AMSU Millimeter-Wave Precipitation Retrievals Trained with MM5 Simulations: Sensitivity to Physical Assumptions

simulation system as a test-bed for precipitation retrieval development for satellite-borne millimeter-wave instruments. The sensitivity study was performed by comparing radiance histograms observed by satellite instruments (AMSU-A/B) over 122 diverse global storms with those simulated by variations of the MM5/RTM system for the same storm set. The radiances simulated by a baseline MM5/RTM model had earlier been shown to agree well with AMSU observations [1]. The simulated radiances were found to be very sensitive to the Mie backscattering fraction and the abundances, altitudes, and models [F(λ)] of both snow and graupel. AMSU retrieval performance was then evaluated assuming large random changes to MM5 and the RTM. The surface precipitation-rate rms retrieval accuracy was found to be robust to those changes, i.e., unchanged within ~± 5 percent and ~± 17 percent for precipitation rates ~< 8 mm/h and ~< 174 mm/h, respectively.

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