Low-Level Polarimetric Signatures of Supercells During EF3+ Tornado Production

Authors

Anthony Noah Began, University of Nebraska-LincolnFollow

First Advisor

Matthew S. Van Den Broeke

Committee Members

Adam Houston, Matt Flournoy

Date of this Version

8-2025

Document Type

Thesis

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, August 2025

Comments

Copyright 2025, Anthony Noah Began. Used by permission

Abstract

Most EF3+ tornadoes are produced by supercells. Understanding of microscale features surrounding intense tornado production remains limited. This research aims to determine commonalities in low-level microphysical processes during intense tornado production via polarimetric radar signature assessment with a Python-based algorithm. Polarimetric variables hypothesized to reflect low-level wind profiles such as the differential reflectivity (Z_DR) arc and the Z_DR–K_DP (specific differential phase) separation vector will be investigated. Supercells which produced EF3+ tornadoes will be examined since these tornadoes pose the highest threat. From 2010 to 2024, the United States recorded approximately 19,100 tornadoes which resulted in 1,260 fatalities. Of these tornadoes, only 476 (~2.5%) were rated EF3+, but these tornadoes were responsible for 1,076 (~85%) of the fatalities.

Z_DR arc area and magnitude, as well as large Z_DR–K_DP separation vectors with orientations nearing 90^o orthogonal to storm motion may indicate strong low-level vertical wind shear, storm-relative helicity (SRH), or storm-relative inflow. This work will compare these “shear signatures” to low-level mesocyclone (LLM) intensity as diagnosed with normalized rotation (N_ROT) in the 20 minutes leading up to EF3+ tornadogenesis. It was hypothesized that N_ROT would increase with increasing Z_DR arc area and magnitude, as with lengthening Z_DR–K_DP separation vectors which approach 90^o from storm motion. Maximum Z_DR arc area and magnitude, and maximum Z_DR–K_DP separation were expected at tornadogenesis. Z_DR–K_DP separation angles were hypothesized to be closest to 90^o from storm motion at genesis.

Before genesis, Z_DR arcs were slightly larger in size, but maintained mostly the same magnitude of values. Z_DR–K_DP separation distances briefly shortened, then suddenly increased around genesis time, with the final five minutes yielding a significant result. Z_DR–K_DP separation angle offsets from 90^o orthogonal to storm motion were greater in the 10 minutes prior to genesis, against the hypothesis.

The findings of this research suggest that National Weather Service (NWS) meteorologists should not place great emphasis on low-level dual-pol signature evolution in tornado warning operations. Rather, given the more robust statistical significance in N_ROT differences preceding strong tornadogenesis, rotational metrics in the velocity field should serve as the primary means of diagnosing tornadic threat in real time nowcasting operations.

Advisor: Matthew S. Van Den Broeke