Water vapour and methane coupling in the stratosphere observed using SCIAMACHY solar occultation measurements

Noël, Stefan; Weigel, Katja; Bramstedt, Klaus; Rozanov, Alexei; Weber, Mark; Bovensmann, Heinrich; Burrows, John P.

doi:https://doi.org/10.5194/acp-18-4463-2018

Articles | Volume 18, issue 7

https://doi.org/10.5194/acp-18-4463-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-18-4463-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 7

Research article

|

03 Apr 2018

Research article |

| 03 Apr 2018

Water vapour and methane coupling in the stratosphere observed using SCIAMACHY solar occultation measurements

Stefan Noël, Katja Weigel, Klaus Bramstedt, Alexei Rozanov, Mark Weber, Heinrich Bovensmann, and John P. Burrows

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Final revised paper (published on 03 Apr 2018)
Preprint (discussion started on 24 Oct 2017)

Interactive discussion

Status: closed

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

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RC1: 'Formal Review by Referee', Anonymous Referee #1, 03 Nov 2017
- AC1: 'Authors' Reply to Referree #1', Stefan Noël, 19 Dec 2017
RC2: 'review of ACP-2017-893', Anonymous Referee #3, 20 Nov 2017
- AC2: 'Authors' Reply to Referree #3', Stefan Noël, 19 Dec 2017
RC3: 'Formal review', Anonymous Referee #2, 21 Nov 2017
- AC3: 'Authors' Reply to Referree #2', Stefan Noël, 19 Dec 2017

Peer-review completion

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

AR by Stefan Noël on behalf of the Authors (20 Dec 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Dec 2017) by Martin Dameris

RR by Anonymous Referee #2 (22 Dec 2017)

Suggestions for revision or reasons for rejection

Second review: Water Vapour and Methane Coupling in the Stratosphere observed with SCIAMACHY Solar Occultation Measurements by Stefan Noël, Katja Weigel, Klaus Bramstedt, Alexei Rozanov, Mark Weber, Heinrich Bovensmann, and John P. Burrows, submitted to ACP

General overview: I don’t feel the authors have adequately responded to the reviewer’s comments. Presenting the new data set and validation would make a fine paper, the added science related to trends and variability has flaws, detailed comments are given below.

Detailed comments:
The abstract states: “A significant positive water vapour trend for the time 2003–2011 is observed at lower stratospheric altitudes with a value of about 0.015 +/-0.008 ppmv year around 17km. “ As noted in my previous review, the time series under consideration is not long enough to talk about trends. If you look at Figure 7 and Figure 6 and Figure 8, it is clear that the end point in the lower stratosphere is a large positive anomaly (see 17 and 25 km), so that any so-called trend calculated is really a consequence of the endpoints used. If data collection had continued for a few more years, the trend wouldn’t be there. If your endpoint was in 2010, you would calculate the opposite trend. You may also have trends induced by the variation in latitude in the time series. From Figure 6, it looks like you start with a latitude around 50N, and ends at 60N; that will induce an age change, which may reflect different entry conditions.

Abstract also says “Above about 20km most of the water vapour is attributed to the oxidation of methane.” What you should say is that, above 20 km, most of the addition of water vapour is due to methane oxidation. Parcels enter the stratosphere with a water vapour mixing ratio between 3 and 6 ppmv. Methane enters with something around 2 ppmv. If all methane were oxidized, that would give 4 ppmv additional, but it isn’t all oxidzed. Your figure 3 shows a peak stratospheric value of about 7 ppmv. Therefore, you may be able to get away with saying that above 20 km, up to have of the water vapour present can be attributed to methane oxidation.

I also don’t think this statement is correct “Further, the increasing methane input into the stratosphere due to the rise of tropospheric methane after 2007 may have contributed to the increased water vapour in the extratropical lower stratosphere as observed by SCIAMACHY.” What is the “lower stratosphere”? Age of air in the extratropical stratosphere (if we’re thinking below 70 mb) is on the order of months. Very little methane oxidation has occurred there (you have a small contribution due to air that has descended from high up in the upper branch of the BDC), so there can’t be much measurable increase in water due to that. You need to look to variations in tropical temperature to understand lower stratospheric water vapour variability.

Introduction: you should just delete the first 3 sentences, they add nothing to this paper. Start with “Water vapour and methane play an important role in the chemistry of the stratophere.”

Page 2, paragraph starting with “The vast majority of….” Delete the first 2 sentences of this paragraph, they are not relevant to the paper. And change “Water vapour enters…” to “The stratospheric entry value of water vapour is set by processes in the tropical tropopause layer (TTL). Also, the statement about the hygropause is not entirely correct. In the tropics, the level of minimum water varies with season (that is the whole point of the tropical tape recorder papers by Mote et al.) Sometimes it is 2 km above the tropopause in the tropics. In the mid latitudes, the level of the hygropause is a function of horizontal transport from the tropical cold point, so it will be elevated relative to the extratroipcal troppause.

You don’t need figure 1 in this paper.

Paragraph starting with line 16: The amount of methane is going to vary with how the tropospheric burden varies. Water vapour will not; it will vary due to tropical tropopause temperature changes, and possible changes in monsoon circulations, mixing in from midlatitudes, convection, microphysics, etc. This sentence “The Brewer-Dobson circulation controls the tropical upwelling.” is also not quite valid. The BDC may reflect tropical upwelling, it doesn’t control it. Note, the description of the driving of the BDC is valid for the upper branch, it’s a much more complicated situation for the lower branch. Also note, the TIL is really irrelevant here, just delete that sentence.

