A clarification of vortex breakdown and tornadogenesis

Recent and past observations of a central downdraft within a mesocyclone ha ve been misinterpreted as evidence of vortex breakdown on the mesocyclone s cale. In this note, the literature is reviewed and model examples are provi ded in order to demonstrate that the mesocyclone dynamics likely precludes mesocyclonic vortex breakdown. It is shown that an adverse vertical pressur e gradient, induced by vertical vorticity that decreases in intensity with height, leads to the formation of a central downdraft, hence, two-celled vo rtex, in absence of vortex breakdown. Two-celled mesocyclones, then, may fo rm when low-level vertical 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.