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<br />Extensive cloud seeding operational projects using hygroscopic particle seeding have and are <br />continuing to be carried out in Thailand, Malaysia, Indonesia, and the Philippines [see <br />RRRDI (1988) for a complete discussion of these projects]. Organizations carrying out these <br />projects are encouraged by their results; however, these projects have not received the <br />statistical and physical evaluation that is required to make the results acceptable to the <br />scientific community. <br /> <br />The general consensus of the scientific community (WMO, 1981b) is that no scientific proof <br />exists of the efficacy of procedures to modify warm clouds so as to increase or redistribute <br />precipitation; however, the results of past warm cloud seeding projects are encouraging, and <br />more study and resource commitment to warm cloud modification is warranted. <br /> <br />3.4 Summary of Warm Cloud Seeding Model Experiments <br /> <br />3.4.1 Introduction <br /> <br />A numerical cloud model that is capable of simulating cloud and precipitation development <br />under natural and seeded conditions was used in the search for a viable warm cloud seeding <br />strategy. The numerical cloud model provided an inexpensive means of screening numerous <br />candidate warm cloud seeding strategies to identify those that are most promising <br />scientifically (WMO, 1981a). The one-and-one-half-dimensional, time dependent, detailed <br />microphysical model originally developed by Silverman and Glass (1973), as extended by <br />Nelson (1979) to include ice processes, and further updated by Silverman to include ice <br />multiplication processes and a variety of hygroscopic chemical seeding options (HAMOD), was <br />used to test the relative effectiveness of a wide range of seeding scenarioso <br /> <br />Ofthe most scientifically promising seeding strategies found in these model experiments, the <br />one that best meets the logistical and economic feasibility requirements is recommended for <br />incorporation in the design of the warm cloud seeding demonstration project. A complete <br />description of the model, the model experiments, and the experimental results is given in <br />appendix D. <br /> <br />3.4.2 Results to date <br /> <br />The key results of the numerical model experiments are summarized <br />as follows: <br /> <br />1. The CCN spectrum. upon which warm and cold clouds develop in the project area has <br />a strong influence on the production of natural rainfall. As the CCN spectra chan~~e <br />from one primarily maritime in character (low number concentration and relatively <br />high numbers in large nuclei sizes) to one that is more continental in character (high <br />number concentration and very few in large nuclei sizes), the amount of natural rain <br />produced decreases markedly. The decrease is greater in warm clouds, which depend <br />solely on the condensation-coalescence process for precipitation development, than in <br />cold clouds, which include precipitation development through ice processes as well. <br />The burning of rice fields and forest areas in the project area produces a smoke <br />containing nuclei which, from evidence in other projects, suggests that the natural <br />CCN spectrum is probably being made more continental in character. <br /> <br />24 <br />