The Role of Surface Drag in Tornadogenesis within an Idealized Supercell Simulation

To investigate the effect of surface drag on tornadogenesis, a pair of idealized supercell simulations is conducted with 50-m horizontal grid spacing. In the first experiment, surface drag is applied to the full wind (full-wind drag case); in the second experiment (environmental drag case), drag is applied only to the background environmental wind, with storm-induced perturbations unaffected. The simulations are initialized using a thermal bubble within a horizontally-homogeneous background environment that has reached a balance among the pressure gradient, Coriolis, and frictional forces. The environmental sounding is derived from a prior simulation of the 3 May 1999 Oklahoma tornado outbreak, but modified to account for near-ground frictional effects. In the full-wind drag experiment, a tornado develops around 25 min into the simulation and persists for more than 10 min; in the environmental-only drag experiment, no tornado occurs.

Three distinct mechanisms are identified by which surface drag influences tornadogenesis. The first mechanism is the creation by drag of near-ground vertical wind shear (and associated horizontal vorticity) in the background environment. The second mechanism is generation of near-ground crosswise horizontal vorticity by drag on the storm scale as air accelerates into the low-level mesocyclone; this vorticity is subsequently exchanged into the streamwise direction and eventually tilted into the vertical. The third mechanism is frictional enhancement of low-level cross-isobaric flow and horizontal convergence, which strengthens the low-level updraft and stretching of vertical vorticity. The second and third mechanisms are found to work together to produce a tornado, while baroclinic vorticity plays a negligible role.

Further analyses are also performed to investigate the intensification and lowering of the mesocyclone proceeding tornadogenesis. Trajectory and circulation analyses are performed. The results suggest that surface drag also contributes positively to the mesocyclone intensification, and the associated low-level updraft is stronger and closer to the ground (below 500 m AGL) in the full-wind drag case than the environmental drag case. This result is evidence that the aforementioned second and third mechanisms for tornadogenesis may work together through a positive feedback.