Mesoscale modeling of meteorological conditions in the lower troposphere during a winter stratospheric ozone intrusion over Zagreb, Croatia

We investigated mesoscale meteorological conditions in the lower troposphere during an episode of winter stratospheric ozone intrusion over Zagreb, Croatia, on 6 February 1990. This particular event was associated with a jet stream over northwestern Europe, and it was accompanied by elevated ozone values at two measuring sites in the greater Zagreb area. A Eulerian nonhydrostatic prognostic mesoscale model has been employed to reproduce significant fine‐scale features of the fold. The initial streamer of the potential vorticity (PV) anomaly, which was found in the lower troposphere from 1.5 to 1 day prior to measured ozone peaks, consisted of a wavelike train of vortices. Horizontal and vertical dimensions of the individual vortices were between 30 and 70 km and between 500 and 1500 m, respectively. At the same time, the vertical PV gradients within the vortices were up to ΔPV = 0.2 × 10−6 K m Pa−1 s−3/100 m. The model also reproduced the streamer's breaking apart and its gradual and irreversible mixing with the adjacent tropospheric air. Both the model results and ozone measurements suggest that the air of stratospheric origin arrived above Zagreb from the southeast.