Microphysics of Arctic Stratiform Boundary-layer Clouds during ARCSIX&nbsp;

Korolev, Alexei V.; Lawson, R. Paul

doi:10.5194/egusphere-2025-5205

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https://doi.org/10.5194/egusphere-2025-5205

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14 Nov 2025

| 14 Nov 2025

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Microphysics of Arctic Stratiform Boundary-layer Clouds during ARCSIX

Alexei V. Korolev and R. Paul Lawson

Abstract. Clouds have a major impact on rapidly decreasing sea ice in the Arctic, yet much is still unknown how cloud microphysics influences cloud development. In situ and remote data were collected by the NASA P-3 and SPEC Inc. Learjet research aircraft in Arctic stratiform boundary-layer clouds over the oceans and sea ice bordering northern Greenland between 25 May and 15 August 2024 during the ARCSIX project. Both aircraft carried a suite of nearly identical state-of-the-art microphysical sensors. Additionally, the P-3 was equipped with aerosol and remote-sensing instrumentation and the Learjet was equipped with a zenith/nadir Ka-band radar. The total length of clouds examined remotely and in-situ by the two aircraft totaled 12,417 km, with 6,266 km of in-situ measurements. Mixed-phase clouds were sampled during 60.5 % of time in cloud, and all-liquid clouds were measured 39.5 % of the time. Cloud- top temperatures were ≥ - 9 °C during 90 % of the stratiform boundary-layer cloud investigations. Single-layer mixedphase clouds sampled with cloud-top temperatures ≥ - 4 °C often contained concentrations of ice particles more than five orders of magnitude higher than measured concentrations of ice-nucleating particles. Despite the high ice concentrations, microphysical conditions supporting secondary ice production were not always present. In contrast, in some clouds where environmental conditions met commonly accepted criteria for secondary ice production, ice particle concentrations were closer to what is expected from primary nucleation. The quality of measurements was unprecedented, but results from our preliminary analyzes raise more questions about primary and secondary nucleation mechanisms than they answer.

Received: 20 Oct 2025 – Discussion started: 14 Nov 2025

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Alexei V. Korolev and R. Paul Lawson

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'Comment on egusphere-2025-5205', Jeff French, 03 Dec 2025 reply
Review of ‘Microphysics of Arctic Stratiform Boundary-layer Clouds during ARCSIX’
Authors: Korolev and Lawson
egusphere-2025-5205

The authors present measurements of cloud microphysical structure from several aircraft-observed cases of stratiform boundary-layer clouds (SBCs) during the recent (mid-2024) ARCSIX campaign. The manuscript focuses on cases with relatively warm cloud tops (T > -9 degC and T=>-4 degC in extreme cases) that contain ice, either widespread or in isolated pockets. The authors assert several times that there are no good explanations, based on our current understanding of ice nucleation, to explain the relatively high ice concentrations that were observed in some of these clouds. The authors present a very well written and thorough discussion providing some conjecture of how ice may have been initiated in these clouds. In the end, the authors admit that the observations along with their interpretation provide more questions than answers. I agree with their assessment.
I found the manuscript well-written and quite enjoyable to read. While I might disagree on a few minor points in the paper, I think this has more to do with style than actual substance. I do find it refreshing to read a paper that isn’t able to ‘solve’ all of the questions raised by the observations and to admit that there are some aspects of cloud evolution, especially in mixed-phase conditions, that we do not fully understand. The authors do a good job of pointing back to previous measurements in the arctic to demonstrate that others have made similar measurements. This provides confidence in the measurements provided here and demonstrates this isn’t a ‘new’ problem, but it is a timely one!
I recommend accepting with minor changes.

Broad/Major comment:
INP measurements quoted in the paper come both from ARCSIX and MOSAIC and are based on filter measurements. From ARCSIX the measurements are quite limited because of the low collection time (length of time filters are exposed), but this is less of a problem with MOSAIC. Regardless, this methodology of collecting filters and processing the samples offline requires assumptions about mode of nucleation and how particles interact with hydrometeors, etc. I am not asserting these assumptions are wrong, but I do wonder out loud if the methodology does not lead to limitations of describing what primary nucleation might be able to produce in conditions that do not adhere strictly to those associated with determining INPs from filter samples. I bring this point up not because I necessarily disagree with the author’s assessment and/or conjecture in the paper, but rather because I think the lack of explanation for the observations based on our current scientific understanding can provide even more opportunity for the author’s to be a little more provocative in exploring ways to explain the measurements.

