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1014 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 A number of case studies have also been analyzed from investigations in the Tushar mountain region of southern Utah. Sassen et al. (1986) documented the passage of a weak cold front over the Tushars and found that supercooled water was present throughout the storm except during the cold frontal passage. The max- imum values of supercooled water occurred in the postfrontal convective region of the storm, consistent with our measurements of weak cold frontal passages in the Sierra Nevada. Rauber and Grant (1987) doc- umented the evolution of supercooled water during an orographic storm associated with a weak cold front. They also found supercooled water to be present con- tinuously within this storm, although in this case, the orographic clouds were prefrontal. The Tushar storm discussed by Long (1986) was particularly interesting because the same storm had affected the SCPP area the day before (7-8 February 1985), and it was well documented by both projects (see Reynolds and Ku- ciauskas 1987). The storm discussed by Long had su- percooled water evolution similar to that observed in SCPP in the early prefrontal region, but differed sub- stantially in the postfrontal region. In California, the passage of the cold front resulted in a reduction in cloud depth and a large increase in supercooled water over the crestline. In Utah, the cold frontal passage was ac- companied by a reduction in supercooled water. The difference appeared related to prevailing wind direction in the postfrontal region in the two areas. Over the SCPP area, winds remained westerly (barrier perpen- dicular) in the postfrontal region, while in Utah the winds became northerly (barrier parallel), eliminating orographic lift. Radiometric measurements of supercooled water have also been reported in a 3-year study over Colo- rado's Grand Mesa by Super et al. (1986). These au- thors provided a climatology of supercooled water measurements similar to that in Fig. 23, but for I-h averages. The magnitude and general characteristics of the distribution of supercooled water measurements in both locations were very similar, an interesting result considering that the annual precipitation at the crest of the Grand Mesa is only two-thirds of that measured in the Sierra Nevada. 8. Implications for cloud seeding Lastly, we discuss the implication of our results for cloud seeding operations in the Sierra Nevada. The success of cloud seeding, when used to augment pre- cipitation, depends on many factors, one of which is the presence of supercooled water. Our results indicate that supercooled water is present in distinct regions of Sierra Nevada storms. The location of these regions, and the magnitude and duration of the presence of supercooled water, depend strongly on the synoptic scale structure, intensity, evolution, and trajectory of the storms. Since the duration of supercooled water in clouds is typically 2-10 hours, it is important to be able to fore- cast these events reliably in other to seed effectively. The postfrontal regions of storms with zonal flow char- acteristics appear to provide the best situation for fore- casting supercooled water events, since the subsidence region associated with a reduction in cloud depth is often evident as a distinct high cloud boundary on sat- ellite photographs. Identifying this boundary was one of the primary forecasting techniques used operation- ally during SCPP. Reynolds and Kuciauskas (1987) have provided detailed analyses of the structure of the atmosphere in the vicinity of this boundary using radar, satellite, and other data. These authors showed that radar and satellite may be used as a tool to predict the onset of supercooled water events when storms. with these characteristics impact the Sierra Nevada. How- ever, the prefrontal regions of storms with meridional flow characteristics are more difficult to forecast. The lower clouds in these cases are frequently obscured by cirrus clouds on satellite photographs, making the onset of the storm difficult to predict. In addition to the existence of supercooled water, another important aspect of cloud seeding opportunity is its magnitude. Seventy percent of the time that ra- diometric measurements of supercooled water were above background, values were below 0.20 mm (Fig. 23). Ninety percent of the time the measurements were below 0.40 mm. It is not clear whether measurements at or below these values represent true cloud seeding opportunity. Definitive studies are still required to in- terpret the meaning of these measurements clearly in terms of the actual cloud supercooled water content. To date, attempts at such studies have met with limited success (see Heggli et al. 1987). 14000 13000 12000 11000 10000 >- 9000 u z 8000 w ::> 0 7000 w cr 6000 II- 5000 4000 3000 2000 1000 0 IJJ 70~ f! Z 601JJ (,) cr w 5OCl. w 40~ !i 305 :::;: ::> 20(,) 100 90 80 10 o 0.1 0,2 0.3 0.4 0,5 0,6 0,7 0,8 0,9 1.0 1.1 1,2 1.3 1.4 INTEGRATED LIQUID WATER (mm) FIG. 23. Frequency distribution ofIiquid water measurements from 1983-84 through 1986-87 with the dual-channel radiometer. The measurements were made at 2-minute intervals. Only measurements > 0.05 mm (background) were included in the analysis. The solid line shows the percentage of measurements with values less than the value on the abscissa. The database includes over 775 hours of ob- servations during which liquid water was observed.