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<br />, <br />!' <br /> <br />2. REVIEW OF THEORY <br /> <br />2.1 Precipitating Ice Crystals <br /> <br />In the cold-cloud precipitation process, ice crystals initially form when <br /> <br />condensed water freezes on or contacts ice nuclei or when water vapor <br /> <br />forms ice directly on ice nuclei by vapor deposition. These ice crystals <br /> <br />then grow by vapor diffusion as the supercooled cloud droplets lose their <br /> <br />mass due to the vapor pressure difference between the ice crystals and <br /> <br />the cloud droplets. As the crystals become larger and begin to fall, <br /> <br />growth by accretion becomes important. <br /> <br />Cloud-base temperature and cloud thickness strongly influence the size of <br /> <br />crystals and the amount of riming. Relatively warm cloud-base temperatures <br /> <br />produce higher liquid water contents which together with a large cloud <br /> <br />thickness increases the degree of riming. A large cloud thickness also <br /> <br />produces favorable conditions for large crystals both from diffusional <br /> <br />and accretional growth. <br /> <br />Cloud-top temperature primarily determines the concentration~f ice <br /> <br />crystals which precipitate from a cloud. Because the number of ice nuclei <br /> <br />which produce ice crystals in a cloud normally increases exponentionally <br /> <br />with colder temperatures, the coldest region of a cloud should overweigh <br /> <br />. any warmer portion of the cloud in the production of ice crystals. Sine <br /> <br />the coldest region in an orographic cloud is normally at cloud top, the <br /> <br />cloud-top temperature, used in conjunction with a measured ice-nucleus <br /> <br />temperature spectrum, should be the most important parameter in predic <br /> <br />the concentration of ice crystals which will precipitate from a cloud <br /> <br />-3- <br />