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Show SECTION 5. 0 EVALUATION OF RESULTS AND DEFINITION OF SEEDABILITY CRITERIA :rnfo~ Numerical Model (iJt 5. 1 la! , Mt. Harris profiles of temperature, humidity, and wind were used as input to i i ~r~ the numerical model for orographic flow and precipitation for each of 44 six ~ see~ ;!eili hour operational blocks during the 1968-69 season. The two primary outputs from the model were (1) a field of orographic vertical velocity, a,nd (2) a profile of orographic precipitation. Layers with positive vertical velocity as predicted by-the model w~re compared to vertical positions of cloud layers as deduced from the Mt. Harris radiosonde. These comparisons revealed that the model usually indicated upward vertical velocity through a deeper layer than clouds actually appeared to occupy. Layers with computed downward motion were seldom indicated by the sounding to possess clouds. Also compared were the computed and observed orographic precipitation profiles. The computer runs were stratified according to cloud top temperature for both seeded and unseeded classes. Then the precipitation profiles were summed for each category and compared to the corresponding sums of observed profiles. The results are shown in Figures 177, 178, 179, and 1800 These figures indicate an unrealistic radical decrease in computed precipitation immediately downwind of the ridge crest, and a location of maximum precipitation further upwind of the crest than is observed. These discrepancies undoubtedly arise partially from the fact that the Park Range crest is broad and flat rather than approximating a half sine wave shape, as used in the model. A scatter plot of a nine-station average precipitation value for ridge stations versus the computed average precipitation for the first 10 kilometers upwind of the ridge crest was made for each of 74 three hour periods (Figure 181). Very little correlation between computed and observed precipitation values is evident. The computed vertical velocity field was also used in the computation of fallout trajectories for seeded snow crystalso The predicted times and locations of seeding effect from these computations agreed well with observed measurements of silver-in-precipitation concentrations, crystal habit, and the number of crystal replicas possessing AgI centers. In summary, the vertical velocity field computed by the model can be used to improve the accuracy of seeded snow crystal trajectory calculations. The 261 |