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1168 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 a. Experiment of 19 Mar 1986 The AgI generator was ignited at 1052 (all times MST) at 2.69 km elevation at Site 1 on the north slope (Fig. 1). Silver iodide consumption was 30 g h -1 until shutdown at 1248 to permit initiation of airborne seeding experiments (see section 4). An aircraft sounding recorded on climbout near 1130 is shown in Fig. 2c. Stability was neutral through the 60-70 kPa layer of interest. The wind and stability during this and the other physical experiments are dis- cussed in more detail by Holroyd et al. (1988). Teth- ersonde observations from the seeding site indicated winds in the 2.7-3.2 km layer of 4-8 m S-I from 3150 to 3550. Wind directions measured by the doppler acoustic sounder ranged from 3200 to 3400, with speeds ranging from 6 to 7 m s -I just 50 m above the mesa top and from 9 to 13 m S-I at the 3.7 km level. Cloud base during the experiment was just below the mesa top at 3.2 km. Cloud tops were near 4.6 km, capped by a dry stable layer. Digital satellite imagery revealed a fairly uniform cloud deck about 25 km wide covering the entire mesa at the time the seeding began. The southern edge appeared slightly broken. The first aircraft sampling pass over the mesa was made near 1150. Some cloud tops were penetrated at 4.6 km, where SLW was less than 0.05 g m -3, maxi- mum IPCwas 5 L -1, and air temperature was -19.50e. Ice particles seldom exceeded 0.2 mm and appeared to be compact plates. Seven crosswind passes followed at 3.8 km altitude, over the Snow Lab and about 6.3 km downwind ofthe seeding generator. After the fifth pass the aircraft climbed to measure cloud tops, which had risen to 4.9 km and cooled slightly to -200e. Peak SLW amounts had increased to 0.15 g m -3 at 4.6 km altitude, with IPC in the 5-15 L -1 range, again with small, compact ice particles. All of the first six crosswind passes at 3.8 km altitude detected a zone of enhanced IPC downwind of the AgI generator. The seventh and final pass, made 28 minutes after generator shutdown, detected little AgI, suggesting that most had been transported beyond the sampling line. This is consistent with the tethersonde wind ob- servations. Total AgI counts recorded during the crosswind passes ranged from 5 (last pass) to 30. The low counts suggest that the 3.8 km sampling level may have been near plume top (Holroyd et al. 1988). The minimum requirement for identification of a plume edge position, 3 counts within 7 seconds (Part I), was achieved on only four passes. These four plume edges were consis- tent, however, ranging from 7.1 to 8.5 km WSW of the Snow Lab. This position agreed well with the ob- served wind direction and enhanced IPC zone. With rare exceptions, the IPC exceeded 10 L -1 only from 5 to 9 km WSW of the Snow Lab. The edges of the seeding-enhanced IPC zone ( hereafter "seeded Ascent AIRCRAFT SOUNDING 18 March 1986 09:38-09:44 mst 50 60 ~ 70 5 II) w 80 f 90 ...0 0 ; TEMPERATURE Oc Ascent AIRCRAFT SOUNDING 18 March 1986 14:38-14:46 mst 50 t. ~ W II: ::) II) II) W II: 80 Q.. Ascent AIRCRAFT SOUNDING 19 March 1986 11:30-11:38 mst 50 . 80 ~ 70 I II) w 80 f 90 Ascent AIRCRAFT SOUNDING 20 March 1986 10:36-10:42 mst 50 .. Q. 60 ~ W II: 70 ~ II) w 80 f 90 Oc FIG. 2. Temperature and dewpoint temperature profiles recorded by aircraft during the three-day experimental period.