Supercell-External Storms and Boundaries Acting as Catalysts for Tornadogenesis

Abstract

It has long been observed that interactions of a supercell with other storms or storm-scale boundaries sometimes seem to directly instigate tornadogenesis. First, the authors explore the frequency of such constructive interactions. WSR-88D radar data are used to categorize 136 tornadic supercells into isolated supercells and supercells that interacted with external factors within 20 min before tornadogenesis. Most cases (80%) showed some form of external influence prior to tornadogenesis. Common patterns of interactions, the typical supercell quadrant that is affected, and changes in azimuthal shear are also identified. To further study these interactions, two sets of idealized CM1 simulations are performed. The first set demonstrates that the speed of the near-ground horizontal flow relative to the updraft can control whether a vortex patch develops into a tornado. A weaker updraft-relative flow is favorable because the developing vortex stays in the updraft region longer and becomes less tilted. Building on these results, it is shown that external outflow can lead to tornado formation by a deceleration of the updraft-relative flow. The deceleration is caused by the pressure gradient force associated with the external outflow, which is already noticeable several kilometers ahead of the outflow boundary. This offers another possible mechanism by which external outflow can act as a catalyst for supercell tornadogenesis.

Supercell low-level mesocyclones: Origins of inflow and vorticity

“…In high streamwise vorticity environments, the probability of tornadogenesis is argued to be high because very little (or no) augmentation from the storm is required in order to instigate the dynamical updraft response that favors tornadogenesis. On the other hand, in low streamwise vorticity environments, the probability of tornadogenesis is argued to be lower because much more augmentation from the storm is required (or from external sources such as storm mergers, e.g., Fischer and Dahl 2022a). The necessity of within-storm supplementation adds to the possible failure points, as there must be favorable storm interactions and/or sufficient dynamic lifting to overcome the negative buoyancy associated with the baroclinic vorticity generation.…”

Idealized Simulations of a Supercell Interacting with an Urban Area

“…Because of the societal hazards associated with these storms, timely and accurate prediction is key. While the overall environmental conditions responsible for supercell formation are well known [2][3][4][5], many studies have found that gradients (or boundaries) in the mesoscale environment can have a substantial impact on the supercell structure and evolution [6][7][8][9][10][11][12][13].…”