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| 28 Jan 2025
Ice formation processes key in determining WCB outflow cirrus properties
Abstract. The macrophysical and radiative properties of cirrus clouds are strongly influenced by their formation pathway. Formation pathways are thought to differ in the dominant ice nucleation mechanism and the thermodynamic regime: liquid origin cirrus are forming at water saturation and ice crystals from by freezing of liquid water drops, while in-situ cirrus form below water saturation at cold temperatures (T < 235 K) and ice crystals form directly from water vapor.
Warm conveyor belts (WCBs) transport liquid droplets and moisture from the boundary layer into the upper troposphere, where cirrus is formed in the outflow. A priori, it is uncertain which ice formation pathway is favoured. We employ a two-moment multi-class cloud microphysics scheme that distinguish between five cloud ice classes. Each ice class is represents ice formed by a unique formation mechanism and therefore we are able to investigate the nucleation process by which ice at an arbitrary location in the model was initially formed. Our analysis suggests that cirrus in the WCB outflow consist predominantly of ice formed by in-situ nucleation processes. However, Lagrangian trajectories show that the cirrus is derived from mixed-phase clouds. The main WCB ascent region was embedded in a slow ascending air mass that resulted in in-situ ice formation above the WCB. This in-situ formed ice sedimented into mixed-phase clouds of the WCB below. We further show that this sedimenting ice substantially alters cirrus properties. Taking into account the combined information on thermodynamic history, ice nucelation processes, and sedimentation is therefore likely vital for cirrus formation and classification.
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RC1: 'Comment on egusphere-2025-185', Anonymous Referee #1, 05 Mar 2025
Review of
"Ice formation processes key in determining WCB outflow Cirrus properties"
by Lüttmer, Miltenberger and Spichtinger, EGUsphere-2025-185
General comments and recommendation:The authors use a newly developed ice-cloud microphysics scheme that allows to treat several groups of ice crystals, that is, to distinguish them. The distinction used for the present purpose is their respective formation origin, in particular, whether their formation is via the freezing of pre-existing water droplets ("liquid origin") or whether they form directly in the ice phase, either heterogeneously (deposition nucleation on solid aerosol) or homogeneously (by freezing of cold solution droplets at high ice-supersaturation, but below water saturation). This new method is applied to the case of a Warm Conveyor Belt (WCB) where moist air is transported within 2 days from the boundary layer to the upper troposphere, which leads to adiabatic cooling, droplet formation, later freezing, and in the outflow region to in-situ ice formation. Simultaneously there is further moist air and cirrus in the same region as the WCB but already initially in the mid to upper troposphere (above 5 km). These airmasses are affected by the strong dynamics below and there is ice formation in these volumes as well. The new method is able to distinguish the formation mechanisms of the ice crystals that are present in Lagrangian air parcels (I assume, that this information is fed in from a basic Eulerian grid).
The obtained results are interesting, in particular, since an alternative distinction method, labelled W16, shows "confusion" once there is sedimentation of ice that is not or cannot be considered in the W16-method. The results show as well, that other simple classification approaches (many small vs. few large crystals) can be misleading, probably again because of the confusing effect of sedimentation.However, this is not yet a good paper and it needs a thorough revision. The paper has two faces, a good one with good scientific content, and a bad one with many sentences, expressions, etc. that are simply not good enough for a scientific publication. Furthermore there are a lot of grammar errors and sloppyness in type-setting. It is a pity! As a first example, please read the title. To my taste this sounds like slang. Shouldn't it read "processes that are key..."? An example for sloppyness is the lone header B2 in the appendix, which has no content following.
There are worse examples in the text. One general comment, I would like to make, is that temperatures are never cold. This is popular speech. Air can be cold, and the corresponding temperatures are low. Please avoid such popular errors in serious writing.Summarising: this paper needs a major revision mainly of its presentation quality.
In the following, I start with a few science questions before I come to a longer list of textual problems. I leave the grammar errors and typesetting problems away.
Scientific comments:Lines 88/89: Please explain the criterion that pressure has to be less than 500 hPa. As a WCB starts in the boundary layer, this is a bit surprising.
Lines 171 ff: Please justify the assumption of equal shapes and relations for all ice modes.
Section 2.2: Perhaps this is explained in the other paper (Lüttmer et al.), but it would be convenient for the reader to learn how in a given air parcel with a mixture of, say DEP and HOM nucleation particles, the dicision is taken which of the principally possible mechanisms wins? And how is it done? Do you use a "winner takes it all" principle, or is there a certain partitioning at work.
Eq. 3 and adjacent text: I understand the integrals, but then the integration is done along trajectories, that is, there must be a time dimension. Where is it?
