Laserfiche WebLink
<br />ABSTRACT <br /> <br />The procedures and results of a five-year program of cloud seeding to in- <br />crease orographic precipitation in the Park Range of Colorado are presented. <br />Winter precipitation systems were seeded with silver iodide from ground <br />generators and aircraft. Measurements in the seeding target area included: <br />total snowfall, snowfall rate, freezing nuclei counts, type of snow crystals, <br />and concentration of silver in snow, as well as radar and rawinsonde obser- <br />vations. As part of the investigation, diffusion tests were conducted to study <br />the transport and dispersion of seeding agent, and a numerical model was <br />developed to predict location and rate of snowfall as functions of observed <br />wind, temperature, and stability. <br /> <br />During the first years of the program, the experimental' design called for <br />pulsed seeding at 1 cph frequency. Power spectra of snOwfall rate were <br />computed in an effort to show significant differences in the time histories <br />of snowfall rate for pulse-seeded and natural snow days. Though seeding <br />effects were evident in some observations from individual cases, the approach <br />was not successful~ largely due to the natural variability of snowfall rate and <br />of seeding agent dispersion in the atmosphere. <br /> <br />For the latter portion of the experiments, experimental days were divided <br />into randomly selected three-hour seeded and unse~ded blocks. This design <br />produced statistically significant conclusions on the effect of seeding. It is <br />concluded that seeding produced snowfall rate increases of 100% or more <br />when cloud top temperature was warmer than -20oC, and snowfall rate de- <br />creases for cloud top temperature colder than -240C. Supporting measure- <br />ments show conclusively that: silver concentration in snow was higher for <br />seedBd than for unseeded periods, hexagonal plate snow crystals were much <br />more numerous in seeded snow, AgI particles acted as nuclei for snow cry- <br />stals, and seeding did frequently raise the surface air nuclei count in the <br />target area. <br /> <br />On the basis of climatological analysis and the results of this program it is <br />concluded that a combination of ground and airborne seeding, utilized when- <br />ever forecast cloud temperatures are appropriate, could, on an operational <br />basis, produce cumulative seasonal snowfall increases of approximately 250/0 <br />in the Park Range. <br /> <br />i <br /> <br />~, <br /> <br />