Idealized Simulations of Mergers Between Squall Lines and Isolated Supercell Thunderstorms

One of the primary elements of a severe weather forecast is determining how storms will organize upon initiation and how that organization will evolve over time. Past research has shown that different organizational modes often produce different types of severe weather, with quasi-linear convective systems producing more straight-line wind damage and isolated supercell storms producing a large fraction of large hail and violent tornadoes. Recent observations-based research has examined the role that mergers between quasi-linear convective systems and isolated supercells play in altering convective mode and severe weather production. This work was able to identify common storm evolutions and environmental characteristics associated with these events; however a detailed analysis of the storm-scale dynamics was not possible using the available observations.

To address this gap in the knowledge, the present study uses idealized cloud model simulations to investigate the storm-scale dynamical processes at work in cases where squall lines and isolated supercells merge into a single system. The simulations demonstrate that a weakening of the squall line's cold pool prior to the merger appears to be a key factor in the maintenance of supercell structures within the merged system. Furthermore, analysis of the vertical vorticity field during the merger reveals a rapid increase in low-level (i.e., below 1 km AGL) vertical vorticity coincident with the merger similar to what was observed in a number of real-world cases. Detailed analysis of model output will be used to examine the processes responsible for this increase in low-level vorticity, along with the implications this has for severe weather production during these types of merger events.