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<br />O\)J22~ <br /> <br />The Grand Mesa also has an upwind barrier for southwest through west flow: the <br />Uncompahgre Plateau. However, the Plateau is about 80 km upwind, so targeting would be <br />expected to be difficult, especially because of the more complex terrain. The Mesa has an <br />east-west orientation and the Uncompahgre Plateau lies from southeast-tD-nDrthwest. Such far <br />upwind seeding during the direct detection phase on the Mesa is not presently recommended. <br />However, numerical model simulations should be examined to further evaluate this possibility. <br /> <br />4.4 Numerical Modeling <br /> <br />Work will commence during the CREST's first year on adaptation of an existing <br />time-dependent, three-dimensional numerical model to the specific experimental areas. This <br />model likely will be the nDnhydrDstatic anelastic numerical model developed at the NCAR <br />(National Center for Atmospheric Research) by Dr. T. Clark and associates (Clark, 1977; <br />Bruintjes et al., 1992). The sophisticated model will be used to examine key physical processes <br />in winter storms and precipitation. Processes of particular interest include the <br />three-dimensional airflow and associated T&D of ground-released seeding agents, and growth <br />and fallout of precipitation particles. The model will be used for in-depth case studies aimed at <br />improving understanding of the physics involved in natural storms, and the perturbations <br />caused by seeding. Field observations and model predictions will be continually compared to <br />test the model and indicate where improvements are needed. <br /> <br />Numerical modeling runs using the field observations will help assess which atmospheric <br />conditions are suitable for cloud seeding and which are not. This process will provide valuable <br />input to the statistical experiment design effort. <br /> <br />A simplified version of the model will be used in real-time to decide which seeding release points <br />to use during individual experiments. The availability of affordable, powerful computer <br />workstations will allow use of sophisticated models for targeting decisions. The State of Arizona <br />weather modification program successfully tested field use of the Clark model during early <br />1992. <br /> <br />,. <br /> <br />H <br /> <br />4.5 Seeding Sites and Agents <br /> <br />The proposed program will exclusively use ground-based seeding because of the relatively high <br />costs and logistic (airframe icing) and volume filling (limited dispersion) problems involved with <br />aircraft seeding of winter orographic clouds. Remote-controlled AgI generators and propane <br />dispensers will be operated at high altitude seeding sites. Silver iodide will be used when SLW <br />temperatures are cold enough for nucleation with AgI. Propane will be used during warmer <br />periods with SLW present. The effectiveness of both seeding agents as a function of cloud <br />temperature and other variables will be investigated during the direct detection phase. The <br />adequacy of T&D of seeding agents and resulting ice crystals from particular sites also will be <br />verified during the direct detection phase. <br /> <br />Recently developed AgI solutions will be tested prior to the statistical/physical phase along with <br />the AgI-NH4I-acetDne solution used in several past experiments and operational programs. As <br />discussed by Super and Heimbach (1983), the Bridger Range Experiment provided little <br />evidence that seeding was effective at crestline temperatures warmer than about -9 oC with the <br />AgI-NH4I-acetDne agent, presumably because ice nucleus activity was too low. On the other <br />hand, Finnegan and Pitter (1988) note that high altitude releases of AgI may cause rapid <br /> <br />21 <br />