Intensification of the Low-Level Updraft in Supercells Preceding Tornadogenesis

As described by R. Davies-Jones, tornadogenesis is a three step process. A mesocyclone forms aloft, followed by the development of near ground rotation and the concentration of this near ground rotation within outflow air into a tornado. A question that remains unanswered, and embedded, in this process is why the updraft intensifies in the low levels immediately prior to tornadogenesis. Using both single and double moment microphysics schemes, CM1 version 18 successfully simulates a storm that displays low-level updraft intensification just prior to the appearance of a tornado-like vortex (TLV) indicated by the modeled storms’ near-ground vertical vorticity. One possible explanation is the development of a pressure deficit within the mesocyclone which is accompanied by the formation of a vertical pressure gradient. This gradient accelerates air upwards enhancing the mesocyclone and lowering the updraft maximum. Vorticity-bearing cold-pool air may be lifted, and tornadogenesis may occur if upward accelerations are sufficiently strong.

The goal is to identify flow features that are paired with an intensifying vertical perturbation pressure gradient force (VPPGF) and the strengthening of the low-level mesocyclone. Analysis of the maximum vertical velocity, maximum vertical vorticity, and the pressure field will be presented.  As would be expected, preliminary results indicate the VPPGF is directed upward beneath the mesocyclone and downward in the simulation’s strongest TLV.  To survey the influence of the components of the pressure field, the Poisson pressure equation has been decomposed and the dynamic and buoyant forcings with respect to low-level updraft intensification will be explored.