Quasi-geostrophic diagnosis of the influence of vorticity advection on the development of upper level jet-front systems

A partition of the geostrophic vorticity into shear and curvature components is employed to consider the influence of differential vorticity advection on the development of upper level jet-front systems in northwesterly flow in an idealized and an observed case. The analysis reveals that negative geostrophic shear vorticity advection by the thermal wind, inextricably coincident with regions of geostrophic cold air advection in cyclonic shear, forces subsidence that is distributed in narrow, quasi-linear, frontal-scale bands aligned along the warm edge of the upper baroclinic zone. In each case examined, this component of the quasi-geostrophic (QG) subsidence makes the largest contribution to upper frontogenetic tilting. Additionally, since QG omega forced by geostrophic vorticity advection by the thermal wind is of the shearwise variety, the analysis shows that the traditional emphasis on the role of laterally displaced transverse circulations is an incomplete description of the upper frontogenetic tilting that arises in such environments. In fact, the results suggest that Mudrick's (1974) emphasis on negative vorticity advection increasing with height combined with Shapiro's (1981) insight regarding the lateral displacement of frontogenetic transverse circulations offers the most comprehensive way to conceptualize the forcings that promote rapid upper level jet-front development in regions of geostrophic cold air advection in cyclonic shear.