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

This article presents a review of 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 begins by tracing 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 1930s. 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 developed by Sawyer and Eliassen is presented and 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. The relative contribution of the along-front ageostrophic circulation to upper-level frontogenesis is considered in the context of the results from three-dimensional β-plane channel models of baroclinic wave growth. Directions for future observational, diagnostic and theoretical investigation are identified, including the scale interactions between upper-level fronts, their environmental baroclinic waves and related low-level cyclones, and between upper-level fronts and mesoscale convective systems. The review concludes with a discussion of the potential role of recent innovations in remote-sensing technology and trends in numerical weather prediction using mesoscale models in motivating continuing interest and future advances in frontal research.