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| 17 Mar 2025
Life cycle studies and liquid-phase characterization of Arctic mixed-phase clouds: MOSAiC 2019–2020 results
Abstract. Height-resolved monitoring of life cycles of mixed-phase clouds (MPCs) was performed in the free troposphere over the central Arctic during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition from October 2019 to September 2020. The research icebreaker Polarstern served as a platform for state-of-the-art remote sensing of aerosols and clouds. The use of the recently introduced dual field-of-view polarization lidar technique in combination with the well-established lidar-radar retrieval technique provided, for the first time, a robust instrumental basis to monitor the evolution of the liquid and the ice phase of MPCs and the interplay between the two phases. We discuss two long-lasting Arctic MPC cases observed close to the North Pole. During the late summer MPC event, most likely three gravity waves strongly disturbed the cloud evolution. We documented this perturbation in detail in terms of liquid and ice-phase properties and the recovery of the strongly disturbed liquid phase within a few hours. For the first time, cloud statistics, covering all seasons of a year, are presented for liquid-bearing stratiform clouds in the central Arctic. The focus is on the optical and microphysical properties of the liquid phase which is of key importance for a long MPC lifetime. The observations confirmed that ice formation occurs predominantly via immersion freezing. We also found that activation of aerosol particles to form droplets is of great importance for the longevity of MPCs and that the free tropospheric reservoirs of cloud-condensation nuclei and ice-nucleating particles seem to be usually well-filled.
Competing interests: Daniel A. Knopf is a member of the editorial board of Atmospheric Chemistry and Physics
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 05 May 2025)
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RC1: 'Comment on egusphere-2025-967', Anonymous Referee #1, 08 Apr 2025
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Review of Jimenez et al., ACPD 2025 (egusphere-2025-967)
General comments to the manuscript
In the study titled “Life cycle studies and liquid-phase characterization of Arctic mixed-phase clouds: MOSAiC 2019-2020 results” by C. Jimenez et al., the authors show results of long-term lidar-radar observations onboard the RV Polarstern during the MOSAiC cruise. Radiosonde profiles helped to interpret the data. Four detailed case studies are presented: two of them explain liquid- and ice phase retrieval results, two only liquid retrieval results. Furthermore, statistical results related to free-tropospheric stratiform liquid-containing cloud layers were presented.
Recommendation:
I would suggest the manuscript to be published after minor revisions considering the remarks below. The authors should address the following points:
General/Major comments:
Title and throughout the text: Is “life cycle studies” the most fitting term? Throughout the study it was not clear to me how the life cycle of the cloud is assessed. Firstly, for the presented statistical analysis the focus is not the life-cycle and thus it is somewhat misleading in the title? Secondly and more generally, I think the term “temporal evolution” is more fitting than life-cycle. You acknowledge on lines 344-346 that from observations at a fixed location (Eulerian perspective) it is hard to perform life cycle analysis (Lagrangian perspective) – but you don’t say how you can be sure you really do study the life-cycle as claimed. I suggest referring to the case study analysis as “temporal evolution” unless you can convincingly show that the onset/end of the observation of the cloud over RV Polarstern marks the formation/dissipation of the cloud and not the times the cloud was advected over the observatory.
Further remark on title: At the same time, “2019-2020” after “MOSAiC” can be omitted. Furthermore, in Section 5 results of “Pure liquid” clouds are presented. This should be added to the title. How about rephrasing the title to sth. like “Characterization of Arctic liquid-containing free-tropospheric clouds observed during MOSAiC”
Third remark on title: “liquid-phase characterization” is somewhat misleading: In the first two case studies, liquid- and ice phase are characterized. Why was the ice phase not characterized in the statistical analysis in Section 5? It is strange that case study analysis is done for liquid- and ice-phase but the statistics are not. Remove the ice-phase analysis in case study 1 + 2?
Section 4.3 with two more case studies comes as a surprise, as the abstract and the conclusions section mention only 2 case studies. Also, why was only the liquid phase analyzed for these case studies? What is the added value of having four instead of two case studies? – I find the two additional case studies do not add much new content to the manuscript, consider removing them.
It would be good to firstly, mention the limitations of the lidar-based retrievals more clearly (briefly done on line 317): Complete lidar attenuation at optical depth > 2.5-3 leads to underrepresentation of multilayer cloud situations.
Minor comments:
Throughout the text many facts are added in brackets – consider removing those or splitting the sentences in two to improve readability.
Line 2: Were MPC really only observed in the free troposphere during MOSAiC? – If not, please remove the “free troposphere” here and refer to it later.
Line 16-17: Is it possible to be more exact than stating “aerosol reservoirs of CCN and INP are well-filled”?
Line 53 – 69: Consider reordering/adding an introductory sentence, so that it becomes clear, that in this paper both, retrievals for liquid phase based on lidar-only observations and ice phase based on lidar-radar observations, are employed.
Line 77: Here you state the focus is on liquid-phase properties. In line 67-68 you state ice phase properties are also retrieved. – So the reader would assume the focus is on both, liquid and ice? – Clarify.
Line 92: Add that the ocean was also studied in depth during MOSAiC.
