Above Cloud CCN Concentrations Help to Sustain Some Arctic Low-Level Clouds

Sterzinger, Lucas J.; Igel, Adele L.

doi:https://doi.org/10.5194/egusphere-2023-1253

Preprints

https://doi.org/10.5194/egusphere-2023-1253

Preprints

15 Jun 2023

| 15 Jun 2023

Above Cloud CCN Concentrations Help to Sustain Some Arctic Low-Level Clouds

Lucas J. Sterzinger and Adele L. Igel

Abstract. Recent studies have reported observations of enhanced aerosol concentrations directly above the Arctic boundary layer, and it has been suggested that Arctic boundary layer clouds could entrain these aerosol and activate them. We use an idealized LES modeling framework where aerosol concentrations are kept low in the boundary layer, and increased up to 50x in the free troposphere. We find that the simulations with higher tropospheric aerosol concentrations persisted for longer and had higher liquid water path. This is due to direct entrainment of the tropospheric aerosol into the cloud layer which results in a precipitation suppression from the increase in cloud droplet number and in stronger radiative cooling at cloud top due to the higher liquid water content at cloud top, which causes stronger circulations maintaining the cloud in the absence of surface forcing. Together, these two responses result in a more well-mixed boundary layer with a top that does not move rapidly in time such that it remains in contact with the tropospheric aerosol reservoir and can maintain entrainment of those aerosol particles. The boundary layer aerosol and cloud droplet concentrations, however, remained low in all simulations. Surface based measurements in this case would not necessarily suggest the influence of tropospheric aerosol on the cloud, despite it being necessary for stable cloud persistence.

Received: 09 Jun 2023 – Discussion started: 15 Jun 2023

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Journal article(s) based on this preprint

20 Mar 2024

Above-cloud concentrations of cloud condensation nuclei help to sustain some Arctic low-level clouds

Lucas J. Sterzinger and Adele L. Igel

Atmos. Chem. Phys., 24, 3529–3540, https://doi.org/10.5194/acp-24-3529-2024,https://doi.org/10.5194/acp-24-3529-2024, 2024

Short summary

Lucas J. Sterzinger and Adele L. Igel

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1253', Anonymous Referee #1, 31 Jul 2023

This study uses LES to simulate idealized mixed-phase clouds in a relatively clean Arctic boundary layer environment. Sensitivity tests are conducted to explore the impact of higher tropospheric aerosol concentrations to cloud properties. Aerosols in the boundary layer is kept 20 per mg while it is increased up to 50x in the free troposphere. Results show that entrained aerosol from the free troposphere can suppress precipitation and help to sustain clouds for longer time. I think the simulation results are clear and make sense to me. My major concern is how those simulations are relevant to real clouds in the atmosphere.
I’m skeptical about the relatively high droplet number concentration at the cloud top from the simulations (Figure 6). Are those simulations realistic? Therefore, I read some MOSAiC-related papers cited in the manuscript to learn more about the observations. I might miss some other papers, but I do not find observational papers to support model setups and conclusions in this study. I recommend the authors to add more observations to justify their model setup and/or conclusions. Some specific comments are listed below:
1. Figure 1 is the only figure related to the observation. I have several questions: do all those days have clouds? Where is the cloud layer on each day? What about the profile of large aerosol particles (those can contribute to droplet formation), instead of all aerosols larger than 12 nm? I recommend the authors plot similar figures like Figures 6, 13, 14 in Lonardi et al. (2022), for all cases chosen in this study. The model setup and conclusions would be more convincing if the authors can show observational evidence of the existence of aerosol layers above the cloud layer, and/or the potential impact of aerosols in the free troposphere to clouds in the boundary layer.
2. Based on Lonardi et al. (2022) Figure 6, clouds on July 23 and July 24 are all above the top of the boundary layer. Therefore, I think Figure 1 might be misinterpreted by the readers that clouds are in the boundary layer and they are affected by aerosols above during the MOSAiC campaign.
3. The authors said that "Here we extend the analysis presented by Lonardi et al. (2022)..." However, based on Lonardi et al. (2022) Figure 6, the large temperature inversion on July 23 and July 24 are at about 600 m and 900 m, respectively. However, in Figure 1 of this study, the top of the boundary layer on these two days are at about 300 m and 100 m. Please explain why they are so different.
4. Figure 2. Are the initial profiles of potential temperature and relative humidity based on the observation durign the MOSAiC campaign? Based on Lonardi et al. (2022) and also Figure 1 in this paper, surface temperature is very close to 0 C and the cloud temperature is just slightly below 0 C. But the initial temerapture profile for simulations in this study is much lower. Please justify the model setup.
5. Please provide formules of the profiles such that simulations can be rerun by others. What about the initial wind profiles? Do you nudge those profiles?
6. Figure 6. Is there any observational evidence to show that cloud droplet number concentration is maximum near the cloud top?
7. Page 4, Line 102, what is the CCN size distribution in the model? Is it fitted based on observations? I think results are sensitive to the CCN distribution. If the authors do not test its sensitivity, it should be clearly stated.
8. Entrainment rate is critical to bring aerosols from the free troposphere to the boundary layer. It would be nice to plot the time series of entrainment rate for different cases.
9. Page 9, Line 180: “This is indicative of a decrease in the amount of aerosol being entrained into the cloud…” Do you know the amount of aerosols entrained from the free troposphere as a function of time?
10. Page 10, Line 215: “Figure 7b shows domain-average time series of \sigma_w^2”. People usually calculate \sigma_w in the boundary layer where turbulence is vigorous. Just want to make sure \sigma_w is averaged in the whole domain or just in the boundary layer? It makes more sense to only average in the boundary layer. It would be better to show the \sigma_w profile.
Other comments:
1) Page 7, Line 127: delete “is”
2) Page 7, Line 130; Page 11 Line 219: “not shown” is not acceptable. Please consider adding it in the main text or supplement.
3) Page 12, Line 230: “Mauritsen et al. (2011) also found that…” This sentence is not clear to me. Please check.
4) Page 12, Line 246: “In these simulations, the boundary layer is …” This sentence is not clear to me. Please check.

