Inference of mean raindrop shapes from dual-polarization Doppler spectra observations

Dual-polarization weather radar measurements depend on mean raindrop shapes. Several recent studies have proposed a technique to estimate mean raindrop shapes from dual-polarization radar measurements. However, the proposed techniques have two limitations. Firstly, a linear shape-size relationship is assumed and dual-polarization measurements are used to estimate a slope of this relationship. Secondly, this procedure is unstable in light rain. In this study we use high elevation angle measurements of spectral differential reflectivity (sZDR), which is measurements of differential reflectivity per Doppler velocity bin, to investigate general relationships between size of raindrops and their shapes. A direct implementation of this method, however, is limited by two factors. Firstly turbulence and cross-wind would introduce broadening to observed Doppler spectra. Secondly the knowledge of the exact fall velocity of raindrops is not generally available and therefore it is difficult to relate observed velocities to the diameters of raindrops. To remove the effect of spectral broadening a specialized deconvolution procedure is used .The fall velocity of the particles is subsequently retrieved by fitting a model drop-size distribution to the deconvolved power spectra. This procedure allows for direct radar observation of raindrop shapes. As a result a study of height and temporal behavior of raindrop shapes can be carried out. The proposed procedure is illustrated by inferences of raindrop shapes from CSU-CHILL observations of several rain events collected during summer 2004 and spring 2005.