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<br />00255~ <br /> <br />Contract Summaries <br /> <br />CONTRACTOR: South Dakota School of Mines and Technology, Rapid City, <br />South D akot a <br />CONTRACT NO. 14-06-D-6796 <br />PRINCIPAL INVESTIGATORS: Richard A. Schleusener and Arnett S. Dennis <br />PERIOD: July 1, 1974, to October 31, 1975 <br />FUNDING: FY75 - $100,000 <br /> <br />When this study began in 1969, the contract objective was to develop <br />precipitation management techniques to increase usable water yields <br />in the northern High Plains. Research from 1969 through 1974 on the <br />South Dakota Cloud Catcher experiment indicated significant rainfall <br />increases from seeding on days when theoretical cloud models showed a <br />potential for dynamic growth of seeded clouds. That work is reported <br />in detail in the Skywater 1973-74 Biennial Report.1! <br /> <br />The contract was amended in 1974 to add analysis and evaluation of <br />data from South Dakota cloud seeding experiments for input to HIPLEX <br />research. More advanced cloud models proved to be practical in <br />reevaluating past experiments. It was learned that assessment of the <br />cloud seeding potential for modifying weather in the northern High <br />Plains requires knowledge of the frequency of convective clouds and <br />their size distribution, since cloud response depends on cloud size <br />and airmass conditions. <br /> <br />l <br />f <br /> <br />I <br /> <br />Comparison of new rainfall analyses with radar echo climatology from <br />the 1972 North Dakota Pilot Project developed a fairly consistent <br />picture of the effect of silver iodide seeding on convective clouds <br />of the northern High Plains, and why seasonal rainfall increases are <br />limited to about 10 to 15 percent. The maximum dynamic effects of <br />seeding with silver iodide are experienced in moderate-sized clouds <br />with depths from 6 to 10 km. These clouds contribute only a rela- <br />tively small fraction of the seasonal rainfall. Seeding larger <br />clouds with silver iodide usually does not make them grow and may <br />damage their precipitation efficiency by increasing updraft speed. <br />Therefore, these studies concluded that seeding should be done <br />sparingly and under areas of new cloud growth. Clouds over 12 000 m <br />high, or those with heavy glaciation, should not be seeded. <br /> <br />Cloud Catcher cell analysis indicated the one-dimensional, steady- <br />state cloud model provides a reasonable picture of the horizontal <br />dimensions of an individual cell and the vertical structure of the <br />updraft. Examination of the 1966-68 South Dakota Rapid Project data <br /> <br />I <br /> <br />1/ "Project Skywater 1973-74 Biennial Report," USOI, Bureau of <br />Reclamation Report No. REC-ERC-76-21, December 1976. <br /> <br />III-5 <br />