Citation: https://doi.org/10.5194/egusphere-2023-1253-RC1
- AC2: 'Reply on RC1', Adele Igel, 08 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1253/egusphere-2023-1253-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1253-AC2
RC2:
'Comment on egusphere-2023-1253', Anonymous Referee #2, 08 Aug 2023

This paper describes a hypothetical numerical Large Eddy Simulation, to test the hypothesis that Arctic clouds are sensitive to entrainment of free troposphere aerosols from remote sources. There is nothing fundamentally wrong with the paper; I just can’t help wonder why this study was perceived, why now, and what new we can learn from it? There are a lot of detailed results that seems a bit in lack of a plan how to use, and I think the study needs to develop before it can be published; hence “major revision”.
Main concerns
The study essentially falls a part into three stories: 1) one that deals with the entrainment of aerosols from the free troposphere into the boundary layer; 2) a second that deals with the effects of those aerosols on the liquid water clouds in this boundary layer, and; 3) third, the results, as a consequence of 1 & 2, on the boundary-layer structure.
For all parts, one problem is the case and the LES. There are no attempts to illustrate or ascertain that this LES in this setup is capable to simulate this cloud and that the processes that makes it tick are adequatly modeled. The case itself looks a lot like a midlatitude marine stratocumulus but is placed at a specific date and specific location in the Arctic. But there is no information on how the case was designed and why these choices were made. The results are dependent on the rate of entrainment, which is dependent on the model formulations, the setup (e.g. subsidence) and the resolution etc. If the entrainment is sensitive to for example resolution, then all the results here will also be.
And what is new here? The second author already a few years ago showed, using another LES but on a well-documented real observed case, that free troposphere aerosols are indeed entrained into the PBL where they have an effect on the clouds. In that paper, as also in this, one main conclusion was that aerosol observations near the surface does not necessarily provide any guidance as to what goes on in the cloud layer, especially if this is dynamically decoupled. So what does this paper add that wasn't shown already in the previous paper? And why should aerosols not be entrained? At the interface of a turbulent boundary layer to a laminar free troposphere there is always entrainment. How much may depend on how vigorous the turbulence is and how well resolved the stably stratified interface is resolved.
So, what is new here? Sure, there are more runs here and more details but how are those used to shed more light on the delicate balances at the cloud top? It looks to me that the synoptic scale divergence prescribed has a large effect on the result; maybe there should have been less runs with different aerosol settings and more runs with different subsidence? And why was not inversion strength varied or the cloud bulk features(?); none of the simulated clouds are very dense. So, the results show that entrainment of free troposphere aerosols happens (provided there is any right at the interface), but any boundary-layer meteorologist could have told you that! What else did we learn?
