater than or equal to 10 m s-1, forecasts based on persistence are best, while for west winds less than 10 m s-1, half climatology and half persistence is the preferable predictor. If the TUTT is weak such that 200-mb easterly winds occur, climatology tends to be the best predictor as it nudges the forecast back to a normal westerly wind regime.
Franklin, J.L. Searching for beta gyres in Omega dropwindsonde data sets. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 55-57, 1995
Gamache, J.F. Preliminary three-dimensional analyses of dual-platform airborne Doppler observations of intense convection in eastern Pacific Hurricane Olivia (1994). Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 254-256, 1995
Gamache, J.F., F.D. Marks, and F. Roux. Comparison of three airborne Doppler sampling techniques with airborne in-situ wind observations in Hurricane Gustav (1990). Journal of Atmospheric and Oceanic Technology, 12(1):171-181, doi:10.1175/1520-0426(1995)0122.0.CO;2 1995
Three different airborne Doppler radar sampling strategies were tested in Hurricane Gustav (1990) on 29 August 1990. The two new strategies were the fore-aft scanning technique (FAST) and airborne dual-platform Doppler sampling. FAST employs radar scans in cones pointing alternately fore and aft of the vertical plane that is perpendicular to the flight track. The airborne dual-platform sampling uses two Doppler radars, each aboard a separate aircraft. The Doppler radars scan strictly in the vertical plane normal to the flight track. The aircraft fly simultaneously along different, preferably perpendicular, tracks. The third strategy tested in Hurricane Gustav was single-platform sampling, which uses one Doppler radar on one aircraft that flies two consecutive, usually orthogonal, flight tracks. The antenna scans in the plane normal to the flight track. The third technique had been used previously in hurricanes and other disturbed weather. The rms differences between the aircraft in-situ winds and the Doppler winds derived near the aircraft by single-platform sampling, dual-platform sampling, and FAST are found to be 7.8, 5.1, and 2.5 m sec-1, respectively. These results suggest that in hurricanes dual-platform, flat-plane sampling and FAST both enable substantial improvements in the accuracy and temporal resolution of airborne Doppler wind fields over those obtained from single-platform, flat-plane scanning. The FAST results should be applicable to dual-beam sampling, which began in 1991. The actual rms errors of Doppler winds from the flight tracks, at levels well above flight level, and in highly sheared environments may be significantly higher than the above differences.
Goldenberg, S.B., and L.J. Shapiro. A new look at the relationship between El Niño, west African rainfall, and North Atlantic tropical cyclone activity. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 585-587, 1995
Gray, W.M., C.W. Landsea, P.W. Mielke, and K.K. Berry. Extended range forecast of the Atlantic seasonal hurricane activity for 1996. HKMetS Bulletin, 5(2):73-81, 1995
This paper presents details of a 6-11 month extended range seasonal forecast of the tropical cyclone activity likely to occur in the Atlantic Ocean basin during 1996. This forecast is based on a forecast scheme developed previously by the authors with several new modifications. This allows estimates of seasonal Atlantic tropical cyclone activity to be made by late November of the prior year. Our ever-evolving forecast schemes are based on l0-month forward extrapolations of the stratospheric Quasi-Biennial Oscillation (QBO) of equatorial zonal winds, two measures of Western Sahel rainfall through late November 1995, an extended range forecast of El Niño conditions in August to October 1996, an extended range forecast of western Sahel rainfall amount for next summer, the November strength of the northeast Atlantic subtropical ridge, and other forecast parameters from the Pacific Ocean and from the Asia-Australia area. Information obtained through late-November 1995 indicates that 1996 Atlantic hurricane activity is likely to be somewhat below the 1950 to 1995 average with five hurricanes (average 5.7), eight named storms (average 9.3), 40 named storm days (average 46), 20 hurricane days (average 23), two intense (category 3-4-5) hurricanes (average 2.1), five intense hurricane days (average is 4.5) and a hurricane destruction potential (HDP) of 50 (average 68). Collectively, net tropical cyclone activity is expected to be 85% of the long period average. The 1996 season should be much less active than the 1995 season but somewhat more active than the four recent inactive hurricane seasons 1991 through 1994.
Gray, W.M., C.W. Landsea, P.W. Mielke, and K.K. Berry. Summary of the 1995 Atlantic tropical cyclone activity and verification of the authors' seasonal prediction. HKMetS Bulletin, 5(2):54-66, 1995
This paper summarizes the tropical cyclone (TC) activity which occurred in the Atlantic Basin during 1995 and verifies the author's seasonal forecast of this activity which was initially issued on 30 November of last year, with updates on 5 June and 4 August of this year. The 1995 hurricane season was a year of near record hurricane activity. There was a total of 19 named storms (average 9.3) and 11 hurricanes (average 5.7) which persisted for a total of 62 days (average is 23). There were five major (intense) hurricanes of Saffir/Simpson category 3-4-5 (average is 2.1 intense hurricanes) with 11.5 intense storm days (average is 4.5). The seasonal total of named storm days was 121 or 262% of average and net tropical cyclone (NTC) activity was 237% of the average for the last 45 years. This unusually active hurricane season was the result of the concurrence of nearly all the physical factors known to enhance seasonal hurricane activity. This convergence of favorable factors typically occurs about once every 10-15 years. Of all these favorable factors, low values of surface pressure during 1995 were the most important. Our 1995 seasonal forecasts made on 30 November 1994, 5 June 1995, and 4 August 1995 all called for above average hurricane activity though not nearly as much hurricane activity as occurred.
Houston, S.H., and M.D. Powell. Real-time surface wind analyses in support of marine warnings in tropical cyclones. Preprints, Second International Workshop on Wind and Earthquake Engineering for Offshore Coastal Facilities, Berkeley, CA, January 17-19, 1995. University of California at Berkeley, 323-328, 1995
Real-time analyses of tropical cyclone wind observations are generated by the Hurricane Research Division (HRD) on an experimental basis. This paper describes analyses that apply to nearshore and offshore commercial activities affected by tropical cyclones. These products can help determine the extent of tropical storm and hurricane force winds for marine warnings. They can also be used as input to storm surge and wave models, as well as for estimating peak wind gusts associated with tropical cyclones.
Houston, S.H., W.A. Schaffer, M.D. Powell, and J. Chen. Comparisons of SLOSH parametric and HRD analyzed surface wind fields in recent hurricanes. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 619-621, 1995
In recent years, the Hurricane Research Division (HRD) has developed new techniques to analyze the surface wind fields in tropical cyclones based on all available surface wind observations, including aircraft flight-level observations adjusted to the surface. As part of NOAA's Coastal Ocean Program research on coastal hazards, the Techniques Development Laboratory (TDL) and HRD are evaluating wind fields used as input to the National Weather Service's (NWS) Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model through comparisons with HRD's surface wind field analyses. The SLOSH model-computed water levels are used primarily by government and emergency management officials to plan for the evacuation of populations from coastal areas prone to flooding from storm surge produced by tropical cyclones. The model was developed for real-time forecasting of hurricane storm surges on continental shelves, across inland water bodies, and along coastlines. The values used to initiate the SLOSH model are each tropical cyclone's position, size, and intensity. Using these input parameters, a wind field is computed by SLOSH, which is used as the primary forcing mechanism for the oceanographic processes. Previously, the SLOSH model has been tested for hurricane landfalls along the U.S. coastlines and has been found to have an accuracy of ±20% when the hurricane is adequately described. Hurricane wind field cases examined in this study include Hugo (1989) at landfall in South Carolina, Bob (1991) at landfall in New England, Andrew (1992) at landfall in south Florida and Louisiana, as well as the closest approach of Bob (1991) and Emily (1993) to the North Carolina Outer Banks.
Jones, R.W., M. DeMaria, and C.R. Hagelberg. A comparison of data assimilation techniques in barotropic track forecasts of west Pacific typhoons. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 141-142, 1995
Kaplan, J. An examination of the role of large-scale forcing on the rapid intensification of Hurricane Emily (1987). Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 332-334, 1995
Kaplan, J., and M. DeMaria. A simple empirical model for predicting the decay of tropical cyclone winds after landfall. Journal of Applied Meteorology, 34(11):2499-2512, doi:10.1175/1520-0450(1995)034<2499:ASEMFP>2.0.CO; 1995
An empirical model for predicting the maximum wind of landfalling tropical cyclones is developed. The model is based upon the observation that the wind speed decay rate after landfall is proportional to the wind speed. Observations also indicate that the wind speed decays to a small, but nonzero, background wind speed. With these assumptions, the wind speed is determined from a simple two-parameter exponential decay model, which is a function of the wind speed at landfall and the time since landfall. A correction can also be added that accounts for differences between storms that move inland slowly and storms that move inland rapidly. The model parameters are determined from the National Hurricane Center best track intensities of all U.S. landfalling tropical cyclones south of 37°N for the period 1967-1993. Three storms that made landfall in Florida prior to 1967 were also included in the sample. Results show that the two parameter model explains 91% of the variance of the best track intensity changes. When the correction that accounts for variations in the distance inland is added, the model explains 93% of the variance. This model can be used for operational forecasting of the maximum winds of landfalling tropical cyclones. It can also be used to estimate the maximum inland penetration of hurricane force winds (or any wind speed threshold) for a given initial storm intensity. The maximum winds at an inland point will occur for a storm that moves inland perpendicular to the coastline. Under this assumption, the maximum wind at a fixed point becomes a function of the wind speed at landfall and the translational speed of motion. For planning purposes, maps of the maximum inland wind speed can be prepared for various initial storm intensities and speeds of motion. The model can also be applied to the entire wind field of an individual storm to provide a two-dimensional field of the maximum wind during a given storm. Examples of each of these applications are presented.
Landsea, C.W. SHIFOR94: Atlantic tropical cyclone intensity forecasting. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 365-367, 1995
Landsea, C.W., W.M. Gray, P.W. Mielke, and K.J. Berry. June to September rainfall in the African Sahel: Verification of our 1995 forecasts and an extended range forecast for 1996. HKMetS Bulletin, 5(2):66-70, 1995
McAdie, C.J., P.P. Dodge, and S.H. Houston. Mesoscale features of Tropical Storm Beryl (15-16 August 1994) as detected by the WSR-88D. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 320-322, 1995
Montgomery, M.T., and L.J. Shapiro. Generalized Charney-Stern and Fjortof theorems for rapidly rotating vortices. Journal of the Atmospheric Sciences, 52(10):1829-1833, doi:10.1175/1520-0469(1995)052<1829:GCAFTF>2.0.CO; 1995
A generalized Charney-Stern theorem for rapidly rotating (large Rossby number) baroclinic vortices, such as hurricanes, is derived based on the asymmetric balance (AB) approximation. In the absence of dissipative processes, a symmetrically stable baroclinic vortex is shown to be exponentially stable to nonaxisymmetric perturbations if a generalized potential vorticity gradient on theta surfaces remains single signed throughout the vortex. The generalized potential vorticity gradient involves the sum of an interior potential vorticity gradient associated with the symmetric vortex and surface contributions associated with the vertical shear of the tangential wind. The AB stability formulation is then shown to yield Fjortof's theorem as a corollary. In the modern view of shear instabilities the theorems admit simple interpretation. The Charney-Sterm theorem represents a necessary condition for the existence of counterpropagating Rossby waves associated with the radial potential vorticity gradient, while Fjortof's theorem represents a necessary condition for these waves to phase lock and grow in strength. Potential application of these results as well as limitations of the slow-manifold approach are briefly discussed.
Ooyama, K.V. A thermodynamic foundation for modeling the moist atmosphere. Part II: Tests of microphysics in the formation of squall lines. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 219-221, 1995
Powell, M.D., S.H. Houston, and I. Ares. Real-time damage assessment in hurricanes. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 500-502, 1995
Pujals, G., C. Bakker, M.D. Powell, and S.H. Houston. Demonstration of object-oriented software for real-time acquisition and presentation of meteorological fields in hurricanes. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 303-305, 1995
Samsury, C.E., and E.J. Zipser. Secondary wind maxima in hurricanes: Air-flow and relationship to rainbands. Monthly Weather Review, 123(12):3502-3517, doi:10.1175/1520-0493(1995)123<3502:SWMIHA>2.0.CO; 1995
Aircraft flight-level data from 787 radial legs in 20 hurricanes are analyzed to identify the composite kinematic structure in the hurricane eyewall, and especially with secondary horizontal wind maxima (SHWM) that occur outside the eyewall. Similar to previous studies, analysis of the flight-level wind data in the eyewall reveals radial convergence near the radius of maximum wind (RMW), and the highest frequency of updrafts and the largest upward mass transport radially inward of the RMW. More than 20% of the flight legs contain substantial secondary horizontal wind maxima of specified strength and length. The kinematic structure associated with SHWM is similar to that of the hurricane eyewall with radial convergence near the radius of maximum wind and a preferred location for maximum upward motions and upward mass transport just inside the RMW. Statistical analysis confirms the similarity in characteristics between radial and vertical velocities of the eyewall and near the SHWM. In addition, for both the eyewalls and the SHWM, the radial velocity composite results show that the radial mass transport in the planetary boundary layer must be largely confined to the lowest 1000 m. Lower fuselage radar reflectivity data from 13 of the hurricanes are used to assess whether the outer wind maxima are associated with rainbands, and vice versa. In the radial legs with SHWM for which radar data were available, the secondary horizontal wind maximum was frequently associated with a mesoscale reflectivity feature (rainband). In contrast, many rainbands, more than 70%, were without wind maxima. The results from this study show that to some extent an outer eyewall or rainband with SHWM can act as a barrier to inflow to the inner wall. Additionally, it is possible that thermodynamic modification of inflow air may occur as a result of convective-scale vertical motions associated with a rainband. In those cases when an outer rainband encircles the eyewall, it is possible that these factors act together with subsidence to weaken the inner eyewall.
Samsury, C.E., M.L. Black, and R.E. Orville. The relationship of cloud-to-ground lightning with radar reflectivity and vertical velocity in Hurricanes Bob (1991) and Emily (1993). Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 257-259, 1995
Shapiro, L.J., and J.L. Franklin. Potential vorticity in Hurricane Gloria. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 524-525, 1995
Shapiro, L.J., and J.L. Franklin. Potential vorticity in Hurricane Gloria. Monthly Weather Review, 123(5):1465-1475, doi:10.1175/1520-0493(1995)123<1465:PVIHG>2.0.CO;2 1995
Potential vorticity (PV) analyses for Hurricane Gloria of 1985 are derived from nested objective wind analyses of Omega dropwindsonde and airborne Doppler radar data. The analyses resolve eyewall-scale features in the inner vortex core and embed analyses of these features within the larger-scale environment. Since three-dimensional geopotential height fields required for evaluation of PV are not available in the core, they are derived using the balance equation. In the process of deriving the heights, the degree of gradient balance is evaluated. The 500-mb tangential winds in the core, averaged azimuthally on the four cardinal points, are close to gradient balance outside the radius of maximum wind. The resulting depiction of PV is the first presented for a real hurricane. Due to data deficiencies immediately outside the Doppler region, as well as inside the eye, smoothing of the wind data using a filter with a minimum 25-km spatial scale is required to derive a balanced geopotential height distribution consistent with a statically stable vortex. The large-scale PV distribution evidences asymmetries in the middle and upper troposphere that appear to be associated with Gloria's translation to the northwest. Eyewall-scale PV in the core, and PV of the azimuthally-averaged vortex, are also presented.
Willis, P.T., and J. Hallett. The cloud microphysical and electrical characteristics of a stratiform melting layer. Preprints, Conference on Cloud Physics, Dallas, TX, January 15-20, 1995. American Meteorological Society, Boston, 240-245, 1995
Willis, P.T., R.A. Black, F.D. Marks, and D. Baumgardner. Airborne raindrop size distributions in TOGA-COARE. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 431-433, 1995
Willoughby, H.E. Eye thermodynamics. Preprints, 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 24-28, 1995. American Meteorological Society, Boston, 357-358, 1995
Willoughby, H.E. Mature structure and evolution. In Global Perspectives on Tropical Cyclones, R.L. Elsberry (ed.). World Meteorological Organization, Geneva, 21-62, 1995
Willoughby, H.E. Normal-mode initialization of barotropic vortex motion models. Journal of the Atmospheric Sciences, 52(24):4501-4514, doi:10.1175/1520-0469(1995)052<4501:NMIOBV>2.0.CO; 1995
An important limitation of numerical hurricane track forecasts is the difficulty in coaxing the vortex to assume the correct initial motion. Results from a semispectral, barotropic, linear model suggest a remedy. When the model is initialized from axisymmetry and rest in a quiescent environment on a northern hemisphere beta plane, the vortex moves toward the northwest. The asymmetric streamfunction field is a dipole such that flow between the cyclonic and anticyclonic gyres advects the vortex. This asymmetry appears to reflect a free oscillation because the asymmetric structure, and the induced motion, persists for a long time in the absence of forcing. When the beta effect is turned off, the motion continues on an f plane, and the dipole can be rotated and scaled to produce any desired initial motion. In the normal-mode interpretation, a vortex with cyclonic circulation throughout accelerates poleward rapidly because the beta effect forces a neutral mode at zero frequency. A vortex with angular momentum reduced to zero by encirclement of the cyclonic core with an annulus of anticyclonic flow experiences weaker forcing of a mode at the most anticyclonic orbital frequency of the axisymmetric circulation. Although the latter mode has a weak barotropic instability, acceleration along the curving track is slow, so that this vortex is promising for track forecasting. By careful choice of vortex position and the normal-mode asymmetry's amplitude and orientation at some time before the beginning of the forecast calculation, it is possible to "preinitialize" the vortex to pass through a target initial position at the initial time with an arbitrarily chosen initial velocity. In completely cyclonic vortices that have asymptotic decay of the swirling flow with radius, radial wave energy propagation damps the mode at zero frequency. Experimentation with a variety of axisymmetric vortex structures suggest that, with this single qualification, existence of the previously described modes is a general property of barotropic vortices scaled to resemble hurricanes.
Yuter, S.E., R.A. Houze, B.F. Smull, F.D. Marks, J.R. Daugherty, and S.R. Brodzik. TOGA COARE aircraft mission summary images: An electronic atlas. Bulletin of the American Meteorological Society, 76(3):319-328, doi:10.1175/1520-0477(1995)076<0319:TCAMSI>2.0.CO; 1995
An electronic atlas of research aircraft missions in TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment) has been prepared and is available on the Internet via World Wide Web browsers such as Mosaic. These maps are in the form of time sequences of color imagery assembled using the NCAR Zebra software. Initial versions of these maps were prepared in the field at the TOGA COARE Honiara Operations Center to aid in the evaluation of each aircraft mission immediately after it was flown. The maps prepared in the field have been updated, corrected, and remapped at standard scales and with common color schemes. They show the meteorological setting of sampling by all seven aircraft participating in TOGA COARE--the two NOAA WP-3D aircraft, the NCAR Electra, the FIAMS C-340, the UK C-130, and the NASA DC-8 and ER-2--by overlaying flight tracks, GMS satellite infrared data, and NOAA WP-3D airborne radar images. The map sequences are combined with text of scientists' notes and other background information on the research flights to form a summary of each aircraft mission. The resulting aircraft mission summaries are intended as a road map to the COARE aircraft dataset. They indicate where and when data were collected and themeteorological context for those data. As an electronic document, the atlas of aircraft mission summaries is available on demand, and it is dynamic: as further information becomes available, the mission summaries will continue to be added to and updated as appropriate, and new releases will be issued periodically.
1994
Aberson, S.D., and M. DeMaria. Verification of a nested barotropic hurricane track forecast model (VICBAR). Monthly Weather Review, 122(12):2804-2815, doi:10.1175/1520-0493(1994)122<2804:VOANBH>2.0.CO; 1994
A nested analysis and barotropic hurricane track forecast model (VICBAR) was run for tropical cyclone cases in the North Atlantic basin during the 1989-1993 hurricane seasons. VICBAR is compared to the other operational hurricane track forecast models and is shown to perform as well as each of these. VICBAR forecasts are stratified by initial date, intensity, and location to assess the variability of model performance. VICBAR produces the best forecasts for hurricane cases, for cases initiated earliest in the hurricane season, for cases moving the most slowly northward, and for those moving westward. The forecasts with the largest errors are examined to illustrate the limitations of the model and to determine whether these cases can be identified operationally.
Atlas, D., and P.G. Black. The evolution of convective storms from their footprints on the sea as viewed by synthetic aperture radar from space. Bulletin of the American Meteorological Society, 75(7):1183-1190, doi:10.1175/1520-0477(1994)075<1183:TEOCSF>2.0.CO; 1994
SEASAT synthetic aperture radar (SAR) echoes from the sea have previously been shown to be the result of rain and winds produced by convective storms; rain damps the surface waves and causes echo-free holes, while the diverging winds associated with the downdraft generate waves and associated echoes surrounding the holes. Gust fronts are also evident. Such a snapshot from 8 July 1978 has been examined in conjunction with ground-based radar. This leads to the conclusion that the SAR storm footprints resulted from storm processes that occurred up to an hour or more prior to the snapshot. A sequence of events is discerned from the SAR imagery in which new cell growth is triggered in between the converging outflows of two pre-existing cells. In turn, the new cell generates a mini-squall line along its expanding gust front. While such phenomena are well known over land, the spaceborne SAR now allows important inferences to be made about the nature and frequency of convective storms over the oceans. The storm effects on the sea have significant implications for spaceborne wind scatterometry and rainfall measurements. Some of the findings herein remain speculative because of the great distance to the Miami weather radar--the only source of corroborative data.
Black, R.A., H.B. Bluestein, and M.L. Black. Unusually strong vertical motions in a Caribbean hurricane. Monthly Weather Review, 122(12):2722-2739, doi:10.1175/1520-0493(1994)122<2722:USVMIA>2.0.CO; 1994
Unusually strong updrafts and downdrafts in the eyewall of Hurricane Emily (1987) during its rapidly deepening phase are documented by both in-situ aircraft measurements and a vertically pointing Doppler radar. Updrafts and downdrafts as strong as 24 and 19 m s-1, respectively, were found. Mean updrafts and downdrafts were approximately twice as strong as those found in other hurricanes. Updrafts had approximately the same width as downdrafts. The most vigorous updrafts were located in the front quadrants of the storm, and most of the strongest downdrafts were found in the rear quadrants. The downdrafts could not be explained in terms of evaporative or melting cooling or precipitation drag. Evidence is presented that moist symmetric instability initiated by precipitation loading may have been responsible for the strong downdrafts.
Burpee, R.W., S.D. Aberson, P.G. Black, M. DeMaria, J.L. Franklin, J.S. Griffin, S.H. Houston, J. Kaplan, S.J. Lord, F.D. Marks, M.D. Powell, and H.E. Willoughby. Real-time guidance provided by NOAA's Hurricane Research Division to forecasters during Emily of 1993. Bulletin of the American Meteorological Society, 75(10):1765-1783, doi:10.1175/1520-0477(1994)075<1765:RTGPBN>2.0.CO; 1994
The Hurricane Research Division (HRD) is NOAA's primary component for research on tropical cyclones. In accomplishing research goals, many staff members have developed analysis procedures and forecast models that not only help improve the understanding of hurricane structure, motion, and intensity change, but also provide operational support for forecasters at the National Hurricane Center (NHC). During the 1993 hurricane season, HRD demonstrated three important real-time capabilities for the first time. These achievements included the successful transmission of a series of color radar reflectivity images from the NOAA research aircraft to NHC, the operational availability of objective mesoscale streamline and isotach analyses of a hurricane surface wind field, and the transition of the experimental dropwindsonde program on the periphery of hurricanes to a technology capable of supporting operational requirements. Examples of these and other real-time capabilities are presented for Hurricane Emily.
DeMaria, M. An evaluation of the hydrostatic version of a new formulation of the primitive equations for atmospheric modeling. Preprints, 10th Conference on Numerical Weather Prediction, Portland, OR, July 18-22, 1994. American Meteorological Society, Boston, 443-445, 1994
DeMaria, M., and J. Kaplan. A statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic basin. Weather and Forecasting, 9(2):209-220, doi:10.1175/1520-0434(1994)0092.0.CO;2 1994
A statistical model for predicting intensity changes of Atlantic tropical cyclones at 12, 24, 36, 48, and 72 h is described. The model was developed using a standard multiple regression technique with climatological, persistence, and synoptic predictors. The model developmental sample includes all of the named Atlantic tropical cyclones from 1989 to 1992, with a few additional cases from 1982 to 1988. The sample includes only the times when storms were over the ocean. The four primary predictors are (1) the difference between the current storm intensity and an estimate of the maximum possible intensity determined from the sea surface temperature, (2) the vertical shear of the horizontal wind, (3) persistence, and (4) the flux convergence of eddy angular momentum evaluated at 200 mb. The sea surface temperature and vertical shear variables are averaged along the track of the storm during the forecast period. The sea surface temperatures along the storm track are determined from monthly climatological analyses linearly interpolated to the position and date of the storm. The verticalshear values along the track of the storm are estimated using the synoptic analysis at the beginning of the forecast period. All other predictors are evaluated at the beginning of the forecast period. The model is tested using a jackknife procedure where the regression coefficients for a particular tropical cyclone are determined with all of the forecasts for that storm removed from that sample. Operational estimates of the storm track and initial storm intensity are used in place of best track information in the jackknife procedure. Results show that the average intensity errors are 10%-15% smaller than the errors from a model that uses only climatology and persistence (SHIFOR), and the error differences at 24, 36, and 48 h are statistically significant at the 99% level.
DeMaria, M., and J. Kaplan. Sea surface temperature and the maximum intensity of Atlantic tropical cyclones. Journal of Climate, 7(9):1324-1334, doi:10.1175/1520-0442(1994)007<1324:SSTATM>2.0.CO; 1994
An empirical relationship between climatological sea surface temperature (SST) and the maximum intensity of tropical cyclones in the North Atlantic basin is developed from a 31-year sample (1962-1992). This relationship is compared with the theoretical results described by Emanuel. The theoretical results are in agreement with the observations over a wide range of SST, provided that the tropopause temperature is assumed to be a function of SST. Each storm is examined to determine how close the observed intensity comes to the maximum possible intensity (MPI). Results show that only about 20% of Atlantic tropical cyclones reach 80% or more of their MPI at the time when they are the most intense. On average, storms reach about 55% of their MPI. Storms that are farther west and farther north tend to reach a larger fraction of their MPI. Storms are also more likely to reach a larger fraction of their MPI in August-November than in June-July. There is considerable interannual variability in the yearly average of the ratio of the observed maximum intensity to the MPI.
DeMaria, M., and S. Aberson. Development of a nested spectral hurricane model. In Research Activities in Atmospheric and Oceanic Modelling, G.J. Boer (ed.). CAS/JSC Working Group on Numerical Experimentation, Report No. 19, WMO/TD-No. 592, 5.36, 1994
Houston, S.H., and M.D. Powell. Observed and modeled wind and water-level response from Tropical Storm Marco (1990). Weather and Forecasting, 9(3):427-439, doi:10.1175/1520-0434(1994)009<0427:OAMWAW>2.0.CO; 1994
The Hurricane Research Division (HRD) analyzes surface wind fields in tropical storms and hurricanes using surface wind observations and aircraft flight-level wind measurements in the vicinity of the storms. The analyzed surface wind fields for Tropical Storm Marco (1990) were compared with the wind fields used for input in the National Weather Service's Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. The HRD wind fields were also used to determine the wind speeds and directions corresponding to the storm surge at tide gauges along Florida's west coast. The observed storm surge at the gauges was compared with the storm surge computed by the SLOSH model. Time series of the SLOSH model winds were compared with the time series based on the analyzed wind field at each tide gauge, because in most cases there were no wind observations available at these gauges. The comparisons of the analyzed and modeled winds and the observed and modeled storm surge show that the SLOSH model reasonably represented the extreme storm tide effects on two basins with relatively complicated coastlines. However, SLOSH overestimated surface winds in areas of offshore flow, resulting in predictions of excessive negative surge. It is suggested that real-time storm surge model calculations, based on input from real-time surface wind analyses, have potential for the support of emergency management response and infrastructure recovery efforts during and immediately following landfall.