Minor comments:
Line 33, abstract: “The total length of clouds…” delete total here, it is redundant.

Line 42, abstract: change analyzes to analysis

Line 79 & 80: Morrison et al. (2012) is missing in references; I’m not sure what is meant by “the predominance of Arctic mixed-phase clouds is largely due to their longevity.” (I was going to dig up the reference to see if I could figure it out from there…) Are the authors stating that if arctic clouds were not long-lived they would be mostly all liquid or all ice? That doesn’t seem correct?

Line 81 and 82: I suggest changing the sentence slightly to state that there exists several reports of observations of mixed clouds containing concentrations of ice crystals greater than what would be expected from primary nucleation at cloud top temperatures (…or the coldest cloud temperature).

Line 86: …temperature of -6…add the word ‘of’

Lines 119-121: clarify that Shupe et al. (2011) note that when boundary layer clouds clouds are present, they are mixed phase 60% of the time and all liquid 40% of the time.

Figure 2: in fig 2b, one of the Learjet tracks is missing (the one directly north of Pituffik around 80 deg latitude).

Line 140-1: “…and produceds gray images at more than 20 times the data rate of previous gray probes.” I don’t understand this statement. Is this asserting that other gray probes ‘miss’ 19 out of 20 particles? Certainly the 2DS has higher resolution than other gray-scale probes such as the 25 micron CIP-gray, but that would lead to 2.5 times better resolution for the 2DS (and hence data rate), not 20 times? Data rate seems an odd measure here…

Lines 148-159: It would be useful to add one or two sentences to describe how the KPR data are processed/presented here. Are the authors using the ‘native’ resolution of the radar, and if so what is it? Are they smoothing the radar measurements along track? Along range?

Line 165: the authors quote drop(let) concentrations averaged over P3 and Learjet measurements of 65+/-59 cm-3. I don’t understand what this measurement represents? Is this all of the incloud measurements averaged together (over all flights)? If so, is the +/-59 then represent 1 standard deviation? One quartile? Or does this measurement represent something else altogether?

In figure 4, and in figure 8, and in figure 10 – in the mixed phase regions, based on the LWC/TWC measurements the ice water content is basically zero…that is not all that surprising that in these regions the vast majority of the mass is being carried by liquid substance…however, I do wonder if these estimates (or my interpretation of them) is consistent to what would be predicted of IWC from the OAP measurements? Also, I think the authors should note in the description of these cases that although in the mixed phase region, the ice particles concentration is high, the amount of mass carried in the ice phase is still incredibly small compared to the total condensed mass.

Line 240 (in reference to figure 6)…the correlation noted in the text between ice concentration, IWC, extinction and lidar measurements are NOT with the lidar surface return, but rather the lidar return above the surface. I found the labeling of ‘lidar surface return’ confusing since we are not interested in the surface return, but rather return between the aircraft flight level and surface.

Line 315: replace ‘ignored completely’ with ‘dismissed’.

Lines 351-2, and 365-6: Why do the authors assert here that all ice crystals result from primary nucleation? I understand there is no apparent mechanism to explain SIP, but then again there is no good understanding to support primary? Since it cannot be explained, I’m not sure it can be assigned all to primary?

Lines 360-4: I think the authors are arguing low concentration of graupel below cloud is because the graupel is not growing via collisions due to its nearly identical fall velocity of drizzle drops. But the graupel would also grow via deposition and as it fell would collect drizzle, so I’m not sure I follow the argument. I don’t necessarily have a better one, though….

Line 374-5: The authors bring up a great point!!!!

Figure 9e: The reflectivity labels on the dBZ colorscale needs to be darker, it is not readable.

Line 438: ‘measurements of temperature, altitude, microphysical measurements and…’ change the second ‘measurements’ to ‘parameters’ (or ‘characteristics’)

Line 488-9: the sentence beginning ‘Of the total….’ Needs to be re-worded somehow.

Line 543: change ‘like’ to likely

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Citation: https://doi.org/10.5194/egusphere-2025-5205-RC1

Alexei V. Korolev and R. Paul Lawson

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Alexei V. Korolev and R. Paul Lawson

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Short summary

The International Panel on Climate Change has concluded that aerosols and clouds are significant contributors to the rate of warming in the Arctic, which is now shown to be more than twice that of the global average. Climate model predictions suggest that the Arctic Ocean will become ice-free sometime between 2030 and 2050. The research presented here increases our knowledge of how aerosols, clouds and surface properties contribute to warming and the melting of sea ice in the Arctic.


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