First results of the TRMM microwave imager (TMI) monthly oceanic rain rate: Comparison with SSIM/I

We evaluated the performance of the Tropical Rainfall Measuring Mission (TR MM) Microwave Imager (TMI) at-launch algorithm for monthly oceanic rain rat e using six months (January - June 1998) of TMI data. Comparison with ocean ic monthly rain rates derived from the Special Sensor Microwave Imager (SSM /I) data shows statistically significant differences. The TMI min rates are lower than the SSM/I rain rates by about 10% overall, except for rain rate s lower than 1.4 mm/day. The low TMI bias may be due to an overestimate of the columnar water vapor as indicated by the estimated rain layer thickness . The superior sampling by the TMI improves the algorithm statistics at the low rain rates. The averaged monthly rain rates over the latitudes between 40 degrees S and 40 degrees N are 2.78 and 3.17 mm/day, respectively for T MT and SSM/I, with RMS difference of 1.35 mm/day and correlation coefficien t of 0.94.