Jones, R.W., and M. DeMaria. Data assimilation for tropical cyclone prediction models. In Research Activities in Atmospheric and Oceanic Modelling, G.J. Boer (ed.). CAS/JSC Working Group on Numerical Experimentation, Report No. 19, WMO/TD-No. 592, 1.33, 1994
Lee, W.-C., F.D. Marks, and R.E. Carbone. Velocity track display: A technique to extract real-time tropical cyclone circulations using a single airborne Doppler radar. Journal of Atmospheric and Oceanic Technology, 11(2):337-356, doi:10.1175/1520-0426(1994)011<0337:VTDTTE>2.0.CO; 1994
The concept and formulation of a real-time Doppler radar wind field analysis technique, velocity track display (VTD), is presented. The VTD algorithm is a harmonic analysis method similar to the velocity-azimuth display technique for ground-based radars; however, it is designed to deduce the primary circulation properties of atmospheric vortices such as tropical cyclones. When an aircraft equipped with a Doppler radar scanning in a track-orthogonal plane penetrates a cyclonic circulation, VTD decomposes Doppler velocities on cylindrical rings into tangential, radial, and the mean cross-track component of the wind velocity. Obtaining estimates of the vortex circulation requires data from only one aircraft flight leg instead of two in the pseudo-dual Doppler radar method. As a test, the VTD technique was applied to two orthogonal legs ("figure 4" pattern) in Hurricane Gloria (1985). The entire computation was completed about 15 min after the end of each flight leg with little or no human interaction. The reconstructed hurricane vortex structure (the mean tangential wind, mean radial wind, and the total tangential wind) is consistent with those documented in the literature by elaborate techniques that demand extensively interactive decisions and intensive computations. The output consists of about 4,000 Fourier coefficients, which can be transmitted from an aircraft to a forecast center via geosynchronous satellite link in real-time for further analysis and as initialization for tropical cyclone models. A version of VTD was run successfully on board a NOAA WP-3D during the 1991 hurricane season.
Lee, W.-C., P.P. Dodge, F.D. Marks, and P.H. Hildebrand. Mapping of airborne Doppler radar data. Journal of Atmospheric and Oceanic Technology, 11(2):572-578, doi:10.1175/1520-0426(1994)011<0572:MOADRD>2.0.CO; 1994
Two sets of equations are derived to (1) map airborne Doppler radar data from an aircraft-relative coordinate system to an earth-relative coordinate system, and (2) remove the platform motion from the observed Doppler velocities. These equations can be applied to data collected by the National Oceanic and Atmospheric Administration WP-3D system, the National Center for Atmospheric research ELDORA system, and other airborne radar systems.
Lighthill, J., G. Holland, W. Gray, C.W. Landsea, G. Craig, J. Evans, Y. Kurihara, and C. Guard. Global climate change and tropical cyclones. Bulletin of the American Meteorological Society, 75(11):2147-2157, 1994
This paper offers an overview of the authors' studies during a specialized international symposium (Mexico, 22 November-1 December 1993) where they aimed at making an objective assessment of whether climate changes, consequent on an expected doubling of atmospheric CO2 in the next six or seven decades, are likely to increase significantly the frequency or intensity of tropical cyclones (TCs). Out of three methodologies available for addressing the question they employ two, discarding the third for reasons set out in the appendix. In the first methodology, the authors enumerate reasons why, in tropical oceans, the increase in sea surface temperature (SST) suggested by climate change models might be expected to affect either (1) TC frequency, because a well-established set of six conditions for TC formation include a condition that SST should exceed 26°C, or (2) TC intensity, because this is indicated by thermodynamic analysis to depend critically on the temperature at which energy transfer to air near the sea surface takes place. Careful study of both suggestions indicates that the expected effects of increased SST would be largely self-limiting, (1) because the other five conditions strictly control how far the band of latitudes for TC formation can be further widened, and (2) because intense winds at the sea surface may receive their energy input at a temperature significantly depressed by evaporation of spray, and possibly through sea surface cooling. In the second methodology, the authors study available historical records that have very large year-to-year variability in TC statistics. They find practically no consistent statistical relationships with temperature anomalies; also, a thorough analysis of how the El Niño-Southern Oscillation cycle influences the frequency and distribution of TCs shows any direct effects of local SST changes to be negligible. The authors conclude that, even though the possibility of some minor indirect effects of global warming on TC frequency and intensity cannot be excluded, they must effectively be "swamped" by large natural variability.
Ooyama, K.V. Another way of modeling the tropical cyclone. Proceedings, International Meeting on Numerical Prediction of Tropical Cyclones, Tokyo, Japan, January 17-21, 1994. Japan Meteorological Agency, Tokyo, 233-237, 1994
Ooyama, K.V. Hurricane track and intensity predictions at HRD. Proceedings, International Meeting on Numerical Prediction of Tropical Cyclones, Tokyo, Japan, January 17-21, 1994. Japan Meteorological Agency, Tokyo, 85-89, 1994
Rosenthal, S.L. Statistical aspects of the precipitation regimes at Miami International Airport (MIA) and Palm Beach International Airport (PBI): 1961-1990. NOAA Technical Memorandum, ERL-AOML-80 (PB94-194289), 40 pp., 1994
Low-frequency components of the time series of mean annual precipitation for 1961-1990 at Miami International Airport (MIA) and Palm Beach International Airport (PBI) are closely in phase. The data show a decadal-scale variation with relatively high precipitation in the 1960s and 1980s and relatively low precipitation in the 1970s. This fluctuation is concomitant with a statistically significant decadal-scale fluctuation in tropical weather system frequency. MIA precipitation is systematically related to tropical weather system frequency on monthly and seasonal time scales but not so precisely that one can conclude a cause and effect relationship exists. Spectral analysis showed well marked peaks in the MIA and PBI spectra (computed from annual precipitation totals) at a period of 15 years. Both stations also showed considerable spectral intensity at a period of 10 years. Cospectral calculations indicated that the covariance of the MIA and PBI annual precipitation totals was primarily the result of fluctuations with periods of 10 years or more. The annual numbers of cloudy days (NCLD), thunderstorm days (NTWS), days with measurable precipitation (NP.01), and the precipitation total for the rainiest day of the year (P24X) at MIA were correlated with those from PBI. Positive correlations (significant at either the 95% or 99% level) were obtained in all four cases. The time series of annual values of NCLD showed statistically significant negative trends at both stations. In contrast, the annual values of NP.01 showed positive trends at these stations. Thus, the number of days with measurable rain at these stations increased over these 30 years while the number of cloudy days decreased. The time series of annual rainfall totals at both stations, when smoothed with five-year running means, showed maxima in the decade of the 1960s. Analyses of year-month cross sections showed that these maxima were the result of anomalously high rainfall in June and September-October during the later half of that decade.
Rosenthal, S.L. Variability of surface-air temperature and precipitation at Miami International Airport including relationships with tropical weather system activity and El Niño: 1941-1990. NOAA Technical Memorandum, ERL-AOML-81 (PB94-216892), 38 pp., 1994
The Miami International Airport (MIA) surface-air temperature and precipitation data for the period 1941-1990 were examined to determine El Niño's impact on the MIA climate. In this sample, El Niño years and the years just prior to El Niño years are relatively warm and the years that follow El Niño years are relatively cool. The MIA winter months in El Niño years tend to be relatively rainy in comparison to those of non El Niño years. The winter and spring of years preceding El Niño years tend to be relatively dry.
Samsury, C.E., and R.E. Orville. Cloud-to-ground lightning in tropical cyclones: A study of Hurricanes Hugo (1989) and Jerry (1989). Monthly Weather Review, 122(8):1887-1896, doi10.1175/1520-0493(1994)122<1887:CTGLIT>2.0.CO;2 1994
Cloud-to-ground lightning characteristics of two Atlantic tropical cyclones of 1989, Hurricanes Hugo and Jerry, are presented. Statistics on the number of flashes, location, polarity, peak currents, and multiplicity (number of strokes per flash) are examined in an 18-hour period divided into pre-landfall and post-landfall categories. Land-based and aircraft lower fuselage radar data are also analyzed to determine the nature of the precipitation in which lightning is detected. Jerry is found to be more electrically active than Hugo, with 691 flashes detected compared with 33 flashes for Hugo. The majority of these flashes, regardless of the polarity, are located in the right front and right rear quadrants of the hurricanes, almost exclusively in outer convective rainbands. One reason for the large difference in the number of flashes between the two storms is the presence of many convective rainbands in Jerry, compared to only a few in Hugo. More than 20% of the flashes in each storm have a positive polarity. Median negative peak currents of the first return strokes are 49 kA in Hugo and 40 kA in Jerry. Median positive peak currents are 65 kA in Hugo and 52 kA in Jerry. The mean multiplicity of the negative flashes is 1.7 in Hugo and 2.6 in Jerry. 20% of the negative flashes detected in Jerry have a multiplicity of 4 or higher.
Shapiro, L.J. Asymmetric evolution of the hurricane. In Research Activities in Atmospheric and Oceanic Modelling, G.J. Boer (ed.). CAS/JSC Working Group on Numerical Experimentation, Report No. 19, WMO/TD-No. 592, 5.33, 1994
Wakimoto, R.M., and P.G. Black. Damage survey of Hurricane Andrew and its relationship to the eyewall. Bulletin of the American Meteorological Society, 75(2):189-200, doi:10.1175/1520-0477(1994)075<0189:DSOHAA>2.0.CO; 1994
A damage map documenting Hurricane Andrew's destructive landfall over southern Florida is presented. Vectors that represent the direction of winds causing damage to trees and structures are shown along with an F-scale rating in order to assess the strength of the near-surface winds. It is hypothesized that increased surface roughness once the hurricane made landfall may have contributed to a surface wind enhancement resulting in the strongest winds ever estimated (F3) for a landfall hurricane. This intense damage occurred primarily during the "second" period of strong winds associated with the east side of the eyewall. For the first time, a well-defined circulation in the damage pattern by the second wind was documented. A superposition of radar data from Miami and Key West on top of the damage map provides the first detailed examination of the relationship between the eyewall and the surface flow field as estimated from the damage vectors.
Willis, P.T., J. Hallett, R.A. Black, and W. Hendricks. An aircraft study of rapid precipitation development and electrification in a growing convective cloud. Atmospheric Research, 33(1-4):1-24, https://doi.org/10.1016/0169-8095(94)90010-8 1994
The rapid initial precipitation growth and initial electrification of a convective cloud, growing as a new cell on the upshear side of a cloud system in Florida, is traced from radar data and aircraft penetrations at the -7°C to -10°C level. This study combines radar, microphysical, and electrical measurements so that an examination of the interactions between the cloud dynamics, microphysics, and electrification is possible. The first pass (-7°C) was characterized by a strong 23 m/s updraft, all liquid cloud water, no precipitation, and no significant electrification. In the 300 s between the two penetrations, precipitation developed very rapidly from 45 dBZ, and the vertical component of the electric field increased from below the measurement threshold to -25 kv/m. The second penetration, which started at -7°C and ended at -10°C, was still exclusively updraft, but with lesser peak velocities and a more complex structure; i.e., no downdraft, but with relative minima in the updraft. The microphysics of the second pass displayed a segment of exclusively cloud liquid water (no precipitation size hydrometeors), a small segment of all liquid precipitation size hydrometeors, a small region of mixed hydrometeors and an extensive region of graupel hyrdometeors, ranging in size from 100 µm to several mm. High cloud liquid water coexistedwith the liquid and graupel by hydrometeors in the strong updrafts. The electrification was observed to occur exclusively in the segments of the cloud pass where graupel were observed. Within this graupel region, where the graupel often coexisted with supercooled cloud liquid water, a significant electric field occurred only at relative minima in the updraft. These relative velocity minima were also minima in the cloud liquid water content. The observed updraft velocities in these relative minima were close to balance velocities for the observed larger graupel hydrometeors. The strongest updrafts, where the formation and the riming growth of graupel was the greatest (maxima in cloud liquid water content), were not the locations of significant electrification at this flight level.
Willoughby, H.E. Nonlinear motion of a shallow water barotropic vortex. Journal of the Atmospheric Sciences, 51(24):3722-3744, doi:10.1175/1520-0469(1994)051<3722:NMOASW>2.0.CO; 1994
Nonlinear motions of a shallow water barotropic vortex on a beta plane differ substantially from the analogous linear motions. The nonlinear model described here, in which wavenumber 1-3 asymmetries interact with each other and the mean vortex, predicts that an initially completely cyclonic vortex will accelerate toward the north-northwest, reaching a speed of 2.5 m s-1 at 48 h. During the rest of the 240-h calculation, the speed varies by -1 as the direction turns from north-northwest to northwest. The vortex accelerations are in phase with temporal changes of vortex-relative angular momentum (LR). The turning back of track coincides with a transition of the wavenumber 1 asymmetry from a single dipole to double dipole. The latter structure appears to be another orthogonal solution of the second-order radial structure equation for a neutral linear normal mode. The corresponding linear model, in which forces only wavenumber 1, shows only the single dipole structure and straight north-northwest accelerating motion that reaches a speed of 9 m s-1 at 240 h. The slower motion in the nonlinear model stems from wave-induced changes in the axisymmetric vortex and vacillation between the orthogonal and modal structures. A nonlinear calculation with zero initial LR on a beta plane follows a curving path dictated by a barotropically unstable linear mode for the first 144 h. Subsequently, the double dipole structure for that mode appears as the track turns toward the northwest and the speed accelerates from 1 to 2 m s-1. A spatially uniform geostrophic environment on an f plane causes vortex motion by advection and by propagation. The potential vorticity (PV) gradient due to the current acts as much as beta does. Although the PV gradient is typically 0.1 of that due to beta, the induced propagation toward high potential vorticity is 1/2-1/4 of that on a beta plane because superposition of the vortex on a geopotential gradient amplifies the PV gradient's effect. In a quiescent environment on an f plane, initial asymmetries that project onto the normal modes induce long-lasting motion that retains about half its speed to 240 h. If the initial speed is -1, vacillation between orthogonal modal structures may cause dramatic turns and accelerations of the vortex track.
Willoughby, H.E. Nonlinear shallow-water vortex motion. In Research Activities in Atmospheric and Oceanic Modelling, G.J. Boer (ed.). CAS/JSC Working Group on Numerical Experimentation, Report No. 19, WMO/TD-No. 592, 5.37, 1994
Willoughby, H.E. The 20th conference on hurricanes and tropical meteorology. Bulletin of the American Meteorological Society 75(4):601-611, 1994
1993
Aberson, S.D., M. DeMaria, and R.E. Kohler. A four year (1989-1992) sample of a nested barotropic hurricane track forecast model (VICBAR). Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 292-295, 1993
Black, M.L. Comparisons of tropical cyclone intensity with eyewall vertical velocities. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 520-523, 1993
Black, M.L., and P.P. Dodge. Time-lapse radar images of Hurricanes Hugo (1989) and Andrew (1992). Preprints, 26th International Conference on Radar Meteorology, Norman, OK, May 24-28, 1993. American Meteorological Society, Boston, 97-99, 1993
Black, P.G., and A.V. Litinetski. Mesoscale fields in Hurricane Gilbert according to data measurements by airborne meteorological laboratories. Meteorology and Hydrology (in Russian), 2:27-37, 1993
Black, R.A., J. Hallett, and C.P.R. Saunders. Aircraft studies of precipitation and electrification in hurricanes. Preprints, Conference on Atmospheric Electricity, St. Louis, MO, October 4-8, 1993. American Meteorological Society, Boston, J20-25, 1993
Black, R.A., P.T. Willis, and J. Hallett. The development of ice particles and the establishment of electric fields in a maritime tropical cumulus cloud. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 81-84, 1993
DeMaria, M., and J. Kaplan. Verification of a statistical hurricane intensity prediction model. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 303-304, 1993
DeMaria, M., and R.W. Jones. Optimization of a hurricane track forecast model with the adjoint model equations. Monthly Weather Review, 121(6):1730-1745, doi:10.1175/1520-0493(1993)1212.0.CO;2 1993
The method of model fitting, or "adjoint method," is tested in a barotropic hurricane track forecast model. The model vorticity field at the beginning of an assimilation period is adjusted to minimize a cost function that is defined as the squared difference between the model vorticity and the vorticity from a sequence of analyses separated by 12 h. After the cost function is minimized, the model vortex closely follows the observed storm track during the assimilation period, indicating that information about the past track of the storm is being included in the model solution. Track forecasts using the assimilation procedure are compared with control forecasts where the model is initialized with a single analysis at the end of the assimilation period. Results from a series of 18 forecasts for Hurricane Hugo (1989) show that within a 12-h assimilation period the average track forecast errors are smaller than those of the control forecasts out to about 48 h. The forecast errors using a 24-h assimilation period are larger than the errors with a 12-h assimilation period. The method described by Derber in which a forcing term that minimizes the cost function is added to the vorticity equation is applied to extend the length of the assimilation period. The forcing function has very localized extrema in the vicinity of the vortex because the scale of the vortex is comparable with the distance that the vortex moves during the assimilation period. This localized forcing interferes with the subsequent motion of the storm during the forecast period. The magnitude of the localized forcing is reduced if the vorticity at the beginning of the assimilation period is first adjusted, and then the forcing term is added to further reduce the cost function. When the combined procedure is used, the average track errors are smaller than the errors in the control simulations out to 72 h. Forecasts from four additional storms from the 1989 Atlantic hurricane season are also presented. In two of these cases, the assimilation degrades the control forecasts. The degradation appears to be related to errors in the operational estimates of the storm positions and to poor first-guess fields used in the analyses.
DeMaria, M., J.J. Baik, and J. Kaplan. Upper-level eddy angular momentum fluxes and tropical cyclone intensity change. Journal of the Atmospheric Sciences, 50(8):1133-1147, doi:10.1175/1520-0469(1993)050<1133:ULEAMF>2.0.CO; 1993
The eddy flux convergence of relative angular momentum (EFC) at 200 mb was calculated for the named tropical cyclones during the 1989-1991 Atlantic hurricane seasons (371 synoptic times). A period of enhanced EFC within 1500 km of the storm center occurred about every five days due to the interaction with upper-level troughs in the midlatitude westerlies or upper-level, cold lows in low latitudes. Twenty-six of the 32 storms had at least one period of enhanced EFC. In about one-third of the cases, the storm intensified just after the period of enhanced EFC. In most of the cases in which the storm did not intensify, the vertical shear increased, the storm moved over cold water, or the storm became extratropical just after the period of enhanced EFC. A statistically-significant relationship (at the 95% level) was found between the EFC within 600 km of the storm center and the intensity change during the next 48 h. However, this relationship could only be determined using a multiple regression technique that also accounted for the effects of vertical shear and sea surface temperature variations. The EFC was also examined for the ten storms from the 1989-1991 sample that had the largest intensification rates. Six of the ten periods of rapid intensification were associated with enhanced EFC. In the remaining four cases the storms were intensifying rapidly in a low-shear environment without any obvious interaction with upper-level troughs.
Dodge, P.P., and R.W. Burpee. Characteristics of rainbands, radar echoes, and lightning near the North Carolina coast during GALE. Monthly Weather Review, 121(7):1936-1955, doi:10.1175/1520-0493(1993)121<1936:CORREA>2.0.CO; 1993
Characteristics of mesoscale rainbands and echoes in radar reflectivity data recorded during the field phase of the Genesis of Atlantic Lows Experiment (GALE) are presented. The primary sources of data were radar microfilm and manually-digitized radar (MDR) reports from the operational National Weather Service (NWS) radars at Cape Hatteras (HAT) and Wilmington (ILM), North Carolina. The data set also included cloud-to-ground lightning flashes that were recorded by the network operated by the State University of New York at Albany. The analyses included rainbands of at least 90-km length with lifetimes of at least 2 h. Nearly all of the rainbands were within 400 km of synoptic-scale or coastal fronts. Warm-sector rainbands predominated. Rainbands were classified by the location of their initial detection relative to the land, coastal shelf, and Gulf Stream. Rainbands were initially identified more frequently over the Gulf Stream and less often over the coastal shelf than the corresponding fractional areas monitored by the radars. Statistical tests determined significant differences in the sample means of the MDR and lightning data between the Gulf Stream and land regions that were largely a consequence of many more hours with MDR and lightning over the Gulf Stream. Composites relative to the beginning and ending of the rainband cases indicated that differences between the Gulf Stream and land were small shortly after the initial detection of rainbands and large just before the detection of rainbands. The largest Gulf Stream-land disparities occurred, on the average, during low-level cold and dry advection at HAT. Trunk and Bosart reported a convective echo maximum over the Gulf Stream near HAT anddiscussed physical processes that can account for the convective maximum.Analysis of one idealized distribution of convection, however, supportsthe likely role of sampling limitations of the NWS radar network indetermining the location of the convective echo maximum near HAT.
Dodge, P.P., M.L. Black, P.A. Leighton, B.A. Christoe, F.D. Marks, and R.W. Burpee. Time-lapse radar images of Hurricane Andrew's landfalls. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 163-166, 1993
Franklin, J.L., S.J. Lord, S.E. Feuer, and F.D. Marks. Multi-scale objective kinematic analyses and the motion of Hurricane Gloria (1985). Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 537-540, 1993
Franklin, J.L., S.J. Lord, S.E. Feuer, and F.D. Marks. The kinematic structure of Hurricane Gloria (1985) determined from nested analyses of dropwindsonde and Doppler radar data. Monthly Weather Review, 121(9):2433-2451, doi:10.1175/1520-0493(1993)121<2433:TKSOHG>2.0.CO; 1993
A set of three-dimensional, filtered, multiple-nested objective analyses has been completed for the wind field of Hurricane Gloria for 0000 UTC 25 September 1985. At this time Gloria was one of the most intense hurricanes ever observed in the Atlantic basin, with a minimum sea level pressure of 919 mb. The nested analyses, based on observations from airborne Doppler radar and Omega dropwindsondes, simultaneously describe eyewall and synoptic-scale features, and are the most comprehensive analyses of a single hurricane constructed to date. The analyses have been used to document the multiscale kinematic structure of Gloria and to investigate the relationship between the kinematic fields and the motion of the vortex. The analyses indicate that the vortex was unusually barotropic. The radius of maximum wind (RMW) was nearly vertical below 500 mb, with a slight inward slope with heights between 750 and 550 mb. The strongest azimuthal mean tangential winds were found well above the boundary layer, near 550 mb, where the RMW was smallest. We speculate that this unusual structure was associated with a concentric eye cycle. A persistent asymmetry in the distribution of eyewall convection was associated with the vertical shear of the environmental flow. The vortex moved approximately 2.5 m s-1 faster than the deep layer mean flow averaged at 667 km radius from the center. Barotropic models have predicted a relationship between the relative motion of the vortex and the gradients of absolute vorticity in the cyclone's environment; however, the predicted relationship was not found in Gloria. The vortex also did not move with the mean flow in the immediate vicinity of the center; the motion of the hurricane was most consistent with the 300-850 mb layer mean flow well outside the eyewall, at a radius of 65 km. The analyses suggest that the environmental flow near the center had been distorted by eyewall convection, with the scale of the distortion determined by the local Rossby radius of deformation.
Gamache, J.F. The angular momentum of the hurricane inner core as observed by airborne Doppler radar. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 533-536, 1993
Gamache, J.F. The effect of global positioning satellites upon the accuracy of dual-aircraft Doppler observations. Preprints, 26th International Conference on Radar Meteorology, Norman, OK, May 24-28, 1993. American Meteorological Society, Boston, 402-403, 1993
Gamache, J.F., R.A. Houze, and F.D. Marks. Dual-aircraft investigation of the inner core of Hurricane Norbert. Part III: Water budget. Journal of the Atmospheric Sciences, 50(19):3221-3243, doi:10.1175/1520-0469(1993)0502.0.CO;2 1993
The hydrometeor water budget of Hurricane Norbert on 24 September 1984 is computed using two microphysical retrieval techniques. Three-dimensional distributions of condensation, evaporation, precipitation, and advection of cloud and precipitation are computed, and a bulk water budget is computed as the volume integral of these distributions. The role of the microphysical retrievals is to provide the three-dimensional distribution of cloud water content, since it cannot be determined with the equipment available. Both retrieval methods use the steady-state continuity equation for water. The first method determines precipitation formation mechanisms from the radar-reflectivity and Doppler wind fields. The cloud water content is determined, through microphysical modeling, to be the amount necessary to explain the rate of precipitation formation. The second method (that of Hauser et al.) solves the water continuity equations as a boundary value problem, while also employing microphysical modeling. This method is applied in three dimensions for the first time. Asymmetries in the water budget of Hurricane Norbert were important, apparently accounting for nearly half of the net condensation. The most condensation and heaviest precipitation was to the left of the storm track, while the strongest evaporation was to the rear of the storm. Many of the downdrafts were unsaturated because they were downwind of the precipitation maximum where little water was available for evaporation. Since evaporation in the downdrafts was significantly less than the condensation in their counterpart updrafts, net condensation (bulk condensation-bulk evaporation) was significantly greater than would be implied by the net upward mass flux. Much of the vapor required to account for the greater bulk condensation appears to have come from enhanced sea surface evaporation under the dry downdraft air to the right of the storm track. The net outflow of condensate from the storm inner core was quite small, although there were appreciable outward and inward horizontal fluxes at certain locations. A maximum of ice outflow to the left of the storm track in front of the storm corresponded well to the ice particle trajectories that Houze et al. suggested were feeding the stratiform precipitation found farther outward from the storm center.
