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<br />Reprinted from JdURNAL OF CLIMATE AND ApPLIED METEOROLOGY, Vol. 22, No. II, November 1983
<br />I American Meteorological. Society
<br />Printed in U.S.A.
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<br />Supe~cooled Liquid Water and Ice Crystal Distributions
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<br />I Within Sierra Nevada Winter Storms
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<br />MARK F. HEGGLI AND LARRY V ARDIMAN1
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<br />us. Bureau of Reclamation, Auburn, CA 95603
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<br />RONALD E. STEWART
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<br />Atmospheric Environment Service. Downsview, Ontario. Canada M3H 5T4
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<br />ARLEN HUGGINS'
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<br />Electronic Techniques Inc" Auburn, CA 95603
<br />(Manuscript received II March 1983, in final form I August 1983)
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<br />ABSTRACT
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<br />Cloud physics d~ta measured by aircraft during two successive winter field seasons (1978-,79 and 1979-80)
<br />of the Sierra Cooperative Pilot Project operating over the Sierra Nevada Range have been examined in order
<br />to determine the distributions of supercooled liquid water and ice crystals. Results indicate that convective
<br />clouds provide the greatest likelihood of significant supercooled water. The Siemi barrier appears to optimize
<br />these conditions 401 to 90 km upwind of the crest within pockets of horizontal extent up to 64 km, although
<br />these conditions were greatly reduced at temperatures less than -lOoC. The dominance ofliquid water content
<br />over ice crystal contentration was maximized 7-10 h after the 700 mb trough passage. Area-wide and banded
<br />clouds, which maktup the remaining precipitation events, showed only small amounts of supercooled water
<br />and general abunda~ce of ice crystals. The largest liquid water contents were observed at the greatest temperatures,
<br />usually 00 to -sod Such climatological information suggests that a weather modification program to enhance
<br />snowfall should coricentrate primarily on the convective clouds.
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<br />1. Introduction
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<br />The potential for enhancing snowfall in winter
<br />orographic clouds is usually associated with the pres-
<br />ence of supercooled liquid water in an ice-deficient
<br />environment. Because dir~ct measurement of liquid
<br />water content (L WC) an~ ice crystal concentration
<br />(ICC) historically has beeQ difficult, many estimates
<br />of seeding potential have b~en made by indirect means
<br />such as cloud top temperature, condensate supply rates,
<br />etc. However, as aircraft: instrumentation has im-
<br />proved, direct measurement ofLWC and ICC has be-
<br />come more prevalent. : '
<br />Some of the more recent direct measurements of
<br />L WC and ICC in winter otographic clouds have. been
<br />made by Hobbs (l975a, ~975b), Hobbs and Radke
<br />(1975), Lamb et al. (l976~, Marwitz (1980), Cooper
<br />and Saunders (1980), Cootfr and Marwitz (1980), and
<br />Marwitz and Stewart (198 ~). These measurements are
<br />leading to the realization th~t ideal conditions for seed-
<br />ing are more limited than Iprevious estimates had in-
<br />dicated, because of lower c~mcentrations and frequen-
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<br />'Current affiliation: Christian ~eritage College, San Diego, CA.
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<br />li>:'~" ,@1983AmericanMeteorolOgiCjklSOCiety
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<br />cies ofL WC and higher concentrations and frequencies
<br />of ICC.
<br />The Sierra Cooperative Pilot Project (SCPP), ad-
<br />ministered by the Bureau of Reclamation, has made
<br />extensive airborne measurements of L WC and ICC
<br />over the central Sierra Nevada range. The SCPP with
<br />the project area shown in Fig. 1, is a research program
<br />directed towards determining the feasibility of weather
<br />modification as a means to supplement the regional
<br />water supply in central California and western Nevada.
<br />This article reports on .these observations of super-
<br />cooled liquid water and ice crystals within central Sierra
<br />storms. Suggestions are made in regards to possible
<br />seeding potential within different cloud types.
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<br />2. Instrumentation platforms
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<br />A collection of meteorological instrumentation was
<br />used to study the Sierra Nevada winter storms. The
<br />equipment was deployed over the American and
<br />Tahoe- Truckee river basins in California and Nevada.
<br />Data were obtained during the 1978-79 and 1979-80
<br />field seasons which coincided with the winter months
<br />of January through March. The two primary data
<br />sources used in this study were the cloud physics aircraft:
<br />and weather radar.
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