The Influence of Environmental Low-Level Shear and Cold Pools on Tornadogenesis: Insights from Idealized Simulations

Idealized, dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near-surface vortices in supercell-like pseudostorms. A cyclonically rotating updraft is produced by a stationary, cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity. Once a nearly steady state is achieved, a heat sink, which emulates the effects of latent cooling associated with precipitation, is activated on the northeastern flank of the updraft at low levels. Cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near-surface vertical vorticity via the baroclinic mechanism, as has been diagnosed or inferred in actual supercells that have been simulated and observed.An intense cyclonic vortex forms in the simulations in which the environmental low-level wind shear is strong and the heat sink is of intermediate strength relative to the other heat sinks tested. Intermediate heat sinks result in the development (baroclinically) of substantial near-surface circulation, yet the cold pools are not excessively strong. Moreover, the strong environmental low-level shear lowers the base of the midlevel mesocyclone, which promotes strong dynamic lifting of near-surface air that previously resided in the heat sink. The superpositioning of the dynamic lifting and circulation-rich, near-surface air having only weak negative buoyancy facilitates near-surface vorticity stretching and vortex genesis. An intense cyclonic vortex fails to form in simulations in which the heat sink is excessively strong or weak or if the low-level environmental shear is weak.