Goldenberg, S.B., and L.J. Shapiro. Relationships between tropical climate and interannual variability of North Atlantic tropical cyclones. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 102-105, 1993
Hallett, J., W. Hendricks, R.A. Black, C.P.R. Saunders, and I. Brooks. Aircraft observations of precipitation development and hydrometeor charge in Florida cumuli. Proceedings, Conference on Atmospheric Electricity, St. Louis, MO, October 4-8, 1993. American Meteorological Society, Boston, 785-790, 1993
Houston, S.H., and M.D. Powell. Surface wind fields during Hurricane Bob's (1991) landfall in New England. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 139-142, 1993
Houston, S.H., F.D. Marks, and P.G. Black. A ship passes through Hurricane Andrew's eye. Mariners Weather Log, 37(1):64-69, 1993
Jensen, R.E., S.H. Houston, C.L. Vincent, and M.D. Powell. Evaluation of a third generation wave model for the U.S. Atlantic coast. Proceedings, Second International Symposium on Ocean Wave Measurement and Analysis, New Orleans, LA, July 25-28, 1993. American Society of Civil Engineers, New York, 433-447, 1993
Jones, R.W., and M. DeMaria. Further results of variational data assimilation with a barotropic hurricane track forecast model. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 333-336, 1993
Kaplan, J., and W.M. Frank. The large-scale inflow-layer structure of Hurricane Frederic (1979). Monthly Weather Review, 121(1):3-20, doi:10.1175/1520-0493(1993)121<0003:TLSILS>2.0.CO; 1993
Aircraft rawinsonde, satellite, ship, and buoy data collected over a 40-h period were composited to analyze the inflow-layer structure of Hurricane Frederic (1979) within a radius of 10° latitude of the storm center. To improve the quality of the composite analyses, the low-level, cloud-motion winds (CMWs) employed in this study were assigned a level of best fit (LBF). A LBF was assigned to each CMW by determining the level at which the closest agreement existed between CMW and ground-truth wind data (e.g., rawinsonde, aircraft, ship, and buoy). The CMWs were then adjusted vertically to uniform analysis levels, combined with ground-truth wind data, and objectively analyzed. These objectively analyzed wind fields were used to obtain kinematically-derived fields of vorticity, divergence, and vertical velocity. An angular-momentum budget was also computed to obtain estimates of surface drag coefficients. The low-level CMWs in this study were found to have LBFs ranging from 300 to 4000 m. It was shown that judicious use of this knowledge leads to substantial improvements in the estimates of the radial flow, but relatively insignificant improvement in the estimates of the rotational component of the wind. These results suggest that the common practice of assigning all low-level CMWs in a tropical cyclone environment to a constant level of 900-950 mb (approximately 500-1000 m) is probably appropriate for computations that depend primarily upon the rotational wind component. These findings, however, also indicate that failure to account for variations in LBFs of low-level CMWs could result in substantial errors in calculations that are sensitive to the radial wind. The kinematic analyses showed that the asymmetric wind structure observed previously in studies of Frederic's inner core extends out to at least 10° latitude radius. Frederic was characterized by strong northeast-southwest radial flow through the storm and a pronounced northwest-southeast asymmetry of the tangential wind field at each analysis level. Analysis of Frederic's surface 560 m angular-momentum budget showed that the mean value of the surface drag coefficient beyond 2° radius was approximately 1.8 × 10-3.
Marks, F.D., D. Atlas, and P.T. Willis. Probability-matched reflectivity-rainfall relations for a hurricane from aircraft observations. Journal of Applied Meteorology, 32(6):1134-1141, doi:10.1175/1520-0450(1993)032<1134:PMRRRF>2.0.CO; 1993
The probability-matching method (PMM) was used to determine the relation between the distribution of equivalent reflectivity Ze measured by an airborne C-band radar and that for concurrently measured rain rate R by a disdrometer on the same aircraft in the eyewall and outer bands of Hurricane Anita in 1977. When the PMM is applied to the disdrometer population of Z's and R's, one finds that the Z-R relations differ significantly from those obtained by linear regression of their logarithms. Such regression relations are deceptive. When PMM is applied to the set of Ze's and R's, we get a family of Ze-R relations as a function of range which differ significantly from the traditional disdrometer-based Z-R relation for hurricanes by Jorgensen and Willis (JW). These new relations are approximate power laws with slope (exponent) which decrease with increasing range. At ranges less than 35 km the reflectivity in the eyewall exceeds that in the outer bands and is consistent with the expectation from the disdrometer-based relations. At greater ranges the converse is true due to beamwidth averaging over a broader beam and different vertical profiles of reflectivity in the eyewall and outer bands. We also devise a method to obtain an "effective zero range" Ze-R relation. This differs from the JW relation by -8.2 dBZ and reflects an error in the radar calibration. This approach is a novel way to calibrate an airborne meteorological radar. The methods may be used with any type of rainstorm and provide a means of using airborne radar and disdrometer systems for air-truthing rainfall measurements from space.
Powell, M.D. Wind forecasting for yacht racing at the 1991 Pan American Games. Bulletin of the American Meteorological Society, 74(1):5-16, doi:10.1175/1520-0477(1993)074<0005:WFFYRA>2.0.CO; 1993
The U.S. Sailing Team competed successfully at the 1991 Pan American Games despite having no previous experience with the sailing conditions off Havana, Cuba. One of the key factors in the team's success was meteorological support in the form of wind climate analysis; application of sea breeze forecasting typical of the south Florida area, modified by tropical weather systems; and effective preregatta briefing.
Powell, M.D. Wind measurement and archival under the automated surface observing system (ASOS): User concerns and opportunity for improvement. Bulletin of the American Meteorological Society, 74(4):615-623, doi:10.1175/1520-0477(1993)074<0615:WMAAUT>2.0.CO; 1993
The National Weather Service, as a part of its modernization effort, is implementing the Automated Surface Observing System (ASOS). Much discussion has occurred about various aspects of ASOS versus the current system of manual and automated observations. Based upon a study of the ASOS specifications and an informal survey of potential ASOS wind data users, defects of the wind sampling and archival strategy chosen for ASOS are discussed in terms of their impact on various user groups. Limitations include: (1) hourly observation average periods that do not conform to international recommendations for wind reporting made by the World Meteorological Organization; (2) no regular archival of high-resolution data--potentially valuable research data are destroyed if not identified within a 12-h period; and (3) no emergency power for operation in severe weather conditions. An alternative sampling and archiving strategy is recommended that benefits a wider cross section of users, without detracting from aviation and forecast service requirements, at a cost of less than 1% of the original ASOS portion of the National Weather Service's modernization budget.
Powell, M.D., and S.H. Houston. Analysis of surface wind fields in Hurricane Andrew. Preprints, 7th National Conference on Wind Engineering, Los Angeles, CA, June 27-30, 1993. Wind Engineering Research Council, College Station, 523-532, 1993
Powell, M.D., and S.H. Houston. Surface wind field analyses in Hurricane Andrew. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 533-536, 1993
Powell, M.D., S.H. Houston, and T.A. Reinhold. Standardizing wind measurements for documentation of surface wind fields in Hurricane Andrew. Proceedings, Conference on Hurricanes of 1992, Miami, FL, December 1-3, 1993. American Society of Civil Engineers, New York, Volume VI, 1-13, 1993
Rosenthal, S.L. Variability of the south Florida mean annual surface air temperature during the last three decades. NOAA Technical Memorandum, ERL-AOML-77 (PB94-130721), 53 pp., 1993
The oberved mean annual surface air temperature at Miami International Airport (MIA) exceeded the 30-year (1961-1990) mean by 1.2 to 2.4 standard deviations between 1989 and 1992. This was the result of a warming trend that started in the late 1960s and accelerated sharply in the mid 1980s. Unfortunately, the MIA surface temperature data cannot be taken at face value. At times during these years, the site for measuring daily surface maximum and minimum temperature was poorly positioned. There was a significant relocation of the equipment in the late 1970s. There have been three important changes of instrumentation from the original liquid in glass extreme thermometers to a series of electronic hygrothermometers, culminating in the installation of the less than ideal HO-83 in 1985. To determine the impact of these activities, the MIA temperature data were compared with data obtained at Palm Beach International Airport (PBI) and Page Field at Fort Myers (FMY). The latter are the NWS first-order stations closest to MIA. Comparisons were also made with data from nearby cooperative (COOP) stations and with data from the FAA station at Fort Lauderdale International Airport (FLL). Comparisons were also made with precipitation data from MIA, PBI and FMY. At least part of the accelerated warming in the 1980s must be attributed to a general south Florida warming during these years. However, MIA and PBI warmed faster than their neighboring COOP stations during the late 1980s, indicating that some artificial warming resulted from the HO-83 installations. The MIA mean annual daily minimum temperatures show systematic fluctuations but not a statistically-significant, long-term trend. These temperatures show a sharp, temporally-local rise starting in the mid 1980s similar to the rise found in the maximum temperatures which, therefore, provides support for the contention that this warming event is, at least partially, a natural event since previous investigators found no bias in the HO-83 minimum temperature data. This conclusion is supported by the results of our precipitation analyses. The MIA and FLL maximum temperatures both showed a warming trend that started in the late 1980s, which lends support to the conclusion that this trend is natural. The analyses support the contention that the MIA mean annual daily minimum temperature warmed somewhat through the effects of nearby jumbo jet operations and parking lot construction during the early 1970s. However, comparisons with other stations indicate that a general natural warming process impacted south Florida during this period and that the warming of MIA's mean annual daily minimum temperature was not entirely a result of human activities. Regardless of the mechanisms that were responsible, the trend ended abruptly in the early 1970s and these temperatures thereafter fell to a new low point in the mid 1980s.
Shapiro, L.J., and M.T. Montgomery. A three-dimensional balance theory for rapidly rotating vortices. Journal of the Atmospheric Sciences,50(19): 3322-3335, doi:10.1175/1520-0469(1993)050<3322:ATDBTF>2.0.CO; 1993
A three-dimensional balance formulation for rapidly rotating vortices, such as hurricanes, is presented. The asymmetric balance (AB) theory represents a new mathematical framework for studying the slow evolution of rapidly rotating fluid systems. The AB theory is valid for large Rossby numbers; it makes no formal restriction on the magnitude of the divergence or vertical advection, which need not be small. The AB is an ordered expansion in the square of the ratio of orbital to inertial frequencies, the square of a local Rossby number. The approximation filters gravity and inertial waves from the system. Advantage is taken of the weak asymmetries near the vortex core as well as the tendency for low azimuthal wavenumber asymmetries to dominate. Linearization about a symmetric balanced vortex allows the three-dimensional asymmetric dynamics to be deduced properly. The AB formulation has a geopotential tendency equation with a three-dimensional elliptic operator. The AB system has a uniformly valid continuation to nonlinear quasigeostrophic theory in the environment. It includes the full inertial dynamics of the vortex core, and reduces to Eliassen's formulation for purely axisymmetric flow. It has a full set of conservation laws on fluid parcels analogous to those for primitive equations, including conservation of potential temperature, potential vorticity, three-dimensional vorticity, and energy. A weakly nonlinear extension of the formulation in the near-vortex region is presented. Appropriate physical applications for the AB system, as well as its limitations, are discussed.
Shapiro, L.J., and M.T. Montgomery. A three-dimensional balance theory for rapidly rotating vortices. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, J45-47, 1993
Shapiro, L.J., and S.B. Goldenberg. Intraseasonal oscillations over the Atlantic. Journal of Climate, 6(4):677-699, doi:10.1175/1520-0442(1993)0062.0.CO;2 1993
Winds at low (near-surface) and 200-mb levels from National Hurricane Center objective analyses are used to elucidate the dynamics of the tropical and subtropical intraseasonal oscillations for the North Atlantic/northeast Pacific regions, including over the continents, for the years 1980-1989. The intraseasonal oscillations are broken into three bands, with long (50-85 day), intermediate (30-55 day), and short (13-29 day) periods. Winter and summer seasons are analyzed separately. A complex empirical orthogonal function technique is used to derive the dominant modes of intraseasonal variability over the region, including their propagation characteristics. Statistically-distinct modes of variability are found only during the winter and only for the long-period and short-period bands. The dominant mode of coupled 200-mb, low-level, long-period variability during winter has a dipole structure. It has a substantial equivalent barotropic component in the subtropics, as well as a baroclinic structure consistent with quasigeostrophic midlatitude systems. Negative outgoing longwave radiation anomalies tend to be in phase with a low-level convergence/upper-level divergence couplet, which lies approximately one-quarter wavelength to the east of the cyclonic vorticity centers. The long-period oscillations during 1981-1988 are dominated by three events, with periods between about 60 and 70 days. There is a negative correlation, explaining about 50% of the variance, between the magnitude of the mode and an index of El Niño based on sea surface temperatures in the eastern equatorial Pacific. The dominant modes of short-period variability during winter appear as zonally-oriented wave trains similar to those found by previous investigators of global-scale fluctuations. Rotation of the modes of 200-mb variability effectively separates them into their propagating and standing components. Approximately one-half of the variance in the meridional wind near teleconnection centers of action is found in the eastward propagating component. The dominant mode of coupled 200-mb/low-level variability propagates to the east, and has a vertical structure similar to that in the long-period band. It has a predominant period near 18 days.
Velden, C., S. Nieman, S.D. Aberson, and J.L. Franklin. Tracking motions from satellite water vapor imagery: Quantitative applications to hurricane track forecasting. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 193-196, 1993
Wakimoto, R.M., and P.G. Black. Damage survey of Hurricane Andrew and its relationship to the radar-detected eyewall. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 54-57, 1993
Willis, P.T., J. Hallett, W. Hendricks, and R.A. Black. Hydrometeor development and structure of a convective cell. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 74-77, 1993
Willoughby, H.E. Nonlinear shallow-water vortex motion. Preprints, 20th Conference on Hurricanes and Tropical Meteorology, San Antonio, TX, May 10-14, 1993. American Meteorological Society, Boston, 533-536, 1993
1992
Black, M.L., and H.E. Willoughby. The concentric eyewall cycle of Hurricane Gilbert. Monthly Weather Review, 120(6):947-957, doi:10.1175/1520-0493(1992)120<0947:TCECOH>2.0.CO; 1992
Hurricane Gilbert of 1988 formed an outer eyewall as it intensified rapidly toward a record minimum pressure of 888 hPa in the western Caribbean. The outer eyewall strengthened and contracted, while the inner eyewall showed some signs of weakening before landfall on the Yucatan Peninsula. Remarkably, both eyewalls survived passage over land, but the storm was much weaker when it entered the Gulf of Mexico. Although the primary cause of weakening was passage over land, the effect of the contracting outer eyewall may have contributed. Later, the outer eyewall completely replaced the inner eyewall. Subsequently, it contracted steadily but slowly as Gilbert maintained nearlyconstant intensity over the cooler waters of the Gulf before final landfall on the mainland of Mexico.
Black, P.G., and A.V. Litinetski. Flight-level and surface wind observations in Hurricane Gilbert from AN-12 "Cyclone" and WP-3D "Orion" aircraft. Proceedings, 5th International Symposium on Tropical Meteorology, Obninsk, Russia, May 27-31, 1992. USSR State Committee on Hydrometeorology, 20-31, 1992
Burpee, R.W., and P.P. Dodge. Analyses of visible satellite imagery of the south Florida sea breeze circulation. Preprints, 5th Conference on Mesoscale Processes, Atlanta, GA, January 5-10, 1992. American Meteorological Society, Boston, 77-80, 1992
DeMaria, M., S.D. Aberson, K.V. Ooyama, and S.J. Lord. A nested spectral model for hurricane track forecasting. Monthly Weather Review, 120(8):1628-1643, doi:10.1175/1520-0493(1992)120<1628:ANSMFH>2.0.CO; 1992
A numerical method for including a wide range of horizontal scales of motion is tested in a barotropic hurricane track forecast model. The numerical method uses cubic B-spline representations of variables on nested domains. The spline representation is used for the objective analysis of observations and the solution of the prediction equations (shallow-water equations on a Mercator projection). This analysis and forecasting system is referred to as VICBAR. The VICBAR model was tested in near real-time during the 1989 and 1990 Atlantic hurricane seasons. For the 1989 season, VICBAR had skill comparable with, or greater than, that of the operational track forecast models. For the 1990 season, VICBAR had skill comparable with that of theoperational track forecast models, except at 72 h when QLM (a three-dimensional mesoscale model) had greater skill than VICBAR. During both 1989 and 1990, VICBAR had considerably more skill for forecasts of hurricanes than for forecasts of tropical storms. For the 1990 season, VICBAR was generalized to include time-dependent boundary conditions from a global forecast model. These boundary conditions improve the VICBAR forecasts, especially for the longer range forecasts (60-72 h). The skill of the VICBAR is sensitive to the choice of the background field used in the objective analysis and the fields used to apply the boundary conditions. The use of background fields and boundary condition fields from a 12 h old global model forecast significantly reduced the VICBAR skill relative to the use of fields from the current global forecast.
Elsberry, R.L., G.J. Holland, H. Gerrish, M. DeMaria, C.P. Guard, and K. Emanuel. Is there any hope for tropical cyclone intensity prediction? -- A panel discussion. Bulletin of the American Meteorological Society, 73(3):264-275, 1992
The outlook for tropical cyclone intensity forecasts from operational and research perspectives was discussed during a panel discussion at the 19th Conference on Hurricanes and Tropical Meteorology. Whereas the operational requirement at the National Hurricane Center is to predict maximum 1-min sustained wind speeds at specific locations, the research community is addressing the prediction of the maximum wind or minimum sea level pressure in the storm. Commonality was found in the forecast strategies for subjectively predicting storm intensity. The panelists suggested improvements may be gained from additional observations, better conceptual and theoretical models of storm structure and behavior, and enhancements in statistical and numerical models. The discussion period brought out opposing viewpoints on a number of topics. Both new observations and better use of the existing observations were believed to be necessary. The limitations and advantages of remotely sensed data for this problem were raised. The most vigorous debates were on the physical processes, such as existence or nonexistence of coupling between outer and inner core structure, and whether convection is simply a response to forcing or is an essential contributor to uncertainty in intensity forecasting. Several participants suggested that uncertainties related to the sea surface temperature and its evolution also contribute to the intensity forecast problem. Some specific suggestions for improving intensity forecasts are given in terms of new observations, new basic understandings, and new applied developments.
Franklin, J.L., and M. DeMaria. The impact of Omega dropwindsonde observations on barotropic hurricane track forecasts. Monthly Weather Review, 120(3):381-391, doi:10.1175/1520-0493(1992)1202.0.CO;2 1992
A scarcity of observations in the hurricane environment is one factor believed to be limiting the improvement in hurricane track forecast accuracy. Since 1982, the Hurricane Research Division (HRD) of the NOAA Atlantic Oceanographic and Meteorological Laboratory has conducted 14 experiments to determine the wind and thermodynamic fields within about 1000 km of tropical cyclones in the Atlantic basin. During these synoptic-flow experiments, Omega dropwindsondes (ODWs) are released from the two NOAA WP-3D research aircraft over a 9-10 h period in the hurricane environment. The ODWs measure pressure, temperature, humidity, and wind as they descend from flight level (about 400 mb) to the surface. These data are then transmitted in real time to the National Hurricane Center (NHC) and the National Meteorological Center (NMC). Recently, a barotropic, nested, spectral hurricane track forecasting model, VICBAR, has been developed at HRD and tested quasi-operationally during the 1989 and 1990 hurricane seasons. Forecasts from this model have compared favorably with other models run at NHC and NMC. In this study, the VICBAR model is used to evaluate the impact of ODW data on track forecast error for the 14 HRD synoptic-flow experiments. The ODW data produced highly consistent reductions in track forecast errors in this sample of cases. Forecast improvements due to single-level midtropospheric (aircraft) data were significantly smaller than those due to the ODWs. At the important verification times of 24-36 h (prior to landfall), when the decision to issue a hurricane warning is being made, the ODWs reduced the model mean forecast error by 12%-16%. These improvements, statistically significant at the 99% level, are comparable to the total improvement in normalized NHC official 24 h forecast error occurring over the past 20-25 years.
Griffin, J.S., R.W. Burpee, F.D. Marks, and J.L. Franklin. Real-time airborne analysis of aircraft data supporting operational hurricane forecasting. Weather and Forecasting, 7(3):480-490, doi:10.1175/1520-0434(1992)007<0480:RTAAOA>2.0.CO; 1992
The Hurricane Research Division (HRD) has developed a technique for real-time airborne analysis of aircraft data from reconnaissance and research flights in tropical cyclones. The technique uses an onboard workstation that analyzes flight-level observations, radar reflectivity patterns, radial Doppler velocities, and vertical soundings from Omega dropwindsondes (ODWs). Many of the workstation analyses are in storm-relative coordinates that depend upon interactive identification of the cyclone center from the radar reflectivity data. Displays of the lower fuselage reflectivity, composited for 1-2 h, provide an overall perspective of the horizontal patterns of precipitation and a framework for interpretation of thermodynamic and kinematic observations. The workstation runs algorithms for estimation of the horizontal wind field in the hurricane core using radial velocities measured by the airborne Doppler radar during one or more penetrations of the storm center. Interactive software also supports real-time processing of ODW wind and thermodynamic data, objective editing of bad data, and automatic dissemination of mandatory and significant-level data in the standard dropwindsonde code. Plans for the 1992 hurricane season include transmission of subsets of the data to the National Hurricane Center (NHC) through the Geostationary Operational Environmental Satellite (GOES) communications system and display of the aircraft analyses for the forecasters at NHC. With the implementation of these plans, NHC will receive two-dimensional analyses of the mesoscale precipitation and wind structure of the storm core and more frequent estimates of the location and recent motion of tropical cyclones.
Griffin, J.S., R.W. Burpee, F.D. Marks, and J.L. Franklin. Real-time airborne analysis of flight-level and radar data supporting operational hurricane forecasting. Preprints, 8th International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, Atlanta, GA, January 5-10, 1992. American Meteorological Society, Boston, 241-246, 1992
Hallett, J., W. Hendricks, and P.T. Willis. Evolution of precipitation spectra in a developing convective cloud. Preprints, 11th International Conference on Clouds and Precipitation, Montreal, Canada, August 17-21, 1992. American Meteorological Society, Boston, 216-219, 1992
Houze, R.A., F.D. Marks, and R.A. Black. Dual-aircraft investigation of the inner core of Hurricane Norbert. Part II: Mesoscale distribution of ice particles. Journal of the Atmospheric Sciences, 49(11):943-962, doi:10.1175/1520-0469(1992)049<0943:DAIOTI>2.0.CO; 1992
Horizontal fields of cloud microphysical parameters, vertical air motion, and horizontal wind at the 6 km level in Hurricane Norbert (1984) were obtained by mapping and interpolating data collected on board a WP-3D aircraft along numerous flight tracks executed within the central region of the storm. Although the storm was characterized by a strong vortex of winds reaching peak values >50 m s-1 all around the storm, the precipitation was concentrated on the southwest side of the storm. A sloping eyewall was located within 20-30 km of the eye. Stratiform precipitation dominated the region outside the eyewall. A band of maximum stratiform precipitation was located 60-70 km southwest of the storm center. The ice particles at flight level tended to be relatively large both in the eyewall and in the outer band of stratiform precipitation. Particles were smaller and more numerous (100-300 l-1) in the zone between the eyewall and outer stratiform band. These particles occurred on the outside edges of the eyewall convective updrafts, indicating that they may have been produced by splintering in association with graupel formation in the updrafts. The large particles in the eyewall tended to be graupel. In the outer stratiform region, characterized by weak, average vertical air motion and an absence of strong convective drafts, the predominant particle type was aggregates. The region of large graupel particles in the eyewall coincided with the radius of maximum tangential wind and was apparently produced by the azimuthal advection of the graupel particles. Since graupel particles fall rapidly, they were not susceptible to advection out of the weaker radial wind component. On the other hand, some of the more slowly falling, less dense aggregates produced in the eyewall region were evidently advected radially as well as azimuthally, thus accounting for the location of the outer region of maximum stratiform precipitation intensity.
Marks, F.D. Kinematic structure of the hurricane inner core as revealed by airborne Doppler radar. Preprints, 5th Conference on Mesoscale Processes, Atlanta, GA, January 5-10, 1992. American Meteorological Society, Boston, 127-132, 1992
Marks, F.D., R.A. Houze, and J.F. Gamache. Dual-aircraft investigation of the inner core of Hurricane Norbert. Part I: Kinematic structure. Journal of the Atmospheric Sciences, 49(11):919-942, doi:10.1175/1520-0469(1992)049<0919:DAIOTI>2.0.CO; 1992
A dedicated experiment to study the details ofthe important physical processes and scale interactions operative within the eyewall regions of Hurricane Norbert was carried out on 24-25 September. The two National Oceanic and Atmospheric Administration (NOAA) Aircraft Operations Center (AOC) WP-3D research aircraft were used for the study. One aircraft, equipped with airborne Doppler radar, flew repeated radial penetrations in and out of the eye, mapping the three-dimensional wind field over the region surrounding the eyewall. This data set provides the first complete mapping of the three-dimensional wind field of the hurricane inner core. The three-dimensional wind field within 40 km of the storm center was derived from a "pseudo" dual-Doppler analysis in each quadrant of the storm. The vertical wind components were derived from the mass continuity equation and the horizontal wind field. The Doppler-derived wind fields for the four quadrants were combined to form a storm composite wind field that was 75 × 75 km on a side and centered on the storm circulation center. The wind-field altitude extended from 0.5-12 km. The Norbert wind field was asymmetric, and the asymmetry varied with altitude. The tangential wind maximum sloped upwind with increasing altitude, from the left of the track (azimuths 147°-327°) at 1 km altitude to the right of the storm track (azimuths 327°-147°) at 3 km altitude. The radial wind at 1 km altitude had inflow in front of the storm (327° azimuth) and outflow behind. This pattern in the radial flow disappeared at 3 km altitude, where the radial flow switched from inflow in the rear of the storm to outflow in the front. The vertical velocity maximum was to the left of the storm track at all levels. The maximum sloped downwind with increasing altitude (along the upper boundary of the reflectivity maximum) from in front of, and to the left of, the track at 2 km altitude, to behind and to the right of the track at 8 km altitude. To investigate the nature of the wind-field asymmetry, a technique was devised to partition the horizontal wind components into a horizontal mean wind as a function of the altitude and a perturbation wind. The cylindrical nature of the wind field permitted further partitioning of the perturbation wind into the mean vortex (a function of radius and height-wave number 0) and a perturbation from the mean vortex (including any higher order wave numbers). The wind partitioning was used to describe the structure of the mean vortex and its interaction with the environmental flow. The partition of the horizontal wind pointed out the complex interactions of the wind components in determining storm motion and in forcing mesoscale convergence/divergence patterns that resulted in the vertical velocity asymmetry.
Peene, S.J., Y.P. Sheng, and S.H. Houston. Modeling tidal and wind-driven circulation in Sarasota and Tampa Bays. Proceedings, International Conference on Estuarine and Coastal Modeling, St. Petersburg, FL, November 13-15, 1991. American Society of Civil Engineers, New York, 357-369, 1992
As part of an effort to quantify the effects of hydrodynamics on water quality within Sarasota Bay, Tampa Bay, and their adjoining waters, a field and modeling study of circulation and transport is being conducted. This paper presents some results from a simulation of barotropic circulation in Tampa and Sarasota Bays during October 1990. Tropical Storm Marco passed just west of the study area on October 11. At that time, NOAA was conducting measurements of tides and currents at a number of stations in Tampa Bay, and USGS had stations monitoring tides in Sarasota Bay. To simulate the circulation, the recently enhanced CH3D model (Sheng, 1989), which was originally developed for Chesapeake Bay, James River, and Lake Okeechobee, was used. To allow sufficient lateral resolution in the vicinity of shorelines and inlets, a boundary-fitted grid with a rather fine grid spacing (between 200 and 1000 m) was generated for the region from Tampa Bay in the north, south to Venice Inlet, and approximately 4 km into the Gulf of Mexico. For the Tropical Storm Marco simulation, a wind field was generated by NOAA's Hurricane Research Center using a two-dimensional least squares fitting algorithm on surface and aircraft winds measured as the storm moved northward along the Florida coast. Based on an objective comparison between simulated and measured results, it is apparent that the model predicts the tidal and wind-driven water surface elevation well, while the prediction of currents is not as good due to the lack of resolution of the navigation channel in the numerical grid.
