06 Jul 2022
06 Jul 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Gravity wave induced cross-isentropic mixing: A DEEPWAVE case study

Hans-Christoph Lachnitt1, Peter Hoor1, Daniel Kunkel1, Martina Bramberger2, Andreas Dörnbrack3, Stefan Müller1,a, Philipp Reutter1, Andreas Giez3, Thorsten Kaluza1, and Markus Rapp3 Hans-Christoph Lachnitt et al.
  • 1Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Germany
  • 2NorthWest Research Associates, Boulder, USA
  • 3Institute of Atmospheric Physics, German Aerospace Center (DLR) Oberpfaffenhofen, Germany
  • anow at: enviscope GmbH, Frankfurt/Main, Germany

Abstract. Orographic gravity waves (i.e. mountain waves) can potentially lead to cross-isentropic fluxes of trace gases via the generation of turbulence. During the DEEPWAVE (Deep Propagating Gravity Wave Experiment) campaign in July 2014 we performed tracer measurements of carbon monoxide (CO) and nitrous oxide (N2O) above the Southern Alps during periods of gravity wave activity. The measurements were taken along two stacked levels at 7.9 km in the troposphere and 10.9 km in the stratosphere. A detailed analysis of the observed wind components shows, that both flight legs were affected by vertically propagating gravity waves with momentum deposition and energy dissipation between the two legs. Corresponding tracer measurements indicate turbulent mixing in the region of gravity wave occurrence.

For the stratospheric data we identified mixing leading to a change of the cross-isentropic tracer gradient from the upstream to the downstream region of the Southern Alps. Based on the quasi-inert tracer N2O we identified two distinct layers in the stratosphere with different chemical composition on different isentropes. The CO-N2O relationship clearly indicates that irreversible mixing between these two layers occurred. Further we found a significant change of the N2O-Θ-profiles from the upstream to the downstream side above the Southern Alps with different gradients of the N2O-Θ-relation just above the tropopause. A scale-dependent gradient analysis reveals that this gradient change is triggered in the region of gravity wave occurrence.

The power spectra of the in-situ measured vertical wind, Θ, and N2O indicate the occurrence of turbulence above the mountains associated with the gravity waves in the stratosphere. The estimated eddy dissipation rate based on the measured three dimensional wind indicates a weak intensity of turbulence in the stratosphere above the mountain ridge. The N2O-Θ-relation downwind the Alps modified by the gravity wave activity provides clear evidence that trace gas fluxes, which were deduced from wavelet co-spectra of vertical wind and N2O are at least in part cross-isentropic.

Our findings thus indicate that orographic waves led to turbulent mixing on both flight legs in the troposphere and in the stratosphere. Despite only weak turbulence during the stratospheric leg, the cross isentropic gradient and the related composition change on isentropic surfaces N2O-Θ gradient change from upstream to downstream the mountain unambiguously conserves the effect of turbulent mixing by gravity wave activity before the measurements. This finally leads to irreversible diabatic trace gas fluxes and thus has a persistent effect on the UTLS trace gas composition.

Hans-Christoph Lachnitt et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-420', Anonymous Referee #1, 30 Aug 2022
  • RC2: 'Referee comment on acp-2022-420', Anonymous Referee #2, 30 Aug 2022
  • AC3: 'Additional clarifications', Hans-Christoph Lachnitt, 25 Nov 2022

Hans-Christoph Lachnitt et al.

Hans-Christoph Lachnitt et al.


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Short summary
We present an analysis of high resolution airborne measurements during a flight of the DEEPWAVE 2014 campaign in New Zealand. The focus of this flight was to study the effects of enhanced mountain wave activity over the Southern Alps. We discuss changes in the upstream and downstream distributions of N2O and CO and show that these changes are related to turbulence induced trace gas fluxes which have persistent effects on the trace gas composition in the lower stratosphere.