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Anton Seimon

An F5 tornado that devastated Plainfield, Illinois, and environs on 28 August 1990, killing 29 people, is shown to be produced by a thunderstorm characterized by highly anomalous cloud-to-ground (CG) lightning activity. Unlike typical summertime convection in which the majority of CG flashes lower negative charge to ground, the Plainfield storm produced predominantly positive-polarity CG flashes during development. Changes in storm structure revealed by radar imagery appear tied to distinct patterns in the CG flash parameters of polarity, flash frequency, first stroke peak current, flash multiplicity, and flash location relative to the parent cumulonimbus. The primary findings are 1) the anomalous predominance (91%) of positive-polarity CG flashes during development; 2) positive CG flashes anomalously occurring mainly within the region of the storm's radar reflectivity core; 3) the onset of a major downburst coinciding with a sudden increase in CG flash rate, from 4 to 17 flashes min-1, and positive percentage, from 91% to 100%; 4) a reduction in flash rate from 17 to 3 flashes min−1 in 3 min, coinciding with the rapid development of a front-flank mesocyclone; 5) a 20-min span of reduced CG activity (1–2 flashes min−1) coinciding with tornado formation and intensification; 6) the reversal in dominant CG flash polarity from positive to negative over the entire thunderstorm domain at the time of tornado touchdown, an occurrence previously undocumented in any other tornadic thunderstorm; 7) steadily weakening mean peak current values (from +100 to +38 kA) leading to the reversal, and steadily strengthening values (from −20 to − 40 kA) following the reversal; and 8) the temporary clustering of all CG flashes within 10 km of the tornadic mesocyclone at maximum (F5) tornadic intensity. These findings suggest the possibility of a relationship between this tornadic thunderstorm's dynamics and electrical activity.

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Anton Seimon, John T. Allen, Tracie A. Seimon, Skip J. Talbot, and David K. Hoadley

Abstract

The 31 May 2013 El Reno, Oklahoma, tornado is used to demonstrate how a video imagery database crowdsourced from storm chasers can be time-corrected and georeferenced to inform severe storm research. The tornado’s exceptional magnitude (∼4.3-km diameter and ∼135 m s−1 winds) and the wealth of observational data highlight this storm as a subject for scientific investigation. The storm was documented by mobile research and fixed-base radars, lightning detection networks, and poststorm damage surveys. In addition, more than 250 individuals and groups of storm chasers navigating the tornado captured imagery, constituting a largely untapped resource for scientific investigation.

The El Reno Survey was created to crowdsource imagery from storm chasers and to compile submitted materials in a quality-controlled, open-access research database. Solicitations to storm chasers via social media and e-mail yielded 93 registrants, each contributing still and/or video imagery and metadata. Lightning flash interval is used for precise time calibration of contributed video imagery; when combined with georeferencing from open-source geographical information software, this enables detailed mapping of storm phenomena. A representative set of examples is presented to illustrate how this standardized database and a web-based visualization tool can inform research on tornadoes, lightning, and hail. The project database offers the largest archive of visual material compiled for a single storm event, accessible to the scientific community through a registration process. This approach also offers a new model for poststorm data collection, with instructional materials created to facilitate replication for research into both past and future storm events.

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