The impact of surface drag on the low-level structure of tornadic storms in idealized simulations

Abstract:

The impact of the surface drag on tornado potential and dynamics needs to be clarified since until recently most of the idealized simulations of supercells historically neglected it. Recent simulation studies that have included surface drag have suggested that it may play important roles in tornadogenesis including the generation of near-surface horizontal vorticity. Yet, many questions remain, including the effects of surface drag on larger-scale near-surface storm structure, such as the kinematic and thermodynamic evolution and relative locations of surface convergence boundaries.

Recent studies have illustrated that the maximum of tornado frequency may have shifted from the traditional Great Plains “Tornado Alley”, to the Southeast United States (SE-US). The variations of surface roughness associated with differing land cover may impact near-surface boundary structure and fundamentally affect the tornado potential. Additionally, typical tornado environments present in the Great Plains are characterized by both high-shear and High CAPE (HSHC) environments, whereas in the SE-US there are a higher proportion of high-shear, low CAPE (HSLC) environments. Differences in storm intensity and structure across these environments may yield different sensitivities to the underlying surface roughness.

In this study, the response of the low-level structure of tornadic storms to the contribution of surface drag will be explored using idealized simulations with the Bryan Cloud Model (CM1). The simulations are initialized with two environmental soundings representative of HSHC and HSLC conditions, respectively, and simulations with surface drag implemented with imposed semi-slip lower boundary conditions with different magnitudes of the drag coefficient are performed and compare with each other. Preliminary results indicate that the inclusion of surface drag substantially alters the structure and intensity of low-level convergence boundaries (LLCB). The forward-flank LLCBs are less well-defined, and dual rear-flank LLCBs are observed due to the effect of surface drag. The detailed evolution and structure of LLCBs are discussed based on trajectory analysis by comparing with each experiment and previous study. We also analyze the attendant change in tornado behavior influenced by these differences.