the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Oct 2024
Quantified ice-nucleating ability of AgI-containing seeding particles in natural clouds
Abstract. For decades, silver iodide (AgI) has been widely used for laboratory ice nucleation experiments and glaciogenic cloud seeding operations due to its ability to nucleate ice at relatively warm temperatures (up to -3 °C). Despite being one of the most well-characterized ice-nucleating substances, gaps remain in the understanding of how its ice nucleation behavior in the laboratory translates to natural clouds. Here, we present, for the first time, measurements of the ice-nucleated fractions (INFs) of AgI-containing seeding particles, derived from in situ measurements of ice crystal number concentrations (ICNC) and seeding particle number concentrations during glaciogenic cloud seeding experiments. The experiments were performed as part of the CLOUDLAB project, in which we used targeted cloud seeding with an uncrewed aerial vehicle to try to answer fundamental questions about ice-phase cloud microphysics. Data from 16 seeding experiments show strong linear correlations between ICNC and seeding particle concentration, indicating relatively constant INFs throughout each experiment. Median INFs (0.07–1.63 %) were found to weakly increase with decreasing cloud temperature at seeding height (range of -5.1 to -8.3 °C). We compare our results with previous key laboratory experiments and discuss the immersion freezing mechanism. This study can help to bridge the gap in understanding of AgI ice nucleation behavior between laboratory and field experiments which further helps to inform future cloud seeding operations.
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RC1: 'Comment on egusphere-2024-3230', Anonymous Referee #1, 23 Nov 2024
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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.
Citation: https://doi.org/10.5194/egusphere-2024-3230-RC1 -
RC2: 'Comment on egusphere-2024-3230', Russell J. Perkins, 10 Jan 2025
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Overall this manuscript describes an extensive set of airborne seeding experiments with downwind sampling capturing changes in cloud properties within the seeding aerosol plume. These experiments are difficult to perform and uncommon in the literature, and provide a very useful validation of seeding techniques in an appropriate real-world environment and some deeper mechanistic insights. The manuscript is publishable as-is, though I will suggest some edits for clarity and minor expansion of discussion:
Introduction or Section 2.2: Ice nucleation by silver iodide based seeding materials will vary with exact aerosol composition, which can be impacted both by burn material formulation and burn conditions. At least a brief discussion of this seems appropriate in one or both of these sections.
There is almost no discussion of secondary ice production in the manuscript, but a few words in the introduction would be welcome considering that there is evidence of secondary ice in some similar clouds to those observed in your study (https://doi. org/10.1029/2021JD036411 and https://doi. org/10.5194/acp-2021-686). Further, Fig 4b shows inverse correlation (or no correlation removing endpoints as discussed) between INF and residence time, which to me implies SIP is not occurring under your study conditions, which seems worth drawing attention to.
Eqn 2 – perhaps a few words around this equation to clarify that the seed_conc is background subtracted to remove ambient aerosol, as discussed at the end of section 2.3.
Do any of these experiments have measurable background ice? The example case does not, but do any of the other 15? Impacts of background ice on the analysis methods should be discussed if it is present.
Line 267-8: Suggest wording change to something like “more seeding aerosols will act as INPs at colder temperatures” rather than discussing activation probability of individual INPs, there is a whole can of worms with heterogeneous classical nucleation theory that you don’t need to get into for this paper. Not a critical change in my opinion though.
Line 271 point a: if this were the primary reason, you could fit an exponential to your data with higher significance. I recommend either including a metric of how fit significance changes here or removing this explanation. I find explanation b more compelling in absence of that, so I think it stands on its own.
Section 4.1:
There is a lot of interesting speculation here, but it feels a little out of place in that it only loosely ties to the experimental results that are presented. I think there is some opportunity for streamlining here, and building this section out to start with results first and how they constrain the potential freezing mechanisms.
The flares generating the particles will locally heat the air and add water vapor from combustion. These thing coupled together makes it very likely that as the flare plume cools to ambient conditions it will generate supersaturated conditions to some extent, so it seems unavoidable that at least some particles will activate through an immersion freezing mechanism, but figuring out what fraction will be difficult.
Can you reach effectively water subsaturated conditions without first depleting all your liquid cloud droplets? I’m not sure this explanation makes sense in light of your data that seems to mostly show persistence of mixed-phase clouds.
Regarding condensation v immersion freezing: My understanding of the general difference is that in immersion freezing, water uptake occurs at a temperature where a given aerosol will not immediately freeze, then drops are cooled until the freeze. For condensation freezing, water uptake occurs at cold temperatures, so as soon as liquid water condenses on an INP it is capable of freezing (as soon as it reaches a requisite water activity). The discussion in this section of the manuscript seems to split these two mechanisms in a different way, however, with liquid water uptake occurring at the same temperatures for both, but the condensation mechanism requiring higher supersaturations for some reason. Some clarification here and in the introduction as to how the authors understand these different mechanisms would be appreciated, but given that the conclusions of the manuscript don’t greatly depend on this interpretation I don’t see this as a major issue.
Section 4.2: this section is a challenging one to discuss, because the different measurements are not directly comparable, as you spend the section outlining, but there is still an understandable desire to compare your measurements to previous literature. Ultimately I think this section is still helpful for the explanations it offers for the discrepancies observed between different studies, but I wonder if the framing at the beginning of the section could be changed to make that clear from the beginning?
For example, lines 350-3: Perhaps it is better to directly state here that the different frozen fraction metrics aren’t directly comparable, but are placed on the same graph for the sake of discussion.
Citation: https://doi.org/10.5194/egusphere-2024-3230-RC2
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