A Climatology of Great Plains Supercells from 2005-2007

Every year, supercell thunderstorms cause damage to property and loss of life across the Great Plains of the United States due to their tendency to satisfy at least one of the criteria for a severe thunderstorm: containing strong winds, large hail, and/or tornadoes. While past research has observed supercells to be the least common mode of thunderstorm overall, it is necessary to understand the spatiotemporal variability of the ratio of supercell thunderstorms to non-supercell thunderstorms. The spatiotemporal distribution of the ratio between supercells and non-supercells is an essential step towards solidifying an understanding of the environmental and geographical conditions that are most favorable for supercells. The distribution of this ratio will also provide a means of assessing the overall threat to life and property by these destructive storms.

This study uses radar data over the Great Plains to identify both supercells and non-supercells. The Warning Decision Support System – Integrated Information (WDSS-II) suite of algorithms is used to process the radar data. Three of the principal WDSS-II algorithms used for this work are the Mesocyclone Detection Algorithm (MDA), “w2merger,” and “w2segmotionll.” The MDA is used to detect mesocyclones in single-radar data; “w2merger” creates a temporally-synchronized composite reflectivity mosaic from multiple radars; and “w2segmotionll” is used to identify thunderstorm clusters from these composite reflectivity mosaics. The output from “w2segmotionll” is used as the input to the Thunderstorm Observation by Radar (ThOR) algorithm which also uses North American Regional Reanalysis storm motion estimates to track the “w2segmotionll” clusters and lightning data to discern between thunderstorm and non-thunderstorm tracks. Finally, the MDA detections are associated with the thunderstorm tracks identified by ThOR, thus discerning supercells from non-supercells. The locations of supercell and non-supercell tracks, as well as the spatial distribution of the ratio of supercells to non-supercells, are then analyzed using GIS.

One criterion that has been found to strongly separate supercell thunderstorms from non-supercell thunderstorms is the amount of vertical wind shear present in their environments. It is anticipated that areas with a high supercell-to-non-supercell ratio will have greater values of bulk shear versus areas with a lower ratio, but analysis must be performed to evaluate the amount of shear in different geographical areas with respect to the ratio, particularly since geography, altitude, etc. itself could be a factor in creating shear. In this work, separate areas of high and low supercell-to-non-supercell ratio are identified via GIS analysis and the bulk shear is identified for each point along thunderstorms' tracks in these areas using RUC data. Analysis of preliminary results is underway and final results will be presented at the conference.