A method for estimating rain rate and drop size distribution from polarimetric radar measurements

Abstract

Since the idea of differential reflectivity was first proposed the polarimetric radar technique has attracted great attention. Polarization radar measurements such as radar reflectivity (Z/sub HH/), differential reflectivity (Z/sub DR/) specific differential phase shift (K/sub DP/), and linear de-polarization ratio (LDR) have been successfully measured. These polarimetric measurements provide more information about precipitation and allow better characterization of hydrometeors. Z/sub DR/ and K/sub DP/ contain drop size information. Rain rate estimation from radar measurements is based on empirical models such as the Z-R relation, R(Z,Z/sub Dr/) and R(K/sub DP/). The empirical relations cannot give accurate results of estimation for various types of rain and drop size distribution (DSD). Accurate rain rate estimation requires detailed knowledge of rain DSD. In the past, rain DSD was commonly assumed to be an exponential distribution. Some observations, however, indicate that natural rain DSD contains fewer of both very large and very small drops than exponential distribution. The Gamma DSD with three parameters is capable of describing a broader variation in raindrop size distribution than an exponential distribution. The problem is how to retrieve the three parameters from limited radar measurements. The authors propose a method for retrieving DSD parameters and use them to calculate rain rate and the characteristic size.