Powell, M.D. Surface wind speeds in hurricanes. Proceedings, ASCE Structures, Congress X, San Antonio, TX, April 13-15, 1992. American Society of Civil Engineers, New York, 246-249, 1992
Shapiro, L.J. Hurricane vortex motion and evolution in a three-layer model. Journal of the Atmospheric Sciences, 9(2):140-153, doi:10.1175/1520-0469(1992)049<0140:HVMAEI>2.0.CO; 1992
A three-layer, multinested numerical model is used to evaluate the asymmetric evolution of a hurricane and its interaction with the large-scale environment. The model uses a compressible fluid in isentropic coordinates. In 72 h the hurricane vortex on a beta plane moves northwest at an average speed of 2.4 m s-1. In the presence of a westerly zonal wind in the upper model layer, the hurricane on an f plane moves to the southeast at an average speed of 0.9 m s-1. A series of experiments establishes that the southeastward drift in the presence of westerly shear is primarily due to the southward isentropic gradient of background potential vorticity (PV) in the middle model layer that is associated with the background temperature field. The cyclonic circulation advects low PV air southward on the west side of the vortex, inducing a negative isentropic PV anomaly to the southwest. This anomaly is associated with a wind field that advects the vortex to the southeast, just as the northward isentropic gradient of PV due to the beta effect advects the hurricane to the northwest. The northward gradient of background PV in the upper layer has little effect on the motion. The westerly wind advects upper layer low PV outside the vortex core to the east, inducing an anticyclonic anomaly that tends to advect the middle-layer vortex to the north; this tendency is secondary to the motion. The role of vertical transports of momentum due to cumulus convection on the hurricane motion is also evaluated. Results are presented that generalize the homogenization of asymmetric absolute vortex and oscillation in relative angular momentum (RAM) found on the beta plane in a previous study with a barotropic model. Outside the vortex core and within ~350 km of the center, the asymmetries reach a near-steady state. The middle-layer asymmetry is associated with a PV gradient that neutralizes the background gradient due to planetary vortex or environmental temperature, thereby insulating the symmetric vortex from distortion. Horizontal fluxes in the presence of the planetary vortex gradient tend to counteract the development of strong anticyclonic total RAM within a large circle about the vortex center.
Velden, C.S., C.M. Hayden, W.P. Menzel, J.L. Franklin, and J.S. Lynch. The impact of satellite-derived winds on numerical hurricane track forecasting. Weather and Forecasting, 7(1):107-118, doi:10.1175/1520-0434(1992)0072.0.CO;2 1992
While qualitative information from meteorological satellites has long been recognized as critical for monitoring tropical cyclone activity, quantitative data are required to improve the objective analysis and numerical weather prediction of these events. In this paper, results are presented that show that the inclusion of high-density, multispectral, satellite-derived information into the analysis of tropical cyclone environmental wind fields can effectively reduce the error of objective track forecasts. Two independent analysis and barotropic track-forecast systems are utilized in order to examine the consistency of the results. Both systems yield a 10%-23% reduction in middle- to long-range track-forecast errors with the inclusion of the satellite-wind observations.
Willis, P.T., J. Hallett, and R.A. Black. Cloud and hydrometeor microphysics at -3°C in a vigorous Florida convective updraft. Preprints, 11th International Conference on Clouds and Precipitation, Montreal, Canada, August 17-21, 1992. American Meteorological Society, Boston, 436-439, 1992
Willoughby, H.E. Linear motion of a shallow-water barotropic vortex as an initial-value problem. Journal of the Atmospheric Sciences, 49(21):2015-2031, doi:10.1175/1520-0469(1988)0452.0.CO;2 1992
This paper revisits calculation of motion for a shallow-water barotropic vortex with fixed mean axisymmetric structure. The algorithm marches the linear primitive equations for the wavenumber 1 asymmetry forward in time using a vortex motion extrapolated from previous calculations. Periodically, it examines the calculated asymmetry for the apparent asymmetry due to mispositioning of the vortex center, repositions the vortex to remove the apparent asymmetry, and passes the corrected vortex motion on to the next cycle. This approach differs from the author's earlier variational determination of the steady-state motion after initial transients had died away. The steady-state approach demonstrated that the vortex had normal modes at zero frequency and, when an annulus of weak anticyclonic flow encircled the cyclonic inner vortex, at the most anticyclonic rotation frequency of the mean flow. Forcing of the former model led to too rapid steady-state poleward motion on a beta plane. At least for the linear problem, the key to more realistic simulation of motion and structure is the normal modes' transient response to diverse forcing: environmental potential vorticity gradients, embedded sources and sinks of mass, and initial asymmetries. The beta effect and other environmental potential vorticity gradients excite the normal modes to induce an acceleration of the vortex center toward and to the left of the direction to maximum environmental vorticity. Times ~100 days would be required to reach the too fast motions predicted in the earlier work. A rotating mass source-sink pair drives the vortex along a cycloidal track, but does not force the normal modes. A nonrotating source-sink forces a motion from the source toward the sink and excites the normal modes, leading to motion that persists after the forcing has ceased. Similarly, initial asymmetries that project onto the normal modes maintain themselves for times >10 days, leading to persistent vortex propagation that evolves as the complex normal-mode frequencies dictate. Understanding ofthese normal modes can contribute to better tropical cyclone motion forecasts through better initialization of numerical track prediction models.
1991
Aberson, S.D., and M. DeMaria. A nested barotropic hurricane track forecast model (VICBAR). Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 81-86, 1991
Baik, J.-J., M. DeMaria, and S. Raman. Observational evidence for upper tropospheric asymmetric eddy momentum forcing and subsequent intensity change. Preprints, 19th Conference on Hurricane and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 478-481, 1991
Baik, J.-J., M. DeMaria, and S. Raman. Tropical cyclone simulations with the Betts convective adjustment scheme. Part III: Comparisons with the Kuo convective parameterization. Monthly Weather Review, 119(12):2889-2899, doi:10.1175/1520-0493(1991)119<2889:TCSWTB>2.0.CO; 1991
Numerical simulations of tropical cyclones in an axisymmetric model with the Betts convective adjustment scheme and the 1974 Kuo cumulus parameterization are compared. It is shown that the storm with the Betts scheme has a slightly more intense mature stage than the storm with the Kuo scheme. For both schemes, the parameterized heating is dominant initially, while the grid-scale heating is dominant at the mature stage. The storms begin to intensify rapidly when the grid-scale heating extends through a deep layer. The Betts scheme is more effective at removing water vapor and delays the onset of grid-scale heating. This results in later development of the storm with the Betts scheme. The storm evolution with both the Betts and Kuo schemes is sensitive to the treatment of the evaporation of liquid water in the grid-scale condensation scheme. This suggests that a prognostic equation for liquid water should be used when simulating tropical cyclones with a model resolution fine enough for grid-scale heating to be important.
Barnes, G.M., and M.D. Powell. The inflow thermodynamics of Hurricane Gilbert. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 486-489, 1991
Barnes, G.M., J.F. Gamache, M.A. LeMone, and G.J. Stossmeister. A convective cell in a hurricane rainband. Monthly Weather Review, 119(3):776-794, doi:10.1175/1520-0493(1991)119<0776:ACCIAH>2.0.CO; 1991
On 10 October 1983 the two NOAA WP-3D aircraft completed a mission designed to provide airborne Doppler radar data for a convective cell embedded in a weak rainband on the trailing side of Hurricane Raymond. Comparisons of the wind field produced from the pseudo-dual Doppler radar technique with in-situ wind measurements suggest that the larger convective-scale features may be resolved if the sampling time is kept to a minimum. The convective cell was found to move downband faster than any environmental winds, but slightly slower than the winds found in the reflectivity core that delineates the cell. In the core of the cell the tangential wind is increased and the radial inflow turns to outflow with respect to the circulation center. The flow field demonstrates that the downband stratiform portion of a rainband is not from cells currently active, since the updraft detrains upwind relative to the cell, but rather it is due to the fallout from ice particles placed into the upper troposphere by clouds that have since dissipated. The mass flux of this cell is estimated to be 5%-10% of the mass flux accomplished by an eyewall of a moderate tropical cyclone. This finding supports the concept that large, convective-active rainbands have a major effect on the subcloud layer air flowing toward the eyewall.
Beryulev, G.P., P.B. Black, and A.V. Litinetski. Intercomparison of wind and temperature data from the research aircraft WP-3D and AN-12BC in Hurricane Gilbert, 1988. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 587-588, 1991
Black, M.L., R.W. Burpee, and F.D. Marks. Vertical motions in tropical cyclones determined with airborne Doppler radial velocities. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 409-411, 1991
Black, P.B., and F.D. Marks. The structure of an eyewall meso-vortex in Hurricane Hugo (1989). Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 579-582, 1991
Burpee, R.W., and P.G. Black. Strong surface winds and mesoscale convective systems in the unnamed tropical storm of 1987. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 412-415, 1991
Burpee, R.W., J.S. Griffin, J.L. Franklin, and F.D. Marks. Airborne analysis of observations from a NOAA P-3 in support of operational hurricane forecasting. Preprints, 7th International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, New Orleans, LA, January 13-18, 1991. American Meteorological Society, Boston, 195-197, 1991
Burpee, R.W., J.S. Griffin, J.L. Franklin, and F.D. Marks. Airborne analysis of observations from a P-3 aircraft in support of operational hurricane forecasting. Proceedings, 4th Interagency Airborne Geoscience Workshop, La Jolla, CA, January 29-February 1, 1991. NASA, Washington, DC, 123-124, 1991
DeMaria, M., and J. Kaplan. A statistical model for predicting tropical cyclone intensity change. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 587-588, 1991
DeMaria, M., and R.W. Jones. Optimization of a hurricane track forecast model using the adjoint model equations. Preprints, 9th Conference on Numerical Weather Prediction, Denver, CO, October 14-18, 1991. American Meteorological Society, Boston, 547-550, 1991
Dodge, P.P., R.W. Burpee, and F.D. Marks. Airborne Doppler radar analyses of the core of Hurricane Gilbert. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 551-552, 1991
Feuer, S.E., and J.L. Franklin. Nested analyses of Hurricane Gloria from dropwindsonde and Doppler radar data. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 130-133, 1991
Franklin, J.L., and M. DeMaria. The impact of Omega dropwindsonde data on hurricane track forecasts using the VICBAR model. Preprints, 9th Conference on Numerical Weather Prediction, Denver, CO, October 14-18, 1991. American Meteorological Society, Boston, 404-407, 1991
Franklin, J.L., M. DeMaria, and C.S. Velden. The impact of Omega dropwindsonde and satellite data on hurricane track forecasts using the VICBAR model. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 87-92, 1991
Gamache, J.F. Inner core budget studies of Hurricane Norbert (1984). Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 545-550, 1991
Gamache, J.F., F. Roux, and F.D. Marks. Comparison of three methods to deduce three-dimensional wind fields in a hurricane with airborne Doppler radar. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 462-465, 1991
Griffin, J.S., R.W. Burpee, J.L. Franklin, and F.D. Marks. Preliminary results of airborne analysis of observations in support of operational hurricane forecasting. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 144-147, 1991
Houston, S.H., and M.D. Powell. Effects of Tropical Storm Marco (1990) on Florida's west coast. Preprints, 5th Conference on Meteorology and Oceanography of the Coastal Zone, Miami, FL, May 6-9, 1991. American Meteorological Society, Boston, 131-133, 1991
Jones, R.W., and M. DeMaria. A variational method for including persistence in a hurricane track forecast model. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 331-334, 1991
Kaplan, J., and J.L. Franklin. The relationship between the motion of Tropical Storm Florence (1988) and its environmental flow. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 93-97, 1991
Lee, W.-C., and F.D. Marks. Real-time display of mean three-dimensional hurricane structure using the VTD technique. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 470-473, 1991
Lee, W.-C., F.D. Marks, and R. Carbone. Real-time display of mean three-dimensional hurricane structure using the VTD technique. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 445-450, 1991
Lhermitte, R., and P.T. Willis. Small Doppler radar as a precipitation gauge. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 790-792, 1991
Marks, F.D., and R.A. Houze. Kinematic structure of the eyewall of Hurricane Emily (1987) as determined from an airborne Doppler radar. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 437-440, 1991
Marks, F.D., D. Atlas, and P.T. Willis. Probability matched Z-R relations for hurricanes from aircraft observations. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 778-781, 1991
Powell, M.D. Meteorological aspects of Hurricane Hugo. In Hurricane Hugo One year Later: Proceedings of a Symposium and Public Forum, Charleston, SC, September 13-15, 1990, American Society of Civil Engineers, New York, 11-40, 1991
Powell, M.D. Surface wind distribution of Hurricane Hugo in the Carolinas. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 441-444, 1991
Powell, M.D., P.P. Dodge, and M.L. Black. The landfall of Hurricane Hugo in the Carolinas: Surface wind distribution. Weather and Forecasting, 6(3):379-399, doi:10.1175/1520-0434(1991)006<0379:TLOHHI>2.0.CO; 1991
Hurricane Hugo struck Charleston, South Carolina, on 22 September 1989 as the most intense hurricane to affect the United States since Camille in 1969. The northeastern eyewall, which contained the maximum winds measured by reconnaissance aircraft shortly before landfall, moved inland over a relatively unpopulated area and there were few fatalities. However, no observations were available to document the surface wind distribution in this part of the storm as it continued inland. To improve specification of surface winds in Hugo, empirically adjusted aircraft winds were combined with coastal, offshore, and inland surface observations and were input to the Ooyama objective analysis algorithm. The wind analysis at landfall was then compared with subsequent analyses at 3 and 6 h after landfall. Reconstruction of the surface wind field after landfall suggests that the maximum (~13 min mean) surface wind at the coast was 50 m s-1 in the Bulls Bay region, ~40 km northeast of Charleston. Surface roughness over land caused wind speeds to drop off rapidly just inland of the coast to only 50% of values measured by reconnaissance aircraft at the same location relative to the storm over water. Despite relatively rapid increases in the central sea-level pressure and decreases in the mean circulation as Hugo progressed inland, hurricane-force wind gusts extended Hugo's damage pattern well past Charlotte, North Carolina, ~330 km inland. Accurate determination of surface wind distribution in landfalling hurricanes is dependent upon the spatial density and quality of surface wind measurements and techniques to adjust reconnaissance flight-level winds to the surface. Improvements should allow forecasters to prepare more accurate warnings and advisories and allow more thorough documentation of poststorm effects. Empirical adjustments to reconnaissance aircraft measurements may replace surface data voids if the vertical profile of the horizontal wind is known. Expanded use of the airborne stepped-frequency microwave radiometer for remote sensing of ocean surface winds could fill data voids without relying upon empirical methods or models. A larger network of offshore, coastal, and inland surface platforms at standard (10 m) elevations with improved sampling strategies is envisioned for better resolution of hurricane wind fields. A rapid-response automatic station network, deployed at prearranged coastal locations by local universities with meteorology and/or wind engineering programs, could further supplement the fixed platform network and avoid the logistical problems posed by sending outside teams into threatened areas.
Rosenthal, S.L. A note on relationships between western Sahel rainfall and U.S. hurricane activity. NOAA Technical Memorandum, ERL-AOML-68 (PB91-176511), 22 pp., 1991
A recent research paper concluded that the probabilities of major hurricane strikes on the east coast of the United States, particularly the east coast of Florida, are greatly enhanced when rainfall over the western Sahel is abundant and that these probabilities are substantially smaller when drought conditions prevail over the western Sahel. This conclusion was based upon a 43-year sample of 1947-1989. In the work presented here, a search is made for simple, statistically significant (at the 5% or better level) relationships between western Sahel rainfall for 1947-1990 and eight types of hurricanes that are defined in the text. When hurricane frequencies for the 11 wettest and 11 driest western Sahel years are compared, statistically significant differences are found for all hurricane types studied, except for Florida landfalling hurricanes. Significant relationships are found for major hurricanes striking the east coast of the United States north of Florida, and for hurricanes of all intensities striking the east coast of the United States north of Florida. However, no significant relationships are found for hurricanes striking Florida. When the wetter 22 years are compared with the drier 22 years, a statistically significant relationship is found for the total of hurricanes of all intensities that strike the Florida Peninsula. For this type of hurricane, the largest frequency is found in the second quartile of western Sahel rainfall years and not in the wettest quartile. This makes the interpretation of the results difficult.
Roux, F., and F.D. Marks. Eyewall evolution in Hurricane Hugo deduced from successive airborne Doppler observations. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 558-563, 1991
Roux, F., F.D. Marks, and J.F. Gamache. Three-dimensional circulation in a hurricane from airborne Doppler radar data: Extended velocity track display. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 466-469, 1991
Shapiro, L.J. The effect of vertical wind shear on hurricane motion in a three-layer model. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 356-357, 1991
Shay, L.K., P.B. Black, J.D. Hawkins, R.L. Elsberry, and A.J. Mariano. Sea surface temperature response to Hurricane Gilbert. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 574-578, 1991
Willis, P.T., and A.J. Heymsfield. Trajectories of hydrometeors in Hurricane Emily. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 192-197, 1991
Willis, P.T., and J. Hallett. Microphysical measurements from an aircraft ascending with a growing isolated maritime cumulus tower. Journal of the Atmospheric Sciences, 48(2):283-300, doi:10.1175/1520-0469(1991)048<0283:MMFAAA>2.0.CO; 1991
The development of precipitation in the top of an isolated marine cumulus is traced by four rapid penetrations with an instrumented aircraft between 400 and 1000 m below the visible top of the growing tower. The hydrometeor distribution evolves from the first appearance of a few large supercooled drops [0.45 l-1, D > 0.5 m] to well-developed precipitation (largely ice) in 500 s. This development results from accretion and coalescent growth in the cloud top volume, not from advection by the updraft of large drops from below. Large supercooled drops precede the appearance of ice at -9°C near the cloud top. The cloud and precipitation water budgets are computed and compared with observed values, which indicate that, once precipitation is well-developed, the convective tower cannot maintain itself as a steady-state entity. The budget computations demonstrate a sensitivity of cloud evolution to the ice particle density.
Willis, P.T., F.D. Marks, and J. Hallett. Tracing the interactions of precipitation evolution and cloud dynamics using airborne Doppler radar and in-situ data. Preprints, 25th International Conference on Radar Meteorology, Paris, France, June 24-28, 1991. American Meteorological Society, Boston, 916-919, 1991
Willoughby, H.E. Reply. Journal of the Atmospheric Sciences,48(9):1209-1212, doi:10.1175/1520-0469(1991)048<1209:R>2.0.CO;2 1991
Willoughby, H.E. Semispectral models of moving hurricane-like vortices. Preprints, 19th Conference on Hurricanes and Tropical Meteorology, Miami, FL, May 6-10, 1991. American Meteorological Society, Boston, 383-384, 1991
1990
Baik, J.-J., M. DeMaria, and S. Ramen. Tropical cyclone simulations with the Betts convective adjustment scheme. Part I: Model description and control simulation. Monthly Weather Review, 118(3):513-528, doi:10.1175/1520-0493(1990)118<0513:TCSWTB>2.0.CO; 1990
A new convective parameterization scheme proposed by Betts is tested in a tropical cyclone model. The convective adjustment scheme adjusts the local temperature and moisture structures towards the observed quasi-equilibrium thermodynamic state and includes nonprecipitating shallow convection, as well as deep convection. The numerical model used for this study is an axisymmetric, primitive equation, hydrostatic, finite difference model with 15 vertical levels and a horizontal resolution of 20 km. The spectral radiation boundary condition, which uses a different gravity wave speed for each vertical mode, is implemented in the model. It is shown that the convective scheme is capable of simulating the developing, rapidly intensifying, and mature stages of a tropical cyclone from a weak vortex. At the mature stage, the minimum surface pressure and maximum low level tangential wind speed are around 923 mb and 58 m s-1. During the early developing stage, the latent heat release is from the convective parameterization, but at the mature stage the latent heat release is mainly due to the grid-scale phase change. For comparison, an experiment is conducted with the parameterized convection excluded, leaving only the grid-scale condensation and evaporation. The results show that the development of a tropical cyclone can be modeled with crude grid-scale condensation and evaporation processes for the 20 km horizontal resolution, similar to other studies. However, the storm with the explicit convective latent heat release is considerably less intense than that with the parameterized convective latent heat release.
Baik, J.-J., M. DeMaria, and S. Ramen. Tropical cyclone simulations with the Betts convective adjustment scheme. Part II: Sensitivity experiments. Monthly Weather Review, 118(3):529-541, doi:10.1175/1520-0493(1990)118<0529:TCSWTB>2.0.CO; 1990
Extensive sensitivity experiments with an axisymmetric tropical cyclone model that includes the Betts convective parameterization scheme are carded out. The sensitivity of the model storm evolution to the convective adjustment parameters is studied. These results show that the model storm leads to earlier development as the adjustment time scale becomes small and the stability weight on the moist adiabat in the lower atmosphere is increased. The model storm evolution is very sensitive to variations in the saturation pressure departure at the lowermost model integer level and the storm at mature stage has a lower central pressure as the magnitude of the saturation pressure departure is increased. The adjustment parameters affect the grid-scale precipitation as well as the convective precipitation and the precipitation is especially sensitive to changes in the saturation pressure departure. Sensitivity of the model to variations in the sea surface temperature, latitude, initial vortex amplitude, initial moisture distribution, and radiation is also investigated. The results of the numerical simulations are similar to previous studies. Sensitivity studies with various horizontal resolutions show that the subgrid-scale heating becomes a larger fraction of the total heating as the horizontal grid size is increased.
Black, R.A. Radar reflectivity-ice water content relationships for use above the melting level in hurricanes. Journal of Applied Meteorology, 29(9):955-961, doi:10.1175/1520-0450(1990)029<0955:RRIWCR>2.0.CO; 1990
Regression of equations linking radar reflectivity (Ze) and ice water content (IWC) were calculated from airborne radar and particle image data that were collected above the melting level in two hurricanes. The Ze-IWC equation from the stratiform areas of Hurricane Norbert (1984) is similar to the composite equation for thunderstorm anvils derived by Heymsfield and Palmer. The Ze-IWC equation from the convective regions of Hurricane Irene (1981) has essentially the same exponent, but a significantly greater coefficient than that from Norbert.The higher density of the graupel and rounded ice in the Hurricane Irene data accounts for the difference in the coefficients. The hurricane Ze-IWC relations have smaller exponents than most of those from midlatitude clouds, which indicates that small ice particles may be more prevalent in these two hurricanes than in midlatitude clouds.
Black, R.A., and J. Hallett. Electric field and microphysical measurements in vigorous hurricane eyewalls. Preprints, Conference on Cloud Physics, San Francisco, CA, July 23-27, 1990. American Meteorological Society, Boston, 662-665, 1990
Burpee, R.W. Radar characteristics of hurricanes. In Federal Meteorological Handbook No. 11, Doppler Weather Radar Observations, Part B: Radar Meteorology and Theory. Federal Coordinator for Meteorological Services and Supporting Research, Washington, DC, 1-14, 1990
DeMaria, M. Normal mode initialization in a tropical cyclone model. Monthly Weather Review, 118(10):2199-2214, doi:10.1175/1520-0493(1990)118<2199:NMIIAT>2.0.CO; 1990
The effect of nonlinear normal mode initialization (NMI) on tropical cyclone simulations is investigated using a three-layer axisymmetric model. It is shown that the balance condition proposed by Machenhauer, which neglects the time tendencies of the gravity-mode amplitudes, is valid in a tropical cyclone simulation. The boundary layer friction, adiabatic nonlinear and diabatic heating terms are important in the balance. A highly truncated version of the model with linearized physical parameterizations is used to analyze the convergence properties of several iterative schemes developed to solve the initialization equations. When diabatic heating is neglected, the schemes will always converge if the linear friction coefficient a is smaller than the Coriolis parameter f. For small horizontal-scale modes, the iterative schemes will also converge for values of a much larger than f. When diabatic beating is included, the rate of convergence of the small horizontal-scale modes becomes extremely slow. The schemes are also tested in the nonlinear version of the model by first running a 7-day tropical cyclone simulation. The initialization schemes are applied at day 5 after the model has produced an intense tropical cyclone. Results show that the tropical cyclone rapidly weakens relative to the uninitialized run during the 6-12 h after the NMI is applied. This weakening occurs because the small horizontal-scale modes do not converge, making the secondary radial circulation much too weak. A scheme is proposed where the NMI is followed by a short integration with the geostrophic modes held fixed. This procedure compensates for the lack of convergence of the small horizontal-scale gravity modes.
DeMaria, M., and R.W. Jones. The use of aircraft observations in a hurricane track forecast model. Preprints, International Symposium on Assimilation of Observations in Meteorology and Oceanography, Clermont-Ferrand, France, July 9-13, 1990. American Meteorological Society, Boston, 191-195, 1990
DeMaria, M., M.B. Lawrence, and J.T. Kroll. An error of Atlantic tropical cyclone track guidance models. Weather and Forecasting, 5(1):47-61, doi:10.1175/1520-0434(1990)005<0047:AEAOAT>2.0.CO; 1990
Mean track forecast errors over the 6-yr period 1983-88 are compared for four tropical cyclone-track forecast models in use at the National Hurricane Center (NHC). The model types represented are statistical, statistical-dynamical, barctropic-dynamical, and baroclinic-dynamical. The statistical-dynamical model (NHC83) had the smallest mean errors at all forecast periods between 12 and 72 h while the baroclinic-dynamical model (MFM) had the largest errors at 12 and 24 h. NHC83 was the only model which had statistically significant forecast skill relative to the CLIPER model. When the forecasts were stratified by latitude, the MFM had significant skill at 36 and 48 h for the northern storms. Further analysis of the forecast errors shows that most of the models have a bias towards the southwest. The MFM has a very large westward bias for low-latitude storms which results from a fast-speed bias in that region. NHC83 has relatively small speed and directional baises, while CLIPER and SANBAR have slow biases. The MFM has been replaced (beginning in 1988) by a new baroclinic-dynamical model, the quasi-Lagrangian model (QLM). A comparison of the MFM and QLM for the 1988 season shows that the QLM has error characteristics similar to the MFM. A barotropic-dynamical model is used to give some insight into the behavior of dynamical track prediction models. Results show that initial-position errors have a negligible effect on the average track forecast errors except at 12 h. Errors in the estimate of the initial storm motion affect the forecast errors out to 72 h. Several spatial filters are applied to the initial analyses which shows that the model storm track is the most sensitive to very large scales (3000-6000 km wavelengths).
Dodge, P.P., J.S. Griffin, F.D. Marks, and R.W. Burpee. Interactive analysis of NOAA P-3 aircraft data in support of operational hurricane forecasting. Preprints, Sixth International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, Anaheim, CA, February 4-9, 1990. American Meteorological Society, Boston, 245-247, 1990
Franklin, J.L. Dropwindsonde observations of the environmental flow of Hurricane Josephine (1984): Relationships to vortex motion. Monthly Weather Review, 118(12):2732-2744, doi:10.1175/1520-0493(1990)1182.0.CO;2 1990
Omega dropwindsonde (ODW) observations from three synoptic-flow experiments in environment of Hurricane Josephine have been analyzed in a research mode using an objective analysis procedure. The nominal times of the analyses are 0000 UTC 10, 11, and 12 October 1984. The filtered, three-dimensional analyses have been used as a basis for several diagnostic and prognostic calculations relating to the motion of the hurricane. Examination of Josephine's environment revealed a strong variability of the flow with distance from the storm center and with pressure. Josephine moved at right angles to the azimuthally averaged wind at 500 mb; the vortex motion was more consistent with the flow near 700 mb. Forecasts made with a barotropic forecast model showed a high sensitivity of the forecast track to the vertical layer used in the initial analysis. These results demonstrate the potential value of vertical sounding information from the ODWs, and show that single-level midtropospheric information is not always representative of a hurricane's environment flow. On each of the three days, the motion of Josephine deviated significantly from its environmental "steering," as measured by an azimuthal average of the 300-850 mb mean flow over the 5 -7 radial band. This deviation from steering (the so-called "propagation" vector) was oriented with components parallel and to the left of the gradient of absolute vorticity in the asymmetric wind field. The magnitude of the propagation was proportional to the strength of the absolute vorticity gradient. These results are consistent with many barotropic modeling studies.
