The structure and evolution of vortex lines in supercell thunderstorms

The evolution of vortex lines in a three-dimensional numerical simulation of a supercell thunderstorm is presented, from the time of storm initiation through the time at which near-ground rotation is well-developed, in order to re-examine the processes of midlevel and low-level mesocyclogenesis, and the relationship between midlevel and low-level mesocyclones. We also will review vortex line structures in observed low-level mesocyclone regions and what the structures might indicate about the vorticity generation mechanisms important to tornadogenesis in supercells.

Vortex lines emanating from the low-level mesocyclones form arches, i.e., they extend vertically from the cyclonic vorticity maximum, then turn horizontally (usually toward the south or southwest) and descend into a broad region of anticyclonic vertical vorticity. This region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low-level mesocyclone; the vorticity couplet straddles the hook echo of the supercell thunderstorm. The arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches, compared to the orientation of the ambient (barotropic) vorticity, are strongly suggestive of baroclinic vorticity generation within the hook echo and associated rear-flank downdraft region of the supercells, and subsequent lifting of the baroclinically altered/generated vortex lines by an updraft.