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13 I I I I I I I I I I I I I I I I I I I temperatures colder than. -20 oC would contain enough ice crystals to use effectively any supercooled liquid water that might be present. Since the effectiveness of artificial ice nucleants increases markedly as the temperature falls below the threshold of activity around -5 oC, it was initially anticipated that most seeding operations would involve supercooled, stable orographic clouds with cloud top temperatures in the -5 to -20 oC range. Evaluation of the precipitation events during the first field season (1984-85) showed that they were usually associated with synoptic scale storms and that the cloud types changed as the storm fronts moved across the target area. Bands of convective clouds were observed frequently. As a part of Al Ghait, the University of North Dakota (UND) Citation II research aircraft was operated in Morocco in 1985. Preliminary examination of the cloud physics data collected by the UNO aircraft indicated that most of the clouds sampled were convective, The typical supercooled liquid water concentrations were between 0.3 and 0.5 g/m3, but concentrations up to 3 g/m3 were observed in small regions. Many of the convective clouds that were sampled developed frozen precipitation particles. However, at least two of the water-rich clouds that were studied intensively failed. to develop appreciable amounts of ice at -12 oC. Some of the supercooled clouds produced small ice particles, but not solid precipitation, at the sampling level. These preliminary observations indicated seed ability for some clouds over the target area (Grainger and Stith, 1987). 2.3.3 Criteria for selection. - A distinction was made between orographic, mainly stratiform clouds and semi-isolated convective clouds in establishing criteria for clouds to be seeded (Hartzell et aI., 1986). The criteria adopted that year, based on cloud top temperature (CIT) and cloud thickness, were: · For stable orographic or layer clouds with only weak embedded convection: -5 oC > CIT > -20 oC and cloud thickness over 1 kilometer. . . For convective clouds: -10oC > CIT > -25 OC and cloud thickness over 2 kilometers. The reasons for establishing different criteria were as follows. Although AgI particles are not very active as ice nuclei at temperatures around -5 oc and diffusional growth is slow, seeding clouds with top temperatures near -5 oC is practical if the clouds last long enough for precipitation to develop. This might be the case for orographic clouds more than 1 kilometer thick, but not for convective clouds, whose lifetimes are often as short as 10 or 15 minutes. Likewise, convective clouds less than 2 kilometers thick probably would not last long enough to develop precipitation. Stable layer clouds (possibly with weak embedded convection) with top temperatures colder than -20 oC and convective clouds with top temperatures colder than ..25 oC usually contain enough natural ice to use effectively the available liquid water (Grant and Elliott, 1974; Gagin, 1981). 2.3.4 Seeding ag~nt. - The team of American scientists who made the. 1983 assessment for the Kingdom of Morocco suggested that because AgI was the most tested and proven agent available for seeding winter storms, it should be used exclusively for the demonstration project. A provision in the Project Grant Agreement made GOM responsible for the purchase of the seeding agent; they agreed to use AgI exclusively for Programme Al Ghait.