A 14-Year TRMM Composite Climatology of Tropical Rainfall with Version 7 Data

The first-time use of both active and passive microwave instruments onboard the Tropical Rainfall Measuring Mission (TRMM, launched in late 1997) has made TRMM the foremost satellite for the study of precipitation in the tropics. One of the key goals of TRMM has been to define the spatial and seasonal climatological rainfall in the tropics as accurately as possible in order to quantify this key component of the hydrological cycle. A climatology of tropical surface rain has been developed based on a composite of fourteen years (1998-2011) of precipitation products from recent TRMM Version 7 (V7) data (Adler et al. 2009, JMSJ). The TRMM Composite Climatology (TCC) V7 consists of a merger of selected TRMM rainfall products over both land and ocean to give a "TRMM-best" climatological estimate. For this new version of the TCC (using TRMM V7 inputs) there have been some changes in the input data sets and in the methods. For the previous TCC V6 product, over ocean, the TCC inputs included the passive microwave retrieval using data from the TRMM Microwave Imager (TMI), the active microwave retrieval using data from the Precipitation Radar (PR), and the combined TMI-PR product, a separate retrieval with different assumptions. Over land, the TMI V6 estimates are known to overestimate surface rainfall, especially in the warm season. Therefore, the TRMM Multi-satellite Precipitation Analysis (TMPA) was used to replace the TMI. However, from our recent analysis of the TRMM products, it is found that TMI V7 rainfall estimates over land have significantly smaller biases and have much better agreement with the PR and the combined TMI-PR products, except in some mountainous regions due to the problematic rainfall screening processes. Therefore, we decided to include the TMI V7 estimates instead of TMPA as one of the components in the TCC V7 over land using some quality control techniques. Consequently, the new version of the TCC becomes a TRMM satellite only rainfall climatology for the TRMM era. In addition to the mean precipitation estimate, the TCC also includes the variation among the three estimates at each point to give an estimate of the uncertainty in the mean value. For evaluation and validation purposes, a series of inter-comparisons will be carried out among the TCC V6 and TCC V7, its components, and ground-based observations. The TCC may have broad applications and should be useful to the user community interested in climate monitoring, climate variability studies, model initialization and verification, and comparison with other non-TRMM rainfall analyses.