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<br />CLOUD PHYSICS At'lD WEATHER MODIFICATION <br /> <br />Roland List <br /> <br />Departm.en t of Physics <br />University of Toronto <br />To,ronto, Ontario, Canada <br /> <br />1. Introduction <br /> <br />Cloud Physics is normally separated into two components: a) the dynamics of <br />clouds and b) the microphysics or the evolution of the particulate matter starting from <br />the aerosol to precipitation. The contiuum and particle aspects are, however, not <br />isolated from each other, they need. to be integrated considering the different interaction <br />and feedback processes. The relate~d aspects will be treated in the following Sections, <br />which will also create the basis for a general discussion of modification techniques by <br />which clouds and precipitation could be affected. The complexity of one single cloud <br />should be obvious from looking at a. thunderstorm with precipitation (Figure 1). How do <br />we explain it, how would we try to alter it? <br /> <br /> <br />F-i.g uft e 1. <br /> <br />ThundeM:toftm in NeblUl.J.>ka, <br />AuguJ.>:t, 1977, bftight: pfte- <br />upUation J.>:tfteaM Il.epfte- <br />.6 en:t hail. <br /> <br />2. Cloud Dynamics <br /> <br />Clouds form by condensation of <br />water vapor on aerosol particles (cloud <br />condensation nuclei or CCN). The necessary <br />slight supersaturation required for this <br />process is a result of cooling. Cooling is <br />caused by lifting of air, an expansion of <br />air at constant pressure or a combination of <br />both. The origins of the cooling process <br />can be found within the large scale motion <br />of the atmosphere including frontal systems, <br />in the locally caused thermals or plumes <br />(leading to Cu and Cu arrays) or in upslope <br />winds leading to orographically caused <br />clouds or cloud systems. <br /> <br />The equations governing the <br />dynamics of clouds are: <br /> <br />1) The Navier-Stokes equation as <br />expressed^in terms of perturbations for <br />pressure p, temperature T and density p from <br />their values in the undisturbed environment which in turn is characterized by the corres- <br />ponding values Pe' Te and Pe (List and Lozowski, 1970): <br /> <br />dv <br />dt <br /> <br />1 <br /> <br />~p <br />P e, <br /> <br />[~ - ~] ~ + d + ~, <br /> <br />Pe e <br /> <br />(1) <br /> <br />., <br /> <br />where v is the velocity, g the gravitational acceleration, d the drag of the liquid water <br />substances or their weight when falling at terminal speeds,-and f the frictional forces. <br />