Snow and Sea Ice Angular Distribution Model From CERES Radiance Measurements

Clouds and the Earth's Radiant Energy System (CERES) instruments on Terra that is on a polar orbit are taking measurements of broadband shortwave, longwave and window radiances since March 2000. Because the CERES instruments can be operated under the rotating azimuth mode, they can take radiance measurements over snow covered surface from a wide range of viewing angles. Angular distribution models (ADM) for snow and sea ice are developed in order to estimate top-of-atmosphere broadband shortwave and longwave irradiance using measurements by CERES instruments. Because of a large difference of angular dependent radiances, ADM is divided in to three types based on surface type, permanent snow, fresh snow, and sea ice. These ADM types are further divided using MODIS-derived scene type; colocation of MODIS image with CERES footprints provides scene identification of CERES footprints such as cloud, snow, and sea ice fraction over a CERES footprint. The shortwave permanent snow ADM depends on cloud fraction and shortwave fresh snow and sea ice ADMs depend on both cloud fraction and snow and ice fraction. The longwave ADMs depends on cloud fraction, surface temperature, and temperature difference between the surface and cloud top. ADM-derived albedo indicates that albedo over clear sky permanent snow is approximately 0.65 and nearly independent of solar zenith angle. Clouds increase the albedo over permanent snow; all-sky albedo over permanent snow is approximately 0.7. As a part of an internal consistency check of ADM-derived irradiance, ADM-derived irradiances and irradiances computed by integrating measured radiances are compared.