Top-of-atmosphere albedo estimation from angular distribution models usingscene identification from satellite cloud property retrievals

The next generation of earth radiation budget satellite instruments will ro utinely merge estimates of global top-of-atmosphere radiative fluxes with c loud properties. This information will offer many new opportunities for val idating radiative transfer models and cloud parameterizations in climate mo dels. In this study, five months of Polarization and Directionality of the Earth's Reflectances 670-nm radiance measurements are considered in order t o examine how satellite cloud property retrievals can be used to define emp irical angular distribution models (ADMs) for estimating top-of-atmosphere albedo. ADMs are defined for 19 scene types defined by satellite retrievals of cloud fraction and cloud optical depth. Two approaches are used to defi ne the ADM scene types. The first assumes there are no biases in the retrie ved cloud properties and defines ADMs for fixed discrete intervals of cloud fraction and cloud optical depth (fixed-tau approach). The second approach involves the same cloud fraction intervals, hut uses percentile intervals of cloud optical depth instead (percentile-tau approach). Albedos generated using these methods are compared with albedos inferred di rectly from the mean observed reflectance field. Aibedos based on ADMs that assume cloud properties are unbiased (fixed-tau approach) show a strong sy stematic dependence on viewing geometry. This dependence becomes more prono unced with increasing solar zenith angle, reaching approximate to 12% (rela tive) between near-nadir and oblique viewing zenith angles for solar zenith angles between 60 degrees and 70 degrees. The cause for this bias is shown to be due to biases in the cloud optical depth retrievals. In contrast, al bedos based on ADMs built using percentile intervals of cloud optical depth (percentile-tau approach) show very little viewing zenith angle dependence and are in good agreement with albedos obtained by direct integration of t he mean observed reflectance field (<1% relative error). When the ADMs are applied separately to populations consisting of only liquid water and ice c louds, significant biases in albedo with viewing geometry are observed (par ticularly at low sun elevations), highlighting the need to account for clou d phase both in cloud optical depth retrievals and in defining ADM scene ty pes. ADM-derived monthly mean albedos determined for all 5 degrees x 5 degr ees lat-long regions over ocean are in good agreement (regional rms relativ e errors <2%) with those obtained by direct integration when ADM albedos in ferred from specific angular bins are averaged together. Albedos inferred f rom near-nadir and oblique viewing zenith angles are the least accurate, wi th regional rms errors reaching similar to 5%-10% (relative). Compared to a n earlier study involving Earth Radiation Budget Experiment ADMs, regional mean albedos based on the 19 scene types considered here show a factor-of-4 reduction in bias error and a factor-of-3 reduction in rms error.