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<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />VOLUME 29
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<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 />
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