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

Abstract 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, and 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. Significance Statement The important role of low-level intense vertical acceleration in tornadogenesis has been highlighted in recent studies because of the resulting near-ground vertical vorticity stretching. However, quantitative analyses on forces causing such vertical acceleration are generally lacking. The flow patterns responsible for the dynamic pressure gradient force (PGF) are in particular not well understood. This study finds that dynamic forcing is the primary driver of low-level vertical acceleration. In the early stage of tornado vortex intensification, the maximum vertical vorticity, being associated with a lowering mesocyclone/tornado cyclone, is located off the ground, and the upward dynamic PGF near the low-level tornado comes mainly from the spin term associated with mesocyclone/tornado cyclone rotation. As the tornado further intensifies, the maximum vertical vorticity shifts to the surface so that the associated spin term reverses sign, and the splat term associated with deformation flows becomes the dominant contributor to upward dynamic PGF. The important role of dynamic PGF associated with the splat term has not been explicitly recognized before in the tornado literature.