Investigation of aerosol–cloud interactions under different absorptive aerosol regimes using Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) ground-based measurements

Zheng, Xiaojian; Xi, Baike; Dong, Xiquan; Logan, Timothy; Wang, Yuan; Wu, Peng

doi:https://doi.org/10.5194/acp-20-3483-2020

Articles | Volume 20, issue 6

https://doi.org/10.5194/acp-20-3483-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-20-3483-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 6

Research article

|

24 Mar 2020

Research article |

| 24 Mar 2020

Investigation of aerosol–cloud interactions under different absorptive aerosol regimes using Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) ground-based measurements

Xiaojian Zheng, Baike Xi, Xiquan Dong, Timothy Logan, Yuan Wang, and Peng Wu

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Final revised paper (published on 24 Mar 2020)
Preprint (discussion started on 07 Jun 2019)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review for Investigation of Aerosol-Cloud Interactions under Different Absorptive Aerosol Regimes using ARM SGP Ground-Based Measurements Zheng et al. 2019,', Anonymous Referee #1, 10 Jul 2019
- AC1: 'Response to reviewer #1', B. Xi, 22 Sep 2019
RC2: 'Review of "Aerosol-Cloud Interactions under Different Absorptive Aerosol Regimes using ARM SGP Ground-Based Measurements”', Anonymous Referee #2, 12 Aug 2019
- AC2: 'Response to reviewer #2', B. Xi, 22 Sep 2019

Peer-review completion

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

AR by Baike Xi on behalf of the Authors (23 Sep 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Oct 2019) by Graham Feingold

RR by Anonymous Referee #1 (25 Oct 2019)

Suggestions for revision or reasons for rejection

I would like to thank the authors for the different answers to the comments and the changes made in the article. The quality of the paper has increased and the results are now more robust and clear. Nevertheless, some of my comments have not been answered adequately or are not mentioned in the final article. I would like the authors to comment on few points I am referring to here.

From the first general comment answer:
The uncertainty for Nd of 25 % is considered by the author, I find this value excessively low, especially when compared with Grovesnor et al. (2017) values: Their review ranges the uncertainty of Nd from 20 % to 75% from ground based radar measurements, please refer to Section 5 from their paper. The uncertainty of LWP leads to an uncertainty on Nd much greater than 25 % (especially for LWP in the range considered in the study). Also in Dong et al. (1997), they conclude to an uncertainty on Nd of 36 %. I would like to see a deeper analysis on the uncertainty of Nd for the different parameters. Also, I think the paragraph on the uncertainty from the answer should appear in the article and not only the last part about re.

Also, can you make explicit the formula you use to propagate the uncertainty of re on ACI? With a simple equation, I find an uncertainty on ACI between 0.01 and 0.01 for a change in re of 10% and Nccn of 0 % (optimistic), which is not negligible considering the value from figure 8-b and the conclusion in paragraph 3.3.6. I would like to see some clarifications here.

From the second general comment answer:
In the article (p. 17, l. 3 of the new document), the lack of vertical velocity measurements is mentioned. But, it can be retrieved from ECMWF (with LTS), it would not be directly at cloud base and will concern a large-scale value but did you look at this parameter? I would also be curious to see the effect of humidity on the results. Did you look at other meteorological parameters than LTS which can impact the ACI?

The stratus formation is enhanced by high LTS (Klein et al. 1993) and, therefore, more prone to high ACI, which is in line with the results presented. The results show that ACI is a function of LTS and a function of wabs. Moreover, from the Figures (a) and (b) from the answer of the second general comment, LTS and wabs seem correlated with each other. The effect of LTS on ACI does not seem negligible, maybe even larger than the effect of wabs on ACI. I think a discussion is needed about the conclusions of the article and the correlations from observation that are not necessarily causality, and that the observed effect can be inhibited or enhanced (Grysperdt, 2016). You mention it but I think it would help to see the ACI for the two regimes of wabs and constrained for two regimes of LTS to ensure that the results described by Figure 8 are not an artifact.

