A Microwave Radiometer Rain Retrieval Method Applicable to Land Areas

Abstract

Multi-spectral, dual-polarization measurements made by the satellite-borne Special Sensor Microwave Imager (SSM/I) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometers cannot discriminate satisfactorily strong, convective rain from weak, stratiform rain. This degrades the quality of the rain information deduced from these radiometers, particularly over land. With the objective to improve the quality of this rain information on land, in this study we have developed a mesoscale area-average rain retrieval method. This method is derived from ATI concepts outlined by Doneaud et al. (1984) and Lopez et al. (1989), and it requires tuning based on calibrated radar, and/or surface rain gauges. As a building block of this method, the fractional rain area, f_R, is first determined in a mesoscale grid box of 2°× 3° with the help of a threshold on the 85 GHz scattering depression. Using SSM/I data, f_R is shown to correlate well with the corresponding ground-truth area-average rain rate, R_g, deduced from 15-minute rain gauge observations. Based on f_R, a method is then developed to retrieve area-average rain rate, R_fR. In order to demonstrate the usefulness of this method, nine-months of R_fR are retrieved from SSM/I data over three grid boxes in the Northeast United States. In the three grid boxes, R_fR can explain about 64% of the variance contained in the corresponding R_g. When average 85 GHz scattering depression in the rain area is also included as a parameter in this method, the variance explained drops to 55%. Thus, we find statistically that the algorithm based solely on fractional rain area gives a better result. With the aid of this tunable method, because of the wider TMI swath, one can use this procedure to fill gaps in space and time present in the rain measurements of the Precipitation Radar (PR) in the TRMM mission.