WMOD00442

• Title: A Model of Hygroscopic Seeding in Cumulus Clouds
• Date: 12/12/1978
• Weather Modification - Doc Type: Report

DECEMBER 1978

GERARD E. KLAZURA AND CLEMENT J. TODD

1761

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LEGEND LESS THAN I-mm DIAMETER
...... I.O-TO 2.5-mm DIAMETER
2.5 TO 5.0-mm DIAMETER
GREATER THAN 5-mm DIAMETER

FIG. 1. Continued

are carried about 500 m higher into the cloud and
thus coalesce faster due to the higher liquid water
contents encountered.
Figs. 1c and 1d illustrate a phenomenon that is
believed to contribute to the success of warm cloud
seeding. Here the updrafts (5.and 10 m S-I) are suffi-
ciently strong to support raindrops big enough to
undergo breakup and set off a Langmuir (1948) chain
reaction. When the drop attains the size of a raindrop
of approximately 5 mm, it breaks up into numerous
smaller drops. Many of these smaller drops would
then be carried by vertical air motions higher up into
the cloud, where they again would grow large enough
to break up, etc. In Fig. 1c the 5-40 ,urn seeds are
able to fall through the updraft as they break up,
finally reaching the cloud base 19-28 min later as

2.5-5 mm drops. However, for hygroscopic seeds
larger than 100 ,urn the breakup mechanism does not
occur since the drops reach cloud base prior to growing
to 5 mm. In the 10 m S-1 updraft case (Fig. 1d),
a large-drop accumulation wne is observed to form
2-5 km above cloud base for 10-400 .urn hygroscopic
seeds. However, the 5 .urn particle would rise high
enough in the cloud to freeze prior to reaching 5 mm
size, and therefore no breakup would occur from it.
The large drops in this accumulation zone would be
expected to fall through to the cloud base as they
migrate outward toward the periphery of the updraft
core (List and Lozowski, 1968), and/or when the up-
draft't become less vigorous in the later stages of cloud
life.
All the particles in Fig. 1e (15 m S-I) are carried