The Effects of Low-Level Wind Shear Orientation, Depth, and Magnitude on Low-Level Rotation in Simulated Supercell Thunderstorms

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.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

Masters

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

Texas A & M University

Year dc:date.issued

2016

Author dc:creator

• Guarriello, Felicia Rose

Advisors dc:contributor.advisor

• Nowotarski, Christopher
• Epifanio, Craig

Committee member dc:contributor.committeemember

• Hetland, Robert

• low-level shear
• supercell

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/1969.1/158140

OAI record oai:identifier

https://oaktrust.library.tamu.edu/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Aoaktrust.library.tamu.edu%3A1969.1%2F158140