Current radar rainfall estimates rely on power-law relationships of rain rate to radar observables. These techniques use best fit relationships based on a range of gamma or exponential rain drop size distributions (DSD’s). Accurate DSD retrievals could provide more accurate rain rate estimates by capturing the natural variability of DSD’s. The DSD retrievals would also be useful in studying the microphysical characteristics and morphology of precipitation events. Recent efforts have resulted in gamma DSD retrieval algorithms using S-band dual-polarization radar and UHF vertical incident profiler observations. The purpose of this paper is to compare the DSD retrievals from the two radars using video disdrometer data.

During the PRECIP98 field program the NOAA 915 MHz profiling Doppler radar was co-located with a video disdrometer and several surface rain gauges. The NCAR S-Pol polarimetric radar was located approximately 38 km away. Several precipitation systems with both convective and stratiform characteristics, passed over the profiler/disdrometer site allowing comparison of DSD retrievals.

In this study time history plots of the measured reflectivity and retrieved parameters of the gamma DSD from the profiler and S-Pol are compared to the video disdrometer. The rain rates and storm total rainfall are also computed from the DSD retrievals and compared to the disdrometer and rain gauge data. The computed rainfall values are then compared to conventional power law radar rainfall estimators. Difference fields and statistics are computed by interpolating the separate data sets to a common time grid, allowing a quantitative analysis. The DSD and rainfall estimates from both radars compared favorably with the disdrometer and gauge data. One notable exception was that the profiler dramatically over-estimated rainfall in the presence of very large, but sparsely populated drops that are common at the edge of convection in Florida.