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Preliminary Investigation of CCN Characteristics in Western North Dakota <br />Andrew Detwiler <br />Institute of Atmospheric Sciences <br />South Dakota School of Mines and Technology <br />Rapid City, SD <br />Darin Langerud and Tracy Depue <br />North Dakota Atmospheric Resource Board <br />Bismarck, ND <br />Introduction: <br />The cloud droplet concentration and size spectrum resulting from activation of cloud <br />condensation nuclei (CCN) in summertime convective cloud updrafts influences the <br />mode by which, and speed with which, precipitation forms. When fewer CCN are <br />present the droplet concentration nucleating and growing in the updrafts of convective <br />clouds is smaller, the droplet size spectrum is broader, and the droplet mean size is larger. <br />This leads to more rapid collision and coalescence of cloud droplets to form drizzle and <br />rain drops. When these drops are carried above the freezing level they freeze at relatively <br />small supercoolings and this leads to the development of graupel at relatively lower <br />levels in the cloud, resulting in earlier ice-phase precipitation. When higher <br />concentrations of CCN are present, the resulting cloud droplet concentration is higher, the <br />size spectrum is narrower, and the mean size is smaller. Collision and coalescence <br />operates much more slowly, rain drop formation is inhibited, and cloud droplets are <br />lofted well above the freezing level before they begin to freeze, followed by growth to <br />graupel by aggregation and riming. Precipitation formation occurs later and higher in the <br />cloud. <br />When low concentrations of active CCN are present, the resulting clouds are <br />characterized as microphysically “maritime”, while if high concentrations of active CCN <br />are present, the resulting clouds are commonly characterized as “continental”, based on <br />characteristic CCN concentrations observed over the ocean and over continents (more <br />details are discussed below). <br />Mather et al (1997) proposed that if appropriate concentrations of smaller (~1 m <br />diameter) hygroscopic nuclei capable of nucleating cloud droplets at small <br />supersaturations are introduced into convective cloud updrafts, they will readily nucleate <br />droplets. These droplets will start to grow and deplete the excess vapor over saturation <br />1 <br />