The structure and evolution of lower stratospheric frontal zones. Part 1: Examples in northwesterly and southwesterly flow

Abstract

The structure, evolution and dynamics of two lower stratospheric frontal zones are examined from a basic state variables perspective. The case studies highlight the asynchronous evolution of the lower stratospheric and upper tropospheric frontal portions of upper level jet-front (ULJF) systems, as well as some substantial differences in lower stratospheric frontal development that occur in southwesterly and northwesterly flow. The evolution of the ULJF in northwesterly flow was characterized by an initially intense but weakening lower stratospheric front along with an initially weak but intensifying upper tropospheric front. Throughout the evolution, geostrophic cold air advection in cyclonic shear characterized a substantial portion of the lower stratospheric front. This circumstance supported subsidence through the local jet core within the cold upper troposphere, weakening the lower stratospheric front via tilting. This subsidence extended downward below the jet core where it is suggested to have played a role in the early stages of upper tropospheric frontogenesis. In the southwesterly flow case, the evolution of the ULJF was characterized by a strengthening lower stratospheric front and a weakening upper

Quasi‐geostrophic diagnosis of the influence of vorticity advection on the development of upper level jet‐front systems

“…This subsidence was responsible for vigorous tilting frontogenesis that extended from the tropopause down to nearly 700 hPa (Figure (b)). The weakening of what appears to be a rather robust lower stratospheric front by this subsidence is consistent with the findings of Lang and Martin () concerning lower stratospheric fronts in northwesterly flow. A wider, wave‐scale subsidence region was associated with curvature vorticity advection and, as at earlier times in this case, the gradient of subsidence was large on the cold side of the upper baroclinic zone (Figure (c)).…”

Climatological and case analyses of lower‐stratospheric fronts over North America

“…Geographically, the climatological analysis suggested a preference for lower‐stratospheric fronts to occur over the Intermountain West: 133 (72%) of all 185 cases were identified in the Intermountain West and 52 (28%) of all 185 cases were identified in eastern North America. Motivated by the work of Lang and Martin (2012, 2013b), which suggested that there is a preference for lower‐stratospheric fronts to develop in southwesterly flow, the composite and case‐study analyses focused on the 54 (29%) lower‐stratospheric fronts that occurred in southwesterly flow. The southwesterly flow cases were further partitioned into geographical location before compositing.…”

“…While the upper‐tropospheric front is often highlighted as the main dynamical structure in a tropopause jet–front system, the analyses of Lang and Martin (2012, 2013b) emphasized the fact that via the thermal wind relationship, frontal development can also occur in the lower‐stratospheric vertical shear zone above a tropopause jet streak (i.e. ‘the region of the jet stream axis with the greatest winds’; American Meteorological Society, ).…”

“…Lang and Martin (2012, 2013b) suggested that while upper‐tropospheric fronts tend to undergo tilting‐induced frontogenesis in the presence of geostrophic cold air advection (CAA), which favours subsidence within the jet core, lower‐stratospheric frontogenesis occurs in the presence of geostrophic warm air advection (WAA), which favours ascent within the jet core (Figure 4(c) of Lang and Martin, 2012). The seminal work on lower‐stratospheric fronts suggested that geostrophic WAA in the lower stratosphere is maximized in southwesterly flow, which creates favourable conditions for lower‐stratospheric fronts to develop downstream of an upper‐level trough.…”

“…The seminal work on lower‐stratospheric fronts suggested that geostrophic WAA in the lower stratosphere is maximized in southwesterly flow, which creates favourable conditions for lower‐stratospheric fronts to develop downstream of an upper‐level trough. It was further suggested by Lang and Martin (2012) that, while upper‐tropospheric front development is favoured in northwesterly flow, lower‐stratospheric fronts and upper‐tropospheric fronts develop asynchronously, with lower‐stratospheric fronts tending to weaken in northwesterly flow.…”