the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ice-nucleating particle concentration impacts cloud properties over Dronning Maud Land, East Antarctica, in COSMO-CLM2
Abstract. Ice-nucleating particles (INPs) have an important function in the freezing of clouds, but are rare in East Antarctica with concentrations between 6 × 10-6 L-1 and 5 × 10-3 L-1 observed at the Belgian Princess Elisabeth Station. These low concentrations offer a possible explanation for the occurrence of supercooled liquid water in clouds observed using the station's Micro Rain Radar and Ceilometer. We used COSMO-CLM² with an added aerosol-cycle module to test the cloud phase’s sensitivity in response to varying prescribed INP concentrations. We tested two cases, one in austral summer, one in austral winter, and analysed the differences resulting from INP concentration changes for an area around the station and over the Southern Ocean within the selected domain. Our results show a strong influence of the INP concentration on the liquid water path in both regions, with higher concentrations reducing the amount of liquid water. Over the ocean, this effect is stronger during winter: During summer, a significant portion of water remains in liquid state regardless of INP concentration. Over the continent, this effect is stronger during summer: Temperatures in winter frequently fall below -37 °C, allowing homogeneous freezing. The largest increase of the liquid water fraction of total cloud hydrometeor mass is simulated over the ocean in winter, from 9.8 % in the highest tested INP concentration to 50.3 % in the lowest. The radiative effects caused by the INP concentration changes are small with less than 3 W m-2 difference in the averages between different concentrations.
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RC1: 'Comment on egusphere-2024-1341', Anonymous Referee #1, 03 Jul 2024
General comments:
The authors present an interesting and relevant study on the impact of modeled INP concentrations on cloud properties in the Antarctic. They assess both the impact on cloud phase and cloud radiative effect. This is a valuable contribution to model development and improvement for polar regions with typically low INP concentrations. Overall, the study holds a very good scientific standard, but the presentation (writing and figures) has shortcomings and should be improved.
Specific comments:
Page 1-2, line 19-22:
This part sounds very specific and slightly simplifying to come that early in the introduction, as there exist other cloud structures than the two-layered one as well. Please consider writing something about the occurrence and presence of mixed-phase clouds in Antarctica in general in this prominent position. This very recent paper by Dietel et al. and references therein might have some inspiration: https://acp.copernicus.org/articles/24/7359/2024/
Page 2, line 29-32:
I suggest rephrasing these two sentences. You may add that you compare a liquid water containing cloud and an ice-only cloud having the same total water content (at least I assume this is what you mean), and possibly a short statement about why that is. The second sentence (As a result […] ice-water conversion.) is currently very vague and might be formulated more specific or eventually omitted.
Page 2, line 32:
Depending on the rephrasing of the previous sentences, make sure that it is clear what “those effects” refers to or reformulate. Also, there might be other potential changes beyond cloud phase that affect uncertainty in global climate models. If “those effects” also should include potential changes in cloud cover, total cloud water content etc. please state that.
Page 2, line 36/37:
Please consider adding against which kind of observations the bias is expressed (satellite observations, I assume).
Page 3, line 55:
Please add some information about the location of Dome C, either in coordinates or in words.
Page 3, line 59-61:
Please add a reference (or move the reference Kretzschmar et al. 2020 to this sentence, if it is the same).
Page 4, line 92-95:
This sentence is very long and a bit complicated. I suggest splitting and rephrasing.
Page 4, line 95-97:
I also suggest changes to this sentence: What is meant by “given their focus on these regions”? Climate and weather models do not automatically have a focus on polar regions.
Page 4, line 98:
Remove “can”, just write “we assess”.
Page 4-5, line 112-122:
Ice nucleation is often described divided into different mechanisms (contact freezing, deposition freezing, immersion freezing). Could you elaborate on which mode your measurements belong to? As the module used in your study is different from the scheme used by Vignon et al. in WRF e.g., this information is not crucial to your model experiments, but it would be interesting to know for the reader to be able to compare to other studies.
Page 6, line 165-166:
I suggest changing the sentence to the following (please check that this equally correct):
This module adds 16 different INP concentration variables corresponding to different activation temperatures.
Page 7, line 188-189:
“Integration” does not sound like the right word to use here. Do you mean simulation, implementation, version or something else?
Page 7, line 188-195:
This whole part is important, but not really suitable in the section Model description. It would fit better either in the end of the Introduction or at the beginning of the Results section. Please consider adapting and moving it.
Page 8, line 209:
You might also mention Schäfer et al. 2024 https://acp.copernicus.org/articles/24/7179/2024/ during the discussion of SIP, as their study (although for the Arctic) included observationally constrained INP concentrations addition to added SIP (again relevant for Discussion part as well).
Page 8, line 216-220:
For which time periods and cloud types/altitudes were the tests performed? Can you tell whether they covered the environmental conditions present throughout the total study?