From specific comment:
I suggest to put the location SGP at the end of the second sentence: "… are selected over the Southern Great Plains region of the United States (SGP). The physicochemical…" Otherwise, the logic of the abstract is difficult to follow.

The answers of the following comment should appear in the text:
- the different resolution and uncertainty between KAZR and MMCR
- The comparison with aircraft measurements from Delle Monache et al. (2004) with a quantification of their results to asses the reliability of the measurements.
- The threshold you are using are from Dong et al. (2015), originally suggested by Jones et al. (2011)
- The uncertainty of ACI corresponds to the 95 % confidence interval.

Delle Monache, L., Perry, K. D., Cederwall, R. T., & Ogren, J. A. (2004). In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 2. Effects of mixing height on aerosol properties. Journal of Geophysical Research: Atmospheres, 109(D6), doi: 10.1029/2003JD004024.

Dong, X., Schwantes, A. C., Xi, B., & Wu, P. (2015). Investigation of the marine boundary layer cloud and CCN properties under coupled and decoupled conditions over the Azores. Journal of Geophysical Research: Atmospheres, 120(12), 6179-6191, doi: 10.1002/2014JD022939.

Dong, X., Ackerman, T. P., Clothiaux, E. E., Pilewskie, P., & Han, Y. (1997). Microphysical and radiative properties of boundary layer stratiform clouds deduced from ground‐based measurements. Journal of Geophysical Research: Atmospheres, 102(D20), 23829-23843, doi: 10.1029/97JD02119.

Grosvenor, D. P., Sourdeval, O., Zuidema, P., Ackerman, A., Alexandrov, M. D., Bennartz, R., ... & Deneke, H. (2018). Remote sensing of droplet number concentration in warm clouds: A review of the current state of knowledge and perspectives. Reviews of Geophysics, 56(2), 409-453, doi: 10.1029/2017RG000593.

Gryspeerdt, E., Quaas, J., & Bellouin, N. (2016). Constraining the aerosol influence on cloud fraction. Journal of Geophysical Research: Atmospheres, 121(7), 3566-3583, doi: 10.1002/2015JD023744.

Jones, C. R., Bretherton, C. S., & Leon, D. (2011). Coupled vs. decoupled boundary layers in VOCALS-REx. Atmospheric Chemistry and Physics, 11(14), 7143-7153, doi: 10.5194/acp-11-7143-2011.

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RR by Anonymous Referee #2 (13 Nov 2019)

Suggestions for revision or reasons for rejection

The authors have revised the manuscript in meaningful ways in response to the first reviews but there are still some passages that give a mixed impression of their understanding of the concerns raised in the reviews. The comments here fall somewhere between suggesting minor and major revisions.

One of my reservations about the first version of this manuscript was the way in which processes for cloud drop (Nd) activation were described relative to measurements of total aerosol number concentrations (Na) and cloud condensation nuclei (CCN.) The analysis of the observations showing the difference in Na conversion to CCN in weakly and strongly absorptive regimes is interesting and fairly straightforward (with the exception noted in the paragraph below.) This analysis is done for an observed %S of 0.2. While there is some discussion of how different %S might affect the results it should be clearly stated in the conclusions that they rest on this chosen value of %S.

With respect to the effect of aerosol composition/absorption on the efficacy of particles to serve as CCN, on P13 L23-25 you interpret the data from Fig 6. You note that there are ranges of Na for which the more absorbing aerosol are below the weakly absorbing aerosol. Are the ranges of number concentration meaningful in some way? What I see are two slopes that are very much alike except for defined excursions that you call out in these ranges. What’s happening for those particular observations? What makes them different? It raises the question of why some highly absorbing aerosol fall within the same relationship of the weakly absorbing aerosol and some do not. Is there another or additional property of this latter set of aerosol has that make them poor CCN?

