Favorability of Tornado-Like Vortex Formation and Duration as a Function of Surface Drag in Idealized Simulations

Abstract Tornado-like vortices (TLVs) are known to be very sensitive to processes in the atmospheric surface layer, where surface drag can play multiple roles. This study is designed to examine the behaviors and dynamics of TLVs in response to a range of surface drag strengths in idealized ensemble simulations and to explore their sensitivities to different storm environments. Despite high variability, a systematic influence of surface drag on the interactions between surface rotation, surface convergence boundaries, and the low-level mesocyclone is found. The inclusion of surface drag facilitates TLV formation by enhancing near-surface vortices and low-level lifting, mitigating the need for an intense updraft gradient developing close to the ground. As surface drag increases, a wider circulation near the surface blocks the inflow from directly reaching the rotating core, leading to a less-tilted structure that allows the TLV to be positioned beneath the pressure minima aloft. A further increase in drag strength discourages TLV intensification due to a downward vertical pressure perturbation gradient force, and it stops benefiting from the support of surrounding convergence boundaries, instead becoming detached from them. We hence propose a favorable condition for TLV formation and duration in which a TLV forms a less-tilted structure directly beneath the low-level mesocyclone but also evolves near surrounding surface boundaries; such a scenario strongly depends on the underlying surface drag strength. Experiments varying convective available potential energy (CAPE) and convective inhibition (CIN) also indicate that the optimum for a simulated TLV depends on how/where these favorable conditions are achieved, but the relationships are more complex than expected. Significance Statement Tornado development is very sensitive to near-surface processes, especially those involving converging air and the upward stretching of rotating air close to the ground. We show that surface friction can affect those processes in multiple ways, altering the mechanisms leading to tornadogenesis. This study elucidates how surface drag can both benefit and hinder tornado formation and determine how long they last in simulated supercells. The overall effect in any particular situation depends on the strength of the surface drag. These findings help us better understand the complex factors behind tornado formation in supercell storms, particularly regarding near-surface processes.