Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet

Langille, Jeffery; Bourassa, Adam; Pan, Laura L.; Letros, Daniel; Solheim, Brian; Zawada, Daniel; Degenstein, Doug

doi:https://doi.org/10.5194/acp-20-5477-2020

Articles | Volume 20, issue 9

https://doi.org/10.5194/acp-20-5477-2020

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

https://doi.org/10.5194/acp-20-5477-2020

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

Articles | Volume 20, issue 9

Research article

|

12 May 2020

Research article |

| 12 May 2020

Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet

Jeffery Langille, Adam Bourassa, Laura L. Pan, Daniel Letros, Brian Solheim, Daniel Zawada, and Doug Degenstein

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Final revised paper (published on 12 May 2020)
Preprint (discussion started on 15 Nov 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Referee Comment', Anonymous Referee #1, 06 Jan 2020
- AC1: 'Author response', Jeffery Langille, 06 Mar 2020
RC2: 'Review of 'Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet' by Langille et al.', Anonymous Referee #2, 10 Jan 2020
- AC2: 'Author Comment', Jeffery Langille, 06 Mar 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Jeffery Langille on behalf of the Authors (06 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Mar 2020) by Farahnaz Khosrawi

RR by Anonymous Referee #2 (03 Apr 2020)

Suggestions for revision or reasons for rejection

Review of 'Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet' by Langille et al. submitted to Atm. Chem. Phys. (acp-2019-220)

The authors have submitted a reviewed version of their manuscript, much improved and better focused. I think that it can be published after a few minor revisions.

Minor corrections:

L35: Please, include a reference to the description of the SHOW instrument

L37-41: It is correct to mention here that the goal is to study the event; however, most of this text is a statement on the results that should be moved to the 'Discussions and Conclusions'

L43-38: Respectfully, the text in these lines shows careless exposition and writing. No doubt, with 'drop' you want to say that the extratropical tropopause is at an altitude lower than the tropical one. However, saying that it 'drops' is not of a proper explanation about climatological characteristics and the physical mechanisms that drive to such behaviour (that have nothing to do with a 'drop')

L51-52: As it is written right now, it could be understood that wave breaking is the only mechanism associated with it. Please, modify the text to make clear that it is part of the existing possibilities "...is associated (among others) with Rossby-wave breaking and large-scale poleward transport." For example, a nice addition to frame the topic here could be to mention that the increase of vertical baroclinicity is also associated with double-tropopauses. This phenomenon was observed by Castanheira et al. (https://doi.org/10.5194/acp-9-9143-2009) using normal modes (therefore filtering Rossby waves), and it is a well-known impact of global climate change.

L52: signature?

L56-65: most of this information is repeated later in Section 2. I suggest to include here only a simple comment on the use of SHOW and refer the reader to such section.

L72: Absolute values are not too informative. I understand that this refers to previous work, but if possible, I would suggest adding relative errors or percentages of bias compared to the values measured by the radiosondes.

L85-90: I support the view of the authors of avoiding the inclusion of unnecessary discussions in the Introduction. I prefer it too. However, this paper is not so long to consider it unreadable. It is seventeen pages long in its current form, including abstract, twenty-nine references (three pages) and another seven pages for figures. This results in roughly six pages for the Introduction, Methods, Results and Discussion. That said, I do not find a good reason to avoid including relevant information that is necessary from the formal point of view. For example, the paper does not have a 'Data availability' section (mandatory in many journals) and in the lines here referred there is no information about the source for the ERA-5 or AURA-MLS data. It must be said which is the source for the data: Is it the ERA5 repository in the ECMWF? A local copy at the University of Saskatchewan or NCAR? Was the dataset retrieved through the Internet? If yes, when was it last accessed?.
Also, it would be desirable to include a Zenodo repository with the data files containing the SHOW measurements used in this work. Do not get me wrong; this information is necessary to assure independent reproducibility of the work. Therefore, I recommend the authors to take into account at least some of these recommendations. They will improve the manuscript.

