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Last modified
7/28/2009 2:38:05 PM
Creation date
4/16/2008 11:07:28 AM
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Weather Modification
Title
Thailand Applied Atmospheric Research Program - Final Report - Volume II
Date
3/1/1994
Weather Modification - Doc Type
Report
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<br />2. DESIGN OF THE THAI COLD CLOUD RAINMAKING DEMONSTRATION <br />PROJECT <br /> <br />2.1 Statement of the Problem <br /> <br />The question to be addressed is whether the seeding of vigorous supercooled convective clouds <br />with an ice nucleant over a moveable area, covering nearly 2,000 square kilometers, Clm <br />produce substantial and statistically significant increases in rainfall over that area. The <br />larger question is whether this proposed methodology might be used to assist Thailand in <br />managing its water resources. <br /> <br />2.2 Principles of Cold Cloud Seeding for Rain Enhancement <br /> <br />People have long dreamed of changing the weather, particularly the precipitation from clouds. <br />Not until the middle of the 20th Century, however, with the discovery of the ice-nucleating <br />properties of dry ice (Schaefer, 1946) and AgI (silver iodide) (Vonnegut, 1947) did this dream <br />approach reality. Tested first in the cold box and later in supercooled-water clouds, dry ice <br />and AgI acted to convert all or a portion of the supercooled water to ice crystals. When tested <br />in supercooled cloud layers, these artificially-induced ice crystals grew and fell from the <br />clouds as precipitation. Before treatment, the clouds contained high concentrations of <br />supercooled water drops, which were too small to fall from the cloud. These drops remained <br />supercooled, despite temperatures below DoC, because of a deficiency in natural ice nuclei. <br />Only after seeding did precipitation occur. The enhancement of precipitation initially seemed <br />as simple as supplying supercooled clouds with artificial ice nuclei to make up for the <br />deficiency in natural ice nuclei. Reality has proved to be much more complex. <br /> <br />Braham (1985) points out that the science of cloud seeding for rain enhancement in mixed- <br />phase cloud (i.e., consisting of water and ice particles) rests on four established facts and two <br />postulates. The first fact is that water drops remain unfrozen in some clouds at <br />temperatures below DoC. The second fact is that the saturation vapor pressure over ice is <br />less than that over supercooled water at the same temperature, which allows ice particles in <br />a supercooled cloud to grow by vapor deposition while the droplets evaporate. The third fact <br />is that precipitation in many areas of the world comes from mixed phase clouds. The fourth <br />fact is that a large number of artificial ice nucleants have been discovered. Those nucleants <br />used most commonly in cloud seeding experiments are dry ice (frozen carbon dioxide) and <br />AgI. A new organic seeding agent (Pseudomonas Syringae), consisting of deactivated, <br />harmless, natural bacteria, has been tested recently in the United States (Woodley and <br />Henderson, 1989). The major advantage this agent has is that it begins to work as an ice <br />nucleant at temperatures as warm as -1 oC; AgI becomes active at temperatures below -4 oc. <br /> <br />The first postulate is that a shortage of ice nuclei limits the PE (precipitation efficiency) of <br />some supercooled clouds to as low as 20 percent for some continental thunderstorms. Some <br />supercooled clouds do not precipitate at all. Such clouds are potential targets for glaciogenic <br />seeding. <br /> <br />Attempts to increase rainfall by improving precipitation efficiency are called "static" seeding, <br />or seeding for microphysical effects. Any concomitant change in cloud dynamics is assumed <br />to be small and of no consequence with this seeding approach. Ice nucleus concentrations on <br />the order of 1 to 10 per liter are thought to be optimal. <br /> <br />4 <br />
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