Review for: Measurement Report: Influence of particle density on secondary ice production by graupel and ice pellet collisions

General comments:

In their experimental study Yadav et al. quantify the number of fragments generated upon collisions between ice pellet and graupel particles and their dependency on particle density. This study extends the work of Grzegorczyk et al. (2023), enhancing our understanding of the collisional break-up mechanism—a critical process in mixed-phase cloud microphysics that is increasingly supported by observational and modeling evidence. Current parameterizations of this process rely on limited and outdated laboratory data, underscoring the importance of this study in providing updated experimental insights to improve its representation in cloud models. Prior to publication, the following major comments should be addressed, followed by minor suggestions and editorial remarks:

Specific comments:

•    In the Experimental section, the authors should discuss the extent to which the ice particles used in the study reflect natural atmospheric conditions. For example: Under what synoptic conditions and cloud configurations would graupel and ice pellets of the sizes used (2.45 mm for graupel; 5 mm and 7 mm for ice pellets) form? Is the liquid water content (LWC) used in the experiments realistic for these scenarios? Additionally, the prevailing supersaturation with respect to ice is not mentioned, yet this parameter is important for ensuring a more accurate implementation of the results in cloud models.

•    The study contains several limitations that should be discussed more thoroughly in the results and conclusions sections:

- First, the experiments focus solely on central collisions, unlike Grzegorczyk et al. (2023), which also examined edge collisions. The potential impact of this choice on the number of fragments generated (e.g., whether head-on collisions are expected to produce more fragments than oblique ones), should be addressed. 

- Second, the authors acknowledge that the collisional kinetic energies (CKEs) used in their experiments represent the upper range of natural CKE values (Lines 124-129), which could influence the applicability of the results to natural conditions. 

- Finally, the detection limit for identifying small fragments (20 μm; Lines 137-140) imposes a constraint, particularly on the observed size of fragments in graupel collisions mentioned in Lines 159-160. 

These limitations should be explicitly acknowledged and discussed in both the results and conclusions sections to provide a balanced interpretation of your findings.

•    The statements made in Lines 192-194 and 216-217 are rather bold and should be tempered by a discussion of the experimental limitations. Specifically, the limited number of experiments conducted, the use of artificial graupel and ice pellets, which may not fully represent natural particles, and the need to examine a broader range of ice particle sizes to cover the phase space relevant for numerical cloud models. Recent work by Gautam et al. (2024) used field observations to constrain the empirical parameters in the theoretical framework developed by Phillips et al. (2017). Comparing the experimental results in this study against these new findings would strengthen the discussion and enhance the robustness of the conclusions.

•    Both the abstract and conclusion sections should better articulate the motivation (the importance of ice multiplication and the understudied contribution of ice pellets within this context), as well as the broader implications of your study.

Minor comments:

•    Line 11 and Line 226: Consider replacing “suffers” with “undergoes” or “experiences” to improve phrasing. 

•    Line 15: Maybe use “formed” instead of “generated” here, to avoid repetition.

•    Lines 17-19: Specify the expected temperature range within the mixed-phase regime where SIP is anticipated to address discrepancies between ice crystal number and ice nucleating particle concentrations.

•    Line 20: Here we could acknowledge not only the lack of systematic laboratory studies but also the challenges in identifying SIP in field observations and accurately representing these small-scale cloud processes in model grid cells across different horizontal resolutions (through empirical parameterizations).

•    Line 22: Note that both droplet diameters <13 µm and >24 µm should coexist in Hallett and Mossop (1974).

•    Line 52: Are you referring to a specific publication here about the importance of SIP in thunderstorm clouds?

•    Line 55: Please clarify “underestimated” environment. Did you mean “undersaturated”?

•    Lines 55-59: Please include references to support this part of the introduction. Are you referring to observational studies (e.g., Korolev et al., 2022; Lachapelle et al., 2024; Lachapelle and Thériault, 2022) or laboratory-based research? 

•    Line 62: Could you clarify what you mean by “fragmentation outcome” here? The number and/or size distribution of the particles produced?

•    Line 70: Maybe “lower” instead of “less”

•    Line 72: Please define what GEORG stands for here.

•    Figure 1 caption: Please provide additional details for the ice pellet shown in panel c, including its density and the temperature at which it was formed, to align with the information given for graupel particles in panels a and b.

•    Table 1: For GG collisions (Experiments 9-11), please confirm whether the density of 0.46 g/cm³ for graupel particles is accurate. It does not appear in Figure 4, Table 3, or the main text. Could this be a typo?

•    Table 3: Please confirm whether results for a density of 0.21 g/cm³ from Grzegorczyk et al. (2023) are also included in Figure 3.

•    Figure 4: Consider adding legends to enhance readability. Additionally, include the temperature at which the experiments were conducted alongside the density information. This will help readers more easily identify the points referenced, such as those mentioned in Lines 207-208.

•    Line 112: Line 112: Have you specified these predefined conditions used for generating graupel particles somewhere in the text?

