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<br />5. NUMERICAL MODELING RESULTS <br /> <br />. <br />. <br /> <br />5.1 Background Information <br /> <br />Numerical modeling provided significant insight into the physical processes involved during winter <br />orographic storms over the Plateau. The model used in these investigations was the sophisticated, three- <br />dimensional, time dependent numerical model developed by T. Clark and associates at the National Center <br />for Atmospheric Research. <br /> <br />Modeling results should be treated with caution until it is demonstrated that they are in reasonable <br />agreement with observations. However, observations are limited in time and space and are impractical to <br />make in some very important locations. Therefore, model results can "fill in" where observations do not <br />exist, provided model results and observations are in good agreement where both exist. <br /> <br />The Clark model was applied to the Mogollon Rim of Arizona, as discussed by Bruintjes et al. (1995). <br />They showed the model was quite successful in reproducing observed plume dispersion. The importance <br />of gravity wave dynamics in the transport and dispersion of seeding material was demonstrated by their <br />work. <br /> <br />". <br /> <br />5.2 <br /> <br />Model Applications to the Wasatch Plateau <br /> <br />Heimbach and Hall (1994) discuss the Clark model and its application to the Plateau. They compared <br />model results with a well-observed case from the early 1991 field season which involved seeding with <br />valley AgI generators. Reasonable agreement was found with AgI plume positioning. Considerable <br />pooling of AgI occurred within the valley, but a shallow layer was eventually transported over the Plateau. <br />The importance of gravity waves for the vertical transport of seeding agent was demonstrated in , <br />agreement with the results of Bruintjes et al. (1995). Gravity waves were also found to be influential in <br />the production of liquid water and its subsequent downwind depletion in zones of subsidence. The <br />horizontal and vertical position of the seeding release point was critical in determining whether the model- <br />simulated seeding agent was transported over the Plateau for particular conditions. <br /> <br />Modeling results also suggested that seeding from the San Pitch Mountains, the next barrier west <br />(upwind) of the Plateau, might provide broader plumes, earlier nucleation, and opportunity for greater <br />vertical transport. These factors might increase seeding effectiveness if embryonic seeded crystals that <br />formed over the San Pitch Mountains were transported into the SL W condensate zone over the Plateau's <br />west slope, where further growth and fallout could occur. A similar approach was apparently successful <br />during the Bridger Range Experiment (Super and Heimbach 1983), although the valley between the <br />barriers was narrower. The approach of seeding on the windward slopes of one barrier to affect another <br />farther downwind should receive further consideration in view oflimited(growth times found over the <br />Plateau (e.g, Huggins 1995). <br /> <br />Reasonable model agreement was found in the case of a high altitude ground release of AgI from the early <br />1991 field season reported by Holroyd et al. (1995). This experiment produced marked IPC enhancement <br />on and above the Plateau top and apparently limited accumulations of snowfall. The heights to which <br />model-simulated AgI plumes reached wef€~ in good agreement with aircraft measurements. The model's <br />smoothed terrain failed to simulate some of the small-scale but important effects of major canyons which <br />funnel the airflow. The model produced weak and shallow clouds which were driven orographically with <br /> <br />21 <br /> <br />