A Methodology for Estimating the Parameters of a Gamma Raindrop Size Distribution Model from Polarimetric Radar Data: Application to a Squall-Line Event from the TRMM/Brazil Campaign

Abstract

A methodology is proposed for estimating the parameters of a gamma raindrop size distribution model from radar measurements of Z h , Z dr , and K dp at S band. Previously developed algorithms by Gorgucci et al. are extended to cover low rain-rate events where both Z dr and K dp are noisy. Polarimetric data from the Sband Dual-Polarization Doppler Radar (S-Pol) during the Tropical Rainfall Measuring Mission (TRMM)/ Brazil campaign are analyzed; specifically, the gamma parameters are retrieved for samples of convective and trailing stratiform rain during the 15 February 1999 squall-line event. Histograms of N w and D o are retrieved from radar for each rain type and compared with related statistics reported in the literature. The functional behavior of N w and D o versus rain rate retrieved from radar is compared against samples of 2Dvideo and RD-69 disdrometer data obtained during the campaign. The time variation of N w , D o , and averaged over a 5 km 5 km area (within which a network of gauges and a profiler were situated) is shown to illustrate temporal changes associated with the gamma parameters as the squall line passed over the network. The gauge-derived areal rainfall over the network is compared against radar using the areal dp method, and the concept of an effective slope of a linear axis ratio versus diameter model is shown to significantly reduce the bias in radar-derived rainfall accumulation.