Intercomparison of tropospheric ozone models: Ozone transport in a complex tropopause folding event
Résumé
The present generation of tropospheric chemistry models applies horizontal and vertical model resolutions that are sufficiently fine to represent synoptic-scale processes. In this study we compare simulations of a tropopause folding event on 20-21 June 2001 from six tropospheric ozone models with tropospheric ozone profiles observed at Garmisch-Partenkirchen (Germany). The event involves air masses of stratospheric origin and of North Atlantic and North American tropospheric origin. Two coupled chemistryclimate models, three chemistry-transport models, and one chemistry-trajectory model participate in the intercomparison. The models do not explicitly include stratospheric chemistry, and stratospheric ozone is parameterized instead. The horizontal resolution of the Eulerian models, T42 (2.8°Â 2.8°) or finer, appears adequate to represent two prominent features, namely, the stratospheric intrusion descending from the upper troposphere to about 4 km altitude on the first day and an ozone-poor air mass of marine origin in the lower troposphere on the second day. The ozone distribution from the Lagrangian model is less representative because of an insufficient air parcel density. Major discrepancies between model results and observations are the underestimation of ozone levels in the intrusion, too strong downward transport of ozone between the lower stratosphere and the upper troposphere on the first day, and too fast and deep descent of the intrusion. Accurate representation of ozone levels in the intrusion depends directly on the accuracy of the simulated ozone in the lower stratosphere. Additionally, for Eulerian models a relatively coarse vertical resolution in the tropopause region may add to inaccuracies in the simulated ozone distributions.
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