Cloud Tomography from Space using MISR and MODIS: Locating the ``Veiled Core'' in Opaque Convective Clouds

For passive satellite imagers, current retrievals of cloud optical thicknessand effective particle size fail for convective clouds with 3D morphology.Indeed, being based on 1D radiative transfer (RT) theory, they work wellonly for horizontally homogeneous clouds. A promising approach for treatingclouds as fully 3D objects is cloud tomography, which has been demonstratedfor airborne observations. However, more efficient forward 3D RT solvers arerequired for cloud tomography from space. Here, we present a path forwardby acknowledging that optically thick clouds have “veiled cores” (VCs). Sunlight scattered into and out of this deep region does not contribute significantinformation about the inner structure of the cloud to the spatially-detailed imagery. We investigate the VC location for the MISR and MODIS imagers.While MISR provides multi-angle imagery in the visible and near-infrared(IR), MODIS includes channels in the short-wave IR, albeit at a single viewangle. This combination will enable future 3D retrievals to disentangle thecloud’s effective particle size and extinction fields. We find that, in practice,the VC is located at an optical distance of ≈5, starting from the cloud boundary along the line-of-sight. For MODIS’ absorbing wavelengths the VC covers a larger volume, starting at smaller optical distances. This concept will notonly lead to a reduction in the number of unknowns for the tomographic reconstruction but also significantly increase the speed and efficiency of the 3DRT solver at the heart of the algorithm by applying, say, the photon diffusionapproximation inside the VC.