| OCR Text |
Show graphi_c areas us_ing syn_optic scale data. The criteria also provide guides to select10n of feasible altitudes of ground generator sites and/ or to the choice of m?st feasible modes of seeding (ground, aircraft, etc. ) when planning a seedmg program. From a seeding evaluation viewpoint, seedability criteria must delineate: (1) targeting limits (wind and stability criteria for the seeding diffusion envelope). (2) the quantity of water becoming available for potential precipitation per unit time. (3) the percentage efficiency of the natural precipitation process. (4) the conditions to which the seeding agent is exposed in its travel to the activation temperature region. To set targeting limits (criterion 1), the geometry of the target area, the relative location of generators, and the local three-dimensional diffusion characteristics must be considered. For the Park Range target area, nominal wind direction limits of 240° and 310° were imposed on the Mt. Harris 11,000 ft. MSL wind vector, which was obtained frequently during seeding operations. The known persistence of valley temperature inversions extending to near the seeding site elevations made it necessary to obtain data on the depth of this inversion. This was done primarily by frequent traverses up and down Rabbit Ears Pass with a truck-mounted recording thermometer. Indirect knowledge of the presence or absence of an inversion between the Emerald Mountain generator altitude and the target altitude was obtained by correlation of Emerald Mountain 100 foot level winds with Mt. Harris 10, 000 ft. MSL winds, and also with ice-nuclei counts observed in the target area. At the Mt. Harris generator site, direct knowledge of the existence of a trapping inversion was obtained during the 1967-68 season by modifying the 1680 mHz radiosondes so that they sensed only temperature up to approximately 9, 500 ft. MSL. The amount of water becoming available for potential precipitation per unit time (criterion 2) is primarily dependent upon the temperature of the saturated air column, the vertical velocity within the column, and the air mass transport speed (horizontal) across the target area. As the air column mean temperature decreases by lOC, the water becoming available decreases by approximately 50%. Thus, it would be desirable to seed at the warmest possible temperature. However, AgI does not become highly active as a seeding agent until exposed to temperatures of -1 OC and colder, and the strongest positive seeding effect should be observed if the seeding agent is dispersed through the maximum crystal growth-rate temperature region near -15C. 315 |
