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1436 JOURNAL OF APPLIED METEOROLOGY VOLUME 35 1992-93 Lake Oraville Runoff Enhancement Program Instrumentation Locations fib ^,"Sh~"~ ^ LAKE ALMANOR Location Map . Mt Hough Chico o ~ . Mr. WEATHER STA. @ RAWINSONDE x PRECIPITA.nON GAUGE . PROPANE DISPENSERS 1 Sacramento o 10 ~o 30 40 50 60 70 Kilometers FIG. 1. LOREP project area and location of field observing equipment. As shown in Fig. 2, the intended target area encom- passes a downwind valley and a second ridge1ine run- ning parallel to and at a slightly lower elevation than the main Sierra crest. The original design assumed that liquid water would form along the crest of the Sierra, allowing initial growth of the crystals. The liquid cloud would then begin to dissipate over the valley, but re- form along the second ridge. It was assumed that along and just over the second ridge most of the seeding ef- fects would be observed. Therefore, most of the pre- cipitation gauges were sited in this area. Many of the results to be presented were derived from an intensive field program conducted during mid- January to mid-March 1993. Emphasis was placed on documenting the transport and dispersion of a tracer and/ or seeding-induced ice crystals from the dispens- ers to the downwind edge of the target area. Sulfur hexaftouride (SF6) was released at approximately 22 kg h -I from two separate high-altitude propane dis- penser sites. Site 7 was located on the crest of the Si- erra, and site 9 was positioned some 5 km west of the crest, as shown in Fig. 2. Two different locations were used to identify which site transports seeded crystals more reliably into supercooled clouds. Only limited propane seeding could be performed due to an exces- sively wet early winter. Measurement systems included a highly instru- mented research aircraft provided by the National Oce- anic and Atmospheric Administration (NOAA). The aircraft was capable of determining horizontal winds using loran navigation but was unable to compute ver- tical motion. A high-altitude mountaintop observatory was established within the target area at Jackson Creek, California (JCC), located along Grizzly Ridge. The aircraft and the observatory were equipped with con- tinuous SF6 analyzers (Benner and Lamb 1985) and an optical ice crystal probe. The analyzer is capable of detecting SF6 concentrations down to 5 ppt (parts per trillion). A verticaly pointing dual-channel microwave radiometer was located at the mountaintop observatory to measure the integrated liquid water and vapor within passing clouds. The radiometer was operated in the ver- tical to sample the integrated liquid and vapor passing over the target ridge. Four time sequential syringe sam- plers (Krasnec et al. 1984) were used to collect 15-min air samples at specified locations within the valley to monitor SF6 (Table 1 and Fig. 2). Each sampler had nine 30-cm3 syringes, providing 2.25 h of sampling time. Locations of weighing precipitation gauges with O.25-mm resolution are also shown in Fig. 2. In addi- tion, three mountaintop weather stations provided mea- surements of supercooled liquid water through the use 120 118 114 114 110 110 106 106 102 102 112 116 FIG. 2. LOREP target area showing lOO-m contours of the terrain. Locations of propane dispensers are labeled dl-dlO, precipitation gauges with three-letter identifiers, and sequential SF6 samplers with a number.