L92: ...is a spatial heterodyne

L115: Langille et al. (2019)

L121: 1 hPa to 1000 hPa

L133: Kunz et al. say that the typical PV values for the tropopause range between 1.5 and 5 PVU and in Fig. 3 the transition values highlighted are 6 and 8 PVU. Moreover, it seems hard that the transition is located along such isolines. I understand that the authors have not performed specific computations for the corresponding tropopause-PV values in this case, Right? This should be acknowledged in the text, saying that the used values are an informed guess.

L141: The WMO (1992) reference is not in the list.

Figure 5. In the caption, 'SHOW' should be capitalized. Also, it should not appear a blank space between the degree symbol and the letter for the cardinal point.

Figure 6. In the caption 'light grey' corresponds to PV, and zonal wind is in black.

L244: HIRDLS

L251: PVU

L261: ' ...which is a sign of irreversible transport.'

Figures 6B and 7C: You use ERA5, so change ECMWF by ERA5 in the titles. The ECMWF has many reanalysis products, and as it is now, it is not clear enough what you mean. Readers could find it confusing.

L327: Langille et al. (2018)

L340: 2018)

The citation style of this journal uses parenthesis, not brackets.

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ED: Publish subject to minor revisions (review by editor) (03 Apr 2020) by Farahnaz Khosrawi

AR by Jeffery Langille on behalf of the Authors (07 Apr 2020) Author's response Manuscript

ED: Publish subject to technical corrections (09 Apr 2020) by Farahnaz Khosrawi

Dear authors,

I am pleased to inform you that your paper can be published after correction of the following minor technical issues:

P1, L25: Reference de Forster and Shine. On the first occasion "shine" should be changed to "Shine"

P1, L31: The reference of Högberg et al. is not really correct here. The Högberg et al. is on water avpour isotopologues and does not show if I remember it correctly any time series. The correct reference on water vapour time series would actually my 2018 paper:

Khosrawi, F., Lossow, S., Stiller, G. P., Rosenlof, K. H., Urban, J., Burrows, J. P., Damadeo, R. P., Eriksson, P., García-Comas, M., Gille, J. C., Kasai, Y., Kiefer, M., Nedoluha, G. E., Noël, S., Raspollini, P., Read, W. G., Rozanov, A., Sioris, C. E., Walker, K. A., and Weigel, K.: The SPARC water vapour assessment II: comparison of stratospheric and lower mesospheric water vapour time series observed from satellites, Atmos. Meas. Tech., 11, 4435–4463, https://doi.org/10.5194/amt-11-4435-2018, 2018.

or if you also would like to mention the water isotopologues I would say the latest WAVAS-II paper would be the better reference:

Lossow, S., Högberg, C., Khosrawi, F., Stiller, G. P., Bauer, R., Walker, K. A., Kellmann, S., Linden, A., Kiefer, M., Glatthor, N., von Clarmann, T., Murtagh, D. P., Steinwagner, J., Röckmann, T., and Eichinger, R.: A reassessment of the discrepancies in the annual variation of δD-H2O in the tropical lower stratosphere between the MIPAS and ACE-FTS satellite data sets, Atmos. Meas. Tech., 13, 287–308, https://doi.org/10.5194/amt-13-287-2020, 2020.

P8, L196: doesn't -> does not.

Happy Easter and saty healthy!

Best wishes, Farahnaz Khosrawi

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Short summary

Water vapour (WV) is a highly variable and extremely important trace gas in Earth’s atmosphere. Due to its radiative and chemical properties, it is coupled to the climate in an extremely complex manner. This is especially true in the lowermost stratosphere (LMS). Despite its importance, the physical processes that control mixing and the distribution of WV in the LMS are poorly understood. This study provides observational evidence of moistening the LMS via mixing across the subtropical jet.