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<br />. <br /> <br />. <br /> <br />CHAPTER I <br /> <br />. <br /> <br />INTRODUCTION <br /> <br />. <br /> <br />Precipitation processes play complex roles in clouds. Latent heats <br /> <br /> <br />released during phase changes of the processes modify the thermodynamics <br /> <br /> <br />and subsequently the dynamics of clouds. These altered thermal and dynamic <br /> <br />fields in the clouds in turn affect formation processes of precipitation. The <br /> <br /> <br />feedback mechanisms betweem precipitation and other cloud processes are <br /> <br /> <br />quite complex and in order to effectively study the links, a proper <br /> <br /> <br />understanding of the microphysics of precipitation is essential. <br /> <br /> <br />The formation of rain can begin by activating cloud condensation nuclei <br /> <br /> <br />(CCN). The CCN are always sufficiently plentiful, and, when activated, they <br /> <br /> <br />grow to form cloud droplets by vapor diffusion. Then, collision with other <br /> <br /> <br />drops may help them grow into even larger drops. However, this liquid <br /> <br /> <br />phase microphysical mechanism is inadequate to explain the development of <br /> <br /> <br />rain outside the tropics. Bergeron (]935) theorized that ice crystals must be <br /> <br /> <br />present in supercooled clouds so that the precipitation particles can grow in a <br /> <br /> <br />sufficiently short period of time to the size of snow to form rain by melting <br /> <br /> <br />and fall without completely evaporating. Subsequent studies, both theoretical <br /> <br /> <br />and experimental, have supported Bergeron's hypothesis that is now widely <br /> <br />accepted in the scientific community. Thus, sound and accurate knowledge of <br /> <br /> <br />the ice phase microphysical processes is crucial in understanding the <br /> <br /> <br />dominant precipitation mechanism. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br />