Clear-sky narrowband albedo variations from VIRS and MODIS data

Clear-sky and surface albedos are important parameters for satellite remote sensing and for modeling the radiative exchange at the surface. The Clouds and Earth's Radiant Energy System (CERES) project is matching broadband radiances with multispectral imager data and derived cloud properties to improve the determination of the Earth’s radiation budget. The imager data include the Visible Infrared Scanner (VIRS) on the Tropical Rainfall Measuring Mission (TRMM) satellite and the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra, operating since spring 2000, and Aqua, operating since summer 2002. This paper presents the results of an analysis of the CERES clear-sky reflectances to derive a set top-of-atmosphere clear sky albedos at 0.65, 0.86, 1.6, and 2.13 um and for the broadband shortwave spectrum (0.2 – 5 µm) for all major surface types using the combined MODIS and VIRS datasets. Surface albedo is derived from the clear-sky albedos using radiative transfer model calculations. The variation of surface type with season and solar zenith angle is examined using the combined dataset. The variability of snow albedo with surface type is examined using MODIS data. Snow albedo was found to depend on the vertical structure of the vegetation. At visible wavelengths, it is least for forested areas and greatest for smooth desert and tundra surfaces. At 1.6 and 2.1-µm, the snow albedos are relatively insensitive to the underlying surface because snow decreases the reflectance. The derived albedo models should be valuable for many remote sensing, simulation, and radiation budget studies.