Objective Identification of Supercellular Internal Momentum Surges Using Idealized Simulations

Supercell thunderstorms are among the most intense forms of convection on the planet and are responsible for the vast majority of significant tornadoes. Tornadogenesis can be broadly described as a three-step process, involving first a mid-level mesocyclone, followed by the introduction of near-ground rotation, and finally the concentration of near-ground rotation into a narrow vortex in contact with the surface. While two of the three steps in the overall tornadogenesis process are generally well understood, a complete explanation of low-level vorticity generation remains lacking. Because it is recognized that downdrafts are important in transporting vorticity toward the surface, the associated cold pool and outflow are critical to understanding the near-ground vorticity distribution in supercells. Previous observations and simulations have revealed that thunderstorm outflow is generally not steady, and that surges of horizontal momentum occur within the cold pool. It is these surges that accompany the near-surface vorticity extrema, which are shown to have the potential to consolidate into or intensify existing surface vortices. Using idealized simulations of a supercell, internal surges are reproduced, and a new algorithm for their objective identification is introduced. Basic statistics of internal surges, including their frequency and source locations, are shown, and their role in near-ground vortex genesis and evolution is examined.