NASA - National Aeronautics and Space Administration

We examine the viewing zenith angle dependent bias (VZA bias) in warm cloud optical thickness (t) retrieved from a plane-parallel approach applied to fused Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging SpectroRadiometer (MISR) data for the months of January and July between 2001 and 2008. The near-simultaneous multiple view-angle observations from MISR offers many advantages over previous t-VZA bias studies: 1) The analysis no longer requires seasonal and latitudinal cloud invariant assumptions, 2) consistent cloudy scene identification with VZA, 3) stratification of VZA-bias with scene characteristics, and 4) a greater range of sun-view geometries. Contrasting results between previous studies are resolved through careful consideration of the relative azimuth angle (RAZ) between sun and view. Relative to nadir-retrieved t, t increases in both forward- and backscatter directions with higher value in backscatter directions for solar zenith angle (SZA) < ~40 . For SZA > ~40 , t increases with increasing VZA in backscatter directions and strongly decreases in forward-scatter directions. For the most oblique views, ~40–100% absolute monthly mean differences relative to nadir-retrieved t is common. This behavior is strongly tied to the sampled RAZ and explained based on three factors tied to the spatial heterogeneity of clouds. These factors also explain the behavior of the t-VZA bias when stratified by nadir-retrieved t and spatial heterogeneity, even in the thin-cloud limit where sun-glint effects are evident. We also observe an underestimation of t relative to nadir in the rainbow-scattering directions and attribute it to an overestimation of the cloud-drop effective radius retrieved from MODIS due to cloud heterogeneity. There remains a need to quantify the bias in nadir-retrieved t as a function of SZA and spatial heterogeneity as a step toward providing bias correction over a wide range of sun-view geometries.