A Clarification of Vortex Breakdown and Tornadogenesis

Abstract

Recent and past observations of a central downdraft within a mesocyclone have been misinterpreted as evidence of vortex breakdown on the mesocyclone scale. In this note, the literature is reviewed and model examples are provided in order to demonstrate that the mesocyclone dynamics likely precludes mesocyclonic vortex breakdown. It is shown that an adverse vertical pressure gradient, induced by vertical vorticity that decreases in intensity with height, leads to the formation of a central downdraft, hence, two-celled vortex, in absence of vortex breakdown. Two-celled mesocyclones, then, may form when low-level vertical vorticity, generated by the vertical tilting of horizontal baroclinic vorticity, exceeds that generated at midlevels. Tornadogenesis occurs in a two-celled mesocyclonic vortex if an instability to which the vortex is susceptible is released.