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Basic Cloud Seeding Concepts Crop duster-turned cloud seeder disperses salt powder milled to 2 to 5 microns for optimal hygroscopic seeding over the Basic Galilee in northern Israel. Photo: D. Rosenfeld Cloud Seeding Concepts William R. Cotton – Colorado State University loud seeding is increasingly About Orographic Clouds How Precipitation Forms being used for both water supply The figure at far right illustrates the Warm cloud precipitation processes (above C enhancement and weather damage formation of an orographic cloud as air 0°C) involve larger-sized droplets settling reduction. In the West, cloud seeding is is forced to lift in order to pass over through the cloud relative to smaller ones performed with the goal of increasing the mountains. Updraft velocities, which and colliding and coalescing to form overall precipitation into a watershed. can be several meters per second, still larger droplets. Precipitation growth Elsewhere, particularly the High Plains depend upon the speed and direction of proceeds very rapidly once droplets exceed of the United States and Canada, it the wind and the height of the barrier. 40 microns in diameter. The efficiency is used for hail suppression to reduce of the process depends on the time damage to crops and urban areas. Orographic clouds may be quite transitory, although with steady winds, they can available for precipitation formation, the Compelling evidence suggests that last for hours. Precipitation can form liquid water content of the cloud, and the seeding supercooled orographic in the time it takes the air parcel to concentration of cloud droplets that form. clouds, those formed by air lifting over move from the upwind lateral boundary Cloud droplets form on hygroscopic mountains, can increase precipitation to the downwind boundary, typically (salt and salt-like, including ammonium on the ground and cause significant around 20 minutes. Because stable, sulfate) particles in the atmosphere increases in the snowpack. Although wintertime orographic clouds have low called cloud condensation nuclei (CCN). the amounts of precipitation increase liquid water content, usually less than CCN concentrations are generally less are under debate, a 10 percent increase 0.5 grams per kilogram, precipitation is conservatively estimated. than 100 per cubic centimeter over production requires efficient conversion oceans, and range from a few hundred to of cloud droplets to precipitation. In the Colorado River Basin, we focus on 1,000 per cubic centimeter over remote glaciogenic seeding (using ice-forming land areas, up to several thousand per The goal of seeding these clouds is to materials) of winter orographic clouds cubic centimeter in areas affected by reduce the timescale of precipitation because the strongest scientific evidence human activities. Clouds with low CCN formation so that precipitation is that seeding can increase precipitation concentrations and high liquid water optimized on the upwind side of the comes from this method. In addition, contents are most efficient at producing mountain crest. Orographic clouds offer western reservoirs are replenished warm rain by collision and coalescence. primarily from snowmelt, derived largely several advantages over cumulus clouds from snowfall from winter orographic for seeding: they are persistent and Ice phase precipitation processes (when clouds, when conditions minimize losses produce precipitation even in the absence most or all of the cloud is below 0°C) to evaporation. In contrast, rainfall from of large-scale meteorological disturbances, include vapor deposition growth of ice summer convective clouds contributes and much of the precipitation is spatially crystals, ice particles collecting cloud much less to reservoirs, as it is largely confined to high mountainous regions, droplets (riming), and collision and absorbed locally by vegetation and lost simplifying set-up of ground-based coalescence of ice crystals (aggregation). via evaporation and evapotranspiration. seeding and observational networks. Because the saturation vapor pressure 16 • March/April 2007 • Southwest Hydrology over ice is less than that over water, ice of opportunity for precipitation crystals that form on ice nuclei (IN) in a enhancement by glaciogenic water-saturated cloud of droplets are in cloud seeding is limited to: a supersaturated environment and grow • clouds that are relatively cold- efficiently by vapor deposition. Riming based and continental; involves ice particles settling through • clouds having top temperatures and colliding with cloud droplets, which in the range of -10°C to -25°C; then freeze onto the particles. Note that for a given liquid water content, the • the time available for Schematic of a stable orographic cloud showing the higher the CCN concentration (such as precipitation formation, trajectory of an air parcel through the cloud, which determines the Lagrangian time scale (t ) for the in polluted air), the smaller the cloud as illustrated at right. p development of precipitable particles. droplets and the lower the efficiency of this process. Ice crystal aggregation The temperature window is The Seeding Process critical: at cloud temperatures colder than occurs most readily under conditions Most cloud-seeding operations use -25°C, natural ice crystal concentrations of high concentrations of ice crystals, silver iodide (AgI), which has a can be high, and seeding could produce relatively warm air (near 0°C), and with crystalline structure similar to ice. Its too many small ice crystals, resulting in complex, dendritic ice crystal structures ice-nucleating ability depends on the an “overseeded” cloud. Alternatively, such that crystals can readily interlock. mode of generation, which typically is seeding materials are less effective in by acetone generators in which AgI is nucleating crystals above -10°C. Concentrations of ice crystals do not suspended in acetone. The acetone is always correspond to the concentrations Timing is also important. If winds are burned, producing a smoke of IN. This of IN. Several mechanisms of ice method allows generators to be located weak, sufficient time may exist for natural multiplication have been proposed that on the ground where they can use natural precipitation processes to occur efficiently. explain many, but not all, differences turbulence to carry IN into the cloud. between IN and ice crystal concentrations. Stronger winds may prohibit efficient natural precipitation, so seeding could Seeding with liquid propane generators is Types of Cloud Seeding speed up precipitation formation. But if also possible, relatively inexpensive, and Hygroscopic seeding is used in warm or the wind is too strong, seeded ice crystals suitable for remote computer-controlled mixed-phase clouds. Large hydroscopic will not have enough time to grow to generation. However, the generators particles (salt powders and hygroscopic precipitation before they are blown over must be located within the cloud to be flare-produced particles; see image the mountain crest and evaporate in the effective; not all supercooled clouds below) are injected into a cloud to sinking subsaturated air on the lee side. reach the surface. Moreover, placement of increase the concentration of “collector Normally National Weather Service model generators at the tops of mountains is not drops” that can grow into raindrops by forecasts and synoptic analyses of winds feasible in designated wilderness areas. collecting smaller droplets and enhancing and temperatures are used to determine if the formation of frozen raindrops and conditions are optimum for seeding clouds. Looking Ahead graupel (snow-like ice) particles. This The application of glaciogenic cloud method of seeding may also be effective seeding to orographic clouds has been in wintertime orographic clouds because shown to increase concentrations of ice it may counteract the negative influences crystals in clouds, reduce supercooled on precipitation of high concentrations liquid water content, and rapidly promote of CCN in polluted airmasses. precipitation. However, further refinement of modeling and forecasting abilities Glaciogenic seeding involves the would help optimize the cloud seeding injection of ice-producing materials process. One model currently being into a supercooled cloud to stimulate evaluated by the author is the RAMS precipitation by ice particle growth. high-resolution mesoscale model, which The objective of glaciogenic seeding is can predict wind speeds, cloud water to introduce seeding material that will contents, natural precipitation amounts, produce the optimum concentration of transport and dispersion, activation of ice crystals for precipitation formation. seeding material, and the amount of That concentration depends on precipitation enhancement by seeding. In particular features of the clouds and Scanning electron microscope image of addition, the role of background aerosol experimental hygroscopic powder milled to concentrations on precipitation formation background aerosol concentrations. the optimal size of 2 to 5 microns, used in a Recent experiments and basic physical seeding experiment conducted in parallel in will be an important area of investigation. modeling suggest that the window Israel and Texas (from D. Rosenfeld). Contact William Cotton at [email protected]. March/April 2007 • Southwest Hydrology • 17.
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