Toward understanding the role of shear instability in tornadogenesis: The origins of vortex sheets along supercell gust fronts that precede low-level mesocyclogenesis

We investigate the origins of the (cyclonic) vertical vorticity within vortex sheets that develop within a numerically simulated supercell in a non-rotating, horizontally homogeneous environment. Vortex sheets are commonly observed along the gust fronts of supercell storms, particularly in the early stages of storm development. The "collapse" of a vortex sheet into a compact vortex often is seen to accompany the intensification of rotation that occasionally leads to tornadogenesis.

The vortex sheets predominantly acquire their vertical vorticity from the tilting of horizontal vorticity that has been modified by horizontal buoyancy gradients associated with the supercell's cool low-level outflow. If the tilting is within an ascending air stream (i.e., the horizontal gradient of vertical velocity responsible for the tilting resides entirely within an updraft), the vertical vorticity of the vortex sheet nearly vanishes at the lowest model level for horizontal winds (5 m). However, if the tilting occurs within a descending air stream (i.e., the horizontal gradient of vertical velocity responsible for tilting includes a downdraft adjacent to the updraft within which the majority of the cyclonic vorticity resides), the vortex sheet extends to the lowest model level. The findings are consistent with the large body of prior work that has found that downdrafts are necessary for the development of significant vertical vorticity at the surface.