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<br />little buoyant contribution. Gravity waves were shown to be important for transport over the Plateau and <br />produced a secondary maximum of tracer over the eastern portions of the Plateau. ' <br /> <br />Heimbach and Hall (1996) and Heimbach et aI. (1997) modeled a well-observed case study during which <br />AgI was released from three valley generators and tracer gas was released in a major canyon mouth. In <br />spite of a surface inversion, the AgI was transported up and over the Plateau in a shallow layer, below <br />minimum aircraft sampling levels. The model results suggest the initial vertical impetus for vertical <br />transport was by the gravity wave mechanism. This was followed by orographic forcing in a more <br />organized westerly flow. The observed confinement of the AgI to a shallow layer was predicted by the <br />.modelsimulation. <br /> <br />5.3 Modeling of Generalized Weather Classes <br /> <br />Valley AgI seeding was modeled for five generalized weather classes by Heimbach et a1. (1998). A total <br />of 46 rawinsonde observations from the early 1991 and early 1994 field programs were grouped into five <br />classes according to temperature profiles. (Nineteen additional soundings did not fit within the five class <br />criteria.) In general, the modeled results were in agreement with well observed case studies selected to <br />represent each sounding. Some of the important results from the modeling include: <br /> <br />a. A frequent tendency existed for a low-level northward drift of the valley-released AgI, parallel to <br />rather than over the Plateau. <br /> <br />b. Poor targeting resulted from valley releases during the two most stable classes. Thirty-seven <br />percent of the classified soundings were in these two classes. <br /> <br />c. The best targeting was with the most unstable class, which also had the coldest temperatures, <br />. thereby resulting in greater AgI effectiveness in ice particle production. Twenty-six percent of the <br />classified soundings were in this class. <br /> <br />d. Frequent negligible Agl effectiveness resulted even when AgI was transported over the Plateau <br />because ofwann prevailing temperatures. <br /> <br />e. Strong upward motion existed over the valley under some stability and wind conditions because of <br />gravity wave transport. This mechanism can significantly aid the transport of valley-released AgI, <br />but its presence, magnitude, and location vary markedly with time. <br /> <br />f. Mechanical forcing is important for AgI transport over the Plateau. <br /> <br />g. In some conditions, there can be a westward and north westward drift of valley-released tracer in <br />spite of organized westerly flow aloft. <br /> <br />5.4 . Summary of Model Results <br /> <br />In summary, the Clark model results were in good agreement with field obserVations. Valley-released AgI <br />was often trliPped by surface-based inversions and usually drifted northward, parallel to the Plateau, rather <br />than over it. Sometimes the drift was westward or north westward, contrary to flow aloft. Very large AgI <br />concentrations were modeled (and observed) along the valley floor on several occasions after generators <br />had been operated for several hours. . <br /> <br />22 <br /> <br />j <br />