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| 19 Aug 2025
Ice-nucleating particle depletion in the wintertime boundary layer in the pre-Alpine region during stratus cloud conditions
Abstract. This study evaluates the regional variability of the number concentration of ice-nucleating particles (INPs) between the two pre-Alpine central-European sites of Eriswil, Switzerland, and Hohenpeißenberg, Germany, supported by INP measurements from Melpitz, Germany, during the winter months of 2024. The aim of the study is to spatially and temporally evaluate INP availability and removal within the planetary boundary layer (PBL) during Bise situations. Target scenario of the study were situations when northeasterly winds (so-called Bise winds) prevailed and layers of stratus clouds formed at the top of the PBL at temperatures down to -10 °C. In these situations, it is expected that INP are depleted along the transport path. The main insights from INP measurements were: First, during the cold-Bise (cloud minimum temperatures as low as -10 °C) and warm-Bise (cloud minimum temperatures above 0 °C), no INP contrast was found between Hohenpeißenberg and Eriswil if both were within the PBL. Also, the INP concentration was overall found to be much lower during the cold-Bise than during the later warm-Bise situation. Second, when the Hohenpeißenberg site was located in the free troposphere during the cold-Bise situation, INP concentrations were much higher compared to Eriswil (still within the PBL) but similar to Melpitz. These observations led to the conclusion that during cold-Bise situations the INP reservoir within the PBL is depleted, likely by the presence of supercooled stratus. The inversion-capped winterly PBL is apparently not capable to replenish the INP reservoir from the free troposphere.
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RC1: 'Comment on egusphere-2025-3675', Anonymous Referee #1, 07 Oct 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3675/egusphere-2025-3675-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-3675-RC1 -
CC1: 'Comment on egusphere-2025-3675', Dongwook Kim, 17 Oct 2025
Review of “Ice-nucleating particle depletion in the wintertime boundary layer in the pre-Alpine region during stratus cloud conditions” by Ohneiser et al.
This review is a combined review from three reviewers (Dongwook Kim, Georgios Dekoutsidis, Jaydeep Singh) as a part of the 2025 EGU Reviewer Training program.
This manuscript investigates the fate of the ice-nucleating particles (INPs) within cloud particles based on the measurements from the two pre-Alpine central-European sites of Eriswil, Switzerland, and Hohenpeißenberg, Germany, during the “Bise winds” periods in the winter of 2024. The study addresses an interesting question: whether supercooled boundary‑layer clouds in wintertime central Europe can deplete INPs along the Bise path. The authors noticed a discrepancy between the model and observations, regarding precipitation in the Swiss plateau under the Bise conditions. They found that INPs are depleted during the transport from Hohenpeißenberg to Eriswil during the cold Bise period, which is perhaps due to the removal after the activation within the supercooled stratus cloud, rather than removal within the PBL. While the study is limited to a very specific area (even more so, a very specific situation in said area), the experimental design, with two stations aligned along the wind and an additional upstream site, is a strength. The findings herein can serve as a basis for future studies with a broader focus and applicability. The topics discussed in this paper are in the scope of the journal and of interest to its readers. We recommend publication after addressing our comments below.
General comments
- Despite it not being common practice, I find the decision of the authors to include a separate section, where they emphasize their scientific questions and running hypotheses, very useful. It provides an easy, direct comparison later in the discussion and provides an easy look-up when studying the results. The introduction is overall well structured, well written, and the results are presented in a clear, concise way.
- In chapter 3, the authors provide an in-depth explanation of the instruments, their capabilities, and how they were used in this study. Although it is a very interesting topic, some parts of the description of the instruments and measurement practices feel overly detailed and unnecessary in order to understand the results and follow the discussion. This is potentially a part that can be shortened if the editor sees fit.
- This study investigates the fate of INPs during the transport from Hohenpeißenberg to Eriswil. While they discuss the potential losses of INPs within the PBL, they do not account for the emission/formation of INPs between the two cities, e.g., resuspension or emission of dust particles. Please discuss such potential sources of INPs between the two cities.
- The proposed removal pathway of INPs is by removal after the interaction within the supercooled stratus cloud (line 305). Does it mean chemical decomposition? Also, according to the introduction (line 31-39), only biological particles may be activated during the cold Bise temperature range during the study (-10 to 0 degree C). Are the activated INPs mostly biological particles?
