Tornado-like Vortexgenesis in a Simplified Numerical Model

Abstract

A novel approach to the modeling of tornado-like vortexgenesis has been developed and is used to articulate the sequence of events that leads to tornadogenesis. The ?pseudostorm? is an idealized thunderstorm representation and emulates the storm-relative flow, into an updraft, of the horizontal streamwise vorticity that is baroclinically generated in cold air outflow. By not explicitly simulating the morphology of a tornadic thunderstorm, but instead concentrating on the development of low-level rotation and tornado-scale vortices, the authors are able to transcend many of the experimental limitations encountered by cloud modelers. Intense, near-ground, cyclonic vortices, which are classified herein as ?tornado like,? evolve solely from horizontal streamwise vorticity due to buoyancy gradients and friction, if present, at the lower boundary. Regardless of the lower boundary condition, none of the vortices exceed the thermodynamic speed limit based on the vertically integrated buoyancy (or convective available potential energy). Sensitivity experiments reveal that the tornado-like pseudostorm vortices develop only when certain updraft propagation rates (and thus storm-relative flow strengths), downdraft intensifies, and fluid viscosities are well matched. The simplicity of the pseudostorm allows one to look closely at the actual genesis of a vortex. In particular, it is found that the such genesis is not ?triggered,? but is instead the outcome of a continuous (albeit rapid) process of amplification of vertical vorticity generated initially through tilting. Also, vertical vorticity intensification proceeds with a large degree of vertical uniformity, obviating the need for a mechanism like the ?dynamic pipe effect? to advance the incipient tornado-like vortex toward the ground.