Analysis of Fig. 4: I am puzzled by the fact that the figure and thus the entire investigation contains or considers or does not filter out ice that is present above the WCB right from the beginning. I think this ice has nothing to do with the ice formation within the WCB and I am not sure how this distorts the analysis. I stumbled over this in the SEC frame. On a previous page I read that SEC has almost no effect in this study, a statement that I find quite plausible (and I wanted to suggest to leave SEC out of the paper at all). Now I see that SEC ice is the first that appears in the WCB. But obviously it was already there and it was not produced since a WCB appears. Isn't it possible to filter these ice masses out and just to consider what was formed directly by the dynamical triggers caused by the WCB? I understand that later such pre-existing ice may fall into the affected clouds. In this case, it would perhaps be better, to introduce a SED (sedimentation) class instead of the SEC class.
Line 295: How large is the excluded fraction?
Lines 319/320: "After ascent...". Please explain this finding. If T and thus saturation pressures are constant (Fig. 3 c), what then reduces RH? Where does the vapour mass go?
Line 404: Shouldn't it read "higher" instead of "lower" supersaturation?
For section 4: Please add somewhere, best already in section 2, an explanation whether air parcles can change their characteristics over time, e.g. from liq.-origin parcel to in-situ-origin parcel and vice versa. Can that happen at all, in both algorithms, and if so, how often does it happen?
Why do you sometimes use mass mixing ratio and sometimes volume mixing ratio?
Comments on text and style:
Line 4: "...and ice crystals directly from water vapor". Nucleation directly from the vapour, that is, without suitable nuclei is possible at several 100% RHi, but this is probably not meant here.
Line 5: It is better to write vapour instead of moisture, since liquid droplets belong to moisture as well.
Line 9: "...ice at an arbitrary location in the model..." What do you mean with "arbitrary location"? I think, "... ice in the model .." would be sufficient.
Lines 9 ff: The two sentences beginning at "Our analysis..." sound contradictory. Please clarify.
Lines 13/14: The last sentence of the abstract states that something that You think about thermodynamics etc. is "vital for cirrus formation". I doubt that nature asks what you think before it forms cirrus.Line 18: "a frequent occurrence"? Better "occurs frequently".
Line 29: word missing
Line 46: "since pure water droplets must exist"? Better replace "must exist" with "would otherwise evaporate".
Line 56: add "of in-situ formed cirrus" after "subcategories".
Line 65: replace the first instance of "available" with "amount of".
Line 85: "originating from the geographic information", What do you mean?
Line 106/7: "...classification methods ... can distort the differences..." What do you mean?
Line 109: "on the other side". If you use such a phrase, then you should also use the phrase "on the one hand" before (and write hand instead of side).
Line 198: Better write "region" instead of "area" when you are talking about three-dimensional things.Lines 253/254: Bad sentence! It reads essentially like " The evolution of... moved over the North Atlantic". Please reformulate.
Line 290: "featured" should be replace by "had experienced".
Line 292: Another example of mixing-up model and nature! ".. tau_600 may impact ice formation ..." How? Does nature first look, which value tau_600 has and then decide?
Line 298: Please rewrite this sentence "Forming the tail...".
Line 315: "below the cirrus temperature level T_c=235K" although this is formally correct, I suggest a rewording. Such expressions are prone to misunderstanding (similar to expressions involving pressure and lower or higher). It always needs a while to understand whether the "below" refers to altitude or to temperature/pressure.
Line 326: Please be consistent with mass mixing ratio vs. mass content, and in line 328 simply "mass". There are more of such instances later.
Line 330: please add "in the parcel" after "ice first occurs".
Line 338: replace "temperature regions" with "temperature regimes".
Lines 339-343: These explanations are not entirely clear to me. In the last line, "where" should probably read "while".
Lines 368/9: Incomplete sentence.
Line 410: what are "view trajectories"?Figure 7, caption: "with colored by" ?
Sect. 4, first par: ones -> once (several instances), also on later pages.
Line 483: "spell" is not the correct word here.
Line 507: Is "outline" not simply a "line"?
Line 511: incomplete sentence "thus the statistic...".
Line 524: one instance of "after" should be dropped.
Line 559: Sentence cannot be understood.Line 632: Drop "remained".
Line 666/7: what is "temperature and supersaturation activation"?
