Consistency of MODIS surface bidirectional reflectance distribution function and albedo retrievals: 2. Validation
Consistency of MODIS surface bidirectional reflectance distribution function and albedo retrievals: 2. Validation
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Date
2003-03-08
Authors
Liang, Shunlin
Lucht, Wolfgang
Jin, Yufang
Schaaf, Crystal B.
Woodcock, Curtis E.
Gao, Feng
Li, Xiaowen
Advisor
Citation
Jin, Y., C. B. Schaaf, C. E. Woodstock, F. Gao, X. Li, A. H. Strahler, W. Lucht, and S. Liang, Consistency of MODIS surface bidirectional reflectance distribution function and albedo retrievals: 2. Validation, J. Geophys. Res., 108(D5), 4159, doi:10.1029/2002JD002804, 2003.
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Abstract
The evaluation of the first available satellite-based global albedo product at 1-km
resolution is essential for its application in climate studies. We evaluate the accuracy of the
Moderate-Resolution Imaging Spectroradiometer (MODIS) albedo product using
available field measurements at Surface Radiation Budget Network (SURFRAD) and
Cloud and Radiation Testbed–Southern Great Plains (CART/SGP) stations and examine
the consistency between the MODIS surface albedos and the Clouds and Earth’s Radiant
Energy System (CERES) top-of-the-atmosphere albedos as well as historical global
albedos from advanced very high resolution radiometer (AVHRR) and Earth Radiation
Budget Experiment (ERBE) observations. A comparison with the field measurements
shows that the MODIS surface albedo generally meets an absolute accuracy requirement
of 0.02 for our study sites during April–September 2001, with the root mean square errors
less than 0.018. Larger differences appear in the winter season probably due to the
increased heterogeneity of surface reflectivity in the presence of snow. To examine the
effect of spatial heterogeneity on the validation of the MODIS albedos using fine
resolution field measurements, we derive an intermediate albedo product from four
Landsat Enhanced Thematic Mapper Plus (ETM+) images at 30-m spatial resolution as a
surrogate for the distributed field measurements. The surface albedo is relatively
homogeneous over the study stations in growing seasons, and therefore the validation
during April–September is supported. A case study over three SURFRAD stations reveals
that the MODIS bidirectional reflectance distribution function model is able to capture the
solar zenith angle dependence of surface albedo as shown by the field measurements. We
also find that the MODIS surface shortwave albedo is consistent with the contemporary
and collocated CERES top-of atmosphere albedos derived directly from broadband
observations. The MODIS albedo is also well correlated with historical surface albedos
derived from AVHRR and ERBE observations, and a high bias of 0.016 and a low bias of
0.034 compared to those of the latter albedos are reasonable considering the differences in
instruments and retrieval algorithms as well as environmental changes.