A global analysis on the view-angle dependence of plane-parallel oceanic liquid water cloud optical thickness using data synergy from MISR and MODIS

摘要

We examine the viewing zenith angle dependent bias (VZA bias) in warm cloud optical thickness (τ) 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 τ-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 τ, τ increases in both forward- and backscatter directions with higher value in backscatter directions for solar zenith angle (SZA) < ~40°. For SZA ~40°, τ 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 τ 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 τ-VZA bias when stratified by nadir-retrieved τ and spatial heterogeneity, even in the thin-cloud limit where sun-glint effects are evident. We also observe an underestimation of τ 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 τ as a function of SZA and spatial heterogeneity as a step toward providing bias correction over a wide range of sun-view geometries.