Investigating the Transition from Elevated Multicellular Convection to Surface-Based Supercells as Observed in the Indiana and Ohio Tornado Outbreak of 24 August 2016 using a WRF Model Simulation and Perturbation Pressure Decomposition

On 24 August 2016, a tornado outbreak impacted Indiana and Ohio with 24 confirmed tornadoes, which caught many meteorologists by surprise. Initially elevated multicellular convection transitioned to surface-based supercells between 1700 and 1830 UTC, after which time tornadoes began to occur. This transition is of particular interest owing to its relatively rare occurrence and because most convection-allowing models failed to predict it, instead depicting a line of storms moving across the affected area.

A WRF model simulation accurately captures the environment during this event, depicting a cluster of elevated thunderstorms developing in the vicinity of an outflow boundary during the early morning hours of 24 August. Trajectory analyses indicate that parcels entering the multicellular updrafts before 1530 UTC originated above 1 km AGL, meaning that the convection was elevated. As diurnal heating progressed, more near-surface parcels entered the multicellular updrafts, each of which moved atop convective outflow and weakened. Since nearly all of the vertical wind shear was confined to the lowest 1 km, mesocyclones did not form via tilting of horizontal vorticity until the storms ingested near-surface air. The storm on the southern end of this cluster became a supercell in the simulation around 1700 UTC, when it became surface based. Some inflow trajectories had large residence times within a theta-e gradient, which originated from anvil shading and leading small convective cells, allowing for the baroclinic generation of streamwise horizontal vorticity. When these helicity-rich parcels entered the supercell updraft, strong near-surface rotation developed. The simulated supercell exhibited at least four mesocyclone cycles over a 5 hour period. Radar data indicate that the southernmost supercell on 24 August also cycled four times.

A novel analysis of the perturbation pressure field from WRF model output indicates that the development of relatively large vertical non-linear perturbation pressure gradients coincided with when near-surface air began to enter the updrafts, resulting in upward accelerations in the lowest 2 km, below the level of maximum rotation, and downward accelerations above this level. In strengthening updrafts, upward-directed perturbation pressure accelerations due to buoyancy may have offset the downward-directed non-linear perturbation pressure accelerations above the level of maximum rotation, allowing the updrafts to deepen and intensify. This simulation suggests that the transition from disorganized elevated convection to surface-based supercells began as near-surface air gradually entered the multicellular updrafts. An increase in updraft rotation in new cells became evident once near-surface air started to be ingested. As the rotation increased, so did the magnitude of the upward-directed non-linear perturbation pressure gradients, which likely aided in the development of a single dominant supercellular updraft.