The dynamical consequences for tropopause folding of PV anomalies induced by surface frontal collapse

Abstract

AbstractIt has been noted previously that during frontal collapse dynamical processes lead to the formation of potential vorticity (PV) anomalies in the vicinity of the surface front. These processes can either be associated with diffusion in the presence of the tight temperature gradients or with intrusion into the atmosphere of the vanishingly thin layer adjacent to the surface. This paper explores the dynamical consequences of these PV anomalies on the parent baroclinic wave cyclone. The method used is to find the vertical motion attributable to these anomalies. The location and magnitude of this vertical motion is clearly key to the dynamical influence of the anomalies. As we are dealing with a three‐dimensional evolving cyclone with an emergent tropopause fold, the calculation of vertical motion needs to be capable of accounting for the role of the highly deformed tropopause. Hence we develop and use a nonlinear balance model approach. The result shows that the PV anomalies near the surface front induce downward vertical motion at the tropopause fold, and thus they act to amplify this feature. This role of diffusion associated with surface frontal collapse is the key finding of this paper. An analysis is made of the role of upper‐ and lower‐level PV anomalies on the overall role of the wave development.