Frontogenesis and Geostrophically Forced Secondary Circulations in the Vicinity of Jet Stream-Frontal Zone Systems

Abstract

Frontogenesis and geostrophically forced secondary circulations in the vicinity of jet stream-frontal zone systems are treated, assuming that the motions conform to the geostrophic momentum approximation and that the secondary circulations are confined to the cross-front plane. Absolute momentum is used as a frontal defining parameter as its spatial discontinuities are shown to bound the tow hyper-gradient domain of upper jet-front systems. The solution of the Sawyer-Eliassen secondary circulation equation for simulated and observed frontal zones illustrates the importance of the geostrophic shearing deformation of an along-front thermal gradient in forcing strong subsidence within and to the warm side of upper fronts. The Miller frontogenesis equations are rederived with the frontal processes partitioned into those arising from geostrophic and ageostrophic motions. The transformation of the frontogenesis equations into geostrophic momentum, isentropic coordinates is introduced to further simplify the diagnosis of ageostrophic frontogenetical processes. A parameterization for clear-air turbulence within upper fronts is incorporated into the Sawyer-Eliassen equation to illustrate the role of turbulent-scale motions in the forcing of secondary circulations about jet-front systems.