Examining the Theorized Role of Lightning and Tornadogenesis

Supercell thunderstorms produce numerous hazardous weather phenomena, including heavy rain, large hail, damaging straight-line winds, lightning and tornadoes. One challenge that both researchers and forecasters face is determining which supercell will go on to produce a tornado and when. Various theories have been proposed that require specific dynamic, thermodynamic and microphysical conditions to be present for tornadogenesis. Alternatively, the “Vonnegut hypothesis” postulates that lightning and associated electrical forces play a key role in tornado formation. More specifically, this class of lightning-based tornado theories require that frequent flashes be spatially localized within an established vortex “core” in order to lead to tornadic activity. Although recent VHF-based lightning studies have shown nearly ubiquitous lightning, high flash rates, and lightning rate jumps prior to severe weather in most supercells, they have also indicated a “lightning hole” in and near the bounded weak echo region (BWER) and mesocyclone. However, few studies have investigated the detailed three-dimensional structure and evolution of lightning in and around tornadoes during their lifetime in order to test a key assumption of the Vonnegut hypothesis. As such, it is unclear if sufficient lightning occurs in and around the incipient vortex and tornado to satisfy the Vonnegut hypothesis. Therefore, this study is specifically designed to analyze tornadic supercells and the role lightning plays in tornadogenesis.

Data from the National Lightning Detection Network (NLDN), Lightning Mapping Array (LMA), dual-polarization radars and other meteorological observations will be used to investigate the potential direct role of lightning discharges in tornadogenesis. The LMA, NLDN and other lightning networks will be used to document the total lightning flash counts (i.e. in-cloud (IC) and cloud-to-ground (CG)), three-dimensional flash structure and other lightning properties in and around the tornado and the larger supercell. LMA source data in the vicinity of the tornado will also be inspected for the coherent presence of so-called “singletons” (or isolated VHF points) and other low source number discharge events that fail to satisfy temporal and spatial clustering or minimum source number criteria typically used to associate VHF sources into flashes. Both Doppler and dual-polarimetric radar signatures, including the tornado vortex signature (TVS) and the tornado debris signature (TDS), in conjunction with NOAA's SPC Severe Storm Reports and detailed damage surveys will be used to identify the location of the tornadic circulations. Dual-polarization and dual-Doppler radar techniques and meteorological observations will also be employed to understand the kinematic, microphysical and thermodynamic structure of these storms and their environment in relation to tornadogenesis and the electrical structure.

Initially, two tornadic supercell case studies, which occurred in northern Alabama on 27 April 2011 and 2 March 2012, will be analyzed. Each supercell will be examined in depth to determine if frequent lightning flashes or other discharge events occur in and around the vortex “core”. Also, understanding of the potential role of cloud thermodynamic, microphysical and dynamical forces present in the storm environment prior to tornadogenesis will be derived from the dual-Doppler radar and meteorological analysis in each case.