Quantifying ZDR-KDP Separation in Severe Convective Storms to Assess Tornadic Potential

Ongoing hydrometeor size sorting in severe convective storms is revealed in polarimetric radar analyses by a separation of specific differential phase (K_DP) and differential reflectivity (Z_DR) enhancement regions. The magnitude of this separation distance and its orientation with respect to storm motion are thought to be linked to storm-relative helicity, and thus may provide information about a storm’s tornadic potential. Whereas previous studies have examined this separation qualitatively, herein we present a technique to quantify this Z_DR- K_DP separation. Within a specified area of interest (“analysis box”), the centroid of all precipitation data points (thresholded with co-polar correlation coefficient ρ_hv) exceeding a given value is found for both K_DP and Z_DR. From these K_DP and Z_DR centroids, the separation distance and orientation (“separation vector”) is computed. The sensitivity of the calculated separation vector and its trend leading up to tornadogenesis to varying the K_DP and Z_DR thresholds is tested. Further, we introduce an “adaptive threshold” that automatically chooses the highest threshold value for both K_DP and Z_DR for which there are a sufficient number of data points in that specific storm, allowing it to be applied generally to a variety of storms modes and environments.