Franklin, J.L., C.S. Velden, J. Kaplan, and C.M. Hayden. Some comparisons of VAS and dropwindsonde data over the subtropical Atlantic. Monthly Weather Review, 118(9):1869-1887, doi:10.1175/1520-0493(1990)118<1869:SCOVAD>2.0.CO; 1990
Omega dropwindsonde and other in-situ (INS) data collected during theNOAA/Hurricane Research Division's (HRD) field program are used as a ground truth data set for the evaluation of VISR Atmospheric Sounder (VAS) soundings over the subtropical Atlantic. The experiments were coordinated with the Cooperative Institute for Meteorological Satellite Services at the University of Wisconsin. The focus of this study is to determine whether soundings derived from VAS radiances are an improvement over the first-guess data used as a starting point in the sounding retrieval process. First-guess inputs for this study are provided by NMC's Regional Analysis and Forecast System (RAFS) nested-grid model (NGM). In a case study, an objective algorithm is used to analyze the INS, VAS, and first-guess data at and below 500 mb from an HRD experiment on 1-2 September 1988. The case study is supplemented by a statistical investigation of data composited from other HRD experiments. In particular, we examine VAS estimates of horizontal temperature and moisture gradients to see if they represent improvements over the first guess. The temperature and moisture descriptions in the vicinity of a 500 mb cold low were improved by the VAS in the case study; however, VAS temperature gradients were found to be generally less accurate than those of the first guess. Temperature gradients from the VAS were also consistently strongerthan INS or first-guess gradients. The composite study found that large-scale VAS moisture gradients were better than those of the first guess. Other results indicate a preferred mode for VAS modifications to the guess; the primary impact of the VAS radiances on the first guess was to improve the description of the phasing of horizontal features. The VAS representation of the amplitude of features, however, was not consistently an improvement. This suggests that in tropical applications, VAS data may be most suitable for subjective forecasting uses; if VAS data are to be used in numerical weather prediction, strongest weight should be given to the representation of the location of weather features (troughs, ridges, etc.), and relatively weak weight should be given to the representation of the strength of these features.
Two-dimensional images of ice particles observed by a NOAA WP-3D research aircraft during the Summer Monsoon Experiment (SMONEX) are examined. These images were obtained in the temperature interval from -25°C to 0°C. The particle structures and size distributions found in convective and stratiform clouds are compared. Branched crystals were located predominantly in stratiform clouds while column-shaped crystals were located commonly in both stratiform and convective clouds. Stratiform clouds, particularly those observed at temperatures warmer than -7°C, had a much greater percentage concentration of large ice particles (>0.8 mm in diameter), and many of these ice particles were aggregates or branched crystals. The importance of aggregation and deposition above the melting level in stratiform clouds is strongly suggested by these findings. Ice particle number concentrations measured with the cloud probe were often very high in convective clouds, with a maximum value of approximately 800 L-1. The average convective-cloud concentration was approximately 230 L-1, while the average concentration in the stratiform clouds was approximately 20 L-1. Liquid water was almost completely absent in the convective updrafts, at temperatures between -10°C and -22°C. This suggests that the convective updrafts may have been nearly completely glaciated, and the microphysics were dominated by deposition. The high particle concentrations in the convective updrafts suggest that the updrafts may provide most of the ice particles found in the stratiform cloud. Significant modification in particle structures and size distributions has occurred, however, by the time these suspended particles fall out of the stratiform clouds. These modifications appear to arise from aggregation and deposition.
Giese, G.S., D.C. Chapman, P.G. Black, and J.A. Fornshell. Causation of large-amplitude coastal seiches on the Caribbean coast of Puerto Rico. Journal of Physical Oceanography, 20(9):1449-1458, doi:10.1175/1520-0485(1990)020<1449:COLACS>2.0.CO; 1990
Sea-level oscillations at supertidal frequency with amplitudes of the order of the mean tidal range have been reported from the Caribbean coast of Puerto Rico. Analysis of a 10-year time series of digital tide data from Magueyes Island, Puerto Rico, demonstrates that sea-level variance at the fundamental normal mode (seiche) frequency of the shelf has a pronounced fortnightly distribution with a maximum occurring 6-7 days after new and full moon. The seiche variance also shows a bimodal seasonal distribution with an inverse relationship to easterly wind stress. It is argued that the seiches are excited by internal waves generated by strong tides in the southeastern Caribbean. Support is provided by airborne radar imagery showing sea-surface patterns suggesting the presence of internal waves near the southern Aves Ridge, and by the results of two field experiments, carried out during times when large-amplitude seiches were expected, to research forevidence of internal wave forcing near the shelf break. During the first experiment, large negative-amplitude, pulse-like internal waves were recorded 6 km seaward of the shelf break during a period of strong seiche activity. Such pulses were not observed during the second experiment. However, high-frequency temperature variance 2.3 km seaward of the shelf break, possibly resulting from internal surf, increased with depth and reached a maximum 6-7 days following new moon, again suggesting the presence of internal waves. The 10-year time series analysis shows that large tides are necessary, but not sufficient, to generate high seiche activity. This is supported by the two field experiments; during the first, large-amplitude seiches occurred as expected, while during the second experiment they did not. We suggest that this behavior is related to variations in stratification, which, in turn, alter the energy transfer from tides to seiches.
Hallett, J., and P.T. Willis. Development of precipitation in isolated maritime cumulus. Preprints, Conference on Cloud Physics, San Francisco, CA, July 23-27, 1990. American Meteorological Society, Boston, 74-79, 1990
Houze, R.A., B.F. Smull, and P.P. Dodge. Mesoscale organization of springtime rainstorms in Oklahoma. Monthly Weather Review, 118(3):613-654, doi:10.1175/1520-0493(1990)118<0613:MOOSRI>2.0.CO; 1990
Radar reflectivity and rain gage data obtained during six springtimes indicates the types of mesoscale organization that occur in association with major rain events in Oklahoma (at least 25 mm of rain in 24 h over an area exceeding 12,500 km2). In these storms, the primary rain area is found to be a contiguous region of precipitation tens to hundreds of kilometers in scale that consists partly of deep convection and partly of stratiform rain. The patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures. About two-thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain. Storm organization was graded according to the degree to which it matched an idealized model of this "leading-line/trailing-stratiform" structure. The precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an axis normal to and passing through the midpoint of the line, or asymmetric, in which case the storm was biased toward having stronger, more discrete convective structure at the upwind (south or southwestern) end of the line and/or the most extensive stratiform precipitation behind the downwind (north to northeastern) end of the line. About one-third of the cases examined displayed much more chaotic, unclassifiable arrangements of convective and stratiform areas. Among the cases with leading-line/trailing-stratiform structure, severe weather was most frequent in systems with (1) a strong degree of leading-line/trailing-strataiform structure, in which a solid, relatively uniform, arc-shaped line had stratiform rain centered symmetrically behind it, and (2) a weaker degree of leading-line/trailing-stratiform structure in which a southwest-northeast line was biased toward having narrow, intensely convective, irregularly spaced cell structure at its southwestern (upwind) end and stratiform rain confined to the region behind the broader northeastern (downwind) portion of the line. Although all mesoscale organization types were characterized by all types of severe weather, the type (2) cases were the most prolific category in terms of tornado and hail production, while type (1) cases were prone to be associated with flooding. The chaotic, unclassifiable cases, which exhibited no line organization, had just as much severe weather as the cases with line organization, but were more likely to produce hail and somewhat less likely to produce tornadoes and flooding than the systems with line structure. Major rain events occurred whenever a mesoscale convective complex (MCC) was dissipating or merely skirting the area. However, 75% of the major rain events occurred under cloudshields that failed to meet the MCC criteria explicitly, although they often resembled MCCs qualitatively. No particular type of mesoscale radar-echo organization was favored when cloud shields meeting the MCC criteria were observed. A slight preference for the more chaotic type of organization was suggested; however, the data sample is not large enough for this finding to be regarded as conclusive. Mean soundings and hodographs generally show no sign of a low-level jet in environments associated with chaotically arranged rain areas that lacked any line structure. On the other hand, a low-level jet and resulting curved hodograph were typically associated with cases in which line organization was evident. The wind shear in the low to mid troposphere, the bulk Richardson number, and other familiar parameters characterizing squall line environments are consistent with results from recent modeling studies. When leading-line/trailing-stratiform structure was present, the cross-line shear in the environment was of a magnitude associated with model simulations in which a rearward sloping updraft circulation favorable to trailing-stratiform anvil formation quickly develops. The along-line componentt of shear was greater when the squall system structure was of the asymmetric type and the degree of leading-line/trailing-stratiform structure was not as strong, i.e., in those mesoscale systems favoring tornado occurrence.
Lord, S.J., and J.L. Franklin. The environment of Hurricane Debby (1982). Part II: Thermodynamic fields. Monthly Weather Review, 118(7):1444-1459, doi:10.1175/1520-0493(1990)118<1444:TEOHDP>2.0.CO; 1990
A three-dimensional analysis of temperature and relative humidity in the environment of Hurricane Debby (1982) has been completed. Observations from Omega dropwindsondes (ODWs) within 1000 km of the storm have been combined with rawinsondes over the continental United States and the Caribbean and with observations from surface ships and aircraft data where possible. The temperature and relative humidity analyses, together with wind analyses from a previous study, form a dataset that can be used as an initial condition in a multilevel prognostic model when combined with analyses over area larger than our analysis domain. In this paper a series of diagnostic tests has been applied to the dataset to evaluate its performance without using a prognostic model. These tests include horizontal maps of the moist convective instability, calculation of the heat and moisture budgets in the vicinity of Bermuda, which was 350 km to the northeast of the storm center, and diagnosis of precipitation from these budgets and from the Arakawa-Schubert cumulus parameterization. Results show that the horizontal distribution of moist convective instability is strongly affected by the low-level moisture field upstream of the main inflow region to the storm. The total surface heat flux, estimated with a bulk aerodynamic method, matches the vertically integrated eddy flux of moist static energy to within observational errors. Precipitation estimates from the budgets give rates of approximately 20 mm day-1, which are consistent with an estimated rate from radar. Partition of the rainfall rate into convective scale and resolvable scale (stratiform) shows about equal contributions. Our results lead us to believe that, within the limitations determined by the horizontal distribution of the observations, the final dataset for Hurricane Debby provides a realistic depiction of the various physical processes that were occurring in Debby's environment. Future work will include data sensitivity experiments with a three-dimensional forecast model.
Marks, F.D. Radar observations of tropical weather systems. In Radar in Meteorology: Battan Memorial and 40th Anniversary Radar Meteorology Conference, D. Atlas (ed.). American Meteorological Society, Boston, 401-425, 1990
Marks, F.D., and P.G. Black. Close encounter with an intense mesoscale vortex within Hurricane Hugo (September 15, 1989). Preprints, 4th Conference on Mesoscale Processes, Boulder, CO, June 25-29, 1990. American Meteorological Society, Boston, 114-115, 1990
Ooyama, K.V. A thermodynamic foundation for modeling the moist atmosphere. Journal of the Atmospheric Sciences, 47(21):2580-2593, doi:10.1175/1520-0469(1990)047<2580:ATFFMT>2.0.CO; 1990
With advances in numerical modeling of the atmosphere, we have experienced that the return to the first principles of physics often enables a model to cope more easily with the complexities of the real atmosphere. The return to the primitive equations of motion from historical balance approximations is an example. This paper proposes a way to return to the "primitive" form of moist thermodynamics, in which prediction is made strictly in terms of conservative properties, such as mass and entropy. There is no conservation law that would apply directly to temperature or pressure. These intensive properties, therefore, should be diagnostically determined by thermodynamics, from the predicted conservative properties. The scope of the paper is limited to the thermodynamics of reversible processes. Irreversible processes, which would make a model alive with real weather, are not discussed here, since each of them requires a separate empirical treatment. It is shown, however, that the proposed formulation of thermodynamics facilitates modularization of various approximations within a model, and among models. For example, both the hydrostatic and nonhydrostatic models can be built under an identical design, differing only in the manner of calculating vertical motion. The proposed formulation is extended to include the ice phase within reversible thermodynamics. Also discussed are numerical problems in the spatial representation of thermodynamic discontinuities, which are caused by the phase transition of water substance.
Powell, M.D. Boundary layer structure and dynamics in outer hurricane rainbands. Part I: Mesoscale rainfall and kinematic structure. Monthly Weather Review, 118(4):891-917, doi:10.1175/1520-0493(1990)118<0891:BLSADI>2.0.CO; 1990
Results of hurricane boundary layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation and kinematic structures in these storms and in Hurricane Floyd (1981) indicate that principal rainbands have common characteristic mesoscale and convective-scale features in the boundary layer. The two-dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements (on the inner, upshear side of the band) and stratiform rain (on the outer downshear side). The bands are oriented perpendicular to the shear above the boundary layer and cells movedownband at about 85% of the density-weighed mean wind speed of the 0.2-6 km layer. The boundary layer windfield is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis, and minima 4-8 km to the inner side. Maximum crossband convergence and cyclonic shear vorticity are also found to the inner side of the rainband axis. Updrafts and downdrafts are preferentially located on the inner side of the band axis, with some downdrafts spreading out at the surface. The band-relative positions of the updraft and perturbation pressure minimum suggest that the minimum may be produced by interaction of the wind shear andthe updraft. Outer hurricane rainbands show many similarities to tropical squall lines; major differences are associated with propagation and the structure of the leading and trailing edges.
Powell, M.D. Boundary layer structure and dynamics in outer hurricane rainbands. Part II: Downdraft modification and mixed-layer recovery. Monthly Weather Review, 118(4):918-938, doi:10.1175/1520-0493(1990)118<0918:BLSADI>2.0.CO; 1990
Recent aircraft boundary layer measurements in the vicinity of principal hurricane rainbands have confirmed that convective downdrafts are capable of transporting cool, dry, low equivalent potential temperature air (θE) to the surface, where the mixed layer is eliminated. The incorporation of this air into convection near the core of the storm may weaken the storm, depending upon the scale of the disturbance and the processes governing the recovery of the air while it is flowing toward the eyewall. This paper examines the thermodynamic characteristics of the boundary layer in outer convective hurricane rainbands, providing evidence for downdraft modification mechanisms and determining the extent to which disturbed boundary layer air may be restored on its trajectory to the storm.
Powell, M.D. Observations of boundary layer structure and mesoscale wind fields in a midlatitude marine rainband during GALE. Preprints, 4th Conference on Mesoscale Processes, Boulder, CO, June 25-29, 1990. American Meteorological Society, Boston, 55-56, 1990
Powell, M.D., and P.G. Black. Meteorological aspects of Hurricane Hugo's landfall in the Carolinas. Shore and Beach, 58(4):3-10, 1990
Powell, M.D., and P.G. Black. The relationship of hurricane reconnaissance flight-level wind measurements to winds measured by NOAA's oceanic platforms. Journal of Wind Engineering and Industrial Aerodynamics, 36(1):381-392, https://doi.org/10.1016/0167-6105(90)90322-4 1990
A well-known problem in hurricane forecasting and in the administration of hazardous weather warnings and advisories concerns what adjustments to make to flight-level reconnaissance wind observations in order to make them representative of sustained surface winds. To solve this problem, a study was initiated comparing NOAA reconnaissance flight-level winds to 10 m level observations from NOAA's oceanic buoys and platforms. A data base was created that consisted of comparisons made whenever the aircraft observation was: (1) within 10 km radial separation from the surface platform (in a storm-relative coordinate system); (2) within ±4 h of the surface observation time; and (3) within ±2 h of the surface observation time. The data base contains all storms flown by NOAA aircraft in the vicinity of the Atlantic and Gulf of Mexico buoy network over the 11-year period from 1975-1986. Comparisons from these criteria are discussed in terms of the ratio of the buoy-measured wind speed (VB) to the aircraft-measured wind speed VA. Results indicate that the 10 m level surface winds over water were within 55%-85% of the winds measured by the reconnaissance aircraft. The ratio VB/VA depended strongly on the low-level atmospheric stability as indicated by the buoy air-sea temperature.
Rosenthal, S.L. A history of NOAA's Hurricane Research Division, including interactions with Florida State University's Department of Meteorology. Proceedings, 40th Anniversary Celebration, Department of Meteorology, Florida State University, Tallahassee, FL, November 30-December 1, 1989. Florida State University, Tallahassee, 42-47, 1990
Rosenthal, S.L. Summary of the special sessions on Hurricane Hugo. 70th Annual Meeting of the American Meteorological Society, Anaheim CA, February 4-9, 1990. Bulletin of the American Meteorological Society, 71(9):1339-1342, 1990
Shapiro, L.J., and K.V. Ooyama. Barotropic vortex evolution on a beta plane. Journal of the Atmospheric Sciences, 47(2):170-187, doi:10.1175/1520-0469(1990)047<0170:BVEOAB>2.0.CO; 1990
A barotropic, primitive equation (shallow water) model is used on the beta plane to investigate the influence of divergence, total relative angular momentum (RAM), and advective nonlinearities on the evolution of a hurricane-like vortex. The multinested numerical model is based on the spectral application of a finite element representation. The undisturbed fluid depth is taken to be 1 km. Scaling of the vorticity equation, in conjunction with a Bessel function spectral decomposition, indicates that divergence should have a very small effect on the hurricane motion. Simulations with an initially symmetric cyclonic vortex in a resting environment confirm this analysis, and contradict previous published studies on the effect of divergence in a barotropic model. During a 120 h simulation, the cyclonic vortex develops asymmetries that have an influence far from the initial circulation. The total RAM within a large circle centered on the vortex decreases with time, and then oscillates about zero. For circles with radii <1000 km, the total RAM approaches, but does not reach, zero. Anangular momentum budget indicates that the horizontal angular momentum flux tends to counteract the net Coriolis torque on the vortex. If the total RAM of the initial symmetric vortex is zero, the weak far-field asymmetries are essentially eliminated. The motion of the vortex is not, however, related to the RAM in any simple way. Within a few days the near-vortex asymmetries reach a near-steady state. The asymmetric absolute vorticity (AAV) is nearly uniform within ~350 km of the vortex center. The homogenization of AAV, which occurs within the closed vortex gyre, is likely due to shearing by the symmetric wind, combined with removal of energy at the smallest scales. Thehomogenization effectively neutralizes the planetary beta effect, as well as the vorticity associated with an environmental wind.
Willis, P.T., and A.J. Heymsfield. Microphysical trajectories in tropical cyclones. Preprints, Conference on Cloud Physics, San Francisco, CA, July 23-27, 1990. American Meteorological Society, Boston, 666-671, 1990
Willoughby, H.E. Gradient balance in tropical cyclones. Journal of the Atmospheric Sciences, 47(2):265-274, doi:10.1175/1520-0469(1990)047<0265:GBITC>2.0.CO;2 1990
Analysis of a large inventory of in-situ observations from research aircraft shows that the gradient wind approximates the axisymmetric swirling flow in the free atmosphere within 150 km of the centers of Atlantic hurricanes and tropical storms. In the middle and lower troposphere, the rms difference between azimuthal mean swirling and gradient winds is typically -1 with zero bias. This balance prevails only for the azimuthal mean, not locally, nor is balance to be expected in either the surface friction layer or the upper tropospheric outflow layer, where the radial flow is comparable with the swirling flow. It is theoretically possible that axisymmetric supergradient flow may occur in response to rapid radial acceleration where the radial flow slows in the friction layer beneath the eyewall or where it converges into intense diabatically-forced updrafts.Nevertheless, the observations in the free lower and midtroposphere show that systematic departures of the azimuthal mean vortex from balance are too small to measure.
Willoughby, H.E. Linear normal modes of a moving, shallow-water barotropic vortex. Journal of the Atmospheric Sciences, 47(17):2141-2148, 1990
Calculations with a linear semispectral model of a moving tropical-cyclone-like barotropic vortex (Willoughby, 1988) show that a vortex with cyclonic circulation throughout exhibits unphysically fast poleward motion on a beta plane, but a vortex with enough anticyclonic circulation at its periphery to make the total relative angular momentum (LR) small moves slowly. The high poleward speed arises because the vortex has a linear normal mode at zero frequency, where the beta effect forces asymmetric perturbations. Advection of planetary vorticity by the axisymmetric circulation forces this normal mode at a rate proportional to LR. Because the governing equations are third-order in time, as many as three linear normal modes are possible. A completely cyclonic vortex has three repeated stable normal modes at zero frequency, whereas one with small LR has a single stable mode at zero frequency and a conjugate pair of barotropically unstable modes. The frequency of the unstable modes lies at the most anticyclonic rotation frequency of the axisymmetric circulation, and the growth rate is slow; the e-folding time is typically 75 days. If the fluid is made very shallow, the stable normal mode moves away from zero frequency. In this situation, the beta effect fails to force the resonance, and the vortex propagates westward much as a planetary Rossby wave does. In this model, meridional motion of vortices with LR = 0 always acts to adjust LR toward zero through conservation of absolute angular momentum. Since the asymmetric perturbations are Rossby waves that propagate upon the radial gradient of mean relative vorticity, the mode at zero frequency experiences critical--radius absorption where the mean swirling wind is zero--at the boundary between cyclonic and anticyclonic mean circulation and at the edgeof the vortex. Regardless of the sign of LR, the wave momentum convergence is concentrated at these critical radii and weakens the circulation while expanding it spatially. When LR = 0, waves emanating from the cyclonic and anticyclonic circulations interfere destructively, so that the vortex radiates no angular momentum to its environment.
Willoughby, H.E. Temporal changes of the primary circulation in tropical cyclones. Journal of the Atmospheric Sciences, 47(2):242-264, doi:10.1175/1520-0469(1990)047<0242:TCOTPC>2.0.CO; 1990
More than 900 radial profiles of in-situ aircraft observations collected in 19 Atlantic hurricanes and tropical storms over 13 years confirm that the usual mechanism of tropical cyclone intensification involves contracting maxima of the axisymmetric swirling wind. Radar shows that annuli of convective echoes accompany the wind maxima. These features, called convective rings, exist and move inward because latent heat released in the rings leads to descent, adiabatic warming, and rapid isobaric height falls in the area they enclose. The radial change in rate of isobaric height fall is concentrated at the inner edge of the wind maximum, causing the gradient wind to increase there and the maximum to contract. Vigorous convection organized in rings invariably causes well-defined, inward-moving wind maxima, but when convection is weak, the rings are also weak or even absent. In this case, the swirling wind may be nearly constant with radius and change slowly in time. Hurricanes that have a single, vigorous, axisymmetric ring strengthen rapidly. Although a series of minor convective rings may support steady strengthening, development is more generally episodic. When asymmetric convection erupts near the center of tropical storms or weak hurricanes, it may cause intensification to falter and the cyclone tracks tobecome irregular. In intense hurricanes, outer convective rings may form around the pre-existent eyewalls, contract, and strangle the original eyewalls, halting intensification or causing weakening.
1989
Black, M.L. Signal loss of WSR-57 radars as a function of range in tropical cyclones. Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 514-517, 1989
Black, M.L., and F.D. Marks. Concentric eyewalls in Hurricane Gilbert (1988). Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 224-225, 1989
Black, P.G., L.K. Shay, R.L. Elsberry, and J.D. Hawkins. Response of the Gulf of Mexico to Hurricane Gilbert. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 226-227, 1989
Burpee, R.W. A summer day without significant rainfall in south Florida. Monthly Weather Review, 117(3):680-687, doi:10.1175/1520-0493(1989)117<0680:ASDWSR>2.0.CO; 1989
Precipitating cumulus clouds occur regularly in the afternoon over the south Florida peninsula during summer months. A day without significant rainfall or radar echoes is rare. This paper discusses one such day, 23 July 1987, during which a dry, stable airmass covered the Florida peninsula. Non-precipitating shallow cumulus formed in a few areas, but there were not any deep, precipitating cumulus clouds over land. The thermodynamic characteristics of the airmass are described and the synoptic-scale patterns that produced the airmass are presented.
Burpee, R.W. Gordon E. Dunn: Preeminant forecaster of midlatitude storms and tropical cyclones. Weather and Forecasting, 4(4):573-584, doi:10.1175/1520-0434(1989)004<0573:GEDPFO>2.0.CO; 1989
Burpee, R.W., and M.L. Black. Temporal and spatial variations near the centers of two tropical cyclones. Monthly Weather Review, 117(10):2204-2218, doi:10.1175/1520-0493(1989)117<2204:TASVOR>2.0.CO; 1989
The Hurricane Research Division collected radar reflectivity data with a portable recorder attached to National Weather Service (NWS) WSR-57 radar as Hurricanes Alicia of 1983 and Elena of 1985 approached the coastline of the United States. The reflectivity data were used to estimate rain rates for the eyewall region, including the rain-free eye, and the rainbands in the annular area outside the eyewall, but within 75 km of the center of the eye. The rain rates include reflectivity corrections that were based upon the variation of average returned power with range in four hurricanes. This study examines the temporal and spatial variations of rain rates in the cores of Hurricanes Alicia and Elena. In Alicia, variations of area-averaged rain rate (R) in the eyewall region were caused by the growth and decay of mesoscale convective areas. In Elena, the life cycles of individual convective cells also accounted for large changes in the eyewall R. In both hurricanes, the time series of R in the rainband region was less variable than the eyewall R, because the rainband region was larger than the eyewall and contained a smaller percentage of convection. The distribution of precipitation in the eyewall and rainband regions was asymmetric. For several hours early in the observing period, the maximum rain rates in the eyewall and rainband regions of Alicia occurred in the left-front quadrant relative to the storm motion. Then, the heaviest rain in the eyewall region shifted to the right-front quadrant and that in the rainband region moved to the right of the storm track. In Elena, the maximum rain rates in the eyewall and rainband regions remained in the right-front quadrant throughout the computational period. About 55% of the precipitation in Elena's eyewall region occurred in the right-front quadrant.
Burpee, R.W., M.L. Black, and F.D. Marks. Vertical motions measured by airborne Doppler radar in the core of Hurricane Elena. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 69-70, 1989
Carbone, R.E., and F.D. Marks. Velocity track display (VTD): A real-time application for airborne Doppler radar data in hurricanes. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 11-12, 1989
DeMaria, M. A nested spectral model for hurricane track forecasting. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 206-207, 1989
DeMaria, M., J.M. Davis, and D.M. Wojtak. Observations of mesoscale wave disturbances during the Genesis of Atlantic Lows Experiment. Monthly Weather Review, 117(4):826-842, doi:10.1175/1520-0493(1989)117<0826:OOMWDD>2.0.CO; 1989
The Portable Automated Mesonet (PAM) data obtained during the Genesis of Atlantic Lows Fxperiment (GALE) are used to document mesoscale wave activity during the 3-day period from 4 to 6 February 1986. From the surface pressure analyses, four cases of wave activity are identified with wavelengths of 200-400 km, phase speeds of 20-40 m s-1 and trough-to-crest pressure amplitudes of 0.5-3.5 mb. Precipitation was associated with the waves in two of the four cases. Detailed analyses of the horizontal structure show that the waves do not have the pressure-wind relationship expected from linear gravity wave theory. The wind vectors are oriented from high to low pressure with a maximum amplitude between the high and low pressure areas. Low-level inversions were present in three of the four cases. In the raw without a low-level inversion, the amplitude rapidly decreased as the wave moved towards the east. In the case which lasted for the longest time period (at least 8 h) and had the largest pressure amplitude, the sounding bad acritical level (where the wind speed equated the wave speed) and a levelwhere the Richardson number was less than 0.25. Vertical velocities as large as 30 cm s-1 were observed and them was some evidence that the wave was vertically tiled towards its direction of motion. Complex principal component analysis (CPCA) is applied to the surface pressure data to determine the applicability of this technique to the study of mesoscale waves. It is shown that CPCA could be used to generalize the results of this study to the entire 60-day period of GALE.
