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

Abstract

The 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.Key Words: lower stratospheric fronts; convection; stability reduction

The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: A Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

“…Grams et al 2013b). A similar impact of strong latent heat release on the upper-level flow has been reported for midlatitude weather systems (e.g., Ahmadi-Givi et al 2004;Grams et al 2011;Lang and Martin 2013;Teubler and Riemer 2016).…”

“…The divergent outflow and initial ridge building results from the diabatically driven strong ascent and associated latent heat release in the TC inner core and later at the midlatitude baroclinic zone yielding a net transport of lower-tropospheric air with low values of potential vorticity (PV) to the tropopause (e.g., DiMego and Bosart 1982;Bosart and Lackmann 1995;Torn 2010;Grams et al 2013b). Also, other weather systems such as predecessor rain events (PREs; Galarneau et al 2010;Moore et al 2013) and warm conveyor belts (WCBs; e.g., Carlson 1980;Madonna et al 2014b) occur surrounding an ET event, exhibit strong diabatic outflow, and may modify the upper-level Rossby wave pattern in a similar manner as the actual ET (Ahmadi-Givi et al 2004;Grams et al 2011;Bosart et al 2012;Lang and Martin 2013;Moore et al 2013;Madonna et al 2014a;Galarneau 2015;Teubler and Riemer 2016). The diabatic nature of this interaction alters the behavior of Rossby waves expected from a purely dry dynamical perspective.…”

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

“…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, ).…”

“…Cross‐section through a characteristic tropopause jet– front system: potential temperature (every 4 K, thin grey contours), wind speed (every 10 m s −1 starting at 40 m s −1 , black contours), the dynamic tropopause (thick black line), and the magnitude of the potential temperature gradient (every 1 K (100 km) −1 starting at 2 K (100 km) −1 , grey shading). The figure is from Lang and Martin (2013b).…”

“…Analyses by Lang and Martin (2012, 2013b) of lower‐stratospheric fronts suggest that a consideration of the separate evolution of the lower‐stratospheric front and the upper‐tropospheric front is necessary in order to fully understand the comprehensive life cycle of a tropopause jet–front system in midlatitude flow. This analysis aims to build upon the limited synoptic understanding of lower‐stratospheric fronts presented in the case‐study analyses of Lang and Martin (2012, 2013b) by constructing and analyzing a climatology and composites of lower‐stratospheric fronts over North America. The goals of this study are thus to elucidate the climatological distribution and structural evolution of lower‐stratospheric fronts over North America.…”