What Forces the Rapid Vertical Acceleration and Vorticity Intensification Near Ground in Tornadoes? Diagnostic Analysis based on a Numerically Simulated Real Tornado

A real-case supercell tornado simulation is analyzed to understand the rapid vertical acceleration of near-surface air parcels leading to intense vertical vorticity stretching and vortex intensification. The vertical acceleration is primarily due to effective buoyancy force and dynamic vertical perturbation pressure gradient force (VPPGF), and the latter is further decomposed into the splat and spin components by solving diagnostic pressure equations. Positive dynamic VPPGF is the dominant forcing responsible for near-ground vertical acceleration, while effective buoyancy is much smaller near ground. In the initial stage of tornado intensification, upward dynamic VPPGF is dominated by the spin term associated with the vorticity of the lowering tornado cyclone embedded within a mesocyclone, because maximum vertical vorticity and associated perturbation pressure minimum are located off the ground. As the tornado further intensifies, the maximum vertical vorticity and corresponding perturbation pressure minimum shift to the ground level, the spin-induced VPPGF becomes negative or downward. At this stage, the upward splat-induced VPPGF is found to be responsible for promoting and supporting continued upward vertical acceleration and vorticity stretching near the ground. The splat component is largest near the ground and close to the corner region of the tornado because of the strong flow deformation there. Trajectory analyses of parcels entering the tornado further substantiate that the dominant term in the upward dynamic VPPGF transitions from the spin term before the maximum vertical vorticity lowers to the ground, to the splat term after the lowering. As the air parcels rise, buoyancy becomes the primary force for continued updraft acceleration, aided by latent heating after reaching saturation.