Editorial Type:
Article
Article Type:
Research Article

Mechanisms for the Generation of Mesoscale Vortices within Quasi-Linear Convective Systems

Morris L. Weisman National Center for Atmospheric Research, Boulder, Colorado

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Christopher A. Davis National Center for Atmospheric Research, Boulder, Colorado

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Print Publication:
01 Aug 1998
DOI:
https://doi.org/10.1175/1520-0469(1998)055<2603:MFTGOM>2.0.CO;2
Page(s):
2603–2622
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Abstract

Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convective systems may be a natural consequence of the finite extent of the convective line, as horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the ambient shear or else generated within the system. In this paper, results are presented from a series of idealized simulations that demonstrate that the source, strength, and scale of these vortices depends on the strength of the ambient vertical wind shear, the strength of the system-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-moderate shear, a line-end vortex pair is generated primarily via the tilting of horizontal vorticity generated within the system-scale cold pool, as the associated vortex lines are lifted within the laterally finite front-to-rear ascending current. Similar mechanisms also operate in environments with stronger or deeper shear, but subsystem-scale vortices can also originate via the tilting of the ambient horizontal vorticity within supercell updraft–downdraft couplets. In all cases, convergence at midlevels enhances Coriolis rotation over the longer term, leading to the preferred development of a cyclonic vortex, as is frequently observed in asymmetric convective systems.

Corresponding author address: Dr. Morris L. Weisman, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307–3000.

Abstract

Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convective systems may be a natural consequence of the finite extent of the convective line, as horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the ambient shear or else generated within the system. In this paper, results are presented from a series of idealized simulations that demonstrate that the source, strength, and scale of these vortices depends on the strength of the ambient vertical wind shear, the strength of the system-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-moderate shear, a line-end vortex pair is generated primarily via the tilting of horizontal vorticity generated within the system-scale cold pool, as the associated vortex lines are lifted within the laterally finite front-to-rear ascending current. Similar mechanisms also operate in environments with stronger or deeper shear, but subsystem-scale vortices can also originate via the tilting of the ambient horizontal vorticity within supercell updraft–downdraft couplets. In all cases, convergence at midlevels enhances Coriolis rotation over the longer term, leading to the preferred development of a cyclonic vortex, as is frequently observed in asymmetric convective systems.

Corresponding author address: Dr. Morris L. Weisman, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307–3000.

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