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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />9 <br /> <br />in Hobbs et ale (197Sb). More concise reviews are available in several <br /> <br /> <br />publications which include theoretical studies of airflow, microphysical <br /> <br /> <br />processes and trajectories of ice crystals in natural and seeded clouds <br /> <br />(Fraser et al., 1973; Hobbs et al., 1973), studies of natural cloud <br /> <br />system structure and precipitation characteristics (Hobbs, 1975a; <br /> <br />Locatelli and Hobbs, 1974; Hobbs et al., 1974), instrumentation and <br /> <br /> <br />analysis techniques (Weiss and Hobbs, 1975; Hobbs and Radke, 1975; Radke <br /> <br /> <br />and Hobbs, 1969; Turner and Radke, 1973), and evaluation of seeding <br /> <br /> <br />effects (Hobbs and Radke, 1975; Hobbs, 1975b). <br /> <br /> <br />With regard to natural cloud system structure, observations during <br /> <br />the program indicated that in prefrontal conditions, ice particles <br /> <br /> <br />dominate over water droplets at temperatures colder than -10oC. The <br /> <br /> <br />ratio of ice to water was lower in post frontal conditions. Hobbs <br /> <br /> <br />(1975a) found that the passage of an occluded or warm front was <br /> <br /> <br />accompanied by a sharp lowering of the diffusional growth layers. As a <br /> <br /> <br />consequence, partfcles reaching the ground were converted from unrimed <br /> <br />to rimed crystals. Hobbs also found that the maximum ice particle <br /> <br /> <br />concentrations in the clouds were often several orders of magnitude <br /> <br /> <br />greater than measurements of ice nuclei. Results of the study also <br /> <br /> <br />indicated that the growth of precipitation particles by riming and <br /> <br /> <br />aggregation is particularly rapid in the last kilometer of fall ~~~~bat <br /> <br /> <br />snow particles reaching the ground originate 10 to 100 kmupwind. <br /> <br /> <br />During the program, particular emphasis was placed on the competing <br /> <br /> <br />processes of riming and aggregation. Case studies presented by Hobbe <br /> <br /> <br />(1975b) showed that seeding typically resulted in a marked decrease ~n <br /> <br /> <br />riming and an increase in aggregation of particles arriving at the <br /> <br />surface. A significant amount of information was compiled conoerning <br /> <br />