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William G. Finnegan and Richard L. Pitter
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William G. Finnegan and Richard L. Pitter

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William G. Finnegan and Steven K. Chai

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A potential molecular mechanism of ice nucleation on AgI and AgI · AgCl particulates involves rearrangement of ordered water molecule clusters associated with hydrated Ag+ ion patches. This nucleation mechanism is thought to occur rapidly at −5° to −20°C on substrate particles wetted by water. This hypothesis is based on analysis of the rates of ice crystal formation and ice nucleus activities observed in experiments previously conducted in a 1-m3 isothermal cloud chamber using chemical kinetics. These experiments examined the chemistry of wetted AgI colloid particles involving electric charge separations between the particles and adjacent solution. The rate of nucleation is slowed by the presence of alkali and alkaline earth halides in concentrations greater than approximately 10−3 M in the water wetting the particles. Match of the crystal structure of the substrate with that of the ice may have no effect on this mechanism.

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Randy D. Horn, William G. Finnegan, and Paul J. DeMott

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Mathematical and experimental errors cast doubt on the ice nucleus activity spectrum of falling dry ice pellets as reported by Fukuta et al. (1971). Preliminary laboratory studies have established that ice embryos or small ice crystals will survive at ice saturation for periods up to 15 min in the Colorado State University Isothermal Cloud Chamber following dry ice seeding. These facts suggest that a re-evaluation be made of the methodology, amounts used, and the effects expected from dry ice seeding of natural clouds.

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Paul T. Scott, William G. Finnegan, and Peter C. Sinclair

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A new ice nucleant aerosol was produced by combustion of a 2% AgI-0.5 mole % Bil3-NH4I-acetone-water solution. The ice nucleating effectiveness of this aerosol is an order of magnitude greater than AgI alone at −10°C. An X-ray powder analysis identified the aerosol as the hexagonal crystal form of AgI having the closest match to ice ever reported for a nucleant of this type.

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Steven K. Chai, William G. Finnegan, and Richard L. Pitter

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In a 1984–85 winter cloud-seeding program at Lake Almanor, California, indium sesquioxide (In2O3) aerosol particle generators were collocated with silver iodide (AgI) aerosol particle generators as a source of inert tracer aerosol. The In2O3 aerosol served as an indicator of the amount of AgI aerosol scavenged. Based on the aerosol emission rates, if AgI aerosol was only captured by scavenging processes, and played no part in forming ice crystals and snowfall, the silver to indium ratio (Ag:In) in the analyzed snow would be 0.8.

Analysis of snow samples from the target area produced frequent Ag:In ratio values in excess of 1.1. In the snowfall at the closest sampling sites to the aerosol generator the high ratios of Ag:In cannot be explained by the contact-freezing ice formation mechanism. A mechanism with a much faster rate than possible by contact freezing is necessary to produce the high Ag:In ratios that were observed. Part of the AgI seeding aerosol functioned rapidly to produce ice crystals by a forced condensation-freezing mechanism immediately after generation, and those ice crystals contributed to the snowfall at those sites closest to the generator.

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William G. Finnegan, Steven K. Chai, and Andrew Detwiler

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Geometrically oriented riming was found in Formvar resin replicas of columnar ice crystals collected in cumulus clouds at −6°C during an aircraft field program in Texas. Rimed cloud droplets were found either on the ends of the crystals or in a girdle around the middle. Oriented riming is attributed to preferential collection on growing ice crystals with charge separations between the crystal body and growing ends. Droplet attraction to separated charge regions of growing ice crystals results in enhanced riming and increases the rate of precipitation development. Effects of this process on cloud electrification depend on whether the cloud droplets carry net charges or are polarized. The impact of this oriented riming process on several cloud electrification scenarios is discussed.

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Paul J. DeMott, William G. Finnegan, and Lewis O. Grant

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Chemical kinetic theory and methodology is applied to examine the ice nucleating properties of silver iodide (AgI) and silver iodide-silver chloride (AgI-AgCl) aerosols in a large cloud chamber held at water saturation. This approach uses temporal data on ice crystals formation with changes in key nucleation parameters such as temperature, water vapor concentration and droplet concentrations. The inter-relationships between ice nucleation effectiveness, nucleation mechanisms, nuclei chemical and physical properties and the rate of appearance of ice crystals can be deduced. The theory and methodology can be applied to atmospheric experimentation.

Ice nucleation effectiveness increases of up to three orders of magnitude over that of AgI aerosols can be achieved with AgI-AgCl solid solution aerosols. Both aerosols are shown to form ice crystals by predominantly contact nucleation at temperatures of −16°C and warmer. Nucleation of the ice phase following collision is identified as a very rapid process, so that the rate of appearance of ice crystals is controlled by the much slower transport rate of nuclei to cloud droplets. The higher efficiency of AgI-AgCl nuclei with respect to the standard AgI nuclei is attributed to an improvement in the relative rates of nucleation versus deactivation or solution following collision of the nuclei with cloud droplets. This increase is most probably due to epitaxy and/or surface “active site” improvements. At a temperature of −20°C, all tested aerosols formed ice crystals by a combination of contact nucleation and deposition nucleation. The percentage of ice crystals formed by deposition correlated well with a minimum particle size of 500 Å for an appreciable deposition rate.

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Rochelle R. Blumenstein, Robert M. Rauber, Lewis O. Grant, and William G. Finnegan

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Ice nucleation by silver iodide-sodium iodide aerosol particles has been characterized in the Colorado State University isothermal cloud chamber using the techniques of chemical kinetics. Two separate mechanisms of condensation-freezing ice nucleation have been observed. One mechanism occurs at water saturation and is a characteristically slow process, with a half-life of the order of 10–30 min. The other mechanism occurs when the environment is supersaturated with respect to liquid water. This mechanism is characteristically fast, requires less than a minute for completion, and results in a higher yield of ice crystals than the slow mechanism.

The mechanism, rate and yield data obtained in the laboratory investigations are applied to an orographic cloud particle trajectory model to assess the ice nucleation characteristics of silver iodide-sodium iodide aerosol particles in the temporal and spatial scale of an orographic cloud. The importance of nucleation mechanism, rate and yield are investigated to determine the control these parameters have on the extent and location of ice nucleation within the cloud and the effect on precipitation distribution. In certain conditional ice crystal production was found to be prolonged over time and space. Resulting precipitation occurred over large areas. In other conditions, ice nucleation occurred primarily within a zone of a few kilometers. Precipitation was then found to occur in a more restricted area. The mechanism and rates of nucleation therefore can affect the targeting and analysis of seeding effects in weather modification experiments.

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