WMOD00571

• Title: Static Mode Seeding of Convective Clouds - A Review
• Date: 5/23/1984
• Weather Modification - Doc Type: Report

vely rapid dispersion of growing ice crystals is required. It is not
uncommon to fi nd peak ice crysta.l concentrations on the order of 1000 1-1
or more following dry ice seeding (Cooper and Lawson, 1984; Hobbs and
Politovich, 1980; Kochtubajda and Rogers, 1984) which reduce in time as
dispersion processes exert their influence. HIPLEX-1 studies (Lawson,
1978; Bureau of Reclamation, 1979; Rodi, 1981) showed that the ice crystals
from dry ice seeding spread through small cumulus congestus in about 5 min.
Silver iodide seeding, on the other hand, produces ice crystals over a
period of time starting with its introduction into the cloud so a time-
varying combination of silver iodide aerosols and ice crystals are involved
in the dispersion process. However, the peak concentration of ice crystals
that droppable silver iodide flares produce initially are also quite high,
reaching values in excess of 1000 1-1 in some cases (Marwitz and Stewart,
1981; Sax et al., 1979). Provided that the required seeding concentration
can be maintained, the timed release feature of silver iodide may be bene-
fi ci al (Marwitz and Stewart, 1981; Engl i sh and Marwitz, 1981) si nce it can
have an effect on clouds where the liquid water continues to be reple-
nished. Otherwise, additional doses of seeding material are required under
these conditions as is the case with dry ice. Experimental evidence
(Jiusto and Holroyd, 1970; Weickmann, 1974) indicates that dry ice seeded
regions spread 2 to 3 times faster than similar volumes seeded with silver
iodide. This was presumably due to the enhanced convection and turbulence
in the cloud resulti ng from the qreater and more rapid heat rel eased by the
induced phase change. Dry ice sl~eding which has a faster nucleation rate
and likely produced a higher concentration of ice crystals at the seeding
temperature levels than the silver iodide.

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