The Influence of Midlevel Shear and Horizontal Rotors on Supercell Updraft Dynamics

Large midlevel (3–6 km) shear is commonly observed in supercell environments. However, any dynamical effect that midlevel shear has on an updraft has been relatively unexplored until now. To examine this effect, we ran ten simulations of supercells in environments across a range of midlevel shear magnitudes. Given that larger midlevel shear results in a storm motion that is faster relative to the low-level hodograph, larger midlevel shear leads to stronger low-level storm-relative flow. We therefore present an analysis of the effects of both midlevel shear and low-level storm-relative flow on supercell updraft dynamics. In our simulations, a larger nonlinear dynamic pressure acceleration develops on the southern flanks of updrafts when low-level storm-relative flow is larger. This dynamical enhancement is driven by larger storm-generated horizontal vorticity associated with rotor-like circulations on the edge of the midlevel updraft, not an increase in mesocyclone strength. In fact, when midlevel shear and low-level storm-relative flow are large, increased updraft width prevents midlevel air and associated shear from entering the updraft core (where the midlevel mesocyclone is located), suggesting that midlevel shear does not affect the midlevel mesocyclone. Furthermore, the largest negative pressure perturbation at midlevels is primarily driven by the horizontal rotors, not the midlevel mesocyclone, at least until the mesocyclone becomes particularly intense. These results clarify the influence of midlevel shear on a supercell thunderstorm, which has previously been the subject of speculation, and provide additional clarity on the role of low-level storm-relative flow.