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2.6 Radiometer :Measurements A dual-frequency 120.6 and 31.54 GHz) microwave radiometer (Hogg et al., 1983), provided by Reclamation, "vas operated at the Johnsville ski area parking lot from December 30,1991, through April 30, 1992. During the 1992-93 winter season, the radiometer was moved to the Jackson Creek Observatory (near the JCC precipitation gauge on fig. 2.6) along Grizzly Ridge, where it was operated from February 5 to March 19, 1993. The radiometer was used to measure path-integrated liquid water in millimeters and precipitable water vapor in centimeters passing over the target area. This information is critical to determining seeding effectiyeness. The radiometer is controlled by a mini-computer, which allows both system control and data collection. Data are recorded to hard disk every 2 min. The disk size is sufficient to record a full season of data. Tipping curves are a means of calibrating the system. They consist of scanning the antenna through various elevation angles that relate to a given number of earth atmospheres (usually 1 to 4). These measurements allow the determination of the attenuation taking place in the entire path from the outside reflector to the microwave detectors. Tipping curves were performed about every 2 weeks under clear sky conditions and a steady water vapor environment. The outside reflector dish must be kept free of snow or water during radiometer operation. A large exhaust fan was positioned to blow over the dish to help maintain a snow-free surface. However, when wet snow was falling, keeping the dish completely dry was nearly impossible. At the Johnsville site, the radiometer was normally operated with the antenna pointing vertically. However, elevation scans were made to the east over Grizzly Ridge periodically to determine if supercooled liquid water was more prevalent over this ridge. The radiomeier was moved to the Jackson Creek site to sample the integrated liquid and vapor passing over the target ridge. An example of integrated liquid and vapor data is shown on figure 2.8. A photograph of the radiometer and ice crystal collection system is shown on figure 2.9. The objecth'es for using the radiometer were: a. Provide real-time indications of supercooled. liquid water for conducting seeding operations. b. Develop a climatology on the frequency and magnitude of supercooled liquid over the target area. c. Determine the impact seeding has on the liquid water. The radiometer experienced a problem throughout the intensive 2-month field program in early 1993. A reference voltage used to compute values of vapor and liquid often drifted outside of specified tolerances. This problem required constant adjustment and compromised data quality. Only a limited set of quality data could be retrieved from this data set. 15