Contrasting ice formation in Arctic clouds: surface-coupled vs. surface-decoupled clouds

Griesche, Hannes J.; Ohneiser, Kevin; Seifert, Patric; Radenz, Martin; Engelmann, Ronny; Ansmann, Albert

doi:https://doi.org/10.5194/acp-21-10357-2021

Articles | Volume 21, issue 13

https://doi.org/10.5194/acp-21-10357-2021

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

Special issue:

Arctic mixed-phase clouds as studied during the ACLOUD/PASCAL...

https://doi.org/10.5194/acp-21-10357-2021

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

Articles | Volume 21, issue 13

Research article

|

09 Jul 2021

Research article |

| 09 Jul 2021

Contrasting ice formation in Arctic clouds: surface-coupled vs. surface-decoupled clouds

Hannes J. Griesche, Kevin Ohneiser, Patric Seifert, Martin Radenz, Ronny Engelmann, and Albert Ansmann

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Final revised paper (published on 09 Jul 2021)
Preprint (discussion started on 12 Nov 2020)

Interactive discussion

Status: closed

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

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RC1: 'A review of "Contrasting ice formation in Arctic clouds: surface coupled vs decoupled clouds" by Griesche et al.', Anonymous Referee #1, 04 Dec 2020
- AC1: 'Reply to RC1 and RC2', Hannes Griesche, 15 Mar 2021
RC2: 'Review of the manuscript "Contrasting ice formation in Arctic clouds: surface coupled vs decoupled clouds" by Griescche et al.', Anonymous Referee #2, 16 Dec 2020
- AC1: 'Reply to RC1 and RC2', Hannes Griesche, 15 Mar 2021

Peer-review completion

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

AR by Sarah Buchmann on behalf of the Authors (17 Mar 2021) Author's response

ED: Referee Nomination & Report Request started (29 Mar 2021) by Matthew Shupe

RR by Anonymous Referee #1 (13 Apr 2021)

Suggestions for revision or reasons for rejection

The authors have addressed my major comments from the previous submission. However, I am now concerned by the new addition of the back-trajectory analysis. I do not find this source analysis conclusive and cannot agree with the authors' conclusions because of several reasons:

1. I don't know how much time aerosols of different types lifted from the surface to the lower troposphere can remain airborne before settling but I doubt it is typically on the order of 10-days. The authors did not provide any reference or reasonable justification for this selection nor have they provided sensitivity test results.

2. Air parcels can generally speaking travel enormous distances over a period of 10-days, much longer than the great circle distance between Svalbard and Leipzig. Thus, I think that without any other contextual analysis (e.g., probability of a common geographic source), from a statistical perspective, it is not surprising that the results from Svalbard could be rather similar to those from Leipzig.

3. I understand that a reception height of 2 km is widely used in the literature. That does not suggest that it serves as a reasonable assumption in every case, and specifically in this case, where most clouds are much closer to the surface, and hence, an altitude of 2 km is not representative and lacks context.

4. The authors did not elaborate on this issue, but HYSPLIT (assuming that this is the model used by the authors) often tends to continue the back-trajectory calculations even after the parcel reaches the surface. Did the authors remove such cases in which the parcel apparently reached its source?

5. In general, a single paragraph is definitely insufficient to introduce, describe, and conclude from a new analysis of the data, but on the other hand, I doubt how much further the authors would like to elaborate on this analysis because it may cause the manuscript to lose some of its focus.

Minor comments:

1. Recommenced swapping sec. 4.1 with 4.2. It seems to me like sec. 4.2 better fits the beginning of the Discussion right after the results are presented, while sec. 4.1 could provide a smoother transition to the Summary & Conclusions.

2. P. 15 l. 2 - Suggest changing "lowest detection limit" to "lowest radar range gate" because detection limit often insinuates intensity limitation.

3. Caption of figure 8: perception --> reception.

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RR by Anonymous Referee #2 (18 Apr 2021)

Suggestions for revision or reasons for rejection

After a detailed reading of the authors’ comments and of the updated version of the manuscript, I am still concerned about the approach adopted by the authors. I must acknowledge that the authors extended the presented data analysis to support their conclusions. Nevertheless, I regret to note there are still not fully transparent aspects and not obvious choices in the data analysis which does not allow the presented analysis to be complete and to improve the accuracy of results.

