Brett Roberts-May 1

The Role of Surface Drag in Supercell Tornadogenesis and Mesocyclogenesis: Studies based on Idealized Numerical Simulations

Idealized numerical simulations of supercell thunderstorms have been employed for decades to study tornadogenesis, providing valuable insights that have helped shape our current understanding of the process. Until the past several years, however, most of these simulations used a free-slip lower boundary condition, effectively disregarding the effects of surface drag. In this study, 50-m idealized simulations of a supercell are performed with parameterized surface drag, and the dynamics of low-level mesocyclogenesis and tornadogenesis are analyzed.

First, a pair of experiments is performed to identify mechanisms by which drag affects storm behavior. In the first experiment (full-wind drag), surface drag is applied to the full wind components; in the second experiment (environmental drag), drag is applied only to the background environmental wind, with storm-induced perturbations unaffected. 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 drag experiment, no tornado occurs. An important mechanism leading to tornadogenesis in the full-wind drag experiment is the generation of near-ground crosswise horizontal vorticity by drag on the storm scale as inflow air accelerates into the low-level mesocyclone; this vorticity is subsequently exchanged into the streamwise direction and eventually tilted into the vertical. Preceding tornadogenesis, the low-level mesocyclone in the full-wind drag experiment also intensifies and lowers rapidly toward the ground, which does not occur in the environmental drag experiment. Circulation budgets for material circuits enclosing the low-level mesocyclone reveal substantial generation of new circulation by surface drag in the full-wind drag experiment, while the mesocyclone circulation in the environmental-drag experiment is primarily barotropic in origin.

A second set of experiments is performed in which the drag coefficient (Cd) is varied over a range of values appropriate for water and land. The initial low-level mesocyclone undergoes earlier and stronger intensification as Cd increases until the final experiment (Cd = 0.05), in which low-level convergence beneath the main updraft is weakened substantially by drag. Circulation budgets for the initial low-level mesocyclone and tornado suggest the storm-scale generation of vorticity by surface drag is more important as Cd increases. Later in the simulations, after precipitation-driven outflow encloses much of the near-ground mesocyclone, tornadoes in experiments with large Cd tend to be weaker and shorter-lived than those in experiments with small Cd.