the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Apr 2025
The extratropical tropopause – Trace gas perspective on tropopause definition choice
Abstract. Aircraft measurement campaigns such as IAGOS-CARIBIC and HALO missions are invaluable sources of trace gas observations in the extratropical Upper Troposphere and Lower Stratosphere (exUTLS), providing simultaneous measurements of multiple substances. To contextualise these observations, the use of dynamic coordinate systems such as tropopause-relative coordinates is highly beneficial. Different approaches to define the tropopause are commonly used in studies, based on either differences in chemical composition, dynamical parameters, or temperature gradients between the troposphere and stratosphere. We examine how different tropopause definitions influence the climatology and seasonality of trace gas observations. Meteorological parameters used in this analysis are obtained from ERA5 reanalysis data interpolated to the flight tracks. Our findings indicate that the thermal tropopause results in larger variability near the tropopause. Different potential-vorticity thresholds result in vertically displaced distributions but similar seasonal variability around the tropopause. Of these, the 3.5 PVU threshold best represents the transport barrier at the tropopause as indicated by the sharpest cross-tropopause gradient. Tracer-based tropopauses using O3 or N2O can be used effectively to differentiate between the troposphere and stratosphere without the use of additional model data. A chemical tropopause tied to a mid-latitude ozone climatology was shown to return a meaningful tropopause-relative coordinate. An investigation of individual flights showed that the tropopauses calculated from model data did not represent small-scale structures well, while the 'in-situ' chemical tropopauses provided more meaningful results. For the calculations of an N2O-based statistical tropopause, however, the case studies highlighted the importance of carefully setting initial parameters.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(5611 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(5611 KB) - Metadata XML
- BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1589', Anonymous Referee #1, 21 May 2025
- AC1: 'Reply on RC1', Sophie Bauchinger, 18 Jul 2025
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RC2: 'Comment on egusphere-2025-1589', Anonymous Referee #2, 06 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1589/egusphere-2025-1589-RC2-supplement.pdf
- AC2: 'Reply on RC2', Sophie Bauchinger, 18 Jul 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1589', Anonymous Referee #1, 21 May 2025
This study is a nice exploration of the utility of various tropopause definitions for characterizing the extratropical upper troposphere and lower stratosphere (ExUTLS). Both airborne field campaign and ozonesonde observations are leveraged and help to provide a comprehensive assessment of ExUTLS composition change relative to the tropopause. The authors find interesting differences in composition change and variability estimates in the ExUTLS for each tropopause definition explored and provide recommendations on their use for future studies. The analysis approach and diagnostics explored are largely robust, though I have some general comments on choices made and argumentation given that the authors should consider for revision. The quality of the writing and presentation of the results are excellent and I have identified very few required technical edits.
General Comments
- On utilizing ozone variability as an assessment tool for reliable discrimination of troposphere and stratosphere observations (i.e., Section 4.2): I'm not convinced this approach is a reliable assessment of tropopause definition performance. One particularly problematic condition to consider (which is not discussed at length in the present draft) is stratosphere-troposphere exchange (STE). Recent STE will undoubtedly result in air that chemically appears to be stratospheric/tropospheric but is located in the troposphere/stratosphere (respectively). Thus, using variability as an indicator of performance/appropriateness would seem to reward definitions that "remove" or otherwise minimize the inclusion of STE events. This complexity should at the very least be acknowledged and considered further in the interpretation of the meaning/significance of these results.
- On the cross-tropopause gradient (i.e., Figure 7): using the magnitude of the ozone gradient across the tropopause as an assessment tool also seems problematic to me. One could achieve the largest gradient by simply introducing a high bias in tropopause altitude such that the ubiquitous sharp stratospheric increase in ozone contributes entirely to the diagnosed cross-tropopause gradient. I think the alternative approach of comparing (or rather, differencing) the gradients/slopes in ozone vs. altitude BELOW and ABOVE tropopause would be a more meaningful and reliable assessment of the appropriateness of each definition. That is nearly accomplished here, but this slight adjustment in approach would resolve an otherwise misleading means of assessing performance.
- On the case study analyses in Section 4.4: this analysis was very brief and I found to be minimally convincing. In particular, where are the observations contextually, especially those in Figure 8? Is the high ozone portion of the flight flagged as tropospheric in panels (e) and (f) of Figure 8 the result of an STE event? For example, could this be a tropopause fold? To convincingly demonstrate appropriateness of these definitions for a case study, more information should be given. Though tracer-tracer analysis was mentioned early, I thought its exclusion from this study was a missed opportunity.
