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<br />./ <br /> <br />3. DESIGN OF THE THAI WARM CLOUD RAINMAKING DEMONSTRATION <br />PROJECT <br /> <br />3.1 Statement of the Problem <br /> <br />The scientific question to be addressed is whether the seeding of warm convective clouds with <br />hygroscopic particles over a target area of900 square kilometers can produce substantial and <br />statistically significant increases in rainfall over that area. An additional scientific question <br />that must be addressed is whether this increase in rainfall over the target area, if any, is in <br />fact an increase in rainfall or a redistribution of the rainfall that would have occurred in the <br />absence of seeding. The larger socioeconomic question is whether the proposed warm cloud <br />seeding methodology can and should be used to assist Thailand in managing its water <br />resources. <br /> <br />3.2 Principles of Warm Cloud Seeding for Rain Enhancement <br /> <br />The evolution of rain in warm convective clouds and the amount of rain falling to the ground <br />involves a number of complex, interacting microphysical and dynamic mechanisms. The <br />process starts with the condensation of water vapor on CCN (cloud condensation nuclei) to <br />form the cloud droplet spectrum. Growth of the cloud droplets continues by condensation and <br />stochastic coalescence between pairs of small droplets to form precipitation embryos <br />(autoconversion). More rapid growth then occurs through the collision-coalescence process <br />(accretion) to form rain drops which are capable of falling against the updraft. Break-up of <br />rain drops may occur as a result of a collision with another drop or because of aerodynamic <br />instability of the rain drop. Some evaporation of the rain drops occurs because of <br />entrainment of dry air in the cloud and as they fall into the drier sub cloud layer. It is <br />important to note that the time required for the rain evolution processes to operate takes <br />place in a cloud whose lifetime is governed by thermodynamic and dynamic processes such <br />as entrainment, water loading, and buoyancy. <br /> <br />The efficiency ofthese rain evolution mechanisms depends on many factors, including the size <br />and number of CCN, the cloud base temperature, the cloud updraft velocity, the size of the <br />cloud, and, in general, the time available for these mechanisms to act. The principles of <br />warm cloud seeding, principally hygroscopic particle seeding, for rain enhancement are rootl~d <br />in these efficiency factors. There are four main approaches to increasing precipitation from <br />warm convective clouds through hygroscopic particle seeding: <br /> <br />1. <br /> <br />Seeding with very small hygroscopic particles, about 0.1 to 1.0 micrometer in radius, <br />in the sub cloud layer to modify the CCN upon which the cloud droplet spectrum first <br />forms, by making it more maritime in character and thereby improving the inherent <br />efficiency of the condensation and autoconversion processes. <br /> <br />1 <br /> <br />2. Seeding the cloud with small hygroscopic particles, about 5 to 10 micrometers in <br />radius, to accelerate the natural auto conversion process of the cloud. <br /> <br />3. Seeding the cloud with large hygroscopic particles, about 50 to 100 micrometers in <br />radius, to bypass the auto conversion process and initiate the collision-coalescenee <br />process earlier in the life of the cloud. <br /> <br />22 <br />