The structure and evolution of lower stratospheric frontal zones in northwesterly and southwesterly flow

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 tropospheric front. A deep column of upward vertical motion resulted from the superposition of lower tropospheric ascent associated with convection along a surface cold front and upper tropospheric-lower stratospheric (UTLS) ascent through the jet core coincident with geostrophic warm air advection in cyclonic shear along large sections of the lower stratospheric front. The UTLS ascent, located on the cold edge of the lower stratospheric baroclinicity, served to intensify the lower stratospheric frontal zone via tilting. The implications of these lower stratospheric frontal processes on the energetics of ULJFs, the topography of the tropopause and downstream sensible weather are discussed.