A method to estimate rain rate over tropical oceans with the TRMM microwave imager radiometer

In this preliminary study, we have developed a method to retrieve rain rate on a scale of 20 km from the brightness temperatures measured by the TRMM microwave imaging radiometer (TMI) over the tropical oceans, using the estimates of rain rate R-PR made by the TRMM Precipitation Radar (PR) as a benchmark. The purpose of this study is to demonstrate with a limited amount of PR and TMI data the feasibility of improving the TRMM operational rain retrieval method V6 over the tropical oceans. This study utilizes the TMI-measured brightness temperatures T-19H, T-37H, and T-85H of horizontally polarized microwave radiances at 19, 37 and 85 GHz, respectively, to deduce a salient non-finear parameter zeta that is highly correlated with R-PR over the oceans. Two additional parameters generated from TMI data, omega and Gamma, add significant amounts of rain information to our retrieval method. The parameter omega is based on T-19V and T-21V, the brightness temperatures measured by TMI for vertically polarized microwave radiances at 19 and 21 GHz respectively. This parameter takes advantage of the independent information contained in T-21V, The parameter F depends on the average horizontal gradient of the TMI-measured T-85V (vertically-polarized 85 GHz radiance) in a 20 km footprint. Initially our TMI rain retrieval algorithm is tuned with the help of R-PR for seven cases of 2 degrees x 3 degrees area over tropical oceans. Then it is applied to 13 other independent tropical ocean cases. For these independent cases, the rain rate R* estimated from our method correlates better with R-PR than the rain rate R-V6 retrieved from the present TMI V6 operational retrieval method. On a 20 km scale, the correlation between R,, and R* is better by about 6% compared to that between R-PR and R-V6 The slope of the regression line between the rain rates R-PR and R-V6 is about 0.5. With respect to R-PR, the rain rate R-V6 retrieved from operational V6 method tends to underestimate high rain rates and overestimate low rain rates. The slope of the regression line between R-PR and the rain rate R* retrieved with our method is about 0.8, another indication of the improvement of R* over R-V6 In addition, the area average rain rate on a scale of 2 degrees x 3 degrees deduced from our method agrees better with that of PR by about 7%.