Near-surface vortexgenesis in idealized three-dimensional numerical simulations involving a heat source and a heat sink in a vertically sheared environment

We present the results of idealized simulations designed to emulate what are believed to be some of the salient aspects of the processes by which strong vortices are produced at the surface in supercell thunderstorms. The simulations are dry but include the generation of near-surface rotation beneath supercell-like (i.e., helical) updrafts in a way consistent with our present understanding of the importance of a downdraft in environments in which vertical vorticity is initially absent at the surface. The horizontal and vertical grids are stretched; the grid spacing is 50 m in the horizontal and vertical in the region of the low-level updraft.

The simulations build on prior numerical experiments conducted by Walko (1993). A stationary, cylindrical heat source is imposed within a horizontally homogeneous environmental wind field containing vertical shear. The vertical wind profile is described by a semicircular hodograph; i.e., the horizontal vorticity is purely streamwise. The interaction of the heat source and wind field results in a cyclonically spinning updraft with maximum rotation at midlevels. The rotation vanishes at the surface because there is no environmental vertical vorticity to stretch at the surface, and because the tilting of horizontal vorticity by an updraft alone cannot produce vertical vorticity at the surface. Once a steady state is achieved, a heat sink is imposed on the western flank of the updraft at low levels. The heat sink produces baroclinically generated vortex rings that sink and spread beneath the updraft. If the heat sink is too strong (e.g., maximum surface temperature deficits >5 K are produced), the parcels originating in the heat sink, as well as the vortex lines generated baroclinically within the temperature gradient along the periphery of the heat sink, simply undercut the updraft and fail to be lifted; only weak vertical vorticity arises at the surface in this case. If the heat sink is too weak (e.g., maximum surface temperature deficits of only 1-2 K are produced), the baroclinic vorticity generation is small and/or parcels that have acquired baroclinic vorticity are unable to spread beneath the updraft from the rear and be lifted by it; only weak vertical vorticity arises at the surface in this case as well. For intermediate heat sink strengths (these yield maximum surface temperature deficits of 2--5 K), significant baroclinic vorticity is generated, yet parcels originating within the heat sink's outflow are able to be forcibly lifted by the updraft; strong surface vortices develop in these simulations, and the wind field kinematically resembles that of a supercell near the time of strong low-level rotation (e.g., an occluded "gust front" structure develops, including an occlusion downdraft, and vortex lines form arches).

The generation of a strong vortex in the simulations can be controlled not only by the strength of the heat sink, but also by the strength of the low-level wind shear. Increasing the shear increases the strength of the low-level updraft because the upward-directed, dynamic vertical pressure gradient force is increased. For a given heat sink strength, as the low-level shear increases, it becomes increasingly likely that the updraft will be able to lift air parcels and vortex lines that originate in the heat sink's outflow, thereby increasing the likelihood of the formation of a strong vortex at the surface.

We believe these simulations provide a plausible explanation for why tornadic supercells are favored in environments containing large low-level wind shear, in addition to environments that limit cold pool production (e.g., environments that have large boundary layer relative humidity).