Review of “Observability of Moisture Transport Divergence in Arctic Atmospheric Rivers by Dropsondes” by Dorff et al. (egusphere-2023-1570)

The manuscript by Dorff et al. investigates moisture transport and its divergence in Arctic ARs and presents a sensitivity study of moisture transport estimates to hypothetical dropsonde deployment strategies. Instantaneous profiles in reanalysis data (“truth”) are compared to a limited set of synthetic dropsonde profiles at different release times as deployed in research campaigns. Beside the role of temporal/spatial separation of the dropsondes, the impact of the flight duration on derived transport variables is addressed. The results of the study may support the conception of research flights in the Artic, which is a region of great interest to current research focussing on Arctic amplification. In addition, the article aims to better characterize transport properties of Artic ARs, which are not well characterized in literature.

The article is a comprehensive piece of work and presents nice and illustrative figures. I think the presented approach is valid and the content is certainly worth for publication. However, in its current form, the study is not sufficiently motivated and the results are not properly discussed in view of related work. Hence, the novelty in terms of applied methods and results and the added knowledge about Arctic ARs and their observation strategy remains unclear. The presentation quality suffers from a confusing writing style. I recommend major revisions and encourage the authors to carefully rewrite their work to improve the readability of the manuscript. I try to give some advice and justification in the detailed comments below:

General comments:

• Writing/Grammar: The grammar is a bit awkward and the article misses coherence and logical order within the paragraphs and sentences. Often, the authors skim over many aspects simultaneously and it is up to the reader to guess potential relationships. The article applies many terminologies that are not defined or described which makes it hard to follow. The writing of this paper would benefit from a grammatical editing and language check. 
  
  
  
  As an example, in the first paragraphs there are no main topics discernible and the topics change rapidly. The first sentence implies that the topic is the AR’s impact on Arctic flooding before a number of other topics are touched (AR impacts, pathways, phase transitions, transport, etc.). However, terms like “significant impacts”, “pathway of ARs” (in a Lagrangian sense or AR displacement?), “moisture transformation processes”, “moisture budget”, “precipitation efficiency”, “divergence of IVT”, “IVT variability”, “horizontal corridors”, “dynamical and thermodynamical processes”, “AR moisture budget components”, “AR evolution” (there are many more examples in the other sections) are not defined or described, which makes it hard to understand the context of this work.
  
  
  
  Several sentences are hard to understand or not logical due to imprecise / unusual language or missing causality. Examples are: L31, L51 (deteriorate … representation?), L57 (monitors transport (…) seen from research aircraft?), L105ff, L152f (why are radiometer/radar relevant?), L182, L204 (putative? inconsistency?), L225 (maintain?), L260 (why intuitive?), L268 (behaves more homogeneous?), L277 (How?), L278 (“long-term aircraft”?), L289-291 (e.g., “carefully correlated observations?”, ”cross-sectoral”), L322 (“narrowed moisture columns here form”?), L333ff (What is the “simplified understanding of divergence”?, “benchmarks…”?), L358 (“behave differently”), L360 (“lower atmosphere”), L364f (“integrate along the vertical axis”), L373ff, L392 (“our arctic AR composition”?), L403f, L485.

