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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 Apr 2019

Submitted as: research article | 15 Apr 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Evidence of small-scale quasi-isentropic mixing in ridges of extra-tropical baroclinic waves

Daniel Kunkel1, Peter Hoor1, Thorsten Kaluza1, Jörn Ungermann2, Björn Kluschat1, Andreas Giez3, Hans-Christoph Lachnitt1, Martin Kaufmann2,4, and Martin Riese2,4 Daniel Kunkel et al.
  • 1Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
  • 2IEK-7, Forschungszentrum Jülich, Jülich, Germany
  • 3Flight Experiments, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany
  • 4Institute for Atmospheric and Environmental Research, University of Wuppertal, Wuppertal, Germany

Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only little attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave driven isentropic exchange) campaign over the North Atlantic in autumn 2017 supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anti-cyclonic side of jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated to small scale waves at the upper edge of the jet-stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained only little attention in terms of mixing in baroclinic waves.

Daniel Kunkel et al.
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Daniel Kunkel et al.
Daniel Kunkel et al.
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Short summary
In this study we present a mixing process around the tropopause in extratropical baroclinic waves. We analyze airborne data from a flight during the WISE campaign in autumn 2017 over the North Atlantic. We further use idealized experiments to study the mixing process. Although the process occurs on small geographical scale, it might be of importance due to its relation to a frequent feature of the extratropical UTLS. The process is relevant for STE but is not fully included in climatologies.
In this study we present a mixing process around the tropopause in extratropical baroclinic...