Comment on EGU-2023-2608 titled “Distinctive aerosol-cloud-precipitation interactions in marine boundary layer clouds from the ACE-ENA and SOCRATES aircraft field campaigns”

GENERAL COMMENTS:

This study uses aircraft in-situ measurements from the ACE-ENA and SOCRATES field campaigns to illustrate vertical profiles of cloud microphysical and precipitation properties and their relationships with above- and below-cloud aerosols. The paper is well-written with appropriate references. There is tremendous detail in Section 2 (uncertainties in airborne observations, formulae used to calculate cloud properties, thresholds used to define in-cloud and above/below-cloud regions) to ensure the results can be reproduced. The discussion of the results is well structured, and the text is backed up with appropriate figures. The study draws proper conclusions and provides appropriate evidence. This is a comprehensive study that deals with numerous elements of aerosol-cloud-precipitation interactions – a single paper is used to show findings that could have spread across multiple studies if the individual elements were investigated further.

While the work is comprehensive, the novelty of this study comes from the fact that these in-situ observations come from regions that have not been sampled extensively by aircraft or the data examined in detail. Additionally, very distinct regions with unique cloud and aerosol characteristics are compared. In isolation, many findings might not strictly be new as they corroborate results from many previous studies that have used aircraft observations. Nevertheless, this study is an important addition to the literature because of the wide range of topics discussed and the fact that the authors contrast cloud and aerosol properties from multiple field campaigns from two different locations. The effort put into this work is commendable and the paper would be a valuable addition to the journal and the literature. Given the number of topics discussed and potential for deeper dives into each topic, I can see multiple studies coming out of further investigation into the results presented here.

I have some considerations for the authors before the study is published:

1. The introduction felt a bit too broad and generic - the authors could better motivate and highlight the unique aspect of this work by guiding the reader through the distinct nature of these regions, by contrasting the existing knowledge of cloud properties from these regions - somewhat done for SOCRATES but not for ACE-ENA, highlighting the ‘climatic significance’ of clouds observed in these two regions – this was only done for MBL clouds in general, and the need for in situ observations from these locations. 

Is there a reason behind comparing these two specific field campaigns? Are we comparing apples to apples? - describing the cloud regime, type, or morphology in these regions could help interpret the differences in the results presented in later sections.

          2. The authors could better and use terminology to refer to cloud processes discussed throughout the text. Terms like “collision-coalescence”, “in-cloud coalescence”, “coalescence-scavenging” are used (often interchangeably it seems) - they can be merged or defined using more commonly used terminology.

Line 57: the authors could separate what they term the ‘coalescence-scavenging effect’ into two parts. While the drizzle drops are within the cloud layer, the process described in Line 58 can be described as the ‘collision-coalescence’ process. Once the drizzle drops are below cloud base, the process described in Line 59 can be described as the ‘precipitation scavenging’ process. Using such terminology would prevent confusion caused by using the same term for two separate processes that occur in-cloud and out-of-cloud, respectively. Please ensure consistency in using the terms as the paper currently uses “collision-coalescence process” in some spots and different terms at other spots while describing a similar process.

SPECIFIC COMMENTS:

ABSTRACT:

Line 15: Could you provide quantitative estimates with the number and size used to state “larger number” and “smaller cloud droplets”?

SECTION 1:

Line 34: The authors mention ‘cloud-top longwave radiative cooling’ here which is very important and should be mentioned but it is not discussed later. In contrast, there is an excellent discussion of cloud top entrainment mixing and droplet evaporation near cloud top later, but it is not introduced here. The authors could better motivate their results by introducing cloud processes in Section 1 if they are discussed later.

Line 42: This statement is only true under conditions of comparable cloud water content. Please update accordingly and provide appropriate references.

Line 44: Instead of listing references at the end of the sentence, a reader would benefit much more if references for specific elements followed the corresponding text. For example, “…investigated by different observational platforms, such as aircraft (Diamond et al., 2018; Painemal et al., 2020), model simulations (Hill et al. 2009)…”.

Please directly state some of the “different maritime regions”. Something like “….over different maritime regions like the southeast Pacific (Painemal and Zuidema, 2011), northeast Pacific (Braun et al., 2018), southeast Atlantic (Diamond et al., 2018), and eastern North Atlantic (Zheng et al., 2022a).”

Line 53: More recent studies of precipitation susceptibility (e.g., Gupta et al., 2022; Jung et al., 2016; Terai et al., 2012) have been built upon the studies cited here. To my knowledge, Feingold and Seibert (2009) introduced the term precipitation susceptibility and the study should be cited here. I see the authors briefly compared their results with some of these previous studies (good to see) - but the studies should be cited in the introductory text. Also suggest discussing the different issues with estimating/interpreting S_o based on results from previous studies.

