The effect of surface drag strength on mesocyclone intensification and tornadogenesis in idealized supercell simulations

Abstract

A suite of six idealized supercell simulations is performed in which the surface drag coefficient Cd is varied over a range of values from 0 to 0.05 to represent a variety of water and land surfaces. The experiments employ a new technique for enforcing a three-force balance among the pressure gradient, Coriolis, and frictional forces so that the environmental wind profile can remain unchanged throughout the simulation. The initial low-level mesocyclone lowers toward the ground, intensifies, and produces a tornado in all experiments with Cd $ 0.002, with the intensification occurring earlier for larger Cd. In the experiment with Cd 5 0, the low-level mesocyclone remains comparatively weak throughout the simulation and does not produce a tornado. Vertical cross sections through the simulated tornadoes reveal an axial downdraft that reaches the ground only in experiments with smaller Cd, as well as stronger corner flow in experiments with larger Cd. Material circuits are initialized enclosing the low-level mesocyclone in each experiment and traced backward in time. Circulation budgets for these circuits implicate surface drag acting in the inflow sector of the supercell as having generated important positive circulation, and its relative contribution increases with Cd. However, the circulation generation is similar in magnitude for the experiments with Cd 5 0.02 and 0.05, and the tornado in the latter experiment is weaker. This suggests the possible existence of an optimal range of Cd values for promoting intense tornadoes within our experimental configuration.