Frontogenesis in a Continuously Varying Potential Vorticity Fluid

Abstract

The geostrophic momentum approximation and a Lagrangian formulation are employed to consider the nature of the fronts that result from the action of a stretching deformation field in a continuously varying potential vorticity fluid. In such a fluid, the tropopause is represented by a shallow region over which the potential vorticity changes from a representative tropospheric value to a representative stratospheric one. Decreasing the depth of this zone resulted in an increase in the intensity of the upper-level front. Reduction in the cross-front temperature contrast reduced the intensity of both the surface and upper-level front. Most notably it resulted in the elimination of the deep tropopause fold in the vicinity of the upper-level front. The environment in which the fronts form may have temperature perturbations that are the result of previous or contemporaneous frontogenetic processes. The effects of two different types of perturbations were studied with the continuous model. The inclusion of a barotropic perturbation, i.e. one that is localized in the cross-front direction but that extends throughout the depth of the fluid, resulted in an increase in the intensity of both the surface and upper-level fronts. The upper-level advection of cold air that results in a localized region of reduced potential temperature has been identified as an additional frontogenetic process. In contrast to the effect of the barotropic perturbation, such a localized perturbation resulted in an increase in the intensity of the upper-level front only.