Quarterly Journal of the Royal Meteorological Society, 2004
Data from two marine eld experiments in the Baltic Sea with stable strati cation have been analys... more Data from two marine eld experiments in the Baltic Sea with stable strati cation have been analysed. The purpose was to test the concept of the 'detached' or 'top-down' eddies and the 'shear-sheltering' mechanism in the presence of a low-level wind speed maximum in the atmosphere. Data used include turbulence and pro le measurements on two 30 m towers and concurrent wind pro les throughout the boundary layer obtained from pilot-balloon soundings.
Quarterly Journal of the Royal Meteorological Society, 2007
In very slightly unstable conditions, when the Obukhov length is much greater than the surface la... more In very slightly unstable conditions, when the Obukhov length is much greater than the surface layer depth, it is observed that the structure of the surface layer turbulence does not accord with standard similarity theory. In particular the efficiency of the turbulent exchange of sensible and latent heat is observed to be more strongly enhanced than is consistent with the standard model. Also the profiles of dissipation of turbulent kinetic energy and temperature fluctuation variance are found to depend on the structure of the whole boundary layer (i.e. are non-local), indicating that a large-scale transport process is at work. At the same time, co-spectral analysis shows how the large-scale eddy motions that determine the heat transport process near the surface are typically 1/5 of the surface layer depth. All these features are found to be similar in measurements at a marine site and at a flat land site, indicating that they are determined by the dynamics of the whole boundary layer rather than being simply dependent on the surface boundary conditions. We hypothesize that in slightly unstable conditions there is a bifurcation in the structure of the large-scale eddy motions. This is a transition from quasi-steady longitudinal roll structures to detached eddies. In the particular regime identified here, the unsteady component dominates the heat transport at the surface. The observed enhancement in the surface layer of the eddy diffusivity of heat is observed to be significantly greater than for momentum. This is shown to be consistent with observations and with a model calculation of the interaction between the surface layer and the descending detached eddies.
Quarterly Journal of the Royal Meteorological Society, 2004
The exchange of sensible heat at the sea surface has been studied with the aid of a comprehensive... more The exchange of sensible heat at the sea surface has been studied with the aid of a comprehensive dataset from the marine siteÖstergarnsholm in the Baltic Sea, with additional data from another site in the Baltic Sea, Nässkär. The measurements include turbulent uxes at about 10 m above the water surface, pro les of temperature and wind data at several levels on towers, sea surface temperatures and wave data.
Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed... more Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Östergarnsholm in the Baltic Sea were used. The situation, which lasted for ~48 hours, appeared in the aftermath of a gale. The wind
Recent research has found that boundary layer turbulence changes its organization as the stratifi... more Recent research has found that boundary layer turbulence changes its organization as the stratification approaches neutral from the unstable side. When the thermal forcing weakens in combination with wind speed above approximately 10 m s Ϫ1 , detached eddies are formed in the upper part of the surface layer. These eddies effectively transport drier and colder air from aloft to the surface as they move downward, thereby enhancing the surface fluxes of sensible and latent heat. This effect has been observed over both land and sea; that is, it is not dependent on the nature of the underlying surface. Here the authors perform a sensitivity study of how this reorganization of the turbulence structure influences the global air-sea heat fluxes. Using modified bulk formulations incorporating this effect, the magnitude of the enhancement in a climatic sense was estimated by the use of 40-yr ECMWF Re-Analysis (ERA-40) data in the bulk formulas. It is found that for the 1979-2001 period, the global increase of the latent and sensible heat fluxes over the ice-free oceans is 3.6 and 1.2 W m Ϫ2 , respectively. These numbers suggest that this effect is of some significance. The results also indicate that the regional and seasonal variability may be large. The largest annual increases are found over the southern oceans between 30°and 60°S where the sensible heat flux increases by 2.3 W m Ϫ2 and the latent heat flux by 6.5 W m Ϫ2 . Ocean areas close to the equator experience almost no increase, whereas the latent heat flux from the Arabian Sea during the monsoon period is enhanced by 11.5 W m Ϫ2 .
Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neut... more Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neutral conditions are shown to agree well with measurements of atmospheric surface-layer variances and spectra. The theory suggests surface-layer turbulence is determined ...
Results from large-eddy simulations and field measurements have previously shown that the velocit... more Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z i , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, u m , has been found to be a function of both z/L and z i /L. At constant z/L, u m decreases with decreasing boundary layer height. Since u m is directly related to the parameterizations of the air-sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m -2 or about 1% and the mean surface stress increase by 1.4 mN m -2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m -2 and an increase of the mean annual surface stress by 2.6 mN m -2 .