• Related work: The authors remain very vague about related work. Although references are given, only rarely a relevant result is described. However, this is needed to understand the motivation of the study and the missing knowledge as well as to assess the novelty of the applied methods and the relevance of this work. For example, information (references) about ARs is spread over many places in the introduction (also L107), but a clear and concise description (definition, shape, evolution, region of occurrence) is missing. Formulations like “are widely assessed over the mid-latitudes” (L33) or “manifold understanding” (L35) are without substance and should be avoided. The authors miss to make clear what is known about Arctic ARs and what might be different in the Arctic compared to the midlatitudes. This also concerns the discussion of results, which needs a more thorough comparison to related work. Here are a few more examples with missing detail about the related work: L49f, L94f, L95f, L102, L109f, L191f, L198f, L266f, L352f, L355f, L358f, L396ff
• Introduction: The motivation of this study remains unclear. I understand that a limited number of dropsondes might affect IVT estimates and I believe that Arctic ARs may be not well characterized, but is that all? Here are a number of questions that would be interesting to know about and that might help motivating this work:
  • Artic ARs: Why should ARs and their observation strategy be different in the Arctic? What is known about Arctic ARs in general and dropsonde-based IVT estimates? How has the problem been addressed (methods)? What do we not know (about IVT, IVT convergence)?
  • Observation strategy: How was the simulated observation strategy defined? How do aircraft limitations (flight duration, number of dropsondes) affect the strategy?
  • Case selection: How and why were the particular nine cases selected (unclear: L68 “predefined in ERA5”, L119ff “picking (…) from catalogue”) and why was only the Atlantic region considered? Please explain the purpose of placing the legs at the sea ice edge (L105ff is unclear). Why is only spring considered?
• Discussion: The paper lacks a thorough discussion of the results, either within the result section or in a separated section at the end. Best of all, the authors add a few references within the result section, however, not detailed enough (see above) so that the added value of the paper becomes clear. A few topics that need a careful consideration and discussion:
  • Reanalysis data: In the introduction the reader is distracted by details about CARRA regional reanalyses. I suggest adding more details about CARRA, but in the methods section and clarify the extent to which km-scale variability of moisture transport can be assessed. The grid spacing (how determined?) and effective resolution of such gridded data are certainly different. What observations of q and wind are available in the Arctic to constrain the wind and q and its variability? Why do you use pressure level data only and how might the rather low number of vertical levels influence the results (L374)? What is the separation of the levels in the lower troposphere? What advantage has been gained by using CARRA instead of ERA5 (one of the results was that 100 km dropsonde spacing are enough to determine AR properties)?
  • Arctic ARs: I recommend adding a more detailed discussion about the determined characteristics of Arctic ARs (e.g., L315ff, results of Fig.1 and 10). This should involve a discussion of the communalities and differences of the presented nine cases. The large case to case variability should be better discussed. Some flight tracks cover the entire AR, some only parts, some reach far into the cold sector, some reach far into the warm sector. They also depict various synoptic situation: some cross the cold front, some extend through the cyclone and include a bent back front. Additionally, the advantages and limitations of the applied methods should be considered in view of other approaches. The differences to the mid-latitudes, surprising findings, etc. should be carved out. It did not become clear to me how the cross-sectional IVT gradients (Fig. 11) are connected to the dynamical situation and the results are contrasting the impression that I got from Fig. 1. I do not get why the convergence analysis in Sect. 5 involves the frontal sectors, which are covered quite differently by the flight pattern. How do divergence and convergence results relate to the synoptic situation and the rain distribution? The TIVT discussion (Fig. 11) could focus more on the AR area: Why is the divergence dominating in what you call warm sector – isn’t that surprising? How is the mid-level drying related to descending air typical for the cold-sector and possibly overrunning the AR? Can you explain the relation of ARs and warm air intrusions (L116)?
  • Flight strategy and dropsonde deployment: I did not get how the flight tracks were defined. Isn’t the zig-zag pattern only the consequence of sufficiently long cross-frontal legs at two latitudes that are required to capture the lateral heterogeneity and to be able to derive divergence? Are all terminologies for the flight pattern (AR corridor, boundaries, boxes, sectors etc.) needed or would it be enough to describe two cross-sections at separate latitudes that are then classified in sectors? What defined the latitudinal spacing? I suggest a separate discussion of how the obtained results may affect future flight planning and the deployment of dropsondes (Q1, Q4). Should there be a strategy to place one dropsonde at a simulated maximum IVT (L223)? I suggest adding a recommendation for the spatial separation (L252, L425) instead of a number per flight which depends on the flight performance. How sensitive are these results to the length of the flight pattern?
• Summary and conclusion: Section 7 should synopsize and synthesise the key results and identify the contribution to research on Arctic ARs. I think it will strongly profit from an improved discussion of the results. So far, the first paragraph is a repetition of what was done. The second paragraph claims that higher resolution reanalyses increase our understanding of arctic moisture transformation and precipitation efficiency, which I don’t see is addressed. The authors should try to better synthesize the central message of their results to each of the RQs in view of the gained knowledge.
• Structure of the article: The structure of the method sections is confusing and I suggest that sections 2 and 3 are merged. Section 2.1 (description of dropsonde data that is actually not used) can be deleted. Sections 2.2-2.4 can be summarized in a data and methods section. The TIVT definition (now in Sec. 4) should be moved to the Sec. 2-3. L233ff should be moved to the method section.