Line 75: Please also provide the months for the ACE-ENA IOPs along with the years.

Line 80: Please provide the duration for the SOCRATES austral summer IOP.

SECTION 2:

I want to commend the authors for the discussion in this section. However, the section does lack some context about the cloud sampling locations. A map of the sampling locations/flights tracks or a list of the range of latitude-longitude coordinates where clouds were sampled during the IOPs would be very useful.

Line 106: I believe by “resolutions” you actually mean - “the size bins of the probe were 1 to 3 um wide”?

Line 111: Can you provide a reference for the phase classification product? If not, a short description of the methodology for it would be useful.

Line 128: Can cite Hansen and Travis, 1974 where the term “effective radius” and the associated formula were introduced.

Line 160: Do your results depend on the selection of the value of 200 m to determine the distance for above-cloud aerosols that are important for the analysis? Gupta et al. (2021) conducted a sensitivity test to determine if their analysis of cloud microphysical properties was affected by this number for distance from above-cloud aerosols. A comment on the sensitivity of the results on this value would be useful.

SECTION 3:

Line 192: These aerosol concentrations for SOCRATES seem high for the Southern Ocean, which was typically viewed to be a pristine environment. The authors refer to previous studies from SOCRATES to explain the aerosol size distribution or composition, do these studies have similar aerosol concentrations?

Line 213: Do you mean that the ‘sub-cloud Nacc values’ are more than double the ‘above-cloud Nacc values’? Suggest rewording this sentence.

Line 268: “These results have further improved the understanding of the aerosol first indirect effect”. This statement is a bit overreaching. There are many studies that have shown similar results. I suggest rewording to “These results are consistent with the understanding of the aerosol first indirect effect”

Line 271: It is interesting that the average Nc for ACE-ENA winter is greater than both the sub-cloud Nacc and Nccn. Do you have any comments on why this is the case? Has this been observed elsewhere? Are the values of Nc influenced by above-cloud aerosols or sub-cloud Na?

Line 310: This is an excellent discussion of entrainment mixing and its competing influence on droplet size/liquid water content and likely highlights two different modes of cloud top mixing – homogeneous and inhomogeneous mixing that depend on the entrainment rate (Lehmann et al., 2009; Lu et al., 2011) – do the authors have any comments based on their calculated entrainment rates? The authors could cite examples of previous studies that show similar vertical profiles of cloud properties where the effects of entrainment mixing on cloud microphysical properties were evident.

Line 370: The skewness of a distribution can actually be calculated as a statistical parameter rather than having a visual comparison. I leave it to the authors to decide if they would like to add this parameter to the study.

SECTION 4:

Line 434: This statement would be more accurate if the Liquid Water Path (LWP; vertical integral of the LWC) values were compared across campaigns rather than the mean LWC. Suggest adding LWP values or rewording the sentence.

Line 468: Do you want to mention some of these aircraft campaigns - VOCALS, ORACLES, ACTIVATE, etc.?

Line 475: These are some very interesting results. While the ACI_r,CT values are close to what one might expect (droplets are too large near cloud top for above-cloud aerosols to exert a significant influence on r near cloud top), it is interesting to note the ACI_{N, CT} values reported here. Do the authors have any explanation or hypotheses for what causes these values to not be closer to 0 like ACI_{r, CT}?

Line 480: I think the authors should also mention cloud top entrainment mixing over here.

Line 484: As mentioned earlier, would be good to also cite Feingold and Seibert, 2009 when defining S_o, which to my knowledge was the first study to define the term.

Line 491: The authors should provide the correlation coefficient values for the S_o calculations and contrast these with previous studies. At least two recent studies did this – Jung et al. 2016 and Gupta et al. 2022.

Line 501: “…due to decreasing 𝑆𝑜 within the thicker cloud (Terai et al., 2012)”. This is oversimplifying the problem. The value of S_o depends not only on cloud thickness but also on the calculation methodology as shown by Terai et al, the cloud type – cumulus versus stratocumulus (Sorooshian et al., 2009; Jung et al., 2016) and the above- and below-cloud aerosol concentration (Duong et al., 2011; Jung et al., 2016; Gupta et al., 2022). Having more information on cloud type/morphology in the introduction would give context to these S_o values and other results in the study.

Line 531: What are the units of the CCN loss rate? Here, the values are reported with units of “cm^-3” which does not include a unit of time, this is likely an error?

SECTION 5:

Line 568: The differences can also be attributed to the different size distributions which are then due to the sources discussed in the following sentences. That would then nicely lead to the discussion of aerosol modes toward the end of the paragraph.

