Cloudy-sky Longwave Angular Distribution Models for the CERES Instrument

The Clouds and the Earth’s Radiant Energy System (CERES) instrument is a scanning broadband radiometer that measures top-of-atmosphere (TOA) reflected solar and emitted terrestrial longwave radiances. Angular distribution models (ADMs) are an important component of estimating TOA radiative fluxes from the radiances measured by CERES instruments. Currently, cloudy-sky longwave ADMs are created by calculating the average observed longwave radiance at each viewing zenith angle for scene types defined by bins of precipitable water, cloud fraction, surface temperature, surface-cloud temperature difference, and a quantity called “pseudoradiance.” Pseudoradiance is calculated based on cloud fraction, cloud temperature, cloud emissivity, surface temperature, and surface emissivity. The expression for pseudoradiance used in the current ADMs neglect the effects of scattering and also does not account for the change of cloud emissivity with viewing zenith angle (VZA). When a formulation for pseudoradiance that takes VZA into account for cloud emissivity is used for Terra daytime cloudy ocean scenes, modeled radiances are slightly closer to those observed, with the RMS error in radiance decreasing from 2.41% to 2.38%. This change produces estimated fluxes that are slightly lower, with a decrease of approximately 0.1 W m^-2. A formulation for pseudoradiance that takes scattering into account will also be evaluated.