The Impact of Squall Line-generated environmental perturbations on isolated convective storms

A number of past studies of severe convective storms have observed isolated storms appearing to intensify as they approach a nearby squall line, prior to the two modes merging into a single system. In particular, recent work examining storm mergers noted increases in low-level storm rotation and a relative increase in tornado production as an isolated supercell draws close to a nearby squall line. Preliminary work using idealized model simulations has revealed that, for the same initial background environment, a supercell evolving ahead of the squall line produces a stronger, more persistent low-level vorticity maximum compared to a supercell that is completely isolated. These observed and simulated storm behaviors imply that the neighboring convective storms are influencing each other despite being separated in space. To date there has been little study of the processes responsible for such interactions. The aim of the present study is to explore one potential avenue for this interaction by examining how a squall line influences the environment of a nearby supercell.

Past modeling and observational studies have shown that squall lines can modify their pre-line environment through a number of avenues, including convectively-generated gravity waves, anvil shading, and storm-induced pressure perturbations. As a first step toward quantifying the impact that such changes may have on pre-line convection, a series of idealized model simulations are being run in which an isolated storm experiences a time-varying background environment representative of that observed ahead of an approaching squall line. These experiments make use of the relatively new idealized method of “Base State Substitution” (Letkewicz et al. 2013) to introduce a time-varying background environment in a controlled manner by modifying the base-state environment of the simulation over time. This method is designed to facilitate controlled testing to compare the relative impacts of the changing thermodynamic and wind profiles on the simulated storm. The background environment is based on a series of soundings launched ahead of an approaching squall line during VORTEX2 field operations on 15 May 2009 (Bryan and Parker 2010). These observations captured the subtle changes to the thermodynamic and wind profiles ahead of the approaching squall line in considerable detail, including the effects of anvil shading and suspected gravity wave activity. This poster will present the results these initial tests, with a focus on identifying which environmental parameters play the largest role in modifying storm evolution ahead of the squall line.