Intercomparison and Interpretation of Satellite-Derived Directional Albedos over Deserts

Abstract

Desert regions are employed in somewhat of a tutorial mode for the purpose of addressing several issues associated with understanding the dependence of planetary (top-of-the-atmosphere) albedo upon solar zenith angle, i.e., the directional planetary albedo. It is emphasized that in evaluating this quantity from satellite data, and with reference to land surfaces, spurious results may be obtained if geographical variations of the planetary, albedo are not isolated from the albedo's solar zenith angle dependence. An atmospheric solar radiation model is then coupled with desert surface bidirectional reflectance measurements to test for consistency with satellite-derived directional planetary albedos. The model is further used to address issues such as the use of narrowband versus broadband instruments, the impact of desert aerosols upon the directional planetary albedo, and to interpret potential differences in the directional planetary albodo associated with different types of deserts. The model results show consistency with satellite measurements, while further suggesting that over desert regions, narrowband instruments should replicate broadband measurement of the directional planetary albedo, as is also consistent with observations. The model shows that the directional planetary albedo is dominated by the directional surface albedo, although surface brightness is a second factor since it influences atmospheric limb brightening and limb darkening processes.