Bulk Analysis of Radar-Derived Mesocyclone Intensity and Transience Preceding Supercell Tornadogenesis and Tornadogenesis Failure

Tornadogenesis is a complex process wherein a combination of environmental conditions and storm processes occur in-tandem for a tornado to form. Many studies have investigated these conditions and processes to assess their importance to tornadogenesis or tornadogenesis failure, with occasional progress made to elucidate a clear reason why most supercells do not produce tornadoes. However, there is still little known about several supercell features and processes thought to be important to the tornadogenesis process. For example, one specific characteristic that has not been the focus of much previous study is the purported transience of low-level mesocyclones, which often undergo rapid and chaotic lifecycles. Upward-directed perturbation pressure gradient forces induced by low-level mesocyclone rotation are thought to be important in stretching and tilting of near-ground rotation prior to tornadogenesis, and that contribution to tornado formation is not instantaneous. Therefore, it stands to reason that the duration over which the low-level mesocyclone, and associated enhancement of dynamic lifting, are present may have some impact on the likelihood of tornadogenesis. Is there a threshold time duration of low-level mesocyclones that makes tornadogenesis much more likely to occur? Do stronger low-level mesocyclones require shorter durations than weaker ones for tornado production? Is the persistence of low-level mesocyclones different preceding tornadic and non-tornadic supercells? What role, if any, does the separation distance and direction between the most intense portions of the low and mid-level mesocyclones play preceding tornadogenesis? These questions remain unanswered from an observational perspective.

In this study, we investigate relationships between mesocyclone intensity and transience, and tornadogenesis or tornadogenesis failure using radar-derived azimuthal shear (a proxy for mesocyclone intensity). We investigate near-ground (0-500 m), low-level (0-1 km, 0-2 km, and 0-3 km), and midlevel (3-6 km) mesocyclones using the National Weather Service WSR-88D radar network to obtain climatologies of isolated tornadic (2013–2017) and non-tornadic (2015) supercells across the contiguous United States during the hour preceding tornadogenesis or failure. Further, we assess any links between near-storm environment characteristics from model analysis fields present during this one-hour period and low-level mesocyclone transience.