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<br />1 '. <br />I <br /> <br />tUee-dimensional structure. Rangno (1986) notes that the cloud variability encountered in <br />several mountainous areas in the United States poses a severe challenge to the forecasting <br />of seeding opportunities and determining a treatment strategy especially when seeding <br />opportunities are short-lived. This statement was confirmed by Super and Holroyd (1989) <br />for the Arizona case where they specifically noted that although CLW was present during <br />all storm. systems, it was highly variable in time. Unless this variability is better understood <br />it will be very difficult to forecast the timing and duration of CLW periods. <br /> <br /> <br />The temporal and spatial variability of CLW also poses a severe problem in <br />targeting the regions of CLW with seeding material. This was especially highlighted <br />in seeding experiments over the Sierra Nevada (Deshler et al., 1990) where in only two <br />of 36 experiments the complete chain of events from seeding to precipitation could be <br />documented. According to the authors the failures can be ascribed to difficult technical <br />and logistic problems, and to the variability of even simple cloud systems. This was <br />particularly evident in the spatial and temporal distributions of CLW and in the natural <br />lluctuations in ice crystal concentrations. Huggins and Sassen (1990) also were unable <br />to document the physical chain of events from seeding to precipitation at the surface in <br />seeding experiments in the Tushar mountains in Utah. Once again insufficient knowledge <br />about the transport and dispersion of the seeding material was quoted as one of the primary <br />reasons for failure. In the Utah experiment as well as in most other experiments seeding <br />generators were located in :fixed positions upwind from the target while the measurement <br />facilities were concentrated in a single location downwind from the seeding generators. <br />These locations were chosen assuming a mean wind direction and assuming that no changes <br />in wind direction occur between the ,seeding generator and the target position. One would <br />expect that with :fixed seeding and target locations, seeding effects would only be detected <br />when the Bow is parallel to a line connecting the seeding generator and the target. This <br />approach also assumes that CLW regions will always be present in the same location and <br />that sufficient amounts are present for seeding material to interact with, and to produce <br />precipitation in the target area. The major dd.ciency in past experiments was that no <br />or very little information was available regarding the detailed windflow patterns over the <br /> <br />, <br />....... <br /> <br />7 <br /> <br />. <br /> <br />. <br /> <br />. <br />