The Roles of Ambient and Storm-generated Vorticity in Near-ground Rotation of a Simulated Supercell

The authors focus on the relative importance of storm-generated vorticity vs. ambient vorticity in producing near-ground rotation in convective storms. Using the Bryan cloud model, convective storms that result in intense near-ground vortices are simulated. A new Lagrangian technique is employed, in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. This contribution is interpreted as barotropic vorticity. The storm-generated vorticity is treated as the residual between the known total vorticity and the barotropic vorticity.

Results for a simulation using the Del City environment confirm the analysis by Davies-Jones and Brooks (1993), who postulated that the barotropic vorticity contributes only to a small extent to the near-ground vortex in environments with predominantly streamwise vorticity. Consistent with their analysis, the near-ground vertical vorticity is due mostly to baroclinic production. The mechanism by which the baroclinic vorticity is brought to the ground relies on the reorientation of initially horizontal baroclinic vortex lines as the air descends ``feet first''. This process is demonstrated by analyzing the reorientation of the local Lagrangian coordinate frame attached to each material fluid volume.