Improvement of Tropical Rainfall Climatology and Definition of Inter-Annual Variations with TRMM and GPM Data

An improved and extended (through 2017) Tropical Composite Climatology (TCC) is presented by combining the new TRMM Version 8 (V8) data with the data (V06) from the Global Precipitation Measurement (GPM) mission. The TRMM V8 rainfall products are produced with algorithms nearly identical to those of GPM V06 providing a consistent set of retrievals for the climatology.

The new TCC consists of a merger of selected TRMM and GPM rainfall products over both land and ocean to give a "TRMM/GPM-best" climatological estimate using 1) the passive microwave retrievals from TMI and GMI instruments, 2) radar-based retrievals using data from the TRMM Precipitation Radar (PR) and GPM's Dual-frequency Precipitation Radar (DPR), and the combined active-passive products. The TRMM V8 and GPM V06 data collected during March-August 2014 when TRMM and GPM overlapped are first examined in the tropics for consistency. In addition, for evaluation and validation purposes, a series of inter-comparisons will be carried out between the TCC and its components as well as other satellite-based precipitation datasets such as GPCP and ocean and land gauge data sets and the previous version of the TCC.

Inter-annual variations with the TCC components are also examined. Previous studies by the authors using TRMM and GPM data have shown that TRMM Precipitation Radar (PR) and GPM Dual-Frequency Precipitation Radar (DPR) surface rain estimates do not have corresponding large amplitudes of ENSO-related, inter-annual variations over the tropical oceans as compared to passive microwave observations. This includes differences in SST-rainfall relations. We re-investigate these relations with the GPM V06 data with an emphasis on understanding these differences with respect to the DPR vertical profiles of reflectivity and rainfall and the associated convective and stratiform proportions. The results (done with earlier versions) indicate that even though the DPR results agree better with the passive microwave results, there still is a significant difference and the radar-based inter-annual variations of surface rainfall are still suspect.