Aircraft Multifrequency Passive Microwave Observations of Light Precipitation over the Ocean

Abstract

Aircraft passive microwave observations at 18, 37, 92, and 183 GHz of light oceanic precipitation are studied in conjunction with visible and infrared observations and ground-based radar data. Microwave signatures for clear, cloudy, and precipitating conditions are defined, with results in general agreement with previous theoretical results. Emission signatures are evident at 18, 37, and 92 GHz with clouds and precipitation producing an increase in brightness temperature (Tb) over that observed over the low-emissivity ocean background. Polarization differences at 18 and 37 GHz also decrease in precipitation areas to minima of 30 K at 18 GHZ and 15 K at 37 GHz. The 92-GHz Tb shows a double-valued relationship, with an increase in cloudy and very lightly raining areas and a subsequent decrease for higher rain rates and deeper clouds where the ice scattering process becomes important. The 183-GHz observations display a distinct sensitivity to small amounts of ice. Simple channel differences are shown to compare favorably to the rain field, including polarization differences at 18 and 37 GHz and frequency differences between 92 and 37 GHz and between 183 and 92 GHz. The cases examined include both stratiform and convective structures, and the results indicate that this difference may be important in microwave Tb-rain rate relationships. Some of the observed differences may be due to the presence or absence of a melting layer, observed as a radar ?bright band.?