Line 569: I don’t think using the words ‘pristine natural environment’ is appropriate when the previous sentence claimed the aerosol concentrations are highest for SOCRATES.

Line 580: Can you also list the percentage increase in r from cloud base to top since these campaigns had different cloud thickness values?

Lines 255 and 576: How does in-cloud coalescence cause an increase in the size of sub-cloud aerosols? In-cloud coalescence would increase the size of a cloud drop as it accumulates water by colliding and coalescing with other droplets. Once this drop evaporates in the sub-cloud to expose the residual aerosol, the aerosol/CCN core size should be the same unless the CCN is modified during droplet growth. Is this related to the condensation of sulfuric gas onto aerosol cores as described in Line 245? If so, is there a way to verify this based on these observations? If not, this should be stated as a hypothesis rather than a conclusion?

Lines 591-597: I don’t fully understand or agree with the conclusion drawn here. The studies that the authors cited here/earlier (among many others) have calculated f_ad using in-situ aircraft data from other locations and shown that assuming f_ad = 0.8 could lead to errors in satellite estimates of droplet concentration. While the calculation of f_ad and stating the regional values is important, these f_ad values do not “shed light on the further understanding of the satellite retrievals, particularly the satellite-based aerosol-cloud interaction assessment”. The authors can state the f_ad values and perhaps add a comment on the need to use these values when calculating droplet concentration for these regions using satellite retrievals, but I suggest removing lines 594-597.

Line 614: I don’t understand what is meant by “the aircraft assessment provides more connected circumstances between the aerosols and cloud layer.”

TECHNICAL CORRECTIONS/CLARIFICATIONS:

Line 12: Could use the term “aerosol-cloud-precipitation” given the terminology in the title? Also, change to “interactions” given the verb “are” in the next sentence?

Line 94: “aerosol, cloud, and drizzle”?

Line 105: “onboard the aircraft”?

Line 110: “large, ice particles”? Large ice particles would be a lot larger than 200 um.

Line 286: “To ensure the representativeness of the vertical profiles”?

Line 443: The sentence should probably end with “during the winter”.

Line 582: Do you mean “The mean cloud-top entrainment rates (𝑤𝑒 ) are a function of cloud top virtual potential temperature and vertical velocity and their values are….”

Figure 1: Please mention which statistical metrics are provided in the legends. Suggest adding that to the figure caption.

Figure 2: The caption lists the incorrect size range for the inner plots. Should be “Aitken mode size distribution (𝐷𝑝 = 0.01 to 0.06 µm)”

REFERENCES:

Feingold, G. and Siebert, H.: Cloud – Aerosol Interactions from the Micro to the Cloud Scale, from the Strungmann Forum Report, Clouds in the Perturbed Climate System: Their Relationship to Energy Balance, Atmospheric Dynamics, and Precipitation, 2, edited by: Heintzenberg, J. and Charlson, R. J., MIT Press, ISBN 978-0-262-01287-4, 2009.

Gupta, S., McFarquhar, G. M., O’Brien, J. R., Delene, D. J., Poellot, M. R., Dobracki, A., Podolske, J. R., Redemann, J., LeBlanc, S. E., Segal-Rozenhaimer, M., and Pistone, K.: Impact of the variability in vertical separation between biomass burning aerosols and marine stratocumulus on cloud microphysical properties over the Southeast Atlantic, Atmos. Chem. Phys., 21, 4615– 4635, https://doi.org/10.5194/acp-21-4615-2021, 2021.

Gupta, S., McFarquhar, G. M., O'Brien, J. R., Poellot, M. R., Delene, D. J., Miller, R. M., and Small Griswold, J. D.: Factors affecting precipitation formation and precipitation susceptibility of marine stratocumulus with variable above- and below-cloud aerosol concentrations over the Southeast Atlantic, Atmos. Chem. Phys., 22, 2769–2793, https://doi.org/10.5194/acp-22-2769-2022, 2022.

Hansen, J. and Travis, L. D.: Light scattering in planetary atmospheres, Space Sci. Rev., 16, 527–610, 1974.

Lehmann, K., Siebert, H., and Shaw, R. A.: Homogeneous and inhomogeneous mixing in cumulus clouds: dependence on local turbulence structure, J. Atmos. Sci., 66, 3641–3659, https://doi.org/10.1175/2009JAS3012.1, 2009. 

Lu, C., Liu, Y., and Niu, S.: Examination of turbulent entrainment-mixing mechanisms using a combined approach, J. Geophys. Res., 116, D20207, https://doi.org/10.1029/2011JD015944, 2011

See attached.