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<br /> <br />the particles being considered. The detailed microphysical approach allows one to follow <br />the evolution of the different particle size distributions. While the Drst method is relatively <br />inexpensive in computer. needs the second can be very expensive depending on the amount <br />of detail that is employed. The ability to realisticaijy simulate the microphysics using <br />either method is primarily limited by our current understanding of the relevant processes. <br />In rR.;prtll enhancement programs the primary questions are how much precipitation falls <br />to the ground and what are the changes in cloud microstructure expected due to cloud <br />seeding. Regarding this question, not only the microphysical processes in the cloud are <br />important but also the interaction of these processes with the mesoscale and larger scale <br />environment and the effect on precipitation intensity and distribution. It is also not yet <br />clear how detailed the microphysical calculations will have to be to provide an acceptable <br />answer to the above question and if bulk water or 'hybrid' methods will or can achieve the <br />same results. <br /> <br />[" <br />i <br /> <br />Methods used to simulate cloud seeding effects have also changed substantially over <br />the past 20 years. Orville et ale (1991) provides an overview or the methods. Early <br />models assumed that all the liquid water was frozen when a threshold temperature was <br />reached. Subsequent models provided for a gradual changeover from water to ice between <br />two temperature regimes through various microphysical interactions. A more realistic <br />method which has been used mainly in the past ten years is to simulate the release of the <br />seeding material and follow the material through the cloud domain. Ice is subsequently <br />initiated only in cloudy regions where the seeding material is present. This is however <br />done in a parameterized manner. A better method would be to include the equations <br />which simulate the release and dispersion or the seeding material in a physically realistic <br />manner. PrpJh";T1&ry tests using this method and comparing it to observed plumes or SFe <br />releases will be discussed in a later chapter. <br /> <br /> <br />Most or the modelling studies in two or three spatial dimensions to date have <br />concentrated on the simulations of seeding e:fFecti in convective clouds (Raie et al., 1980; <br />Orville and Chen, 1982, Orville and Kopp, 1990; Fritsch and Chappel, 1981; Koenig and <br />Murra.y, 1976; Levy and Cotton, 1984 amongst others). An overview or these modelling <br /> <br />19 <br /> <br />. c <br /> <br />. <br /> <br />. <br /> <br />. <br />