Quantitative Comparison of TRMM Precipitation Algorithms in Tropical Cyclones

The rainfall characteristics of Tropical Cyclones (TCs) are quantified using the TRMM Tropical Cyclone Precipitation Feature (TCPF) database. The database consists of over 13,000 TC overpasses of 1013 TCs from December 1997 through December 2009. Using TCPF data, two TRMM rainfall retrieval algorithms are compared: The Version 6 TRMM Precipitation Radar (PR) 2A25 algorithm and the Version 6 TRMM Microwave Imager (TMI) 2A12 algorithm. Consistent with previous studies, considerable differences are found to exist between the algorithms. For all TCs over oceans, the PR 2A25 algorithm produces the most volumetric rainfall, as it contains the highest mean rain rates in all TC intensity categories. The TMI 2A12 algorithm contains a higher percentage of light (< 5 mm/hr) rain rates, but compensates with a greater total rain area. The difference between Version 6 and 7 will also be compared for all overpasses and for specific cases if the Version 7 data is available by early 2012.

A semi-manual method was used to further divide the TC overpasses into inner core (IC), inner rainband (IB) and outer rainband (OB) regions. The difference in rain rates between the algorithms was found to be most significant in the IC region, with an average difference of 4 mm/hr and a maximum difference of over 20 mm/hr in some cases. Further analysis was conducted using additional characteristics from the TCPF database including storm intensity, vertical radar reflectivity profile, 37 GHz and 85 GHz brightness temperature, convective vs. stratiform rain, and geographic location. The algorithms will be further compared by dividing the storms into radial quadrants and adjusting the coordinates in the direction of forward motion. Using this method, the difference in the algorithms can be calculated relative to the location from the TC center. The wind shear over the storm will also be derived from the NCEP reanalysis data to measure the effect of highly sheared convection on the TMI 2A12 algorithm.