Thermodynamic and dynamic responses of the hydrological cycle to solar dimming

Smyth, Jane E.; Russotto, Rick D.; Storelvmo, Trude

doi:https://doi.org/10.5194/acp-17-6439-2017

Articles | Volume 17, issue 10

https://doi.org/10.5194/acp-17-6439-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The Geoengineering Model Intercomparison Project (GeoMIP):...

https://doi.org/10.5194/acp-17-6439-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 10

Research article

|

30 May 2017

Research article |

| 30 May 2017

Thermodynamic and dynamic responses of the hydrological cycle to solar dimming

Jane E. Smyth, Rick D. Russotto, and Trude Storelvmo

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Final revised paper (published on 30 May 2017)
Preprint (discussion started on 07 Oct 2016)

Interactive discussion

Status: closed

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

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RC1: 'Review of Smyth et al.', Anonymous Referee #1, 19 Oct 2016
RC2: 'Nothing new, and poorly diagnosed', Anonymous Referee #2, 30 Oct 2016
AC1: 'AR by Jane E. Smyth on behalf of the Authors (13 Jan. 2017)', Jane Smyth, 13 Jan 2017

Peer-review completion

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

AR by Jane Smyth on behalf of the Authors (13 Jan 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Jan 2017) by Ulrike Lohmann

RR by Anonymous Referee #1 (23 Jan 2017)

RR by Anonymous Referee #2 (29 Jan 2017)

Suggestions for revision or reasons for rejection

I recommend this paper be rejected. It does not present interesting new science, and there are still flaws in the figures. The analysis of differences in climate change simulations in which the global average temperature does not change, with a scaling that depends on temperature differences, does not make sense to me. I do not understand why that part is in the paper.

I am very annoyed that the authors did not respond to one of the items in my previous review. The maps are still not plotted correctly. The longitude labels are still in the wrong place. And there is a border at the top, bottom and right edge of the maps with no shading. This gives me no confidence that the results are plotted correctly. You don’t have to use GrADS, which would not have this problem, but there are many other graphics programs, such as NCL, ferret, and even Matlab that can do this. This refusal to fix this aspect of the paper alone makes me recommend to the Editor that this paper be rejected, and that if resubmitted the Editor makes sure the maps are of an acceptable quality.

Fig. 4 has no significance measures or error bars. How different from zero would the values have to be to merit consideration?

p. 4, last paragraph. No, the small differences between simple and extended scaling, and the large disagreement between them and the actual results, mean that this is not an appropriate way to analyze the results. First of all, you need statistical tests to show how different the scalings need to be from each other to even deserve consideration. To say that relative humidity (RH) plays a modest role is incorrect, and certainly should not be in the abstract. What is correct is that you cannot tell how important RH is.

In various places in the paper, the authors say data are not available. But did they write to the modelers to obtain the data? Just because they are not posted to the websites they looked at does not mean they do not exist. In my experience, modelers are happy to send data in response to a request.

If the authors are going to analyze RH and ITCZ location, and their changes with geoengineering, it is incumbent on them first to analyze the piControl runs to see if the models do a good job of simulating these in the first place. If not, then how can we trust small changes. It is traditional to through out models in such a comparison if their current climate differs quite a bit from observations, not because you were not able to get the model output.

The ITCZ shifts found here are very small (<1°) and completely expected given the N-S temperature change differences. Since the models differ so much in their simulation of the ITCZ, this does not seem an important result.

For the seasonal analysis, why did the authors choose the unconventional JFM and JAS or the seasons rather than the more traditional DJF and JJA? Without any special reason this was the wrong decision and prevents comparison with the results of others.

There are 15 more comments in the attached annotated manuscript that would need to be addressed.

Referee Report: PDF

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RR by Anonymous Referee #3 (16 Feb 2017)

Suggestions for revision or reasons for rejection

"Thermodynamic and dynamic responses of the hydrological cycle to solar dimming" by Smyth et al addresses the important question of how geoengineering schemes, designed to regulate global surface temperature, affect the hydrological cycle. I find this paper to be particularly interesting because when we discuss P-E changes the focus is often on the thermodynamic component. However in this case, by construction, the thermodynamic component of changes in P-E is close to zero and the dynamics dominates. If we are to better predict future changes in the hydrological cycle then we need to understand both the thermodynamic and dynamic components, and this paper is a valuable contribution to understanding the latter component. I recommend the manuscript be accepted pending minor revisions.

