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Paul W. Mielke Jr., Lewis O. Grant, and Charles F. Chappell

Abstract

An orographic cloud seeding experiment conducted in the vicinity of Climax, Colo., has been continued for five additional wintertime periods from 1965–70. A comparison of this new independent information is made with previously discussed wintertime operations of the experiment from 1960–65. As a whole, agreement between these independent data sets is good. In particular, the agreement in temperature and wind partitions is consistent with a previously reported model which describes seeding effects under various physically defined conditions. These comparisons have been made using pooled groups of precipitation sensors having similar elevations and locations.

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

Abstract

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

Abstract

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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Hermann E. Gerber, Paul A. Allee, Ulprich Katz, Charles I. Davis, and Lewis O. Grant

Abstract

The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000Å In diameter.

Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuclei.

Also, field measurements were conducted on 12,000-ft Chalk Mountain (Climax, Colo.) for the purpose of measuring the sizes of active AgI-NaI nuclei emanating from acetone ground generators located at least 6 mi upwind. The size distribution of the nuclei on seeding days proved similar to what might he expected from this generator type. On non-seeding days, the number of active nuclei decreased sharply while the peak of the size distributions shifted to larger sizes.

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Robert M. Rauber, Lewis O. Grant, Da Xiong Feng, and J. B. Snider

Abstract

The spatial and temporal evolution of supercooled water fields in ten wintertime storm systems occurring over the northern Colorado Rocky Mountain region have been examined using data collected by the recently developed scanning dual-channel microwave radiometer. These data were supported by several independent datasets including vertically pointing radar data, mountaintop liquid water content measurements, low and high altitude measurements of crystal rime characteristic rawinsonde data and precipitation intensity measurements.

The ten case studies discussed in this paper represent various stages in the synoptic scale evolution of storms that affect the northern Colorado Rockies. Liquid water was found to occur in nearly all stages of most of these storms. The temporal variations in the magnitude of the liquid water content were significant.

Three common features concerning the evolution of the liquid water field were observed in the prefrontal cloud systems: 1) an inverse relationship between precipitation rate and liquid water content occurred; 2) a direct relationship between cloud top temperature and liquid water content was observed; and 3) the magnitude of the liquid water content was consistently higher over the mountain slopes.

In the postfrontal cloud systems studied, the liquid water content exhibited little variability upwind of the mountain base but varied considerably in the vicinity of the mountain. In these three storms, the magnitude of the liquid water content over the ridge was inversely related to the precipitation rate at mountain base. Liquid water production near the ridgeline was associated with both orographic and convective forcing.

Three orographic cloud systems are discussed in this paper. These clouds formed in similar synoptic environments. The three systems were shallow, had tops warmer than −22°C, and had limited horizontal extent. As in the previous cases, the changes in the liquid water field were inversely associated with changes in precipitation rate. In one case, a decrease in liquid water content was also associated with a decrease in cloud top temperature.

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Roger L. Steele, C. P. Edwards, Lewis O. Grant, and Gerhard Langer

Abstract

The NCAR acoustical ice nucleus counter was calibrated against a Bigg-Warner Weather Bureau type chamber modified as a mixing chamber. The mixing chamber was in turn calibrated against the CSU-NSF isothermal diffusion cloud chamber. This work was carried out using a 300-liter aluminized mylar bag into which known samples of silver iodide nuclei were introduced. Nuclei were transferred from the bag to the NCAR counter in a carrier gas, at a flow rate of 10 liters min−1. It was found that the NCAR counter measured from 16–52% of the count given by the mixing chamber. An NCAR unit was modified with a velvet liner to test the feasibility of eliminating the glycol system, and measurements were made as described above. The modified unit did not count reliably.

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Paul W. Mielke Jr., Glenn W. Brier, Lewis O. Grant, Gerald J. Mulvey, and Paul N. Rosenzweig

Abstract

A reanalysis of the Climax I and II experiments is described. The concern prompting this reanalysis is a suggestion arising from Colorado State University analyses of extended area effects. Those analyses suggested a regionwide pattern of precipitation that, by chance, may have favored the randomly selected seeding days for some of the important meteorological partitions used in earlier analyses. In order to address this concern, this reanalysis employs excellent covariate relationships developed before the initiation of Climax II and which account for over half of the target variability for most meteorological partitions of major interest (e.g., warm 500 mb temperatures and southwest 700 mb wind directions). The statistical evidence of seeding-induced increases for this reanalysis is generally much stronger than in the previous analysis, which did not utilize covariates. For example, the joint one-sided Wilcoxon test statistic P-value for testing the null hypothesis that seeding did not induce a precipitation increase during warm 500 mb temperatures of the Climax I and II experiments is now 0.0013, compared to 0.0550 in the previous analysis. However, the reanalysis also indicates that previous estimates of increases attributed to seeding based strictly on ratios of seeded to non-seeded precipitation amount means are apparently too large. For example, the estimated precipitation increase of the combined Climax I and II data for the warm 500 mb temperature partition is reduced from 41 to 25% when the full set of data is employed in this reanalysis.

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