Comparisons of Measurements from the TRMM PR and the WSR-88D Ground-based Radar

The TRMM Precipitation Radar (PR), a spaceborne 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. The relative calibration accuracy of the radars is evaluated by checking the reflectivities at the storm top, where the PR attenuation is negligible. Near the surface, where the PR attenuation effects can be significant, the data are used to assess the performance of the PR attenuation correction algorithm. In this study the ground-based measurements are taken from the S-band WSR-88D radar during 1998 and 1999 for the more than 40 overpasses of the TRMM satellite over the Melbourne, Florida site during times when significant precipitation was present in the overlap region of the PR and WSR-88D. Resampling the PR and WSR-88D data sets to a common high-resolution grid provides a mean by which the radar reflectivity factor can be compared at different heights and for different rain types over ocean and land. While the examples of the instantaneous comparisons of the radar reflectivities are illustrated in CAPPI and RHI displays, the statistical results of comparisons are analyzed. 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. Furthermore the agreement improves in the comparison of area-averaged rain rates.