Determination of Broadband Albedos of Partially Snow-Covered Sites for Validation of MODIS Snow Albedo Retrievals

Over the period from February 24-28, 2003, the albedo of two partially snow-covered sites in northwestern Iowa was characterized to validate snow albedos retrieved from data collected by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Below average snowfall provided the opportunity to evaluate the accuracy of snow albedos retrievals from the current MODIS snow-albedo algorithm for pixels containing a mixture of snow and other surface covers. Over a frozen lake and agricultural fields, heterogeneity in albedo was observed within a single 500 m MODIS pixel. Over the frozen lake, the primary source of spatial variability was associated with snow patches alternating with lake ice over distances of a few meters. Small-scale variability in albedo was also observed on the agricultural fields, but larger-scale spatial variability due to differing land covers and redistribution of snow by wind also occurred. At both localities, snow albedo was measured at random sampling localities within a series of 10 m grid cells. On the agricultural fields, albedo measurements were also made on a 250 m grid. Geographic Information System (GIS) techniques were employed to integrate these in situ albedo measurements into estimates of broadband albedo representative of MODIS pixels for comparison with albedos retrieved from MODIS. In general, MODIS snow albedos were within 10% of those measured in situ. Decreases in the MODIS retrieved albedos over the study period were also consistent with those observed on the ground.