Review of the Structure and Dynamics of Upper-Level Frontal Zones

摘要

This article reviews upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves. The discussion traces present day concepts concerning the structure of upper level frontal systems, which are based on composite analyses of radiosonde and aircraft data, from their origins in the pioneering analyses of upper air data in the 1930's. Perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis. The dynamics of upper level fronts are shown to comprise the interactions between the primary (geostrophic) and secondary (ageostrophic) circulations. To elucidate the mechanisms and feedbacks contributing to the evolution of upper level fronts in relation to their setting within baroclinic waves, the two-dimensional theory of forced secondary circulations in the cross-front plane is presented ad interpreted, and theoretical and numerical examples of the formation of upper-level fronts in idealized two-dimensional flows are reviewed. In the three dimensional case, the presence of along front ageostrophic circulations superimposed upon the cross-front ageostrophic circulations treated by the two-dimensional theory is discussed in terms of the gradient wind. (Reprints)