Intrusion of stratospheric ozone to the free troposphere through tropopause folds - a case study

Simultaneous ozone radiosoundings at Jægersborg north of Copenhagen, Denmark, and at Gardermoen north of Oslo, Norway, at 12 UTC on 21 March, 1994, both show a pronounced peak of ozone in the free troposphere. A study involving routine radiosoundings as well as analysed numerical weather prediction (NWP) model data shows that a tropopause folding occurred over the Davis Strait and the southern part of Greenland about two days before the radiosoundings. Trajectory calculations performed using the same NWP model data indicate that air parcels corresponding to the observed ozone peaks at Jægersborg and at Gardermoen have passed trough the tropopause fold thus bringing down ozone rich air from the stratospheric ozone layer to the free troposphere. A quantitative simulation of this transport phenomenon is performed using an atmospheric three-dimensional long-range dispersion model. By fitting the simulated vertical profiles of ozone concentration to the observed profiles, an estimate is obtained of the rate of ozone intrusion through the tropopause fold.