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<br />328 <br /> <br />JOURNAL OF APPLIED METEOROLOGY <br /> <br />VOLUME 29 <br /> <br />results from seeding with a curtain of CO2 or a line <br />source of AgI burned in acetone. Considering the rather <br />warm temperatures at which cloud seeding opportu- <br />nities exist in the Sierra Nevada, CO2 was used almost <br />exclusively during these experiments. Now, based on <br />the two cases presented, there is evidence that AgI is <br />as effective as CO2 up to temperatures as warm as <br />-60C, and that the snowfall produced by these two <br />different materials is quite similar, consisting of high <br />concentrations of rimed particles < 1 mm in diameter. <br />Considering the simpler logistics of seeding with AgI <br />in acetone and the continued nucleation of new ice, <br />compared with instantaneous nucleation by CO2, AgI <br />would be the best choice. This choice also increases <br />the time an aircraft can conduct seeding and reduces <br />restrictions on location of the seeder aircraft. Seeding <br />does not have to be done in the liquid cloud, but can <br />be done upwind when icing or air traffic problems are <br />encountered. <br />Although the results of these experiments would ap- <br />pear disappointing based on the overall success rate of <br />the several measuring platforms, it is conceivable that <br />the magnitude of seeding effects were the same on days <br />when measurable effects were masked by a cloud's nat- <br />ural processes. The best that this type of experiment <br />can do is to provide a documented estimate of the <br />amount of precipitation that can be expected from air- <br />craft seeding when conditions are favorable. Viewed <br />in that light, experiments such as this are successful if <br />they can provide even a few cases where the links in <br />the chain of physical effects from seeding are clearly <br />documented. The failures in this endeavor are partly <br />a reflection of the difficult logistical and technical <br />problems encountered when undertaking such an ef- <br />fort, and partly a reflection of the natural variability <br />of even simple cloud systems, particularly in their spa- <br />tial and temporal distribution ofliquid water. As Hobbs <br />(1975) pointed out, experiments like this cannot re- <br />place long-term statistical experiments, which are nec- <br />essary to establish the overall increases in precipitation <br />that can result from routine cloud seeding. However, <br />experiments like this are necessary to lay the ground- <br />work for designing statistical experiments, by refining <br />seeding techniques and measurement methods, and by <br />providing estimates of the effects anticipated. Unfor- <br />tunately, based on the magnitude of effects measured <br />during this experiment, a statistical experiment would <br />have to be quite long. This fact, coupled with the dif- <br />ficulty and expense of field work, makes such an ex- <br />periment a formidable undertaking. <br />These experiments provide physical plausibility to <br />the current thinking that in selected wintertime oro- <br />graphic clouds seeding can provide a small addition to <br />the winter snowpack (American Meteorological Soci- <br />ety, 1985). Unfortunately a clear assessment of a <br />cloud's seedability requires sophisticated in situ and <br />remote measurements of cloud liquid water, a capa- <br />bility unavailable to routine cloud-seeding operations. <br /> <br />To extend this knowledge to those operations will re- <br />quire efforts directed at developing simpler and less <br />expensive means of recognizing appropriate opportu- <br />nities. <br /> <br />Acknowledgments. This research was sponsored by <br />the Bureau of Reclamation, U.S. Department of the <br />Interior. Gratitude is extended to all Sierra Cooperative <br />Pilot Project field personnel. Without their dedicated <br />efforts this research would not have been possible. The <br />research aircraft was supplied and operated by the <br />University of Wyoming, the seeding aircraft by Aero <br />Systems Inc. Electronics Techniques Inc. was respon- <br />sible for the rawinsonde, radar, and precipitation data; <br />the Desert Research Institute, University of Nevada, <br />for the Ka band radar and snow chemistry data; and <br />the Bureau of Reclamation for measurements at King- <br />vale. Gratitude is also extended to the reviewers. Their <br />thorough reviews contributed significantly to the final <br />manuscript. <br /> <br />REFERENCES <br /> <br />American Meteorological Society, 1985: Planned and inadvertant <br />weather modification, a policy statement of the American Me- <br />teorological Society, Bull. Amer. Meteor. Soc., 66,447-449. <br />Blumenstein, R, R., R. M. Rauber, L. O. Grant and W. G, Finnegan, <br />1987: Application of ice nucleation kinetics in orographic clouds, <br />J. Climate Appl. Meteor., 26, 1363-1376. <br />Bureau of Reclamation, 1983: The design ofSCPP-1. Rep., Division <br />of Atmospheric Resources Research, U,S. 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