The Dependence of TOA Reflectance Anisotropy on Cloud Properties Inferred from ScaRaB Satellite Data

Abstract

An angular dependence model (ADM) describes the anisotropy in the reflectance field. ADMs are a key element in determining the top-of-the-atmosphere (TOA) albedos and radiative fluxes. This study utilizes 1-yr satellite data from the Scanner for Radiation Budget (ScaRaB) for overcast scenes to examine the variation of ADMs with cloud properties. Using ScaRaB shortwave (SW) overcast radiance measurements, an SW mean overcast ADM, similar to the Earth Radiation Budget Experiment (ERBE) ADM, was generated. Differences between the ScaRaB and ERBE overcast ADMs lead to biases of ∼0.01–0.04 in mean albedos inferred from specific angular bins. The largest biases are in the backward scattering direction. Overcast ADMs for the visible (VIS) wavelength were also generated using ScaRaB VIS measurements. They are very similar to, but a little smaller at large viewing angles and a little larger at nadir, than the SW overcast ADMs. To evaluate the effect of cloud properties on ADMs, ScaRaB overcast observations were further classified into thin, thick, warm, and cold cloud categories to generate four subsets of ADMs. The resulting ADMs for thin and thick clouds show opposite trends and deviate significantly from the overall mean ADM by more than 10%. Deviations from the mean ADM were also noted for the ADMs developed for warm water clouds and cold ice clouds. These deviations were attributed to the different scattering phase functions of water and ice particles and were compared with results from model simulations. Use of a single mean overcast ADM results in albedo biases of 0.01–0.04, relative to the use of specific ADMs for particular cloud types. The biases reduced to ∼0.005 when averaged over all cloud types and viewing geometry.