Deep stratospheric intrusions: a statistical assessment with model guided analyses

A statistical assessment of deep intrusions of stratospheric air based on records of two adjacent mountain stations of the northern Alps at different altitudes is presented. Ten years recordings of beryllium activity, ozone concentrations, and relative humidity at the Zugspitze summit (2962 m a.s.l.), as well as ozone and relative humidity at the Wank summit (1776 m a.s.l., 15km distance) were analyzed. 195 stratospheric intrusion events could unambiguously be identified for the Zugspitze, whereas 85 intrusion events were found for the Wank. No event could be reliably identified at the valley floor station at Garmisch-Partenkirchen (740m a.s.l.). There is a pronounced seasonal cycle of the frequency of events showing highest activity during fall, winter, and spring, whereas low activity is found during summer. By assessing average events it was possible to infer the monthly mean enrichment rate of the lower tropospheric ozone concentration by deep stratospheric intrusions. It was found that at least 3% of the ozone burden is replaced every month on an annual average. Three events of moderate strength were taken to be further analyzed by mesoscale meteorological model simulations with subsequent trajectory studies. In two cases the intrusion of stratospheric air was induced by tropopause foldings. In the third case a cut-off low with an associated fold was responsible for the increased exchange. All three cases revealed that the ingress of stratospheric air observed at the mountain station is a non-local process induced more than 2000 km apart. Transport over these distances took about 2–4 days. Along the pathways through the tropopause the air parcels are shown to subside from the tip of the folds at 400–500 hPa down to about 700 hPa to reach the Zugspitze measurement station.