A Climatology of Lower Stratospheric Fronts in North America

Robust lower-stratospheric fronts (LSFs) are often observed during the winter season in conjunction with tropopause jet-front (TJF) systems. Associated via the thermal wind relationship with vertical shear above the level of maximum winds, LSFs are a significant structure within a TJF located approximately at 200 hPa. The dynamical processes associated with LSFs have the potential to alter the structure and evolution of a TJF, thus implicating them in the life cycle of surface cyclones. However, the development and dissipation of LSFs is currently not well understood. This study aims to accurately document the life cycle of LSFs within the context of ULJF systems and downstream weather.

Using twice daily NCEP/NCAR Global Forecast System (GFS) 1-degree analyses, a synoptic climatology of LSFs was constructed from 184 cases identified during the 2004-05 to 2011-12 winter seasons. LSFs, like their tropospheric counterparts, are characterized by larger than background horizontal potential temperature gradients, cyclonic shear, and static stability. The cases were then divided into their synoptic scale flow environment (northwesterly, westerly, etc.) and composited. These composite analyses revealed that LSFs tend to form in two distinct environments, which led to further analysis of the development of LSFs over (1) the Rocky Mountains and (2) the Eastern United States. The results of this comparative analysis and the proposed mechanisms involved in lower stratospheric frontogenesis and frontolysis will be presented.