There are many typing and grammar errors throughout the text, too much to list them here. Please note that a spell checker will not find them all.Citation: https://doi.org/10.5194/egusphere-2025-185-RC1 -
RC2: 'Comment on egusphere-2025-185', Anonymous Referee #2, 05 Mar 2025
In this manuscript, Lüttmer et al. use a novel multimodal ice microphysical scheme combined with offline trajectories to study the formation mechanisms of cirrus in the outflow of a warm conveyor belt. This allows a much more accurate estimation of microphysical sources compared to previous studies. In particular, they show that most cirrus is formed by in situ ice nucleation at T<-35°C, which differs from the results of a commonly adapted method used in several previous studies.
This is a valuable study that adds an important perspective on the microphysical properties of ice clouds in warm conveyor belts. This will be a valuable contribution to the community. However, I have several points that I would like to see clarified/addressed before acceptance. Many of my general comments relate to the writing. The manuscript, especially the second part, could be tightened up to help the reader. Some more storytelling that would connect the initial results to those presented in later parts of the publication might help, as well as possibly a summary sketch.
General comments:
1. My main takeaways from the paper are the following:
I. Most cirrus are formed by in situ ice nucleation
II. Sedimentation of ice crystals is very important and affects the properties of WCB cirrus.
III. Beware of the W16 classification criterion, microphysically it doesn't make that much sense.I think point II. could be better clarified in the abstract and the final discussion.
More importantly, while the study makes a detailed comparison with the W16 criterion, there is no discussion about it. The authors give no guidance to the reader about it.More specifically, suppose we are planning a new field campaign. Should we rely on W16 for our cirrus origin classification? If not, what should we do? Such brief comment would be an important manuscript's message that largely increases the manuscript's relevance.
2. This study focuses on a single WCB. Can the results be generalized, as is more or less implied in the text? Is the variability between different WCBs and their cirrus formation/properties large? For example, would the cirrus of a very strong WCB be very different from that of a rather weak WCB?
3. The title could better indicate the main result. The title alone implies to me a study that compares the importance of ice formation processes with other processes (e.g. large-scale dynamics, turbulence, etc.) in determining cirrus properties. I guess the key finding is that cirrus is formed by in situ ice nucleation. This could go in the title.
4. The manuscript could be shortened and streamlined in places to allow the reader to get to the last part of the results, which give unique insights into microphysics that you can't get with a standard microphysical scheme. Mentioning/reminding the reader of this in a few places may help.
Minor: It is more appropriate to use the term "cloud radiative effect" rather than "cloud radiative forcing". CRF has been used a lot in the past, but since the word "forcing" implies a perturbation in the climate system, the community has moved to using CRE instead, since clouds are an internal component of the climate system.
Specific comments:
Line 6: I would suggest simplifying the sentence beginning with "A priori" to something like: The dominant ice formation pathway remains uncertain.Lines 10-14: I think these sentences could be improved, the message could be expressed in a more direct, clearer way.
Line 20: Do we know what fraction of the total mid-latitude cirrus comes from WCB? Have any studies looked at this? If determining radiative forcing is a broader goal here, it would be important to know.
Lines 28-38: It would be good to add references to some satellite studies on impacts of cirrus on CRE. E.g. Hong et al., 2016 (10.1175/JCLI-D-15-0799.1), or Matus and L'Ecuyer, 2017 (10.1002/2016JD025951).
Line 60: Insolation is at least as important in determining their CRE as their altitude (and thus temperature). Don't forget that the CRE also depends a lot on the surface albedo and the presence/absence of lower lying clouds, e.g. clouds over a snowy landscape will have a very low SW CRE.
Lines 61-67: Since the intro is already quite long (but nicely written), I would suggest removing this paragraph. It goes way beyond the work presented here.
Line 70: Remove the parentheses
Line 81: What is analysis data? I would just stick to reanalysis there.
Line 100: Since Krämer et al. mainly used the previously processed satellite data, it would be more fair to cite the work of Sourdeval et al., 2018 (10.5194/acp-18-14327-2018) and Gryspeerdt et al., 2018 (10.5194/acp-2018-20) here.
Line 126: Don't most 3D Eulerian atmospheric models account for sedimentation?
Beginning of section 2.3: Could the authors explicitly write whether they are using offline or online trajectories (this becomes clear later in the text, but not immediately).
Section 2.3: It may be helpful to add some more schematic figure/sketch/illustration of the WCB forward and above WCB backward trajectories. A sketch could also highlight cirrus clouds.
Lines 221, 227, 228: "...integrated mass (number) ratios..." implies that mass is synonymous with number. Please rewrite, e.g. mass and number.
Line 222: If the ultimate goal is to study radiation effects, some sort of 2D area normalization would also be useful.
Line 225: This should come at the beginning of section 2.4, but I would not use the term "completely glaciated" for clouds that can only be composed of ice.
