BEHAVIOR OF AN INVERSION-BASED PRECIPITATION RETRIEVAL ALGORITHM WITHHIGH-RESOLUTION AMPR MEASUREMENTS INCLUDING A LOW-FREQUENCY 10.7-GHZ CHANNEL

A microwave-based, profile-type precipitation retrieval algorithm has been used to analyze high-resolution passive microwave measurements ov er an ocean background, obtained by the Advanced Microwave Precipitati on Radiometer (AMPR) flown on a NASA ER-2 aircraft. The analysis is de signed to first determine the improvements that can be pined by adding brightness temperature information from the AMPR low-frequency channe l (10.7 GHz) to a multispectral retrieval algorithm nominally run with satellite information at 19, 37, and 85 GHz. The impact of spatial re solution degradation of the high-resolution brightness temperature inf ormation on the retrieved rain / cloud liquid water contents and ice w ater contents is then quantified in order to assess the possible biase s inherent to satellite-based retrieval. The tests are conducted on a dataset obtained during a preliminary flight experiment that took plac e on 18 October 1990 over a Gulf of Mexico squall line that developed south of the Florida Panhandle. Careful inspection of the high-resolut ion aircraft dataset reveals five distinctive brightness temperature f eatures associated with cloud structure and scattering effects that ar e not generally detectable in current passive microwave satellite meas urements. Recovery of such high-resolution information by satellites w ould generally be expected to improve precipitation retrieval, but the se improvements have never been quantified and thus are addressed in t his study. Results suggest that the inclusion of 10.7-GHz information overcomes two basic problems associated with three-channel retrieval. First, unresolved rainfall gradients in the lower cloud layers due to 19-GHz blackbody saturation effects are recovered when the 10.7-GHz ch annel data are included. Second, unrealistic oscillations in the retri eved rain liquid water contents that arise from the highly variable sc attering signatures at 19, 37, and 85 GHz are eliminated by virtue of the 10.7-GHz Rayleigh frequency probing into the lower cloud containin g the bulk of the liquid water. Intercomparisons of retrievals carried out at high-resolution and then averaged to a characteristic satellit e spatial scale to the corresponding retrievals in which the brightnes s temperatures are first convolved down to the satellite scale suggest that with the addition of the 10.7-GHz channel, the rain liquid water contents will not be negatively impacted by special resolution degrad ation. That is not the case with the ice water contents as they appear to be quite sensitive to the imposed scale, the implication being tha t as spatial resolution is reduced, ice water contents will become inc reasingly underestimated. The overall implications of this study in th e context of the upcoming United States-Japan Tropical Rainfall Measur ing Mission are that the inclusion of a 10.7-GHz frequency on the pass ive microwave radiometer and the relatively higher spatial resolution of the low and intermediate frequencies at 10.7, 19, and 35 GHz result ing from the relatively low orbit (approximately 350 km) will lead to significantly improved microwave-based rainfall measurements over what are currently available today.