An extensive set of turbulence data from the 3-and 12-m heights taken over an agricultural site (... more An extensive set of turbulence data from the 3-and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from 'ideal' sites.
An extensive urban meteorology project in Uppsala, Sweden, is described. The city itself is consi... more An extensive urban meteorology project in Uppsala, Sweden, is described. The city itself is considered suitable as a kind of model city, being almost circular, having sharp urban/rural boundaries and being situated in a relatively flat area. The project incorporates numerous measurements. A main station with a 100-m mast is situated at the NE urban/rural border. Its instrumentation consists of a slow-response ('profile') system and a turbulence system. The profile instrumentation is described in detail. In addition to the main station, the project comprises: measurements from two 14-m masts, one on the top of a centrally located building, the other mobile; pilot balloon ascents; tethered balloon soundings, car-borne temperature traverses etc. Some preliminary results are presented: analyses of wind profiles from the mast and three-dimensional heat island studies. Boundary-Layer Meteorology 15 (1978) 69-80. All Rights Reserved Copyright @I 1978 by D. Reidel Publishing Company, Dordrecht, Holland 70 ULF HOGSTROM ET AL.
Some of the fundamental issues of surface layer meteorology are critically reviewed. For the von ... more Some of the fundamental issues of surface layer meteorology are critically reviewed. For the von Karman constant (Ic), values covering the range from 0.32 to 0.65 have been repotted. Most of the data are, however, found in a rather narrow range between 0.39 and 0.4 1. Plotting all available atmospheric data against the so-called roughness Reynolds number, Rq, = U+Z~/Y or against the surface Rossby number, Roa = G/f .zo gives no clear indication of systematic trend. It is concluded that k is indeed constant in atmospheric surface-layer flow and that its value is the same as that found for laboratory flows, i.e. about 0.40.
Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea ha... more Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea have been analyzed to estimate the turbulence energy budget. It is concluded that the jets had the same origin as found in an earlier study from the same general area: inertial oscillation caused by frictional decoupling when relatively warm air flows out over much colder water.
A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was charact... more A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the presence of a pronounced low level jet at very low height, 30-150 m, above the surface of the sea, The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temperature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the same site at similar stability conditions but with no low level jet. Instead these quantities are shown to be governed by scales characteristic of stable shear flow away from the surface. The height to the jet centre appears to be an important quantity. Thus, for the cases with the lowest jet height values (30-50 m), some turbulent characteristics of the flow (non-dimensional velocity standard deviations and the correlation between the longitudinal and vertical velocity) have values similar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called 'canonical' boundary layer) rather than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as 'inactive' turbulence, as well as gravity waves, were suppressed in this case.
Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fl... more Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jiidra&s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under 'ideal conditions' (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding nondimensional potential temperature gradient (equivalent to KH/KW) is found to be unity for (I -d)/L, less than about -0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large. 198 ULF H6GSTRdh.l ET AL. 200 ULF H&XTR&U ET AL.
A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of n... more A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.
Previous results of non-dimensional wind and temperature profiles as hmctions of c( = z/L) show s... more Previous results of non-dimensional wind and temperature profiles as hmctions of c( = z/L) show systematic deviations between diierent experiments. These discrepancies are generally believed not to re8ect real differences but rather instrumental shortcomings. In particular, it is clear that flow distortion has not been adequately treated in most previous experiments. In the present paper, results are presented from a surface-layer field experiment where great care was taken to remove any effects from this kind of error and also to minimize other measuring errors. Data from about 90 30-min runs with turbulence measurements at three levels (3,6, and 14 m) and simultaneous profile data have been analysed to yield information on llux-gradient relationships for wind and temperature.
Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have ... more Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have been studied. The data were gathered at a flat coastal site (Ngsudden on the island of Gotland, Sweden) during three consecutive days in May 1980 which were characterized by a steady, very stable stratified marine approach flow. The site is situated on a flat area ca. 1500 m from the shoreline. Only daytime runs have been analysed in the present paper. The sensible heat flux at the ground was typically 200 W mm2 and was found to decrease more or less linearly with height throughout the I.B.L., being slightly negative at greater heights. The momentum flux was also found to decrease with height, but nevertheless shear production of turbulent kinetic energy was found to be large throughout the entire I.B.L.
Data from two IW-min runs on the turbulent atmospheric wind, temperature and humidity fields abov... more Data from two IW-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events. in fact constitute ;I large part of the total Huxcs. about YO% during the actual events.
