Simulated Effects of an Isolated Supercell on the Evolution of a Nearby Squall Line

Effectively forecasting convective storm organization is critical for the safety of lives and property. Such convective storms include supercells, which have the potential to produce large hail and destructive tornadoes, as well as squall lines, which can yield damaging winds and flash flooding. Forecasting storm organization has generally focused on the background environment, but other factors such as forcing mechanisms and storm mergers are important as well. Since it is not well known how neighboring storms may influence each other, this study plans to shed some light on understanding how a supercell thunderstorm may alter the organization of a nearby squall line. Previous research has shown that the majority of the severe weather produced by squall lines occurs when three-dimensional structures, such as bowing segments or mesovortices, develop within the squall line. One way this can happen is when the squall line interacts with a mesoscale boundary, such as a frontal or outflow boundary. This can lead to variations in convective intensity along the squall line, and also represents a source of enhanced horizontal vorticity for mesovortex formation. What is less clear is whether a supercell's outflow, which is generally weaker and shallower, will be sufficient to alter the squall line.

Past studies have shown that in cases of squall line-supercell mergers, there was a weakening of the squall line near the onset of the merger due to the outflow from the supercell cutting off inflow to the squall line. In these cases the two storms were in close proximity, and it is unclear how the effect may change for a supercell further ahead of the line. In cases of supercell-supercell interactions, the outflow from one supercell has been shown to enhance the tornado production in a trailing supercell. This study hypothesizes that the cold pool produced in the wake of an isolated supercell will be strong enough to promote localized heterogeneity along the nearby squall line, and may favor the development of mesovortices or small bowing segments. The effect of the supercell's outflow is, however, expected to vary based on its intensity due to the range from the squall line or type of supercell (high precipitation, low precipitation, and classic).

To examine these effects, squall line-supercell interactions have been simulated using the Bryan Cloud Model (CM1) configured with a double moment microphysics scheme, convection-resolving grid spacing, and an environment favorable for supercell structures and bow echoes. The Base-State Substitution method was used to effectively simulate both convective modes in the same horizontally homogeneous environment. A pair of benchmark simulations will be discussed that compare the squall line in isolation to one in close proximity to a supercell. Special attention will be paid to the effects that the presence of the supercell has on the squall line's cold pool and low-level lifting, along with the development of any mesovortices. These results will be compared with a series of sensitivity tests run to explore the effects of the proximity of the two storm types, as this is likely to impact the depth and strength of the supercell's outflow.