•    Line 252: “possess”

•    Line 256: Maybe you could reiterate the Hallett-Mossop temperature range here to avoid potential confusion with the wider -20°C < T < -5°C range mentioned earlier.

References

- Gautam, M., Waman, D., Patade, S., Deshmukh, A., Phillips, V., Jackowicz-Korczynski, M., Paul, F. P., Smith, P., and Bansemer, A.: Fragmentation in Collisions of Snow with Graupel/Hail: New Formulation from Field Observations, J Atmos Sci, 81, 2149–2164, https://doi.org/10.1175/JAS-D-23-0122.1, 2024.

- Grzegorczyk, P., Yadav, S., Zanger, F., Theis, A., Mitra, S. K., Borrmann, S., and Szakáll, M.: Fragmentation of ice particles : laboratory experiments on graupel-graupel and graupel-snowflake collisions, Atmospheric Chemistry and Physics Discussion, https://doi.org/10.5194/egusphere-2023-1074, 2023.

- Hallett, J. and Mossop, S. C.: Production of secondary ice particles during the riming process, Nature, 249, 26–28, https://doi.org/10.1038/249026a0, 1974.

- Korolev, A., Demott, P. J., Heckman, I., Wolde, M., Williams, E., Smalley, D. J., and Donovan, M. F.: Observation of secondary ice production in clouds at low temperatures, Atmos Chem Phys, 22, 13103–13113, https://doi.org/10.5194/acp-22-13103-2022, 2022.

- Lachapelle, M. and Thériault, J. M.: Characteristics of Precipitation Particles and Microphysical Processes during the 11-12 January 2020 Ice Pellet Storm in the Montréal Area, Québec, Canada, Mon Weather Rev, 150, 1043–1059, https://doi.org/10.1175/MWR-D-21-0185.1, 2022.

- Lachapelle, M., Thompson, H. D., Leroux, N. R., and Thériault, J. M.: Measuring Ice Pellets and Refrozen Wet Snow Using a Laser-Optical Disdrometer, J Appl Meteorol Climatol, 63, 65–84, https://doi.org/10.1175/JAMC-D-22-0202.1, 2024.

- Phillips, V. T. J., Yano, J. I., and Khain, A.: Ice multiplication by breakup in ice-ice collisions. Part I: Theoretical formulation, J Atmos Sci, 74, 1705–1719, https://doi.org/10.1175/JAS-D-16-0224.1, 2017.

Measurement report: Influence of particle density on secondary ice production by graupel and ice pellet collisions by Yadav et al.

In the measurement report results from collisions of different larger ice particles agents such as graupel and ice pellets, at different temperature and ice particle density or surface roughness are summarized and discussed. The Phillips (2017)[1] approach was applied to relate the number of secondary ice fragments to the kinetic energy of the collision. In addition, the results were compared to a previous study where bare graupel- bare graupel collisions were investigated [2]. The topic is relevant, interesting and fits within the scope of Atmospheric Chemistry and Physics. However, the manuscript needs revision in a few points before I recommend publication.

The measurement report is comprehensive and discusses some aspects very well, but it lacks precision in its formulation and explanation. Sometimes some unimportant aspects are described in too much detail. The main outcome can be presented more clearly in the abstract.

One general question is: How often do graupel-graupel or even graupel-ice pellet collisions take place in a real cloud or in modelled clouds? How likely is that? I strongly suggest, that the authors give a short elaboration on that in the introduction.

P1 l9: The number of ice fragments produced by drop freezing experiments is not clear yet as it is so far based only on detection with high speed microscopy [3, 4]. In Kleinheins et al. (2021) [5] indications were found for pressure-release events likely producing secondary ice splinters, which could not be seen high speed microscopy detection. A more detailed discussion on the detection limits and value of the statement is required when such a comparison is made.

P5 l138: The detection limit has to be mentioned already in the abstract.

P12 l250-251: Again. This cannot be states, see comments above.

P2 l27-28: It is important to note and present properly in the report, that the number of secondary ice particles could only be detected visually and might have therefore a large bias towards larger ice splinters. The quantification of smaller ice particles is not possible at the moment. Care should therefore be taken to ensure that no false impression is created.

P3 l59: Why is this temperature range explicitly given? Why is it not larger and goes up to 0°C? I would expect the temperature to be of minor relevance and thus only the existence of these two colliding objects and their kinetic energy might be of major importance.

Minor comments:

P1 l1/l4: The difference of ‘graupel’ and ‘ice pellet’ has to be clarified already in the abstract. Are ice pellets referred to freezing rain? This is necessary to understand the concept of these measurement reported.

P1 l9-10: Information of the detection limit is needed. A real comparison is only possible if the detection limits are clear and comparable and if no smaller secondary ice particles would exist, which is not clear. The detection should be mentioned and the statement should be changed or removed (see major concern).

P2 l50-52: Could you confirm this statement with a reference?

