The response of a simulated supercell thunderstorm to environmental perturbations induced by a nearby squall line

It is well established in the literature that convective storm organization, intensity, and evolution are strongly modulated by environmental profiles of buoyancy and wind shear. Many past studies have focused on how convective storms respond to wholesale changes to the environment, such as what would be experienced as a result of meso- or synoptic-scale environmental variations. Less is known, however, about how a convective storm may respond to subtle perturbations to the local environment, such as those generated by the presence of other nearby convective storms. The present study addresses this problem from the perspective of an isolated supercell thunderstorm responding to environmental perturbations resulting from the approach of a nearby squall line. Squall lines can affect their pre-line (inflow) environment via a number of mechanisms, including convectively-generated gravity waves, anvil shading, and flow changes in response to local pressure perturbations. These effects may potentially be important in cases where a squall line develops in close proximity to isolated supercells, as is often observed during severe weather outbreaks.

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.