The effect of barotropic vorticity in a supercell within unidirectional shear

In the absence of initial vertical vorticity, the development of ground-level rotation in convective storms requires downdrafts, which reorient horizontal vorticity into the vertical and simultaneously advect it to the ground. This horizontal vorticity is a combination of ambient vorticity (represented by the ‘barotropic vorticity') and vorticity generated within the storm due to baroclinity and possibly surface friction. Fundamentally, these contributions can either reinforce or cancel each other, and quantifying each contribution is thus an important part of our understanding of the development and maintenance of surface mesocyclones and tornadoes. In a previous study using the Del City base state, Dahl et al. (2014, JAS, in press) found that the ambient vorticity barely contributed to the development of near-ground vertical vorticity. This is the case because the ambient storm-relative vorticity was streamwise, thus remaining aligned with the velocity vectors, which become horizontal near the ground. This raises the question of how the role of the ambient vorticity changes if it is allowed to have a substantial crosswise component. To this end, the simulation of an isolated supercell in unidirectional shear will be presented, where parcels have a much larger storm-relative crosswise vorticity component than in the Del City case. Forward trajectories are integrated over a period of 40 minutes and the vorticity along the trajectories is separated into storm-generated and imported (barotropic) contributions. It is found that the vertical near-ground vorticity due to baroclinic generation dominates as in the Del City case. However, in contrast to the Del City simulation, the barotropic effects act to weaken the near-ground vorticity. Further, the experiment supports the idea that the sign of the barotropic near-ground vertical vorticity depends on the initial orientation of the vorticity relative to the storm-relative ambient flow at the parcels' original heights.