Application of Spectral Polarimetry to a Hailstorm at Low Elevation Angle

Polarimetric radar has proven to be an effective instrument to improve rainfall estimation, to enhance data quality, and to classify different types of hydrometeors. Polarimetric radar employing simultaneous transmission mode can measure differential reflectivity, co-polar correlation coefficient, and differential phase in additional to spectral moments of reflectivity, mean radial velocity, and spectrum width. These measurements contain important dynamical and microphysical information of hydrometeors integrated over radar resolution volume. Spectral polarimetry, which can unveil the polarimetric variables as a function of Doppler velocity within the resolution volume by combining Doppler and polarimetric measurements simultaneously, recently has gained more interest in the community. 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 angles, where polarimetric properties are maximized. An interesting feature was observed in polarimetric spectra from a hailstorm. A hypothesis of shear-induced size sorting is proposed and the theory for such process is derived.

The hypothesis is first supported by observations of a hailstorm on 24 April 2011 from the C-band OU-PRIME (Polarimetric Radar for Innovations in Meteorology and Engineering). By analyzing the polarimetric data from OU-PRIME, a mixture of rain and hail is suggested. This is further supported by the data collected by the closely located S-band KOUN radar, which is the upgraded polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D). Further evidence of vertical shear and turbulence is provided from radar measurements and Mesonet wind data.

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. Doppler spectra from both horizontal and vertical channels can be generated from given drop size distribution(s), wind shear, turbulence broadening, elevation angle, etc. The radar cross section from each hydrometeor is obtained from a T-matrix approximation. The hypothesis is further verified using simulations. Furthermore, the impact of size sorting due to vertical shear is investigated and demonstrated using this simulation for various scenarios.