Page 9, line 242:
The beginning of the Results section should be improved. What do you mean by “In the simulation”, possibly “In our simulations/experiments”? As indicated earlier, some of the discussion of the end of 3.1 might fit here and it would be useful to introduce the structure of the Results section to the reader.
Figures 2 and 3:
Please add panel labels (a), (b) etc. to the individual panels and use them when referring to the figures in the text.
For the ceilometer colorbar: Please write the exponents as exponents and not ^-1.
You might consider removing the long headings for the six individual plots and instead add the variable (Attenuated Backscatter, Radar Reflectivity, Cloud Ice Content, Cloud Liquid Water Content) to the x-axis label and the instrument/simulation name (Ceilometer, MRR, VL, VH) in the upper left corner of each plot.
Page 10, line 246:
What do you mean by frontal cloud? If you mean a certain time period or altitude region, please indicate that. If you mean the whole (shown) cloud, I suggest removing “frontal”.
Page 10, line 260:
The terms VL concentration and VL setting are used aside throughout the manuscript, sometimes also VL INP concentration. I recommend choosing one nomenclature and sticking to it.
Page 11, line 267:
The terms TQC and TQI are nowhere explained in the text apart from figure captions. Please add or remove the term in brackets.
Page 14, line 298-301:
Change beginning of the sentence to: When applying the constraint to only count timesteps with at least […]. I also suggest making the last part starting with “highlighting the stronger” a new sentence.
Page 14, line 304:
Switch order of VH and M in text to match order in figures.
Page 15, line 308-310:
This sentence should be improved, potentially avoiding the repetition of close to 0 Wm-2 and instead repeating what the increase refers to (from VL to VH).
Page 17, line 359-360:
I am not sure if I understand what you want to convey with this sentence. The prescribed CCN concentration may also influence vertical cloud structure through different size distributions and sedimentation velocities. But mainly, I am again wondering about the conditions of the tests you performed and wrote about earlier. If they covered all possible conditions and you didn’t see an influence on liquid water content, the CCN concentrations might really not be of importance, but if you still consider the prescribed (low) CCN concentration a relevant source of error/uncertainty, some more discussion/references might be good. Please clarify.
Page 18, line 363:
Could you use a stronger formulation than “could likely mean”?
Page 18, line 377:
Please replace “Hallet-Mossop” by “rime splintering”. (“Hallet-Mossop” has never been introduced as a term, although you correctly cite them in line 174.)
Page 18, line 379:
I suggest replacing “alleviating” by “reducing”.
Page 18, line 380:
I assume that by thicker clouds you mean the clouds where the water content exceeds your threshold. Consider a better formulation so that it can’t be confused with geometrically thicker.
Figure A1:
Please remove “Temperature” from the heading and put it into the colorbar label instead.
Figure A2:
Same comment as to Figure 2 and 3 applies, but is less important in the appendix.
Technical corrections:
Title/page 4, line 101:
The term “Dronning Maud Land” appears only in the title and then never again. Instead, the term “Queen Maud Land” appears in the first sentence of the Observations part. Please make that consistent and use the same term both places.
Page 4, line 97:
Please correct spelling: Antarctic sea ice (no capital letters in sea ice).
Page 4, line 98:
Please correct spelling: Southern Ocean (with capital letters at the beginning).
Page 5, line 147:
Correct spelling: ground-based lidar.
Page 6, line 162:
Please correct spelling: Antarctic ice sheet (no capital letters in ice sheet).
Page 7, line 174:
Please correct spelling: rime splintering (no capital letters).
Page 7, eq. 2:
There is one redundant closing bracket at the end of the equation. You might also consider to write the exponent in superscript instead of in brackets.
Page 8, line 210:
What is the plural “SIPs” meaning here – secondary ice productions? I suggest writing SIP modes instead.
Page 8, line 214:
Citation Bigg and Hopwood 1963 should be without brackets.
Figure 1:
Consider moving the labels (a), (b), (c) in the caption text in front of the dates instead of behind.
Figures 4, 5, 6:
Units on axis labels should not be italic.
Page 14, line 298:
Correct spelling: with.
Page 16, line 311:
Please correct spelling: optically.
Page 17, line 327:
Citation Ricaud et al. 2024 should be without brackets.
Page 17, line 353:
Correct spelling: ice crystal number concentration (no capital letters).
Page 17, line 352, 355:
Please correct spelling: secondary ice processes (no capital letters). Or use the abbreviation SIP that you introduced for secondary ice production earlier.
As you probably notice, I commented on the spelling regarding small or capital letters quite a few places and I noticed that this was not consistent for many terms throughout the manuscript. I recommend going through the whole manuscript carefully checking which terms to use capital letters for and which not, as I might have overseen some places.
