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<br />Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 17, No, 12, December 1978
<br />American Meteorological Society
<br />Printed in U. S. A.
<br />
<br />Generalized Criteria for Seeding Winter Orographic Clouds
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<br />LARRY V ARDIMAN AND JAMES A. MOORE
<br />
<br />Bureau of Reclamation Denver, CO 80225
<br />(Manuscript received 3 May 1978, in final form 25 August 1978)
<br />
<br />ABSTRACT
<br />
<br />An a posteriori analysis was conducted utilizing precipitation, rawinsonde and seeding generator data
<br />from seven randomized winter cloud-seeding research projects conducted in orographic settings in the
<br />Rocky Mountain West and on the Pacific Coast of'the United States, Variables were developed and
<br />investigated to establish generalized seedability criteria that are applicable to a variety of meteorological
<br />and topographic conditions, The variables were divided into four general categories: time available,
<br />water available, nuclei available and mixing available. This approach established stratifications under which
<br />positive (increases) or negative (decreases) seeding effects occurred, The study showed that positive seeding
<br />effects occurred at the crest under stable or unstable conditions when a "crest" trajectory was present,
<br />moderate-to-high cloud moisture was present and the cloud-top temperature was - between -10 and
<br />-300C, Decreases occurred at the crest for unstable clouds with a "blow-over" trajectory, with low
<br />cloud moisture and cloud"top temperature colder than -300C, The precipitation for upwind and downwind
<br />regions of a barrier was also increased or decreased depending on stability, trajectory, cloud moisture
<br />and cloud-top temperature, Other stratifications are discussed in the paper.
<br />
<br />1. Introduction
<br />
<br />Since Bergeron's (1949) analysis of the effect of
<br />artificial ice nuclei on supercooled clouds and Ludlam's
<br />(1955) classic study on artificial snowfall from mountain
<br />clouds, considerable progress has been made in the
<br />field of winter orographic snowpack augmentation.
<br />Bergeron's and Ludlam's studies were theoretical
<br />concepts based on laboratory and field observations.
<br />By combining related facts pertaining to mountain
<br />clouds and the production of artificial ice nuclei, they
<br />were able to suggest possible seedability criteria which
<br />would allow successful augmentation of orographic
<br />snowfall.
<br />Since the late 1940's, experimental and operational
<br />cloud seeding to increase precipitation in the mountains
<br />has been carried out to refine these theories. These
<br />efforts have been conducted under widely differing
<br />topographical and meteorological conditions. Additional
<br />seedability criteria have been developed which have
<br />permitted application of artificial seeding to progres-
<br />sively more suitable cloud conditions. Some of these
<br />seedability criteria are simplified indices based on a
<br />single major cloud condition, while other are combina-
<br />tions of conditions, Today, most investigators recognize
<br />that seedability depends on a number of different
<br />meteorological and topographical conditions such as
<br />cloud moisture, cloud-top temperature and wind speed.
<br />However, it is not sufficiently clear how these interact
<br />" with each other in different locales and the relative
<br />importance of each.
<br />
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<br />
<br />A large data base is available from field projects
<br />with which seedability criteria can be studied. Ran-
<br />domized winter orographic cloud-seeding projects have
<br />been conducted extensively in the western United
<br />States from the Rocky Mountains to the Sierra and
<br />from New Mexico to the State of Washington. These
<br />projects have seeded many different types of clouds
<br />and have targeted their efforts on mountain barriers
<br />of varied shape and size.
<br />Attempts should now be made to integrate results
<br />from several of these projects into a comprehensive set
<br />of seedability criteria. Along these lines, Grant and
<br />Elliott (1974) looked at cloud-top temperature. After
<br />adjustments were made for cloud height differences
<br />among the projects studied, a general cloud-top tem-
<br />perature" window" was found to exist in which seeding
<br />increased precipitation, Such studies are needed for
<br />many other variables.
<br />The Bureau of Reclamation has conducted an a
<br />posteriori analysis of seven randomized winter, oro-
<br />graphic seeding projects conducted in the western
<br />United States, The study involved identifying physically
<br />definable, generalized seedability criteria for varying
<br />meteorological and topographical conditions.
<br />
<br />2. Data base
<br />
<br />Of the numerous winter orographic projects which
<br />could have been included in an analysis of this type,
<br />seven projects were selected. The criteria used to select
<br />the projects were that the meteorological and topo-
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