Cloud physical and microwave radiative properties of tropical stratiform precipitation inferred from multichannel microwave radiances

Principal component analysis was performed on SSM/I multichannel radiances derived from approximately 50 regions of horizontally extensive, moderate-to-heavy precipitation over the tropical oceans. Three modes of multichannel variability explain 97% of the correlation between the 7 channels of the SSM/I. A parametric one-dimensional stratiform rain cloud model was developed and coupled with a forward radiative transfer model in order to retrieve characteristic microphysical properties associated with the sample of rain events. Findings of note include the following: (1) highly distinct vertical profiles of hydrometeors can, under some conditions, lead to indistinguishable 7-channel radiances from the SSM/I. There is therefore an inherent physical ambiguity in multichannel retrievals of hydrometeor structure which can only be resolved through the use of a priori information. (2) The addition of an arbitrary number of microwave channels at higher and lower frequencies does not remove the ambiguity in the cases under consideration. (3) Multi-channel consistency between observations and models can only be achieved by assuming frozen precipitation particle sizes much smaller than those implied by 'standard' particle size distribution models, such as that of Sekhon-Srivastava.