Abstract
Abstract. A large-scale comparison of water-vapour vertical-sounding instruments took place over central Europe on 17 October 2008, during a rather homogeneous deep stratospheric intrusion event (LUAMI, Lindenberg Upper-Air Methods Intercomparison). The measurements were carried out at four observational sites: Payerne (Switzerland), Bilthoven (the Netherlands), Lindenberg (north-eastern Germany), and the Zugspitze mountain (Garmisch-Partenkichen, German Alps), and by an airborne water-vapour lidar system creating a transect of humidity profiles between all four stations. A high data quality was verified that strongly underlines the scientific findings. The intrusion layer was very dry with a minimum mixing ratios of 0 to 35 ppm on its lower west side, but did not drop below 120 ppm on the higher-lying east side (Lindenberg). The dryness hardens the findings of a preceding study (“Part 1”, Trickl et al., 2014) that, e.g., 73 % of deep intrusions reaching the German Alps and travelling 6 days or less exhibit minimum mixing ratios of 50 ppm and less. These low values reflect values found in the lowermost stratosphere and indicate very slow mixing with tropospheric air during the downward transport to the lower troposphere. The peak ozone values were around 70 ppb, confirming the idea that intrusion layers depart from the lowermost edge of the stratosphere. The data suggest an increase of ozone from the lower to the higher edge of the intrusion layer. This behaviour is also confirmed by stratospheric aerosol caught in the layer. Both observations are in agreement with the idea that sections of the vertical distributions of these constituents in the source region were transferred to central Europe without major change. LAGRANTO trajectory calculations demonstrated a rather shallow outflow from the stratosphere just above the dynamical tropopause, for the first time confirming the conclusions in “Part 1” from the Zugspitze CO observations. The trajectories qualitatively explain the temporal evolution of the intrusion layers above the four stations participating in the campaign.
Highlights
The complexity of stratospheric air intrusions into the troposphere has been investigated with lidar systems in great detail
An open question has been how much of the tropospheric air originates already from the so-called “mixing layer” around the thermal tropopause (e.g. Danielsen, 1968; Lelieveld et al, 1997; Hintsa et al, 1998; Zahn et al, 1999, 2014; Fischer et al, 2000; Hoor et al, 2002, 2004; Pan et al, 2004, 2007; Brioude et al, 2006, 2008; Sprung and Zahn, 2010; Vogel et al, 2011) prior to the descent and how much of the admixture occurs during the descent of an intrusion layer into the lower troposphere
The new instrument design, which is described in more technical detail in Wirth et al (2009a), features a robust, highly compact, and efficient transmitter system, which fulfils all spectral requirements for a water vapour DIAL
Summary
The complexity of stratospheric air intrusions into the troposphere has been investigated with lidar systems in great detail. Turbulent mixing has been identified as an important source of tropospheric air in tropopause folds (Shapiro, 1976, 1978, 1980). Danielsen, 1968; Lelieveld et al, 1997; Hintsa et al, 1998; Zahn et al, 1999, 2014; Fischer et al, 2000; Hoor et al, 2002, 2004; Pan et al, 2004, 2007; Brioude et al, 2006, 2008; Sprung and Zahn, 2010; Vogel et al, 2011) prior to the descent and how much of the admixture occurs during the descent of an intrusion layer into the lower troposphere. In some cases mixing of polluted or convectively lifted air into intrusions within the free troposphere has been reported (e.g. Parrish et al, 2000; Brioude al., 2007; Homeyer et al, 2011; Sullivan et al, 2016)
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