Spectral analysis of dual-polarization radar signals in a tornadic supercell storm

The upgrade of the Weather Surveillance Radar-1988 Doppler (WSR-88D) to be dual-polarization is underway. The polarimetric variables provide the insight of the scatterers' shape, size, and orientation, which can be used for retrieving drop size distribution, classifying the types of hydrometeors, and improving rainfall rate estimation, for example. The polarimetric variables such as differential reflectivity (Z_DR) and cross correlation coefficient (ρ_hv) are typically estimated in the time domain, which provide integrated information of scatterers within the radar volume. Using spectral analysis, the Z_DR and ρ_hv can be revealed as a function of radial velocities. Together with the Doppler spectra, they have the potential to simultaneously study the microphysics and dynamics within the radar volume. The KOUN radar, operated by the National Severe Storms Laboratory (NSSL), is a research prototype of the dual-polarization WSR-88D. The Level I time series data of a tornadic supercell,collected by the KOUN radar on 29 May 2004, are processed to demonstrate the application of both Doppler and polarimetric variables in spectral domain. Doppler spectra with dual-peak are observed from a region of high reflectivity at the lowest elevation angle, that is offset from the region of high Z_DR. The dual-peak spectra are transited to single-peak spectra with wide skirt at a higher elevation angle 2.5^o. Moreover, significant decrease of ρ_hv is evident in the skirt of the spectra, while high values of ρ_hv are still observed around the main peak.Many other interesting cases will be presented and hypothesis will be discussed.