The Role of Surface Drag in Supercell Tornadogenesis and Mesocyclogenesis: Insights from Idealized 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. The initial sounding is produced such that it represents a balance between the horizontal PGF, Coriolis, and frictional forces, so the background environment does not change as the storm evolves. Five experiments are performed in which the surface drag coefficient (Cd) is varied over a range of values (0 ≤ Cd ≤ 0.05). For nonzero Cd, the initial low-level mesocyclone produced by the supercell intensifies and lowers toward the ground, ultimately producing a tornado; this occurs more rapidly for larger Cd. Circulation budgets for material circuits initialized enclosing the low-level mesocyclone reveal that surface drag generates positive circulation in the near-ground inflow east of the mesocyclone, and this circulation constitutes a larger proportion of the total circulation for experiments with larger Cd. Later in the simulations, after cool outflow wraps into the near-ground mesocyclone, experiments with smaller Cd tend to produce stronger and longer-lived tornadoes than experiments with large Cd. Qualitative differences between experiments in mesocyclone behavior are also discussed, along with implications of these findings for operations and future modeling studies.