WMOD00311

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

possible effects and the relative degrees to which they
are realized results in a highly non-linear system which
is difficult to predict. The net result on precipitation or
hail formation can be either beneficial or not. Figure 4
illustrates the various outcomes that may result.

2. HYGROSCOPIC SEEDING

Hygroscopic seeding involving salt or other
hygroscopic aerosols has been in and out of favor over
the last 40 years; it is currently undergoing a period of
renewed interest. The current rebirth is largely due to
work conducted in South Africa under the leadership of
the late Graeme Mather (Mather et al., 1997; Cooper et
at. 1997). The premise of hygroscopic seeding is that
the introduction of hygroscopic particles in the region

Ample
IOSIIOW
flaJccs

Fig. 4. Various outcomes of ice-phase cloud seeding.

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below cloud base modifies the cloud droplet
distribution and increases the efficiency of the warm
rain process.

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Because hygroscopic seeding is directed at the
waml rain process, most numerical simulations of its
effects have involved the application of detailed
microphysical techniques using a large number of size
categories to represent the liquid water, and possibly
CCN, spectra. The IAS modeling group was involved in
some of the early work in this area as described in
Farley and Chen (1975). During the current rebirth of
interest in hygroscopic seeding, the use of detailed
microphysical modeling has been continued and
expanded to include ice particles. Zev Levin's group in
Israel has been very active in this area.

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Modifleadonof IN
AcdVauCin Temperalure

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Otau~

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(Jsc lip allot
SUpctlXlQledW, II. cr
in Small Particles

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and Freeze
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Wet and
Pl'Y,
Oto\\llh

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