Evaluating the contribution of storm-scale dynamic and microphysical processes to the intensification and longevity of interacting cells

Lee et al. (2006) studied the April 19th, 1996, tornado outbreak that affected parts of Iowa, Missouri, Illinois and Indiana. In this event, storm initiation and interaction occurred along the cold front, warm front, and dryline, and the tracks of individual cells, splitting cells and all subsequent mergers were mapped extensively. According to this study, 57% of mergers were associated with tornadogenesis, and out of the 57 tornadoes that occurred on this day, 54% formed within 15 minutes of a cell merger. These results suggest differential cell motion, cell splitting and merging, and favorable cell interaction can have a prominent role in determining the severity of a particular weather event.

Current research efforts focus on determining storm-scale dynamic and microphysical processes that are responsible for modulating the intensity and longevity of interacting cells. Using the Weather Research and Forecasting Model (WRF v3.2.1), horizontally uniform idealized simulations with 540- and 180-meter resolutions are being analyzed. The sounding used for these simulations was extracted from a MM5 simulation of the April 19th, 1996, Illinois tornado outbreak. In the control simulation, only one thermal is initiated in order to study the behavior and intensity of an isolated storm. In the other simulations, a second warm thermal is initialized to the southwest of the first, and the initial position of the second cell is varied between each simulation.

By varying the orientation of the initial cell pair between each simulation, an intricate spatial pattern of storm intensification and weakening emerges. The presence of a second cell results in stronger winds and rotation at the surface over a larger area relative to the single-storm control simulation in 42 of the 51 two-storm simulations. One storm pair configuration characterized by a strong, persistent mesocyclone differs in initial cell position by only 8 km from a pair exhibiting significantly weaker rotation. Additional preliminary results show pulse-like fluctuations in cold pool intensity and a slight warming trend in surface theta-e measurements within three minutes of maximum measured surface vertical vorticity. Future work includes trajectory analyses and simulations at tornado-scale resolution.