A Method to Estimate Rain Rate over Tropical Oceans with the TRMM Microwave Imager Radiometer

[摘要] 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 RPR 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 T19H, T37H, and T85H of horizontally polarized microwave radiances at 19, 37 and 85 GHz, respectively, to deduce a salient non-linear parameter ξ that is highly correlated with RPR over the oceans. Two additional parameters generated from TMI data, ω and γ, add significant amounts of rain information to our retrieval method. The parameter &omega is based on T19V and T21V, 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 T21V. The parameter Γ depends on the average horizontal gradient of the TMI-measured T85V (vertically-polarized 85 GHz radiance) in a 20 km footprint. Initially our TMI rain retrieval algorithm is tuned with the help of RPR for seven cases of 2° × 3° 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 RPR than the rain rate RV6 retrieved from the present TMI V6 operational retrieval method. On a 20 km scale, the correlation between RPR and R* is better by about 6% compared to that between RPR and RV6. The slope of the regression line between the rain rates RPR and RV6 is about 0.5. With respect to RPR, the rain rate RV6 retrieved from operational V6 method tends to underestimate high rain rates and overestimate low rain rates. The slope of the regression line between RPR and the rain rate R* retrieved with our method is about 0.8, another indication of the improvement of R* over RV6. In addition, the area average rain rate on a scale of 2° × 3° deduced from our method agrees better with that of PR by about 7%.

[学科分类] 大气科学