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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3 <br /> <br />The tacit assumption is made that these simplifications are close <br /> <br />enough to reality that the results of the simulation will represent the <br /> <br />atmospheric situation correctly. Such assumptions are typically crude, <br /> <br />at best, and frequently not basically adequate. This is the case for <br /> <br />many aspects of ice nucleation in clouds. <br /> <br />Uncertainties about ice nucleus activity and nucleation mechanisms <br /> <br />are reflected in the way they are incorporated into computer <br /> <br />simulations. Nucleation mechanisms are usually differentiated in <br /> <br /> <br />microphysical models (e.g., Young, 1974a; Cotton et al., 1986), and <br /> <br /> <br />some regions of cloud are theoretically expected to favor some <br /> <br />mechanisms of nucleation over others. For example, Young (1974b) has <br /> <br />shown theoretically that contact-freezing nucleation would proceed <br /> <br />faster in an evaporating cloud because of enhanced thermophore tic <br /> <br />collection of aerosol and because of evaporative cooling of cloud <br /> <br />drops; however, there is little experimental evidence of this <br /> <br />preferential activity. For a particular type of ice nucleating aerosol, <br /> <br />there is as yet no theoretical basis for differentiating between its <br /> <br />ability to act by, respectively, condensation-freezing or immersion- <br /> <br />freezing modes of nucleation. This needs to be determined <br /> <br />experimentally. Likewise, the difference in nucleating activity of an <br /> <br />aerosol which can arrive in a supercooled cloud by several different <br /> <br />thermodynamic paths is largely unresolved; there exists little <br /> <br />theoretical or experimental information to infer any difference in ice <br /> <br />nucleus function. Matthews et al. (1972) theoretically predicted ice <br /> <br />nucleating aerosols would have short lifetimes within cloud droplets <br /> <br />because they would dissolve; however, this was not evident in <br /> <br />preliminary studies by DeMott et a1. (1985). <br />