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than on non-seed days.' Since the first level of confirmation was not achieved, progressive consideration of the ether confirmatory ana- lyses was precluded. Ex~oratory analyses were then conducted to determine if seeding effects were indicated. Most of the FACE-l analyses wre repeated on the FACE-2 data, especially the covariate analyses which were ~recluded from the con- firmatory specifications. The most important of the replicated analyses indicated that the seed day FT and TT rainfalls were larger than the respective non-seed day rainfalls, but the rainfall increases were small and sta- tistically non-significant (Woodley et al., 1983). More recently, additional exploratory analyses have attempted to assess treatment effects in FACE-l and FACE-2 through a guided exploratory linear modelling approach (Flueck et al., 1984). The individual and residual linear model results suggest that positive target area treatment effects are indicated in both FACE-l and 2. The comparable yearly results indicate that seven of the eight sum- mers during which FACE-l and 2 was conducted had indications of rainfall increases due to seeding. In one year a rainfall decrease due to seeding is indicated. Woodley et al., (1983) suggest that the pro- bable reasons for the relatively weak FACE-2 results are: a) an unknown and possibly intermittent seeding effect, b) inadequate predictor equations that account for natural rainfall variability and, c) a limited sample size. Since the FACE program did not directly and systematically measure the meteorological proce,ses relevant to the pre- cipitation process in the FACE clouds and the intermediate responses to seeding, understanding of the physical reasons for the tantalizing FACE-l and 2 results will not be possible. 4. SCPP The SCPP (Sierra Cooperative Pilot Project) is a winter orographic cloud research program with the goal of developing a practical cloud seeding capability for increasing the streamflow of the American River Basin of the Central Sierra Nevada of California. The primary scientific objective ofSCPP is to define those conditions giving rise to preci- pitation increases, decreases or no change by the prescribed treatment of winter orographic clouds and cloud systems. Since 1976, SCPP has been making detailed m~ieorological measurements using in-situ and ,remote observation platforms, e.g., cloud physics aircraft, conventional and Doppler radar, radiometers, a ground precipitation network, a rawinsonde network, etc., and con- ducting calibration and randomized seeding studies in order to meet its primary objec- tive. The basic hypotnesis now developed by which cloud seeding is expected to produce additional precipitation is based on the finding that available condensate in the form of supercooled liquid water is' lost in winter cloud systems via entrainment and/or sub- sidence to the lee of the Sierra crest. By adding glaciogenic seeding material (in SCpp's case aerial dry ice seeding) at an appropriate place and time within these r ~ :,2 ,J:!:,~~, q GO c1ou-ds or cloud systems, iCfs'"hypothesi zed that additional ice embryos will be formed al1d grow at the expense of this available,' liquid producing precipitation particles that will fall-out within the specified target area, thereby increasing the total efficiency of the precipitation process. One common synoptic pattern in the Sierras giving rise to seedable events is shown in Figure 4 (after Browning and Monk, 1982). It has been found that in the major, deep por- tion of a cyclonic storm, regions 1, 2, 3', of Figure 4, that very little seedability exists. This is caused by an abundance of ice being generated within the deep cloud' system by primary and secondary ice produc- t i on, and the presence of a barri er pa ra ll.e 1 low level jet (Parish, 1982) within a strong stable layer along the lower levels of the barrier, minimizing liquid water production thru lift by the barrier. As the upper level front (or split-front as described by Browning and Monk) moves by, the low-level condensate supply increases, liquid water is produced and seedability increases rapidly in this shallow orographic cloud. An example of such an event is shown in Figure 5 for a storm observed at SCPP's Kingvale high- altitude observatory on 21 January, 1984. Note the rapid increase in liquid as the ~er- tica1ly pointing Ka band radar showed cloud top heights dropping substantially. In the Sierras it appears this supercooled liquid is distributed close to the barrier and quite often exists between the -5 ac and 0 oc level, Figure 6. The extent of these liquid water episodes have been observed to last from three to twenty-four hours. It has been observed that the droplet spectra for the liquid water observed can take on either a a maritime or continental distribu- tion depending on air mass source regions. When maritime distributions occur, the cloud may quickly transition to ice naturally due to the onset of secondary ice crystal produc- tion mechanisms. When seeding these shallow orographic clouds using vertical curtains of dry ice, vertical wind shears increase the diffusion of these curtains substantially over what turbulent diffusion alone could accomplish thus allowing substantial vertical and horizontal regions containing liquid to receive increased ice concentrations (Stewart and Marwitz, 1982). This has been verified both by aircraft and ground based observations of seeding signatures. 'J / / Upper cold frontel precip. /' /Worm frontel prec:p. ~ /,------^---, , ~' ~::::::::::::::-=- , ~'I' ,'..,.,.,",..'...',',','..- DRY~_,~b~~I:;i;~:~'~i;;;ll:II:::::~:~.~,:" ~0.,..."lllll,I.II,II'~' ,fi'I7'l,,~;.;.~"II:,::....:I'1 II ,,'., DRY 5 I ::\'i:;:: ,1~:::IIp.:,,:...:::. ,3,'" I , /:f}"",I, .."",I,,:,.......l,lI',',11I ~ I A '-----v--' Shallow moist zone B FIGURE 4 -- CROSS SECTION THRU A SPLIT FRONT. MOST SEEDABLE REGIONS DETERMINED FROM RADIOMETRIC AND AIRCRAFT OBSERVATIONS WITHIN THE STORr1 ARE ANNOTATED. (AFTER BROWNING AND HONK, 1982). ~) --. ;;. ;':".!.H .. ",r II..... .