Quantifying variations in shortwave aerosol–cloud–radiation interactions using local meteorology and cloud state constraints

Douglas, Alyson; L'Ecuyer, Tristan

doi:https://doi.org/10.5194/acp-19-6251-2019

Articles | Volume 19, issue 9

https://doi.org/10.5194/acp-19-6251-2019

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

https://doi.org/10.5194/acp-19-6251-2019

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

Articles | Volume 19, issue 9

Research article

| Highlight paper

|

13 May 2019

Research article | Highlight paper |

| 13 May 2019

Quantifying variations in shortwave aerosol–cloud–radiation interactions using local meteorology and cloud state constraints

Alyson Douglas and Tristan L'Ecuyer

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Final revised paper (published on 13 May 2019)
Preprint (discussion started on 23 Nov 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Understanding Aerosol-Cloud-Radiation Interactions Using Local Meteorology and Cloud State Constraints', Anonymous Referee #1, 04 Jan 2019
- AC1: 'Response to Reviewer One', Alyson Douglas, 19 Feb 2019
RC2: 'Review of Douglas and L'Ecuyer (2018)', Anonymous Referee #2, 07 Jan 2019
- AC2: 'Response to Reviewer 2', Alyson Douglas, 19 Feb 2019

Peer-review completion

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

AR by Alyson Douglas on behalf of the Authors (19 Feb 2019)

ED: Referee Nomination & Report Request started (28 Feb 2019) by Timothy Garrett

RR by Anonymous Referee #2 (06 Mar 2019)

RR by Anonymous Referee #1 (15 Mar 2019)

Suggestions for revision or reasons for rejection

General comment:
The authors improved significantly the manuscript. There are still major concerns regarding fundamental concepts as listed below.

Major comments:
1. LWP vs meteorology. I am still not convinced why LWP is not meteorology.
The authors write in their response to reviewer 1 that “While boundary layer depth determines the maximum cloud depth, there are variations in the LWP of warm boundary layer clouds. Decoupling, cloud breakup, and precipitation can alter the LWP of the cloud independent of the boundary layer height. We therefore wanted to account for these processes separately from the influences of the meteorology like stability and entrainment of free atmospheric air.” The mentioned processes can be also meteorology related: decoupling can occur due to entrainment and warm advection, cloud breakup can be related to enhanced entrainment and drying, mixing due to wind shear, etc. and precipitation was shown to related to cloud depth.
Furthermore, in section 2.4.2 page 7, line 9 the authors write “we consider the LWP separately from the local meteorology as it represents the cloud thermodynamics more than the local environmental conditions” - thermodynamics are derived from environmental conditions. Perhaps the term mircophysical processes fits better what the authors mean.
The LWP can indeed be changed due to aerosols, however the sing of the response varies between studies (e.g.: https://doi.org/10.5194/acp-2018-885).
In my opinion the authors decision that LWP is not meteorology needs to be properly justified or changed. Alternatively, the context of the results should be rephrased.

2. The authors response to how CF is calculated seems not to answer my question.
The CF depends on the chosen area: if one would calculate the CF over a region of 4x4 km, while the typical clouds in the broader regions are much larger (say each cloud is 20x20km), the CF in the sub regions would tend to be mostly 0 or 1. If one would take the CF over 100x100km instead for the same broader region the total CF may be something else. One should be careful because when correlating the CF to other parameters it matter whether the CF is 1, 0 or in between (all CF may be correct, depending on the size of the averaged region). The authors choose segments of 12km which I think is too small to represent warm boundary layer cloud sizes. Perhaps showing the sensitivity of that choice (12 km segment) to the calculated CF would be enough.

3. The authors say that the thinnest clouds are less sensitive to aerosols. This is in contrast to what one would expect: the albedo is not yet saturated for thin clouds and therefore both Twomey and LWP effects would tend to increase the albedo more than in thicker clouds.
Unless having a plausible physical explanation for this result, I doubt the authors analysis. What can change the CRE and reduce the thin clouds sensitivity? Is it evaporation of the smaller drops? This would mean a negative sensitivity?

Minor comments:
1. P9 L7: 99% fell below a LWP of 4 kg/m^2?
2. P9 L26: “The sensitivity is only calculated if there are 100 observations within the regime and the linear regression Pearson correlation coefficient is greater than .4.“ Why 100? why 4?
3. The relationship between turbulence and activation efficiency is still unclear.
4. P20 L24: “Surface winds were not included in analysis because the dependence of the warm cloud radiative response to aerosols depends most on LWP, RH, and stability, with only some regions showing a dependence on surface winds in our initial analysis.” Can you show these results (supporting information)?

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ED: Reconsider after major revisions (18 Mar 2019) by Timothy Garrett

AR by Alyson Douglas on behalf of the Authors (17 Apr 2019) Author's response Manuscript

ED: Publish as is (25 Apr 2019) by Timothy Garrett

AR by Alyson Douglas on behalf of the Authors (29 Apr 2019)

Short summary

Aerosols are released by natural and human activities. When aerosols encounter clouds they interact in what is known as the indirect effect. Brighter clouds are expected due to the microphysical response; however, certain environments can trigger a modified response. Limits on the stability, humidity, and cloud thickness are applied regionally to investigate local cloud responses to aerosol, resulting in a range of indirect effects that would result in significant cooling or slight warming.