Impacts of Low-Level Environmental Shear Upon Surface Vorticity in Supercells

Supercell tornadogenesis has been described as a three-step process (Davies-Jones and Brooks 1993). First, an updraft acquires net rotation through tilting of streamwise vorticity. Second, a downdraft produces vertical vorticity at the surface. Finally, a tornadic circulation spins-up at the ground as vertically oriented vortex lines in the storm's outflow are converged and stretched (Walko 1993; Wicker and Wilhelmson 1995). These final two steps are still not well 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 (Brooks et al. 2003; see their Fig. 3). In recent studies (Dahl et al. 2012) it appears that all parcels reaching the surface vortex have a history of descent. If 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. Using the Bryan Cloud Model 1 (CM1; Bryan and Fritsch 2002), we are currently performing sensitivity experiments varying the magnitude of the bulk shear in the lowest 1000m within full-physics supercell simulations. Parcel trajectories and tracers are being used to identify the “participation” of outflow parcels in low-level vortices and in the parent supercell's main updraft. Our poster and extended abstract will demonstrate the basic sensitivities to low-level shear and present dynamical analyses to explain them.