Cloud Seeding Concepts

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					Basic
                                                                                     Crop duster-turned cloud seeder disperses salt powder milled
                                                                                     to 2 to 5 microns for optimal hygroscopic seeding over the
                                                                                     Galilee in northern Israel. Photo: D. Rosenfeld




Cloud Seeding Concepts
William R. Cotton – Colorado State University



C
        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
        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                                                        through the cloud relative to smaller ones
                                                is forced to lift in order to pass over
performed with the goal of increasing the                                                       and colliding and coalescing to form
                                                mountains. Updraft velocities, which
overall precipitation into a watershed.                                                         still larger droplets. Precipitation growth
                                                can be several meters per second,
Elsewhere, particularly the High Plains                                                         proceeds very rapidly once droplets exceed
                                                depend upon the speed and direction of
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
                                                Orographic clouds may be quite transitory,      of the process depends on the time
damage to crops and urban areas.
                                                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.             1,000 per cubic centimeter over remote
In the Colorado River Basin, we focus on
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         warm rain by collision and coalescence.
western reservoirs are replenished
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
in polluted air), the smaller the cloud        as illustrated at right.                 determines the Lagrangian time scale (tp) for the
                                                                                        development of precipitable particles.
droplets and the lower the efficiency
of this process. Ice crystal aggregation     The temperature window is
                                             critical: at cloud temperatures colder than
                                                                                                   The Seeding Process
occurs most readily under conditions                                                               Most cloud-seeding operations use
of high concentrations of ice crystals,      -25°C, natural ice crystal concentrations
                                                                                                   silver iodide (AgI), which has a
relatively warm air (near 0°C), and with     can be high, and seeding could produce
                                                                                                   crystalline structure similar to ice. Its
complex, dendritic ice crystal structures    too many small ice crystals, resulting in
                                                                                                   ice-nucleating ability depends on the
such that crystals can readily interlock.    an “overseeded” cloud. Alternatively,
                                                                                                   mode of generation, which typically is
                                             seeding materials are less effective in
                                                                                                   by acetone generators in which AgI is
Concentrations of ice crystals do not        nucleating crystals above -10°C.
                                                                                                   suspended in acetone. The acetone is
always correspond to the concentrations                                                            burned, producing a smoke of IN. This
of IN. Several mechanisms of ice             Timing is also important. If winds are
                                                                                                   method allows generators to be located
multiplication have been proposed that       weak, sufficient time may exist for natural
                                                                                                   on the ground where they can use natural
explain many, but not all, differences       precipitation processes to occur efficiently.
                                                                                                   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 cotton@atmos.colostate.edu.

                                                                                                         March/April 2007 • Southwest Hydrology • 17

				
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