Numerical investigations of supercell mergers

A few recent studies have suggested that some tornadogenesis events happen shortly before or after a storm merger involving a supercell. However, not all mergers yield tornadic supercells. Rather, many mergers have a detrimental effect on the structure and evolution of the resulting storm, and the underlying dynamics and physics determining the outcome of a merger are poorly understood. Given the current ambiguity in expectation, a better understanding of the processes controlling the outcome of storm mergers involving supercells may be of immense help to the operational community. In order to advance our understanding of these dynamic processes, we have performed a series of idealized simulations using the cloud model CM1 in which the relative maturity of the interacting storms is varied. The simulated mergers have a spectrum of outcomes ranging from cases in which the newer storm updraft disrupts and replaces the older one to cases in which the newer storm updraft joins with the older one. The dynamical processes responsible for these differences are investigated with Lagrangian calculations of forcings for vertical momentum along parcel trajectories. Changes in low-level vorticity owing to outflow interactions also are investigated.