My concerns can be summarized in the two following major points.

The authors state that they initially “did not provide quantitative thresholds about how we separated ice and liquid clouds because of the challenges in their estimation. As a consequence their preferred to use manpower to manually analyze the data set and decide where ice is or not.“ Nevertheless, In their response, the authors also provides an approach fundamentally based on the definition of a threshold on lidar volume depolarization ratio for the detection of ice-containing clouds.The comparison between the “manual” data selection and those based on the automatic data classification, independently on the difficulties intrinsic to the definition of credible thresholds for the automatic algorithm, opens the way to the following thought: considering that with the automatic selection the difference in the percentage of surface-coupled and decoupled clouds is lower compared to the manual analysis (i.e a factor of about 1.5-4 vs a factor of 2-6 on average in between 0°C and -10°C) can we consider this difference as the results of the level of subjectivity of the analysis? Or is this an indication that the analysis is largely affected by the irreducible uncertainties due to the assumptions done in the retrieval of lidar products? Assuming the error bars in Fig.5 are a good estimation of statistical uncertainty according to Seifert et al. (2010), the variability between the “manual” and “automatic” data processing may be representative of a bias which may affect your manual approach. Although the results demonstrates that majority of clouds in the height corresponding to the interval 0°C and -10°C are surface-coupled, the quantification of their fraction must be as accurate as possible and potential systematic effects, such as those due to a manual data analysis, should be discussed in the manuscript.

2. The authors removed from the manuscript the analysis on the estimation of INP concentration and added a new analysis to demonstrate the hypothesis that the aerosol source acting as INPs in Arctic are the similar to those of a continental site, i.e. Leipzig, which is used as a comparison term in the data analysis.
First of all it is not clear to me why, using a multi-wavelength Raman lidar, aerosol extinction profiles in clear sky, presented in the updated manuscript version, and consequently, lidar ratio profiles are not retrieved from Raman channels. The capability of a multi-wavelength Raman lidar are fully neglected with consequent increase of the uncertainties in the retrieved products. Assuming a constant value of the lidar ratio may be considered acceptable only for ice cloud, although also I that case the variability of lidar ratio could affect at smaller extent the selected thresholds. The retrieval of the aerosol lidar ratio from Raman lidar measurements from could definitely, coupled with the particle depolarization ratio and air mass back-trajectories, clarify the role of different aerosol types involved the ice clouds formation (several example from literature can be provided, but I am sure the authors knows all of them very well). This could also avoid to include the comparison with Leipzig in support of the authors interpretation of a major role for the continental aerosol in the ice cloud formation in the Arctic and increasing the credibility of the presented analysis, which appears still too speculative.
I want also to add that aerosol backscatter profiles in Fig. 7 must be shown over a longer vertical range to ensure the reader can have a clear idea of the calibration accuracy of lidar profiles.

About the presented investigation of the aerosol sources using FLEXPART model, if it is true that marine fraction decrease with height, above 2 km, it remains the major source at all levels, while the other aerosol types slightly increases with the height, except for "grass cropland" aerosol which has a bit larger increase. I do not see any reference to support the authors’ statement of similarity with the aerosol composition typical for the Leipzig site, which is not a marine site. For this part, the analysis looks carried in out in hurry and must be deeper.

At this point, although the statistics presented in the manuscript on the surface-coupled and decoupled ice-containing clouds are interesting, I am not sure if the content of the manuscript is sufficient for publication on ACP. Major revisions are still required and if points above cannot be fulfilled in a short time by the authors, I’d suggest to re-submit the manuscript once data are more consolidated.

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ED: Reconsider after major revisions (19 Apr 2021) by Matthew Shupe

AR by Hannes Griesche on behalf of the Authors (31 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (09 Jun 2021) by Matthew Shupe

Short summary

Heterogeneous ice formation in Arctic mixed-phase clouds under consideration of their surface-coupling state is investigated. Cloud phase and macrophysical properties were determined by means of lidar and cloud radar measurements, the coupling state, and cloud minimum temperature by radiosonde profiles. Above −15 °C cloud minimum temperature, surface-coupled clouds are more likely to contain ice by a factor of 2–6. By means of a literature survey, causes of the observed effects are discussed.