Once inside the boundary layer these aerosols have more or less the expected effects. Ice is there but IN is so small that ice plays no important role to the dynamics here. So why have it there? Solar radiation is also present, but so little it (probably) do not have an effect. So why have it there at all? More free troposphere aerosol results in more CCN in the boundary layer – obviously – and this results in denser clouds (larger LWP) with smaller and more numerous droplets. And this in turn leads to more cloud top buoyancy and more mixing; Nothing spectacular here; mostly the expected effects. But lots of details that may – or may not – reveal something. Problem is those are never explored in any detail; the analysis is unimaginative and the result is rather boring. Probably right, at least in principle, but still rather boring. With this effort in numerical simulation, there just has to be something more you can do.
Minor comments:
Line 8: That the cloud top does not move is controlled by subsidence. In reality this mean that the the cloud does move upward by mixing, but that i is immediately advected back down again by synoptic-scale advection. The bulk result may be close to net zero, but mixing and advection are not the same thing.
Line 15: Clouds are indeed important for Arctic climate but I have seen no study that shows them to have a large effect on “Arctic amplification”. If these authors have, I’d like to see a reference here.
Lines 20-21: A reference should be inserted here, as this statement comes from one single study (SHEBA) and most other summer studies I have seen does not show this warming regime, unless the ice has melted completely.
Line 45: “argued” is better than “found”; there is no way in which Matt could have proven this, but it is a good argument.
Line 64: What “scale” is that? LES is a numerical technique and is appropriate for flows with large enough eddies but there is no specific scale; it all depends on the problem.
Lines 73-74: So what happens with coalescence? Each droplet forms on one CCN each, but after having collided, forming a larger drop, there is no way the new aerosol formed by evaporation of that drop can be assigned back to the original CCNs.
Line 69: I’d like to know more about the aerosols scheme. How is formation of new aerosols handled and how are aerosols that become CCN or are entrained replaced? How is size distribution consndered?
Line 81-83: This is a different motivation to that given earlier, and does not rely on assumptions on entrainment.
Line 84: Is this radiation code sensitive to effective droplet radius?
Line 86-87: Setting the heat fluxes to zero is a reasonable assumtions; they are never very large anyway. But what about the momentum flux? Is this a free-slip simulation? How is this “reasonable” for ice surfaces?
Line 94: It looks like the cloud top is decreasing, at least for the less vigorous boundary layers. So, how was this value selected and how is the LES upper boundary and the magnitude of the entrainment affected? Is it a special target to have a constant cloud top constant, and should that then not require different divergence? And how would that affect entrainment?
Line 102: In the whole intro with references and all, the units for aerosol concentreations is cm^-3. Here all of a sudden, in the LES, it is mg^-1. Why?
Line 105-106: Since IN and ice processes are hardly considered, why even bother with IN? This is hardly a prototypical mixed-phase cloud as it is.
Line 156: Delete space between “salt” and “20”.
Line 165: Do you mean “dropets” and not “aerosols” here?
Line 176: What “cloud radius”?
Line 179: What “cloud number concentration”?
Lines 198-199: Why choose such an awkward date? While it is true that aerosol concentrations are low in autumn, this is a hypothetical LES and it would not be more or less hypothetical with the sun switched off completely.
Line 200: The flux divergence cannot have the unit “W m^-2”.

Citation: https://doi.org/10.5194/egusphere-2023-1253-RC2
- AC1: 'Reply on RC2', Adele Igel, 08 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1253/egusphere-2023-1253-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1253-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1253', Anonymous Referee #1, 31 Jul 2023

