Impacts of Increasing Low-Level Shear on Supercells During the Evening Transition

Supercell tornadogenesis has been described as a three-step process. First, an updraft acquires net rotation through tilting of streamwise vorticity. Second, a downdraft produces vertical vorticity at the surface. Finally, a tornadic-strength circulation spins-up at the ground as vertically oriented vortex lines in the storm's outflow are converged and stretched . This final steps is perhaps the least understood.

Several environmental forecast parameters have been shown to possess substantial skill in forecasting tornadogenesis. One such parameter that discriminates well between strongly tornadic supercells and nontornadic supercells is the environmental bulk shear in the lowest kilometer. However, since tornadoes are formed solely due to downdraft processes, then why are the kinematic properties of the inflow a statistically significant predictor of strong tornadoes? A physical relationship or interaction that helps facilitate the transition between tornadogenesis steps two and three must exist , likely due to changes in the storm's profile of vertical vorticity and/or changes in the low-level dynamic lifting of air.

Furthermore, the diurnal maximum in tornado frequency is in the late afternoon and early evening hours. Storm chasers often refer to this phenomenon anecdotally as “six-o'clock magic”, because supercells seem to have the propensity to produce tornadoes as the sun sets. There is also an increase in shear in the lowest one kilometer during this time as the surface layer decouples from the rest of the planetary boundary layer. It is unclear how storms directly respond to this change in shear. Perhaps this is related to why tornadoes most frequently occur at this time of day.

Using the Bryan Cloud Model 1, we have been investigating this problem with full-physics supercell simulations initialized with observational soundings obtained from VORTEX2. For instance, supercells simulated in base-state environments characterized by a transition from weak to strong low-level shear, produces a noticeably more intense surface vortex compared to the control supercell, which remains in weak low-level shear. Vertical velocities in the lowest few kilometers increase on the order of 10 m/s, and the 0-1km mean dynamic vertical perturbation pressure gradient is higher as well. Parcel trajectories and tracers are being used to identify the “participation” of outflow parcels that acquire significant near-surface vertical vorticity in the parent supercell's main updraft. Our poster and extended abstract will expand on these ideas and will present further dynamical analyses to explain them.