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The Impact of Midlevel Shear Orientation on the Longevity of and Downdraft Location and Tornado-Like Vortex Formation within Simulated Supercells

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  • 1 a Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana, Illinois
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Abstract

Despite an increased understanding of environments favorable for tornadic supercells, it is still sometimes unknown why one favorable environment produces many long-tracked tornadic supercells and another seemingly equally favorable environment produces only short-lived supercells. One relatively unexplored environmental parameter that may differ between such environments is the degree of backing or veering of the midlevel shear vector, especially considering that such variations may not be captured by traditional supercell or tornado forecast parameters. We investigate the impact of the 3–6-km shear vector orientation on simulated supercell evolution by systematically varying it across a suite of idealized simulations. We found that the orientation of the 3–6-km shear vector dictates where precipitation loading is maximized in the storms, and thus alters the storm-relative location of downdrafts and outflow surges. When the shear vector is backed, outflow surges generally occur northwest of an updraft, produce greater convergence beneath the updraft, and do not disrupt inflow, meaning that the storm is more likely to persist and produce more tornado-like vortices (TLVs). When the shear vector is veered, outflow surges generally occur north of an updraft, produce less convergence beneath the updraft, and sometimes undercut it with outflow, causing it to tilt at low levels, sometimes leading to storm dissipation. These storms are shorter lived and thus also produce fewer TLVs. Our simulations indicate that the relative orientation of the 3–6-km shear vector may impact supercell longevity and hence the time period over which tornadoes may form.

Significance Statement

We explore how the orientation of the 3–6-km vertical wind shear vector impacts the longevity of and thus the potential for near-surface vortex formation within simulated supercell thunderstorms. Our investigation is significant because these impacts have been relatively unexplored and because the midlevel winds dictate where outflow surges develop within supercells. We found that the storm-relative location of outflow surges can affect the magnitude of the convergence beneath an updraft, the buoyancy of the air flowing into an updraft, and the potential tilting and undercutting of an updraft by outflow, all of which can influence supercell longevity and thus the potential for near-surface vortex formation.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Kevin Gray, kevintg2@illinois.edu

Abstract

Despite an increased understanding of environments favorable for tornadic supercells, it is still sometimes unknown why one favorable environment produces many long-tracked tornadic supercells and another seemingly equally favorable environment produces only short-lived supercells. One relatively unexplored environmental parameter that may differ between such environments is the degree of backing or veering of the midlevel shear vector, especially considering that such variations may not be captured by traditional supercell or tornado forecast parameters. We investigate the impact of the 3–6-km shear vector orientation on simulated supercell evolution by systematically varying it across a suite of idealized simulations. We found that the orientation of the 3–6-km shear vector dictates where precipitation loading is maximized in the storms, and thus alters the storm-relative location of downdrafts and outflow surges. When the shear vector is backed, outflow surges generally occur northwest of an updraft, produce greater convergence beneath the updraft, and do not disrupt inflow, meaning that the storm is more likely to persist and produce more tornado-like vortices (TLVs). When the shear vector is veered, outflow surges generally occur north of an updraft, produce less convergence beneath the updraft, and sometimes undercut it with outflow, causing it to tilt at low levels, sometimes leading to storm dissipation. These storms are shorter lived and thus also produce fewer TLVs. Our simulations indicate that the relative orientation of the 3–6-km shear vector may impact supercell longevity and hence the time period over which tornadoes may form.

Significance Statement

We explore how the orientation of the 3–6-km vertical wind shear vector impacts the longevity of and thus the potential for near-surface vortex formation within simulated supercell thunderstorms. Our investigation is significant because these impacts have been relatively unexplored and because the midlevel winds dictate where outflow surges develop within supercells. We found that the storm-relative location of outflow surges can affect the magnitude of the convergence beneath an updraft, the buoyancy of the air flowing into an updraft, and the potential tilting and undercutting of an updraft by outflow, all of which can influence supercell longevity and thus the potential for near-surface vortex formation.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Kevin Gray, kevintg2@illinois.edu
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