Retrieval of Boundary Layer Cloud Water Path and Effective Radius using High Resolution Imagery and 3D Radiative Transfer

Differences in Boundary Layer Cloud (BLC) responses between climate models have been identified as one of the largest sources of uncertainty in climate model projections. Satellite retrievals of BLC water path and effective radius from MODIS and other Visible-and-Near-Infrared satellite imagers are being widely used in the assessment of climate and other atmospheric models. However, operational satellite imager retrievals for BLCs are based on one-dimensional radiative transfer (1D RT), which nominally limits application of retrievals to homogeneous stratocumulus. This limitation has long been recognized as a major source of error and uncertainty in satellite imager retrievals. Primarily due to BLCs, ~25% of the ocean surface (at 1 km resolution) is only partly-filled-by-cloud or on-a-cloud-edge [Cho et al. 2015, Marchand et al. 2010]. Conditions that result in both significant uncertainties in cloud fraction and large errors in cloud microphysical retrievals based on 1D RT.

While higher resolution visible imagery (that is, significantly better than the 250 m to 1 km used by MODIS) can almost certainly reduce uncertainties in BL cloud fraction (at least over ocean), it is not clear that such high resolution imagery can be used to improve retrievals of cloud water path, effective radius, or even optical depth due to three dimensional (3D) scattering effects. In this presentation we report on our efforts to retrieve BL cloud water path and effective radius using full 3D RT rather than relying on 1D RT. The analysis will include examination of an iterative 3D retrieval applied to both idealized and observed clouds. The approach is an extension of the retrieval originally published by Marchand and Ackerman 2004 (JGR).