A global view of the plane-parallel nature of oceanic water clouds through MISR and MODIS fusion

The plane-parallel assumption is commonplace in weather and climate research in both modeling and remote sensing applications. While observational and 3-D radiative transfer modeling studies have revealed some shortcomings in the application of the plane-parallel assumption, we continue to lack a global perspective on how often and to what degree this assumption is valid, as well as an ability to identify the cloud heterogeneity conditions under which its validity occurs. Gaining this perspective will require a globally representative dataset of the true optical properties of clouds. Given the lack of such a dataset, we must seek alternative ways to reach our global perspective.

One way to gain this perspective is through the fusion of MISR and MODIS data, whereby MODIS-retrieved cloud optical properties are examined for view-angular consistency in the MISR observations. Metrics are developed to examine angular consistency in the BRF, optical depth, and spherical albedo and applied to a statistically representative dataset for oceanic water clouds. Our results reveal, for example, that clouds are angularly consistent in BRF to within 5% and 10% of their plane parallel-value 65% and 93% of the time, respectively. Global maps of these metrics show solar zenith angle dependence and large spatial variability, with stratiform regions being more plane-parallel-like than cumuliform regions. We relate angular consistency to sub-pixel variability of nadir-BRF, allowing us to potentially identify, with a prescribed confidence level, which MODIS microphysical retrievals within the MISR swath meet the plane-parallel assumption to within any desired range in view-angle consistency.