Review of “Overview: Quasi-Lagrangian observations of Arctic air mass transformations – Introduction and initial results of the HALO–(AC)³ aircraft campaign” by Wendisch et al., for publication in Atmospheric Chemistry and Physics

Summary

This paper provides an overview of the recent HALO-(AC)³ aircraft campaign highlighting the flight plans/strategy, measurements, and various measurement/sampling techniques of Arctic air masses. Some of the novelties of this study include quasi-Lagrangian measurements from eight warm-air intrusion (WAI) and twelve cold-air outbreak (CAO) cases, derived surface heating/cooling and moistening/drying estimates, comprehensive aerosol/CCN data, and estimates of mesoscale divergence using dropsondes released during circular flight patterns. The abstract is very well written, concise, and clearly conveys the novelties and (initial) results of the AC³ campaign. The quasi-lagrangian sampling strategy is very clearly defined, thought out, and easy to follow in the text/results. The section on Arctic clouds nicely highlights cloud phase as a function of the underlying surface (open water versus sea ice) as well as a simultaneous retrieval of effective radius for both ice crystals and liquid drops. These results, in my view, appropriately highlight and contextualize the various datasets as well as set the table for a number of planned (and likely interdisciplinary) analyses across a wide array of Arctic climate science sub-disciplines. Aside from a couple of very minor comments (indicated in the Specific Comments) and with a few of the figures being quite “busy” with lines and markers, every figure – in my view – is justified in its content with each figure adding very clear and rich context to the paper. Another strength of this manuscript is that, given volume of data and analysis in this manuscript, all sources of uncertainty (e.g., LWP and snowfall) are well characterized and quantified.

This is a commendable effort by all authors and contributors. For a very lengthy manuscript with 17 figures and 3 appendices, this was a very fun read with a lot of concise, “to-the-point” information that many sub-disciplines within the Arctic science community will be eager to read. I liken this manuscript to a fine 7-course dinner: it may take a while before you’re finished, but every course delivers masterfully crafted dishes by world-class chefs with each dish delivering a palette of flavors certain to whet every appetite in the Arctic climate community. The manuscript in its present form is perfect in the sense that it captures just the right amount of detail (in my view) for an overview paper. While I have a number of very specific comments that would improve clarity in a few spots, they are extremely minor and can be addressed quickly without the need for a second review. I have no general concerns/comments for this manuscript, and overall, I believe this manuscript is publishable in its present form to Atmospheric Chemistry and Physics.

I look forward to many more in-depth studies following and building upon the excellent work presented in this manuscript.

Specific Comments

L23-24: “... was more than 1.5 K warmer than during pre-industrial times” though it’s stated “Data published by the Copernicus Climate Change Service show...”, this statement needs a citable reference.

L24: “numerous feedback mechanisms in the Earth’s climate system” it would be good to list 2-3 or so of these feedback mechanisms here.  

L44: A reference or two here would be good.

L105: This is a very lengthy introduction, but a necessary one as each paragraph here has a clear focus and motivation for the AC3 campaign.

L124: Add latitude/longitude coordinates for Kiruna and Longyearbyen here.

L134: Add latitude/longitude coordinates for Ny-Ålesund.

Section 2, like the introduction, is very well structured and written.

L171: Casual readers may not fully understand what a “Lagrangian” frame of reference is and how it ties into the sampling strategy described in this paragraph. A sentence to open up this paragraph describing what “Lagrangian” is, in my view, would lead the rest of this paragraph better and make the sampling strategy clearer to the reader in its objective.

L174: “Because of their...” I would lead this sentence with “For example, ...” as this would more clearly lead the reader into a discussion of balloon-related drawbacks described in the previous sentence.

Figure 2 Caption: Recommend changing “enables to observe the changes” to “enables observational changes”

L248: I am slightly confused by the writing here – what do you mean by a “quality of possibilities”? I think “provides unprecedented quantity of possibilities” would work here.

Figure 5: This is a very well-constructed figure that clearly contrasts CAOs with WAIs.

L305: How exactly is the “ice growth process” inferred or done using measurements here?

L306: Can you point to or reference where “we also detect stronger riming”?

Figure 6: Very picky comment here... “weak” should be capitalized in the Figure Title.

L317-319: Very interesting result!

Figure 7: I love the setup of this figure – it is definitely one of the most informative figures I’ve ever seen relating ice index and distance from the ice edge to actual cloud morphology. I hope to see versions of this figure in your future papers.

Figure 8 caption: Is it really necessary to call this a “Shapiro-Keyser cyclone” here? I think it would be better if this were referenced (including the citation) in the main text rather than the figure caption.

L374 and Figure 10 caption: One of the other prevailing cloud phase/microphysics algorithms for ground-based cloud remote sensors follows the widely-used Shupe et al. (2008, and references therein). I think it would be useful for the Arctic cloud/climate community to comment on how your algorithm compares with the Shupe et al. algorithm (and perhaps discuss how a comparison of these algorithms might be done in a future AC3-related study which would also be very interesting!).

Shupe, M. D., and Coauthors, 2008: A Focus On Mixed-Phase Clouds. Bull. Amer. Meteor. Soc., 89, 1549–1562, https://doi.org/10.1175/2008BAMS2378.1.

L382: Just say “Future studies” rather than “near future studies”.

L390: Following my previous comment for L374, this might be a good spot to discuss potential differences in these algorithms.

L430: How typical are RHi values of 140%? Might be good to add a reference or two here for comparison sake.

L455-456: I’d merge these two sentences.

L512-513: I agree with this conclusion.

Review Criteria

1. Does the paper address relevant scientific questions within the scope of ACP? Yes, all objectives and questions fit within the scope of ACP.
2. Does the paper present novel concepts, ideas, tools, or data? Yes, many ideas/concepts and tools are novel.
3. Are substantial conclusions reached? Yes, for an overview paper, the initial results and findings represent substantial conclusions.
4. Are the scientific methods and assumptions valid and clearly outlined? Yes.
5. Are the results sufficient to support the interpretations and conclusions? Yes, and all interpretations/conclusions are well characterized in terms of potential sources of uncertainty or other limitations.
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Yes.
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes.
8. Does the title clearly reflect the contents of the paper? Yes.
9. Does the abstract provide a concise and complete summary? Yes.
10. Is the overall presentation well-structured and clear? Yes, the presentation is of extremely high quality and very clear.
11. Is the language fluent and precise? Yes, the language is very clear, fluent and precise.
12. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Yes, all mathematical formulae, etc. are correctly defined and used.
13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? No, I believe the length of all parts of the paper are appropriate and/or sufficient in length.
14. Are the number and quality of references appropriate? Yes. The number of figures and references are a bit on the high side, but understandably so as this is an overview paper of a major, multi-platform field campaign.
15. Is the amount and quality of supplementary material appropriate? Yes, the supplementary material adds rich quality and material to the manuscript.