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Bulletin American Meteorological Society tion in the ARB falls between October and May, while more than 75 percent falls in the colder winter months of No- vember through March. Therefore, SCPP has concentrated on winter cloud systems, with intensive field observations taken from early January through March. The storms affecting this region usually move in from the Pacific Ocean, with the associated air masses having had long residence times over the ocean. The freezing level usually is between 1000 m and 3000 m. The warmer storms usually move in from the southwest. Colder storms, with the freezing level occasionally as low as 300 m, come from the Gulf of Alaska. These storms often stagnate just off the coast, bring- ing showery weather for several days. The average annual precipitation from the Sacramento Valley floor to the Sierra Nevada crest and beyond Lake Tahoe is shown in Fig. 2. As one proceeds eastward, the pre- cipitation initially increases with elevation on the west or up- wind side of the Sierra Nevada, but shows little change from Blue Canyon to the crest. Then it decreases rapidly down- wind of the crest. The specific objectives in designing SCPP were to identify the conditions under which cloud seeding causes precipita- tion increases or decreases in the Sierra Nevada and to esti- mate the magnitude of these changes. To achieve these objec- tives a network of in situ measuring systems, as well as weather radars and aircraft, have been put into the field. Be- cause cloud-seeding studies deal in space and time scales var- ying from millimeters and seconds (sampling changes in snow crystals) to thousands of kilometers and days (monitor- ing storm systems moving inland from the Pacific Ocean), a varied mix of equipment has been required. Table 2 gives a complete list of equipment used in SCPP at one time or another and their applications. Most measurement systems employed have been concen- trated in or near the ARB as shown on Fig. 3 (top), which locates the sites with major equipment installations. An ele- vation transect across the Sierra Nevada is shown as Fig. 3 (bottom). . Data collection for SCPP began during the winter of 1976/77. The SCPP field office was set up at Auburn, Cali- fornia in September 1977. SCPP has been in the field each winter since then with the exception of 1980/81, which was a stand-down year for analysis of the data collected during the three preceding years. Cloud-seeding experiments have been conducted on SCPP since the winter of 1977/78, but on a lim- ited scale and with close attention given to suspension cri- teria that were designed to prevent seeding during hazardous weather situations. 4. Special equipment a. Aircraft The project aircraft have been maintained at McClellan AFB (near Sacramento) during each of the intensive field seasons conducted since 1976. In most winters, a seeder aircraft and a cloud-physics aircraft have been used. Both are high-per- formance, twin-engine planes capable of extended flight in severe wind and icing conditions. The seeder aircraft is cap- able of seeding clouds by dispensing dry-ice pellets from a 515 TABLE 2. Equipment and instrumentation used on SCPP, 1976-85. Equipment Purpose PRECIPITATION GAUGES Telemetered Weighing Gauge Recording Belfort Weighing Gauge Heated Tipping Bucket Gauge Disdrometer Various wind, temperature, humidity, and pressure sensors Rawinsonde Station (4) Pibal SNOW CRYSTAL SAMPLING Photo-Replicator Photomicrographs Aspirated Two-dimensional Laser Probe AEROSOL AND TRACER NCAR Ice-nucleous Counter High Volume Samplers Snow Samplers (manual) SF6 Detector AIRCRAFT CLOUD PHYSICS University of Wash. B-23 University of Wyoming King Air SEEDER/fRACER University of Wash. Cessna 207 Aero Systems, Inc. Aero Commander University of North Dakota Piper Cheyenne II REMOTE SENSING Acoustic Sounders (2) National Weather Service (Sacramento) WSR-57 S-band Weather Radar Skywater C-band Weather Rada r NCAR C-band Doppler Radar NOAA X-band Doppler Radar Microwave Radiometer K-band Radar GOES Weather Satellite Real-time precipitation intensity Document precipitation intensity Precipitation intensity Droplet spectra Real-time and recorded to determine character of storm system Measure the vertical profile of temperature, humidity, wind Determine the low-level wind profile Determine habit and rime of snow crystals Determine habit and rime of snow crystals Continuous observations of crystal concentration and size Nucleation efficiency of AgI generators Collects AgI particles and determine concentration Concentration of trace elements in snow including silver Measure concentration and movement in trace gas within cloud Transport and diffusion Precipitation physics Transport and diffusion Kinematic studies Precipitation physics Release tracer material Observe cloud top Dispense dry ice and silver iodide Release radar targets for wind studies Physical observations in cloud Dispense dry ice Silver iodide Precipitation physics Monitor stability, inversions, turbulence in boundary layer Monitor precipitation and organization of storm Monitor and record precipitation patterns Monitor precipitation intensity and motion within clouds Passive detector of liquid water and vapor Monitor depth of precipitating clouds Monitor and record satellite observed cloud patterns