Dodge, P.P. The precipitation structure of Hurricane Elena. Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 522-524, 1989
Franklin, J.L. Objective analyses of Omega dropwindsonde data from Hurricane Josephine (1984). Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 204-205, 1989
Franklin, J.L., C.S. Velden, C.M. Hayden, and J. Kaplan. A comparison of VAS and ODW data around a subtropical cold low. Preprints, 4th Conference on Satellite Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 141-144, 1989
Gamache, J.F. Retrieval of thermodynamic and microphysical variables from airborne Doppler observations in Hurricane Norbert (1984). Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 525-528, 1989
Gamache, J.F. The eyewall water budget of Hurricane Norbert (1984) as determined from airborne Doppler radar. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 73-74, 1989
Hirsh, M.A., and H.A. Friedman. Creating an awareness of the hurricane problem in at-risk coastal communities of south Florida. Preprints, Second International Conference on School and Popular Meteorological and Oceanographic Education, Crystal City, VA, July 12-16, 1989. American Meteorological Society, Boston, 160-162, 1989
Landsea, C.W., H.E. Willoughby, and J.M. Masters. Analysis of Hurricane Gilbert at its maximum intensity. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 222-223, 1989
Lord, S.J. Vorticity advection from nested analyses of the hurricane environment. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 202-203, 1989
Lyons, W.A., M.G. Venne, P.G. Black, and R.C. Gentry. Hurricane lightning: A new diagnostic tool for tropical storm forecasting? Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 113-114, 1989
Marks, F.D. Kinematic structure of the inner core of hurricanes as viewed by airborne Doppler radar. Proceedings, Third Interagency Airborne Geosciences Workshop, La Jolla, CA, February 21-24, 1989. American Meteorological Society, Boston, IV79-IV81, 1989
Marks, F.D. Three-dimensional structure of the eyewall of Hurricane Emily (1987) as determined from an airborne Doppler radar. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 71-72, 1989
Marks, F.D., and S.J. Lord. Kinematic structure of Hurricane Gloria as viewed by airborne Doppler radar and Omega dropwindsondes. Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 529-532, 1989
Ooyama, K.V. Thermodynamics in the primitive form for modeling the moist atmosphere. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 157-158, 1989
Powell, M.D. Boundary-layer kinematic structure in outer hurricane rainbands. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 67-68, 1989
Powell, M.D. Boundary-layer structure and dynamics in outer hurricane rainbands. Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 533-536, 1989
Rappaport, E.N., and P.G. Black. The utility of special sensor microwave/imager data in the operational analysis of tropical cyclones. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, J21-J24, 1989
Shapiro, L.J. The relationship of the quasi-biennial oscillation to Atlantic tropical storm activity. Monthly Weather Review, 117(7):1545-1552, doi:10.1175/1520-0493(1989)117<1545:TROTQB>2.0.CO; 1989
Monthly averaged 30 and 50 mb zonal winds at Balboa are used to determine objectively the relationship of the quasi-biennial oscillation (QBO) to seasonal (August through October) Atlantic tropical storm activity during the years 1952-1986. The largest correlations between storm activity and the 30 mb wind are found in June, which is three months before the center of the season. Extrapolation and direct calculation confirm a near in-phase relationship between tropical storm activity and the zonal wind at about 50 mb. Zonal winds filtered to remove periods 1 yr are used to establish correlations between the QBO and tropical storm activity for 1955-1983 that are essentially independent of the month considered. A correlation at 30 mb is established with a conservative estimate of true skill, from both in-phase and out-of-phase information, that explains 30% of the variance in storm activity. The skill is much greater than that estimated from seasonal classifications of the QBO. The statistics are resilient to removal of the effects of the El Ni¤o cycle. When El Ni¤o years are explicitly excluded, the true skill explains an estimated 32% of the variance. Low-latitude storms are even more strongly related to the QBO. Physical mechanisms possibly responsible for the observed associations are discussed in light of these results. A mechanism for the observed correlations is suggested that emphasizes the difference between lower-tropospheric steering and the lower-stratospheric zonal wind. The relationships of the results, and suggested physical mechanism, to those of Gray are considered.
Shapiro, L.J. Vortex evolution on a beta plane. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 98-99, 1989
Shay, L.K., R.L. Elsberry, and P.G. Black. Vertical structure of the ocean current response to a hurricane. Journal of Physical Oceanography, 19(5):649-669, doi:10.1175/1520-0485(1989)019<0649:VSOTOC>2.0.CO; 1989
During the passage of Hurricane Norbert in 1984, the Hurricane Research Division of NOAA conducted a planetary boundary-layer experiment that included the deployment of airborne expendable current profilers (AXCP). A total of 16 AXCPs provided, for the first time, high-resolution vertical profiles of currents and temperatures in hurricanes wind conditions. This study focuses on the vertical structure of the near-inertial baroclinic currents excited by the passage of this hurricane. The transient, hurricane-induced currents are isolated from the AXCP profiles in Norbert by subtracting a spatially-averaged current. Near the center of Hurricane Norbert, the WKBJ-scaled vertical wavenumber spectra are a decade greater than the Garrett-Munk spectra (GM75). The first ten linear, baroclinic-free modes are calculated from the spatially-averaged, Brunt-Vaisala frequency. To allow a more direct comparison with the AXCP observations in the high-wind regime, the near-inertial response for the three-dimensional velocities is simulated by superposing a hurricane-like wind stress field onto the first ten baroclinic modes. About 70% of the current variance in Hurricane Norbert can be explained by a sum of only the first four near-inertial modes. Most of the ocean current variability can be accounted for by the wind stress curl, although the direct effect of the wind stress and the stress divergence do contribute to the observed current variance within 30-60 km from the storm. However, these last two effects rapidly diminish after one inertial period. Although the energy input by the hurricane forcing is spread over all of the vertical wavelengths, most of the energy is contained in the gravest four vertical modes which then govern the dynamics in the wake region.
Venne, M.G., W.A. Lyons, C.S. Keen, P.G. Black, and R.C. Gentry. Explosive supercell growth: A possible indicator of tropical storm intensification? Preprints, 24th Conference on Radar Meteorology, Tallahassee, FL, March 27-31, 1989. American Meteorological Society, Boston, 545-548, 1989
Willis, P.T., and A.J. Heymsfield. Hurricane microphysical trajectories. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 75-76, 1989
Willis, P.T., and A.J. Heymsfield. Structure of the melting layer in mesoscale convective system stratiform precipitation. Journal of the Atmospheric Sciences, 46(13):2008-2025, doi:10.1175/1520-0469(1989)046<2008:SOTMLI>2.0.CO; 1989
This study examines the aircraft observations and theoretical evolution of particles above, through, and below the melting layer in the stratiform region associated with a mesoscale convective system (MCS). The aircraft data were obtained from an advecting spiral descent where the descent rate approximately corresponded to the typical hydrometeor fall speeds. The microphysical and thermodynamic measurements not only allowed us to characterize the particle evolution, but also enabled us to compare them with the theoretical evolution of the particles in the melting layer and to quantify the associated heating and cooling rates. Even though complete melting requires a fairly deep layer, most of the mass melts, and thus most of the cooling occurs, in a thin layer above the location of the radar bright band. Based upon the magnitude of vertical velocity fluctuations, the layers below the melting layer appear to be decoupled from those above. The ice water content above the melting layer is two to three times the liquid water content below the melting layer. The production of a few, very large, aggregates is dramatic after the onset of melting, due in part to a melting-induced increase in the terminal velocity difference between similar-sized hydrometeors. The radar reflectivity maximum (bright band) is due to these relatively few, very large, aggregates that survive to warmer temperatures. The reflectivity maximum is depressed well below the isothermal layer and the level where most of the ice mass is melted. Above the melting layer, small crystals are replenished by a fragmentation or breakup process.
Willis, P.T., and P. Tattleman. Drop-size distributions associated with intense rainfall. Journal of Applied Meteorology, 28(1):3-15, doi10.1175/1520-0450(1989)028<0003:DSDAWI>2.0.CO;2 1989
The probability of occurrence of extreme rainfall rates is reviewed. The drop-size distributions associated with a range of high rainfall rates are examined using data from tropical storms and hurricanes. Mean drop-size distributions are presented for a range of high rainfall rates, as well as a Gamma-distribution fit to the entire set of normalized drop-size distributions. This fit forms the basis for a model drop-size distribution for intense rain. The goodness of fit of the model is examined by comparing it with independent drop-camera measurements of high-rain-rate distributions from several geographic locations. The slope of exponential fits to the distributions are examined for constancy with rainfall rate and are generally found to decrease with increasing rainfall rate.
Willoughby, H.E., J. Masters, and C.W. Landsea. A record minimum sea level pressure observed in Hurricane Gilbert. Monthly Weather Review, 117(12):2824-2828, doi:10.1175/1520-0493(1989)117<2824:ARMSLP>2.0.CO; 1989
On 13 September 1988, Hurricane Gilbert attained an extreme minimum sea level pressure, estimated to be 885 hPa from aircraft reconnaissance reports at the time. Post-season analysis indicates that the flight-level pressure, P, upon which this figure is based requires correction upward. In typhoons with sea level pressure P, the estimation has a bias toward low pressure. Although the aircraft did not release a dropsonde in the eye at minimum pressure, it is possible to calculate hydrostatic sea level pressures by assuming a variety of plausible thermal structures below flight level. With corrected P, both the statistical extrapolation with its bias removed and the hydrostatic calculations show that a revised value of 888 ± 2 hPa is closer to the true minimum sea level pressure. The standard deviation of the various approximations means that the probability is <3% that the actual minimum failed to reach a value below 892 hPa, the old record for a hurricane in the Atlantic Basin set by the Labor Day Hurricane of 1935.
Willoughby, H.E., W.P. Barry, and M.E. Rahn. Real-time monitoring of Hurricane Gilbert. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 220-221, 1989
Wood, V.T., and F.D. Marks. Hurricane Gloria: Simulated land-based Doppler velocities reconstructed from airborne Doppler radar measurements. Preprints, 18th Conference on Hurricanes and Tropical Meteorology, San Diego, CA, May 16-19, 1989. American Meteorological Society, Boston, 115-116, 1989
1988
Black, P.G., R.L. Elsberry, and L.K. Shay. Airborne surveys of ocean current and temperature perturbations induced by hurricanes. In Advances in Underwater Technology, Ocean Science, and Offshore Engineering, Vol. 16, Oceanology '88. Graham and Trotman, London, 51-58, 1988
Black, P.G., R.L. Elsberry, L.K. Shay, R.P. Partridge, and J.D. Hawkins. Atmospheric boundary layer and oceanic mixed layer observations in Hurricane Josephine obtained from air-deployed drifting buoys and research aircraft. Journal of Atmospheric and Oceanic Technology, 5(6):683-698, 10.1175/1520-0426(1988)005<0683:ABLAOM>2.0.CO;2 1988
Three drifting buoys were successfully air-dropped ahead of Hurricane Josephine. This deployment resulted in detailed simultaneous measurements of surface wind speed, surface pressure and subsurface ocean temperature during and subsequent to storm passage. This represents the first time that such a self-consistent data set of surface conditions within a tropical cyclone has been collected. Subsequent NOAA research overflights of the buoys, as part of a hurricane planetary boundary-layer experiment, showed that aircraft wind speeds, extrapolated to the 20 m level, agreed to within ±2 m s−1, pressures agreed to within ±1 mb, and sea-surface temperatures agreed to within ±0.8°C of the buoy values. Ratios of buoy peak 1 min wind (sustained wind) to one-half h mean wind >1.3 were found to coincide with eyewall and principal rainband features. Buoy trajectories and subsurface temperature measurements revealed the existence of a series of mesoscale eddies in the subtropical front. Buoy data revealed storm-generated, inertia-gravity-wave motions superposed upon mean current fields, which reached a maximum surface speed >1.2 m s−1 immediately following storm passage. A maximum mixed-layer-temperature decrease of 1.8°C was observed to the right of the storm path. A temperature increase of 3.5°C at 100 m and subsequent decrease of 4.8°C following storm passage indicated a combination of turbulent mixing, upwelling and horizontal advection processes.
Burpee, R.W. Forecaster biography: Grady Norton--hurricane forecaster and communicator extraordinaire. Weather and Forecasting, 3(3):247-254, doi:10.1175/1520-0434(1988)003<0247:GNHFAC>2.0.CO; 1988
DeMaria, M. The effect of physical processes on the convergence of Machenhauer's normal mode initialitzation scheme. Preprints, 8th Conference on Numerical Weather Prediction, Baltimore, MD, February 22-25, 1988. American Meteorological Society, Boston, 751-757, 1988
DeMaria, M., and J.D. Pickle. A simplified system of equations for simulation of tropical cyclones. Journal of the Atmospheric Sciences, 45(10):1542-1554, doi:10.1175/1520-0469(1988)045<1542:ASSOEF>2.0.CO; 1988
A simplified system of equations which can simulate the development and mature stages of tropical cyclones is presented. The model is similar to that presented by Ooyama, except that the assumption of incompressible fluid layers is relaxed. Instead, the governing equations for a compressible fluid in isentropic coordinates are discretized vertically by considering three fluid layers with constant potential temperature. This makes the inclusion of thermodynamic processes more straightforward. The governing equations in the adiabatic case are mathematically equivalent to the equations used by Ooyama, except with an extra term in the pressure gradient force. The model equations are solved using a spectral method where the basis functions are the normal modes of the linearized equations. Numerical simulations show that the model sensitivity to vertical stability, sea surface temperature and midlevel moisture are similar to results from more general models. The sensitivity to these factors can be explained qualitatively by consideration of the vertical stability factor used in the cumulus parameterization. The sensitivity to latitude is investigated in more detail in previous work. Low-latitude storms are smaller than high-latitude storms, but intensify more rapidly initially. This difference is related to the radial positioning of the diabatic heating which is much closer to the storm center for low-latitude storms. This occurs since the air in the boundary layer can penetrate closer to the storm center when the Coriolis force is smaller. The model is also initialized with climatological values of sea surface temperature as a function of latitude appropriate for the western Pacific. Despite its simplicity, the model can reproduce the observed variations of storm size and intensity with latitude.
Dodge, P.P. A climatology of rainbands observed by coastal radars in GALE. Reports of GALE/CASP Preliminary Analysis Workshop, Virginia Beach, VA, November 2-6, 1987. GALE Project Office, NCAR, Boulder, 19-22, 1988
Franklin, J.L., and S.J. Lord. Comparisons of VAS and Omega dropwindsonde thermodynamic data in the environment of Hurricane Debby (1982). Monthly Weather Review, 116(8):1690-1701, doi:10.1175/1520-0493(1988)116<1690:COVAOD>2.0.CO; 1988
Synoptic-scale thermodynamic fields in the environment of Hurricane Debby (1982) determined from two sets of VAS soundings (VAS1, VAS2) are compared with those obtained from in-situ data (INS). VAS1 sounding were derived from an iterative solution of the radiative transfer equation with manual quality control. VAS2 soundings, which represent the present state-of-the-art, were derived from a simultaneous solution of the transfer equation with objective quality control. In situ data were obtained primarily from Omega dropwindsondes. Comparisons are made for 0000 UTC 16 September 1982 at the mandatory pressure levels up to 400 mb. The integrated effect of VAS-INS differences is estimated by comparing 400 mb geopotential height fields and their associated gradient winds. The comparisons show that the VAS1-INS temperature differences are not spatially uniform at most levels, due largely to the influence of moisture. The quality of the VAS2 data is much improved over VAS1; the effect of moisture is not noticeable. However, the VAS2 analyses still show spatially nonuniform differences from INS at some levels. Thus, VAS gradient data may be of irregular quality on the synoptic scale. Geopotential height fields at 400 mb imply gradient wind differences from INS of up to 12 m s-1 for VAS 1 and 6 m s-1 for VAS2. The VAS2 sounding set could be improved further by the use of manual data editing, and a more accurate first-guess of the surface temperature analysis.
Franklin, J.L., S.J. Lord, and F.D. Marks. Dropwindsonde and radar observations of the eye of Hurricane Gloria (1985). Monthly Weather Review, 116(5):1237-1244, doi:10.1175/1520-0493(1988)116<1237:DAROOT>2.0.CO; 1988
Two soundings from the eye of Hurricane Gloria (1985) during a period of rapid deepening are described. The soundings were made by Omega dropwindsondes (ODWs) during research flights of the NOAA Hurricane Research Division on 24-25 September 1985. During the 4.7 h between the two ODW drops, Gloria's minimum sea-level pressure fell from 932 to 922 mb. The ODWs indicate substantial warming due to dry adiabatic descent from 580-660 mb. Descent rates are estimated to be about 11 cm s-1. Near 500 mb, ascent is indicated. Approximately 60% of the 10 mb pressure fall is associated with thermodynamic changes below 500 mb.
Gamache, J.F., F.D. Marks, and R.A. Black. The bulk water budget of Hurricane Norbert (1984) as determined from thermodynamic and microphysical analyses retrieved from airborne Doppler radar. Preprints, 10th International Cloud Physics Conference, Bad Homburg, Federal Republic of Germany, August 15-20, 1988. American Meteorological Society, Boston, 711-713, 1988
Houze, R.A., F.D. Marks, and R.A. Black. Mesoscale patterns of ice particle characteristics in Hurricane Norbert. Preprints, 10th International Cloud Physics Conference, Bad Homburg, Federal Republic of Germany, August 15-20, 1988. American Meteorological Society, Boston, 708-710, 1988
Lord, S.J., and J.L. Franklin. Diagnostics of thermodynamic and wind fields in the environment of Hurricane Debby (1982). Preprints, 8th Conference on Numerical Weather Prediction, Baltimore, MD, February 22-25, 1988. American Meteorological Society, Boston, 605-610, 1988
Lord, S.J., and J.M. Lord. Vertical velocity structures in an axisymmetric, nonhydrostatic tropical cyclone model. Journal of the Atmospheric Sciences, 45(9):1453-1461, doi:10.1175/1520-0469(1988)045<1453:VVSIAA>2.0.CO; 1988
A statistical analysis of several experiments with different microphysical parameterizations in an axisymmetric, nonhydrostatic tropical cyclone model illustrates the impact of icc-phase microphysics on model vertical velocity structure. The parameterizations are designed to illustrate the effects of (1) thermodynamic input through latent heating, (2) vertical sorting of microphysical species by fall speed, and (3) different rates of the parameterized microphysical conversion processes. The results confirm previous studies on the thermodynamic effect of melting, but they also show that the other factors, namely, fall speed and microphysical conversion rates, are important in determining model vertical velocity structure and evolution. Statistical summaries of updrafts and downdrafts show distinct increases in the intensity and horizontal scale of downdrafts near the melting level when parameterized snow is included. Model storms without snow show a greater percentage of convective-scale updrafts and downdrafts; they intensify more slowly but ultimately become stronger than those that have larger scale vertical velocity structures.
Powell, M.D. Boundary-layer structure and dynamics in outer hurricane rainbands. Ph.D. thesis, Florida State University, Tallahassee, 227 pp., 1988
Results of hurricane boundary-layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation, kinematic, and thermodynamic structures in these storms indicate that principal rainbands have common characteristic mesoscale and convective-scale features in the boundary layer. The two-dimensional mesoscale structure suggests that rainbands are made of a linear aggregate of cellular reflectivity elements (on the inner, upshear side of the band) and stratiform rain (on the outer downshear side). The band is oriented perpendicular to the shear above the boundary layer and cells move downband at about 80% of the maximum wind. Alongband and crossband wind and equivalent potential temperature maxima are located on the outer side of the band axis. Updrafts and downdrafts are preferentially located on the inner side of the band axis. Downdraft transport of cool and dry air from middle levels on the inner side of the rainband was responsible for modifying mixed-layer structure adjacent to the band on alongband scales of 100 km. An undisturbed mixed layer of 500 m was present on the outer side of the band. Application of a mixed-layer model to low-level flow trajectories from the outer rainband to the eyewall indicates that under some conditions, the mixed layer may not recover sufficiently and low surface equivalent potential temperature air may reach the eyewall. These conditions are associated with suppressed flow in a region of positive divergence with moderate rainfall from a middle level anvil cloud. Differential evaporation cooling over the transition layer drives entrainment of dry air, resulting in a drier mixed layer (with lower surface equivalent potential temperature). The model results suggest that incomplete recovery may be responsible for transitional changes in hurricane intensity.
Shapiro, L.J., D.E. Stevens, and P.E. Ciesielski. A comparison of observed and model-derived structures of Caribbean easterly waves. Monthly Weather Review, 116(4):921-938, doi:10.1175/1520-0493(1988)116<0921:ACOOAM>2.0.CO; 1988
A linear primitive equation model has been used to test the hypothesis that the vertical structure of observed Caribbean easterly waves is determined by the interaction between convective heating and the environmental wind. The model determines the response to a propagating heat source in a specified basic state. The model allows for the inclusion of diffusion and cumulus momentum transports. The linear perturbations are assumed to have the form of a single Fourier component in the zonal direction. The frequency and zonal wavelength of the disturbance are taken from observations of the three-dimensional structure of a series of Caribbean easterly waves made by Shapiro. The structure of the basic state zonal wind, assumed to be a function of height, is based on observations near the latitude of largest observed wave amplitude. The maximum heating rate is 5 K day-1, centered at about 19°N. Very good agreement is found between the model-derived vertical structure of the waves and that observed by Shapiro (1986). In particular, the observed 90° westward phase shift between the 200 mb and near-surface troughs, and the westward tilt of the trough axis with height, are reproduced in the model solutions. Although linearization is not strictly valid for the observed wind amplitudes of 5 m s-1, the model's linear dynamical framework appears to represent the wave's structure well. The westward phase shift is found to depend on the downward flux of wave energy toward a near-critical layer near the ground. Experiments also suggest that the latitude of the disturbance may be as important a factor in the determination of the westward tilt of the trough axis as is the structure of the basic state zonal wind. An eastward tilt of the trough axis in the lower troposphere, such as that in the classical model of a Caribbean easterly wave, can occur at low latitudes, when the westward phase shift is in a narrow layer near the level of maximum heating. Cumulus momentum transports do not substantially change the structure of the forced wave disturbance. The model solutions are compared with similar experiments of Holton, and are related to results of Stevens, Lindzen and Shapiro.
Willis, P.T., and A.J. Heymsfield. Melting-layer structure in MCC stratiform precipitation. Preprints, 10th International Cloud Physics Conference, Bad Homburg, Federal Republic of Germany, August 15-20, 1988. American Meteorological Society, Boston, 699-701, 1988
Willoughby, H.E. Linear motion of a shallow-water, barotropic vortex. Journal of the Atmospheric Sciences, 45(13):1906-1928, doi:10.1175/1520-0469(1988)045<1906:LMOASW>2.0.CO; 1988
A shallow-water barotropic model of tropical cyclone motion allows calculation of linear wavenumber one perturbations on a maintained, moving axisymmetric vortex. The perturbations are Rossby waves that depend upon the radial gradient of axisymmetric relative vorticity rather than the meridional gradient of absolute vorticity. Although the motion of the vortex is a parameter for calculation of the perturbations, the motion in a particular situation is determinate because it minimizes the Lagrangian of the system. The motion in an environmental current matches the current, except at frequencies where the vortex is barotropically unstable. Imposed sources and sinks of mass simulate the effects of convection. The "convectively-induced"motions excite the barotropic instability plus a mode that depends uponforcing at the Rossby wave critical radius. This mode has largest amplitude and fastest vortex motion at the orbital frequency of the axisymmetric flow where forcing is imposed. It seems to correspond with the trochoidal motion of real tropical cyclones. For cyclonic frequencies only, perturbation in the stream-function field resembles a solitary Rossby wave and exhibits counterrotating gyres isolated from the relative flow due to the vortex motion. The vortex motion on a beta plane is largely meridional with speed proportional to the total relative angular momentum of the vortex. When the vortex has cyclonic circulation throughout, the northward motion is much too fast. This unreasonable result highlights the importance of nonlinear processes in tropical cyclone motion.
Willoughby, H.E. The dynamics of the tropical cyclone core. Australian Meteorological Magazine, 36(3):183-191, 1988
The core of a tropical cyclone occupies the inner 100-200 km of the vortex. It is dominated by a cyclonic primary circulation in balance with a nearly axisymmetric, warm core low-pressure anomaly. Superimposed on the primary circulation are weaker asymmetric motions and an axisymmetric secondary circulation. The asymmetries, which may be either internal gravity waves or Rossby waves, modulate precipitation and cloud into trailing spirals. The axisymmetric secondary circulation, driven by latent heat release and surface friction, comprises the following parts: surface inflow that extracts latent heat from the sea and replaces the frictional loss of angular momemtum (M) to the sea; diabatically forced deep inflow that supplies an excess of M above frictional loss; the eyewall, an outward sloping locus of convective ascent; diabatically forced descent inside the eye; and upper tropospheric inflow. The eyewall usually moves inward as a result of differential adiabatic heating across the wind maximum. Eyewall succession occurs in intense cyclones when two concentric eyewalls are present and the outer replaces the inner. Because of their semibalanced dynamics, the primary and secondary circulations are relatively simple and well understood. These dynamics are not valid in the upper troposphere where the outflow is comparable to the swirling flow, nor do they apply to the asymmetric motions. Since the synoptic-scale environment appears to interact with the vortex core in the upper troposphere by means of the asymmetric motions, future research should emphasize this aspect of the tropical-cyclone dynamics.
1987
Barnes, G.M., G.J. Stossmeister, M.A. LeMone, and J.F. Gamache. A rainband on the trailing side of a fast-moving hurricane. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 377-380, 1987
Black, M.L., and R.W. Burpee. Temporal and spatial variations of precipitation near the center of tropical cyclones. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 170-173, 1987
Black, P.G., and F.D. Marks. Environmental interactions associated with hurricane supercells. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 416-419, 1987
Bluestein, H.B., and F.D. Marks. On the structure of the eyewall of Hurricane Diana (1984): Comparison of radar and visual characteristics. Monthly Weather Review, 115(10):2542-2552, doi10.1175/1520-0493(1987)115<2542:OTSOTE>2.0.CO;2 1987
Features seen in aerial and satellite photographs of the inside edge of the eyewall of Hurricane Diana (1984) are compared with features seen in digitized three-dimensional airborne radar reflectivity data. The photographs show regularly spaced, upwind (downshear) tilted striations in the northeast, east, and southeast sectors of the eyewall that are nearly collocated with upwind (downshear) tilted axes of relative reflectivity maxima of approximately 15 dBZ.
