The total quasi-geostrophic (QG) vertical motion is partitioned into transverse and shearwise couplets oriented parallel to, and along, the geostrophic vertical shear, respectively, in order to examine the role of rotational frontogenesis, and its associated shearwise circulation, in the life cycle of an upper-front/jet system in northwesterly flow. In particular the analysis emphasizes two aspects of that role: (1) the influence of shearwise ω on scalar frontogenesis, and (2) the effect of rotational frontogenesis, and its associated vertical circulation, on the initiation of along-flow cold air advection during upper frontogenesis.
The case-study analysis reveals that shearwise subsidence persistently and significantly contributes to scalar frontogenesis throughout the life cycle. The transverse subsidence contribution to scalar frontogenesis is initially weak but grows more substantial after the establishment of along-flow cold air advection near the base of the thermal trough.
Prior work has argued that the development of along-flow cold air advection arises from a cyclonic rotation of ∇θ promoted either by contributions from tilting or horizontal kinematics. From the QG perspective adopted in the present study, the tilting and kinematic rotations are seen as interconnected aspects of a single, underlying dynamical process: rotational frontogenesis, in which the rotation of ∇θ produced by vertical vorticity is accompanied by a discrete vertical circulation (the shearwise circulation) that provides the subsidence upshear of the vorticity maximum required to initiate along-flow cold air advection. It is further suggested that the characteristic distribution of the kinematic and tilting contributions to rotation of ∇θ in the vicinity of an upper-tropospheric vorticity maximum underlies the observed preference for the development of upper-level fronts in northwesterly flow. Copyright © 2010 Royal Meteorological Society