Numerical simulation of an HP supercell—bow echo transition

A nested grid primitive equation model (RAMS version 3b) was used to simulate the transition of a High-Precipitation supercell into a bow echo. A unique aspect of this simulation is that the model was initialized with synoptic data from June 30, 1993. Six telescoping nested grids allowed atmospheric flows ranging from the synoptic-scale down to the tornadic vortices to be represented. All convection in the simulation was initiated with resolved vertical motion and subsequent condensation/latent heating from the model microphysics; no warm bubbles or cumulus parameterizations were used.

The simulation initially produced a classic supercell which developed at the intersection between a stationary front and an outflow boundary. As the simulation progressed, additional storms developed west of the main storm along the stationary front. One of these storms interacted with the main storm to produce a single supercell storm. This storm had many characteristics of a high-precipitation (HP) supercell, and eventually evolved into a bow-echo.

An analysis of the storm's transition into a bow echo suggests that the interaction between convective cells triggered a series of events which played an important role in the transition. Following cell merger, the precipitation rate increased and the resulting evaporative cooling and increased precipitation loading increased the pressure behind the gust front. This led to an acceleration of the gust front marking the beginning of the transition.

The simulated storm structure and evolution differed significantly from that of classic supercells produced by idealized simulations. Several vertical vorticity and condensate maxima along the flanking line moved northward and merged into the mesocyclone at the northern end of the convective line during the bow echo transition. During these merger events, the low-level mesocyclone briefly intensified. The merger events were part of a larger process whereby vertical vorticity was advected into the mesocyclone from the flanking line, suggesting that the flanking line may be a vorticity source for the mesocyclone in these types of storms. Results from the simulation and these analyses will be presented.