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<br />""0: <br /> <br />Since 1976, SCPP has been making detailed meteorological measurements using in- <br />situ and remote observation platforms, e.g., cloud physics aircraft, conven- <br />tional and Doppler radar, radiometers, a ground precipitation network, a <br />rawinsonde network, etc., and conducting calibration and randomized seeding stu- <br />dies in order to meet its primary objective. The basic hypothesis now developed <br />by which cloud seeding is expected to produce additional precipitat"ion is based <br />on the finding that available condensate in the form of supercooled liquid water <br />is lost in winter cloud systems via entrainment and/or subsidence to the lee of <br />the Sierra crest. By adding glaciogenic seeding material (in SCPP's case aerial <br />dry ice seeding) at an appropriate place and time within these clouds or cloud <br />systems, it is hypothesized that additional ice embryos will be formed and grow <br />at the expense of this available liquid producing precipitation particles that <br />will fall-out within the specified target area, thereby increasing the total <br />efficiency of the precipitation process. <br /> <br />One common synoptic pattern in the Sierras giving rise to seedab1e events is <br />shown in Figure 2 (after Browning and Monk, 1982). It has been found that in <br />the major, deep portion of a cyclonic storm, regions 1, 2, 3, of Figure 2, that <br />very little seedabi1ity exists. This caused by an abundance of ice being <br />generated within the deep cloud system by primary and secondary ice production, <br />and the presence of a barrier parallel low level jet (Parish, 1982) within a <br />strong stable layer along the lower levels of the barrier, minimizing liquid <br />water production thru lift by the barrier. As the upper level front (or sp1it- <br />front as described by Browning and Monk) moves by, the low-level condensate <br />supply increases, liquid water is produced and seedability increases rapidly in <br />this shallow orographic cloud. An example of such an event is shown in Figure 3 <br />for a storm observed at SCPP's Kingva1e high-altitude observatory on 21 January, <br />1984. Note the rapid increase in liquid as the vertically pointing Ka band <br />radar showed cloud top heights dropping substantially. In the Sierras it <br />appears this supercooled liquid is distributed close to the barrier and quite <br />often exists between the -5 oc and 0 oc level, Figure 4. The extent of these <br />liquid water episodes have been observed to last from 3 to 24 hours. <br /> <br />It has been observed that the droplet spectra for the liquid water observed can <br />take on either a a maritime or continental distribution depending on air mass <br />source regions. When maritime distributions occur, the cloud may quickly tran- <br />sition to ice naturally due to the onset of secondary ice crystal production <br />mechani SolS. <br /> <br />When seeding these shallow orographic clouds using vertical curtains of dry ice, <br />vertical wind shears increase the diffusion of these curtains substantially over <br />what turbu1 ent diffusi on alone cou1 d accompl i sh thus all owi ng substanti a1 ver- <br />tical and horizontal regions containing liquid to receive increased ice con- <br />centrations (Stewart and Marwitz, 1982). This has been verified both by <br />aircraft and ground based observations of seeding signatures. <br /> <br />As the storm further transitions into the post frontal air mass, further desta- <br />bilization occurs and strong convective elements or bands develop on the upwind <br />edge of the orographic cloud. The seedabi1ity then moves to the foothill region <br />of the Sierra Nevada where liquid water is being lost to entrainment in convec- <br />tive clouds. This seedable region translates down the barrier because, as these <br />convective clouds continue to develop and merge on the barrier, rapid deve10p- <br /> <br />6 <br />