The sensitivity of mesocyclonic near-surface vertical vorticity to the low-level shear: A theory based on an analytic density current solution

A physical explanation for the associative relationship between low-level vertical shear and the occurrence of significant tornadoes is proposed and tested using idealized experiments that rely on an analytic solution for density currents in sheared flow. The theory endorses the philosophy that the primary role of low-level shear in the generation of near-surface vertical vorticity is to generate/amplify lateral gradients in the vertical velocity, not to serve as a source of horizontal vorticity. As long as gust front-parallel (quasi-horizontal) vortex lines are in place in the near-surface layer, the theory predicts that the interaction of a gust front with the vertical shear will generate positive vertical vorticity near the gust front inflection (the transition point from forward-flank gust front to rear-flank gust front) through tilting of horizontal vorticity.