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

All current operational retrievals of cloud optical properties with reflected solar radiation from satellites suffer from biases that depend on solar-view geometry, which is in part due to the plane-parallel assumption of clouds. This study examines the viewing zenith angle (VZA) dependence of plane-parallel retrieved cloud optical thickness (τ) with a fused global dataset from the Multiangle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS), for the months of January and July between 2001 and 2008. With the unique near-simultaneous multi-angle observations from MISR, we are able to overcome many shortcomings in previous observational studies on τ-VZA relationships derived from wide-swath, single-view scanning instruments. Unlike previous studies, we are able to exclude cloud seasonal and latitudinal invariant assumptions, eliminate inconsistency in cloudy scene identification across multiple view-angles and minimize the impact of pixel expansion with viewing obliquity on τ retrievals. Qualitatively, our results confirmed major observed τ-VZA relationships in previous observational studies. For example, in January, for oblique Sun (solar zenith angle (SZA) >40°), plane-parallel retrieved τ decreases with VZA in forward-scatter directions with VZA, whereas τ is less sensitive to VZA in the backscatter directions. Extending our analysis to include larger SZA (SZA>68° in January and SZA>73° in July), larger VZA=70°, and stratifying the data by nadir-τ and cloud spatial heterogeneity reveals additional complexities not observed before. To understand the complexities in τ-VZA relationships requires a careful consideration of (1) the various 3-D radiative transfer pathways, (2) the increased viewing of more cloud-sides with viewing obliquity, (3) the relative azimuth angle between sun and view, (4) the concavity of the radiance-τ non-linear relationship with view-angle, and (5) other non-3-D radiative transfer effects, such as sunglint.