Idealized simulations of supercell demise based on VORTEX2 observations

On 9 June 2009, VORTEX2 targeted a supercell thunderstorm that formed just to the cool side of a remnant outflow boundary and exhibited low-level rotation. However, as the storm propagated deeper into the cool air, the updraft was observed to shrink and completely dissipate. Analysis of the inflow soundings illustrated an increase in low-level convective inhibition over time; yet, an elevated layer containing sufficient instability and modest inhibition was also present, suggesting that other factors may have played a role in storm demise. 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. We hypothesize that these modifications impacted storm maintenance via weakening of dynamic lifting. However, the extent to which these processes played a role in storm demise in comparison to the influence of the cooling environment was not clear from the observations. Thus, an idealized approach was adopted in order to assess the relative contributions of the trends in the thermodynamic and kinematic profiles to the demise of the supercell, as well as analyze the relevant processes.

The simulations employed a homogeneous base-state defined by the first observed near-storm sounding, allowing the model to develop a mature supercell within two hours. Once the simulated storms matured, their environment-relative perturbations were extracted and inserted into a new base state environment with an altered wind profile or thermodynamic profile (or both). The experiments were designed to isolate the effects of a wind profile with weakening shear and helicity from the effects of an increasingly stable boundary layer. Our presentation will demonstrate the relative importance of kinematic versus thermodynamic modifications in instigating storm demise, and will describe the key processes that occur during dissipation.