WMOD00176

• Project Name: Annual Report
• Title: Seeding Operations & Atmospheric Research, 2005 Annual Report
• Prepared By: Duncan Axisia
• Date: 12/31/2005
• Weather Modification - Doc Type: Report

condensation nuclei (CCN) suppress
precipitation in the warm layer due to the large
concentration of small droplets and will
precipitate more slowly than a similar cloud
ingesting clean air, which forms small
concentrations of larger droplets that coalesce
faster into raindrops (Rosenfeld, 2001).

To improve precipitation efficiency, a
mechanism has to be present to disrupt the
microphysical stability and lead to larger cloud
particles, which, in turn, have greater fall
velocities and can fall out as precipitation
(ASCE, 2004). The next section will discuss
the precipitation augmentation concepts of
cold-cloud seeding and warm cloud seeding for
the purposes of increasing precipitation
efficiency.

Precipitation augmentation concepts

Static seeding concept

The static mode of cloud seeding is based on
the purely microphysical concept that clouds
are deficient in ice nuclei and therefore
additions of silver iodide crystals that mimic the
structure of ice should result in a more efficient
precipitation producing cloud system. Cotton
and Pielke (1995) suggest that seeding using
this hypothesis is limited to:

1. clouds which are relatively cold-based and
continental;
2. clouds having top temperatures in the
iCinge -10 to -25 DC;
3. a time scale limited by the availability of
significant supercooled water before
depletion by entrainment and natural
precipitation processes.

This hypothesis has been tested and
scrutinized during the last decade in
experiments with mixed results. Although
there are constant indications that seeding can
increase precipitation, a number of recent
studies have questioned many of the positive
results, weakening the scientific credibility of
some of these experiments. As a result, there
is some uncertainty as to the methodology of
such a hypothesis.

Dynamic seeding concept

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The concept of dynamic seeding is a physically
plausible approach that offers an opportunity to
increase rainfall by much larger amounts than
the static concept. This concept is to seed
supercooled clouds with large enough
quantities of ice nuclei to cause glaciation of
the cloud. Due to seeding, supercooled liquid
water is converted into ice particles, releasing
latent heat, increasing buoyancy, and thereby
invigorating cloud updrafts. In favorable
conditions, this will cause the cloud to grow
larger, process more water vapor, and yield
more precipitation (Bruintjes, 1999). The
enhanced updraft may also promote the
initiation of convection in the surroundings.

Cloud modeling studies have shown that the
formation of precipitation in a cloud will
normally affect the dynamics of the cloud,
which in tum feeds back into the microphysics.
This suggests that cloud seeding for
microphysical effects cannot be achieved
without some dynamic effect. Hence, the term
"static seeding mode" is somewhat misleading,

Hygroscopic seeding concept

In the case of the warm rainfall process
(collision-coalescence mechanism),
opportunities exist for providing large
hygroscopic nuclei to promote initial droplet
growth. One other opportunity is to use much
larger hygroscopic salt particles to form
raindrop embryos directly. Therefore, the term
"hygroscopic seeding" has been associated
with two seeding methods. One method
applies hundreds of kilograms of salt particles
(dry sizes of 1 0 to 30 microns) near cloud base
to produce drizzle-sized drops almost
immediately. The other method uses salt flares
to disperse 1 micron or smaller sized particles
into cloud updrafts. The objective is to enhance
rainfall by altering the size distribution of CCN
in the updraft creating a more maritime type
cloud, enhancing the coalescence process,
forming rainfall in the seeded volume and
eventually spreading throughout the cloud.
Cooper et al. (1997) identifies these
precipitation mechanisms and states that
hygroscopic seeding might have a beneficial

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