Cloud Feedbacks from CanESM2 to CanESM5.0 and their influence on climate sensitivity

Virgin, John G.; Fletcher, Christopher G.; Cole, Jason N. S.; von Salzen, Knut; Mitovski, Toni

doi:https://doi.org/10.5194/gmd-14-5355-2021

Articles | Volume 14, issue 9

https://doi.org/10.5194/gmd-14-5355-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

The Canadian Earth System Model version 5

https://doi.org/10.5194/gmd-14-5355-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 9

Model evaluation paper

|

31 Aug 2021

Model evaluation paper |

| 31 Aug 2021

Cloud Feedbacks from CanESM2 to CanESM5.0 and their influence on climate sensitivity

John G. Virgin, Christopher G. Fletcher, Jason N. S. Cole, Knut von Salzen, and Toni Mitovski

Download

Final revised paper (published on 31 Aug 2021)
Preprint (discussion started on 08 Mar 2021)

Interactive discussion

Status: closed

RC1: 'Comment on gmd-2021-11', Anonymous Referee #1, 31 Mar 2021

This paper analyses the changes in radiative feedbacks between CanESM2 and CanESM5.0. The authors show that cloud feedbacks are the main driver for the change in feedbacks. Then, they decompose the cloud feedback into components (fraction, optical depth, altitude) to investigate the main drivers for the changes. Given that CanESM5 is the model with the largest effective climate sensitivity of the CMIP6 ensemble, and that it shows a large change with respect to CanESM2, the topic of this paper is very relevant for the climate community. The results are important because they add to the recent body of literature on the reasons for the increase in EffCS between CMIP5 and CMIP6 models. The paper is logically structured, clearly written, and the conclusions are supported by a clear methodology. I recommend publication after minor revision.

Perhaps my main criticism is the explanation of differences in SW low cloud feedback between amip-p4K and amip-Future (Figure 6, L271-280). I the discussion speculative and not very convincing, as one cannot clearly see how the population of feedback and pattern anomalies is distributed. I suggest changing how this is presented and explained.

SPECIFIC COMMENTS

Figure A3: given that this figure is referenced before Figure 2, I'd recommend using the full figure caption, not "As in Figure 2".

Figure A2: This figure is referenced after Figure A3, I suggest swapping the order.

Figures: Please increase text size of axes titles and tick labels.

L17: Can the Chaney models be considered ESMs?

L24-25: - "particularly with regards to properties such as cloud optical depth, which reflect shortwave (SW) radiation and cool the planet". This reads weird, clouds reflect radiation, cloud optical depth is a property that changes how much it is reflected.

L43: (CanESM2). (Flynn and Mauritsen, 2020). I'd suggest re-arranging this sentence to avoid model acronym and the citation being together.

L70-71: Strictly speaking, ECS doesn't need the definition of a CFP (time-varying or not). One can run a model to equilibrium (with a practical definition of "equilibrium") and calculate ECS without invoking a feedback parameter.

L165-167: optical thickness and emissivity are not the only properties, cloud fraction and top pressure/temperature can produce radiative feedbacks (as discussed later in the paper).

L169-174: Figure 3 -> Figure 2

L193-194: Perhaps worth noting that, at the same time, the SW feedback (especially for CanESM2) shows differences of similar magnitude in the opposite direction.

L195-198: I think the term 'emissivity' is wrongly used here (perhaps you mean emission). As explained in the second sentence, the altitude feedback is related to the temperature difference between cloud tops and the surface. That mechanism doesn't need a change in cloud emissivity.

L311: The new developments are included in HadGEM3-GC3.1.

Citation: https://doi.org/10.5194/gmd-2021-11-RC1
RC2: 'Comment on gmd-2021-11', Anonymous Referee #2, 13 Apr 2021

Please find the attached file.

Citation: https://doi.org/10.5194/gmd-2021-11-RC2
AC1: 'Comment on gmd-2021-11', John Virgin, 01 Jul 2021

Please see the attached file that has responses for both authors in line by line format.

Citation: https://doi.org/10.5194/gmd-2021-11-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by John Virgin on behalf of the Authors (01 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Jul 2021) by Axel Lauer

RR by Anonymous Referee #2 (23 Jul 2021)

ED: Publish subject to minor revisions (review by editor) (26 Jul 2021) by Axel Lauer

Dear authors,

Thank you for the revised version. Before accepting the manuscript for publication, I would like to ask you to consider the minor comments by reviewer #2 on the revised manuscript. Please see the reviwer's comments below.

Thank you,

Axel Lauer
(handling topical editor)

---

The manuscript is better organized and presented by including more analysis on the impact of surface warming pattern on cloud feedback. Most of my comments are addressed to my satisfaction and I recommend its publication after minor revision.

Minor:

Figure 5c and d: Why do you choose Years 130-150 to calculate EIS response rather than the full 150 years? Since you are studying the cloud feedback, why not showing the EIS response normalized by the global-mean surface temperature change?

I feel the abbreviation ‘CFP’ is slightly strange because it is never used in the community as far as I know. Why not keep it as the ‘cloud feedback parameter’ or \lambda?

Technical:

L58: ‘a updated’ -> ‘an updated’

L93: sstClim and sstClim4xCO2 are experiment names for CMIP5. For CMIP6, it should be piClim-control and piClim-4xCO2.

L151: keep consistent: ‘abrupt-4xCO2’ -> ‘abrupt-4xCO2’. Also in the caption of Figure 1.

L199: “a compensating differences” -> ‘compensating differences’

L257: “Figure 4a and c” → “Figure 3a and c”

L263-264: “it does exert a major not influence on“ -- sounds strange. What is ‘major not influence’?

L307: ‘as a cloud controlling factor’ -> ‘as a cloud controlling factor.’

L332: “and a produces” → “and produces”

L359: “CanESM2-abrupt-4xCO2” → “CanESM2 abrupt-4xCO2”

L362: What is the “said pattern”?

L363: “play” should be “plays”

Hide

AR by John Virgin on behalf of the Authors (26 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (27 Jul 2021) by Axel Lauer

AR by John Virgin on behalf of the Authors (03 Aug 2021) Author's response Manuscript

Short summary

Equilibrium climate sensitivity, or the amount of warming the Earth would exhibit a result of a doubling of atmospheric CO₂, is a common metric used in assessments of climate models. Here, we compare climate sensitivity between two versions of the Canadian Earth System Model. We find the newest iteration of the model (version 5) to have higher climate sensitivity due to reductions in low-level clouds, which reflect radiation and cool the planet, as the surface warms.