Pioneering research by Reed and Sanders in the 1950s and Danielsen in the 1960s established that tropopause folding and upper-level frontogenesis occurred preferentially in northwesterly flow aloft. They also showed that cold-air advection in the northwesterly flow contributed to subsidence that maximized along the warm boundary of the baroclinic zone and resulted in a positive contribution of the twisting term to upper-level frontogenesis. Furthermore, they showed that stratospheric air was entrained deep into the troposphere in a tropopause fold in conjunction with strongly sinking air on the basis of isentropic trajectories constructed using potential vorticity (PV) conservation techniques.

The purpose of this talk is to document examples of tropopause folding that can occur outside of the now-classical northwesterly flow events documented by Reed, Sanders, and Danielsen. Three nonclassical cases of tropopause folding will be described. These cases occurred in conjunction with: (1) a landfalling and transitioning tropical cyclone, (2) a surge of low PV air from the tropics into midlatitudes, and (3) confluent frontogenesis in southwesterly flow. The PV perspective is adopted for the presentation of the cases.