The structure and evolution of lower stratospheric frontal zones.

摘要

The structure and evolution of the lower stratospheric portion of upper level jet-front systems (ULJFs) are examined in a variety of cases. Following a historical overview, the first half of the analysis examines two lower stratospheric frontal zones. The asynchronous evolution of the lower stratospheric and upper tropospheric frontal portions of ULJFs, as well as some substantial differences in lower stratospheric frontal development that occur in southwesterly and northwesterly flow are highlighted.;The northwesterly flow case was characterized by an initially intense but weakening lower stratospheric front along with an initially weak but intensifying upper tropospheric front. Geostrophic cold air advection in the lower stratospheric cyclonic shear supported subsidence within the cold upper troposphere. This subsidence resulted in lower stratospheric frontolysis via tilting and extended downward below the jet core where it is suggested to have played a role in the early stages of upper tropospheric frontogenesis.;The southwesterly flow case 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 ascent through the jet core, coincident with geostrophic warm air advection in the lower stratospheric cyclonic shear. The upper tropospheric ascent, on the cold edge of the lower stratospheric front, acted frontogenetically in the lower stratosphere via tilting.;The second half of the analysis highlights four recent southwesterly flow cases and suggests a role for tropospheric convection in the development of each lower stratospheric front. The analysis suggests that the lower stratospheric frontogenetic ascent may become enhanced in response to a convective destabilization of the upper troposphere. As latent heating associated with surface frontal convection redistributed the thermal field, the static stability of the near-tropopause upper troposphere decreased. Such a destabilization was associated with a robust response to lower stratospheric QG forcing, in the form of ascent within the cold upper troposphere, and led to lower stratospheric frontogenetic tilting in each case. The implications of the lower stratospheric frontogenetic processes on tropicalextratropical interactions and downstream sensible weather are discussed.