The circulation associated with a cold front Part 1: Dry case.

Abstract

The transientbehaviour of an idealised dry frontal system is investigated using a two-dimensional numerical model. The development of a cross-stream circulation within stationary and moving cold fronts is determined for various frontal and synoptic conditions. In the stationary front, a circulation is generated by symmetric baroclinic instability, but nonlinear effects restrict this circulation to remain very weak. In the moving cold front, the vertical shear of the synoptic wind which advects the front produces an ageostrophic residue as a result of the differential advection of the vertical shear of the frontal jet and the horizontal temperature gradient across the front. This residue, which depends upon the vertical synoptic shear and the thermal wind structure of the frontal system, will generate a cross-stream circulation which maintains the cold front in a quasi-steady state. The resulting motion field is described well by the streamfunction balance equation. The lifting produced by the cross-stream circulation in the moving cold front system may be sufficient to trigger deep convection under favourable conditions in the moisture and synoptic wind fields. (A)