Lifting of Ambient Air by Density Currents in Sheared Environments

Abstract

Abstract Two aspects of vorticity associated with cold pools are addressed. First, tilting of horizontal vortex tubes by the updraft at a gust front has been proposed as a means of getting near-ground rotation and hence a tornado. Theory and a numerical simulation are used to show that this mechanism will not work because warm air parcels approaching the gust front decelerate in strong adverse pressure gradient. The near-surface horizontal vorticity available for upward tilting is greatly reduced by horizontal compression before it is tilted. Consequently, uplifting of vortex tubes produces little vertical vorticity near the ground. Second, it is shown that the baroclinic vorticity generated at the leading edge of the cold pool is transported rearward in the vortex sheet along the interface between cold and warm air, and the barotropic vorticity associated with environmental shear is conserved along streamlines. Warm parcels away from the interface do not acquire baroclinic vorticity to offset their barotropic vorticity, as assumed in a theory for long-lived squall lines. The vortex sheet has a far-field effect on the circulation in the warm air. A steady-state vortex method is used to propose why there is a steady noncirculating density current only when a lid is present and at a specific height.