Arctic clouds in ECHAM6 and their sensitivity to cloud microphysics and surface fluxes

Kretzschmar, Jan; Salzmann, Marc; Mülmenstädt, Johannes; Quaas, Johannes

doi:https://doi.org/10.5194/acp-19-10571-2019

Articles | Volume 19, issue 16

https://doi.org/10.5194/acp-19-10571-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-19-10571-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 16

Research article

|

21 Aug 2019

Research article |

| 21 Aug 2019

Arctic clouds in ECHAM6 and their sensitivity to cloud microphysics and surface fluxes

Jan Kretzschmar, Marc Salzmann, Johannes Mülmenstädt, and Johannes Quaas

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Final revised paper (published on 21 Aug 2019)
Supplement to the final revised paper
Preprint (discussion started on 12 Nov 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review on Kretzschmar et al.', Anonymous Referee #1, 10 Dec 2018
- AC1: 'Response to RC1', Jan Kretzschmar, 29 Mar 2019
RC2: 'Review for "Arctic cloud cover bias in ECHAM6 and its sensitivity to cloud microphysics and surface fluxes" by Kretzschmar et al.', Anonymous Referee #2, 04 Jan 2019
- AC2: 'Response to RC2', Jan Kretzschmar, 29 Mar 2019

Peer-review completion

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

AR by Jan Kretzschmar on behalf of the Authors (29 Mar 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Apr 2019) by Jui-Yuan Christine Chiu

RR by Anonymous Referee #1 (16 Apr 2019)

RR by Anonymous Referee #2 (07 May 2019)

Suggestions for revision or reasons for rejection

I thank the authors for responding to my comments. In my opinion, the manuscript has improved, however, unfortunately, some of the analyses still do not support the conclusions and are rather speculative. I also do not see how tuning of WBF or surface heat fluxes result in overall improvement of the model or that it is physically justifiable. Many of my major original concerns regarding the manuscript still remain — in particular, I am still not convinced that the Arctic low-cloud bias exists in the first place without the further analysis I suggested in the first round of revisions. The cloud phase comparison is also not meaningful as the scales are different, and the new figure only further strengthens my concern regarding the impact of the WBF tuning on cloud phase partitioning. More details follow below.

* The authors are missing the point of my first comment — I am aware that the authors used the COSP simulator, but my point is that since the focus of this study is on low-level clouds, the lowest 1-2 km of Arctic clouds will not be visible by GOCCP or the model with the COSP simulator on, whereas these clouds may exist according to ground-based observations (see the references listed in my original comment). Just because the COSP or GOCCP cannot “see” those low-clouds, does not mean that they are not actually there. Again, nothing prevents the authors from analyzing collocated Arctic ground-based observations to get a sense of the extent to which the claimed underestimate of low-cloud fraction actually holds, which was part of my original recommendation. At this point, I’m still not convinced that this high bias in cloud cover exists in the first place.

* I would recommend that the authors exclude the year 2007 and include another year (2011) to avoid the issue of the change in nadir viewing angle. That way, the comparison between model and observations would at least be more consistent.

* Comparison between Figure 6 and the figure from Korolev et al. (2017) is not meaningful because the latter observations were spatially averaged over 100 m, while the GCM grid box used in the manuscript has a horizontal resolution that is orders of magnitude larger. In fact, the authors seem to acknowledge the difference in spatial scales. A comparison with satellite observations would therefore be more of an apples to apples comparison. Furthermore, the plot over Siberia only adds to my initial concern that the WBF tuning exacerbates the phase partitioning problem by increasing ice fraction to values close to 1.0 between temperatures -5C and -35C for the lower threshold value plotted.

* I see the logic of the authors in comparing profiles of temperature and relative humidity to the model to support their claim of the low-cloud overestimate, but because there is no clean causal relationship that can be teased apart, this additional piece of information is too speculative and not very useful in my opinion.

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ED: Reconsider after major revisions (24 May 2019) by Jui-Yuan Christine Chiu

Dear Jan,

Thank you very much for your revised manuscript. I've received comments from two reviewers. As you can see, one reviewer has the same concerns that were raised in the first round of the review process, and he/she feels that those concerns have not been addressed properly.

After reading your manuscript carefully, I think that reviewer#2's comments are valid and important to address.

Firstly, I agree that it is important to demonstrate that there is indeed a model bias in low-cloud cover and liquid cloud cover. I appreciate your new figure, but unfortunately, I don't quite see your logic. The newly-added arguments (including the comparison in temperature and humidity), as mentioned in the reviewer's 4th bullet point, don't provide watertight evidence to support the existence of cloud cover biases. It looks like you have done some analyses using ground-based measurements. Could you please explain what prevents you from doing a more direct comparison here? Also, are you aware of the work presented in Forbes and Ahlgrimm (MWR, 2014)? They used DARDAR, which can help alleviate the limitation in Calipso. In short, since the main goal of the manuscript is to reduce the cloud cover bias, I feel Reviewer#2's 1st bullet point must be properly addressed.

Secondly, regarding the data time periods used in your analysis, I understand the reviewer's concern. This may have been taken care of in your GOCCP dataset, but it is better to double check with the data provider.

Thirdly, while I appreciate a lot of work has been done in this manuscript and I believe your work will be valuable for the community, I do worry that these model developments have been applied to other models. It is unclear to me what is new in terms of our scientific understanding. To fit the scope of ACP, the new part needs to be shown more clearly to readers.

Fourthly, some bits of writing are confusing and require more precise wording. For example, when mentioning "difference" in figure captions, please define it clearly. Also, Page7, line 5-10. When you say "constant" throughout the manuscript, do you sometimes mean "unchanged"? Page 11, first paragraph about Fig. 7. Could you please also check your Fig. 7? Are they supposed to be consistent to Fig. 5? The numbers don't seem to match to me.

Finally, please incorporate Reviewer1's comments.

Please feel free to let me know if you have any question. I look forward to receiving your revised manuscript!

Kind Regards,
Christine Chiu

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AR by Jan Kretzschmar on behalf of the Authors (11 Jul 2019) Author's response Manuscript

ED: Publish as is (27 Jul 2019) by Jui-Yuan Christine Chiu

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Short summary

This study aims to explore Arctic cloud properties in the atmospheric circulation model ECHAM6. We compare cloud properties in the model to satellite observations using a satellite simulator and show that ECHAM6 overestimates low-level liquid-containing clouds. In sensitivity studies, we show that this bias can be related to cloud microphysics and surface fluxes.