Fuzzy Logic Classification of Three-Body Scattering from S-band Polarimetric Radar Measurements

The three-body scatter signature (TBSS) is a radar artifact that appears downrange from a high radar reflectivity core in a thunderstorm. The TBSS at S-band occurs as a result of multiple (three-body) scattering of large hail when the radar-transmitted electromagnetic wave is scattered by hail to the ground, bounced back to the hail and then scattered to the radar. Previous studies have shown that TBSSs have been precursors to severe weather such as large hailstones and damaging winds at the surface. Since the TBSS is a radar artifact, identification is also useful for quality control of radar data used in numerical weather prediction (NWP) and quantitative precipitation estimation (QPE). Therefore, it is advantageous to develop a method to identify the TBSS in radar data for operational use.

Polarimetric radar measurements can be utilized to identify the TBSS in radar data. In this study, we developed a fuzzy logic classification algorithm for the TBSS identification. Polarimetric radar data collected by the dual-polarization KOUN Weather Surveillance Radar-1988 Doppler (WSR-88D) were extracted to develop S-band trapezoidal membership functions for a TBSS class of radar echo. Nearly 3000 radar gates were extracted from 50 TBSSs to statistically develop the membership functions. Five variables were investigated for the discrimination of the radar echo: 1) horizontal radar reflectivity factor (Z_H); 2) differential reflectivity (Z_DR); 3) copolar cross-correlation coefficient (ρ_hv); 4) the standard deviation of horizontal radar reflectivity factor (SD[Z_H]); and 5) the standard deviation of differential phase (SD[Φ_DP]). These membership functions were added to a modified hydrometeor classification algorithm (HCA) to identify TBSSs. Hard thresholds for the TBSS class were also implemented into the algorithm. After the development of the modified HCA, testing was conducted on radar data collected by dual-pol WSR-88Ds from multiple severe weather events that were associated with TBSSs.