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<br />t <br />I <br />, <br />J <br />1 <br />I <br />1\ <br />t <br />I, <br />f <br />I <br />1\ <br />" <br />I <br />I <br />I <br />.,. <br />I <br />1 <br /> <br />4.0 SULFUR HEXAFLUORIDE TRACER EXPERIMENTS <br /> <br />4.1 Design of Experiments <br /> <br />North American Weather Consultants was contracted by <br />the Bureau of Reclamation to provide essential services to <br />carry out experimental cloud seeding over locations in the <br />Colorado River Basin. A "first look" winter program wa& <br />conducted 1n early 1987 over the high plateau region of central <br />Arizona. A major portion of this effort incorporated the <br />use of a continuous tracer gas analyzer mounted inside a high <br />performance aircraft (King Air 200) capable of sustaining <br />flight in supercooled liquid water cloud and measuring various <br />microphysical parameters. <br /> <br />The project design called for either ground based or <br />airborne releases of pure sulfur hexafluoride (SF6) typically <br />at points upwind of the Mogollon Rim. Release rates of 20-25 <br />kg/hr. were calculated to be necessary in order to have detectable <br />SF6 concentrations at least 55.5 km (30 n.m.) downwind. SF6 <br />releases typically began 30 to 60 minutes prior to aircraft <br />sampling to allow the advection of the plume1 downwind from <br />the release site. The King Air aircraft would then be flown <br />at fixed intervals of 18.5, 37, 55.5 km (10, 20, and 30 n.m.) <br />downwind perpendicular to the mean lower level wind. The <br />first pass at each of the three fixed distances downwind would <br />be flown as low as terrain would allow. Once a plume was <br />identified, the aircraft would then step up in increments <br />until the plume was lost. The aircraft would then descend <br />1n an attempt to document plume top height. <br /> <br />1Note: the term "plume" is used throughout this section to <br />describe an encounter of SF6 above the background level of <br />the analyzer. <br /> <br />4-1 <br /> <br />