Investigating the Development and Characteristics of Streamwise Vorticity Currents Produced by Outflow Surges in Simulated Supercell Thunderstorms

Kevin T. Gray aDepartment of Atmospheric Sciences, University of Illinois Urbana–Champaign, Urbana, Illinois

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Jeffrey W. Frame aDepartment of Atmospheric Sciences, University of Illinois Urbana–Champaign, Urbana, Illinois

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Abstract

Streamwise vorticity currents (SVCs) have been hypothesized to enhance low-level mesocyclones within supercell thunderstorms and perhaps increase the likelihood of tornadogenesis. Recent observational studies have confirmed the existence of SVCs in supercells and numerical simulations have allowed for further investigation of SVCs. A suite of 19 idealized supercell simulations with varying midlevel shear orientations is analyzed to determine how SVC formation and characteristics may differ between storms. In our simulations, SVCs develop on the cold side of left-flank convergence boundaries and their updraft-relative positions are partially dependent on downdraft location. The magnitude, duration, and mean depth of SVCs do not differ significantly between simulations or between SVCs that precede tornado-like vortices (TLVs) and those that do not. Trajectories initialized within SVCs reveal two primary airstreams, one that flows through an SVC for the majority of its length, and another that originates in the modified inflow in the forward flank and then merges with the SVC. Vorticity budgets calculated along trajectories reveal that the first airstream exhibits significantly greater maximum streamwise vorticity magnitudes than the second airstream. The vorticity budgets also indicate that stretching of horizontal streamwise vorticity is the dominant contributor to the large values of streamwise vorticity within the SVCs. TLV formation does not require the development of an SVC beforehand; 44% of TLVs in the simulations are preceded by SVCs. When an SVC occurs, it is followed by a TLV 53% of the time, indicating not all SVCs lead to TLV formation.

Significance Statement

Streamwise vorticity currents (SVCs) are features within thunderstorms hypothesized to strengthen updraft rotation and increase the likelihood of tornado formation. SVCs in a suite of 19 thunderstorm simulations are analyzed to investigate how they develop, if their characteristics differ between storms, and how often they precede tornado production. The rotation in an SVC is amplified as air accelerates toward the updraft, which is the main process contributing to SVC formation. The likelihood of SVCs may vary with differences in the winds 3–6 km above the ground. These findings may aid in developing strategies for better observing SVCs.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Kevin T. Gray, kevintg2@illinois.edu

Abstract

Streamwise vorticity currents (SVCs) have been hypothesized to enhance low-level mesocyclones within supercell thunderstorms and perhaps increase the likelihood of tornadogenesis. Recent observational studies have confirmed the existence of SVCs in supercells and numerical simulations have allowed for further investigation of SVCs. A suite of 19 idealized supercell simulations with varying midlevel shear orientations is analyzed to determine how SVC formation and characteristics may differ between storms. In our simulations, SVCs develop on the cold side of left-flank convergence boundaries and their updraft-relative positions are partially dependent on downdraft location. The magnitude, duration, and mean depth of SVCs do not differ significantly between simulations or between SVCs that precede tornado-like vortices (TLVs) and those that do not. Trajectories initialized within SVCs reveal two primary airstreams, one that flows through an SVC for the majority of its length, and another that originates in the modified inflow in the forward flank and then merges with the SVC. Vorticity budgets calculated along trajectories reveal that the first airstream exhibits significantly greater maximum streamwise vorticity magnitudes than the second airstream. The vorticity budgets also indicate that stretching of horizontal streamwise vorticity is the dominant contributor to the large values of streamwise vorticity within the SVCs. TLV formation does not require the development of an SVC beforehand; 44% of TLVs in the simulations are preceded by SVCs. When an SVC occurs, it is followed by a TLV 53% of the time, indicating not all SVCs lead to TLV formation.

Significance Statement

Streamwise vorticity currents (SVCs) are features within thunderstorms hypothesized to strengthen updraft rotation and increase the likelihood of tornado formation. SVCs in a suite of 19 thunderstorm simulations are analyzed to investigate how they develop, if their characteristics differ between storms, and how often they precede tornado production. The rotation in an SVC is amplified as air accelerates toward the updraft, which is the main process contributing to SVC formation. The likelihood of SVCs may vary with differences in the winds 3–6 km above the ground. These findings may aid in developing strategies for better observing SVCs.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Kevin T. Gray, kevintg2@illinois.edu
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