2006

Remote Sensing

Surface albedo is a fundamental radiative parameter as it controls the Earth’s energy budget and directly affects the Earth’s climate. Satellite observations have long been used to capture the temporal and spatial variations of surface albedo because of their continuous global coverage. However, space-based albedo products are often affected by errors in the atmospheric correction, multi-angular bi-directional reflectance distribution function (BRDF) modelling, as well as spectral conversions. To validate space-based albedo products, an in situ tower albedometer is often used to provide continuous “ground truth” measurements of surface albedo over an extended area. Since space-based albedo and tower-measured albedo are produced at different spatial scales, they can be directly compared only for specific homogeneous land surfaces. However, most land surfaces are inherently heterogeneous with surface properties that vary over a wide range of spatial scales. In this work, tower-measure...

Remote Sensing, 2016

We present the first comparison between new fAPAR and LAI products derived from the GlobAlbedo dataset and the widely-used MODIS fAPAR and LAI products. The GlobAlbedo-derived products are produced using a 1D two-stream radiative transfer (RT) scheme designed explicitly for global parameter retrieval from albedo, with consistency between RT model assumptions and observations, as well as with typical large-scale land surface model RT schemes. The approach does not require biome-specific structural assumptions (e.g., cover, clumping, understory), unlike more detailed 3D RT model approaches. GlobAlbedo-derived values of fAPAR and LAI are compared with MODIS values over 2002-2011 at multiple flux tower sites within selected biomes, over 1200ˆ1200 km regions and globally. GlobAlbedo-derived fAPAR and LAI values are temporally more stable than the MODIS values due to the smoothness of the underlying albedo, derived via optimal estimation (assimilation) using an a priori estimate of albedo derived from an albedo "climatology" (composited multi-year albedo observations). Parameters agree closely in timing but with GlobAlbedo values consistently lower than MODIS, particularly for LAI. Larger differences occur in winter (when values are lower) and in the Southern hemisphere. Globally, we find that: GlobAlbedo-derived fAPAR is~0.9-1.01ˆMODIS fAPAR with an intercept of~0.03; GlobAlbedo-derived LAI is~0.6ˆMODIS LAI with an intercept of~0.2. Differences arise due to the RT model assumptions underlying the products, meaning care is required in interpreting either set of values, particularly when comparing to fine-scale ground-based estimates. We present global transformations between GlobAlbedo-derived and MODIS products.

Remote Sensing

Land surface (bare soil, vegetation, and snow) albedo is an essential climate variable that affects the Earth’s radiation budget, and therefore, is of vital interest for a broad number of applications: Thematic (urban, cryosphere, land cover, and bare soil), climate (Long Term Data Record), processing technics (gap filling, data merging), and products validation (cal/val) [...]

International Journal of Digital Earth, 2017

Upscaling ground albedo is challenged by the serious discrepancy between the heterogeneity of land surfaces and the small number of ground-based measurements. Conventional ground-based measurements cannot provide sufficient information on the characteristics of surface albedo at the scale of coarse pixels over heterogeneous land surfaces. One method of overcoming this problem is to introduce high-resolution albedo imagery as ancillary information for upscaling. However, due to the low frequency of updating of high-resolution albedo maps, upscaling time series of ground-based albedo measurements is difficult. This paper proposes a method that is based on the idea of conceptual universal scaling methodology for establishing a spatiotemporal trend surface using very few high-resolution images and time series of ground-based measurements for spatial-temporal upscaling of albedo. The construction of the spatiotemporal trend surface incorporates the spatial information provided by auxiliary remote sensing images and the temporal information provided by long time series of ground observations. This approach was illustrated by upscaling ground-based fine-scale albedo measurements to a coarse scale over the core study area in HiWATER. The results indicate that this method can characterize the spatiotemporal variations in surface albedo well, and the overall correlation coefficient was 0.702 during the study period.

Remote Sensing

Surface albedo, as a key parameter determining the partition of solar radiation at the Earth’s surface, has been developed into a satellite-based product from various Earth observation systems to serve numerous global or regional applications. Studies point out that apparent uncertainty can be introduced into albedo retrieval without consideration of surface anisotropy, which is a challenge to albedo estimation especially from observations with fewer angular samplings. Researchers have begun to introduce smoothed anisotropy prior knowledge into albedo estimation to improve the inversion efficiency, or for the scenario of observations with signal or poor angular sampling. Thus, it is necessary to further understand the potential influence of smoothed anisotropy features adopted in albedo estimation. We investigated the albedo variation induced by BRDF smoothing at both temporal and spatial scales over six typical landscapes in North America using MODIS standard anisotropy products wi...

2012 IEEE International Geoscience and Remote Sensing Symposium, 2012

This paper describes the algorithm underlying the ESA DUE globAlbedo product. The purpose of the project is to produce a global 8-day land surface albedo product with associated uncertainty on a 1 km grid with continuous spatial coverage using data from European sensors. The product covers the period 1999-2011.

Abstract. Multi-spectral and multi-directional data acquired during the ReSeDA experiment thanks to the airborne Pol-DER sensor were used to retrieve surface albedo over the experimental site, for 16 days over the year 1997. The data were available in four wave-bands (10 or 20 nm width) centered at 443 nm, 550 nm, 670 nm, and 865 nm. Zenith view angles ranged from 0 to 50 o. This study aimed at evaluate a procedure based on the use of multi-directional and multi-spectral information to retrieve surface albedo. Multidirectional ...

Remote Sensing of Environment, 2000

This study describes the use of ground-based albedome-lator data over the 1.1ϫ1.1-km area of an Advanced ter measurements for validating albedo measurements ac-Very High Resolution Radiometer pixel shows that the quired from space at the 1-km spatial resolution of Modinstrument tower at the field site, in the vicinity of which erate Resolution Imaging Spectroradiometer (MODIS). the ground albedo measurements were made, was located This work forms an element of the Grassland Prototype in an area where the local spatial heterogeneity of albedo Validation Exercise initiative, a prelaunch Earth Observis similar to that at the coarser satellite resolution. This ing System field experiment conducted in May 1997 at explains the agreement between the ground-based and Jornada, NM, USA. In lieu of MODIS data, we present the satellite-derived albedo values despite the difference albedo comparisons using 1.1-km Advanced Very High in spatial scale. These results provide the first step in es-Resolution Radiometer and 6ϫ7-km Polarisation and Ditablishing appropriate strategies for albedo validation for rectionality of the Earth's Radiation data. To derive MODIS. ©Elsevier Science Inc., 2000 broadband albedo from the satellite data, we calculate spectral

The two parts that follow were written in 2001 and 2003, when I was teaching for a term near Chicago and the Iraq war was launched. The first is rather sententious, the second a series of episodes that I participated in and observed. The numbering of the sections reflects the chronological sequence of when they were written. Substack shows the most recent post first and I prefer reading them backwards: from the most contemporary into the past, a method of extension from concrete to abstract, local to global, with the summary as conclusions, having not seen the trailer in advance of the movie. You choose.