Review “A closed-to-open cell mixed-phase cloud transition observed over the Nordic Seas under high aerosol loading and strong surface fluxes”

The authors present a well-written manuscript that analyzes one specific closed-to-open cell transition in a marine cold air outbreak. The analysis appears thorough and detailed and contains appropriate figures. My comments below are mostly of minor character.

General comments

• The abstract reads quite technical, mostly due to the frequent use of specific numbers/values. Please consider giving a broader picture first before going into the details. E.g., it could be more interesting to mention that the studied case is a relatively polluted and cold one, compared to other studies, than only giving the numbers in the abstract. Also, you could mention in the abstract already that you identified the decrease in aerosol number due to precipitation and entrainment as a key factor for the timing of the closed-to-open cell transition (if I understood you correctly from the conclusions).
• I am not entirely sure yet, what role the output of AROME-Arctic plays in their study. While it is definitely valuable to compare the archived model output to the observations, the simulations are not described in great detail in the manuscript. Specifically, I am missing information about the aerosol (CCN and INP) concentrations applied in the model. As I see it, the statement in the beginning of the conclusions that the transition is incorrectly depicted by the model (line 499), is both unclear (convective bands have not been introduced earlier) and lacks discussion beforehand. How do the CCN and INP number concentrations in the model compare with observations and may differences there explain differences in the depiction of the transition?
• The CCN concentrations in Fig. 13 and 14 (h) show an irregular frequency of scattered points. Sometimes points with larger distance than 3-4 km are connected by lines, and for larger distances between points the line is interrupted. Why does the distance of the points vary (different aircraft speed, values not shown due to not overcome thresholds, instrument failure or other reasons?) and how are larger distances/gaps treated? Please also confirm whether the concentrations shown in (h) and (i) are all from in-situ observations.

Specific/minor comments

Line 187: Typo: should be “Scandinavia”, not Scandanavia.

Fig. 4, 13, 14, 15 and others: The “rainbow”/jet colormap is not perceptually uniform. Please consider shifting to a colormap that is perceptually uniform (this refers to the gradients inside the colormap).

Line 224: What do you mean by “not yet shown”?

Line 249: Could you add information here, what causes the higher aerosol concentrations downwind? Which sources are there between Villum and the flight path?

Fig. 7, 11 and 17: Do you need the color bar? If the curves are of only one color each, you could also consider to just give the distance in the legend or caption.

Line 263: Consider adding that you talk about “vertical” profiles here.

Line 272/3 and 290: How certain are the altitude estimations for inversion and cloud top height? Is the overshoot robust?

Fig. 9: For the sake of colorblind-friendliness, I recommend changing one of the colors red or green.

Fig. 13/14(f): Could you use the same color scale for both figures anyway? SL1 goes hardly ever below 1 g kg^-1 anyway, as far as I can see.

Fig. 14: Typo in caption: Locations where cold pools (entrainment) are identified “are” underlined in blue (gold) in (d)-(f).

Line 411: Typo “Aitken” mode, not Aiken.

Section 7: The answer to the question proposed in the section title seems hidden in line 435-437. You could consider repeating that at the end of the section.

Line 485-489: Do you mean that collisional breakup is followed by increased transfer from the liquid to the ice phase through the Wegener-Bergeron-Findeisen process? The ice-ice collisions can’t be causing the depletion of LWC themselves.

Line 493/4: This is an interesting suggestion. Do you have observational evidence for how frequent the recirculation of falling into updrafts occurs or on which scales this process is relevant in Arctic marine cold-air outbreaks? Does this e.g. depend on the width or intensity of the cold pool?

Line 537: Typo “THREDDS” server, not TREDDS.

Line 570/1: There seems to be a slight underestimation of droplet number concentration in the model compared to observations, even though the ranges overlap (as you correctly state). Could you comment on that possible underestimation?

The manuscript presents observations from a single research flight comprising three low-level flight segments. The authors combine SL1 and SL3 to provide an approximate quasi-Lagrangian view of a closed-to-open cell transition in mixed-phase marine clouds. The dataset is rich, combining aerosol, cloud microphysical, thermodynamic, and remote-sensing measurements. The combination of these complementary measurements is a major strength of the study and it provides a useful basis for future modeling work. The figures are informative, the manuscript is well written, and the study is suitable for publication in ACP. I am not an expert in all the measurement techniques used and therefore cannot fully assess instrument-specific uncertainties. Nevertheless, the observations appear physically consistent, and the proposed interpretation is plausible. 

The manuscript is in parts very technical and descriptive and contains perhaps too many numerical details in the main text, which sometimes obscure the key points. I suggest moving some to tables or to the Supplement and retaining only the most important numbers in the main text. The abstract could also be revised to improve readability and emphasize the main findings more clearly. In this respect, the conclusion section provides an effective and concise synthesis of the results.

The manuscript appropriately acknowledges that aerosol, microphysical, and boundary-layer processes are strongly coupled and that modeling will be required to quantify their relative roles. However, some statements appear to present the proposed sequence as causal. I suggest clarifying which links in the sequence from aerosol changes to enhanced precipitation, cold-pool formation, and open-cell organization are directly supported by the observations, and which remain interpretations or hypotheses.

Figure 1 indicates the satellite acquisition time and the aircraft position at that time. However, because the flight segments were sampled over a substantially longer period, the authors should discuss to what extent the cloud morphology observed in the instantaneous satellite image is representative of the clouds sampled during the different parts of the flight. If suitable satellite observations are available, an image sequence spanning the flight period could help demonstrate the temporal evolution of the clouds.

Line 245: the units appear to be incorrect.