This study uses LES to simulate idealized mixed-phase clouds in a relatively clean Arctic boundary layer environment. Sensitivity tests are conducted to explore the impact of higher tropospheric aerosol concentrations to cloud properties. Aerosols in the boundary layer is kept 20 per mg while it is increased up to 50x in the free troposphere. Results show that entrained aerosol from the free troposphere can suppress precipitation and help to sustain clouds for longer time. I think the simulation results are clear and make sense to me. My major concern is how those simulations are relevant to real clouds in the atmosphere.
I’m skeptical about the relatively high droplet number concentration at the cloud top from the simulations (Figure 6). Are those simulations realistic? Therefore, I read some MOSAiC-related papers cited in the manuscript to learn more about the observations. I might miss some other papers, but I do not find observational papers to support model setups and conclusions in this study. I recommend the authors to add more observations to justify their model setup and/or conclusions. Some specific comments are listed below:
1. Figure 1 is the only figure related to the observation. I have several questions: do all those days have clouds? Where is the cloud layer on each day? What about the profile of large aerosol particles (those can contribute to droplet formation), instead of all aerosols larger than 12 nm? I recommend the authors plot similar figures like Figures 6, 13, 14 in Lonardi et al. (2022), for all cases chosen in this study. The model setup and conclusions would be more convincing if the authors can show observational evidence of the existence of aerosol layers above the cloud layer, and/or the potential impact of aerosols in the free troposphere to clouds in the boundary layer.
2. Based on Lonardi et al. (2022) Figure 6, clouds on July 23 and July 24 are all above the top of the boundary layer. Therefore, I think Figure 1 might be misinterpreted by the readers that clouds are in the boundary layer and they are affected by aerosols above during the MOSAiC campaign.
3. The authors said that "Here we extend the analysis presented by Lonardi et al. (2022)..." However, based on Lonardi et al. (2022) Figure 6, the large temperature inversion on July 23 and July 24 are at about 600 m and 900 m, respectively. However, in Figure 1 of this study, the top of the boundary layer on these two days are at about 300 m and 100 m. Please explain why they are so different.
4. Figure 2. Are the initial profiles of potential temperature and relative humidity based on the observation durign the MOSAiC campaign? Based on Lonardi et al. (2022) and also Figure 1 in this paper, surface temperature is very close to 0 C and the cloud temperature is just slightly below 0 C. But the initial temerapture profile for simulations in this study is much lower. Please justify the model setup.
5. Please provide formules of the profiles such that simulations can be rerun by others. What about the initial wind profiles? Do you nudge those profiles?
6. Figure 6. Is there any observational evidence to show that cloud droplet number concentration is maximum near the cloud top?
7. Page 4, Line 102, what is the CCN size distribution in the model? Is it fitted based on observations? I think results are sensitive to the CCN distribution. If the authors do not test its sensitivity, it should be clearly stated.
8. Entrainment rate is critical to bring aerosols from the free troposphere to the boundary layer. It would be nice to plot the time series of entrainment rate for different cases.
9. Page 9, Line 180: “This is indicative of a decrease in the amount of aerosol being entrained into the cloud…” Do you know the amount of aerosols entrained from the free troposphere as a function of time?
10. Page 10, Line 215: “Figure 7b shows domain-average time series of \sigma_w^2”. People usually calculate \sigma_w in the boundary layer where turbulence is vigorous. Just want to make sure \sigma_w is averaged in the whole domain or just in the boundary layer? It makes more sense to only average in the boundary layer. It would be better to show the \sigma_w profile.
Other comments:
1) Page 7, Line 127: delete “is”
2) Page 7, Line 130; Page 11 Line 219: “not shown” is not acceptable. Please consider adding it in the main text or supplement.
3) Page 12, Line 230: “Mauritsen et al. (2011) also found that…” This sentence is not clear to me. Please check.
4) Page 12, Line 246: “In these simulations, the boundary layer is …” This sentence is not clear to me. Please check.

Citation: https://doi.org/10.5194/egusphere-2023-1253-RC1
- AC2: 'Reply on RC1', Adele Igel, 08 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1253/egusphere-2023-1253-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1253-AC2
RC2:
'Comment on egusphere-2023-1253', Anonymous Referee #2, 08 Aug 2023

