Assessing Tornadic Potential in Nonsupercell Storms By Quantifying the Separation of ZDR and KDP Enhancement Regions

Studies of supercells with polarimetric radars have documented several repeatable signatures, including the separation between enhancement regions of K_DP, known as the “K_DP foot”, and Z_DR, known as the “Z_DR arc.” This separation is thought to arise from hydrometeor size sorting. Past studies have shown that hydrometeor size sorting is caused by storm-relative flow in the sorting layer and, by extension, is quite often correlated to storm-relative helicity. We employ a new method that quantifies both the magnitude of the separation between Z_DR and K_DP enhancement regions as well as the orientation of separation relative to storm motion. The magnitude and orientation comprise what we call the “separation vector”. This method was applied to 30 nonsupercell tornadic cases from the eastern and southeastern United States using polarimetric WSR-88D data in order to analyze the evolution of the separation vector leading up to tornadogenesis. Results from these tornadic cases show a preferred orientation of 0-90° to the right of storm motion, with a median separation distance of about 4 km between Z_DR and K_DP enhancement regions. For a given storm, the separation distance tends to maximize at the time of the tornado report. Correlations between the separation vector and environmental parameters derived from high-resolution operational model analyses, as well as differences between tornadic and non-tornadic nonsupercell cases will also be discussed.