Comparing Size-Sorting Signature Algorithm Performance in Distinguishing Tornadic and Nontornadic Supercells

Several previously identified and well-documented polarimetric radar signatures have been related to ongoing storm-scale processes such as size sorting. Two polarimetric signatures that result from size sorting are an enhanced area of differential reflectivity (Z_DR) located along the inflow edge of the forward flank, known as the Z_DR arc, and an enhanced area of specific differential phase (K_DP) found in the forward flank, known as the K_DP foot. Recently, the orientation of the vector connecting the K_DP foot and the Z_DR arc centroids (termed the K_DP–Z_DR separation vector) relative to storm motion has been the subject of many studies and found to be useful in distinguishing between tornadic and nontornadic supercells. Two methodologies developed to quantitatively analyze the separation vector are the “dynamic thresholding” approach and the Supercell Polarimetric Observation Research Kit (SPORK). These provide powerful tools for interrogating past cases but are not properly suited for use in the operational warning decision-making process. To address this, a novel algorithm known as “w2sepvec” has been developed to identify and track the K_DP–Z_DR separation vector. This study compares the output of the three methodologies and evaluates the performance of w2sepvec. All three methodologies produce quantitatively similar trends between tornadic and nontornadic supercells; separation vectors tend to be more orthogonal relative to storm motion for tornadic storms and closer to parallel for nontornadic storms. However, separation vectors produced by w2sepvec demonstrate more variability than the other two methodologies. Despite the higher variability, w2sepvec shows promise as a tool for forecasters to use in warning operations.