Quarterly Journal of the Royal Meteorological Society, 2004
Data from two marine eld experiments in the Baltic Sea with stable strati cation have been analys... more Data from two marine eld experiments in the Baltic Sea with stable strati cation have been analysed. The purpose was to test the concept of the 'detached' or 'top-down' eddies and the 'shear-sheltering' mechanism in the presence of a low-level wind speed maximum in the atmosphere. Data used include turbulence and pro le measurements on two 30 m towers and concurrent wind pro les throughout the boundary layer obtained from pilot-balloon soundings.
Quarterly Journal of the Royal Meteorological Society, 2007
In very slightly unstable conditions, when the Obukhov length is much greater than the surface la... more In very slightly unstable conditions, when the Obukhov length is much greater than the surface layer depth, it is observed that the structure of the surface layer turbulence does not accord with standard similarity theory. In particular the efficiency of the turbulent exchange of sensible and latent heat is observed to be more strongly enhanced than is consistent with the standard model. Also the profiles of dissipation of turbulent kinetic energy and temperature fluctuation variance are found to depend on the structure of the whole boundary layer (i.e. are non-local), indicating that a large-scale transport process is at work. At the same time, co-spectral analysis shows how the large-scale eddy motions that determine the heat transport process near the surface are typically 1/5 of the surface layer depth. All these features are found to be similar in measurements at a marine site and at a flat land site, indicating that they are determined by the dynamics of the whole boundary layer rather than being simply dependent on the surface boundary conditions. We hypothesize that in slightly unstable conditions there is a bifurcation in the structure of the large-scale eddy motions. This is a transition from quasi-steady longitudinal roll structures to detached eddies. In the particular regime identified here, the unsteady component dominates the heat transport at the surface. The observed enhancement in the surface layer of the eddy diffusivity of heat is observed to be significantly greater than for momentum. This is shown to be consistent with observations and with a model calculation of the interaction between the surface layer and the descending detached eddies.
Quarterly Journal of the Royal Meteorological Society, 2004
The exchange of sensible heat at the sea surface has been studied with the aid of a comprehensive... more The exchange of sensible heat at the sea surface has been studied with the aid of a comprehensive dataset from the marine siteÖstergarnsholm in the Baltic Sea, with additional data from another site in the Baltic Sea, Nässkär. The measurements include turbulent uxes at about 10 m above the water surface, pro les of temperature and wind data at several levels on towers, sea surface temperatures and wave data.
Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed... more Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Östergarnsholm in the Baltic Sea were used. The situation, which lasted for ~48 hours, appeared in the aftermath of a gale. The wind
Recent research has found that boundary layer turbulence changes its organization as the stratifi... more Recent research has found that boundary layer turbulence changes its organization as the stratification approaches neutral from the unstable side. When the thermal forcing weakens in combination with wind speed above approximately 10 m s Ϫ1 , detached eddies are formed in the upper part of the surface layer. These eddies effectively transport drier and colder air from aloft to the surface as they move downward, thereby enhancing the surface fluxes of sensible and latent heat. This effect has been observed over both land and sea; that is, it is not dependent on the nature of the underlying surface. Here the authors perform a sensitivity study of how this reorganization of the turbulence structure influences the global air-sea heat fluxes. Using modified bulk formulations incorporating this effect, the magnitude of the enhancement in a climatic sense was estimated by the use of 40-yr ECMWF Re-Analysis (ERA-40) data in the bulk formulas. It is found that for the 1979-2001 period, the global increase of the latent and sensible heat fluxes over the ice-free oceans is 3.6 and 1.2 W m Ϫ2 , respectively. These numbers suggest that this effect is of some significance. The results also indicate that the regional and seasonal variability may be large. The largest annual increases are found over the southern oceans between 30°and 60°S where the sensible heat flux increases by 2.3 W m Ϫ2 and the latent heat flux by 6.5 W m Ϫ2 . Ocean areas close to the equator experience almost no increase, whereas the latent heat flux from the Arabian Sea during the monsoon period is enhanced by 11.5 W m Ϫ2 .
Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neut... more Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neutral conditions are shown to agree well with measurements of atmospheric surface-layer variances and spectra. The theory suggests surface-layer turbulence is determined ...
Results from large-eddy simulations and field measurements have previously shown that the velocit... more Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z i , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, u m , has been found to be a function of both z/L and z i /L. At constant z/L, u m decreases with decreasing boundary layer height. Since u m is directly related to the parameterizations of the air-sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m -2 or about 1% and the mean surface stress increase by 1.4 mN m -2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m -2 and an increase of the mean annual surface stress by 2.6 mN m -2 .
