P4.6
The relative importance of lower-level and upper-level shear on the lifting of environmental air by gravity currents
George H. Bryan, NCAR, Boulder, CO
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.
Poster Session 4, Bow Echoes and Derecho Producing Systems, BAMEX
Tuesday, 7 November 2006, 3:00 PM-4:30 PM, Pre-Convene Space
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