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<br />releases of seeding agent, as discussed in Sec. 2. This is the zone which should routinely be <br />targeted by seeding agents. <br /> <br />3) The primary SLW zone rapidly evaporates downwind of the crest because of warming <br />produced by subsidence, and by depletion from conversion to snowfall. However, secondary <br />ridges and peaks often produce local "pockets" of additional SL W downstream. There is <br />evidence that SL W production is common wherever moist airflow is forced up and over local <br />terrain. While topography varies considerably, typical in-cloud growth rates for crystals fonned <br />over the windward slope should be about 15 to 30 minutes before passage through the lee <br />subsidence zone where cloud droplets rapidly evaporate. That is sufficient time for seeded <br />crystals to grow to precipitable sizes. But as a general rule, the sooner that seeded crystals can be <br />introduced into SL W cloud, the greater the snowfall production. All seeding is a "race" between <br />time needed for crystal fonnation, vertical and horizontal dispersion of the seeded crystal plume, <br />and snowflake growth plus fallout before SL W disappears to the lee of mountain barriers. Larger <br />ice crystals sublimate relatively slowly in the lee subsidence zone compared to the rapid <br />evaporation of tiny cloud droplets. But non precipitating seeded crystals will eventually <br />sublimate downwind of mountain barriers unless exposed to a secondary upslope region with <br />additional SL W cloud. <br /> <br />4) Examination of mountain top temperature observations revealed that SLW cloud is mildly <br />supercooled in a large portion of all stonn passages. That is, the SL W is too warm for effective <br />seeding with AgI which requires temperatures colder than about -6 to -80C depending upon <br />where the AgI is released, and on the type of fonnulation and combustion. This was noted by <br />Boe and Super (1986) and Holroyd et al. (1988), both using Grand Mesa observations. Similar <br />concerns about the limited temperature window for AgI seeding were given by Sassen and Zhao <br />(1993), using observations over the Tushar Mountains of southern Utah. While a significant <br />fraction of stonn phases would be seedable with AgI, many others could be treated only by an <br />alternate seeding method like expansion of liquid propane. The percentage of periods seedable <br />with AgI is likely overestimated by mountain top observations because seeded crystals need to be <br />formed a sufficient distance upwind (downslope) of crestlines to allow for growth and fallout. <br /> <br />5) Because saturation water vapor content increases with temperature, clouds only slightly <br />colder than OOC can be expected to contain the highest SL W amounts. Observations have shown <br />that the highest SL W amounts are usually in cloud only a few degrees colder than OOC while <br />SL W amounts are generally limited by temperatures -1 OOC and quite limited by -150C. This <br />trend has been noted by Boe and Super (1986), Super (1994), Solak et al. (2005) and others, and <br />at least partially explains why SL W maximums are at the lowest and warmest elevations over <br />mountains. This has important implications in selecting the most appropriate seeding technology. <br />The altitudes of maximum SLW are sometimes too wann for AgI to be effective. Consequently, <br />it is strongly recommended that alternative seeding methods be considered in addition to the <br />traditional use of AgI. <br /> <br />6) The best ground seeding situation occurs where two parallel ridges exist perhaps 10-15 Ian <br />apart. Seeding on the windward slope of the upwind ridge can not only enhance snowfall on its <br />windward and lee slopes, but can also provide relatively large seeded crystals which will rapidly <br />grow in the next SL W zone over the downwind ridge. The Bridger Range Experiment in <br />southwest Montana is a good example of this situation (Super and Heimbach 1983). <br /> <br />7) It has been documented that SL W cloud varies rather rapidly with time over any given <br />point. While some storm episodes may provide semi-continuous SL W presence for as long as a <br />day or more, such events are rare. More typically, SL W presence may alternate a number of <br /> <br />5 <br />