| OCR Text |
Show for variations in cloud bases and tops (elevation and temperature variations) and stability characteristics. The knowledge of stability values is necessary in order to determine whether seeding material released from the ground could diffuse vertically into the precipitating cloud (it is trapped in any existing inversions or near isothermal layers) and to qualitatively explain the variability of precipitation rates (a lapse rate ?: moist adiabatic supports convective elements and increases the short period variability). 4. 5. 1. 3 Computed Seeded Precipitation Fallout Area The first step in computing the area of seeding effect consists of obtaining a field of orographic vertical motion. This was done by using the most representative Mt. Harris vertical profile of wind, temperature, and humidity, as input to the numerical model of orographic flow and precipitation. Next, for ground or airborne Agl releases computations of the cross-wind and vertical spread of the seeding plume are necessary (as well as the downwind transport). These calculations were made using (1) the Mt. Harris wind profile, (2) the field of orographic vertical motion, and (3) vertical and horizontal (cross-wind) dispersion coefficients for a moist adiabatic (neutral) temperature lapse rate (see Section 3. 2 of this report and also Turner, 1967), A plume half-width of four standard deviations ( 4 0) from the central axis was assumed to obtain the seeded precipitation fallout patterns. 1 1 The final step in determining the area of seeding effect (seeded precipitation fallout area) consisted of calculating trajectories of seeded snow crystals forming at various selected temperatures and obtaining the intersection of these crystals with the terrain. This was accomplished by (1) determining ' the level of the selected temperatures using the appropriate Mt. Harris sounding, (2) selecting an arbitrary crystal travel time, t, (3) finding crystal fall distance vs time in Figure 84 (Nakaya and Terada, 1935), (4) adding the.1 numerically computed vertical motion to step (3), and (5) moving the crystal along with the mean wind in the layer through which the crystal comes in contact with the ground. Maximum vertical distance change for each computation was < 1000 ft. An envelope enclosing the ground interceptions of the various crystal trajectories defines the computed seeded precipitation fallout area (area of seeding effect). Timing of arrival of the se e ding effect was made simply by adding the diffusion time required to reach the selected t e mperature and the subsequent crystal travel time to fallout point. Isochrones of arrival time were then drawn on the experimental area maps, $,long with the boundaries of the area of seeding effect. 146 |