Line 30, I would rewrite to say “Water vapour production in the stratosphere is largely a consequence of methane oxidation.

Page 3: discussion of potential water: the authors should note that 2CH4+H2O is effectively conserved following a stratospheric parcel, not in the stratosphere. For example, if you look at distributions from an Eulerian rather than a Lagrangian standpoint, you’ll see variations related to variations in water vapour entry…ie, the seasonal cycle tape recorder, variations related to the QBO impacting lower stratospheric temperatures and mixing.

Page 3, line 6…delete “probably”

Page 3, line 9…Age of air is a function of altitude and latitude and season. 2 years at 15 km, even at mid latitudes is too long. Note recent work by Ray et al. (http://onlinelibrary.wiley.com/doi/10.1002/2016JD026198/abstract) detailing mesospheric sinks of SF6 that can contaminate polar age of air estimates.

Page 3, line 12. This statement is wrong “However, the mixing of air masses during transport does not affect the total hydrogen balance such that potential water should still be conserved.” That depends on whether you are mixing air masses with the same value of 2CH4+H2O. Consider at the edge of the subtropical, near 400K in January. The tropical 2CH4+H2O will be at an annual minimum. On the poleward side of the jet, you’ll have air that entered a few months earlier with warmer tropical tropopause temperatures, and that value of 2CH4+H2O will be larger. Mixing of those will not “conserved” potential water, but it will conserve the sum of the potential water.

Page 3 line 16. The paper states “Ideally, both water vapour and methane should be retrieved from measurements by the same instrument.” I see no reason why this is true. You want both water vapour and methane to be accurate, and co-located, but they don’t have to be from the same instrument.

Figure 7 caption…make it clear that you are looking at varying latitudes in this plot (perhaps refer back to latitude plot in figure 6)

Figure 7: Does the satellite coverage give the same latitude for the same day in the year for each year? If it doesn’t, I’m not sure what the anomalies actually mean.

Page 11, line 9, the age of air at 30 km is only calculated to be 8 years if you use the MIPAS SF6 derived ages. If you look at the Haenel paper plots of balloon derived ages, they are significantly less. Also, the larger than a 2-year shift doesn’t make sense. Any variations you’re seeing that are QBO related in the high latitudes are largely going to be dynamical (and conservations of mass related). There could possibly be some entry value related signal due to QBO induced tropical tropopause temperature changes, but those should be fairly small, and mixed out by the time air has reached an age in the range of 4-8 years. I recommend deleting this whole discussion.

Page 12: potential water discussion: 2CH4+H2O will be conserved in a Lagrangian sense, not a Eulerian sense. Your measurements are effectively Eulerian, so I’m not convince this discussion is meaningful.

Page 13, trends section. The time series isn’t long enough to do trend analysis, and the variations of sampling with latitudes make this really complicated, and that isn’t properly acknowledged. I recommend deleting this entire section. In particular, potential water is conserved in a parcel, thereby giving a potential water trend of zero. But, this data set isn’t looking at parcels, so the whole discussion doesn’t make sense. I agree that water vapor is only produced in the stratosphere by methane oxidation, but I don’t think this analysis proves that.

Discussion…methane, QBO, and BDC are not the only factors impacting stratospheric water vapour. ENSO has been shown to play a role, SSWs can play a role, I’m not sure what the authors mean by “varying tropical tropospheric water vapour”, but I would expect that would only play a role if you’ve changed the fraction of air that bypasses the cold point (via overshoots), or that has actually changed the tropical tropopause temperature radiatively.

Last paragraph on page 15: I think this needs to be stated more clearly. What do you mean by “water vapour changes”? Is this in a Lagrangian sense (ie, the only way you change water vapour in a parcel is via production due to methane oxidation…and effectively that is correct if you’re ignoring molecular hydrogen), or are you considering variation at a given location (in an Eulerian sense), in that if water increases, you’re looking at air that is effectively older, so it has a larger amount of water (and lower amount of methane.) I also don’t think the statement “Above 20 km , in the region of the deep branch of the Brewer-Dobson circulation, air is older. This enables oxidation of methane to water vapour to be completed rapidly.” is correct. It is not the fact that the air is older that allows the methane oxidation, it’s a function of the lifetime relative to altituden (see LeTexier et al, QJRMS fig 1, and it should probably be 30 km instead of 20 km).

First paragraph page 16, delete this paragraph, you don’t have the length of time series to be considering 5-6 year oscillations.

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RR by Anonymous Referee #3 (16 Jan 2018)

ED: Reconsider after major revisions (25 Jan 2018) by Martin Dameris

AR by Stefan Noël on behalf of the Authors (27 Feb 2018) Author's response Manuscript

ED: Publish as is (05 Mar 2018) by Martin Dameris

AR by Stefan Noël on behalf of the Authors (05 Mar 2018)

Short summary

The combined analysis of stratospheric methane and water vapour data derived from SCIAMACHY solar occultation measurements shows the expected anti-correlation and a clear temporal variation related to waves in equatorial zonal winds. Above about 20 km most of the additional water vapour is attributed to the oxidation of methane. The SCIAMACHY data confirm that at lower altitudes water vapour and methane are transported from the tropics to higher latitudes.