Localized forcing of slantwise motion at fronts

A cold front is here modelled as a baroclinic zone in which there is a local low‐level forcing of ageostrophic motion. Such forcing may result from the outflow from line‐convection at the surface front, or from synoptic‐scale frontogenesis, and is represented by a localized fixed region in which anomalies of the inertia and/or buoyancy of the air are imposed, either continuously or impulsively. The response to this forcing is described when the baroclinic flow is either unstable or weakly stable to symmetric instability. Such conditions are thought to be typical of cold fronts. The flow is examined using classical parcel theory, a (new) more complete parcel theory, a primitive‐equation numerical model, and the so‐called Geometric (or balanced) Model. The predicted motion involves a lens‐shaped area oriented along the sloping isentropes, with geometry dictated by the background potential vorticity and the imposed anomalies in inertia or buoyancy. The lens has zero or weakly negative potential vorticity, in the case of a background flow which is symmetrically stable. This lens, grown in a symmetrically stable atmosphere, is offered as an alternative explanation for the observed state of (moist) slantwise neutrality at a cold front, in contrast to that explanation which assumes an irreversible adjustment to symmetric instability .