Large Sensitivity of Near-surface Vertical Vorticity Development and Amplification to Heat Sink Location in Idealized Simulations of Supercell-like Storms

Markowski and Richardson (2014, hereafter MR14) showed that a simple numerical model consisting of only a heat source and heat sink can reproduce many aspects of the supercell thunderstorms observed in nature (the updraft produced by the heat source rotates cyclonically owing to the vertical shear in the environmental wind profile).  In the MR14 simulations, the development of an intense cyclonic vortex at the lowest model level is the result of circulation-rich, near-surface air being associated with weak negative buoyancy and also experiencing a large upward-directed vertical perturbation pressure-gradient force owing to its proximity to the midlevel mesocyclone.  This is most likely to happen when the heat sink is of intermediate strength, and the environmental low-level shear is strong.  However, intensification of near-surface vertical vorticity also is sensitive to the location and dimensions of the heat sink. 

One could interpret this sensitivity as implying that outflow with relatively small negative buoyancy and strong low-level dynamic upward forcing are necessary but insufficient conditions for the development of intense near-surface vertical vorticity. (In actual supercell environments, low LCLs and strong low-level shear are, of course, insufficient conditions for tornadogenesis as well.) Though the amplitude of the negative buoyancy and the strength of the low-level dynamic upward forcing are relatively easily controlled in the model by the heat sink strength and environmental low-level vertical shear, respectively, the magnitude of the near-surface circulation and the potential colocation of circulation-rich air with strong dynamic upward forcing are extremely sensitive to the details of the buoyancy field, its gradients, and trajectories, all of which are very sensitive to the location and three-dimensional structure of the heat sink.

Even though the MR14 simulations contain no moist processes, the simulations are potentially revealing about the sensitivity of tornadogenesis to subtleties in the shape and location of the cold pool, and by implication, microphysical processes.  [In an actual supercell, the location, size, and amplitude of the “heat sink” (i.e., the distribution of negative buoyancy) is affected by the deep-layer wind shear, storm-relative flow, hydrometeor fall speeds, and hydrometeor species.] We will investigate this sensitivity further in our conference presentation.