Application of spectral polarimetry to a hailstorm at low elevation angle

Spectral polarimetry has the potential to unveil the polarimetric variables as a function of Doppler velocity within the resolution volume by combining Doppler and polarimetric measurements simultaneously. This provides a unique opportunity to study the hydrometeors' microphysical properties in relation to the dynamics of the environment. The past study of spectral polarimetry has focused on using measurements from higher elevation angles, where both the size sorting from the hydrometeors' terminal velocities and the polarimetric characteristics are maintained. In this work, spectral polarimetry is applied to data from the lowest elevation angle (0 degrees), where polarimetric properties are maximized. Specifically, the data collected by the C-band OU-PRIME (Polarimetric Radar for Innovations in Meteorology and Engineering) during a hailstorm on 24 April 2011 are used. The slope of the spectral differential reflectivity (i.e., differential reflectivity as a function of radial velocity) exhibits interesting variations across the hail core, which suggests the presence of size sorting of hydrometeors caused by vertical shear in a turbulent environment. A close-by S-band KOUN radar, which is the upgraded polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D), is also used to provide insight of this hailstorm. Moreover, a flexible numerical simulation is developed for this study, in which different types of hydrometeors such as rain, hail, snow, etc. can be considered individually or as a combination for different shear and turbulence conditions. The radar cross section of each hydrometeor is obtained from a T-matrix approximation. As a result, Doppler and differential reflectivity spectra as well as reflectivity and differential reflectivity are produced. The hypothesis of shear-induced size sorting is further investigated by the simulation for different conditions. Additionally, the simulation suggests that the negative slope of spectral differential reflectivity can be obtained if additional large raindrops are included in the exponentially distributed drop size distribution.