Cross-Tropopause Mass Exchange and Potential Vorticity Budget in a Simulated Tropopause Folding

To study stratosphere–troposphere exchange, an approach based on the nonconservation of potential vorticity (PV) is developed; this approach arises naturally if one defines the tropopause in terms of PV. The evolution of a tropopause fold simulated by a mesoscale model is studied, as well as the evolution of PV at the tropopause level. The PV framework also permits the identification of the physical processes responsible for the cross-tropopause exchange as either diffusive or diabatic. In this model simulation, the diabatic processes are found to be the most important in the exchange. In particular, it is found that the negative heating gradient in the region of the warm sector of the surface cyclone is responsible for most of the diabatic exchange across the tropopause. The mass exchange during the tropopause folding event is estimated to be around 4.9 × 1014 kg in four days over the domain considered (1600 × 2000 km). This number is shown to correspond to the net difference between exchange from stratosphere to troposphere (23.5 × 1014 kg) and exchange from troposphere to stratosphere (18.6 × 1014 kg). Using the results from the exchange of a passive tracer, the exchange of ozone is estimated to be of the order of 1.1 × 108 kg. Finally, the origin of the air exchanged is found to be from the lower stratosphere and the upper troposphere, for the period of four days studied.