An Analysis of ZDR Arc Characteristics in a Large Sample of Supercell Storms

Differential reflectivity (Z_DR) arcs are one of the most prominent dual-polarization features of supercell storms, and are manifest as an arc-shaped area of high Z_DR along a supercell’s forward flank reflectivity gradient. Since previous modeling studies have hypothesized that the magnitude of the drop-size sorting by the storm-relative wind which creates the arc signature is related to the strength of the low-level shear and SRH in a storm’s environment, the presence of a strong Z_DR arc is often said to indicate that a storm may have the potential to develop strong low-level rotation and potentially become tornadic. However, observational studies of Z_DR arc characteristics in large (n > 100) samples of supercells and the relationship of these characteristics to environmental parameters and whether a storm produces a tornado have not yet been conducted. This study intends to fill that knowledge gap, using an automated Python algorithm to identify, track, and analyze Z_DR arc characteristics in a large sample of supercells. This dataset is then used to examine the impact of various environmental parameters (obtained from proximity RAP soundings) on arc size and intensity, as well as whether arc characteristics and their changes over time differ between tornadic and nontornadic storms. Finally, a similar analysis is performed using another proxy for drop-size sorting in supercells—the separation angle between the Z_DR arc and K_DP foot centroids. Results of these analyses indicate that Z_DR arc characteristics are much more dependent on instability and moisture parameters than on low-level shear and SRH. Furthermore, Z_DR arc size and intensity are not meaningfully different between tornadic and nontornadic supercells. However, a consistent increase in arc areal extent was found shortly before tornadogenesis (and shortly before peak normalized rotation (NROT) in nontornadic storms), and the K_DP-Z_DR separation angle was found to be substantially larger in tornadic supercells than in those which did not produce a tornado.