Model study of a stratospheric intrusion event at lower midlatitudes associated with the development of a cutoff low

A tropopause folding event associated with the development of a cutoff low over southeastern Europe at the end of March 1995 enhanced the ozone concentrations in the upper troposphere. The episode has been simulated using a coupled chemistry-general circulation model that has been "nudged" toward actual meteorology using a simple four-dimensional assimilation technique based on European Centre for Medium-Range Weather Forecasts data. Model results are compared with ozone observations and analyzed meteorological data. In particular, simulated geopotential heights show a realistic representation of the synoptic development of the cutoff low system. The model also succesfully reproduces the space-time evolution of potential vorticity and specific humidity fields associated with the tropopause fold. Furthermore, modeled ozone concentrations during that period show a pronounced increase in the upper troposphere associated with downward fluxes from the stratosphere. The total amount of stratospheric ozone transferred into the troposphere during the event is estimated at 6.4 x 10(8) kg. The results of the present simulation indicate that the model provides a useful tool to simulate stratosphere-troposphere: exchange and its effect on global tropospheric ozone, and that the model representation of the large-scale wave forcing is more important than the fine details of tropopause foldings.