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<br />The seeding hypothesis of the Utah operational program raises three <br /> <br />key questions. <br /> <br />They concern the spatial and temporal distributions of <br /> <br />supercooled liquid water in the clouds which may be seeded in the program, <br /> <br />the delivery of seeding materials to these clouds, and the trajectories of <br /> <br />cloud and precipitation particles. The first two questions are treated in <br /> <br />the present report. The third question on trajectories is not treated in <br /> <br />order that adequate resources may be applied to the first two questions. <br /> <br />a. <br /> <br />Cloud and precipitation characteristics. <br /> <br />The clouds over the <br /> <br />Tushar Mtns are usefully classified in terms of precjf2itation echo types <br />~ - . .~~ r, ttlf. ..-~~ <br /> <br />(PETs) '. The amount of supercooled liquid water and its distribution in <br />~'r- <br /> <br />the clouds depend on the PET. <br /> <br />The "area-wide" or A PET originates in a synoptic disturbance. Echo <br />~. ~~... ---~ ~ ---- .....~ <br /> <br />tops of A PETs are the highest (median height ~ 4780 m AGL) and coldest <br /> <br />(median temperature ~ -28C). Average and maximum Ku-band radar reflec- <br /> <br />tivities are the greatest. <br /> <br />Maximum reflectivities are located near cloud <br /> <br />base. A PETs are relatively similar to each other in terms of height and <br /> <br />reflectivity. They are consistent producers of precipitation particles. <br /> <br />The "cell" or C PET is believed to originate in convection. The C PET <br /> <br />has the second highest (~ 3740 m AGL) and coldest (~ -21C) tops. Average <br /> <br />and maximum Ku-band radar reflectivities are moderate. Maximum reflec- <br /> <br />tivities are located over a wide range of heights. C PETs are relatively <br /> <br />dissimilar one from the other in terms of height and reflectivity. They <br /> <br />are variable producers of precipitation particles. <br /> <br />The "no echo-precipit~tionll .o,~ NP ':~T _ is a shallow echo lying over ~~e <br /> <br />Tushar Mtns. The NP PET is unique to the mountains and is largely respon- <br /> <br />sible for the 44% greater frequency of echoes observed there than in <br /> <br />xxi i <br />