Content area
This study examines a trend in the 7-year seasonal (i.e., trimonthly) rainfall amounts (September 2001 to August 2008) observed by the TRMM Precipitation Radar (PR). The PR-observed rainfall amounts averaged in the range of 36°S to 36°N tend to increase slightly over this period. This tendency can be caused not only by natural variations but also by sampling errors (random errors) due to the PR's sparse observations in time. To separate the natural variations from the sampling errors, we developed a method of evaluating the sampling errors by a bootstrap method using the actual data observed by the PR. This method enables an evaluation of regional difference and characteristics of the sampling error in 5° grid boxes. As an application of the simulated sampling errors, we tested the significance of the trends in the averaged rainfall amounts by using Akaike's Information Criterion (AIC) and concluded that the positive trends were significant in all three cases; the trend in rainfall amounts averaged over all observed area (0.283 mm (3 months)2), that over land (0.518 mm (3 months)2) and that over the ocean (0.208 mm (3 months)2). We also tested the significance of the trends in the individual 5° grid boxes during this period and found that regions with the largest trends correspond to those in which the earlier studies have found the precipitation patterns caused by the El Niño-Southern Oscillation (ENSO).