I support publication of the manuscript.

The authors present a detailed study of the effect of silver iodide containing flares on supercooled liquid water clouds, during an experiment conducted in Eriswil, Switzerland as part of the CLOUDLAB project. The primary result reported in this manuscript is the fraction of silver iodide containing particles which nucleated ice in the experiments.

As noted just above, I support publication. I also think there are alternative perspectives for some of the results that should be considered

The flares were carried by drones, and burned while the drone was in-cloud. This is a very important point to keep in mind, in my opinion. In many cloud seeding studies, the seeding material (usually some sort of flare) is released (i.e. burned) below cloud base, or on on the leading edge of the cloud deck, with the expectation that wind or updraft will carry the material into the cloud. There is definitely a dispersion problem to contend with in these cases. (More on dispersion below)

The authors make the assumption that saturation ratio in the cloud is 1. I disagree with this assumption on two levels. I think that the mean saturation ratio is slightly above 1 (a quasi-steady state supersaturation). See for example, Yang et al, 2019, in particular figure 4. I agree that the saturation ratio is certainly close to 1, but exceeding 1 by just a tiny bit could be very important for this case. The flare material includes some very hygroscopic components and even a slight supersaturation could be enough to activate those particles. The second level of my disagreement with the assumption that the saturation ratio is 1 is that the cloud is turbulent, so there must be fluctuations in the temperature and water vapor concentration, and thus the saturation ratio. Figure 4 in Yang et al shows a distribution of values. You can also see it in Figure 8 of Siebert and Shaw, 2017. The fraction of ice nucleating particles that the authors are deriving is very likely to be a convolution of the fraction of particles that encountered a supersaturation high enough (in the fluctuating environment) to activate them, AND the fraction of particles that had a silver iodide particle within them sufficient to catalyze nucleation. The authors state that a saturation ratio of 1 is enough to hydrate the particles by some substantial amount, and I agree with that. That said, they also appeal to a freezing point depression. If droplets are activated, they will be getting steadily more dilute, even in a slightly supersaturated environment.

I appreciate the inclusion of Table 1 in the manuscript. Could the authors comment on cases 58 and 62? I note that the aggregation factor in case 58 was 7.35 when the residence time was 8.2 minutes. In case 62, the aggregation factor was 4.36 (substantially lower) even though the residence time was five minutes longer. Was there some significant difference in conditions for case 58 that would cause the aggregation factor to be so much higher?

In line 193, the authors state that their derived ice nucleating fraction should be representative of the initial conditions despite the fact that they are measuring ice crystal concentration and interstitial aerosol after an elapsed time of about 10 minutes. The stated rationale for that statement is that both aerosol and ice crystals will disperse, and the dispersion will be similar. I disagree. The crystals are growing – becoming much larger. Their gravitational settling will be much more pronounced. That’s acknowledged later in the paper, but it should be mentioned here as an uncertainty.

Minor points

line 17: the reference to Pruppacher and Klett. That’s a 900+ page book. At least provide a chapter in the reference, please.

Line 345: “...the deterministic component of heterogeneous nucleation...” There is no deterministic component to nucleation. Nucleation is stochastic. Ice nucleation does tend to be favored in certain places for some samples, but even there, ice doesn’t always form in the same place. See figure 3 in Holden et al., 2019. (I know the title of the paper is that it proves the existence of active sites, but figure 3 clearly shows that ice may form at one spot with a preference, but it isn’t deterministic.)

References

Holden, M.A., Whale, T.F., Tarn, M.D., O’Sullivan, D., Walshaw, R.D., Murray, B.J., Meldrum, F.C. and Christenson, H.K., 2019. High-speed imaging of ice nucleation in water proves the existence of active sites. Science Advances, 5(2), p.eaav4316.

Siebert, H. and Shaw, R.A., 2017. Supersaturation fluctuations during the early stage of cumulus formation. Journal of the Atmospheric Sciences, 74(4), pp.975-988.

Yang, F., McGraw, R., Luke, E.P., Zhang, D., Kollias, P. and Vogelmann, A.M., 2019. A new approach to estimate supersaturation fluctuations in stratocumulus cloud using ground-based remote-sensing measurements. Atmospheric Measurement Techniques, 12(11), pp.5817-5828.