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

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Abstract
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 × 108 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.