Technical Edits
Line 9: "larger variability near the tropopause" would be better stated as "larger composition variability near the tropopause"
Line 107: "substances" should be "substance"
Line 151: "across the tropopause" would be better stated as "from troposphere to stratosphere in the extratropics"
Line 153: "the vast majority of" could be stated simply as "most"
Line 179: suggest revising "last decades" to "last several decades"
Line 304: I find the statement "a large number of" to be somewhat misleading. What is considered large? Could you express this as a fraction of all aberrations? From my interpretation of the analysis, it appears to have a minimal impact on the statistics, so that gives the impression that these observations are still few though certainly more numerous than remaining definitions.
Line 313: "stratosphere" should be "stratospheric"
Lines 322-323: this sentence also appears to be an overstatement. The variability in ozone is not largest for the thermal tropopause in all seasons, though the range appears to be. In particular, the mode is largest for the N2O definition in autumn and winter.
Line 338: delete floating paren after "Fig. 6b"
Citation: https://doi.org/10.5194/egusphere-2025-1589-RC1 - AC1: 'Reply on RC1', Sophie Bauchinger, 18 Jul 2025
- On utilizing ozone variability as an assessment tool for reliable discrimination of troposphere and stratosphere observations (i.e., Section 4.2): I'm not convinced this approach is a reliable assessment of tropopause definition performance. One particularly problematic condition to consider (which is not discussed at length in the present draft) is stratosphere-troposphere exchange (STE). Recent STE will undoubtedly result in air that chemically appears to be stratospheric/tropospheric but is located in the troposphere/stratosphere (respectively). Thus, using variability as an indicator of performance/appropriateness would seem to reward definitions that "remove" or otherwise minimize the inclusion of STE events. This complexity should at the very least be acknowledged and considered further in the interpretation of the meaning/significance of these results.
-
RC2: 'Comment on egusphere-2025-1589', Anonymous Referee #2, 06 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1589/egusphere-2025-1589-RC2-supplement.pdf
- AC2: 'Reply on RC2', Sophie Bauchinger, 18 Jul 2025
Peer review completion
Journal article(s) based on this preprint
Data sets
IAGOS-CARIBIC whole air sampler data (v2024.01.12) T. Schuck and F. Obersteiner https://doi.org/10.5281/zenodo.10495039
IAGOS-CARIBIC MS files collection (v2024.10.28) A. Zahn et al. https://doi.org/10.5281/zenodo.14000090
Mission PHILEAS (Probe High Latitude Export of air from the Asian Summer Monsoon) HALO https://doi.org/10.17616/R39Q0T
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Sophie Bauchinger
Andreas Engel
Markus Jesswein
Timo Keber
Harald Bönisch
Florian Obersteiner
Andreas Zahn
Nicolas Emig
Peter Hoor
Hans-Christoph Lachnitt
Franziska Weyland
Linda Ort
Tanja J. Schuck
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(5611 KB) - Metadata XML
This study is a nice exploration of the utility of various tropopause definitions for characterizing the extratropical upper troposphere and lower stratosphere (ExUTLS). Both airborne field campaign and ozonesonde observations are leveraged and help to provide a comprehensive assessment of ExUTLS composition change relative to the tropopause. The authors find interesting differences in composition change and variability estimates in the ExUTLS for each tropopause definition explored and provide recommendations on their use for future studies. The analysis approach and diagnostics explored are largely robust, though I have some general comments on choices made and argumentation given that the authors should consider for revision. The quality of the writing and presentation of the results are excellent and I have identified very few required technical edits.
General Comments
Technical Edits
Line 9: "larger variability near the tropopause" would be better stated as "larger composition variability near the tropopause"
Line 107: "substances" should be "substance"
Line 151: "across the tropopause" would be better stated as "from troposphere to stratosphere in the extratropics"
Line 153: "the vast majority of" could be stated simply as "most"
Line 179: suggest revising "last decades" to "last several decades"
Line 304: I find the statement "a large number of" to be somewhat misleading. What is considered large? Could you express this as a fraction of all aberrations? From my interpretation of the analysis, it appears to have a minimal impact on the statistics, so that gives the impression that these observations are still few though certainly more numerous than remaining definitions.
Line 313: "stratosphere" should be "stratospheric"
Lines 322-323: this sentence also appears to be an overstatement. The variability in ozone is not largest for the thermal tropopause in all seasons, though the range appears to be. In particular, the mode is largest for the N2O definition in autumn and winter.
Line 338: delete floating paren after "Fig. 6b"