This paper describes a hypothetical numerical Large Eddy Simulation, to test the hypothesis that Arctic clouds are sensitive to entrainment of free troposphere aerosols from remote sources. There is nothing fundamentally wrong with the paper; I just can’t help wonder why this study was perceived, why now, and what new we can learn from it? There are a lot of detailed results that seems a bit in lack of a plan how to use, and I think the study needs to develop before it can be published; hence “major revision”.
Main concerns
The study essentially falls a part into three stories: 1) one that deals with the entrainment of aerosols from the free troposphere into the boundary layer; 2) a second that deals with the effects of those aerosols on the liquid water clouds in this boundary layer, and; 3) third, the results, as a consequence of 1 & 2, on the boundary-layer structure.
For all parts, one problem is the case and the LES. There are no attempts to illustrate or ascertain that this LES in this setup is capable to simulate this cloud and that the processes that makes it tick are adequatly modeled. The case itself looks a lot like a midlatitude marine stratocumulus but is placed at a specific date and specific location in the Arctic. But there is no information on how the case was designed and why these choices were made. The results are dependent on the rate of entrainment, which is dependent on the model formulations, the setup (e.g. subsidence) and the resolution etc. If the entrainment is sensitive to for example resolution, then all the results here will also be.
And what is new here? The second author already a few years ago showed, using another LES but on a well-documented real observed case, that free troposphere aerosols are indeed entrained into the PBL where they have an effect on the clouds. In that paper, as also in this, one main conclusion was that aerosol observations near the surface does not necessarily provide any guidance as to what goes on in the cloud layer, especially if this is dynamically decoupled. So what does this paper add that wasn't shown already in the previous paper? And why should aerosols not be entrained? At the interface of a turbulent boundary layer to a laminar free troposphere there is always entrainment. How much may depend on how vigorous the turbulence is and how well resolved the stably stratified interface is resolved.
So, what is new here? Sure, there are more runs here and more details but how are those used to shed more light on the delicate balances at the cloud top? It looks to me that the synoptic scale divergence prescribed has a large effect on the result; maybe there should have been less runs with different aerosol settings and more runs with different subsidence? And why was not inversion strength varied or the cloud bulk features(?); none of the simulated clouds are very dense. So, the results show that entrainment of free troposphere aerosols happens (provided there is any right at the interface), but any boundary-layer meteorologist could have told you that! What else did we learn?
Once inside the boundary layer these aerosols have more or less the expected effects. Ice is there but IN is so small that ice plays no important role to the dynamics here. So why have it there? Solar radiation is also present, but so little it (probably) do not have an effect. So why have it there at all? More free troposphere aerosol results in more CCN in the boundary layer – obviously – and this results in denser clouds (larger LWP) with smaller and more numerous droplets. And this in turn leads to more cloud top buoyancy and more mixing; Nothing spectacular here; mostly the expected effects. But lots of details that may – or may not – reveal something. Problem is those are never explored in any detail; the analysis is unimaginative and the result is rather boring. Probably right, at least in principle, but still rather boring. With this effort in numerical simulation, there just has to be something more you can do.
Minor comments:
Line 8: That the cloud top does not move is controlled by subsidence. In reality this mean that the the cloud does move upward by mixing, but that i is immediately advected back down again by synoptic-scale advection. The bulk result may be close to net zero, but mixing and advection are not the same thing.
Line 15: Clouds are indeed important for Arctic climate but I have seen no study that shows them to have a large effect on “Arctic amplification”. If these authors have, I’d like to see a reference here.
Lines 20-21: A reference should be inserted here, as this statement comes from one single study (SHEBA) and most other summer studies I have seen does not show this warming regime, unless the ice has melted completely.
Line 45: “argued” is better than “found”; there is no way in which Matt could have proven this, but it is a good argument.
Line 64: What “scale” is that? LES is a numerical technique and is appropriate for flows with large enough eddies but there is no specific scale; it all depends on the problem.
Lines 73-74: So what happens with coalescence? Each droplet forms on one CCN each, but after having collided, forming a larger drop, there is no way the new aerosol formed by evaporation of that drop can be assigned back to the original CCNs.
Line 69: I’d like to know more about the aerosols scheme. How is formation of new aerosols handled and how are aerosols that become CCN or are entrained replaced? How is size distribution consndered?
Line 81-83: This is a different motivation to that given earlier, and does not rely on assumptions on entrainment.
Line 84: Is this radiation code sensitive to effective droplet radius?
Line 86-87: Setting the heat fluxes to zero is a reasonable assumtions; they are never very large anyway. But what about the momentum flux? Is this a free-slip simulation? How is this “reasonable” for ice surfaces?
Line 94: It looks like the cloud top is decreasing, at least for the less vigorous boundary layers. So, how was this value selected and how is the LES upper boundary and the magnitude of the entrainment affected? Is it a special target to have a constant cloud top constant, and should that then not require different divergence? And how would that affect entrainment?
Line 102: In the whole intro with references and all, the units for aerosol concentreations is cm^-3. Here all of a sudden, in the LES, it is mg^-1. Why?
Line 105-106: Since IN and ice processes are hardly considered, why even bother with IN? This is hardly a prototypical mixed-phase cloud as it is.
Line 156: Delete space between “salt” and “20”.
Line 165: Do you mean “dropets” and not “aerosols” here?
Line 176: What “cloud radius”?
Line 179: What “cloud number concentration”?
Lines 198-199: Why choose such an awkward date? While it is true that aerosol concentrations are low in autumn, this is a hypothetical LES and it would not be more or less hypothetical with the sun switched off completely.
Line 200: The flux divergence cannot have the unit “W m^-2”.

