Climatological and case analyses of lower-stratospheric fronts over North America.

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. [ABSTRACT FROM AUTHOR]