WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

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The existence of SLW in excess of that converted naturally to ice particles and snowfall is a
necessary but not sufficient condition for seeding to have potential. (Diffusional ice crystal
growth can occur when the atmosphere is saturated with respect to ice, but the presence of
some SLW will be considered necessary for significant seeding potential.) Abundant SLW is
formed in the lowest several hundred meters above windward slopes when moist air is forced to
ascend mountain barriers. This process produces liquid water droplets at rates determined by
the temperature, humidity, and upward motion ofthe air; these tiny droplets tend to evaporate
rapidly in the descending air in the lee of the mountain barriers. In addition, the forced uplift
will sometimes release latent instability in the atmosphere, resulting in (gen€~rally weak)
convection with additional SLW production.

Much of the SLW produced by forced ascent over mountain barriers is not naturalJly converted
to ice particles during portions of some winter storms. The SLW can exist at temperatures well
below 0 oC because of the scarcity of ice-forming nuclei capable of converting the supercooled
cloud droplets to ice crystals. Ice nuclei are highly temperature dependent and the
concentration of effective ice nuclei increases by about a factor of 10 with a 4 oC decrease in
cloud temperature.

Once ice particles are formed in SLW cloud, they initially grow at the expEmse of the
surrounding supercooled droplets because of the difference in saturation vapor pressure over
liquid and ice (diffusional growth). Later growth may be accelerated by accretion, ifmany cloud
droplets collide with and freeze on falling ice particles, and/or by aggregation, :if many ice
crystals chain together into large snowflakes. Larger snowflakes and snow pellets formed by
accretion are more likely to reach the swface as snow because of their higher terminal fall
velocities.

Natural ice particle production is often too low for efficient conversion of the SLW to snowfall.
In some inefficient clouds, ground-based seeding from high altitude AgI generators or propane
dispensers can create a significant increase in the ice particle concentration in the lowest
several hundred meters above the windward slopes; that is, within the primary SLW zone. The
SLW and ground-based seeding zones are shown conceptually on figure 1. Cloud above the
primary SL W zone consists mostly of ice crystals with perhaps limited liquid in some portions.
Ice crystals initiated by seeding will grow as the crystals are transported along quasi-horizontal
trajectories through the SLW zone. Growth will be especially rapid when SLW is abundant
across the entire barrier, but will continue when at least ice saturation exists. Cloud
temperature also influences ice particle growth rates (Redder and Fukuta, 1989).

When the environment is suitable for adequate growth of some ice particles to permit their
fallout to the target swface, the seeding will cause additional snowfall. Depending upon the
growth environment and three-dimensional wind field, additional snowfall may occur for some
distance beyond the mountain barriers, although evidence is weak for far downwind effects
(e.g., Grant et al., 1992) On a long-term basis, the great majority of seeded snowfall is expected
to settle on the higher elevations of the target barrier.

Conditions which favor seeded crystal growth and fallout include limited natural ice crystals,
abundant and nearly continuous SLW from the generators to the target, and moderate winds.
Exceptionally strong winds may produce abundant SLW, but transport time across the barrier
may be too short for many snowflakes to settle to the swface before being carried to the lee
evaporation/subsidence zone.

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