The total quasi-geostrophic (QC) 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 QC rotational frontogenesis, and it’s associated shearwise circulation, in the life cycle of an upper-front/jet system in northwesterly flow. The analysis reveals that shearwise subsidence is the predominant mode of frontogenetical subsidence throughout the life cycle. Given the physical relationship between shearwise vertical motions and rotational frontogenesis, a two stage conceptual model of the northwesterly flow upper frontogenesis process emerges. In the first stage, the pre-cold advection stage, the thermal advection along the front is weak but entirely positive. Positive rotational frontogenesis, characterized by along-shear Q-vectors, dominates as the vertical vorticity associated with the nascent upper-front/jet feature rotates the isentropes cyclonically into an orientation that eventually allows cold air advection along the jet. The shearwise vertical motions directly associated with this process provide frontogenetical subsidence on or to the warm side of the upper baroclinic zone, just upshear of a developing thermal through. During this pre-cold advection stage, little contribution of fontogenetical tilting is made by the transverse vertical motions.
The cold air advection stage begins after rotational frontogenesis has reoriented the isentropes sufficiently to establish cold air advection along the jet. This circumstance results in the establishment of a transverse circulation with subsidence on or to the warm side of the baroclinic zone, the well know Shapiro effect. Though the transverse subsidence becomes larger during the cold air advection stage, it is accompanied by an equally large, or larger, contribution to frontogenetic subsidence delivered by continued rotational frontogenesis. Late in this stage, the two circulations delivered by continued rotational frontogenesis. Late in this stage, the two circulations overlap just upstream of the thermal trough where rapid upper frontogenesis occurs.
|
