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Physical mechanisms of extra area effects from weather modification




Mulvey, Gerald J., author

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One of the complexities of weather modification, namely extra area effects have long posed an opportunity for the long-term control of the earth's weather. This study investigates the physical mechanisms by which cloud seeding projects may cause extra area effects. The investigations center on one of the simplest of precipitating systems, namely the cold wintertime orographic clouds of the central Rocky Mountains. Three lines of investigation are followed: (1) field studies of seeding material movement in the atmosphere and receiver cloud characteristics, (2) numerical simulation, and (3) historical studies of the affected cloud system. The field observations consist of case studies of the movement and dispersion of silver iodide from ground based generators. These studies, during the winters of 1974-75 and 1975-76, used nuclei counters aboard two aircraft. Aerosol silver concentration measurements were also made during the last experimental year. The surface observations made as part of the field studies included snow collection for silver analysis, radar observation and ice nuclei measurements. The aircraft studies established the fact that regions of above backround ice nuclei concentrations extend from the target cloud systems as far as 240 km downwind while exhibiting concentrations from 10 to over 700 ice nuclei per liter active at -20°C. The analysis of silver concentrations in snow confirmed above backround silver concentrations exist in snow samples on days during which cloud seeding occurred in the mountains. The numerical cloud models were used to investigate the mode of seeding and the seeding requirements of the downwind cloud systems . Case study runs using a cumulus model suggested that seeding the upslope cloud would cause little dynamic intensification. It was therefore inferred that the seeding mode was static. The second cloud model, a rapid glaciation model, estimated the seeding requirements in terms of active ice nuclei or ice crystals for precipitation augmentation to be between 1.0 and 5 No1 -1. An ice crystal transport model was used to predict the survival time for a spectrum of crystal sizes under a variety of conditions. The results indicate that under certain meteorological conditions crystals typically observed in orographic conditions can survive long enough to reach the downwind upslope cloud in concentrations between .5 and 50 No1-1. The historical studies established characteristics of the typical upslope clouds as well as the surface features controlling their formation. The radar observations showed convective-like echoes migrating within the upslope cloud over the eastern plains of Colorado downwind of Climax. These studies show the existence of at least two feasible mechanisms through which mountain orographic clouds can affect the precipitation on the eastern plains. The studies also outline the meteorological conditions under which the mechanisms investigated are operative.


August 1977.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1977.

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Weather control -- Colorado


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