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990 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 Santa Clara County t miles o 50 I. ..' b' ~ . eO kilometers 190 . 160 Lake Almanor Drainage Bosin Portions of the Americon River Basin :rruckee- Tahoe Basin Upper American River Upper Mokelumne River Watenhed Carson-Walker Drainage Upper San Joaquin River Watershed Upper Kings River Watershed () ~ FIG. I. River basins where operational weather modification programs have been conducted in California during 1975/85 (source: California Dept. of Water Resources). These new observational tools provided the tech- nology for renewed exploration of supercooled water within storms. Measurements of supercooled water us- ing these instruments led to the realization that aircraft measurements provided an incomplete description of the supercooled water structure and distribution in Sierra Nevada storms. Supercooled water was found in many parts of storms where it had not been previ- ously identified. In particular, significant amounts of supercooled water often appeared at altitudes that were below the minimum clearance for safe flight (Heggli 1986). Another study by Heggli and Reynolds (1985) found that storms having split-front characteristics de- veloped supercooled water following the passage of the upper-level front. In a series of cases studied by Heggli (1985), supercooled water was shown to increase near the time of cirrus passage. Studies like this, however, are limited by the case-study approach, which empha- sizes the features of a few or sometimes a single storm. A broader study, evaluating all storms, was warranted. In this paper, we provide a comprehensive analysis of the evolution and distribution of supercooled water in Sierra Nevada storm systems that occurred during the period the radiometer was available to SCPP. In performing this analysis, storm types were identified that accounted for the majority of storms occurring in the project area from 1983/84 through 1986/87. The storm typing was based on synoptic characteristics, such as storm trajectory and cloud features. The objective of this study is to describe how su- percooled water evolves near the Sierra Nevada crest- line during each of the storm types. The relationship between liquid water presence in clouds and large-scale storm features is considered. Altitudes and tempera- tures of liquid-bearing cloud layers are also estimated using data from the radiometer and collocated rawin- sondes. The results clarify previous findings reported by the SCPP and provide a basis for improving regional weather modification programs. 2. Research site and instrumentation The SCPP program operated in the American River Basin of the central Sierra Nevada Mountains in Cal- ifornia. A map of the research site showing the topog- raphy, the location of the study area in the state of California, and the location of instrumentation used in this paper, is given in Fig. 2. Because the complete network of instrumentation used during SCPP and a thorough discussion of the SCPP program are provided by Reynolds and Dennis (1986), only instrumentation important to this paper will be described.