In the previous version of the manuscript there was not a clear message that the conversion of Na to CCN was dependent on aerosol properties and the conversion of CCN to Nd primarily dependent on cloud state and dynamics. In the discussions in Section 3.3. and later this has been largely rectified. The authors provide an explanation that the CCN to Nd relationship is impacted by environmental heating of absorbing aerosol and the resulting effects on cloud microphysics (rather than differences in cloud microphysics being due to the activation process associated with strongly and weekly absorbing aerosol.) The details of this dynamic are not explored due to a lack of relevant observation.

Given that the latter is not an aerosol-cloud microphysical effect there are passages earlier in the paper that are still confusing. On P2 L5-8 the implication is still that there is some microphysical effect for CCN to Nd that is different for absorbing and non-absorbing aerosol.

Some of the problem may just be the use of terminology. For Example, on P3 L1 “the efficacy of the activation of CCN” would be clearer as “the efficacy of the activation of aerosol.” For the CCN measurements as they are made here, at a given supersaturation, the number concentration is assumed to be 100% efficacious for cloud droplet activation. Size and composition, however, will affect the activation rate of aerosol.

On P10 L28-29 to P11 first paragraph there is a statement that Na does not serve as a realistic CCN proxy for calculating ACIr but attributes the poor result on decoupled boundary layer condition. Is this the author’s assumption or the assumption made by Feingold et al. 2003? What about the fact that Na can include large numbers of very small particles that will not activate, especially at these supersaturations? The difference between Na and scattering as a CCN proxy and coupling of the BL are mixed up in this discussion. Scattering (or aerosol index) have long been used as a more reliable CCN proxy due to their sensitivity to larger particle sizes. It just contributes to the confusion with how the manuscript characterizes total aerosol and CCN with respect to droplet activation.

This continues in the next paragraph, P11 L10. Is there an assumption that a constant fraction of aerosol effective activates as you state? At what %S? You’ve shown yourself this relationship can depend on composition and it certainly will depend on size and that is known to be variable at SGP. New particle formation events can push aerosol concentrations up with a large number concentration at very small sizes that you would not see in the CCN or scattering measurements. I don’t think the paragraph is the right way to lead into your results.

Ultimately I would suggest leaving the Na measurements out of the discussion when covering CCN to Nd or ACI. Na should be the focus when discussing how absorption impacts how well aerosol serve as CCN. Because you have CCN measurements and can use these directly to reference ACI in terms of drop numbers or effective radius just leave it at that. You can still go back and show that ACI differs in high and low absorption regimes by using the co-SSA quantity and leave total number concentration out of it. I don’t see the need for referring to a CCN proxy when you have CCN. It’s really confusing the discussion and leaves the reader wondering how the authors are considering the cloud drop activation process. Related, P20 L29-P21 L1 - not sure this statement is worth making.

In addition, some discussion of the heating effect of absorbing aerosol on the environment leading to differences in stability and cloud state/dynamics (updraft speeds and moisture availability) early in the manuscript might help to clarify the different effects of aerosol absorption on the cloud properties that you are postulating.

I think this manuscript still need work before being considered for publication.

Specific:
P1 L 26: I think this is supposed to read “…suggests a diminished cloud microphysical response to aerosol loading”
P6 L8: Aerosol Observing System
P7 L5 “question of whether surface aerosols can be linked to what actually happens in clouds aloft.”
P15 L15 – switch position of Nccn and Na

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ED: Reconsider after major revisions (26 Nov 2019) by Graham Feingold

AR by Baike Xi on behalf of the Authors (07 Jan 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (26 Jan 2020) by Graham Feingold

AR by Baike Xi on behalf of the Authors (05 Feb 2020) Author's response Manuscript

ED: Publish as is (15 Feb 2020) by Graham Feingold

AR by Baike Xi on behalf of the Authors (19 Feb 2020) Manuscript

Short summary

The continental low-level stratiform cloud susceptibilities to aerosols were investigated under different absorptive aerosol regimes. The weakly absorbing aerosols, which are more hygroscopic, can better activate as cloud condensation nuclei. The favorable thermodynamic condition enhances the cloud susceptibility, while the cloud-layer heating effect induced by strongly absorbing aerosols dampens the cloud susceptibility. Overall, the clouds are more susceptible to the weakly absorbing aerosols.