Specific comments:

1. Derivation of Eqn (2): The authors should be careful when describing the assumptions that go into this equation - the derivation assumes small meridional AND zonal gradients of CHANGES in temperature, and further assumes that the vertically-integrated atmospheric moisture scales with the near-surface specific humidity (this assumption is not currently mentioned in the manuscript). The text should be modified to state these assumptions more carefully and in full.

2. Page 3, lines 27-29: The difference between simulated P-E anomalies and the extended scaling does not perfectly isolate the role of dynamics as it also includes the horizontal temperature and RH gradient terms [see the last two terms on the RHS of Eqn (7) in Byrne & O'Gorman (2015)]. These terms are particularly important over land regions, and can also be important at high latitudes over oceans (because of polar-amplified warming). This is an issue for the paper as interpreting the dynamic component of changes in P-E is difficult because it currently includes these gradient terms. I suggest the authors calculate the full extended scaling of Byrne & O'Gorman (2015) and then compare that to the simulated d(P-E) to truly isolate the dynamic component (which can also be calculated explicitly). Calculating the gradient terms may also gives insights into how large RH changes over land influence P-E.

3. Page 4, lines 26-27: Byrne & O'Gorman (2015) also found, in global warming simulations, that local changes in RH do not affect P-E - it might be worth connecting to that result here.

4. Page 5, lines 10-17: A recent paper focusing on land relative changes in a range of climate models might be useful for the discussion here: Byrne & O'Gorman (2016): "Understanding Decreases in Land Relative Humidity with Global Warming: Conceptual Model and GCM Simulations", J. Climate

5. Page 6, lines 7-10: Calculating the impact on P-E of gradients in RH changes will allow you to check explicitly whether these land-surface affects are important for hydrological cycle changes under solar dimming conditions.

6. Section 2.3: It is interesting that you find narrowing tendencies for the ITCZ in these simulations. The physical processes causing changes in the width of the ITCZ under global warming have received some attention in recent years [e.g., Byrne & Schneider (2016a,b) -> see https://climatedynamics.ethz.ch/people/mike/publications.html for copies of these papers). It would be good to connect with this evolving literature when discussing the changes you see in ITCZ - would also be really cool to use the analytical framework of Byrne & Schneider to diagnose what processes in these solar dimming simulations are driving the ITCZ width changes! This is beyond the scope of the current article but could be an interesting avenue to explore in the future.

7. Page 9, lines 14-16: I think it is difficult to be confidant in this statement without calculating how gradients of changes in RH affect P-E over land - they may be an important influence compared to just the local RH changes [as is the case in Byrne & O'Gorman (2015)].

8. Page 8, lines 27-29: A damped seasonal ITCZ migration is one possible explanation for reduced monsoon precipitation but there are several others (e.g., reduced monsoon circulation strength, zonal shifts in the monsoon, reduced moisture content). Without further analysis I think the authors should remove this statement (and a similar statement in the Conclusions).

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RR by Anonymous Referee #4 (22 Feb 2017)

ED: Reconsider after major revisions (26 Feb 2017) by Ulrike Lohmann

AR by Jane Smyth on behalf of the Authors (16 Apr 2017) Author's response Manuscript

ED: Publish as is (22 Apr 2017) by Ulrike Lohmann

AR by Jane Smyth on behalf of the Authors (22 Apr 2017) Author's response Manuscript

Short summary

Geoengineering is a controversial proposal to counteract global warming by reducing the incoming solar radiation. Solar dimming could restore preindustrial temperatures, but global rainfall patterns would be altered. We analyze the global rainfall changes in 11 climate model simulations of solar dimming to better understand the underlying processes. We conclude that tropical precipitation would be substantially altered, in part due to changes in the large-scale atmospheric circulation.