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 the angular dependence of 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 Moderate Resolution Imaging Spectrometer (MODIS) derived properties; collocation of MODIS radiances with CERES footprints provides scene identification of CERES footprints such as cloud, snow, and sea ice fraction over a CERES footprint, as well as cloud optical thickness. 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. To include variations of optical properties of snow and sea ice, the surface is further classified either as bright or dark snow and sea ice using the MODIS-derived nadir view reflectance at 630 nm. Longwave ADMs depend on the cloud fraction, surface temperature, and temperature difference between the surface and cloud top. ADM-derived albedo indicates that the mean 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 mean albedo over permanent snow is approximately 0.7. A time series of the albedo over arctic regions from March 2000 to February 2002 will be presented.