Citation: https://doi.org/10.5194/egusphere-2023-1253-RC2
- AC1: 'Reply on RC2', Adele Igel, 08 Nov 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1253/egusphere-2023-1253-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-1253-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Adele Igel on behalf of the Authors (08 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Nov 2023) by Lynn M. Russell

RR by Anonymous Referee #1 (27 Nov 2023)

RR by Anonymous Referee #2 (25 Dec 2023)

Suggestions for revision or reasons for rejection

This is the second time I read this, and very I'm happy to say it reads much better this time. After my first read i felt the changes still needed could be easily done by editing and I was leaning towards "minor revision". However, summarizing my finding they now appear somewhat more substantial and there are issues that must be resolved. Hence, I'm going for "major revision" but will let the editor know I would survive if the decision came down to be "minor".

Major comments:
While my first reaction to a summertime case without solar radiation was quite negative, and I don't thing I explicitly suggested that, I must say as long as its clear that this is a hypothetical experiment, is is quite successful.

However, the sensitivity case with a stable near-surface PBL is quite unrealistic; I've seen many profiles of clouds like this and I've never seen one like it. I can't see what process that could lead to this shape of profiles, and it does have the unfortunate consequence of making a much juicier cloud than all the other cases. I instead suggest lowering the surface temperature by that amount, rather than increasing the cloud temperature. That would serve to preserve - roughly - the cloud thermodynamics and avoid changing too many things at the
same time. In addition it would be more realistic. Fortunately, I don't see the results from the sensitivity tests playing a large role. In fact, I also suggest dropping Section 4.3 entirely. As it is now, it doesn't add much to the conclusions, but it also seems to me to contain a lot more than we get to see. Maybe seeds for another paper, rather than "waste" it here?

I also have two more substantial comments. The first relates to the experimental set up up in relations to reality and what is explored. In reality, entrainment tends to elevate the top of the cloud layer by mixing in free troposphere air into the boundary layer, and so for a constant cloud-top height, subsidence is required to balance this. Hence, if entrainment is
varied while subsidence is kept constant the cloud top height will vary. And they do; this is also what the simulations show and - in my opinion - while realistic it adds two complications.

First, at the end of the manuscript there is a discussion about how the subsidence taking over from entrainment and "collapses" the boundary layer such that the "plume" of aerosols in the free troposphere separates from the boundary-layer top. Now, for the life of me I can't understand why the free troposphere aerosols are not advected along with the PBL top so that the "plume" follows it. These results to me seem unrealistic and I would like that this is properly checked before the manuscript is accepted. I understand that this could lead to a major revision, but I hope not.

Second, if this is realistic it contaminates - if you will - the results. There is the effect of entraining aerosols, which to a first order is related to the aerosol concentration in the free troposphere and, to a second order, has the feedback that a more "active" and persistent cloud, entrains more efficiently thereby contributing to its own persistence. But with the PBL collapse and the separation, which also generates a feedback by additionally suppressing cloud formation, follows yet another aspect that, if realistic, seems to be somewhat artificial. Hence, one could debate if it had been better to tune the different experiments such that the PBL top remained unchanged by changing the aerosols aloft, the same in all runs. I'm not
saying this is the right way to go, but it should at least be discussed and this would be the minor revision.

Detailed coments:
Figure 1: Maybe plot the height axis scaled to the main inversion base would make things clearer?

