Wednesday, 7 November 2012
Symphony III and Foyer (Loews Vanderbilt Hotel)
Casey E. Letkewicz, United States Air Force Academy, USAF Academy, CO; and M. D. Parker
On 9 June 2009, VORTEX2 targeted a supercell thunderstorm that formed just to the cool side of a quasi-stationary synoptic boundary. The storm exhibited strong low-level rotation, yet as it moved deeper into the cool air, the updraft was observed to shrink and completely dissipate. Three inflow soundings launched over the lifetime of the supercell illustrated an increase in low-level convective inhibition over time, likely contributing to demise. However, an elevated layer containing sufficient instability and modest inhibition was also present, suggesting that other factors may have played a role. In addition to the observed thermodynamic changes, the near-storm environment demonstrated a notable decrease in bulk vertical wind shear and storm-relative helicity over the lifetime of the storm, which is also hypothesized to have impacted storm maintenance due to weaker dynamic lifting. While the role of an increasingly stable near-storm environment is fairly straightforward in terms of maintenance, the extent to which the evolving kinematic profile influenced storm dissipation, as well as the relevant processes at work, is less certain and not easily extracted from the observations. Thus, an idealized approach was adopted in order to isolate the trends in the thermodynamic and kinematic profiles and understand their relative contributions to storm demise, and also assess the relevant processes.
Given the evolving near-storm environment on 9 June 2009, a new modeling technique was utilized that incorporated a varying base-state while maintaining a degree of control over the experiments. The simulations employed a homogeneous base-state defined by the first observed near-storm sounding, resulting in the development of a mature supercell within two hours. Once the simulated supercell reached a quasi-steady state, its environment-relative perturbations were extracted and inserted into a new base-state environment with an altered wind profile or thermodynamic profile (or both). This procedure was performed multiple times in order to gradually nudge the base-state towards another observed near-storm environment. A series of experiments were performed to isolate the effects of the changing wind profile from those of the increasingly stable boundary layer. The results of the experiments demonstrate the relative importance of kinematic versus thermodynamic changes in instigating storm demise. A statistical approach was employed to analyze how dynamical processes within the supercell are modified in response to the evolving inflow environment.
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