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<br />Printed January 30, 1990 <br /> <br />Nevada. Much effort was devoted to searches for SL W. Originally it was thought that embedded <br />convective bands in the winter storms would contain abundant SL W, but most convective bands <br />penetrated by the project's instrumented aircraft turned out to be glaciated and contain little SL W <br />(Heggli et aI., 1983). <br /> <br />Some new pieces of equipment were used during the last few years of the project (Reynolds and <br />Dennis, 1986) (slides). In particular, a radiometer was located near the crest of the Sierra Nevada <br />to determine when supercooled clouds were present (Heggli and Rauber, 1988). Data from the <br />SCPP radiometer and icing rate meters on exposed peaks showed that SLW is present for several <br />hundred hours each winter over the higher parts of the Sierra Nevada. It often occurs so close to <br />the ground that it escapes detection by aircraft. In meridional storms, SLW is most common ahead <br />of the associated fronts; in zonal storms, it often occurs after a front moves through the region and <br />leaves behind a fairly shallow orographic cloud layer with top temperature around -5 to _lOOC <br />(Heggli and Rauber, 1988). Many such clouds produce little or no precipitation unless they are <br />seeded, so they constitute one type of seeding opportunity. Satellite data have proved useful in <br />identifying them. <br /> <br />After some preliminary seeding trials on both post-frontal cumulus and layered orographic clouds, <br />a targeting model was developed to direct snowflakes produced by aircraft seeding to an <br />instrumented site at Kingvale, California (Rauber et aI., 1988). A second aircraft was used to follow <br />the seeding plume; a radar set scanned the treated cloud volume to record echoes from both natural <br />and artificial precipitation particles. Equipment at Kingvale included a very sensitive gauge capable <br />of weighing individual snowflakes and an aspirated 2-D probe to record changes in particle size, <br />shape, and concentration over times as short as a few minutes. In addition, snow samples were <br />collected for chemical analysis. <br /> <br />Despite the care exercised, only 16 percent of the seedlines passing over Kingvale produced effects <br />completely consistent with the seeding hypothesis (Dennis and Reynolds, 1989). Winter storm <br />clouds over the Sierra Nevada are deeper than the orographic clouds over the Grand Mesa, and <br />the Sierra Nevada topography is more complex than that of the Grand Mesa. These differences <br />may account for the difficulties experienced in tracking plumes of seeding material and the resultant <br />ice crystals on SCPP. <br /> <br />Other important studies of winter storms in recent years include work in the Bridger Range of <br />Montana (Super and Heimbach, 1988), Utah (Sassen, 1985), the Park Range of Colorado (Uttal <br />et aI., 1985), and the Mogollon Rim of Arizona (Super et aI., 1989). A consistent picture is <br />emerging from these studies. In general, cloud seeding opportunities are found in persistent, <br />shallow, orographic clouds with cloud top temperatures from -5 to about _120e. These clouds may <br />be purely stratiform, or may contain small, embedded convective elements and qualify as <br />stratocumulus. Deep, precipitating clouds occurring inland generally show little SL W that could be <br />exploited by seeding. Cloud seeding opportunities may exist in other orographic clouds, for <br />example, in embedded convection over the California coastal mountains as suggested by Smith <br />(1962). However, present indications are that clouds with fairly warm tops should receive top <br />priority in programs to augment orographic precipitation and snowpack. <br /> <br />6 <br />