Craig–Gordon model validation using stable isotope ratios in water vapor over the Southern Ocean

Dar, Shaakir Shabir; Ghosh, Prosenjit; Swaraj, Ankit; Kumar, Anil

doi:https://doi.org/10.5194/acp-20-11435-2020

Articles | Volume 20, issue 19

https://doi.org/10.5194/acp-20-11435-2020

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

https://doi.org/10.5194/acp-20-11435-2020

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

Articles | Volume 20, issue 19

Research article

|

06 Oct 2020

Research article |

| 06 Oct 2020

Craig–Gordon model validation using stable isotope ratios in water vapor over the Southern Ocean

Shaakir Shabir Dar, Prosenjit Ghosh, Ankit Swaraj, and Anil Kumar

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Final revised paper (published on 06 Oct 2020)
Supplement to the final revised paper
Preprint (discussion started on 16 Dec 2019)
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Interactive discussion

Status: closed

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

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RC1: 'review of: Craig-Gordon model validation using observed meteorological parameters and measured stable isotope ratios in water vapor over the Southern Ocean.', Anonymous Referee #1, 21 Feb 2020
- AC1: 'Response to RC1', Shaakir Dar, 28 May 2020
RC2: 'Review of Dar et. al. submission to ACP', Anonymous Referee #2, 02 Mar 2020
- AC3: 'Response to RC2', Shaakir Dar, 28 May 2020
AC2: 'Author Comments', Shaakir Dar, 28 May 2020

Peer-review completion

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

AR by Shaakir Dar on behalf of the Authors (24 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (06 Jul 2020) by Manvendra Krishna Dubey

RR by Anonymous Referee #1 (17 Jul 2020)

Suggestions for revision or reasons for rejection

The authors have appropriately responded in their letter to most of my main concerns and have taken adequate steps in the paper to respond to them.

However, there are a few points that need further action so that the readers will be better able to get full benefit of this interesting study.

One important point is that what is compared in the 'observation-model' comparison is not obvious as written. I understood that it was the result of Craig-Gordon (or its extended version) with the 'global closure assumption’ (at least, I thought that these were equations 7 and 18 which are used, and not 1 and 11). This should be specified, for example in the introduction on line 47 (and maybe in the captions for some of the corresponding figures).

I did not fully understand also what is done with Pfahl and Wernli's study. It is cited as if they were providing diffusivity coefficients. However, it is not what the 2009 paper is about. The values cited on line 136 are close to be the inverse of their non-equilibrium fractionation factors (a power-law m independent of wind speed, as discussed inPfahl and Wernli (2003) paper). The issue discussed in the paper is the dependence of the fractionation factor as a function of wind which is specific to each study (Merlivat and Jouzel (1979), Cappa et al (2003) and Pfahl and Wernli (2003). In this sense, discussing using Pfahl and Wernli is different from just a question of diffusivity coefficients, for which I am only aware of the two studies (Merlivat, 1978; Cappa et al., 2003). It is in itself a parameterization. How is this taken into account here?

What is variable 'x' is not really defined (cf lines 123 and 136: the definition of ‘x’). As there is no equation for x, it is hard to estimate quantitatively what is the index x. A definition equation should be introduced once, also better defining the index and how it is used.

The conclusion is rather short, and should be slightly expanded. I still think that one can mention there possible caveats (or modifications) to TCG or UCG models used for the evaporative flux. For example, sea spray formation and evaporation by very high winds are not taken into consideration. Same for condensation/deposition close to sea-ice at near-freezing temperatures (but probably only encountered at the most southern latitudes during these cruises). The advective mixing model is a very nice addition, nonetheless there can be other sources of advective humidity to the surface layers than from Antarctica (such as from upper atmospheric layers with different properties, in particular because of condensation/precipitation...) or reevaporation of rainfall. I am not saying by that these are necessarily important to take into consideration (after all, there still exists an average misfit wit hteh evaporation model both ind18O and d-excess on Figure 9, even north of 60°S). Also, the non-local source of the evaporation was discussed (with the back trajectories). This could be commented upon in the conclusion section (difference between local and 'non-local' sources.

Also, there were some statistical relations which hold better when separating the two cruises. Any idea why?

Detailed comments:
In the reply letter, it is mentioned that some samples are from bucket collection, other from CTD cast. For buckets, could there be possible biases (in S and d18O, both too high), but also the two samples (S and d18O) are not collected with the same bucket. This should mentioned in the S table caption (adding for example a * to the salinities not collected from same bucket as the sample for d18O analysis). Otherwise, there are some anomalies in d18O/S that cannot be understood. For example; the low value d18O near an iceberg is not associated with low- S , which I would have assumed for this season, and if there no refreezing along the iceberg at depth (in this case, is it the S-value from a CTD cast or from a bucket; and is there or not refreezing, which would be interesting per se).

At the end of the introduction, line 47, ‘... and different fractions of molecular vs turbulent diffusion.’ I think that it is important to add ‘in the framework of the global closure assumption’ (at least, I understood that these were equations 7 and 18 which are used, and not 1 and 11; this needs to be specified)
Line 96, suggestion to replace starting at ‘ABove the ocean one can assume...’ by ‘The global closer assumption is commonly done, by which ...’
Line 136: I would change citation to Pfahl and Wernli.
Fig. 1: add year and dates in caption of fig. 1
Fig. 4: the end of the caption is unclear, as well as the response in the response letter. There may be a need to specify more how the data are grouped based on the trajectories (is the source latitude three days before considered, for example?)
FIgure 10: interesting, but I find what is written along the horizontal axis hard to read. Also, always the same sign (except for slope middle pattern). Nothing cut? Any idea why?
Figure 11: why the choice of a blue column, and not just blue dots? This would be more consistent with earlier figures and also easier to visualize.

l. 151: remove ‘were’
l. 152: replace ‘like’ by ‘such as’
l. 167: remove ‘caused’
l. 170, and L. 187: SST instead of sst
l. 197: ‘that’ instead of ‘than’
l. 215: ‘d-excess of’ to be replaced by ‘and d-excess’
l. 217: ‘observations’ and later ‘links’
l. 225: ‘that predict’
l. 229: end of sentence ‘are considered’?
l. 234: ‘... is insufficient...’
l. 235: remove ‘The process like’ and start sentence by ‘Advective mixcing...’
l. 253: replace ‘the contribution of which’ by ‘its contribution....’ or something equivalent

Suppl. Material:
for humidity, sling psychrometer used. What is the accuracy expected for its reading?
SST from bucket thermometer. The authors mention that it is accurate to 0.2°C? How is the bucket collected and its temperature read when there is high wind (more risk of cooling... evaporation?) I dont think that it will be as acurate with winds of 25 m/s or more that were sometimes encountered.
In table 4, the dates starting in the middle of the table invert month and day. There is also an incorrect date one line before the end.

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ED: Publish subject to minor revisions (review by editor) (17 Jul 2020) by Manvendra Krishna Dubey

AR by Shaakir Dar on behalf of the Authors (27 Jul 2020) Author's response Manuscript

ED: Publish as is (07 Aug 2020) by Manvendra Krishna Dubey

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