Raindrop size distribution obtained from polarimetric radar measurements

Ever since the introduction of differential reflectivity (Zdr) measurement, one of the long-standing goals of polarization diversity measurements, has been the estimation of the raindrop size distribution (RSD). Seliga and Bringi (1976) showed that, for an exponential RSD, Zdr is directly related to the median volume diameter (Do). Careful inter-comparisons between radar measurements of Zdr and Do, derived from surface disdrometers, have shown that Do can be estimated to an accuracy of about 10-15 % . A general Gamma distribution model was introduced by Ulbrich (1983) to characterize the natural variation of the RSD. The nonspherical nature of raindrops also results in anisotropic propagation of electromagnetic waves, with a difference in the propagation constant at horizontal and vertical polarization states. The specific differential propagation phase (Kdp) is the forward scatter measurement (Seliga and Bringi, 1978), whereas, the radar reflectivity and Zdr are backscatter measurements. All three of these measurements weight the RSD in different ways, thereby making them very useful for estimating the RSD itself. The weighting of the RSD by Zdr and Kdp is controlled by the variation of the mean raindrop shape and size. Thus, a combination of the three polarimetric radar measurements can be utilized to estimate the RSD - or, in other words, the parameters of a parametric form the RSD, such as the Gamma RSD. This paper presents algorithms for the estimation of parameters of a Gamma RSD from polarimetric radar measurements.