OCR Text |
Show with continuous ice nucleus counters, were used to track released silver tl iodide plumes, either from the ground-based generators or from pyrotechnics . aboard an aircraft. Also, extensive field calibration of the ground-based gen-;: erators was made. These tests showed that both the ground generators and 1 the airborne pyrotechnic devices reliably release adequate numbers of AgI particles for treating a cloud volume of the dimensions of the Park Range tar- '.~ get area. The question of whether or not the proper volume of precipitating •· cloud was treated in any particular case is thus a problem of diffusion rather ;1 than of Agl particle generation. ,i Due to known temperature inversion characteristics in a local mountain-vallei 1 circulation regime, plus the general complexity of diffusion processes in ' ,w mountainous terrain, there are some obvious advantages in making airborne trl as compared to ground releases of seeding material. For instance, a temperature inversion is frequently present from the valley floor to slightly above i the ground generator location on either Emerald Mountain or Mt. Harris. Available data indicates the presence of this condition during at least half of the winter precipitation hours over the Park Range. This inversion totally traps any seeding material released below it, thus preventing treatment of the precipitating cloud above by ground based releases. Under this regime, airborne delivery is necessary to obtain cloud treatment. 1 On the other hand, airborne seeding is markedly more expensive. Both airborne and ground releases are subject to targeting problems under conditions of rapidly changing wind regimes. However, the airborne droppable pyrotechnics distribute seeding agent vertically over a 7000 to 9000 ft. depth, and a number of these can easily be released along any desired seeding track. Thus, a specified target area is more certain to be seeded when using droppable pyrotechnics, as compared to releasing from a single ground-based generator. There is evidence from the special support cloud physics data (in. the form of silver concentration in precipitation samples) that both ground-based and air- · borne short period ( 1 hour) releases 'of Agl can affect the in.t ended target area for as much as 6 hours and frequently for 3 to 4 hours. When economy, simplicity of operation, and delivery reliability are all considered tegether, it is c oncluded that a reasonable operational project design should include both ground and airborne delivery systems, with the ground delivery system to be used in the absence of trapping stable layers and with . feasible targeting winds. Airborne releases should be made during unfavorable ground-based targeting wind regimes, trapping stable layers, and when ground· released material would not likely reach properly cold cloud temperatures. 6. 4 Modeling Results Versus Observations A numerical model of orographic flow and precipitation was developed and 332 |