Doppler radar observations concerning the generation of vertical vorticity in tornadic thunderstorms are documented for two case studies and the role of downdrafts in intensifying vertical vorticity is examined. The observations, supporting recent numerical simulations, show that production of vertical vorticity begins at the very roots of updraft as horizontal vorticity in low-level inflow regions is tilted toward the vertical. Then, as the flow passes through the updraft, the tilted vorticity and preexisting vertical vorticity is amplified by convergence to create the tornado parental circulation or mesocyclone.
The low-level mesocyclone intensification that heralds tornadogenesis seems to result from interaction between spreading rainy downdraft air and inflow air from the storm's right flank. Amplification of vertical vorticity by convergence surges in the region of interaction. Rear downdrafts, which develop at approximately the time of tornadogenesis, do not transport significant vorticity; rather, their divergent character reduces vertical vorticity. Rear downdraft formation reverses the horizontal gradient of the vertical wind across thelow-level mesocyclone and increases vorticity generation by twisting within the mesocyclone; but the generation rate is at least a factor of 2 less than the amplification rate by convergence. Thus, tornadoes are most likely to be triggered by the vorticity amplification that follows from outflow-inflow interaction.
During dissipation, updrafts and rainy downdrafts weaken, and rear downdraft air fills the mesocyclone. Vertical vorticity rapidly dissipates toward ground via the convergence mechanism, and the association between the mesocyclone and updrafts ends.