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<br /> <br />development via the ice process can proceed via either the coalescence-freezing mechanism <br />(Braham, 1986), or ice crystal growth by vapor cWf'usion followed by riming into graupel <br />(mG) mechanism (Silverman, 1986). Cloud base temperature and buoyancy are usually <br />good indicators as to whether precipitation will develop via the coalescence-freezing <br />mechanism or the IRG mechanism (SilvemJ.an, 1986; Mather et al.,1986). The cloud <br />droplet concentration and size spectrum can also vary dramatically depending on the <br />CCN concentration and size spectrum. <br /> <br />I. <br /> <br /> <br />Knowledge about the evolution of the different precipitation formation processes and <br />how they interact is extremely important in order to determine the optimum treatment for <br />each situation. Some major gaps in our knowledge regarding these processes has become <br />evident during the last few years. The development of precipitation via the coalescence <br />process and the subsequent onset of the ice process has received a lot of attention lately, <br />in particular to explain the development of high ice concentrations recently measured in <br />clouds (Rangno and Hobbs, 1991). High supersaturations (in the order of 5 to 10%) in <br />clouds due to the onset of coalescence given as the reason for high ice concentration by <br />Rangno and Hobbs is still a much debated issue. According to calculation. by Clark(1973) <br />and Young(1974c) a rapid decrease in the number of cloud droplets occurs with the onset <br />of coalescence. The sudden decrease in cloud droplet concentration. subsequently leads to <br />the increase in cloud supersaturations according to their calculations. Rangno and Hobbs <br />(1991) argues that these high supersaturations can in turn lead to the enhancement of'ice <br />crystal concentrations by nucleating more ice crystals. There is however, still doubts about <br />these high supersaturations and there presently exist no instrument that can accurately <br />measure it in cloudy enviromnents. The argument i. that these high supersaturations are <br />unrealistic and that in .reality new cloud condensation nuclei would get activated much <br />quicker to provide the balance for the droplets swept out by coalescence. Further research <br />is needed in this area. <br /> <br />! ' <br /> <br />,... <br />j <br />I <br />1, <br /> <br />f' <br />i <br /> <br />: ., <br /> <br /> <br />Another important distinction that haa to be made i. the temperature levels at which <br />the ice crystals nucleate. In clouds with cold bases and a narrow cloud droplet spectra <br />ice tends to develop between the -9 to -120C levels (Krauss et ale 1987, Bruintjes et al., <br /> <br />15 <br /> <br />.' <br /> <br />.. <br /> <br />. <br />