Line 96 - 101: Who is “we”? - It is very uncommon to refer to a group of co-authors as “we” and to focus the literature study on own publications that are not pertinent to the study subject of the manuscript. Consider removing reference to wildfire smoke publications. Consider merging this paragraph with the one on lines 102 – 108 and extend your references to other studies using the MOSAiC atmospheric remote-sensing instrumentation, e.g. https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2193/egusphere-2024-2193.pdf or https://acp.copernicus.org/articles/23/14521/2023/ , https://doi.org/10.1525/elementa.2021.000071 among others.
Line 120: The acronym MOSAiC has been introduced before and does not need to be explained again here.
Line 127: add “profiles of”
Line 128: “retrievals of” instead of “observations of”
Line 147: “vertical profiles” instead of “height profiles”
Line 149: Either expand on the “even” by explaining what is special about summer aerosol conditions” or remove it
Line 152: I am confused by the wording “reservoir” – Why not call it “proxy for INP concentration”?
Line 156: replace “our” by “the”
Line 157: add “troposphere”, add a sentence on why altitude ranges below 500m and above 7 km are excluded from the analysis.
Line 161: add “liquid-containing” before cloud layer
Line 172: move the “well” to the end of the sentence
Line 175, line 232, line 237 etc: “On” the order of
Line 182-183: This sounds confusing: What is the MPC top layer? Is it the liquid-containing layer? - Then you could refer to the base of it as “base of the liquid-containing layer of the MPC” instead of as “cloud base” (here and elsewhere, e.g. line 314)
Line 184: What are these virga representative for? The evolution of ice properties (e.g. IWC, ice particle effective radius) with increasing distance from the base of the liquid-containing layer is e.g. dependent on the relative humidity. – Expand/clarify.
Line 210: Are only single-layer stratiform clouds considered or also multi-layer scenarios? Why are clouds > 7 km excluded from the analysis?
Line 218: add “interpolated” to the radiosonde temperature information
Line 223: Why is a cloud still counted as the same cloud if the cloud-free gap is almost an hour? It seems like a very high allowed gap time. Please clarify.
Line 236: What is meant by “time interval of ice nucleation of 60s”?
Line 240-241: The sentence “the CCN and INP reservoirs are well-filled” is used 4 times throughout the manuscript. – I still don’t understand it. Please only this phrase once and rephrase elsewhere to give readers the chance to understand the meaning once differently expressed.
Line 227 – 242, Fig 1: You previously mention that you consider clouds with tops up to 7 km. Please motivate clearly, why you only show the particle number concentrations at 2 km height instead of at different altitudes.
Line 243: Begin what? – Consider removing the phrase.
Line 257: Is the start of the winter-time MPC Dec 30 as stated here or Dec 29 as stated on line 246?
Line 260: clarify if you mean horizontal or vertical wind velocities
Line 263: what do you mean by “a few percent of the air mass were advected from 30-60 N”?
Line 264: I suggest adding “likely” in front of “soil material” as soil moisture content also plays an important role in lifting of soil dust that is not considered
Line 310: Can you substantiate your hypothesis that riming occurred with the available observations? Also, explain how would riming lead to strong ice production?
Line 312: Substantiate your claim of homogeneously distributed ice crystals in the virgae column.
Line 321: add “lidar volume” in front of depol ratio
Line 321-324: Following your explanation in this paragraph, the low lidar volume depolarization ratios marked in green at the lower end of the virgae are caused by droplets backscattering. – Clarify/expand.
Line 332: Consider displaying the cloud radar mean Doppler velocity time-height display to see if you can identify the same virgae structure as well as cycles of up- and downdrafts (superimposed on particle fall velocity) in it. This might substantiate your hypothesis of decreased up- and downdraft strength in the later part of the case study observation as well (lines 355-359).
Line 336-337: consider discussion of the Sep 21, 2020 case study to its corresponding section. What can we learn from differing horizontal separation of the updrafts in the two considered case studies?
Line 340: What is the other measured depolarization ratio?
Line 341-343: This was mentioned earlier and can thus be removed.
Line 353-354: You attributed the enhanced ice virgae at the beginning of the observation period to potential ice seeding from the cloud above. The strong virgae extends to after when the upper left cloud was not observed anymore (until around 9 UTC). Why?
Line 364ff: Please add the definitions of the ice-phase fractions from IWC, LWC, CDNC, and ICNC. – ok, partly shown on line 401, move here at first mention
Line 38ff: To me the conclusion that a time-dependent INP activation is central for the longevity to MPC should be added to the abstract.
In the discussion of the wintertime case study (Section 4.1.), the feature at 22-23 UTC below 1 km with increased values of several parameters is not mentioned yet and should be discussed.
Line 418: 88.5°N is “near” the North Pole, not “over” the North Pole. Please correct it.
Line 420: Rephrase “the air mass came from Iceland, Greenland, northern Canada, and even from Alaska” – unclear how the same air mass can come from all of these different directions.
Line 431: Do you have a reference to substantiate your assumption of gravity wave crossing over RV Polarstern?
