WMOD00311

• Title: Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
• Date: 4/26/2003
• Weather Modification - Doc Type: Report

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assessments of "seedability" and cloud seeding
potential, experimental design considerations such as
the location, amount, and time of seeding, and studies
leading to greater understanding of cloud seeding
effects.

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II. SEEDING AGENTS AND TECHNIQUES
I. ICE-PHASE SEEDING

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Glaciogenic, or ice-phase, cloud seeding is the
most common form applied, and has been the subject of
nearly continuous study for over 50 years. It has been
used in both precipitation enhancement and hail
suppression applications. Silver iodide is by far the
most widely-used seeding agent in this category,
although dry ice has also been quite popular. A few
other chemical agents have fallen in and out of favor
over the years. The basic premise is that natural clouds
are normally deficient in ice particles at relatively low
supercoolings. Ice-phase cloud seeding aims to increase
the temperature at which ice is initiated, and increase
the concentration of ice particles in the relatively warm
region from OOC to -150 or -20oC where much 'of the
growth of precipitation-sized particles occurs.

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The degree of sophistication employed in cloud
seeding simulations has evolved over time. Early efforts
were relatively simplistic, being based upon the
temperature at which cloud liquid would transform to
cloud ice, or a temperature range over which this
transformation would occur. This is frequently referred
to as a first generation seeding treatment, and is typified
by Chang and Orville (1972) and Kopp and Orville
(1972).

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-40

...35

.- -30
(,)
2...
-25
l-20
a:
;:) -15
~
a:
~-IO
:l5
~ -5
o

I Cl..OUO WATER.
RAIN

The next level of improvement in the realism of
seeding simulations was to base the transformation of
liquid to ice on ice nuclei activation functions as in
Orville and Kopp (1974). This second generation
seediilg technique was based on a more solid physical
foundation, that being the direct response of the
processes of ice crystal nucleation and drop freezing to
modified ice nuclei activity spectra, Subsequent ice
crystal interactions including contact freezing of
raindrops, aggregation of ice crystals and the growth of
ice crystals to precipitation-sized particles via the
Bergeron process perpetuate the seeding effects.

Figure 1 provides a crude depiction of the different
particle regimes for first generation seeding techniques.
The regimes resulting for second generation techniques
are similar, but somewhat less arbitrary. It should be
noted that second-generation techniques are still quite
popular although most current applications also apply a
time and/or space window to the effects.

A significant improvement in realism is attained
when the seeding agent is treated as a separate field
variable in the simulations, as in Hsie et al. (1980) and
Kopp et at. (1983). This allows for realistic
representation of the introduction of the seeding
material, its subsequent transport within the flow field
and depletion as it is consumed. The conservation
equation for the seeding agent in third generation
seeding treatments contain appropriate source and sink
terms which vary depending on the type of seeding
agent being simulated.

]I[ CLOUD ICE,
'GRAUPEL
t t t
(vt-R-1r-B-4E)
:nb b.ooo WATEA.
GRAUPEL
(INClUDING
FROZEN RAIN)
-1C7C
Ila CLOUD WATER,
RAIN.GRAUPEL
I CLOUD WATER,
RAIN

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II CLOUD ~TER,
RAIN.
GRAUPEL

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(0 )
NATURAL

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m ClOUD ICE.
GRAUPEL

(b)
UGHT ICE-
PHASE SEEDING

(c)
HEAlY ICE-
PHASE SEEDING

Fig. 1. Moisture regimes in the model cloud~ (a) natural, (b) light ice-phase seeded; and (c)
heavy ice-phase seeded

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