- The description of HPB as “in the free troposphere” is misleading; it is more accurate to state that HPB was above the local inversion (decoupled from the PBL), avoiding any implication of a zero-thickness boundary layer. The site setting also remains unclear—please specify whether HPB and Eri are on ridge/slope/valley, provide station elevations, and relate these to inversion height (a simple time–height plot would help). An apparent inconsistency exists where HPB is described as within the PBL during cold-Bise but “free troposphere” during warm-Bise, although the PBL typically deepens in warmer conditions; a physical mechanism (e.g., terrain-driven decoupling, subsidence, radiosonde timing) should be provided or the claim revised. Finally, link this more explicitly to the INP interpretation: depleted in-cloud INPs at Eri versus higher INPs above the inversion at HPB are consistent with cloud scavenging plus episodic entrainment from aloft; consider secondary ice production and avoid wording that suggests an absent PBL.
Specific comments
- Line 10: Please elaborate on what the “INP contrast” means.
- Line 71-72: This is the motivation for studying INP in the region. This can be mentioned in the abstract.
- Line 157-158 & Fig. 5: Please provide references regarding the deactivation of biological particles after heating. Also discuss how the heating treatment affects non-biological particles. Also, for Fig. 5 spectra, please use more distinguished markers for the two methods, such as triangles and circles.
- Line 195: Please explain the implications of the inversion for INPs measurements
- After introducing the acronyms for the two cities (line 79-80), use these acronyms throughout the manuscript, rather than mixing them with the original names (e.g., lines 209 & 211).
- Line 208-212: How was the back trajectory in February?
- Line 220: I could only find a very brief mention of artificial cloud seeding in line 75, but also without any details there. I think it is very important to provide more information on that, even if there is a publication cited. What type of seeder was used? Can it be an INP, and under which conditions? Were these conditions present in any case you also analyzed? How would you expect this could affect your study?
- Line 222: How did you check the absence of the particles from artificial seeding?
- Line 228: Melpitz INPs were higher for the T range from -15 to -5 degree C. However, it is unclear if the total identified INPs were higher than those of the other two cities. Why is Melpitz INPs data below -15 degrees C not shown? Please explain in the main text.
- Line 299–301: It is unclear how the dusty-cirrus mechanism supports INP mixing with supercooled stratus in the boundary layer. Could this not be explained directly as the interaction of INPs with supercooled droplets? The dusty-cirrus mechanism usually describes mixing of dusty air with clear moist air.
- Table 1: Are these instruments at the Eriswil site or both Eriswil and Hohenpeißenberg sites? It would be better to list all the relevant instruments at the three measurement sites in the table.
- Figure 2: In panels c) – f). I am a bit confused regarding the dates that are presented. In both cases, the authors do not include profiles from the whole time period, and the dates do not match between c) and e). Is that a result of data availability/quality? Are all days included in the analysis, even if not shown here?
- Figure 2 caption for e-f: Does it mean that the shown profile is for Munchen, not HPB? How close were these two locations?
- Figure 4: Why are only cold period trajectories calculated?
- Figure 4 caption: mention that the figure also shows precipitation along the back trajectory path.
- Figure S1-S2: consider adding a third panel that shows the difference (and/or ratio) between INP number concentration from Hohenpeißenberg and Eriswil.
Technical comments
- Line 8: INP → INPs
- Line 9: First → first
- winterly PBL → wintertime PBL
- Lines 21-30: Although this section is very well written and supported by many relevant references, I personally find the structure a bit odd. Crystal growth is followed by secondary ice formation and then by ice nucleation. In my opinion, starting with introducing HOM and HET nucleation and then ice crystal growth and secondary formation would be a more logical sequence.
- Line 31–32: Please provide a reference for the statement: “At a temperature of –20 °C, the fraction of aerosol particles able to act as INP is on average one per million.”
- Line 32: A reference could be added here to support this claim.
- Line 41: …mainly on ambient temperature, ice supersaturation and…
- Line 45: …their shape and size during…
- Abbreviations: Repeated or undefined (e.g., CLOUDLAB Line 75). Define abbreviations consistently (e.g., PM10, PolarCAP, PCR).
- Line 132: “Uni Wyoming (2024)” → University of Wyoming (2024)
- Line 211: HBP? → Please check the abbreviation.
- Line 272: ICNC repeated too often; reduce repetition across the manuscript.
- Line 276: “300 m” but cloud top varies up to 400 m, please clarify.
- Line 283: Fig. 5b,c)) → delete repeated parentheses.
- Figure 2: Legends are barely legible
- Figure 2c–f: y-axis label missing/unclear, please.
- Figure 6: extend width; hours are hard to read, though results are discussed in short time ranges.
- Figure S4-S7: Please indicate the three measurement sites on the map.
