Very high stratospheric influence observed in the free troposphere over the Northern Alps – just a local phenomenon?

Abstract

The atmospheric composition is strongly influenced by a change in atmospheric dynamics, which is potentially related to climate change. A prominent example is the doubling of the stratospheric ozone component at the summit station Zugspitze (2962 m a.s.l., Garmisch-Partenkirchen, Germany) between the mid-seventies and 2005, roughly from 11 ppb to 23 ppb (43 %). Systematic efforts for identifying and quantifying this 15 influence have been made since the late 1990s. Meanwhile, routine lidar measurements of ozone and water vapour carried out at Garmisch-Partenkirchen (German Alps) since 2007, combined with in-situ and radiosonde data and trajectory calculations, have revealed that stratospheric intrusion layers are present on 84 % of the yearly measurement days. At Alpine summit stations the frequency of intrusions exhibits a seasonal cycle with a pronounced summer minimum that is reproduced by the lidar measurements. The summer minimum disappears 20 if one looks at the free troposphere as a whole. The mid-and upper-tropospheric intrusion layers seem to be dominated by very long descent on up to hemispheric scale in an altitude range starting at about 4.5 km a.s.l.Without interfering air flows, these layers remain very dry, typically with RH ≤ 5 % at the centre of the intrusion. Pronounced ozone maxima observed above Garmisch-Partenkirchen have been mostly related to a stratospheric origin rather than to long-range transport from remote boundary layers. Our findings and results for 25 other latitudes seem to support the idea of a rather high contribution of ozone import from the stratosphere to tropospheric ozone.

Three Decades of Tropospheric Ozone Lidar Development at Garmisch-Partenkirchen

“…The layer descended to southern Spain and then turned north-eastward towards the Alps, slightly rising. Due to the long travel the minimum relative humidity was as high as 6 %, as measured by both our water-vapour DIAL and the Munich radiosonde (Trickl et al, 2020).…”

“…However, in addition to the low RH around 1 CET the corresponding HYSPLIT trajectories indicate for both layers a descent over at least 13 d from high altitudes over the North Pacific, confirming the idea of stratospheric intrusions. Intrusions with just a low rise in ozone are not rare during the cold season (Trickl et al, 2020). They can be resolved at least in the range covered by the less noisy 277 -313-nm wavelength pair.…”

“…After the second major system upgrading routine measurements were started in 2007. Almost 5000 ozone profiles were obtained from 1991 to February 2019, numerous examples can be found in our publications (see Appendix, the most recent one, on the period 2007 to 2016, being (Trickl et al, 2020)).…”

“…In Figure we show the profiles for three measurements under complex conditions (Saharan dust up to 4 km and a stratospheric air intrusion around 5.7 km) obtained within less than three minutes on 18 June 2013. The intrusion originated at 10 km or more over the United States roughly 13 days backward in time (Trickl et al, 2020). The layer descended to southern Spain and then turned north-eastward towards the Alps, slightly rising.…”

“…The ozone peak between 2.5 and 3.0 km is not necessarily caused by a subsiding stratospheric air intrusion: The relative humidity (RH) at the Zugspitze summit rose from 38 % to 66 % until 17:00 CET, when the Zugspitze ozone reached the mixing ratio of the 11:30 peak above the summit. Subsidence is not very likely under prefrontal conditions anyway (Trickl et al, 2020). Also contributions from Northern Italy could have been picked up.…”