Dr. Matthew Van Den Broeke
Date of this Version
Wilson, M. B., 2019: An Analysis of Differential Reflectivity Arc Characteristics in 109 Supercell Storms. MS thesis, University of Nebraska.
Differential reflectivity (ZDR)arcs are one of the most prominent dual-polarization features of supercell storms, and are manifest as an arc-shaped area of high ZDR along a supercell’s forward flank reflectivity gradient. Since previous modelling 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 ZDR 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 ZDR arcs characteristics in large (n > 100) samples of supercells and the relationship of these characteristics to environmental parameters, low-level rotation strength, and whether a storm produces a tornado or not have yet to be conducted. This study intends to fill that knowledge gap, using an automated Python algorithm to identify, track, and analyze ZDR arc characteristics in 109 supercells. This dataset is then used to examine the impact of various environmental parameters (obtained from proximity RAP analyses) on arc size and intensity, as well as whether arc characteristics can indicate whether a storm will develop strong rotation and whether arc characteristics 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 ZDR arc and KDP foot centroids. Results of these analyses indicate that ZDR arc characteristics are much more dependent on instability and moisture parameters than on low-level shear and SRH and that changes in ZDR arc size and intensity do not reliably foreshadow low-level rotation changes on timescales of up to 15 minutes. Furthermore, ZDR 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 (peak normalized rotation (NROT)) in tornadic (nontornadic) storms, and the KDP-ZDR separation angle was found to be substantially larger in tornadic supercells than in those which did not produce tornadoes.
Advisor: Matthew S. Van Den Broeke