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WINTER OROGRAPHIC STORM 13 j y ~ V 555 C:V t t.:p. this 8 kIn stretch called the "upwind highway" (Hi way 31, south of DOT). Silver iodide had been released on the valley floor by 8 enerators for over 48 hours~~ J()'O hexiflouride was released within the mouth of ., Canyon at a rate of~ g s'! from about 1000-1120 (all times MST) and 1250-1420 to simulate AgI. The SF6 tracer gas was measured by fast-response detectors (Benner and Lamb, 1985) in a van and on the aircraft; both of which monitored their position with Global Positioning System receivers. The aircraft also obtained observations of ice nuclei (IN), air temperature, liquid water content and horizontal winds. This approach allowed for testing of both valley-floor and canyon-mouth releases during the same experiment. Several instruments were operated near the north end of the upwind highway at the "DOT" site. A dual-channel microwave radiometer (Hogg et al. 1983) provided supercooled liquid water (SLW) amounts. An acoustical counter (Langer. 1973) monitored the IN concentration. (Similar counters were operated in the aircraft and a second instrumented van). Wind velocity and temperature also were measured. A network of 5 precipitation gauges were maintained across the Plateau. each in a sheltered location to minimize wind-induced undercatch. Wind velocity and air temperature were monitored by 3 automatic weather stations in addition to the DOT observatory. One was on the valley floor, one was in the entrance to Fairview Canyon, and one was 3.3 kIn further up that Canyon. Experiment of March 2. 1991 - Mternoon On the morning of March 2 a NW-SE stationary front was positioned NE of Utah and a weak cold front trailed through Colorado and New Mexico from a small closed low to the NE of Utah. The weak front had crossed Utah before daylight. The air mass over Utah was maritime Pacific. Most of precipitation occurred before noon with rates tapering off as the shortwave and associated surface features moved to the east and weakened. The atmosphere was conditionally unstable which produced embedded convection over Utah in the post-frontal environment. Observations were made from early afternoon until the storm ended about 1600. Fixed Surface Observations: Precipitation was very light during this period. Average hourly amounts between 1200-1600 for the Plateau-top gauges downwind of the DOT were 0,004 inches. Snowfall was negligible on the west slope of the Plateau. The DOT wind speed was steady during the experimental period at about 5 m sol. Wind direction gradually backed from 290 degrees true at 1200 to 245 degrees at 1430, and thereafter was between 245-255 degrees. The wind speed ranged from 10-15 m S'l at the head of.the north fork of Birch Creek Canyon, consistently from the west. Winds on the valley floor were between 205-240 degrees near 5 m sol until after 1500 when they began to decrease. Winds in the lower portion of Fairview Canyon were consistently up-canyon with speeds of 1-4 m sol. Until 1540 the vertically-integrated SLW varied markedly, typical of embedded convection, with an average value near 0.15 mm. The SLW thereafter was negligible as skies cleared with the end of the storm. The IN count rate averaged about 500 min-l prior to 1540 and thereafter rapidly approached zero. The abrupt decrease is believed due to the end of convection, which presumably was transporting the IN from the valley to the Plateau top, rather than to AgI generator network turnoff between