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Last modified
7/28/2009 2:40:35 PM
Creation date
4/24/2008 2:52:55 PM
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Weather Modification
Title
The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
Date
12/1/1991
Weather Modification - Doc Type
Report
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<br />The mean value of vertically integrated SL W from midnight to 0900 hours was about 0,8 mm. Winds <br />in the SLW zone during that period were westerly and at least 15 m S.l. This results in a SLW flux <br />exceeding 12,000 g S.l per meter crosswind distance. If that aIDount of flux were converted to <br />precipitation of unifonn depth across the approximate 10 km width of the plateau downwind of the <br />radiometer, a precipitation rate of 4.3 nun per hour (0.17 in per hour) would result Snowfall rates <br />gradually approached that value after 0400 as the clouds became increasing efficient. However, <br />precipitation rates were much lower than possible with the available SL W flux before 0400, and, with the <br />exception of a single hour, similar low precipitation rates had existed since 2000 the previous day. This <br />extended period with high SL W flux and very low precipitation rates likely had large seeding potential. <br />The problem would be to separate enhanced snowfall due to seeding (a "seeding signal") from the natural <br />variations with time. A seeding signal might have been obvious if a pulse of AgI had been released prior <br />to 0400 but what if an AgI release had been started about 0400? Then one might have been tempted to <br />claim as a seeding effect the apparently natural increase in snowfall after 0400. <br /> <br />Obviously other data must be examined that ate unaffected by seeding but related to snowfall rates in the <br />target in accessing the reality of seeding signals. For example, the decrease in SL W with time in figure <br />4-1 was associated with an abrupt decrease in windspeed after 0900, and a veering of the wind from <br />southwest to northwest so the airflow gradually became less orthogonal to the barrier. These wind <br />velocity changes would be expected to provide less forced uplift and associated reduced condensate <br />production. The atmosphere also became colder during the afternoon and the clouds appeared to be quite <br />efficient in converting the limited liquid condensate to light snowfall. <br /> <br />4.3 Observational Considerations for Physical Cloud Seeding Experiments <br /> <br />Recent improvements in instrumentation have made it practical to monitor key physical processes, <br />provided mountain ranges are chosen that are logistically practical for experimentation. For example, <br />microwave radiometers can monitor SL W above mountain barriers, and two-dimensional laser imaging <br />probes can observe vast numbers of ice particles for later computer processing. These key observations <br />were impractical to make on a routine basis until approximately the past decade. <br /> <br />. <br /> <br />A very important factor common to successful physical experiments is airborne tracking of the seeded <br />volume between the release point or line and the target area. The complexities of the three-dimensional <br />airflow over mountains are sufficient to render suspect any windspeed and direction estimates based on <br />upwind soundings or a few local surface measurements. It is almost essential that an aircraft monitor <br />where the seeding material is going until it passes over the target, or is shown to have missed the target. <br />An alternate approach would be to have sufficient ground sampling to monitor the position of a surface- <br />released seeding plume but this is impractical in most mountain regions. <br /> <br />Implicit in the need for aircraft tracking is that experiments be conducted over mountain barriers where <br />airborne sampling is practical at low levels. Some regions have serious airspace conflicts among aircraft; <br />for example, within "Victor" routes between major airports. Such high traffic areas should be avoided in <br />planning experiments because needed airspace blocks will frequently be unavailable. <br /> <br />An even more seriou~ consideration is to avoid barriers that preclude flight near the surface. The usual <br />restriction is that aircraft flying in-cloud must stay at least 2,000 ft (600 m) above the highest terrain <br />within 5 miles (8 kIn) of the flight path. Special wavers can be obtained in some locations to allow flight <br />within 1,000 ft (300 m) of the terrain, but either nearby navigational aids are required or a sophisticated <br />on-board navigation system must be employed (Loran-C, inertial navigation system). Even if the target <br />is at a relatively high elevation, nearby higher peaks may preclude aircraft sampling within a kilometer <br />. .' <br /> <br />36 <br />
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