Line 226: "Sub cirrus clouds" sounds a bit too abstract to me. What about fully glaciated mixed phase clouds? That's a term that has already appeared in the literature.
Line 244: "is being used" => are used
Figure 1:
I would prefer a perceptually uniform colormap here (which is also an EGU journal requirement). Also, areas with no satellite data could be shaded gray to distinguish them from areas with no clouds, which appear as white.Figure 2:
I. Is there a reason for a jump in color from blue to yellow at q_tot of 1e-5?
II. The short descriptions under a-d could be more descriptive.
III. When I look at these plots, I think of a WCB breaking out of a prison cell (or a WCB in a prison, for Figure 1). I would slightly reduce the visibility and/or number of gridlines.
IV. Would it make sense to mark t_out could on the plotted trajectories?Figure 3: Tau was earlier used for cloud optical depth, here for the ascent timescale. I would prefer to call it t_600, also for consistency with t_out.
Line 278: Why is the intersection with WCB trajectories needed for the selection of "above WCB" trajectories? Wouldn't even those that never intersect WCB be relevant for WCB trajectories and their properties due to ice sedimentation?
Figure 4: Would it make sense to show the same figure but for ICNC and/or ice radius? Perhaps in the appendix?
Lines 333, 334: I think it's Figure 4c, not 4b, and Figure 4d, not 4c Figure 5: Panel a colors are the same as in other panels, but show different quantities. Please change them! Then we will only need 2 legends, not one for each plot.
Figure 5: I don't think it's necessary to show illegible very small numbers (e.g. 0.00 or 0.02). Maybe just state that anything smaller than a certain threshold, e.g. 0.05, is not printed in the plot?
General comment: I would like to make it very clear that DEPO is deposition nucleation at cirrus temperatures. Many microphysical schemes have legacy code that (unjustifiably) uses mixed phase deposition freezing.
Line 425: I think this can be linked to the results in Fig. 5.
Figure 6d: FRZ only forms at 235 K, right? So the distribution around that temperature is caused by sedimentation and vertical transport, right?
Figure 6e, lines 431-432: Why is there no/little change in total cloud ice content at T>230 K? (different from the comment). What should we expect based on the deposition growth equation/theory? At what temperatures is the total cloud ice content consistent with this, and when is it not? If not, is sedimentation the reason for the deviations? Or vertical transport?
Figure 6f: may be more effective with a linear x axis
Line 434: "...ice derived mixed-phase clouds" sounds odd
Lines 437-438: I see 6-8 cm/s and 10 cm/s from the plots. Please look at this!
General comment on section 3.5 and other results: It would be good to better point out what this novel scheme can do that other schemes cannot; what insights we get because of the multiple ice modes?
Line 510: evaporation => sublimation
After section 4.2 or so, the reader starts to get saturated. Shortening the text would help, as would turning parts of the text more into a story that refers to not just one but several figures. Examples:
1. Section 4.3 starts with an introductory paragraph describing what will be shown. I don't think that's bad, but I also don't think we need such a long intro paragraph for every results subsection.
2. Section 4.3 is very long, could be shortened.Lines 555-558: So the sedimentation rate of the overlying cirrus (and, I assume, relatively small) ice crystals is greater than the WCB rise. Is this correct? And what temperature range are we talking about here?
Figures 6-8: The first figures show the ice water content in units of kg/kg, here the units are changed to ppmv. It would be good to be consistent, unless there is a strong reason for the unit change (which should be mentioned).
Section 4.3: Is it really useful to compare a WCB with climatologies?
Section 4.3: Some sort of summary sketch might help guide readers who are tired at this point in the manuscript.
Line 644: "As discussed above, these particles..." ==> "These particles..."
Lines 648-649: Does this make sense overall? How does it compare to the expected sedimentation rates for the ice crystals?
Lines 650-: I'm missing a general message about cirrus classifications. See also General Comment #1.
Section 5: How does section 5 relate to the problems outlined in the Introduction? It would be good to round this up, or simply remove some parts of the long intro, and keep the focus on the main results of this work. Microphysics, cirrus origin, and not radiative relevance. Since there is a lot of discussion about radiative relevance, I would expect this to be shown in the results, or at least speculated in the discussion.
PS. While I indeed agree with reviewer 1 that temperatures are technically never cold or warm, I think popular language can be used in scientific publications if it helps get the message across. In any case, clear communication with a touch of storytelling is a hard-to-achieve goal, even for very experienced researchers. I wish the authors all the best in their work on revisions!
Citation: https://doi.org/10.5194/egusphere-2025-185-RC2
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