The relative importance of lower-level and upper-level shear on the lifting of environmental air by gravity currents

Using time-dependent numerical simulations, this study investigates the relative importance of lower-level versus upper-level shear on the ability of gravity currents to lift environmental air. Herein, "lower-level" shear is defined as wind shear from the surface to roughly 1-2 times the depth of the cold pool, and "upper-level" shear is defined as wind shear from this point to the upper troposphere. In conditions that are characteristic of derechoes in the central United States, lower-level shear is 4-8 times more effective at enhancing the lifting of environmental air. Hence, a 1 m/s increase in lower-level shear has roughly the same dynamical impact as a 5 m/s increase in upper-level shear. Furthermore, upper-level shear is only beneficial in a small part of the parameter space that is studied herein. In fact, negative upper-level shear (i.e., "reverse" shear) can also be beneficial for lifting of environmental air, and appears to be particularly helpful for convective initiation.