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Lewis O. Grant

Abstract

The hypothesis used for the initial Climax wintertime cloud seeding experiment and for subsequent Climax replication-type experiments are described and briefly discussed. More recent physical studies of Colorado orographic clouds and seeding hypotheses are briefly summarized. These later tests and studies of orographic cloud seeding hypotheses emphasized direct and remotely sensed cloud and precipitation measurements utilizing instrumentation and modeling capabilities not available during the Climax statistical experiments. The conclusions suggested from the hypothesis testing, considering both the statistical experiments and the later physical studies, are summarized.

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Robert M. Rauber and Lewis O. Grant

Abstract

The Physical and microphysical structure of the supercooled water fields in wintertime storms over the Park Range of the northern Colorado Rocky Mountains is examined using aircraft and ground-based measurements. Cloud top, cloud base, and zones of strong orographic lift are identified as regions in stratiform systems where supercooled water production can occur. Cloud systems over Colorado's Park Range were found to have low droplet concentrations (≪300 cm−3). In clouds with the lowest droplet concentrations (<100 cm−3), broad droplet spectra were consistently observed. Significant numbers of large (<20 μm) droplets were present in these cases.

The data presented here and in Part I are used to construct conceptual models of the structure and evolution of the liquid water fields in 1) shallow cloud systems with warm cloud tops, 2) deep stratiform clouds with cold tops, and 3) deep convective regions.

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Robert M. Rauber and Lewis O. Grant

Abstract

A case study of an orographic cloud system that developed over the mountains or southern Utah is presented. The storm system contained supercooled liquid water over several hours, and produced almost no precipitation. Because of the high liquid water content, low ice particle concentrations, minimal precipitation, and long duration, the storm appears to have been a good candidate for seeding to augment precipitation. A preliminary analysis of the climatological frequency of orographic cloud systems over these mountains is discussed.

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Lewis O. Grant and Roger L. Steele

The output of effective ice nuclei from silver iodide generators can vary by at least three to four orders of magnitude as a function of the type of generator device employed and its operational efficiency. A detailed consideration of each phase of the process of calibrating silver iodide generators shows that with care meaningful calibrations to within a factor of 1.5 are possible.

The repeatability of actual calibration experiments together with space closed volume experiments support the validity of generator testing procedures.

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Lewis O. Grant and Robert E. Elliott

Abstract

The greatest potential for seeding with artificial ice nuclei to augment precipitation should occur with cloud summit temperatures in the range from about −10C to about −25C. This is the temperature region where there may be a deficiency of natural ice-forming nuclei. This cloud-top temperature range therefore constitutes a “temperature window” for seeding effectiveness. This article considers the results from a number of cloud seeding experiments reported in the literature with respect to this temperature window. The analysis of seven randomized experiments and references to four other experiments indicates that there is a window in the cloud-top temperature range for which precipitation increases are indicated. This extends from about −10C to about −24C for seeding conducted in the modes employed on these projects. At the coldest cloud-top temperatures, generally less than about −30C, decreases in precipitation are indicated. There are variations among the samples which appear to be explainable in terms of differences in the degree of convection present, the seeding methods used, or in the type of nucleant employed. No evidence is presented to show that the temperature window concept applies where there are strong dynamic effects, either natural or due to seeding, such as those in relatively large and isolated cumuli.

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

Abstract

The nature of precipitation changes resulting from seeding cold orographic clouds is examined by separating the observed total precipitation change into duration and intensity change components. The total precipitation change and its two components are then evaluated as functions of cloud temperature using precipitation data recorded in the primary target area during the cloud seeding experiment conducted near Climax, Colo. The results show that the total change in observed precipitation is mainly controlled by changes in precipitation duration, rather than intensity. The main effects of seeding appear to be the initiation of a precipitation release for the warmer clouds during many hours when it would not have occurred naturally, and the suppression of precipitation for the coldest clouds during some hours when it would have occurred naturally. These results are consistent with the concepts of cloud microstability and cloud over-seeding.

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

Abstract

This study is concerned with the elevation and spatial variation effects of wintertime orographic cloud seeding over an area encompassing Fremont, Hoosier and Vail mountain passes in the central Colorado mountains during a period from 1960 to 1965. The observation network consisted of 65 precipitation stations distributed over the three passes. Depending on the grouping of precipitation stations used to represent the prime target area of the study, the average daily precipitation for all 120 seeded days was from 6 to 11% greater than the average daily precipitation for all 131 non-seeded days. There is a high probability that these differences could have occurred by chance alone.

Analyses have also been made according to physically defined stratifications based on a model which describes the seeding effects ascribed to the various strata. Statistically significant increases (decreases) were observed over much of the area for the seeded periods in comparison with the non-seeded periods when 500 mb temperatures were −20C and warmer (−27C and colder). Little or no effects were noted in the intermediate temperature range. When 500-mb wind velocities were from 22–28 m sec−1, statistically significant increases were observed during the seeded period in comparison with the non-seeded period throughout the area.

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David W. Reynolds, Thomas H. Vonder Haar, and Lewis O. Grant

During the past several years, many weather modification programs have been incorporating meteorological satellite data into both the operations and the analysis phase of these projects. This has occurred because of the advancement of the satellite as a mesoscale measurement platform, both temporally and spatially, and as the availability of high quality data has increased. This paper surveys the applications of meteorological satellite data to both summer and winter weather modification programs. A description of the types of observations needed by the programs is given, and an assessment of how accurately satellites can determine these necessary parameters is made.

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Guy G. Goyer, Lewis O. Grant, and Thomas J. Henderson

Abstract

Weathereord, a 40-grain detonating fuse containing about 20 per cent of silver iodide, has been evaluated as a cloud seeding nuclei generator in the laboratory, in the field, and in aircraft cloud seeding. Comparative data on the output efficiencies of several types of silver iodide generators are presented and show that, as a dispersal system, Weathercord provides in unit time and unit volume the highest concentration of nuclei available from any known source. The tests in supercooled logs at Yellowstone National Park and two test cases of the seeding from aircraft of orographic cumuli are also described. Although of a preliminary nature these field tests suggest the effectiveness of the large concentrations of AgI nuclei, generated by Weathercord, in modifying relatively thin supercooled clouds.

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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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