Raindrop shapes as seen by a polarimetric Doppler radar

Dual-polarization radar rainfall observations are affected by changes in rain drop shapes. Despite of raindrop shapes influence on dual polarization radar measurements there is little empirical data on effective raindrop shapes in naturally occurring conditions. Buy using radar data, one might be able to study evolution of raindrop shapes with time, altitude, etc. and therefore trace back changes in the shapes to underlying physical processes. However, this approach is limited by the uncertainties in raindrop size distributions. To overcome this problem combination of spectral and dual-polarization measurements can be used.

In this study we propose to use dual-polarization spectral radar observations collected at high elevation angles (typically between 30 and 60 degrees) to retrieve effective raindrop shapes. This study is based on observation of spectral differential reflectivity,that is the ratio of hh to vv power spectra. The spectral differential reflectivity can directly be related to raindrop shapes, in the absence of spectral broadening. Since observed spectra are always affected by some type of broadening, e. g. due to turbulent motion or cross-wind, it is necessary to remove its effect.

The proposed procedure is divided in two parts. Firstly, Doppler power spectrum measurements are used to estimate DSD parameters, magnitude of spectral broadening and ambient air velocity. Secondly, hh and vv power spectra deconvolved using retrieved magnitude of spectral broadening. Then deconvolved hh and vv power spectra are used to estimate spectral differential reflectivity and raindrop shapes are estimated. This method is evaluated on stratiform rain measurements collected by the CSU-CHILL radar.