Precipitation Retrieval over Land and Ocean with the SSM/I: Identification and Characteristics of the Scattering Signal

Abstract

The subject of this study is the identification of precipitation in warm and cold land and ocean environments from the Defense Meteorological Satellite Program's (DMSP) Special Sensor Micmwave/Imager (SSM/I). The high sensitivity of the SSM/I 85.5 GHz channels to volume scattering by precipitation, especially ice above the freezing level, is the basis for this identification. This ice scattering process causes SSM/I 85.5 GHz brightness temperatures to occasionally fall below 100 K. It is demonstrated that the polarization diversity available at 85.5 GHz from the SSM/I allows discrimination between low brightness temperatures due to surface water bodies versus those due to precipitation. An 85.5 GHz polarization corrected temperature (PCT) is formulated to isolate the precipitation effect. A PCT threshold of 255 K is suggested for the delineation of precipitation. This threshold is shown to be lower than what would generally be expected from nonprecipitating cloud water alone, yet high enough to sense relatively light precipitation rates. Based upon aircraft radiometric measurements compared with radar derived rain rates, as well as model calculations, the corresponding average rain rate threshold is approximately 1?3 mm h?1. The majority of precipitation that falls on the earth exceeds this rate. Because the 85.5 GHz measurements of oceanic storms are often dominated by scattering due to precipitation above the freezing level, while the 19.35 GHz radiances are dominated by emission due to rain below the freezing level, there is independent information about the gross vertical structure of oceanic precipitation systems from the SSM/I. Apparent differences between storms in formative, mature, and dissipating stages are inferred from the diagnosed amounts of ice versus raindrops, and supported by time lapse GOES imagery. Deviations from the average relationship between 19.35 GHz warming and 85.5 GHz cooling are suggested for use as a diagnostic tool to evaluate lower level rain/upper level ice relative abundances. As an example of this capability, overrunning precipitation shows a horizontal offset between the advancing ice layer and the trailing rain area, consistent with idealized conceptual models of warm frontal precipitation. Part II of this study will address global screening for the precipitation scattering signal, its statistical characteristics, and the false rain signatures frequently caused by snow cover and cold land.