Citation: https://doi.org/10.5194/egusphere-2025-3675-CC1 -
RC2: 'Comment on egusphere-2025-3675', Anonymous Referee #2, 22 Oct 2025
Ohneiser et al. provides a unique study of measuring ice nucleating particles (INPs) over time and distance in a region of the world where stratus clouds can persist with Bise winds and models cannot accurately reproduce them. Therefore, I believe this study is a valuable addition given the novelty of the measurements, despite more cases being valuable to reinforce the hypotheses and initial findings presented here. I think it is suitable for publication, after consideration of my comments. There are a few issues regarding clarity, and I take issue with how some of the statements are worded (e.g. need more qualification or interpretation of the data). Some of the statements can be argued against as currently written.
I like how the authors have laid out the text, with hypotheses early on, and coming back to the initial hypotheses at the end of the text. I especially like the schematics in Figure 1 and 5 to improve understanding. I think it is an important conclusion and nice finding to have evidence of INP removal under the cold Bise conditions. Nice work.
Lines 22-24: Secondary ice processes are mentioned here but are not mentioned anywhere else in the manuscript. See below where I suggest including it later on.
Lines 31-32: “At a temperature of -20 °C the fraction of aerosol particles able to act as INP is on average one per million.” This statement needs a citation as it is very dependent on the global region of the study as well as vertical location of measurement.
Lines 39-40: You should cite Tobo et al. 2024 here, which found greater warm temperature INPs with snow-free conditions: https://www.nature.com/articles/s43247-024-01677-0
Lines 64-65: “Therefore, INPs active at temperatures between 0 °C and -10 °C, which is a typical Bise cloud minimum temperature, are required to form precipitation.” This statement seems too restrictive with later text that discusses the seeder-feeder effect. My interpretation of your text suggests that these INPs would not necessarily be required, especially in the times of year with fewer bio sources.
Line 88: I know it is defined in the abstract but given you define what a warm Bise is in this section, it is best to define cold Bise again.
Lines 87-89: It would be good to add any information of typical precipitation or snow accumulation amounts in the region, as this is unfamiliar for me, so I have no idea what “typically no significant precipitation” means.
Line 116: Good to put a brief sentence discussing these other projects (I know CLOUDLAB makes an appearance in the intro).
Lines 122-124: How did you determine when you were in a Bise situation? It would be good to include at least briefly in this section.
Line 125 and subsequent uses: Is “blind” a typo? I have only seen field “blank”.
Section 3.1 General: Why were the pore sizes of the INP samples different? I would recommend including in this section how many filters were collected.
Line 132 and subsequent uses: It is better to use “University of Wyoming” instead of “Uni Wyoming” at least at the first use, and likely the full name in the references “University of Wyoming Atmospheric Science Radiosonde Archive”. I have never heard it called “Uni Wyoming” before. It is commonly abbreviated as UW. Also, the link you put in the reference is a general link, can you include information such as the station number somewhere so readers can more easily reproduce the data?
Line 150: “Well” is probably better here than “tube”.
Line 151: I assume you don’t mean aluminum foil here since you view it optically, something like a “clear adhesive film” might be clearer than “a foil”.
Lines 157-158: Have you compared or seen any effect of “reusing” aliquots for the heat test versus pipetting out fresh aliquots and heating those?
Figure 2: The legends in c and e are smaller than the rest and are hard to read.
Lines 196-199: How can you know that is supercooled liquid from the reflectivity alone? It sounds like you may have other data to inform this statement, so I would suggest including that here.
Line 208-212: All supplemental trajectory figures should be labeled the same way as Figure 4. Why are the heights listed in the figure captions different from those described in Lines 169-170? I think it should be qualified that some of the trajectories (especially the green ones) do not go toward Hohenpeißenberg.
Lines 251-253 and Figure 5: I don’t agree that the heat fraction is low during the cold Bise, especially in Figure 5b and c, as the log scale can make the spectra look closer than they actually are. I would suggest providing some numbers here of heat fractions at a given temperature between the cases to make a better statement. 5b and c suggest Eriswil has quite a high heat fraction, while I agree the heat fraction is quite low in 5e at the downwind sites. Also, the heat data for 5h Hohenpeißenberg site looks strange below -15, could that plateau be an artifact of dilution? It may be best to cut that one where the untreated sample for the site stops since it is hard to compare.
Line 260: Can you please clarify what you mean by natural cloud seeding here?
Line 279: You could include a statement about potential secondary ice production here in this paragraph.
Line 288: I don’t agree with the low biogenic INP fraction except in 5e. Even 5f you could make the argument of a high heat fraction at Eriswel above -15 °C. It depends on what temperature you are looking at, and quantitative information earlier would inform if this statement could be made. At the very least, it should be qualified.
Abstract: Can the “no INP contrast was found between Hohenpeißenberg and Eriswil if both were within the PBL” really be made given the higher values at Eriswil above -15 °C in 5b, and higher values Hohenpeißenberg in 5c? I think it needs some more definition and characterization.
Citation: https://doi.org/10.5194/egusphere-2025-3675-RC2
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