A potential vorticity diagnostic approach to upper-level frontogenesis within a developing baroclinic wave

The process of tropopause folding is studied in the context of the life cyc le of baroclinic waves. Previous studies of upper-level frontogenesis have emphasized the role of the vertical circulation in driving stratospheric ai r down into the midtroposphere. Here, a potential vorticity-based approach is adopted that focuses on the generation of a folded tropopause. To facili tate comparison of the two approaches, the diagnosis is applied to the uppe r-level front previously simulated and studied by Rotunno et al. The potent ial vorticity approach clarifies the primary role played by the horizontal nondivergent wind in producing a fold and explains why folding should be a common aspect of baroclinic development. Between the trough and upstream ridge, prolonged subsidence within a region of weak system-relative flow generates a tropopause depression oriented at an angle to the large-scale flow. The large-scale vertical shear then loca lly increases the slope of the tropopause, eventually leading to a tropopau se fold. In contrast, tropopause folding in the base of the trough is cause d by the nondivergent cyclonic circulation associated with the surface ther mal wave. The winds associated with the thermal wave amplify the potential vorticity wave aloft, and these winds, which decrease with height, rapidly generate a tropopause fold within the trough.