| dc.description.abstract | Supercell thunderstorms simulated using the numerical model CM1 are used to analyze the effects of low-level vertical wind shear on near-ground rotation in the storm. In particular, the parameters being assessed are the orientation, magnitude, and depth of the low-level vertical wind shear. Particular emphasis is given to the effects of the shear in determining the position of the low-level outflow relative to the midlevel mesocyclone/updraft.
The simulations are initialized using idealized soundings of quarter-circle, clockwise turning, with unidirectional westerly shear above 2 km hodographs. A control simulation is run without any low-level vertical wind shear to compare to the other runs. Experiments are then conducted in which the background sounding is modified by adding a low-level shear layer at one of three different orientation angles: 0° (easterly shear), 90° (southerly shear), or 180° (westerly shear).
Comparing a set of simulations run for a shear layer depth of 500 meters with a shear magnitude of 7 m s^−1 , the most favorable orientation for intensifying near-ground rotation based on positioning of the outflow relative to the midlevel mesocyclone was the 0° case. The 90° case became more favorable after being run for another hour, where as the control and 180° cases did not develop favorable conditions. Changing the shear layer depth to 250 meters gives similar results, but when the shear layer depth is changed to 1 kilometer the most favorable simulation became the control. Finally, when the magnitude of the shear was increased to 15 m s^ −1 , none of the cases was found to be favorable, as the outflow was not found to be positioned in any of the simulations below the midlevel mesocyclone.
Statistically, a significant negative correlation was found between the maximum near-ground vertical vorticity/circulation and the distance between the maximum near-ground rotation and the midlevel mesocyclone. When this distance decreased, the vertical vorticity/circulation increased, suggesting that the positioning of the outflow beneath the midlevel mesocyclone plays a key role in amplifying the surface rotation. For the particular sounding used in this study, a low-level shear orientation produces the most favorable positioning of the outflow beneath the mesocyclone, and thus lead to the strongest surface rotation among the cases considered. | en |
