Polarimetric Radar Signatures in Significant Severe Left-moving Supercells

Authors

Raychel Nelson, University of Nebraska-LincolnFollow

First Advisor

Matthew S. Van Den Broeke

Date of this Version

5-2024

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Earth and Atmospheric Sciences

Under the supervision of Professor Matthew S. Van Den Broeke

Lincoln, Nebraska, May 2024

Comments

Copyright 2024, Raychel Nelson. Used by permission

Abstract

Left-moving (LM) supercells, though rarer than right-moving (RM) supercells, may produce significant severe weather. However, there are very few existing studies on LM supercells, particularly polarimetric radar analyses. The upgrade of the nationwide Weather Surveillance Radar-1988 Doppler (WSR-88D) network to polarimetric capability and subsequent studies vastly improved understanding of RM supercells, but similar efforts have largely not been made for LM supercells. This study employs an automated polarimetric radar signature detection algorithm to examine a dataset of significant severe (hail ≥ 2.00”, wind ≥ 65 kts) LM supercells to quantify their polarimetric signatures. Comparisons are made with RM supercells to determine any differences in typical presentation. LM supercells were found to be significantly smaller than RM supercells. Hailfall area was similar in both populations, though LM supercells produced hail more often. These results provide substantiation to the claim often made in the literature that LM supercells are prolific hail producers. Their tendency to produce large amounts of hail may have contributed to significantly smaller differential reflectivity (Z_DR) columns and arcs compared to RM supercells, as hailfall can obscure these signatures. The angle between the storm motion vector and the vector between the specific differential phase (K_DP) foot and Z_DR arc was also smaller in LM supercells, and not negative as often as expected. This study also found that LM and RM supercells occur in similar environments, with only measures of low-level wind shear and storm-relative helicity being significantly different. A comparison of LM supercell signatures before and after significant severe outcomes suggests that there may be useful indications of imminent severe weather in the Z_DR arc, but more detailed study is needed. This novel study establishes the typical polarimetric presentation of significant severe LM supercells to aid in the development of a conceptual model for LM supercells in the context of how they differ from the long-established and widely used RM supercell model. These results can aid operational warning forecasters in anticipating the potential for LM supercells to produce severe outcomes and may help improve warning lead time.

Advisor: Matthew S. Van Den Broeke