P2 l52-54: This sentence is confusion. Which agents are colliding with each other? What is the relation to the abundance? Thunderstorms are also mixed-phase clouds. maybe you can use the term of 'convective' for differentiation.

P2 l54: What is meant by strength? Please clarify.

P3 l55: What is meant by ‘underestimated environment’? Subsaturated with respect to ice? Clarify.

P3 l80: It is unclear what 'unwanted size or density' means. Shorten the sentence and avoid unnecessary information.

P3+ section 2.1: A table presenting all these multiple values would be much clearer and easier to understand.

P4 l104-105: I think that this information is not needed.

P5 l111-112: I don’t think that this information is understandable in its actual form ‘during the characterization measurements after producing several particles in GEORG under predefined conditions, like temperature, growth time, LWC.’

P5 l113-116: Is this a relevant information? How does it affect the collision as the mass distribution is different? In case, a discussion would be needed.

P5 l117: Why is it random?

P5 l 121: I strongly recommend to rephrase this part to : 'no new ice fragments could be observed considering the detection limit of ...?'

P5 l122: I don't understand why a new fragile graupel does not produce ice fragments after collision.

P5130: Does the oil conserve the shape of ice particles, so that a size distribution can be derived for larger fragments? Or do they melt and only equivalent droplet diameters can be derived from that?

P5 l134-136: I don't think that this is the reason. Maybe the fragments were too small? Or the shape was not conserved? Or I could simply leave the last part of the sentence out.

P5 l139-140: I would delete this sentence. It is confusing and give no further insights.

P8 l178: What is meant by ‘bulk structure’. Do you mean a ‘more compact structure’?

P12 l240-243: That is not completely true. The information of the frequency of graupel-graupel collisions in atmospheric clouds is missing. Please clarify and elaborate a bit more on this.  Do you mean ‘unsaturated’ with respect to ice? What is the consequence of this argument (242-243)?

P12 l255-256: Specify the reference to which you refer. H-M experimental results are very different and inconsistent [6, 7].

Technical corrections:

P3 l55-56: You can include an 'also' as this sentence is not directly related to the previous sentence, or reformulate.

P3 l62: Remove ‘outcome’.

P4: Change ‘into’ to ‘of’? and delete ‘any’.

P4 l100: I am not sure if ‘kept stagnant’ is a proper formulation.

P8 l175: I would delete ‘change in’.

P10-11 l204-205: I would delete the sentence in brackets.

P11 l233: Do mean ‘consequences’ instead of ’outcomes’?

[1]          V. T. J. Phillips, J.-I. Yano, and A. Khain, "Ice Multiplication by Breakup in Ice–Ice Collisions. Part I: Theoretical Formulation," (in English), Journal of the Atmospheric Sciences, vol. 74, no. 6, pp. 1705-1719, 01 Jun. 2017 2017, doi: https://doi.org/10.1175/JAS-D-16-0224.1.

[2]          P. Grzegorczyk, S. Yadav, F. Zanger, A. Theis, S. K. Mitra, S. Borrmann, and M. Szakáll, "Fragmentation of ice particles: laboratory experiments on graupel–graupel and graupel–snowflake collisions," Atmos. Chem. Phys., vol. 23, no. 20, pp. 13505-13521, 2023, doi: 10.5194/acp-23-13505-2023.

[3]          A. Lauber, A. Kiselev, T. Pander, P. Handmann, and T. Leisner, "Secondary Ice Formation during Freezing of Levitated Droplet," Journal of the Atmospheric Sciences, vol. 75, no. 8, pp. 2815-2826, Aug 2018, doi: 10.1175/jas-d-18-0052.1.

[4]          A. Keinert, D. Spannagel, T. Leisner, and A. Kiselev, "Secondary Ice Production upon Freezing of Freely Falling Drizzle Droplets," Journal of the Atmospheric Sciences, vol. 77, no. 8, pp. 2959-2967, 2020, doi: 10.1175/jas-d-20-0081.1.

[5]          J. Kleinheins, A. Kiselev, A. Keinert, M. Kind, and T. Leisner, "Thermal Imaging of Freezing Drizzle Droplets: Pressure Release Events as a Source of Secondary Ice Particles," (in English), Journal of the Atmospheric Sciences, vol. 78, no. 5, pp. 1703-1713, 01 May. 2021 2021, doi: https://doi.org/10.1175/JAS-D-20-0323.1.

[6]          A. Korolev and T. Leisner, "Review of experimental studies of secondary ice production," Atmospheric Chemistry and Physics, vol. 20, no. 20, pp. 11767-11797, Oct 2020, doi: 10.5194/acp-20-11767-2020.

[7]          J. S. Seidel, A. A. Kiselev, A. Keinert, F. Stratmann, T. Leisner, and S. Hartmann, "Secondary ice production – no evidence of efficient rime-splintering mechanism," Atmos. Chem. Phys., vol. 24, no. 9, pp. 5247-5263, 2024, doi: 10.5194/acp-24-5247-2024.