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The Influence of Convective Thermal Forcing on the Three-Dimensional Circulation around Squall Lines

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  • 1 National Center for Atmospheric Research,Boulder, Colorado
  • | 2 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 3 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Midlatitude squall lines are typically trailed by a large region of stratiform cloudiness and precipitation with significant mesoscale flow features, including an ascending front to rear flow; a descending rear inflow jet; line-end vortices; and, at later times, mesoscale convective vortices. The present study suggests that the mesoscale circulation in the trailing stratiform region is primarily determined by the time-mean pattern of heating and cooling in the leading convective line. Analysis of the line-normal circulation shows that it develops as thermally generated gravity waves spread away from the leading line. Midlevel line-end vortices are the result of diabatically driven tilting of horizontal vorticity generated by the time-mean thermal forcing. In the presence of the Coriolis force, a symmetric thermal forcing generates an asymmetric stratiform circulation and a pattern of vertical displacement that resembles the comma-shaped stratiform anvil observed in real systems; this suggests that asymmetries in the cloud and circulation behind midlatitude squall lines are not necessarily the result of asymmetries in the convective leading line.

* Current affiliation: Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania.

Corresponding author address: Rajul Pandya, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: rpandya@wcupa.edu

Abstract

Midlatitude squall lines are typically trailed by a large region of stratiform cloudiness and precipitation with significant mesoscale flow features, including an ascending front to rear flow; a descending rear inflow jet; line-end vortices; and, at later times, mesoscale convective vortices. The present study suggests that the mesoscale circulation in the trailing stratiform region is primarily determined by the time-mean pattern of heating and cooling in the leading convective line. Analysis of the line-normal circulation shows that it develops as thermally generated gravity waves spread away from the leading line. Midlevel line-end vortices are the result of diabatically driven tilting of horizontal vorticity generated by the time-mean thermal forcing. In the presence of the Coriolis force, a symmetric thermal forcing generates an asymmetric stratiform circulation and a pattern of vertical displacement that resembles the comma-shaped stratiform anvil observed in real systems; this suggests that asymmetries in the cloud and circulation behind midlatitude squall lines are not necessarily the result of asymmetries in the convective leading line.

* Current affiliation: Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania.

Corresponding author address: Rajul Pandya, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: rpandya@wcupa.edu

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