A numerical investigation of convective storm evolution in cases of mergers between squall lines and isolated supercells

Recent observational work has identified two evolutions that occur when squall lines and isolated supercells merge into a single system, both of which result in the development of bow-echo structures. Additionally, it was found that as these mergers occur, the primary severe weather threat shifts from one of tornadoes and large hail (associated with isolated supercells) to one of widespread damaging wind (associated with the resultant bow echo). This finding underscores the importance of understanding how storm interactions can lead to changing storm organization, and with it an evolving severe weather threat. The present study is focused on understanding the storm-scale processes at work that govern the development of these bow-echo structures when squall lines and supercells merge. These processes are being investigated with high-resolution numerical simulations of several merger cases using the Weather Research and Forecasting (WRF) model. Initial results indicate that fluctuations in the intensity of the squall line's cold pool may play an important role in the observed behaviors. Specifically, an initial local weakening of the cold pool appears to facilitate maintenance of features associated with the supercell, including its mesocyclone, during the merger. Subsequently, a local enhancement of the cold pool south of the merger location, owing to enhanced precipitation, appears to be important to the development of the bowing structures. Additional analysis is underway to investigate the post-merger evolution of the supercell's mesocyclone, and what role, if any, that it plays in facilitating bow echo development. We plan to present a combination of full-physics WRF case study simulations and idealized cloud model simulations run using the Bryan Cloud Model (CM1) to address these topics.