WMOD00461

• Title: Weather Modification Assessment - Kingdom of Thailand
• Date: 9/1/1986
• Country: Thailand
• Weather Modification - Doc Type: Report

B. Precipitation Enhancement Opportunities in Thailand

1. Cloud types

The seasonal variation in cloud types that occurs in Thailand is
described in table 1-1. It can be seen that nearly all of the precipi-
tation falls from convective stor-m systems with widely varying cloud
characteristics and precipitation patterns depending on the source and
depth of the airmass in which they develop and oln how the streams of
moisture-laden air interact with the coastlines and mountains. Most of
the precipitation falls from organized convective cloud lines and bands
associated with the southwest monsoon, intertropical convergence zone,
tropical depressions and the occasional intrusion of a typhoon. Some
precipitation also falls as showers from relatively isolated airmass
convective clouds and thunderstorms. The depth of the convective clouds
is mainly determined by the strength of the southwest monsoon, the depth
of the moist layer and/or the magnitude of the convective instability of
the airmass.

The cloud-base temperature in virtually all seasons and locations in
Thailand is 20 oC. Therefore, it is likely that coalescence is an
important mechanism in precipitation development in all clouds (Johnson,
1982). On many days the cloud tops do not reach the freezing level so
the precipitation that falls develops solely through the condensation-
coalescence mechanism, or the so-called warm rain process. On other
days, the cloud tops extend well above the freezing level and ice pro-
cesses play an important part in precipitation developmento In these
clouds, condensation-coalescence is followed by freezing and growth of
graupel which melt and fall to the ground as rain, sometimes referred
to as the mixed phase precipitation process. Both cloud types represent
opportunities for precipitation enhancement but by different seeding
strategies involving different seeding agents. Cloud seeding operations
in Thailand to date employ a combination of chemicals in a three-step
seeding strategy developed in Thailand which is applied to both cloud
types alike.

2. Technical feasibility

In the absence of specific data on the dynamical and microphysical pro-
perties of clouds in Thailand, estimates of the probable results of
seeding them can, at this writing, only be obtained by extension from
the results of precipitation enhancement projects in other areas of the
world involving similar cloud types. Previous sleeding operations in
Thailand and other nearby countries using their unique seeding strategy
provide very limited experience for making these estimates since they
were narrow in scientific scope, short in duration and only crude eva-
luations were possible. Thus it should be recognized that the analogs
to be examined represent seeding strategies not yet attempted in
Thailand.

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The closest analogs to the seeding of cloud types involving the warm
, rain process, which have been subjected to a scientific evaluation, are
several projects that have been conducted in India and the United

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