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I I I I I I I I I I I I I I I I I I I This document details a plan for a program designated the CREST (Colorado River Enhanced Snowpack Test), designed to significantly improve the scientific basis for increasing winter mountain snowpack in the Col~rado River Basin. The objective of the CREST is to obtain scientific proof that properly conducted seeding of winter orographic clouds in the Colorado River Basin will beneficially and cost-effectively enhance snowpack and subsequent runoff without deleterious effects to the environment. The program is designed to produce results in the shortest time, for the least cost, and with the highest probability of success. The CREST will follow and exceed the recommendations of the AMS and WMO policy statements. Cloud seeding attempts to mimic natural snow production processes in clouds that are inefficient in producing ice crystals and snowfall. A brief description of snow forming processes follows. During winter storm passages, moist air is forced to ascend over mountain ranges by prevailing winds. This upward motion causes the air to cool. The rising and cooling of moist air often results in water vapor condensing into vast numbers of liquid water droplets that form a cloud. The rate of production of liquid water is determined by the airs temperatur4:!, humidity, and upward motion. The tiny cloud droplets have insignificant fall speeds so they are suspended in the airstream. If not converted to snowflakes while passing over the mountain range, cloud droplets quickly evaporate immediately downwind of the mountain range where downward motion and associated wanning prevail (lee subsidence zone). Cloud droplets often remain in the liquid state at temperatures lower than 0 oc (the freezing point of bulk water) because of the scarcity of effective ice forming nuclei in the atmosphere. Cloud droplets colder than 0 oc are referred to as supercooled droplets, and the clouds they form are called SLW (supercooled liquid water) clouds. When effective natural ice nuclei are present, ice crystals will form and grow at the expense of the supercooled cloud droplets, initially because of the difference in vapor pressure over water and ice. A"i ice crystals grow larger, their fall speed increases and they may collide with large numbers of SLW droplets that freeze onto them, resulting in snow pellets which fall rapidly. The collision of many small ice crystals can result in large snowflakes which also have significant fall speeds. If ice particle growth and fallout are sufficient before the wind transports the cloudy air into the lee subsidence zone, some snowflakes will settle to the mountain surface as snow. Most of the SLW in mountain-induced clouds is within a few thousand feet of the crests where water vapor content and forced uplift are greatest. Deep clouds with cold tops usually have abundant natural ice nuclei. Their resulting ice crystals can fall into the SLW zone" converting some of it into snowfall. Observations have indicated that even deep clouds can have more SLW available than is naturally converted to ice. Shallow clouds are often too warm to have many effective ice nuclei and such clouds are inefficient in snowfall production. In some situations where natural ice nuclei are too scarce to efficiently convert SLW into snow, s.eeding can improve the conversion process. The most commonly used seeding agent is silver iodide, which has a crystalline structure similar to ice. When released as very tiny particles, silver iodide is an effective ice nucleant in SLW cloud colder than about -8 oC. However, recent observations over several mountains in the West have revealed a large fraction of the low-level SLW is between 0 oc and -8 oC. Propane seeding can be effective at these warm temperatures. The expansion of the propane gas momentarily chills the cloudy air enough (below -40 OC) so vast numbers of ice crystals form. Both of these seeding agents will be used in the CREST. x~u