A potential vorticity diagnostic approach to upper-level frontogenesis within a devloping baroclinic wave

Abstract

According to the traditional approach, upper-level frontogenesis results from a geostrophic forcing and its ageostrophic response acting in concert to tighten the horizontal temperature gradient. Particular focus is placed on the sinking branch of this transverse circulation and the descent of stratospheric air into the frontal zone. Rotunno et al. (1994) used a primitive equation, f-plane, channel model to apply such an approach to an upper-level frontogenesis event within a simulation of baroclinic wave growth. In potential vorticity (PV) thinking, frontogenesis is more appropriately viewed in terms of tropopause folding: changes in the temperature field are implied by the reorientation of the PV field. Consequently, a method to diagnose tropopause folding is developed and applied to the same model of baroclinic wave growth. The PV analysis finds the vertical motion important for generating a slope where the tropopause is initially flat. The location and orientation of this new slope in the northwesterly flow region is determined by the confluent streamline. However, while vertical motions can bring the tropopause to nearly vertical, only vertical shear of the horizontal wind can generate a fold. In the northwesterly flow region upstream of the trough, this vertical shear is provided primarily by the mean wind. Piecewise potential vorticity inversion techniques reveal the nondivergent wind associated with the developing surface cyclone to provide the necessary vertical shear to create the fold in the base of the trough.