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Larry Vardiman

Abstract

The number of fragments generated by crystal collisions in a cloud is a product of the number of fragments produced per collision and the collision frequency. The first term, called the fragment generation function, was obtained experimentally by taking high-speed photographs of collisions of natural ice crystals with a fixed plate. The number of fragments in a collision was found as a function of the change in momentum on impact with a fixed plate and as a function of crystal type and degree of rime. The difference in the change in momentum for collisions in a cloud compared to the fixed plate is treated theoretically and developed into a mathematical model. The collision frequency is incorporated into the model and rates of fragment generation studied for different crystal combinations, sizes and concentrations.

The generation of secondary particles by mechanical fracturing does not explain the presence of large concentrations of ice crystals in relatively warm clouds. The additional crystals generated in smooth stratiform clouds may reach a maximum of 10 times the expected natural crystal concentration at or near cloud base. Isolated convective clouds do not appear to contain the proper conditions to produce significant additional crystals by mechanical fracturing. Stratiform clouds with embedded convection appear to provide the greatest opportunity for second” particle generation. Here the secondary crystal concentrations could reach 100 to 1000 times the expected natural concentrations.

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Larry Vardiman and James A. Moore

Abstract

No abstract available.

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Larry Vardiman and James A. Moore

Abstract

An a posteriori analysis was conducted utilizing precipitation, rawinsonde and seeding generator data from seven randomized winter cloud-seeding research projects conducted in orographic settings in the Rocky Mountain West and on the Pacific Coast of the United States. Variables were developed and investigated to establish generalized seedability criteria that are applicable to a variety of meteorological and topographic conditions. The variables were divided into four general categories: time available, water available, nuclei available and mixing available. This approach established stratifications under which positive (increases) or negative (decreases) seeding effects occurred. The study showed that positive seeding effects occurred at the crest under stable or unstable conditions when a “crest” trajectory was present, moderate-to-high cloud moisture was present and the cloud-top temperature was between −10 and −30°C. Decreases occurred at the crest for unstable clouds with a “blow-over” trajectory, with low cloud moisture and cloud-top temperature colder than −30°C. The precipitation for upwind and downwind regions of a barrier was also increased or decreased depending on stability, trajectory, cloud moisture and cloud-top temperature. Other stratifications are discussed in the paper.

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Donald Rottner, Larry Vardiman, and James A. Moore

Abstract

A serious systematic bias of the data set used in the original Vardiman and Moore (1978) work was detected while performing a continuing investigation of seeding “windows.” This bias occurred because a large number of no-seed cases were used in the Climax I and Climax II data sets that were not within the original strict randomization. Removal of this systematic bias produced a significant change in both the overall and stratified results. The nature of the bias and its impact on the original results are discussed.

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Donald Rottner, Larry Vardiman, and James A. Moore

Abstract

No abstract available.

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Larry Vardiman, Everett D. Figgins, and Herbert S. Appleman

Abstract

This paper describes the equipment, the theory, and the results of an operational fog-dissipation system installed at Fairchild AFB, Washington, during the winter of 1969–70. An array of stationary ground dispensers was employed to determine the effectiveness of such a system for providing operational support to aircraft landings and take-offs. Usable clearings followed 25 of 29 seeding operations conducted to bring field conditions above minima. In 17 cases the clearings were conclusively the result of fog seeding. Natural clearing appeared to be at least partially responsible in the eight other cases. Seeding failed to produce usable results in four cases in which the temperature was 31F or higher. In an additional five cases preventive seeding was carried out when initial conditions were above minima. In these cases no attempt was made to evaluate the results due to the uncertainty in knowing the conditions which would have occurred without seeding. A total of 68 aircraft departures and 35 landings were made possible during the project.

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Mark F. Heggli, Larry Vardiman, Ronald E. Stewart, and Arlen Huggins

Abstract

Cloud physics data measured by aircraft during two successive winter field seasons (1978–79 and 1979–80) of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective clouds provide the greatest likelihood of significant supercooled water. The Sierra barrier appears to optimize these conditions 40 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although these conditions were greatly reduced at temperatures less than −10°C. The dominance of liquid water content over ice crystal concentration was maximized 7–10 h after the 700 mb trough passage. Area-wide and banded clouds, which make up the remaining precipitation events, showed only small amounts of supercooled water and general abundance of ice crystals. The largest liquid water contents were observed at the greatest temperatures, usually 0° to −5°C. Such climatological information suggests that a weather modification program to enhance snowfall should concentrate primarily on the convective clouds.

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