Rain rate retrieval using airborne imaging radiometry during CAMEX3/TEFLUN-B

Abstract

A multiband Polarimetric Scanning Radiometer (PSR) was integrated onto the NASA DC-8 aircraft (N717NA) and flown from August through September of 1998 during the third Convection and Moisture Experiment (CAMEX3) and Texas-Florida Under-flight (TEFLUN-B) campaign. The PSR is a unique conically-scanned imaging radiometer with channels at 10.7, 18.7, 21.5, 37.0 and 89.0 GHz, measuring both vertical and horizontal polarizations at each of these frequencies. These channels correspond to several key bands of the DMSP (Defense Meteorological Satellite Program) SSM/I (Special Sensor Microwave Imager) and the NASA TRMM (Tropical Rainfall Measuring Mission) TMI (TRMM Microwave Imager). The PSR was developed by Georgia Institute of Technology and the NOAA Environmental Technology Laboratory. It is the first airborne imaging radiometer to provide a research quality set of high spatial resolution multiband polarimetric microwave imagery within and around a hurricane. A nonlinear statistical emission algorithm was developed for rain rate retrieval, similar to that described by Skofronick-Jackson and Gasiewski (1995). We use the PSR/A 10.7 GHz channels of horizontal and vertical polarization because of their predominantly monotonic response to near-surface rain rate from 0 to /spl sim/50 mm/hr. An initial comparison of the PSR retrieved rain rate to coincidently observed rain rate retrieved using NASA JPL's Airborne Rain Mapping Radar (ARMAR) shows favorable agreement over at least an order of magnitude in rain rate intensity. Discrepancies are within the standard deviations of the PSR retrieval algorithm and can be explained by differences in the observation geometries of the sensors and by differences in the physical measurement principals between the two instruments. Further comparison of the PSR rain rate retrieval with the TMI level 2A12 rain rate product is similarly favorable. Some discrepancies can be explained by the differences in spatial resolution of the two passive microwave instruments and the different nature of the two retrieval algorithms. Despite these differences, the correlation coefficient between the TMI and the PSR coincidentally observed rain rates is 0.92 and 0.91 for PSR horizontal and vertical channels (respectively), for rain rates from /spl sim/1 to /spl sim/16 mm/hr.