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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Oct 2020

26 Oct 2020

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This preprint is currently under review for the journal ACP.

Mixing at the extratropical tropopause as characterized by collocated airborne H2O and O3 lidar observations

Andreas Schäfler, Andreas Fix, and Martin Wirth Andreas Schäfler et al.
  • Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany

Abstract. The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the mid-latitudes, largely depends on dynamical processes fostering the exchange of air masses. Here we follow the need to better characterize the ExTL in relation to the dynamic situation using the first-ever collocated airborne lidar observations of ozone (O3) and water vapour (H2O) across the tropopause. The potential of such lidar profile data, required for a novel, two-dimensional depiction of the complex trace gas distributions and mixing along cross-sections, is illustrated for a perpendicular jet stream crossing during a research flight over the North Atlantic conducted on 1 October 2017 in the framework of the Wave-driven Isentropic Exchange (WISE) field campaign.

The analysis of the ExTL shape and composition uses a combined view of the lidar data in geometrical and tracer-tracer (T-T) space, which was so far not possible from existing observations. For this particular case study, which is considered to be representative for the climatological distribution, the T-T depiction allows to identify distinct mixing regimes that suggest mixing of air masses with differing origin: we find clearly separated mixing of stratospheric air with moist extratropical air as well as with dry tropical air in the surrounding of the extratropical jet stream. This separation is indicative for differing transport pathways in the troposphere which need to be further elaborated using Lagrangian diagnostics. The O3 and H2O distributions confirm strongest mixing above and below the maximum jet stream winds, while it is suppressed in-between. The interrelation of chemical and dynamical discontinuities is investigated and strongest isentropic trace gas gradients are found to be better correlated with maximum isentropic PV gradients than with classical dynamical tropopause definitions. Although the methods neither allow conclusions on the individual mixing process nor on the location and time of the event, the consideration of data subsets allows discussing the formation and interpretation of isentropic and vertical mixing lines in T-T space and to develop hypotheses on mixing at different time-scales. The presented two-dimensional lidar data is considered to be of relevance for the investigation of further synoptic situations leading to mixing across the tropopause and for future validation of chemistry and numerical weather prediction models.

Andreas Schäfler et al.

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Andreas Schäfler et al.

Andreas Schäfler et al.


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Publications Copernicus
Short summary
First-ever, collocated ozone and water vapor lidar observations across the tropopause are applied to investigate the ExTL in new detail. The combined view of a quasi-instantaneous cross-section and its tracer-tracer depiction allows to analyze the shape and composition ExTL in relation to the dynamic situation. We discuss distinct mixing regimes and the interpretation of mixing lines. Such lidar data is relevant for future UTLS investigations and validation of chemistry and NWP models.
First-ever, collocated ozone and water vapor lidar observations across the tropopause are...