Assessing the MODIS snow albedo products over tundra burn regimes

Surface Bidirectional Reflectance Distribution Function (BRDF) and albedo products (MCD43) derived from MODerate Resolution Imaging Spectroradiometer (MODIS) observations, from NASA's Terra and Aqua platforms, have been successfully validated during the growing seasons of various vegetated land surface types. This study assesses the MODIS standard BRDF/Albedo product and the daily Direct Broadcast BRDF/Albedo at tundra locations under large solar zenith angles and high anisotropic diffuse illumination and multiple scattering conditions. Low illumination angles and persistent cloudiness, as well as lower-quality atmospheric correction and cloud discrimination, limit the number of high quality retrievals that are obtained during snow-covered periods at these high latitudes. An integrated validation strategy, including analysis of the representativeness of the surface heterogeneity, is performed to decide whether direct comparisons between field measurements and 500-m satellite products were appropriate or if the scaling of finer spatial resolution airborne or spaceborne data was necessary. Landsat ETM+ data are used to generate finer scale representations of albedo to fully link the ground data with the satellite data. Results show that high quality MODIS products (with solar zenith angles of less than 70º) do achieve a high accuracy during snow-covered periods at several Alaskan tundra areas including differing burned and unburned regimes. At the start of snowmelt, the MODIS albedo values at burn sites are generally lower than those at an unburned site due to the dark residue as a result of fire. The daily albedo product is particularly well suited to capture the rapidly changing surface conditions during the spring snow melt. Results also show that a full expression of the blue sky albedo is necessary at these large SZA snow covered areas because of the effects of anisotropic diffuse illumination and multiple scattering.