Validation of TRMM Precipitation Radar through Comparison of Its Ten-Year Measurements to Ground-Based Radar

The TRMM Precipitation Radar (PR), space-borne radar operating at a frequency of 13.8 GHz, is attenuated by cloud water, rain and partially melted hydrometeors. The correction for attenuation is therefore one of the most important procedures in the estimation of rain rate. Validating the TRMM PR estimates and examining some of TRMM standard algorithms require comparisons with well-calibrated non-attenuated ground-based radar measurements. One of the critical steps of validation is to accurately resample data from space-borne and ground-based instruments to a common grid. Because of differences in the scattering geometries of space-borne and ground-based observations, accurate registration of data is a challenging problem. This problem is further complicated if there are offsets in time among the measurements. As a result, one or more of the data sets may require translation, rotation, or scaling to ensure reliable comparisons among them. A technique for transforming data acquired from different platforms to a common coordinate is developed and applied to the measurements of PR and ground-based radar. In this study the ground-based measurements are taken from the S-band WSR-88D radar for the period from 1998-2007 for the overpasses of TRMM satellite over the Melbourne, Florida site during times when significant precipitation was present in the overlap region of the PR and WSR-88D. Instantaneous and statistical features of the PR products such as radar reflectivity factor and rain rate are compared to the same products obtained from the ground-based observations. Comparisons of PR to and WSR-88D provide a means to examine the radar calibrations and attenuation correction procedures adopted in the satellite radar algorithms. Because the spatial (sample) resolution of the ground-based weather surveillance radar varies as a function of radar range (while the satellite radar yields nearly uniformly spatial sampling), uniform gridding over the overlap area of satellite and ground radar observations might lead to biases or ambiguities particularly because of variability of precipitation in space. These uncertainties will be checked through the results of comparisons that are broken into several radar ranges relative to the WSR. The analysis of results indicates that the reflectivity fields derived from the PR data after attenuation correction agree well with those obtained from the WSR-88D. Comparisons of rain rates also yield good agreement with the conditional mean rain rate from the PR and WSR-88D. This agreement improves when area-averaged rain rates, rather than high resolution rain rates, are compared.