The Response of a Simulated Supercell to Environmental Perturbations Induced by an Approaching Squall Line

Squall lines can modify their pre-line (i.e. inflow) environment in a number of ways, including high-and low-frequency convectively-generated gravity waves, anvil shading, and hydrostatic pressure perturbations. These modifications can lead to changes in the vertical profiles of wind shear, stability and humidity within the pre-line environment; parameters that are important for convective storm organization. This study is investigating whether or not squall line-generated perturbations to the local (pre-line) environment are sufficient to modify the structure and severe weather potential of a supercell thunderstorm in that environment. In particular, will these perturbations lead to changes to storm scale features that are linked to tornado production such as rear flank downdraft characteristics and outflow temperature? Based on past studies of the effects of squall lines on their local environment, one hypothesis for how this may work is that a gradual moistening of the lower troposphere ahead of the squall line will favor warmer cold pools in the supercell, leading to a higher likelihood for tornadogenesis and tornado sustenance.

To address this research question, this study is employing idealized numerical simulations using the Bryan Cloud Model (CM1). These simulations are run using convection-resolving horizontal and vertical resolution and utilize a recently developed method called Base State Substitution to simulate a supercell in a time-varying background environment representative of that found ahead of an advancing squall line. This environment was created by retrieving soundings ahead of a simulated squall line every five minutes, sampling how the environment changed as the squall line approached. A range of initial sounding locations relative to the squall line were tested, along with several configurations where the soundings were taken at a fixed distance ahead of the line throughout the simulation (i.e., the sounding location moved with the squall line). These runs were compared with a control simulation where a supercell was simulated using the same temporally static background environment as the squall line simulation.

This presentation will detail the effects of these changing environments on supercell structure and severe weather production. Key results include an observed increase in low level vertical vorticity and an overall decline in cold pool intensity as the environment changes (e.g. the squall line approaches); however the details of these changes vary based on the sounding locations. The relative impacts of the changes to the wind and thermodynamic profiles will also be explored to shed light on potential processes responsible for the storm-scale changes.