Frontogenesis Processes in the Middle and Upper Troposphere

Abstract

Basic issues regarding upper-level frontogenesis addressed in this paper are: (i) simulated frontogenesis influenced by the initial flow, (ii) upper-level frontogenesis as essentially a two-dimensional process, and (iii) frontal-scale positive feedback between vertical advection of momentum and vorticity advection by the ageostrophic wind, which is important for the intensification of upper-level frontal zones. The methodology for investigation is based on analysis of simulated upper-level frontogenesis with a three-dimensional primitive-equation model. The model is a simplified version of the UCLA GCM, with 21 layers in the vertical, horizontal resolution of 1.2° lat ? 1.5° long; a 60° sector of one hemisphere as periodic domain, and physics reduced to horizontal diffusion and dry convective adjustment. Simulations initialized with jet streams symmetric about the latitude of maximum wind at each pressure level produce?in the middle troposphere?the strongest frontal zones downstream of the trough of growing baroclinic waves. Strongest upper-level frontal zones originating upstream of the wave trough, as observed, are produced when initial jet streams and perturbations are chosen so that the growing waves have small meridional phase tilt in the initial stages. In the simulations, tilting associated with divergence of the across-jet ageostrophic flow is the dominant frontogenetical process upstream of the wave trough. Further, tilting associated with divergence of the ageostrophic wind along the jet also contributes to frontogenesis, but to a lesser extent. The former result is similar to that obtained with two-dimensional models in which frontogenetical vertical motions are associated with divergence of the ageostrophic wind across the front. No definitive evidence is found proving that the simulated frontogenesis is enhanced by a positive-feedback process involving vertical advection of momentum and vorticity advection by the ageostrophic wind. It is found, however, that both of these processes are nonnegligible contributors to the frontal intensification.