On the representation of aerosol activation and its influence on model-derived estimates of the aerosol indirect effect

Rothenberg, Daniel; Avramov, Alexander; Wang, Chien

doi:https://doi.org/10.5194/acp-18-7961-2018

Articles | Volume 18, issue 11

https://doi.org/10.5194/acp-18-7961-2018

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

https://doi.org/10.5194/acp-18-7961-2018

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

Articles | Volume 18, issue 11

Research article

|

06 Jun 2018

Research article |

| 06 Jun 2018

On the representation of aerosol activation and its influence on model-derived estimates of the aerosol indirect effect

Daniel Rothenberg, Alexander Avramov, and Chien Wang

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Final revised paper (published on 06 Jun 2018)
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Preprint (discussion started on 04 Aug 2017)
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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 Rothenberg et al.', Anonymous Referee #1, 08 Oct 2017
- AC1: 'Reply to Reviewer #1', Daniel Rothenberg, 04 Jan 2018
RC2: 'Review of ACP-2017-680: On the representation of aerosol activation and its influence on model-derived estimates of the aerosol indirect effect', Anonymous Referee #2, 30 Nov 2017
- AC2: 'Reply to Reviewer #2', Daniel Rothenberg, 04 Jan 2018

Peer-review completion

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

AR by Daniel Rothenberg on behalf of the Authors (04 Jan 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Jan 2018) by Graham Feingold

RR by Anonymous Referee #2 (29 Mar 2018)

Suggestions for revision or reasons for rejection

My apologies for sending this back for another iteration, but I don't think the authors quite understood the gist of my previous comments.

The key thing I was trying to highlight is that even though this study uses the same driving model to test the sensitivity of a variety of activation schemes, the response of the model may lead to different meteorology - which then may swamp the signal from the details of a particular activation scheme. Your Figure 2 in your response is a case in point: although the convective precipitation over the eastern Pacific doesn't appear to change much between runs, I would suggest that the convective activity over the Maritime Continent and Indian Ocean does see a marked change for those schemes that see an increased optical depth in the tropics in figure 5. This increased optical depth is then likely due to the change in frequency/intensity of the convection scheme rather than a local aerosol effect. (One could also hypothesize that a teleconnection may suppress some ascent in the Eastern Pacific and lead to the increased CDNC as seen in Figure 4.)

Having re-read the paper several times, I don't think this a major concern for the key results of PI versus PD emissions scenarios (primarily because of the hemispheric dominance of the signal). However, statements such as that in the abstract that '...differences in the activation sensitivity lead to a spread of over 0.8 Wm-2 in global average shortwave indirect effect...' are perhaps misleading when not given the context of any change to the model climatology/meteorology. This change may be a physically realistic interpretation and as such a valid (but very complex AIE), but it may also be an idiosyncrasy of the particular model formulation and, given a different convection scheme for example, this result may not be robust.

So what do I suggest the authors do in response to this criticism? The point is not that the paper should necessarily go into details on mechanisms such as that speculated on above, but to acknowledge and (briefly) discuss the fact that meteorology (and parametrization of that meteorology) is a key component in understanding the role of aerosol activation schemes and the sensitivities thereof. One suggestion would be to add a short subsection in section 3 to outline this caveat. This could be with reference to your Figure 2 (along with equivalent plot for large scale precipitation or any other indicative evidence you may have) to show that the convective behaviour may have changed in those simulations that see significant changes in the optical depth in the tropics. Even better would be if you are able to use other diagnostics you have available to more decisively tie down a particular mechanism such as I speculate on above – but I'm not suggesting this latter aspect is necessary for acceptance of the manuscript.

Given this discussion, the abstract should then contextualize the resulting spread in AIE, e.g. '...differences in the activation sensitivity may lead different evolution of the model meteorology and ultimately to a spread of over 0.8 Wm-2 in global average shortwave indirect effect...'.

Typographical errors

Page 9, line 4: '...play online a minor role...' should be '...play only a minor role...'?

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ED: Publish subject to minor revisions (review by editor) (29 Mar 2018) by Graham Feingold

AR by Daniel Rothenberg on behalf of the Authors (13 Apr 2018) Author's response Manuscript

ED: Publish as is (01 May 2018) by Graham Feingold

AR by Daniel Rothenberg on behalf of the Authors (11 May 2018) Manuscript

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Short summary

Estimates of how much the particles we emit into the atmosphere cool the climate depend on how those particles influence the relative number of cloud droplets. Those estimates are strongly influenced by how many droplets a given climate model predicts under clean conditions, even more so than how much that human emissions increase droplet concentrations. Because of this, observations of particles influencing clouds in clean conditions could help constrain their climate-cooling potential.