A TRMM microwave radiometer rain retrieval method based on fractional rain area

Abstract

Recent studies based on aircraft observations made over ocean by radar and multi-channel passive microwave radiometer, each with a field of view (fov) of a few kilometers, indicate significant, problems in relating the radar derived rain rate with the brightness temperature measurements of the microwave radiometer. We arrive at, a similar conclusion from extensive observations of rain rate made by ship-borne radars and rain rate deduced from observations of the satellite-borne, multi-channel Special Sensor Microwave Imager (SSM/I) radiometer, which has a much larger fov (∼30 km). The principal reason for these problems is that the signal due to rain drops contained in the radiometer measurements is non-linearly mixed with that of other hydrometeors that could be present in the radiometer fov. These other hydrometeors include liquid droplets, and dry and melting ice and snow particles of different densities, sizes, and shapes in clouds. Observations made by SSM/I are not adequate to uncouple the rain signal satisfactorily from such a non-linear mix of signals on the scale of the radiometer footprint. These problems are complicated further by meteorological conditions, which can significantly alter the amount and spatial distribution of these other hydrometeors. For these reasons an empirical method is developed to estimate area-average rain rate in a mesoscale region of about 300 x 300 km2, based on SSM/I data. One parameter of this empirical method fR relates to the fractional rain area in a mesoscale region, while the second parameter, χ, reflects in a weak way the scattering and emission properties of the hydrometeors in that region. This method requires tuning with the help of radar data. Over the TOGA-COARE area, the rain rates retrieved from this method can reproduce the radar observed rain rates with a correlation coefficient of about, 0.85. Furthermore, monthly total rainfall estimated from this method for the TOGA-COARE area has an error of about 13 %. This rain retrieval method is applicable to the Tropical Rainfall Measuring Mission (TRMM), in which multi-channel dual-polarization microwave radiometer observations over a 760 km wide swath are overlapped with those of radar, which has a swath of 220 km.