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<br />~,. <br /> <br />.'." <br /> <br />:e <br /> <br />f' <br />, <br /> <br />. <br /> <br />J .. <br />1 <br />I <br /> <br />! <br />i <br />I <br />t..__ <br /> <br />" <br />, <br /> <br />:. <br /> <br />[/..;u <br /> <br />1.4 Cloud Modelling <br /> <br />Understanding of the complex interactions between the mesoscale and cloud envi- <br />ronment and between microphysical and dynamical processes in summertime convective <br />clouds and in winterstorms in mountainous regions can be substantially increased with <br />the help of sophisticated numerical models, especially when supported by comprehensive <br />field measurements. To simulate the airfiow and subsequent cloud and precipitation <br />development in a realistic manner requires time-dependent models with two dimensions in <br />space (Orville et al., 1991). However, to study the initiation, organization and complete <br />life cycle of convective and orographic clouds over complex terrain requires three spatial <br />dimensions (Rasmussen et al., 1988 and 1989). This will be evident in the South African <br />cases and for the development of clouds over complex terrain in Arizona which will be <br />described in following chapters. <br /> <br />This sophistication'is necessary to obtain a better und~anding' of the many <br />important interactions that can occur in a cloud, interactions which are often times non- <br />linear and sel! compensating. Orville et al.(1991) notes that this is perhaps why so many <br />final results of cloud seeding can be hypothesized and yet are so hard to test. In addition, <br />processes that initiate cloud and precipitation development are difficult to formulate and <br />simulate. Very little has as yet been done in three-dimensional modelling of cloud seeding <br />effects primarily due to a lack of computer resources. However, the situation has improved <br />in the past several years with computer technology more available to run sophisticated time <br />dependant numerical models in three space dimensions which both address the dynamics <br />and microphysics of cloud and precipitation development. <br /> <br />One of two methods are usually used to model the microphysics of precipitation <br />processes. They are the bulk water method or a more detailed method. The bulk <br />water method parameterizes the processes by assuming a size distribution function for <br /> <br />18 <br />