The Role of Surface Drag in Tornadogenesis: Idealized Supercell Simulations

A pair of idealized, single-sounding supercell simulations are performed at a horizontal resolution of 50 m to explore the impacts of surface drag on tornadogenesis. In the first experiment, the standard formulation for surface drag is employed, with the drag force proportional to the total horizontal wind speed at ground-level. In the second experiment, surface drag is proportional to only the base-state horizontal wind components at ground-level from the background sounding, such that any perturbations in the wind field arising from the storm are not subject to drag. In this way, different physical processes related to surface drag can be separated and their relative effects evaluated. A warm bubble is used to initiate deep moist convection in each experiment, and a storm with supercell characteristics develops over the first 30 min of simulation time. For the first 20 min, the qualitative evolution of both experiments is very similar. In the experiment with standard surface drag, a tornado-like vortex (TLV) develops by 25 min and persists for over 10 min, with maximum vertical vorticity values in excess of 2 s^-1. In the second experiment with surface drag modified to remove perturbation effects, a TLV fails to develop during the same period. The physical mechanisms responsible for this result will be explored, including detailed trajectory analyses for parcels entering the TLV which reveal a strong role for surface drag in vorticity generation.