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

Abstract

Recent case‐study analyses produced a conceptual model suggesting that lower‐stratospheric fronts, the lower‐stratospheric half of a tropopause jet–front system, develop preferentially in southwesterly flow in the presence of lower‐stratospheric geostrophic warm air advection and ascent within the jet core. This conceptual understanding is examined by performing climatological and case analyses of 185 objectively identified lower‐stratospheric fronts that occurred over North America during the winter seasons (December, January and February) of 2004–2012 in the 1.0°×1.0°NCEP/NCAR Global Forecast System analysis. The climatological analysis shows a preference for strong southwesterly flow lower‐stratospheric fronts to occur over the Intermountain West region and relatively weaker southwesterly flow lower‐stratospheric fronts to occur over eastern North America. The southwesterly flow lower‐stratospheric fronts were composited according to their geographical location and the analysis shows that the composite lower‐stratospheric and tropospheric structures in these two locations have statistical differences. The case‐study analyses of southwesterly flow lower‐stratospheric fronts in these two regions highlight different, geographically dependent, frontal evolutions. The strong lower‐stratospheric front identified in the Intermountain West region interacted with an orographic gravity wave and associated lower‐stratospheric ascent that rearranged the local thermal field. The lower‐stratospheric front in eastern North America developed parallel to a surface front and was associated with diabatically reduced static stability in the upper troposphere, geostrophic warm air advection, and enhanced ascent.