The Effects of Low-Level Wind Shear Orientation, Depth, and Magnitude on Low-Level Rotation in Simulated Supercell Thunderstorms
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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.
Guarriello, Felicia Rose (2016). The Effects of Low-Level Wind Shear Orientation, Depth, and Magnitude on Low-Level Rotation in Simulated Supercell Thunderstorms. Master's thesis, Texas A & M University. Available electronically from