WMOD00404

• Title: Thailand Applied Atmospheric Research Program - Final Report - Volume II
• Date: 3/1/1994
• Weather Modification - Doc Type: Report

The second postulate centers on the observation that the updraft regions of convective clouds
frequently contain substantial amounts of supercooled liquid water. If this water can be
frozen at a faster rate and at warmer temperatures than would have occurred naturally, the
latent heats of fusion and deposition will be released, leading to increased cloud buoyancy
and greater cloud growth. (Note: Depositional heating does not always add to cloud
buoyancy; Orville and Hubbard, 1973) This larger cloud would then process more water
vapor, leading to increases in precipitation.

The second postulate is called "dynamic" seeding, or seeding for dynamic effects. With this
approach, concentrations of ice nuclei on the order of 100 per liter are thought optimum.
This approach is called "dynamic" because the primary result of seeding is the invigoration
of the internal circulations that sustain the cloud and the ingestion and processing of more
water substance. In theory, at least, this approach can lead to the precipitation of more
water than was present in the cloud at the time of seeding.

In reality, neither approach to seeding for rain enhancement produces only "static" or
"dynamic" effects. All cloud processes are interactive. For example, microphysical changes
(e.g., rapid glaciation) are a prerequisite for the production of dynamic effects, and dynamic
changes (e.g., increased updraft) are necessary for the production of more cloud condensate.
The cloud processes leading to precipitation are far more complex and interactive than was
believed at the time these seeding conclspts were being developed. For ease of reference, the
terms "static" and "dynamic" seeding are still used to indicate whether the primary purpose
of the seeding is to induce microphysical changes that enhance precipitation efficiency, or to
induce dynamic changes that enhance cloud size or duration, respectively.

By extrapolation from seeding experiments in climatologically similar areas and preliminary
results from cloud model and field seeding experiments on Thai clouds, it. has been concluded
that the most promising cold cloud seeding concept for Thailand is dynamic-mode seeding.

2.3 Summary of Results Relevant to Thailand

The most systematic investigation of the potential of "dynamic seeding" for rainfall
enhancement began in clouds overthe Caribbean Sea in the mid-1960s (Simpson et aI., 1967),
and continued in Florida in the series of experiments that came to be called the FACE
(Florida Area Cumulus Experiment). Although the FACE program did not provide conclusive
proof that seeding had increased areal precipitation, the estimated rainfall increases ranged
between 10 and 25 percent for the target area, covering 1.3 x 104 square kilometers and
between 20 percent and 50 percent for groups of treated convective clouds within the target
area (called the "floating target") (Woodley et aI., 1982; 1983). The FACE program also
provided strong evidence for substantial increases in rainfall from individual convective
clouds and cells. The first experiments (in 1968 and 1970) indicated that the rainfall from
individual clouds could be increased by over 100 percent (Simpson and Woodley, 1971). A
major breakthrough in the second of the two experiments (1978 to 1980) was made with the
development of a method to identify, track, and assess the properties of the treated clouds
throughout their lifetimes. Use of this technique permitted a more comprehensive analysis
of the effect of seeding on the individual convective cells. Again, the results indicated rain
increases of over 100 percent (Gagin et al.,1986).

These results for tropical clouds in Florida provided the impetus for continuation of dynamic
seeding research in Texas. The Texas research to date indicates that dynamic seeding has

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