The present study uses a series of idealized numerical simulations to compare a supercell that evolves in a temporally-static large-scale environment (i.e. a base-state environment that does not vary in time, as is typically used in idealized cloud models) to a supercell that evolves in a temporally-varying large-scale environment. The latter of these is produced by extracting soundings from the inflow environment of a simulated squall line, and using these soundings to update the background environment of the supercell simulation every 5 minutes. In effect, the supercell experiences changes to the wind and thermodynamic profile consistent with an approaching squall line, but without the squall line physically present in the simulation. This allows us to isolate the impacts of squall-line generated environmental perturbations from more direct interactions such as hydrometeor seeding or outflow mergers. All three simulations, the static supercell, variable supercell and squall line, are started with the same horizontally homogenous environment based on an observed case of a squall line-supercell interaction from 24 May 2008.
Initial results from comparing the temporally-varying supercell simulation with the static environment simulation indicate that the approach of the squall line has several impacts on the supercell. Overall, storm organization appears to decline over the course of the 3 hour simulation, evolving from a classic supercell structure to a hybrid multicell/supercell structure. This is consistent with a reduction in convective available potential energy (CAPE) and deep-layer (0-6 km AGL) vertical wind shear as the squall line approaches. Prior to evolving into this hybrid structure, however, the supercell produces a region of strong low-level vertical vorticity that is stronger, longer-lived, and occurs prior to any organized regions of low-level vorticity in the static environment simulation. This may be partially due to the variable-environment supercell being characterized by a rear flank cold pool that is generally warmer than that in the static environment. This warmer outflow is consistent with increasing relative humidity over the depth of the troposphere as the squall line approaches. The increase in low-level vertical vorticity is consistent with observations of enhanced low-level storm rotation in real-world cases of supercells encountering squall lines.