On the Development of Large Surface Vorticity in High-Resolution Supercell Simulations

Tornadogenesis has been described as three-stage process, involving i) the genesis of the mid-level mesocyclone, ii) development of rotation at the surface, and iii) amplification of this vorticity to tornadic strength by convergence. This study is concerned with the second step, i.e., the mesocyclogenesis at the surface. Although our understanding of this process has been steadily advanced over the past years, several details remain unexplained. Some of these include the source regions of parcels that become part of the mesocyclone at low levels as well as details of the processes that lead to vertical vorticity in downdrafts. Moreover, the role of low-level shear in the genesis of intense low-level circulations has not conclusively been explained. Another question is why barotropic processes, which have been shown to be a viable source of rotation at the surface in idealized models, apparently are not realized in observed supercells and in full-physics simulations.

To elucidate the processes that lead to intense vertical vorticity at the surface, high-resolution cloud-scale simulations have been carried out using the Bryan cloud model (CM1). It is found that multiple surges of horizontal momentum (which are associated with vertical vorticity at their flanks) precede the development of compact low-level circulation centers at the lowest model level. These circulations are fed by the vorticity associated with the momentum surges. Some of these circulation centers become vertically connected to the mesocyclone aloft. Although the rear-flank downdraft (RFD) may temporally coincide with some of these surges, the RFD does not cause them. The surges (and the vertical vorticity) emanate from the main downdraft northwest of the updraft. Using the ability of CM1 to calculate forward trajectories with high accuracy, we find that the parcels in the low-level circulation centers originate east of the storm, are passing through its forward flank, and gain horizontal vorticity as they traverse the eastern edge of the main downdraft. Their vorticity vectors are reoriented, leading to vertical vorticity at the foot of the downdraft. Subsequently, the parcels are swept southwards and their vertical vorticity is amplified by horizontal convergence. In contrast to previous studies, all of the parcels in the low-level circulation are processed by the main downdraft and none of them originate directly from the inflow sector. Details of how the vortex lines are reorientated in the main downdraft are investigated using simulations of idealized downdrafts in different wind-shear regimes.