Specific points

• Q2 addresses correlations of wind and moisture, which has not been motivated by the introduction. What is (un)known? For understanding Q3, the relevance of IVT divergence needs to be explained more carefully.
• I do not understand the sector classification: Please specify the “requirements” in Cobb et al. (L198ff). In L194 the prefrontal, core and postfrontal are differentiated. Then you come up with a threshold definition for the AR edges. How does this all fit together and how are the sectors defined? Please move relevant information about Arctic ARs to the introduction.
• L202: I cannot see the three dropsondes that calculate IVT.
• Fig. 6: The median lines for the grey boxplots are hard to see. I guess that these distributions are calculated from the boot-strapping method (add information to caption). How many cross-sections? Please add what percentiles the box and whiskers represent. Change “seconds” to “minutes”.
• Add more references to figure panels within the text whenever appropriate.
• L274f: How can you see this in Fig.7? Winds also strongly vary and the transport distribution (grey shading) resembles the wind distribution (red shading). The sentence in L275f contains redundant information.
• L285f: Do you have an explanation for the increased variability in the free troposphere? Fig. 10 shows that your cross-sections pick up dry post-frontal subsidence regions and also dry Arctic air eastward of the AR feature, which likely impacts this result. Or maybe this is what your last sentence wants to say? How much sense does it make to calculate horizontal means for such heterogeneous sections? Wouldn’t it be more interesting to focus on the AR itself and check how much the fluctuations at small scales contribute to IVT?
• L289-291: Better explain the meaning of “correlated” and “coherent”.
• L322f: If there is little information from small scale fluctuations, why should one care about supplementary q observations?
• Fig. 11 “frontal specific AR sectors” is unclear. I see dashed and dashed/dotted lines and wonder that the dotted lines at negative distances are warm pre-frontal areas? I do not understand the two sentences “Leg specific … (lines)” – please rephrase. What is “corridor IVT convergence”?
• It is sometimes confusing what data is used. I actually thought that the flight duration was not considered for the “continuous” (L394, 426). “Continuous” was also used earlier (see e.g., Fig. 14 caption), however, I think it referred to the high-resolution cross section profiles. I suggest a clear structure and description.

General 

This paper provides a contribution to advancing our understanding of arctic atmospheric rivers by presenting an analysis of them using different reanalysis products, and suggesting ideal targeting strategies for the purpose of understanding and closing the moisture budget.

Major

In general, I think it is important for this paper to provide some additional context and motivation for the exercise of synthetic sampling. Is there a possibility for in situ sampling of arctic ARs? Is the paper calling for this capability as a requirement for us to meaningfully further our understanding in this region?

How would this papers’ findings be different if the synthetic sampling wasn’t a part of it? Does this framing potentially distract from the findings regarding the structure of arctic ARs? I suggest the authors consider strengthening their case for structuring the paper in this way and referring to more papers studying arctic ARs and their structure in addition to observational studies covering the midlatitudes if they would like to keep this framing. I suggest considering a reframing where the authors discuss what can be learned about arctic AR structure from appropriate reanalyses at different resolutions, and then recommend sampling strategies to verify/supplement this knowledge.

Do your results regarding non-instantaneous sampling change if you take into account the observations in time and space where and when they occur, as is possible in many assimilation systems now?

I very much like how the authors identify key questions and then revisit them in the summary with their answers to synthesize the paper for the readers. I also think the figures overall and really nice and clear, although I do have some suggestions for the authors to consider.

Minor

Line 24 – flood may not be the best word choice here, please revisit (“affect”?)

Figure 1, Figure 4: locate us in space with lat/lon

Figure 3 – suggest including a box in (a) to illustrate where the box in (b) comes from.

Line 252 – does this suggestion of 5 sondes at minimum depend on the AR width?

Figure 6 – suggest this would be better presented as spatial interval to not require so much information regarding assumptions about plane speed etc. Indicate what the colors represent in the caption.

Line 268 – isn’t this larger difference in q expected given the colder air?

If the AR is more moist or more windy, does that affect the spacing requirements to fully capture the structure?

Line 323 – constant winds in time or in space? Can you refer to one of your figures here?

Figure 13 – what is the purpose of the colors in the box-whiskers plot? Add to caption.

Editorial

A general quick read for grammar/word choice (clarity)/readability is warranted although generally the paper is in good shape. A few suggested changes are below (non-exhaustive).

Line 214 – suggest changing “infer” to “investigate”

Line 219 – suggest rephrase “arises the question, how” to “raises the question whether”

Line 264 – suggest rephrase “contributes to IVT with roughly 50%” to “contains roughly 50% of the IVT magnitude”

Line 265 – remove “even”

Line 321 – suggest rephrase “are little coherent” to “exhibit little coherence”

Line 348 – suggest rephrase “neither it considers” to “it considers neither”

Figure 11 – suggest removing “corridors in the” from the caption