The structure and evolution of lower stratospheric frontal zones. Part II: The influence of tropospheric ascent on lower stratospheric frontal development

Abstract

AbstractThe intensification of frontal characteristics in the region above the mid‐latitude jet core within the lower stratospheric portion of an upper‐level jet front system (ULJF) is known as lower stratospheric frontogenesis. Four recent cases of lower stratospheric frontogenesis in southwesterly flow are examined in order to elucidate the interaction between lower stratospheric dynamical processes and tropospheric ascent that characterizes such developments. In all of the cases examined the lower stratospheric front was (1) characterized by lower stratospheric quasi‐geostrophic forcing for ascent on its cold side, and (2) parallel to a surface cold front.As latent heating associated with ascent along the surface cold front redistributed the potential temperature field within the upper troposphere, the stability of the near‐tropopause upper troposphere decreased, thus intensifying the response to lower stratospheric frontal forcing by enhancing the frontogenetic ascent within the upper troposphere. It is therefore suggested that ascent originating in the lower troposphere is able to influence the development of lower stratospheric fronts and substantially alter the structure of the mid‐latitude tropopause and its associated horizontal potential vorticity gradient at and above the jet core. The implications of lower stratospheric frontogenetic processes on facilitating tropical–extratropical interactions is discussed.