Line 432: In which way does “the gravity wave significantly disturb the
development of the liquid and the ice phase of the MPC deck and the interaction between both phase for hours.” – In Fig 5, I don’t see evidence of a disturbed development, if anything, the development of ice phase seems enhanced (enhanced radar reflectivity) and the lidar volume depol ratio seems to have similar values in the liquid-containing cloud-top layer.
Line 434-435. +section 454-459: Please list in which products you see perturbations. – I don’t see any at the indicated times. Also, this paragraph seems very speculative. Often the word “expected” is mentioned and then it is acknowledged that the expected behavior of variables was not observed. – As the information content is thus limited, I suggest shortening this section considerably.
Line 446: The term “precipitation fields” sounds not appropriate, the radar reflectivity is very low suggesting that just few ice crystals fell below the main virga features without sublimating, I suggest rephrasing. Please mention if any precipitation was observed by ground-based sensors.
Line 450-451: The scale of Fig 5 is too coarse to see the mentioned features.
Line 460-463: The methodology was previously introduced and can be omitted here.
Line 466: State why you think riming occurred. – Do you see that in specific variables?
Line 472: You mention that “The stable phase in the MPC evolution could not establish before 15:00 UTC.” – The ice production from 12 – 15 UTC lasted three hours and thus seems pretty stable to me. Why is this not considered as stable?
Line 479-480: Explain why the alternative hypothesis is not convincing.
Line 481-482: It is stated that ice crystal effective radius was 50 microns during the stable phase of the MPC. In Fig 6, it looks like as if this was the case for the entire observation period. – Clarify.
Line 485: 2x “alpha_liq” used
Line 490: Not quite true, IWC peaked again at 15 UTC.
Line 488-489: description of LWP time series is incomplete (10-15 UTC is missing). Why is there no LWP from 11-12 UTC in Fig7 Panel c).
Line 504: does the “most” refer to the entire MOSAiC observation period or to June and July?
Line 517: add a verb to the sentence
Line 521-22: at which times do you expect seeding to play a role (not clear to me in Fig. 8 as most pronounced virgae are mostly not at the same time as lower-liquid-containing cloud layers
Line 553-554: The sentence is unclear.
Line 555: What were the criteria for the selection of the subset?
Line 561: Why are cloud layers observed for < 20min not considered?
Line 565-566: How do the statistics of your analysis compare to these values?
Line 574: I don’t think the pure liquid layers refer only to clouds before ice nucleation sets in: In Fig 11 you show that quite a large fraction of PL layers have CCT of > 0C – so there won’t be ice formation setting in. rephrase.
Line 665: repeat the height range for “low cloud layers” here
Comments on Tables:
Table 1: Very good that a table regarding uncertainties is included. Please expand by adding two more columns: One indicating if the parameter is lidar-derived or lidar-radar derived and one more adding a reference in which the uncertainty is derived.
Comments on Figures:
Consider using logarithmic scale units for displaying radar reflectivity instead of linear units as commonly done in order to allow visual comparison of reflectivity with other manuscripts focusing on detailed case study analysis.
Fig. 1: Please explain the reason for the data gaps in the caption.
Fig. 2: Shorten the caption by removing sentences giving an analysis of the figure (virga is formed etc). Also, it is mentioned that the black vertical lines in b refer to the radiosonde launches at 5 and 17 UTC on Dec 31 etc. In panel b), the vertical black lines are at 0, 6, 12, and 18 UTC though and I count five (instead of four mentioned) black vertical lines. – Correct.
Also, in the description of Fig.2, pls comment on the cause of the layers of enhanced signal strength between roughly 1.5-2km altitude and 0-3 UTC.
Fig 2, 3 etc: Consider rephrasing “life-cycle” to “temporal evolution” unless you can prove that the onset/end of the observation of the cloud over RV Polarstern marks the formation/dissipation of the cloud and not the times the cloud was advected over the observatory.
Fig 3: In panel b,c add “lidar” to the title to make it coherent with panel a where you state the instrument name (radar)
Fig 4: add a horizontal line at 250m below liquid-containing layer base as well as 75m above it to show at which altitudes the values presented in Fig 3 are from.
Fig5. +line 452: In the caption refer to Panel c) as “Mean Doppler Velocity” as “vertical velocity” could be mistaken as “vertical air velocity”. Shorten the caption by removing the last sentence (“The orange regions may indicate upwind areas when taking permanent ice crystal sedimentation into account.”) since it is an interpretation of the figure which belongs to the main text.
Fig.7: “and produced strong ice virgae and triggered strong ice production.” is discussion and should thus be avoided in the caption. What happens at 15 UTC? IWC as well as IWP show a peak and should also be discussed.
Fig 8: shorten the caption by removing “All cloud layers show a blue color at cloud base (not always visible) in panel b, in an unambiguous sign for liquid-dominated cloud layers so that ice is produced by immersion freezing. The strong increase of the depolarization ratio with height (from blue to light greem yellow or even red) is caused by multiple light scattering by the water droplets.”
Fig. 11: Panel e) should have CDNC as x-axis label.
Citation: https://doi.org/10.5194/egusphere-2025-967-RC1
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