EFFECTS OF PRECIPITATION AND CLOUD ICE ON BRIGHTNESS TEMPERATURES IN AMSU MOISTURE CHANNELS

Extinction by ice and rain at the AMSU frequencies used in water vapor profile retrievals is investigated with DMSP observations and brightn ess temperature simulations of a convective storm system, The simulati ons are based on mesoscale forecast model output of atmospheric, cloud , and rain profiles from which the absorption and scattering due to bo th liquid and frozen hydrometeors are calculated, Comparison with sate llite observations indicates discrepancies of more than 90% (up to 60 K), of which only about 20% results from ignoring scattering by model- prescribed ice, The major source of error is the inability of the fore cast model to produce the spatially localized high ice concentrations which cause the low microwave brightness temperatures, A criterion bas ed on the difference between measured brightness temperatures at 183.3 1 +/- 3 and 183.31 +/- 1 GHz is suggested to screen out convective eve nts before water vapor retrieval, Application to the case study examin ed here improved agreement between simulated and observed brightness t emperatures by up to a factor of two.