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Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 27, No.9, September 1988 American Meteorological Society The Characteristics and Evolution of Supercooled Water in Wintertime Storms over the Sierra Nevada: A Summary of Microwave Radiometric Measurements Taken during the Sierra Cooperative Pilot Project MARK F. HEGGLJ Bureau of Reclamation, Auburn, California ROBERT M. RAUBER Electronic Techniques, Inc., Auburn, California (Manuscript received 10 August 1987, in final form 18 February 1988) ABSTRACT A comprehensive examination of the evolution and vertical distribution of supercooled liquid water in Sierra Nevada winter storms was completed as part of the Sierra Cooperative Pilot Project. Measurements of supercooled water in 63 storms, which affected the area from the 1983/84 through the 1986/87 winter field seasons, were made with a dual-channel microwave radiometer located near the Sierra Nevada crestline. The analyses were carried out in two parts. In the first part, winter storms were grouped into two general categories according to the prevailing flow and resultant storm trajectory. The two storm types consisted of those propagating in zonal flow and those in meridional flow. A complete description of the storm typing is provided. Storms in zonal flow were either developing, occluding, or dissipating during the time they affected the Sierra Nevada. The stage of evolution of the storm dictated the characteristics of supercooled water observed during the storm's passage over the Sierra Nevada. The presence of supercooled water in meridional storms was influenced mainly by the trajectory of the storm. Results indicated that zonal storms had the most sustained periods of supercooled water in the postfrontal region, while sustained supercooled water measurements in meridional storms were usually greatest within the prefrontal portions of the storm. These periods of supercooled water were associated primarily with orographic clouds. Studies of the vertical supercooled water distribution showed that saturation often existed within I km of the local terrain when supercooled water was measured by the radiometer. Fifty percent of the rawinsondes launched through supercooled water-bearing clouds measured water-saturated layers deeper than 0.5 km. Saturated layers were found to exist most often between -80 and -lOoC, although they were often present over a much wider temperature range. Conditions favorable for ice crystal growth were common, which supports the pursuit of precipitation en- hancement as a means of supplementing regional water supplies. 1. Introduction The Sierra Cooperative Pilot Project (SCPP), ad- ministered by the Bureau of Reclamation and designed to develop a firmer foundation for operational weather modification programs in the region, has measured and studied supercooled liquid water within winter storms affecting the central Sierra. Nevada mountains of Cal- ifornia and Nevada. Historically, operational programs have been conducted in ten Sierra Nevada river basins (see Fig. 1) over the past 10 yr. These operational pro- grams emphasize the interest in using weather modi- ... Current address: University of Illinois, Department of Atmo- spheric Science, Urbana, IL 61801. Corresponding author address: Mark HeggIi, 1277 Coon Ct., Cool, CA 95614. @ 1988 American Meteorological Society fication to solve long-term water needs in California and Nevada. The SCPP has attempted to respond with a scientific investigation intended to provide a basis for weather modification operations. The results pre- sented here provide comprehensive analyses of SCPP research concerning cloud seeding opportunity rec- ognition. Early studies in the SCPP concentrated on airborne investigations of supercooled water and ice crystal dis- tributions in clouds (Heggli et al. 1983). These studies indicated that the largest ratios of supercooled water content to ice crystals existed in the postfrontal con- vective clouds. Very little supercooled water was ob- served in area-wide frontal clouds or shallow orographic clouds. Soon after the Heggli et al. results were pub- lished, supercooled water measurements from ground- based instrumentation, particularly the dual-channel radiometer, and icing rate measurements from moun- taintop weather stations became available.