Sensitivity of Deep Ascent of Cold-Pool Air to Vertical Shear and Cold-Pool Buoyancy

Abstract

The tilting and stretching of solenoidally generated vorticity that is hypothesized to be a necessary condition for supercell tornadogenesis is predicated on the presence of ascent of cold-pool air. Results are presented from experiments designed to test the sensitivity of this ascent to the temperature deficit of the cold pool and the environmental vertical shear. Experiments use idealized 2D numerical simulations involving a density current and a parameterized non-rotating deep convective updraft. Experiments conducted with only the density current demonstrate that simulated cold-pool upward motion generally exhibits a highly correlated direct relationship to both environmental vertical shear and cold-pool temperature deficit. Thus, despite increased negative buoyancy, colder cold pools are theoretically characterized by faster ascent of cold-pool air. In the presence of the parameterized, non-rotating, deep convective updraft, cold-pool upward motion is found to exhibit a strong linear relationship to both environmental shear and cold-pool temperature deficit. A cold-pool tracer is also used to measure the depth of transport of cold-pool air. Maximum tracer depth is found to increase linearly with environmental vertical shear but is found to decrease with increasing cold-pool temperature deficit. These sensitivities are attributed to the degree of phasing between deep positively buoyant ascent and the density current dynamics: for stronger shear and smaller cold-pool temperature deficits, the deep updraft and the gust front remain in close proximity, resulting in deep transport of cold-pool air.