An extensive set of turbulence data from the 3-and 12-m heights taken over an agricultural site (... more An extensive set of turbulence data from the 3-and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from 'ideal' sites.
An extensive urban meteorology project in Uppsala, Sweden, is described. The city itself is consi... more An extensive urban meteorology project in Uppsala, Sweden, is described. The city itself is considered suitable as a kind of model city, being almost circular, having sharp urban/rural boundaries and being situated in a relatively flat area. The project incorporates numerous measurements. A main station with a 100-m mast is situated at the NE urban/rural border. Its instrumentation consists of a slow-response ('profile') system and a turbulence system. The profile instrumentation is described in detail. In addition to the main station, the project comprises: measurements from two 14-m masts, one on the top of a centrally located building, the other mobile; pilot balloon ascents; tethered balloon soundings, car-borne temperature traverses etc. Some preliminary results are presented: analyses of wind profiles from the mast and three-dimensional heat island studies. Boundary-Layer Meteorology 15 (1978) 69-80. All Rights Reserved Copyright @I 1978 by D. Reidel Publishing Company, Dordrecht, Holland 70 ULF HOGSTROM ET AL.
Some of the fundamental issues of surface layer meteorology are critically reviewed. For the von ... more Some of the fundamental issues of surface layer meteorology are critically reviewed. For the von Karman constant (Ic), values covering the range from 0.32 to 0.65 have been repotted. Most of the data are, however, found in a rather narrow range between 0.39 and 0.4 1. Plotting all available atmospheric data against the so-called roughness Reynolds number, Rq, = U+Z~/Y or against the surface Rossby number, Roa = G/f .zo gives no clear indication of systematic trend. It is concluded that k is indeed constant in atmospheric surface-layer flow and that its value is the same as that found for laboratory flows, i.e. about 0.40.
Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea ha... more Four aircraft measurement sets made in late May 1989 within low level jets over the Baltic Sea have been analyzed to estimate the turbulence energy budget. It is concluded that the jets had the same origin as found in an earlier study from the same general area: inertial oscillation caused by frictional decoupling when relatively warm air flows out over much colder water.
A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was charact... more A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the presence of a pronounced low level jet at very low height, 30-150 m, above the surface of the sea, The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temperature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the same site at similar stability conditions but with no low level jet. Instead these quantities are shown to be governed by scales characteristic of stable shear flow away from the surface. The height to the jet centre appears to be an important quantity. Thus, for the cases with the lowest jet height values (30-50 m), some turbulent characteristics of the flow (non-dimensional velocity standard deviations and the correlation between the longitudinal and vertical velocity) have values similar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called 'canonical' boundary layer) rather than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as 'inactive' turbulence, as well as gravity waves, were suppressed in this case.
Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fl... more Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jiidra&s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under 'ideal conditions' (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding nondimensional potential temperature gradient (equivalent to KH/KW) is found to be unity for (I -d)/L, less than about -0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large. 198 ULF H6GSTRdh.l ET AL. 200 ULF H&XTR&U ET AL.
A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of n... more A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.
Previous results of non-dimensional wind and temperature profiles as hmctions of c( = z/L) show s... more Previous results of non-dimensional wind and temperature profiles as hmctions of c( = z/L) show systematic deviations between diierent experiments. These discrepancies are generally believed not to re8ect real differences but rather instrumental shortcomings. In particular, it is clear that flow distortion has not been adequately treated in most previous experiments. In the present paper, results are presented from a surface-layer field experiment where great care was taken to remove any effects from this kind of error and also to minimize other measuring errors. Data from about 90 30-min runs with turbulence measurements at three levels (3,6, and 14 m) and simultaneous profile data have been analysed to yield information on llux-gradient relationships for wind and temperature.
Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have ... more Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have been studied. The data were gathered at a flat coastal site (Ngsudden on the island of Gotland, Sweden) during three consecutive days in May 1980 which were characterized by a steady, very stable stratified marine approach flow. The site is situated on a flat area ca. 1500 m from the shoreline. Only daytime runs have been analysed in the present paper. The sensible heat flux at the ground was typically 200 W mm2 and was found to decrease more or less linearly with height throughout the I.B.L., being slightly negative at greater heights. The momentum flux was also found to decrease with height, but nevertheless shear production of turbulent kinetic energy was found to be large throughout the entire I.B.L.
Data from two IW-min runs on the turbulent atmospheric wind, temperature and humidity fields abov... more Data from two IW-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events. in fact constitute ;I large part of the total Huxcs. about YO% during the actual events.
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Papers by Ulf Högström