Page 17, line 357:
MARCUS is an acronym for a campaign. It should be written with capital letters only and the long name should also be given.
References:
Many references are missing clickable doi or url links which makes it harder to quickly click and check something for the reader/reviewer. This is just meant as a tip that it is nice to provide the links from the beginning and not first when the copyediting/typesetting office demands it.
Citation: https://doi.org/10.5194/egusphere-2024-1341-RC1 -
RC2: 'Comment on egusphere-2024-1341', Anonymous Referee #2, 10 Jul 2024
The authors present the results of a modeling study aimed at understanding the controls exerted by ice nucleating particles (INPs) on liquid water contents of Antarctic / Southern Ocean mixed phase clouds. Winter and summer numerical modeling simulations of cloud systems were set up, using prescribed CCN and INP concentrations, and varying the latter to investigate the impacts. The runs are guided by observational case studies. As the authors note, the simulations are generally qualitatively representative of the observations but “the model fails to accurately represent the long-lived, dense liquid and mixed-phase layers observed at the (continental) station”. Indeed this is an ongoing issue in the simulation of polar mixed phase clouds that this work cannot address. Further, radiative biases are present in the model, but are noted to be on the same order as those reported in prior modeling studies.
The first main finding of this work is that contrasting air temperatures over land and ocean led to different seasonal impacts. Cold wintertime temperatures over the content supported homogeneous freezing and thus variations in INP had little impact, a perhaps expected finding. Over the ocean in wintertime, temperature were moderate enough such that liquid water contents responded strongly to INP concentrations, whereas in summertime, temperature were warm enough to limit ice formation regardless of INP concentrations. The second main finding is that the simulated changes in liquid water content have only a small radiative effect. This result seems to indicate that concern over changes in the radiation balance in the Antarctic induced by changes in cloud phase might be overstated.
An aspect of this work that represents an advance over prior treatments is that an INP budget is included, that has 16 different temperature activation bins, enabling a more realistic representation than fixed-INP concentrations. However, the slope of the cumulative distribution is fixed, using the DeMott et al. (2010) parameterization.
Overall, there is a nice description of the modeling system which is a useful addition to the literature, and the work takes a methodical approach to explore sensitivities. I have the following suggestions for changes to the paper. A number of these questions relate to the INP observations, although in the end they are used primarily to provide bounds on the variation input to the model. If those measurements are not described elsewhere then I suggest to improve that discussion so that this work can serve to report them.
Comments:
- Abstract: when citing the INP concentrations (line 3) please indicate these are at -20 C. and that the analysis method used refers to immersion freezing INPs.
- line 48: the Kay et al. (2012) reference is over a decade old; have there been updates to CESM that have reduced the cited bias?
- line 113: what is the pore size of the filters? not much detail is provided about the INP observations. Does the implementation begin at -15 C because the samples were below the limit of detection at warmer temperatures? could the DeMott (2010) parameterization be extended to “fill in” some reasonable values? why not use the slope of measured INP spectra, instead of the slope of this fit based on the DeMott global (not polar) fit?
- line 114: how is the inlet oriented? was there no precipitation shield? what sampling inlet losses might be expected (the dimensions of the tube are not provided)?
- line 118: how were blank corrections handled?
- Section 3.1: how is the INP budget handled during model spin-up time (is such spin up time considered?) In other words, as INP are removed from the domain (although also regenerated from evaporating precipitation and advected in), are the INP concentrations during the analyzed period markedly different from the initial condition?
- It would be interesting to see a timeline / contour plot showing the budget of INPs in the simulations, perhaps selecting the -20C point in the spectrum for this, and including a discussion of any influence on the findings.
- line 174: is rime splintering ever active in these simulations? This mechanism relies on the presence of specific hydrometeor types and of certain size. Evidence for secondary ice production (SIP) in Southern Ocean clouds has been published, so excluding relevant SIP processes might be a shortcoming of this work. I was confused by the statement on line 208 that increasing INPs was equivalent to representing some secondary ice production modes (although the authors note in line 201 “this is not a very accurate assumption” and again qualify this approach in the Discussion (line 355)).
- Line 258: “ice droplets” should be “ice crystals”?
Citation: https://doi.org/10.5194/egusphere-2024-1341-RC2
Data sets
Ceilometer observations taken at Princess Elisabeth Station, Dronning Maud Land, East Antarctica Florian Sauerland et al. https://doi.org/10.48804/07SS6R
MRR observations taken at Princess Elisabeth Station, Dronning Maud Land, East Antarctica Florian Sauerland et al. https://doi.org/10.48804/MDDKU0
Replication Data for: Ice-nucleating particle concentration impacts cloud properties over Dronning Maud Land, East Antarctica, in COSMO-CLM² Florian Sauerland and Nicole van Lipzig https://doi.org/10.48804/XGJVIZ
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