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APRIL 1990 DESHLER, REYNOLDS AND HUGGINS 289 measurements suggesting that shallow orographic clouds are suitable for seeding to increase snowfall (Hobbs 1975a; Hill 1980b, 1982, 1986; Rauber and Grant 1986; Reynolds and Kuciauskas 1988). Leonov and Perelet ( 1967) and Leskov ( 1974) pre- sented more direct measurements of effects, both in cloud and at the ground, from glaciogenic seeding of stratus and stratocumulus during the winter months in southern Russia. Weickman ( 1974) provided direct physical observations in Great Lakes supercooled clouds. Hobbs (197 5b ), however, was the first to at- tempt to determine the magnitude of seeding effects within orographic clouds by documenting the complete chain of physical processes resulting from seeding. Hobbs was successful at tracking seeded precipitation from cloud to ground on several occasions. On those occasions the clouds were visibly glaciated at aircraft altitude, and surface snowfall was observed to change from graupel and other heavily rimed particles to lightly rimed dendrites and stellars. Precipitation increases up to 0.5 mm h -1 were observed during the predicted pe- riod of effect, and precipitation was thought to be re- distributed to higher elevations. Recently these types of detailed physical observations have been reported in other mountainous regions (Super and Heimbach 1988; Super and Boe 1988). Research directed at obtaining physical evidence to quantify the effects of seeding orographic clouds, and to develop appropriate seeding technologies, has been conducted in the central Sierra Nevada of California since 1976 (Reynolds and Dennis 1986). Descriptions of the two exploratory seeding experiments conducted by the Sierra Cooperative Pilot Project (SCPP) are given in Bureau of Reclamation (1983, 1985). Target clouds were selected based on observations of SL W. The cloud types centered on post-frontal cumulus, stratus, and stratocumulus that form over the Sierra Nevada. Cloud seeding was done from aircraft using either dry ice, droppable pyrotechnic flares containing AgI, or by burning an AgI NH41 mixture in acetone. Airborne and ground-based instruments were em- ployed to measure the effects of seeding. The project conducted: 1) exploratory research and preliminary seeding trials until 1981 ; 2) randomized seeding of post- frontal convection during the winters of 1982/83 and 1983/84; and 3) randomized seeding of widespread orographic clouds during the winters of 1984/85 through 1986/87. Results from the seeding experiments on postfrontal convective clouds indicated that only marginal benefits could be obtained (Huggins and Rodi 1985; Rodi and Flueck 1986). While this experiment was being con- ducted it was observed that shallow orographic clouds over the central Sierra Nevada contained SL W for long periods of time over a wide area. These clouds were like the type Ludlam (1955) proposed to seed for snowpack augmentation. To treat these clouds the sec- ond randomized experiment was conducted January- March 1985, January-March 1986, and November- December 1986. Twenty-one experiments were com- pleted during this time, including 8 deliberate seeding experiments. An additional 15 similar experiments were completed prior to 1985. The purpose of this pa- per is to present the major findings from these exper- iments by presenting the experimental design and pro- cedure, by discussing individual cases yielding defini- tive results, and by presenting an overview of all seeding experiments. 2. Design of the fixed target experiment The experimental concept was based on the idea that the project had the technology necessary to phys- ically trace the evolution of packets of seeded ice crys- tals, from origin, through growth, to fallout at the sur- face. The effects of seeding could thus be demonstrated and the technology developed by measuring the evo- lution of seeded cloud elements. Obtaining statistically significant results from this experiment was not possible because of the limited time available to conduct field experiments and the wide natural variability of the pa- rameters expected to be sensitive to seeding. a. Physical structure of the candidate clouds Shallow orographic clouds formed by forced ascent of moist air over the Sierra Nevada were selected for the experiment. Natural precipitation rates vary from a trace to as much as 3-4 mm h -1. Various synoptic weather patterns are conducive to generating these clouds (Heggli and Reynolds 1985; Heggli and Rauber 1988; Reynolds and Kuciauskas 1988). The clouds typically exhibit neutral stability and have been ob- served to exist for 12 to over 72 hours. Cloud top tem- peratures range from -80 to -150C and are rarely be- low -200e. Depending on the strength of the stable layer existing at cloud top, a barrier-parallel wind max- imum can occur in the lowest 3 km above ground from the valley up to the crest (Parish 1982). Widespread, fairly continuous SL W has been ob- served in these cloud types by both a cloud physics aircraft and a dual-channel microwave radiometer lo- cated at either Blue Canyon (1983) or Kingvale (KGV) (1984, 1985, 1986). The cloud droplet spectra can be either continental or maritime (Marwitz 1987). Su- percooled droplets as large as 200-300 ~m in diameter have been observed. Precipitation is produced by either diffusional growth of ice crystals followed by accretion and / or aggregation for continental cloud droplet dis- tributions, or coalescence growth followed by droplet freezing and perhaps ice multiplication for maritime cloud droplet distributions. There is evidence suggest- ing that a Hallett-Mossop ice multiplication mecha- nism can develop, especially for maritime clouds with mean droplet diameters >20 ~m (Mossop 1985). Al- though turbulence is weak, the spreading of artificially nucleated crystals is enhanced by differential growth