How Stratospheric are Stratospheric Air Intrusions?
Thomas Trickl1, Hans-Eckhart Scheel1, Michael Sprenger2, Andreas Stohll3, Hannes Vogelmann1
1Karlsruher Institut für Technologie, IMK-IFU, Garmisch-Partenkirchen, Germany
2Eidgenössische Technische Hochschule (ETH), Zürich, Switzerland
3Norwegian Institute for Air Research, Kjeller, Norway
The change in atmospheric composition is partly driven by changing atmospheric dynamics, in potential relation to climate change. A prominent example is doubling of the stratospheric influence on ozone at the summit station Zugspitze (2962 m a.s.l., Garmisch-Partenkirchen, Germany) between the mid-seventies and 2000. Systematic efforts for identifying and quantifying this influence have been made since the late 1990s (e.g., H. E. Scheel in: http://www.forst.tu-muenchen.de/EXT/LST/METEO/staccato) based on data filtering ozone, relative-humidity (RH) and 7Be measurements. The stratospheric impact is much lower at the neighbouring site Wank (1780 m a.s.l.). In recent years we have refined the data filtering approach, based on 7Be and relative humidity, which will lead to a revision of the analysis. The quality of the results, however, significantly depends on the degree of mixing of the stratospheric air tongues with tropospheric air. It is important to note that the humidity measurements with our differential-absorption water-vapour lidar, carried out since 2004, show that even in very thin intrusion layers arriving at low altitudes the minimum RH mostly stays in the range between 0 and 2 %. Given the long distance between the arctic source regions and the Alps, this result suggests that free-tropospheric mixing is rather slow. It is also difficult to explain the characteristic strong drop in ozone in the subsiding stratospheric ozone layers in the time series of our ozone lidar to typically 50 to 80 ppb by mixing. We think that our observations in the Northern Alps to some extent map the vertical trace gas distributions in the lower stratosphere of the Arctic source regions. A smooth transition from tropospheric to stratospheric air in the source region is also consistent with the small to moderate drop of the Zugspitze CO mixing ratio during intrusion periods. In addition, the average CO in direct stratospheric intrusions has slightly grown since the beginning of the measurements in 1990, probably indicating increased input into the lower stratosphere from regions with growing air pollution, such as Asia.