A New Spin on Vorticity Advection: Application to Upper Frontogenesis

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 both 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 frontogenetical tilting. Additionally, since QG omega forced by geostrophic vorticity advection by the thermal wind is of the shearwise variety, the analysis suggests that the traditional emphasis on transverse circulations, the conceptual heart of the Shapiro effect, may be an incomplete description of upper frontogenetic tilting. In fact, the results point toward 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 as 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.