Tropical rainfall distributions determined using TRMM combined with other satellite and rain gauge information

A technique is described to use Tropical Rainfall Measuring Mission (TRMM) combined radar-radiometer information to adjust geosynchronous infrared sat ellite data [the TRMM Adjusted Geostationary Operational Environmental Sate llite Precipitation Index (AGPI)]. The AGPI is then merged with rain gauge information (mostly over land) to provide finescale (1 degrees latitude X 1 8 longitude) pentad and monthly analyses, respectively. The TRMM merged est imates are 10% higher than those from the Global Precipitation Climatology Project (GPCP) when integrated over the tropical oceans (37 degreesN-37 deg reesS) for 1998, with 20% differences noted in the most heavily raining are as. In the dry subtropics the TRMM values are smaller than the GPCP estimat es. The TRMM merged product tropical-mean estimates for 1998 are 3.3 mm day (-1) over ocean and 3.1 mm day(-1) over land and ocean combined. Regional d ifferences are noted between the western and eastern Pacific Ocean maxima w hen TRMM and GPCP are compared. In the eastern Pacific rain maximum the TRM M and GPCP mean values are nearly equal, which is very different from the o ther tropical rainy areas where TRMM merged product estimates are higher. T his regional difference may indicate that TRMM is better at taking into acc ount the vertical structure of the rain systems and the difference in struc ture between the western and eastern (shallower) Pacific convection. Comparisons of these TRMM merged analysis estimates with surface datasets s hows varied results; the bias is near zero when compared with western Pacif ic Ocean atoll rain gauge data, but is significantly positive as compared w ith Kwajalein radar estimates (adjusted by rain gauges). Over land the TRMM estimates also show a significant positive bias. The inclusion of gauge in formation in the final merged product significantly reduces the bias over l and, as expected. The monthly precipitation patterns produced by the TRMM merged data process clearly show the evolution of the El Nino-Southern Oscillation (ENSO) trop ical precipitation pattern from early 1998 (El Nino) to early 1999 (La Nina ) and beyond. The El Nino-minus-La Nina difference map shows the expected e astern Pacific maximum, the "Maritime Continent'' minima, and other tropica l and midlatitude features, very similar to those detected by the GPCP anal yses. However, summing the El Nino-minus-La Nina differences over the globa l tropical oceans yields divergent answers for interannual changes from TRM M, GPCP, and other estimates. This emphasizes the need for additional valid ation and analysis before it is feasible to understand the relations betwee n global precipitation anomalies and Pacific Ocean ENSO temperature changes .