Burpee, R.W., J. Ward, and D.G. Marks. A preliminary evaluation of Omega dropwindsonde data in track forecasts of hurricanes. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 98-100, 1987
Ciesielski, P.E., L.J. Shapiro, and D.E. Stevens. A comparison of the observed and model-derived structures of tropical easterly waves. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 249-252, 1987
DeMaria, M. Tropical cyclone track prediction with a barotropic spectral model. Monthly Weather Review, 115(10):2346-2357, doi:10.1175/1520-0493(1987)115<2346:TCTPWA>2.0.CO; 1987
A barotropic spectral model (BSM) is developed to investigate the possibility of forecasting tropical cyclone tracks with global, general circulation models. The model is governed by a barotropic vorticity equation in spherical coordinates which is solved with a spectral method with spherical harmonic basis functions. The model was run with a triangular truncation of 128 on half of the northern hemisphere (180°W-0°W), and was initialized using horizontal winds from the NMC analyses vertically averaged from 1000 to 100 mb. The storm circulation is represented by a specified axisymmetric vortex and the model was tested by making 30 track forecasts of Atlantic tropical cyclones (13 storms) which occurred from 1979 to 1984. The skill of the model was assessed by comparing the track forecasts to forecasts from a model based on climatology and persistence (CLIPER). The BSM has statistically significant skill for 24 and 36 h track forecasts and longer range of skill for forecasts of low-latitude storms. For low-latitude storms, the BSM had than the operational SANBAR and moveable fine mesh (MFM) models. The sensitivity of the model to the horizontal resolution is tested. These results suggest that track forecasts could be made with a global spectral model with a triangular truncation of about 96. It might then be feasible to make track forecasts with a global spectral model similar to the operational model at the European Centre for Medium Range Weather Forecasts which uses a triangular truncation of 106. The sensitivity of the model to the domain size and to the specification of the initial vortex is also investigated. These results show that when simple lateral boundary conditions are used, the track forecast errors rapidly increase when the model domain is made smaller than half of a hemisphere. These results also show that the track errors are very insensitive to the size of the vortex, provided that the vortex is not unrealistically large. When the shape of the vortex profile is changed to include an anticyclonic circulation at large radii, the track errors are smallest when the total angular momentum of the vortex is close to zero. The errors rapidly increase as the total angular momentum becomes negative. The effect of modifying the initial analyses so that the deep-layer mean wind in the storm region is consistent with the previous storm motion is also studied. The track errors show the most reduction when the analyses within a radius of about 1000 km of the norm am modified.
Dodge, P.P., M.L. Black, R.W. Burpee, and F.D. Marks. Time-lapse radar imagery from landfalling hurricanes. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 166-169, 1987
Esbensen, S.K., L.J. Shapiro, and E.I. Tollerud. The consistent parameterization of the effects of cumulus clouds on the large-scale momentum and vorticity fields. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 48-51, 1987
Esbensen, S.K., L.J. Shapiro, and E.I. Tollerud. The consistent parameterization of the effects of cumulus clouds on the large-scale momentum and vorticity fields. Monthly Weather Review, 115(3):664-669, doi:10.1175/1520-0493(1987)115<0664:TCPOTE>2.0.CO; 1987
A physical and mathematical framework for the mutually consistent parameterization of the effects of cumulus convection on the large-scale momentum and vorticity fields is proposed. The key to achieving consistency is the understanding that the vorticity dynamics of the clouds below the spatial resolution of a large-scale dynamical model may be neglected in the vorticity budget when the clouds are considered to be independent buoyant elements sharing a common large-scale environment. This simplified approach is used to obtain a consistent pair of large-scale momentum and vorticity equations based on Ooyama's theory of cumulus parameterization. The results focus attention on the need to obtain a better understanding of the detrainment process and the pressure interactions between the clouds and their environment.
Franklin, J.L. Reduction of errors in Omega dropwindsonde data through postprocessing. NOAA Technical Memorandum, ERL-AOML-65 (PB87-173308), 22 pp., 1987
The postprocessing of Omega dropwindsonde (ODW) data at the NOAA Hurricane Research Division (HRD) is described. The errors common to ODW data are illustrated with examples, and the improvements to ODW accuracy through postprocessing are estimated.
Franklin, J.L., K.V. Ooyama, and S.J. Lord. Two improvements in Omega windfinding techniques. Journal of Atmospheric and Oceanic Technology, 4(1):214-219, doi:10.1175/1520-0426(1987)004<0214:TIIOWT>2.0.CO; 1987
A one-dimensional local spline smoothing technique is applied to Omeganavigational signals for the purpose of windfinding. Wind profiles soproduced depend largely on two parameters of the smoothing procedure:the nodal spacing, which determines the smallest resolvable scale, and afiltering wavelength, which produces the necessary smoothing of the phasedata, and prevents representational distortion of any power from theunresolved scales. Phase "noise" from stationary test sondes issuperimposed on synthetic Omega signals to compare wind profiles obtainedwith this new procedure with profiles computed using other techniques.It is shown that the effect of aircraft maneuvers on Omega wind accuracyis not completely removed by the normal practice of evaluating all phasederivatives at a common time. Additional improvements in accuracy of 2-3m s-1 can be obtained by a "rate-aiding" technique using aircraft navigational data.
Gamache, J.F. The bulk water budget of Hurricane Norbert (1984). Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 351-354, 1987
Goldenberg, S.B., S.D. Aberson, and R.E. Kohler. An updated, fine-grid version of the operational barotropic hurricane-track prediction model. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 86-89, 1987
Jin, H.-L., H.E. Willoughby, and J.M. Piotrowicz. Modeled hurricane's structure and evolution by a nonhydrostatic thermo-convective hurricane model (no ice phase). Proceedings, 7th National Conference on Typhoons, Shanghai, China, November 4, 1985. Shanghai Typhoon Institute, 221-230, 1987
By using an axisymmetric, nonhydrostatic, thermo-convective storm model the genesis/evolution processes of tropical storms were simulated. The simulated hurricane structure, particularly in the inner region of the modeled storm, is very similar to those observed. The eyewall is composed of continuously altering convective cloud cells and the radius of the eye core discontinuously varies with their life cycle and their substitution. Three contract simulations have shown that thermal convections act vigorously, thus an intense hurricane is developed as the sea surface temperature increases while convections delay beginning and proceed slowly. Thus a longer pregnant (genetic) period is needed for the storm formation as the upper tropical environment above 700 hPa dried.
Jones, R.W. A simulation of hurricane landfall with a numerical model featuring latent heating by the resolvable scales. Monthly Weather Review, 115(10):2279-2297, doi:10.1175/1520-0493(1987)115<2279:ASOHLW>2.0.CO; 1987
A nested grid hurricane model is used to transport a strong vortex over a straight coastline at about 4 m s–1. The track, at landfall, of the vortex is about 20 km to the left of a control simulation without land. Just before landfall, a 15 km amplitude trochoidal oscillation of the vortex track occurs. This amplitude is nearly double that of similar oscillations of the control simulation. About 10 h before landfall, a spiral rainband nearly surrounds the vortex at radii of about 135 km. This rainband has a weak secondary maximum in the tangential wind and is the model analog of the secondary eyewalls observed by Willoughby et al. in several hurricanes. The rainfall in spiral rainbands diminishes during the 7 h before landfall. However, rainfall in the inner core of the vortex is greater during landfall than in the control simulation. The greatest rainfall accumulation is to the right of the vortex. However, compared with the control simulation, rainfall is greater to the left and less to the right of the vortex. This may be the result of an increase of the relative radial inflow in the boundary layer in the left-front quadrant near landfall. To the extent which is possible, these characteristics of landfall are related to observations.
Lord, S.J. The 17th Conference on Hurricanes and Tropical Meteorology, 7-10 April 1987, Miami, FL. Bulletin of the American Meteorological Society, 68(11):1431-1437, 1987
Lord, S.J., and J.L. Franklin. The environment of Hurricane Debby (1982). Part I: Winds. Monthly Weather Review, 115(11):2760-2780, doi:10.1175/1520-0493(1987)115<2760:TEOHDP>2.0.CO; 1987
A three-dimensional, nested analysis of wind fields in the environment of Hurricane Debby (1982) has been completed. The basic analysis tool uses a two-dimensional least-squares fitting algorithm combined with a derivative constraint that acts as a spatial low-pass filter on the analyzed field. Gridded results of horizontally analyzed fields are combined into vertical cross sections and then analyzed to produce vertical continuity. Consequently, a three-dimensional analysis is obtained. The database for the analysis comes primarily from Omega dropwindsondes (ODWs), rawinsondes, and satellite-derived winds above 400 mb in the environment of Hurricane Debby near 0000 UTC 16 September 1982. Since these data come from many different sources, and thus are not evenly distributed in the horizontal or vertical, techniques have been developed to alleviate difficulties associated with inhomogeneous data. The analyzed wind fields provide an independent evaluation of satellite-derived winds at and below 400 mb. General features of the environmental wind fields surrounding Debby are described. The wind analyses are then used to diagnose terms in the vorticity equation. The spatial orientation of a calculated dipole in the horizontal vorticity flux convergence term indicates that it is an approximate indicator of Debby's observed short-term motion. Finally, to provide an initial assessment of the wind analysis quality, experimental track forecasts with a barotropic model are performed with the layer-mean wind fields and operationally available data outside the analysis domain. Initial errors in the forecast tracks are directly related to the orientation of the diagnosed vorticity flux convergence dipole. The research wind analysis results in a substantial reduction in track error for short-term (12 h) forecasts compared to analyses from operationally available data. This reduction is due to an improved representation of the wind fields in the near-storm environment.
Lord, S.J., and J.L. Franklin. Wind analyses for the environment of Hurricane Debby (1982): Some diagnostic calculations and forecast experiments. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 94-97, 1987
Marks, F.D., and R.A. Houze. Inner core structure of Hurricane Alicia from airborne Doppler radar observations. Journal of the Atmospheric Sciences, 44(9):1296-1317, doi:10.1175/1520-0469(1987)044<1296:ICSOHA>2.0.CO; 1987
Airborne Doppler radar measurements are used to determine the horizontal winds, vertical air motions, radar reflectivity, and hydrometer fallspeeds over much of the inner-core region (within 40 km of the eye) of Hurricane Alicia (1983). The reconstructed flow field is more complete and detailed than any obtained previously. The data show both the primary (azimuthal) and secondary (radial-height) circulations. The primary circulation was characterized by an outward sloping maximum of tangential wind. The secondary circulation was characterized by a deep layer of radial inflow in the lower troposphere and a layer of intense outflow above 10 km altitude. The rising branch of the secondary circulation was located in the eyewall and sloped radially outward. Discrete convective-scale bubbles of more intense upward motion were superimposed on this mean rising current, and convective-scale downdrafts were located throughout and below the core of maximum precipitation in the eyewall. Precipitation particles in the eyewall rainshaft circulated 18-20 km downwind as they fell, consistent with the typical upwind slope with increasing altitude of eyewall precipitation cores. Outside the eyewall, the precipitation was predominantly stratiform. A radar bright band was evident at the melting level. Above the melting level, ice particles were advected into the stratiform region from the upper levels of the eyewall and drifted downward through a mesoscale region of ascent. Hypothetical precipitation particle trajectories showed that as these particles fell slowly through the mesoscale updraft toward the melting level, they were carried azimuthally as many as 1½ times around the storm. During this spiraling descent, the particles evidently grew vigorously. The amount of water condensed by the ambient mesoscale ascent exceeded that transported into the stratiform region by the eyewall outflow by a factor of 3. As the particles fell into the lower troposphere, they entered a mesoscale region of subsidence, the top of which coincided with the radar bright band.
Marks, F.D., and R.A. Houze. Three-dimensional structure of the eyewall of Hurricane Norbert as determined from an airborne Doppler radar. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 347-350, 1987
Ooyama, K.V. Numerical experiments of steady and transient jets with a simple model of the hurricane outflow layer. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 318-320, 1987
The major topic of this paper is the resolvable spatial scales that can be analyzed by statistical interpolation of an undersampled data set. The inquiry was motivated by the need to design the most appropriate procedures for spatial analysis of the upper air sounding data from the GARP Atlantic Tropical Experiment. A reliable representation of horizontal scales in the analyzed wind fields was a matter of utmost concern, since the derived fields of vorticity, divergence, and vertical motion were also of vital interest. To achieve our goal, it was found that the traditional premise of statistical interpolation had to be reexamined. The main conclusions of this theoretical inquiry are: (1) resolvable scales are determined by the geometrical distribution of observing stations; (2) precise knowledge of the second-moment statistics improves the analysis by de-aliasing the amplitude of resolvable scales, but has no effect on the definition of resolvable scales; (3) residual effects of unresolvable signals in the data are removable by a spatial filter and must be so removed; and (4) spatial phases of de-aliased resolvable scales may still be in error. On the basis of these findings, the objective analysis procedures we have developed are targeted on the best achievable analysis of resolvable scales. The procedures include the following: an adequate estimate of “true” statistical fields from the given ensemble of data, a search for the optimum spatial filter by monitoring the targeted error variance, and a rational method of desensitizing the analysis to statistically errant data. In order to reduce the spatial phase error of propagating disturbances, the procedures are extended to the analysis of the time-wise Fourier-transformed data set (actually in the frequency-band analog). Since the wind is a physical vector, the entire procedure for the wind analysis is given in the tensor-invariant form, which is decidedly advantageous for very practical reasons. For example, the tensor approach eliminates the notorious ambiguity in normalization that is encountered in the multivariate approach. The paper also describes, in the Appendix, a method of filtered mechanical interpolation, which is specifically designed, with a variety of optional boundary conditions, for application to analysis in a finite domain.
Powell, M.D. Boundary-layer structure in convective hurricane rainbands. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 373-376, 1987
Powell, M.D. Changes in the low-level kinematic and thermodynamic structure of Hurricane Alicia (1983) at landfall. Monthly Weather Review, 115(1):75-99, doi:10.1175/1520-0493(1987)115<0075:CITLLK>2.0.CO; 1987
Aircraft, land station, and buoy data were composited with respect to the center of Hurricane Alicia (1983) for three 8 h periods corresponding to prelandfall in the open Gulf of Mexico, landfall in the Galveston area, and postlandfall in the vicinity of Houston. Comparison of the wind analyses before, during, and after landfall emphasizes the land-sea frictional asymmetry at landfall. In addition, other asymmetries in the surface wind field and differences between the flight-level and the surface wind fields are revealed. The asymmetric structure of the surface wind field may be interpreted as having resulted from the combined effects of land-sea roughness differences, background environmental flow, and storm translation. The land-sea frictional difference acted to oppose the mean vortex flow over land and reinforce it over water. The southwest background environmental flow acted nearly parallel to the coastline, producing surface inflow on the left side and outflow on the right side, while the effect of the storm translation increased winds on the right and decreased winds on the left. At landfall, the analysis revealed a broad region of high wind speeds and a mesoscale divergence-convergence couplet along the outer rainband axis just offshore on the northeast (right) side of the storm. The outer rainband axis acted as an obstruction to the surface flow, separating the warmer central core of the storm from the environment through which the storm moved. In contrast to recent numerical model studies, surface convergence was also noted on the left side of the storm just offshore, despite outflow at flight level. Analyses of temperature, dew point, and equivalent potential temperature indicate that loss of the oceanic heat and moisture source, combined with advection of drier air on the landward side of the storm, was responsible for cooling and drying of the inflowing boundary layer air. Upon introduction of this air into the core convection and vertical ascent, a decrease in the release of latent heat could then lead to cooling in the middle levels of the storm and a subsequent increase in the central sea-level pressure.
Powell, M.D., and P.N. Georgiou. Response of the Allied Bank Plaza tower during Hurricane Alicia (1983). Journal of Wind Engineering and Industrial Aerodynamics, 26(2):231-254, https://doi.org/10.1016/0167-6105(87)90046-8 1987
As Hurricane Alicia passed over Houston on August 19, 1983, a record lasting approximately 90 minutes was obtained of the wind-induced accelerations of the tallest building in the downtown area, the Allied Bank Plaza. Coincidently, the building had been the subject of a detailed wind tunnel model study several years earlier, the results of which included the prediction of building accelerations as a function of wind speed and wind direction. From the many wind observations made during Alicia’s passage inland, it was possible to reconstruct the wind speeds and directions experienced at the Allied Bank Plaza site which overlapped the period of the acceleration record. This reconstructed wind history was combined with the wind tunnel test data to compute a time series of estimated accelerations sustained by the building during Alicia’s passage inland. The resulting favorable comparison of actual and predicted accelerations provides a valuable case study, illustrating the reliability of wind tunnel modeling within the design process for tall buildings.
Sanford, T.B., P.B. Black, J.R. Haustein, J.W. Feeney, G.Z. Forristall,and J.F. Price. Ocean response to a hurricane. Part I: Observations. Journal of Physical Oceanography, 17(11):2065-2083, doi:10.1175/1520-0485(1987)017<2065:ORTAHP>2.0.CO; 1987
The response of the ocean was investigated using aircraft-deployable expendable current profilers (AXCP). The goals were to observe and separate the surface wave and surface mixed-layer velocities under the storms and to map the across-track and along-track velocity and temperature response in the mixed layer and thermocline. Custom instrumentation was prepared, including slower falling AXCPs, and the AXCP equipment was installed on NOAA WP-3D aircraft. Research flights were made into two 1984 hurricanes: Norbert, in the western Pacific off Baja, California (19°N, 109°W); and Josephine, off the west coast of the United States (29°N, 72°W). Thirty-one probes were deployed in each hurricane. All but four AXCPs survived the 220-knot launch and wave-zone impact (surface winds up to 75 knots) and produced basic RF transmissions. About half the AXCPs provided temperature and velocity profiles. Most velocity profiles exhibited strong surface wave contributions, slab-like velocities in the SML, strong shears beneath the SML, and only weak flows in the upper thermocline. Separation of the surface gravity wave velocities from the steady and inertial motions was obtained by fitting the profiles to steady flows and shears in three layers and to a single surface wave at all levels. The velocity profiles displayed large divergences to the horizontal SML velocities in the wake of the hurricanes. The observations show a strong enhancement to the right of the storm as expected from numerical simulations. The largest SML velocities were 1.1 m s-1 in Norbert and 0.73 m s-1 in Josephine. Numerical simulations will be compared with the observations in Part II.
Shapiro, L.J. Month-to-month variability of the Atlantic tropical circulation and its relationship to tropical storm formation. Monthly Weather Review, 115(11):2598-2614, doi:10.1175/1520-0493(1987)115<2598:MTMVOT>2.0.CO; 1987
Monthly mean winds have been derived from 200 mb and Analysis of the Tropical Oceanic Lower Layer (ATOLL) winds over the southern North Atlantic, Caribbean, Gulf of Mexico, and eastern Pacific during the hurricane seasons (June-November) of 1975 through 1985. After removal of the seasonal cycle, the winds are expressed in terms of empirical orthogonal functions. The dominant mode of variability for the combined 200 mb/ATOLL circulation strongly resembles part of a Walker cell confined near the equator. This mode is strongly correlated with El Niño index (Weare, 1986), and is associated with the El Niño/Southern Oscillation. A positive (El Niño-like) index tends to be associated with more anticyclonic vorticity at the ATOLL level in the tropics and increases in the vertical shear between about 10° and 30°N. This circulation is unfavorable for tropical storm formation. Correlations are derived between the monthly mean winds and monthly tropical storm frequency in the Atlantic basin. Contemporaneous correlations in August, September and October, the three most active months, as well as correlations between winds and tropical storm formation one and two months later, are computed. Predictability of monthly tropical storm frequency at the two-month lead is statistically significant, with true skill approximately 45% of the variance. Only one-sixth of this skill is associated with the El Niño/Southern Oscillation A favorable environment for storm formation is apparently established at least two months before the given month of formation. The results extend and complement predictions of seasonal tropical storm activity and previous hypotheses concerning the influence of El Niño on storm formation.
Shapiro, L.J. Month-to-month variability of the Atlantic tropical circulation and its relationship to tropical storm formation. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 231-233, 1987
Shay, L.K., R.L. Elsberry, and P.B. Black. Mesoscale ocean temperature and current patterns induced by hurricanes. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 388-392, 1987
Simpson, R.H., J. Simpson, and S.L. Rosenthal. Hurricane. In Encyclopedia of Science and Technology, 6th Edition, Volume 8, McGraw-Hill, New York, 544-557, 1987
Tanner, A., C.T. Swift, and P.B. Black. Operational airborne remote sensing of wind speeds in hurricanes. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 385-387, 1987
Velden, C.S., and S.B. Goldenberg. The inclusion of high-density satellite wind information in a barotropic hurricane forecast model. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 90-93, 1987
Willis, P.T., and F.D. Marks. Convective-scale transports in a mature hurricane. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 343-346, 1987
Willoughby, H.E. Tropical cyclone track prediction: Some theoretical aspects. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 262-265, 1987
Willoughby, H.E., and W.P. Barry. Real-time data acquisition and analysis in Hurricane Charley of 1986. Preprints, 17th Conference on Hurricanes and Tropical Meteorology, Miami, FL, April 7-10, 1987. American Meteorological Society, Boston, 341-342, 1987
1986
Black, P.G., F.D. Marks, and R.A. Black. Supercell structure in tropical cyclones. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP255-259, 1986
Black, P.G., R.A. Black, J. Hallett, and W.A. Lyons. Electrical activity of the hurricane. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, J277-J280, 1986
Black, P.G., R.W. Burpee, N.M. Dorst, and W.L. Adams. Appearance of the sea surface in tropical cyclones. Weather and Forecasting, 1(1):102-107, doi10.1175/1520-0434(1986)001<0102:AOTSSI>2.0.CO;2 1986
Black, R.A. Microphysical investigations above the melting level in Hurricane Norbert (1984). Preprints, 23rd Conference on Cloud Physics, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP252-254, 1986
Black, R.A., and J. Hallett. Observations of the distribution of ice in hurricanes. Journal of the Atmospheric Sciences, 43(8):802-822, doi:10.1175/1520-0469(1986)043<0802:OOTDOI>2.0.CO; 1986
Observations of the type and distribution of particles above the 0°C isotherm in three Atlantic hurricanes are presented. Supercooled drops, graupel, columns, and aggregated snowflakes were observed. The supercooled drops were found only in convective updrafts stronger than 5 m s-1, but not all updrafts >5 m s-1 contained appreciable liquid. Graupel was found in all updrafts at temperatures <-2°C, and small columns were sometimes found in downdrafts. Nonconvective rainbands contained 15-30 L-1 of snow composed of columns and what appeared to be large aggregates. Other stratiform regions contained 1-15 L-1 of medium and large aggregates; columns were occasionally found there also but only within about 15 km of convection. Hurricane convection is almost completely glaciated at the -5°C level. It is suggested that the ice particles observed at 6 km inside the convection result primarily from downward mixing on both sides of the eyewall updraft of ice formed in the convective areas at higher, colder levels. The ice in the stratiform areas is believed to have fallen from the high-level (6 km and higher) eyewall outflow.
Burpee, R.W. Mesoscale structure of hurricanes. In Mesoscale Meteorology and Forecasting, P.S. Ray (ed.). American Meteorological Society, Boston, 311-330, https://doi.org/10.1007/978-1-935704-20-1_14 1986
Friedman, H.A., and O.E. Thompson. First International Conference on School and Popular Meteorological Education, Oxford, England, July 2-4, 1984. Bulletin of the American Meteorological Society, 67(4):422-425, 1986
The First International Conference on School and Popular Meteorological Education convened at Lady Margaret Hall, one of the colleges of Oxford University. The conference, held from July 2-4, 1984, was organized by the Royal Meteorological Society (RMS) and was cosponsored by the American Meteorological Society (AMS) and the World Meteorological Organization (WMO). The AMS Board of School and Popular Meteorological and Oceanographic Education (BSPMOE) played an active, though relatively minor, role in helping to structure the conference by developing interest among potential participants in the United States, including educators, meteorological instrument manufacturers and distributors, scientists, school career counselors, representatives of the electronic and print media, and publishers of meteorological and related textbooks and other educational materials. The chairperson of the Conference Organizing Committee was J.M. Walker, education secretary of RMS.
Gamache, J.F. Bulk water-budget components in Hurricane Norbert as determined from airborne radar and Doppler observations. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP244-247, 1986
Gamache, J.F. Particle characteristics in stratiform and convective clouds observed during summer MONEX. Preprints, Conference on Cloud Physics, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, J143-J146, 1986
Jones, R.W. Mature structure and motion of a model tropical cyclone with latent heating by the resolvable scales. Monthly Weather Review, 114(6):973-990, doi:10.1175/1520-0493(1986)114<0973:MSAMOA>2.0.CO; 1986
The mature structure of a model tropical cyclone is presented with emphasis on convection in the eyewall and spiral rainbands. Representative patterns of rainband activity occur during the last two days of a 13-day experiment and are shown for 288 h or day 12. These model rainbands seem to be forced by a pair of quasi-stationary spiral bands of upward motion that appear in the low troposphere and boundary layer ahead of and behind the westward moving vortex. These forcing bands, in turn, may be the result of a non-linear interaction of azimuthal wavenumber one with itself. The convective band elements ahead of the vortex do not reach the outflow layer, but are capped by a mesoscale subsidence band in the upper troposphere. Elements behind the vortex, however, reach the outflow layer and have greater precipitation intensity. During the last two days of the experiment, three times as much rain fell behind the vortex and outside of the inner core as fell in front of the vortex. The mesoscale subsidence band that caps the convection in bands in front of the vortex is the direct result of a characteristic asymmetry of the 250 mb layer. This asymmetry forms between simulation hours 60 to 84 and results from bands outside of the inner core of the vortex. The model vortex structure, incross-section format, is compared with recent aircraft observations of strong hurricanes (Jorgensen, 1984a,b), particularly Hurricane Allen on 5 August 1980. The model data represent a time and space average with respect to real data. When this averaging is taken into account, reasonable agreement is found between the model and Allen. The model updraft maximum occurs inside of the radius of maximum wind and the precipitation maximum is located outside of the maximum wind at lower levels, in agreement with the real data. The time average motion of the vortex is compared with the mass weighted mean environment current at 990 km radius. For periods of four days or more, the vortex speed agrees with the current speed and the vortex path is mostly to the right of the environment current. For 10 h averages, vortex speed deviations <12% occur and show effects of vortex environment current interactions.
Krueger, S.K., R.M. Wakimoto, and S.J. Lord. Role of ice-phase microphysics in dry microburst simulations. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP73-76, 1986
Marks, F.D. Three-dimensional wind structure of the eyewall of Hurricane Norbert as determined from an airborne Doppler radar. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP248-251, 1986
McBride, J.L., and H.E. Willoughby. Comment: An interpretation of Kurihara and Kawase's two-dimensional tropical-cyclone development model. Journal of the Atmospheric Sciences, 43(24):3279-3283, doi:10.1175/1520-0469(1986)043<3279:CIOKAK>2.0.CO; 1986
This comment presents a detailed examination of the published model results of Kurihara and Kawase (1985) in an attempt to clarify the role of wave-CISK in the development of tropical cyclones. Kurihara and Kawase’s model simulates the development of a tropical depression, although the vertical structure differs significantly from observations. The physical roles of vertical shear and nonlinear dynamics in the development in this model are unclear. The authors propose that the nonlinear terms in the equations promote rapid growth by increasing the “inertial stiffness.” A major concern, however, is that the enhanced development may occur because the nonlinear terms excite modes with high horizontal wavenumbers. These modes grow rapidly through wave-CISK. From considerations of the climatological importance of horizontal shear to tropical-cyclone development in nature, this model may be less relevant to tropical cyclogenesis than one that allows horizontal shears of the environmental flow. The authors discuss the model’s response to changes in the vertical shear of the basic state, which appears to have the opposite effect in the model from what it has in nature.
