Dual-polarization signatures in nonsupercell tornadic storms

Open Access
Loeffler, Scott David
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 29, 2017
Committee Members:
  • Matthew Robert Kumjian, Thesis Advisor
  • Paul Markowski, Committee Member
  • Yvette Pamela Richardson, Committee Member
  • Severe storms
  • Radar
  • Polarimetric radar
  • Tornadoes
Tornadoes associated with nonsupercell storms present unique challenges for forecasters. These tornadic storms, although often not as violent or deadly as supercells, occur disproportionately during the overnight hours and the cool season, times when the public is more vulnerable. Additionally, there is significantly lower warning skill for these nonsupercell tornadoes compared to supercell tornadoes. Thus, these storms warrant further attention. This study utilizes dual-polarization WSR-88D radar data to analyze nonsupercell tornadic storms over a three-and-a-half-year period focused on the mid-Atlantic and southeastern United States. The analysis reveals three repeatable signatures: the separation of specific differential phase (KDP) and differential reflectivity (ZDR) enhancement regions owing to size sorting, the descent of high KDP values preceding intensification of the low-level rotation, and rearward movement of the KDP enhancement region prior to tornadogenesis. This study employs a new method to define the ``separation vector," comprising the distance separating the enhancement regions and the direction from the KDP enhancement region to the ZDR enhancement region, measured relative to storm motion. The median separation distance between the enhancement regions is found to be around 4 km and tends to maximize around the time of tornadogenesis. A preferred quadrant for separation direction is found to be between parallel to and 90 degrees to the right of storm motion. Furthermore, it is shown that, for a given separation distance, the storm-relative helicity increases as the separation direction increases from 0 degrees toward 90 degrees. Discussions on the implications of the other two signatures (i.e., descending high KDP values and rearward movement of KDP enhancement regions) are presented, although higher temporal resolution data are crucial for further analysis of these signatures.