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812 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 Beginning in 1984/85, SCPP initiated a randomized exploratory fixed target experiment (Flueck 1982) to test the effects of aerial seeding on precipitation en- hancement from orographic clouds. The target was designated as a 750 km2 area above 1.5 km elevation in the northeast comer of the ARB (see Fig. I). The purpose of this experiment was to observe, as com- pletely as possible, the chain of events leading from nucleation to precipitation for particles created by seeding within the orographic clouds. Preliminary re- sults of this experiment have been discussed by Martner (1986) and Deshler and Reynolds (1987). Targeting seeding effects within the fixed target area was critical to experiment evaluation. For SCPP, tar- geting was particularly important, since the ground- based microphysical observations were limited to one location at Kingvale (Fig. 1). Targeting required a technique that could 1) accurately reproduce the major characteristics of the airflow across the mountain within the seeded region of the cloud; 2) closely reproduce the growth and fallout trajectories of ice particles created by seeding; 3) predict the location where aircraft should seed to produce effects at the target; 4) account for dispersion of a seeding curtain by vertical wind shear and particle fall velocity spectra variations (see Stewart and Marwitz 1982a); 5) be initialized with typical data available to the program; and 6) run in real time. The problem of targeting was complicated by the complex structure and rapid changes that characterize Sierra Nevada winter storms. Many storm systems that pass over the Sierra Nevada were found to have similar features to those observed over the Pacific Northwest (Hobbs 1978) and the British Isles (Browning and ~R NV CA o o in ,., SH - LN- Monk 1982). During such storms, cloud systems often evolved in a sequence similar to that shown in Fig. 2 (Reynolds and Dennis 1986). Maximum supercooled liquid water content (and, by inference, maximum precipitation enhancement potential) was often found to occur following the passage of the upper-level cold front as the cloud top lowered (Heggli and Reynolds 1985; Reynolds and Kuciauskas 1987). During this phase of the storm, winds at cloud level generally changed continually with time in response to the pas- sage of mesoscale and synoptic scale features. In many storms, a low-level barrier jet was present (Parish 1982). The speed of this jet, and its horizontal and vertical extent, varied from storm to storm. · Calculating accurate wind fields over the Sierra Ne- vada in a time frame useful for targeting calculations proved to be difficult. During the years of the fixed target experiment (1984/85 through 1986/87), SCPP scientists experimented with simple parameterizations of the airflow (Elliott 1981; Elliott and Rhea 1984; Rhea and Elliott 1986), detailed simulations using the model of Anthes and Warner (1978) as adapted by Parish (1982) and Waight (1984), and detailed more complex simulations using the nested grid model of Clark (1977) (Smolarkiewicz et aI., unpublished). The latter models proved valuable for research, but no method could consistently reproduce accurate wind fields within the seeded cloud region in a time frame necessary for an airborne seeding experiment. Two basic problems hampered progress in solving the targeting problem: I) Sierra Nevada storms often contain mesoscale features with horizontal scales on the order of the \ 120:30 \ 'b 'Co \ /'.20:00 ~ r, ,I : ~ \ 101 \ \ I~~ \' II;' ) I 1(- '-~ I ..-_ - I ..\\ \ ,\ ,I \. \ 'b 'Co \ \ I , I ,,~ , CO",,' I ~ , 1 \ ,I / --=...... / t,o ,..f- / \ I ,'I I \0 ~i1ometers 20 \ I FIG. I. Map of the SCPP research area showing the location of the area in California, the location of the Lincoln, Sheridan and Kingvale rawinsonde launch sites, the mean topography of the region, and the target area (dashed box) for the fixed target seeding experiment.