Polarimetric Spectral Analysis of Tornadic Debris Signatures

Tornadic Debris Signatures (TDS) from polarimetric radars can provide near real-time tornado damage estimation and improve detection, especially when visual identification is not possible (e.g., in heavy rain or at night). While previous studies have catalogued typical values of polarimetric variables corresponding to the TDS, it is yet unknown how the size, concentration, and shape of different debris types affect them. Furthermore, the Doppler velocity measurement of the wind is biased by debris motion, such that the characterization of tornado intensity by radar remains a challenge. The polarimetric spectral densities (PSD) are useful tools for extracting information about the different scatterers within a radar resolution volume. The PSDs represent the distribution of polarimetric variables as a function of Doppler velocity, providing a means to identify the polarimetric characteristics of scatterers moving with different Doppler velocities (e.g., debris and hydrometeors in tornadoes). Albeit the increasing number of polarimetric observations of tornadoes with the recent upgrade of the NEXRAD network, the analysis of the TDS using polarimetric spectral densities has not been fully developed, which is one of the objectives of this study. Different methods to estimate the PSD are presented and their limitations assessed. Additionally, a technique to identify debris within the PSD is proposed. The proposed technique is applied to data from a polarimetric time-series radar emulator capable of simulating debris signals, as well as real data from research and operational radars for cases with reported TDS verified by ground observations. The results from the polarimetric spectral analyses of simulated data are compared against those from the real data, and the validity of the proposed method is assessed.