Laserfiche WebLink
<br />Little infonnation is published concerning direct observations of the T &D of seeding agents from low <br />level (valley) generators such as used in the Utah operational program. The limited evidence that exists <br />is mostly indirect; for example, the analysis reported by Long (1984) indicated most target area snow <br />samples, analyzed for trace levels of silver (Ag), had only background levels present. This suggests that <br />targeting of the AgI seeding agent was frequently unsuccessful. However, direct measurements of the AgI <br />plume, or a simulated seeding plume using a tracer gas, would provide more conclusive infonnation. <br /> <br />Preliminary examination of the early 1991 data from the Wasatch Plateau, when valley-released plumes <br />were tracked by aircraft and truck, has yielded the following indications. During some stonns, or at least <br />some stonn stages, valley-released AgI was trapped into a very shallow layer by a surface-based <br />temperature inversion. High concentrations of ice nuclei were observed near the valley floor, but transport <br />to plateau-top levels was essentially eliminated by the stable aunosphere and calm-to-light winds. <br />Sometimes downslope drainage winds were observed near the valley floor during stonn periods which <br />would carry seeding material away from the intended target. <br /> <br />During a series of storm passages during the first half of March 1991, valley-released AgI often was <br />detec~ on top of the Wasatch Plateau, and several times at aircraft sampling levels 700 to 800 m above <br />the Plateau. These were relatively wann stonn sequences, generally with embedded convection present. <br />The vertical convective motions produced abundant SL W, and apparently also transported AgI from the <br />valley generators to well above the plateau. Concentrations of AgI, measured by continuous cloud <br />chambers operated near -20 oe, frequently exceeded several hundred IN (ice nuclei) per liter. However, <br />cloud chamber calibrations of the type of generator used indicate about two .orders of magnitude less <br />effective IN at -10 oe and another order of magnitude less IN at -8 oe. Thus, 500 per liter IN measured <br />by the cloud chambers represents only about 5 per liter at -to oe and 0.5 per liter at -8 oe. (It should <br />be noted that the instruments used may only be semiquantitative so these values should be considered <br />initial estimates subject to possible revision after further analysis and comparison with tracer gas <br />measurements.) <br /> <br />Temperatures at 9,000 ft m.s.l. on the plateau were generally between +2 and -5 oe when SLW was <br />present during the early March stonns. The sampling aircraft often found AgI up to 12,000 ft but not <br />much higher. With typical lapse rates of 2 oe per 1,000 ft, temperatures at 12,000 ft would be expected <br />to range between -4 and -13 oe for the March stonns sampled. If the cloud chamber IN measurements <br />approximate actual cloud response to the AgI, cloud levels wanner than about -to oe likely were not <br />effectively seeded most of the time. Thus, the current impression is that convective periods are very <br />effective in vertically transporting valley-released AgI just at the times and places that abundant SL W is <br />being produced. However, it is not yet clear whether current seeding rates and generator spacings are <br />routinely adequate for production of enough effective IN at the temperature levels reached. <br /> <br />A tracer gas was released in the mouths of two different canyons during early 1991 aircraft missions over <br />the Wasatch Plateau. The initial impression is that this simulated seeding was successful somewhat more <br />frequently than valley-floor seeding, but certainly less frequently than the high altitude seeding. The <br />canyon mouth seeding resulted in relatively wide crosswind plume spreading, presumably because of flow <br />up several small canyons that branch off the main canyons. That may offer a distinct advantage in treating <br />a large cloud volume with a single generator. <br /> <br />Direct observations of the T &D of AgI from high altitude generators has been published from some sites <br />similar to Utah mountains, For example, Super and Heimbach (1988) presented observations of AgI <br />plumes and resulting ice particles from a seeding site well up the windward slope of the Bridger Range <br />of Montana. All six aircraft missions documented that the AgI was transported over the target area. The <br /> <br />18 <br />