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520 sion was reached after only one season of physical studies by the University of Washington B-23 aircraft during the drought year of 1976/77 (Rango et aI., 1977). A decision was made in 1981 that a randomized "explora- tory" (Flueck, 1982) experiment would be initiated on cellu- lar convection. This experiment, sometimes called SCPP-l, was to go on while research continued on other cloud types to serve as the basis for additional randomized experiments. A design for a floating-target experiment was begun in the summer of 1981 (Bureau of Reclamation, 1983). After pre- liminary trials during the winter of 1981/82, the floating- target experiment began officially in 1982/83. The floating-target experiment was designed to investigate the effects of airborne delivery of dry-ice pellets or AgI pyro- technics to convective clouds over a large portion of the American- Truckee- Tahoe river basins. Clouds are treated along 37-km lines oriented parallel to the Sierra Nevada. The seeding rate is now set at 0.4 kg' m -I for CO2 or one 20-gm flare per 10 seconds of flight time. A classification run is made by the cloud-physics aircraft prior to seeding to deter- mine the relative seedability of the clouds. The clouds are classified A, B, or C, with A surmised to be the most seedable and C the least seedable. Each experimental unit lasts 30 minutes. The basic seeding hypothesis includes the creation of more ice crystals in cloud, formation of more precipitation particles, and changes in radar characteristics. Increases in the amount and coverage of precipitation on the ground from this experiment are expected to be negligible. This expecta- tion is due to the analysis of both aircraft and radar data from nonrandomized seeding experiments showing that, for 30-minute treatment periods, any additional precipitation falling would be below the level of detectability of the SCPP precipitation gauges. Primary seeding-response variables used in the statistical analysis range from ice-crystal concen- trations soon after seeding (three minutes) to changes in the radar-echo coverage. The ability to distinguish seed from no- seed cases is greatest from a few minutes to tens .of minutes after seeding, but decreases as the associated precipitation approaches the ground. Because of suspended operations, lack of adequate cloud conditions, and changes in seeding priorities, only three ran- domized events in the floating-target experiment have been performed over the last two years. Nevertheless, results from calibration trials and these randomized events indicate that seeding signatures in these convective clouds are detectable by both aircraft instruments and radar (Huggins and Rodi, 1985). Many of these clouds have lifetimes too short for de- veloping additional precipitation from seeding. This is mainly due to the mixing of dry air into the cloud as it tries to grow in the post-storm environment. Some cumulus clouds last long enough to develop augmented precipitation from seeding but it is difficult to predict which of the many clouds that form will in fact do so. The impact of these results on future convective cloud studies will be discussed in a later section. b. Fixed-target experiment Calibration seeding trials have been conducted on the more- stable orographic clouds also. In some cases seeding signa- tures were detected by the cloud-physics aircraft (Stewart Vol. 67, No.5, May 1986 . r, p ~~ ::<~~ 6~ j ~ "::'! ,'"!:'--- - ; I I I o 30 60 90 120 Distance (km) FIG. 6. Schematic mass-flow channels shown normal to barrier as produced by the GUIDE model flow routine. Upwind sounding is extrapolated through these channels. Under stable conditions, crest of flow shown to slope back aloft and blocked flow is shown as dead layer. jSeeding Curtain 1 km t c: o i > CD iii Distance ~ FIG. 7. Symbolic depiction of trajectories resulting from simul- taneous initiation (from six levels) of six packets of crystals where each packet contains several crystals which evolve with differing time- dependent fall-speed functions. and Marwitz, 1982) and possibly by the GML, indicating that precipitation at the ground might also have been affected. A randomized fixed-target experiment to test effects of aerial seeding on orographic clouds was initiated in 1984/85. The fixed target has been set up near the upper reaches of the ARB (Fig. 3, top). Within this target are 11 precipitation gauges along with the Kingvale site, where both remote-sens- ing and direct-observation platforms (GML) are operated to quantify the effects of seeding. The Auburn forecast office also has access to the four remote PROBE (Portable Remote Observation of the Environment) weather stations, whose data are telemetered via GOES (Geostationery Operational