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<br />.If. <br /> <br />. <br /> <br />. <br /> <br />ABSTRACT <br /> <br />The field of weather modification and more specifically. precipitation <br />enhancement has had a history of ups and downs for the past forty years. This <br />thesis attempts to provide an overview of the work that has been done in this <br />field over this period with particuiar emphasis on the last ten years. In <br />addition, key areas where scientific knowledge is lacking are identified and <br />addressed in later chapters. The thesis attempts to link the cloud dynamics <br />of cloud development with the microphysics of precipitation development. A <br />combination of observational data analyses together with numerical modelling <br />studies were used to increase our understanding of cloud and precipitation <br />development. <br /> <br />ie <br /> <br />A sophisticated three-dimensional time-dependent numerical model that <br />incorporated all the relevant physical processes was used to conduct numerical <br />studies of winterstorms over mountainous regions and of summertime convective <br />cumulus clouds. Good agreement was found between the modelling results and <br />observational data. The modelling results also indicated the importance of <br />the interaction between the airflow and orography in determining preferred <br />areas of cloud and precipitation development. The simulation of the <br />dispersion of tracer/seeding plumes proved to be highly successful and could <br />be used in future seeding simulations. It was shown that different <br />microphysical processes might result in differences in seeding responses and <br />should be taken into account in cloud seeding programs. <br /> <br />1 . <br />i <br />t,,_, <br /> <br />, .- <br />J <br /> <br />:. <br />