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

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.