Line 64: No black lines in figure, but I guess that would just make if messy? Maybe suggestion above would help?

Lines 66-67: Suggest "All aerosol profiles except that of 24 July 2020 (Fig. 1b) have higher ... inversion."

Line 70: Drop "can"; you do, don't you?

Lines 107-108: What do you mean by "ice is negligible"? Experience is that much of the precip from even from summer Sc is frozen, rather than drizzle, so in what way is it negligible.

Line 113: Again, change to "Surface heat fluxes were..."; surface moment flux is not zero!!

Lines 121-122: Inversions "5K per 100 m or stronger" seems to me to be on the strong side, looking at published studies. Its not unusual, but probably not "frequent" either.

Lines 152-153: The stable configuration is somewhat unrealistic, for three reasons: 1) Keeping the same surface temperature makes the whole PBL quite warm. It would seem more natural to instead lower the surface temperature and keeping the cloud layer as in control; 2) The structure with a deep quite stable layer is something that I've never seen. Instead the common structure would be two well mixed layers, one associated with the cloud and one with near-surface shear produced turbulence, separated by a shallow inversion; 3) Together, 1) & 2) makes the liquid water content unrealistically high.

Lines 163: I suggest "the six aerosol sensitivity simulations", since there are more simulations coming.

Lines a63-164: Suggest "appear to attain quasi-steady cloud tops", since this doesn't happen until after some 15h.

Line 171: Do you mean "too small to be observed, or "too small to be measured"? These are not the same you know...

Lines 177-179: This seems to contradict earlier statements, and previously published studies by the second author, that near-surface aerosol observations are not representative for the cloud layer

Para beginning /w line 203: This needs to be stated carefully. Many of these simulations are essentially optically thin clouds, clouds that are not black bodies, at least large parts of the time. This is especially true for the low aerosol cases. But here these clouds are kept appearing in a well mixed PBL, in part by the initial conditions. In reality, without solar radiation the surface temperature would drop, and the clouds would become decoupled. But the set up here artificially prohibits this development by i) keeping the surface temperature fixed and ii) by the zero heat flux at the surface. One could argue that while "gray clouds" certainly are realistic in the Arctic, such clouds in a (relatively) deep well mixed PBL isn't.
Instead decoupling would occur, possibly with fog forming in the lower layer.

Para beginning /w line 235 & fig6: Variance of vertical velocity most certainly does not go to zero at the surface in reality; is this perhaps only the resolved-scale variance? If so that needs to be clear; scaled with friction velocity, this variance tends towards a fixed rather well-known value.

Line 237: Drop "average" or rephrase; the average vertical wind is always close to zero, minus the subsidence. It is the strength of updrafts and downdrafts that become stronger or weaker, which is manifested in the variance, but the average vertical velocity remains (essentially) unaffected.

Line 252: Choice of words; this has nothing to do with "success" or "failure". Maybe "efficiency"?

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ED: Reconsider after major revisions (25 Dec 2023) by Lynn M. Russell

AR by Adele Igel on behalf of the Authors (05 Feb 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 Feb 2024) by Lynn M. Russell

RR by Anonymous Referee #1 (17 Feb 2024)

RR by Michael Tjernström (19 Feb 2024)

ED: Publish as is (19 Feb 2024) by Lynn M. Russell

AR by Adele Igel on behalf of the Authors (22 Feb 2024)

Journal article(s) based on this preprint

20 Mar 2024

Above-cloud concentrations of cloud condensation nuclei help to sustain some Arctic low-level clouds

Lucas J. Sterzinger and Adele L. Igel

Atmos. Chem. Phys., 24, 3529–3540, https://doi.org/10.5194/acp-24-3529-2024,https://doi.org/10.5194/acp-24-3529-2024, 2024

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Lucas J. Sterzinger and Adele L. Igel

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lsterzinger/sterzinger-igel-2023-scripts Lucas J. Sterzinger https://zenodo.org/record/8010973

Lucas J. Sterzinger and Adele L. Igel

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We find in our study using idealized large eddy simulations that clouds forming in the Arctic in environments with low concentrations of aerosol particles may be sustained by mixing in new particles through cloud top. Observations show that higher concentrations of these particles regularly exist above cloud top in concentrations that are sufficient to promote this sustenance.


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