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.