The mechanisms and rate of transfer of mass and chemical species between the stratosphere and troposphere (stratosphere-troposphere exchange) are currently uncertain. In the extratropics exchange appears to be dominated by processes associated with tropopause folds and cut-off lows.

The limitations of some commonly used model-based methods of diagnosing of extratropical cross-tropopause transport in tropopause folding events are revealed by performing mesoscale resolution forecasts using the Met Office Unified Model including an idealised passive tracer. Evidence is presented that the transport and mixing which occurs in parameterized convection and turbulent mixing can lead to significant cross-tropopause transport. In this case study, the inclusion of transport by parameterized convection leads to an increase in domain averaged transport of around 27%. The importance of these processes depends on the synoptic system under consideration and local effects can greatly exceed the domain average. These processes are neglected by methods of diagnosing cross-tropopause transport based on trajectories calculated using the resolved scale winds or based on the nonconservation of potential vorticity. The sensitivity of transport estimates to the horizontal and vertical resolution of the model and to the explicit diffusion required for model stability is determined.