Impacts of Lapse Rates on Low-Level Rotation in Idealized Storms

Adiabatic lapse rates appear to be a common feature in the lower troposphere on tornado days. This article reviews physical reasons why lapse rates may influence surface vortex intensification and reports on numerical simulations designed to study the key processes. In the idealized numerical model, an initial mesocyclone-like vortex and nonvarying convection-like heat source are used in different environmental stability profiles. The scales of interest in these simulations typify those of a parent supercell, and the developing circulations constitute direct responses to the imposed heating. Downward parcel displacements are needed for surface vortex development in environments with no preexisting surface vorticity. In the simulations, under neutral stratification there is strong heating-induced subsidence anchored near the storm edge, whereas under stable stratification there are instead gravity waves that propagate away to the far field. In addition, under weak or neutral low-level stratification there is very little resistance to downward parcel displacements. In the simulations, these two effects combine to bring high angular momentum air from aloft downward to the surface under neutral lapse rates; this in turn leads to surface vortex genesis, even without precipitation processes. When the lower troposphere is stable, surface vortex intensification is only simulated when there is already preexisting vertical vorticity at the ground. When the initial vortex is elevated (vertical vorticity falls off to zero above the ground), surface vortex intensification is only simulated under neutral low-level stability. The results are interpreted within the controlled experimental framework, after which the possible ramifications to processes in real storms are discussed.