Ooyama, K.V. A spectral prediction model on nested domains and its application to asymmetric flow in the hurricane boundary layer. WMO/IUGG International Symposium on Short and Medium-Range Numerical Weather Prediction, Tokyo, Japan, August 4-8, 1986. PSMP Report Series No. 19, 451-454, 1986
Powell, M.D. Airborne Doppler radar observations in the hurricane boundary layer. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP260-263, 1986
Shapiro, L.J. The three-dimensional structure of synoptic-scale disturbances over the tropical Atlantic. Monthly Weather Review, 114(10):1876-1891, doi:10.1175/1520-0493(1986)114<1876:TTDSOS>2.0.CO; 1986
Twice-daily analyses of low-level and 200-mb winds over the tropical Atlantic region, archived by the National Hurricane Center, are used to diagnose the structure of synoptic-scale disturbances in the 3-5 day period band. The large-scale disturbances, extracted by a complex empirical orthogonal function technique, are found to have a preferred shift at 200 mb relative to the low-level troughs of somewhat less than one-quarter cycle. The presentation concentrates on July 1975, during which a repeated series of strong disturbances propagated through the region. The relationship between these disturbances and systems in the eastern Pacific is discussed. An analysis of the vorticity propagation characteristics for the disturbances during the month indicates a very different balance from level to level. At the lower level, advection by the mean wind plays a major rate; at 200 mb, the meridional advection of mean vorticity is more important. Rawinsonde data from several island stations are used to resolve the vertical structure of the disturbances. After adjustment for lower density aloft, the kinetic energy at the lower and upper levels is found to be almost equal. The systems propagate westward faster than the mean zonal wind at any level, with a zonal phase speed that is relatively constant with height. It is inferred that the disturbances most likely propagate as a coherent system due to vertical coupling by convection. Evidence is found that the influence of low-level waves on the evolution of 200-mb systems may be stronger than has been previously described.
Shapiro, L.J., and D.B. Chelton. Comments on "Strategies for assessing skill and significance of screening regression models with emphasis on Monte Carlo techniques." Journal of Applied Meteorology, 25(9):1295-1298, doi:10.1175/1520-0450(1986)025<1295:COFASA>2.0.CO; 1986
In a recent paper, Lanzante reviewed methods for estimating the skill and significance of screening regression models through the use of Monte Carlo simulations. The strategies reviewed have several limitations that were not specified by the author. Due to the influence of true model skill, the Monte Carlo method provides an upper bound on the expected artificial skill, not the expected artificial skill itself as assumed. Lanzante emphasizes the advantages of the use of independent (uncorrelated) predictors. However, the disadvantages of their use and the advantages of dependent predictors in a screening regression were not considered. The review of the effects of serial correlation on estimates of skill is misleading. The assertion that the formulations developed by Davis and Chelton are erroneous is incorrect. Moreover, contrary to the implication of the review, the use of effective sample size in tests of model significance has practical utility in applications including the Monte Carlo method.
Szoke, E.J., E.J. Zipser, and D.P. Jorgensen. A radar study of convective cells in mesoscale systems in GATE, Part I: Vertical profile statistics and comparison with hurricanes. Journal of the Atmospheric Sciences, 43(2):182-198, doi:10.1175/1520-0469(1986)043<0182:ARSOCC>2.0.CO; 1986
This is part 1 of a two-paper describing the structure of the vertical profile of radar reflectivity in convective cells which are of part of mesoscale convective systems in GATE. Earlier work has established that characteristic mean vertical velocities in such convective clouds in GATE were rather weak, <3-5 m s-1. The microphysical implications of the weak updrafts have been proposed to include the rarity of large particles above the freezing level. As a working hypothesis for these papers, it is proposed that cells with weak updrafts have characteristic vertical reflectivity profiles exhibiting modest reflectivities at low levels and decreasing rapidly with height above the freezing level. Vertical radar reflectivity profiles are compiled from 296 convective cells having at least 40 dBZ surface reflectivity, and it is found that the profiles are consistent with the above reasoning. The mean profile is of modest strength, 45 dBZ echo at the surface and a 20 dBZ echo top of 8.2 km, and the reflectivity decreases rapidly with height above the freezing level. Statistics of surface reflectivity, echo top, and height of the maximum echo aloft all lend support to the above picture of the GATE cell. The GATE radar profiles are similar to mature hurricane radar profiles, consistent with their updraft velocities being weak as well. In contrast the GATE profiles are markedly weaker than profiles from continental thunderstorm cells, consistent with the much higher vertical velocities in the continental cells.
Wiggert, V., and B.R. Jarvinen. A storm surge atlas for the Sabine Lake area. NOAA Technical Memorandum, NWS-NHC-30, 102 pp., 1986
Willis, P.T. Characteristics of hurricane melting layers. Preprints, 23rd Conference on Radar Meteorology, Snowmass, CO, September 22-26, 1986. American Meteorological Society, Boston, JP264-267, 1986
1985
Black, P.G., R.L. Elsberry, L.K. Shay, and R.M. Partridge. Hurricane Josephine (1984) surface winds and ocean response determined from air-deployed drifting buoys and concurrent research aircraft data. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 22-24, 1985
Black, P.G., V.J. Cardone, R.C. Gentry, and J. Hawkins. SEASAT microwave wind and rain observations in severe tropical midlatitude marine storms. In Advances in Geophysics: Satellite Oceanic Remote Sensing, B. Saltzman (ed.). Elsevier Publishers, 27:197-277, https://doi.org/10.1016/S0065-2687(08)60406-9 1985
This overview presents initial results of studies concerning SEASAT measurements in and around tropical and severe midlatitude cyclones over the open ocean and provides an assessment of their accuracy and usefulness. Sensors flown on SEASAT provided complementary measurements of surface wind speed direction, rainfall rate, significant wave height and wave length, and sea surface temperature. These measurements were made with the SEASAT-A Satellite Scatterometer (SASS), the Scanning Multichannel Microwave Radiometer (SMMR), The SEASAT altimeter, and the SEASAT Synthetic Aperture Radar (SAR). This is the first time that such a sophisticated array of microwave instruments has been used to study tropical cyclones.
Burpee, R.W., R.E. Kohler, and D.G. Marks. An evaluation of Omega dropwindsonde data in track forecasts of Hurricane Josephine. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 162-163, 1985
Dodge, P.P., R.W. Burpee, and F.D. Marks. Convective-scale and mesoscale structure of hurricanes during landfall. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 7-8, 1985
Franklin, J.L., and P. Julian. An investigation of Omega windfinding accuracy. Journal of Atmospheric and Oceanic Technology, 2(2):212-231, 10.1175/1520-0426(1985)002<0212:AIOOWA>2.0.CO;2 1985
The major sources of error in Omega-derived wind estimates are examined and illustrated. Sample dropwindsondes and local Omega signals are used to illustrate the effects of several types of phase propagation anomalies. A stationary test sonde and synthetic Omega phases are used to determine the accuracy of three Omega phase-smoothing algorithms and their associated error estimates and to determine the impact of base station motion for sondes released from aircraft. Omega windfinding errors can be classified as either "internal" or "external" errors. Internal errors are associated with signal quality and transmitter-sonde geometry, while external errors are caused by anomalous phase propagation. Estimates of wind error (wind uncertainties) are provided by the equations of Omega windfinding. These uncertainties, however, estimate only the effects of internal errors. Precise assessment of errors caused by anomalous phase propagation requires the measurement of phase data by a stationary receiver. Such measurements show that errors from external sources range from about 1 m s-1 for diurnal changes in ionospheric height to 20-30 m s-1 for sudden ionospheric disturbances. Methods for dealing with these problems in sonde postprocessing are described. Data from a stationary test sonde show that the effect of aircraft maneuvers on real-time Omega wind estimates is substantial; during turns, errors in real-time wind estimates increase by over 50%. The comparison of phase-smoothing algorithms shows that cubic-spline smoothing produces wind estimates 20-50% more accurate than those obtained with other methods. Hence, it is recommended that this smoothing algorithm be used in dropwindsonde postprocessing. It is estimated that such postprocessing will reduce errors by 60% during aircraft turns and by 30% at other times.
Franklin, J.L., S.J. Lord, L.J. Shapiro, and K.V. Ooyama. An objective analysis of Omega dropwindsonde data from Hurricane Debby (1982). Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 186-187, 1985
Friedman, H.A. Educational program encourages students to seek hurricane precautions (instructional programs to encourage family involvement). In Hurricane Awareness Workbook: Perspectives on Hurricane Preparedness (a monograph). Federal Emergency Management Agency/National Oceanic and Atmospheric Administration, Washington, DC, A27-A28, 1985
Friedman, H.A. Meteorological education as a window on science and technology: Activities of the AMS Board of School and Popular Meteorological and Oceanographic Education. Proceedings, First International Conference on School and Popular Meteorological Education, Oxford, England, July 2-4, 1984. Royal Meteorological Society, 6-9, 1985
Many U.S. scientists and educators have long held the belief that our educational system has failed to provide its students with the mathematics and science skills needed to compete successfully in today’s technologically-oriented society or to maintain our nation’s position of technological leadership in the world. Only recently has this belief motivated members of the meteorological profession, through the American Meteorological Society (AMS) and other scientific organizations, to undertake a number of educationally-related activities designed to reverse the disturbing trend toward math and science illiteracy in the nation’s schools. The Board of School and Popular Meteorological and Oceanographic Education (BSPMOE) has joined with other boards of the AMS Education and Manpower Commission to develop a resource guide for use with school-aged children and the general public. The guide is designed to help promote an awareness of meteorology as a science and the importance of “weather” in everyday life. We believe that such awareness will serve as a “window” on mathematics, science, and technology, especially for school-aged children, and will awaken their scientific curiosity, enhance their scientific literacy, and heighten their enthusiasm for continued learning. The activities, accomplishments, and programmatic goals of the BSPMOE are discussed.
Friedman, H.A. School-based and community-wide education and public information programs to increase tropical cyclone awareness and preparedness. Proceedings, First International Conference on School and Popular Meteorological Education, Oxford, England, July 2-4, 1984. Royal Meteorological Society, 79-85, 1985
Education and public information are recognized as critical elements in the design, organization, and implementation of effective tropical cyclone warning systems. Decision-makers and citizens must have a clear understanding of the dangers associated with tropical cyclones. Otherwise, even in nations that have a high level of preparedness, citizens are likely to take all measures necessary to protect themselves, or to mitigate against, the destructive effects of future landfalling storms. In response to this recognition, a number of education, public information, and preparedness programs have been proposed or are now in progress. The goals and strategies of three such programs, namely, (1) a cognitive and effective learning model (CALM), to create an awareness of the hurricane problem in at-risk coastal communities of south Florida (HRD/AOML-NOAA), (2) Pan Caribbean Disaster Preparedness and Prevention Project (CARICOM, UNDRO, WHO), and (3) Tropical Cyclone Programme Project No. 14: Public Information and Education (WMO, UNDRO, LRCS), are discussed in this paper. These efforts represent, respectively, programs with local, regional, and international focus.
Friedman, H.A., P. Stephens, J. Williams, and O.E. Thompson (editors). Guide to Establishing School and Public Educational Activities. American Meteorological Society (first edition, limited distribution), Boston, 21 pp., 1985
The first edition of this guide to local AMS chapters and university departments is designed to help develop school and public educational outreach programs in their communities. The guide was produced cooperatively by the AMS Board of School and Popular Meteorological and Oceanographic Education and the Board on Women and Minorities. The authors invite readers of this document to comment on its usefulness in conducting and implementing community educational outreach programs. Formal publication of the guide is planned.
Gamache, J.F., and R.A. Houze. Further analysis of the composite wind and thermodynamic structure of the 12 September GATE squall line. Monthly Weather Review, 113(8):1241-1260, doi:10.1175/1520-0493(1985)113<1241:FAOTCW>2.0.CO; 1985
An objective analysis technique is applied to the time-composite wind and thermodynamic fields of the 12 September GATE tropical squall line. Previous subjective analyses described by Gamache and Houze are confirmed and several new results are obtained. In the previous analyses, mesoscale upward motion was found in the upper troposphere of the stratiform precipitation region immediately trailing the squall line. Mesoscale downward motion was found in the lower troposphere of the stratiform region. The convective clouds were found to be the source of condensate for more than half of the stratiform precipitation, but mesoscale-updraft condensation was also found to be substantial. In these previous studies, thermodynamic structure was not analyzed, the wind analyses were limited by the number of levels included and vorticity was not analyzed. By employing an objective analysis method in the present study, we have refined and extended the previous work by including more levels, computing vorticity and analyzing the thermodynamic fields. In the stratiform region, the level of zero vertical motion separating the mesoscale updraft in the upper troposphere from the mesoscale downdraft below is found to be at the 520 mb level (a higher altitude than was indicated by the previous subjective analyses). Maximum convergence in the stratiform region occurred near this level (at 500 mb), but maximum positive vorticity is found to have been at a somewhat lower altitude (650 mb). The thermodynamic structure of the mesoscale updraft in the stratiform region is indicated by the objective analysis to have been more complex than previously estimated. In its central layer the mesoscale updraft contained a warm anomaly with a humidity that was saturated with respect to ice. Cool anomalies are indicated to have existed near the top of the stratiform cloud deck and (possibly) at the base of the mesoscale updraft. The structure of the squall system was apparently strongly affected by interaction with the wake of an earlier squall line and with a convective line existing immediately ahead of the squall and intersecting it at nearly right angles. The portion of the squall line feeding on the stabilized wake air associated with these two convective lines was characterized by systematically lower cell tops, as determined by radar, than the remainder of the line. The portion of the stratiform region trailing this part of the line exhibited a distinctly different thermodynamic stratification than was observed to the rear of the deeper-cell section of the squall line. This difference is attributed to the lower altitudes at which condensate and water vapor were determined from this portion of the line are inferred to have advected into the stratiform region.
Goldenberg, S.B., S.D. Aberson, and R.E. Kohler. Incorporation of Omega dropwindsonde data into SANBAR: An operational barotropic hurricane-track forecast model. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 44-45, 1985
Houze, R.A., F.D. Marks, R.A. Black, P.T. Willis, and J.F. Gamache. Airborne Doppler radar and cloud microphysical measurements in Hurricane Norbert. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 5-6, 1985
Jorgensen, D.P., E.J. Zipser, and M.A. LeMone. Vertical motions in intense hurricanes. Journal of the Atmospheric Sciences, 42(8):835-856, doi:10.1175/1520-0469(1985)042<0839:VMIIH>2.0.CO;2 1985
Hurricane vertical motion properties are studied using aircraft-measured 1 Hz time series of vertical velocity obtained during radial penetrations of four mature hurricanes. A total of 115 penetrations from nine flight sorties at altitudes from 0.5 to 6.1 km are included in the data set. Convective vertical motion events are classified as updrafts (or downdrafts) if the vertical velocity was continuously positive (or negative) for at least 500 m and exceed an absolute value of 0.5 m s-1. Over 3000 updrafts and nearly 2000 downdrafts are included in the data set. A second criteria was used to define stronger events, called cores. This criteria required that upward (or downward) vertical velocity be continuously greater than an absolute value of 1 m s-1 for at least 500 m. The draft and core properties are summarized as distributions of average and maximum vertical velocity, diameter, and vertical mass transport in two regions: eyewall and rainband. In both regions updrafts dominated over downdrafts, both in number and mass transport. In the eyewall region, the draft and core strength distributions were similar to data collected by aircraft in GATE cumulonimbus clouds. Unlike GATE clouds, however, the largest updraft cores (larger than 90% of the distribution) were over twice as large and transported twice as much mass as did the corresponding GATE updraft cores. Eyewall ascent was highly organized in a channel several kilometers wide located a few kilometers radically inward from the radius of maximum tangential wind. As in GATE, the strongest hurricane updraft cores were weak in comparison with the strongest updrafts observed in typical midlatitude thunderstorms. Mean eyewall profiles of radar reflectivity and cloud water content are discussed to illustrate the microphysical implications of the low updraft rates.
Lewis, J.M., C.M. Hayden, C.S. Velden, T.R. Stewart, S.J. Lord, S.B. Goldenberg, and S.D. Aberson. The use of VAS winds and temperatures as input to barotropic hurricane-track forecasting. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 40-41, 1985
Lord, S.J., and J.M. Piotrowicz. Vertical velocity structures in an axisymmetric, nonhydrostatic tropical cyclone model. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 138-139, 1985
Marks, F.D. Evolution of the structure of precipitation in Hurricane Allen (1980). Monthly Weather Review, 113(6):909-930, doi:10.1175/1520-0493(1985)113<0909:EOTSOP>2.0.CO; 1985
Reflectivity data from the airborne radar systems on board the three NOAA aircraft were gathered during six consecutive days in Hurricane Allen of 1980. The data have been used to specify the horizontal and vertical precipitation distribution within 111 km radius of the hurricane center. The evolution of the structure and intensity of the precipitation in the storm is described from representative time composite radar maps for seven research flights made during the 6-day period. The eyewall was characterized by a narrow ring (12-15 km wide) of intense reflectivity (42-47 dBZ) surrounding the center of the storm at a radius that varied in time from 12-40 km. The eyewall had steep radial gradients of reflectivity (4-5 dB km–1) and tilted radially outward in height. The rainbands were characterized by areas of enhanced reflectivity embedded in a region of stratiform rainfall that contained a distinct bright band at the height of the 0EC isotherm. The most striking changes in structure during the 6-day period were the rapid contraction in eyewall radius and the development of a secondary ring of intense reflectivity 80-100 km from the storm center. These changes in eye radius appeared to be related to the vortex evolution, as discussed by Willoughby and others. Changes in storm intensity, coincident with the eyewall radius changes, seemed to have little effect on the total storm rainfall or latent heat release. The maximum storm rainfall occurred when the storm had a double eyewall structure. After the period of the double eyewall, the mean rain rate in the eyewall increased as the storm approached maximum intensity. However, coincident with the increase in eyewall rain rate, the eyewall area decreased, resulting in little change in the total storm rainfall. The sequence of time composites provided the first opportunity to describe, quantitatively, the precipitation distribution within 111 km of the center of a mature hurricane that was away from land influences. The rainfall analysis showed that the mean rain rates in the eyewall were a factor of 6 greater than those outside the eyewall (11.3 mm h–1 versus 1.8 mm h–1), but because the eyewall region encompassed such a small area, it only contributed 40% of the total rainfall within a radius of 1E latitude of the storm center. The precipitation distribution around the storm was asymmetric; more rainfall occurred ahead of the storm than behind. In general, the maximum precipitation in the eyewall region was within 15-20° of the storm track. The maximum rainfall in the rainband region was 40-50° to the right of that in the eyewall.
Ooyama, K.V. The polar representation of tensor cross-spectra of winds. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, 182-183, 1985
Powell, M.D. Airborne Doppler radar observations of the hurricane boundary layer. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, 3-4, 1985
Shapiro, L.J. Objective analysis of winds in the three to five-day band over tropical Atlantic. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 184-185, 1985
Tattelman, P., and P.T. Willis. Model vertical profiles of extreme rainfall rate, liquid water content, and drop-size distribution. Environmental Research Papers, Air Force Geophysics Laboratory, No. 928, AFGL Technical Report 85-0200, Hanscom AFB, MA, 42 pp., 1985
This report provides a new model of hydrometeors and associated cloud-water content from the surface to 20 km. The model profiles at altitude were developed based on five surface rainfall rates: 36, 84, 168, 432, and 1872 mm/hr. The first three rates correspond to a frequency of occurrence of 0.5%, 0.1%, and 0.01% of the time during the worst month in the most severe area of the world for intense rainfall. The last two are the 42- and 1-min world record rainfalls. The surface rainfall rates were extrapolated aloft using results from previous studies. A large sample of drop-size distributions from intense rainfall collected during reconnaissance of Atlantic hurricanes/tropical storms was analyzed. The data set was normalized and fit by a gamma distribution. This was used to specify the drop-size distributions and liquid water content for rainfall rates specified at the surface and aloft. Concurrent cloud-water content was estimated. Results are presented at 2-km intervals.
Willis, P.T. Microphysics of a stratiform melting layer in Hurricane Alicia. Proceedings, 16th Conference on Hurricanes and Tropical Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 14-15, 1985
Willis, P.T. Reply. Journal of the Atmospheric Sciences, 42(12):1349-1350, doi:10.1175/1520-0469(1985)042<1349:R>2.0.CO;2 1985
Willoughby, H.E. Confirmatory observations of concentric eyes in hurricanes. Proceedings, 16th Conference on Hurricanes and Meteorology, Houston, TX, May 14-17, 1985. American Meteorological Society, Boston, 1-2, 1985
Willoughby, H.E., D.P. Jorgensen, R.A. Black, and S.L. Rosenthal. Project STORMFURY: A scientific chronicle, 1962-1983. Bulletin of the American Meteorological Society, 66(5):505-514, 1985
Between 1962 and 1983, research in hurricane modification centered on an ambitious experimental program, Project STORMFURY. The proposed modification technique involved artificial stimulation of convection outside the eyewall through seeding with silver iodide. The artificially invigorated convection, it was argued, would compete with the convection in the original eyewall, lead to reformation of the eyewall at larger radius, and thus produce a decrease in the maximum wind. Since a hurricane's destructive potential increases rapidly as its maximum wind becomes stronger, a reduction as small at 10% would have been worthwhile. Modification was attempted in four hurricanes on eight different days. On four of these days, the winds decreased by between 10 and 30%. The lack of response on the other days was interpreted to be the result of fault execution of the experiment or poorly selected subjects. These promising results have, however, come into question because recent observations of unmodified hurricanes indicate: (1) that cloud seeding has little prospect of success because hurricanes contain too much natural ice and too little supercooled water; and (2) that the positive results inferred from the seeding experiments in the 1960s probably stemmed from inability to discriminate between the expected effect of human intervention and the natural behavior of hurricanes.
1984
Burpee, R.W., D.G. Marks, and R.T. Merrill. An assessment of Omega dropwindsonde data in track forecasts of Hurricane Debby (1982). Bulletin of the American Meteorological Society, 65(10):1050-1058, doi:10.1175/1520-0477(1984)065<1050:AAOODD>2.0.CO; 1984
Omega dropwindsondes (ODWs) were released from two NOAA WP-3D aircraft to measure the environmental wind field in the middle and lower troposphere within 1000 km of the center of Hurricane Debby on 15 and 16 September 1982. The observations were coded in standard formats and transmitted from the aircraft to the National Hurricane Center (NHC) and the National Meteorological Center (NMC) before operational forecast deadlines. The ODW winds clearly indicated the location and strength of a midtropospheric trough in the westerlies that was the major synoptic-scale feature affecting Debby's motion. On 16 September, the dropwindsondes also identified a smaller scale cutoff low in the northern part of the trough. The cutoff low that was centered about 500 km to the north northwest of Debby affected the hurricane's motion from midday on the 16th to midday on the 17th. The ODWs provided NHC with timely information that was used subjectively in determining the official forecasts of Debby's track. The potential of the ODWs to improve the track models that serve as guidance for the forecasters at NHC depends upon both the quality of the ODW data and the ability of the operational objective analyses to respond to the ODW data. In 1982, the objective analysis that initialized several of the track models was a spectral analysis with a global domain. At 500 mb, the scale of the wind circulations; of Debby and the cutoff low was approximately 500 km. The global operational objective analysis did not resolve these important features. The ODW data can help to improve the objective guidance for the hurricane forecasters only if the operational objective analyses and the track models are designed to make use of the ODW information. to obtain the data needed to revise current models and to develop new models, ODW experiments are planned in the next few years when hurricanes threaten the Atlantic or Gulf coasts of the United States.
Friedman, H.A., C.A. Arnhols, N.M. Dorst, C.J. Nelson, and W.J. Brown. Airborne research meteorological data collected by the National Hurricane Research Laboratory (Hurricane Research Division/AOML) during the 1982-1983 hurricane seasons: Inventory and availability. NOAA Data Report, ERL-AOML-3, 168 pp., 1984
The history, nature, use, and availability of in-situ research meteorological data that have been gathered by specially instrumented aircraft have been described in several publications. In 1982, the National Hurricane Research Laboratory (NHRL), now Hurricane Research Division (HRD)/Atlantic Oceanographic and Meteorological Laboratory (AOML), published NOAA Data Report ERL AOML-2 (Friedman et al.) to document the airborne research meteorological data collected in support of its hurricane field programs during the WP-3D era (1976-1981). The present publication continues this documentation with an inventory and description of the research data that were obtained during the 1982 and 1983 hurricane seasons, and which are available at HRD/AOML.
Lord, S.J., H.E. Willoughby, and J.M. Piotrowicz. Role of a parameterized ice-phase microphysics in an axisymmetric, nonhydrostatic tropical cyclone model. Journal of the Atmospheric Sciences, 41(19):2836-2848, doi:10.1175/1520-0469(1984)041<2836:ROAPIP>2.0.CO; 1984
Results of an axisymmetric, nonhydrostatic hurricane model are analyzed with emphasis on the role of a parameterized ice-phase microphysics Inclusion of ice processes produces dramatic differences in the structure and evolution of the simulated hurricane vortex. Mesoscale convective features are wore plentiful with ice, and the simulated vortex grows more slowly. Time and space-averaged budgets of key model variables show that cooling due to melting ice particles can initiate and maintain model downdrafts on a horizontal scale of tens of kilometers. This scale depends critically on both the horizontal advection of the parameterized snow particles detrained from the tops of convective updrafts and the mean fall speed of the particles toward the melting level. In situ production of snow particles results from a wide variety of parameterized microphysical processes and is significant factor in maintaining upper-level snow concentration. These processes are strongly height-dependent.
Willoughby, H.E., F.D. Marks, and R.J. Feinberg. Stationary and moving convective bands in hurricanes. Journal of the Atmospheric Sciences, 41(22):3189-3211, doi:10.1175/1520-0469(1984)041<3189:SAMCBI>2.0.CO; 1984
Aircraft observations in hurricanes indicate that the hurricane vortex may be subdivided into an inner gyre where the air trajectories form closed paths and an outer envelope where they do not. In the closed gyre, a core of air moves with the vortex; in the envelope, environmental air passes through the vortex and around the core. A system of spiral bands, termed the stationary band complex (SBC), forms near the boundary between the core and the envelope where the Rossby number is of order unity. The SBC differs dynamically both from convective rings because it is asymmetric and from propagating gravity-wave bands because its Doppler-shifted frequency is below the local inertia frequency. In more intense systems with stronger convective instability, the SBC may evolve into a convective ring and move into the vortex core. Outward propagating gravity-wave bands have also been observed. Such bands are often associated with track oscillations as the storm makes landfall or recurves. Spiral-shaped entities within the SBC tend to lie across the streamlines when the convective instability is small and along them when it is large. Storms moving through an environmental flow with westerly vertical shear exhibit an east-to-west drift across the vortex. This phenomenon is expressed in the asymmetric streamfunction as an anticyclonic eddy northeast of the center and a cyclonic eddy south of the center. The velocity potential has a divergent cell west of the center and convergent cell that extends along the inside of the SBC east of the center. This pattern is apparently forced by potential vorticity conservation along the trajectories of the rotational flow and by heating in the SBC. The irrotational flow between the two cells substantially cancels the rotational drift within the vortex core.