WSPC06829 - Laserfiche WebLink

Home Browse Search

UiHllb9 which makes ground-based seeding feasible from relatively low elevation sites. Significant but less amounts of liquid water are usually present during the neutral stage of the storm and is also seedable. Airborne seeding is more effective in producing ice particles in this case. The other stages of the storm have less seedabi1ity because they contain high ice crystal concentrations and low liquid water contents. Local orographic clouds form over the mountain barrier with the onset of moist flow, grow to their maximum size, and then diminish. Super- cooled water zones are confined to a narrow region directly above the slopes and crests of the barrier and are associated with the zone of mountain-induced updrafts. Individual ice crystals are frequently horizontally stratified; that is, their rates of fall are in approxi- mate balance with the general upward motion approaching the main barrier. Modification potential is greatest in orographic cloud systems with warm cloud tops where ice crystal concentrations are low and the natural precipitation process is relatively inefficient. Clouds with high cold tops, on the other hand, are generally more efficient and naturally produce heavy precipitation. In order to be confident that cloud seeding is responsible for increases in snowfall from winter orographic clouds, several links in a chain of physical events must be documented. First, the cloud must have super- cooled water, or at least ice supersaturation, in excess of that which can be converted into snow by naturally occurring ice particles. Such portions of the orographic cloud are considered to be Iseedab1e." Second, seeding material must be successfully and reliably produced. Third, the seeding material must be transported to the seedab1e region in a timely manner, and must have dispersed sufficiently upon reaching this region, so that a significant cloud volume is affected by the desired concentration range of ice nuclei or the resulting ice crystals. Fourth, in the case of silver iodide seeding, the temperature must be cold enough for substantial nucleation to occur. Fifth, once ice crystals form, they must remain in an environment suitable for growth long enough to enable fallout to occur, generally prior to their being carried beyond the mountain barrier where downslope motion, cloud evaporation, and ice crystal sublimation normally exist. It is not necessary that each 1 ink be monitored throughout each storm. However, sufficient physical evidence should be obtained concerning each link so that a prudent observer would have reason to expect similar performance in other similar storm situations. Therefore, the program design will provide the means to obtain and evaluate such physical evidence. While such physical evidence is necessary, it is essential to demon- strate, with reasonable certainty, that the seeding program is actually increasing precipitation over the intended target area. This requires precipitation measurements throughout the target area, and a statisti- cal analysis of those measurements due to the large natural variability which occurs with precipitation amounts. 2. Subbasin design adaptation. - Prior to operation in those subbasins chosen as the demonstration sltes, seeding agent and delivery systems 28