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MODELS OF THE DEVELOPMENT OF THE ELECTRICAL STRUCTURE OF CLOUDS 144 ure 10.4 (c)]. However, the observations of Williams and Lhermitte (1983) pointed out that the Musser and Krider results can also be explained by the convective charge transport. Their observations showed that falling precipitation may not be the only cause for the electrification of thunderstorms. All the models agree that the inductive process requires higher precipitation rates in order to operate effectively. Some models show that the most effective method to produce strong fields is to let both inductive and noninductive mechanisms operate simultaneously. While noninductive mechanisms can be powerful, particularly early in the development of the electric field, it is difficult to see how one can ignore the inductive process altogether. This process should operate in general whenever an ambient electric field is present. In some cases, it may discharge the particles, while in others it will charge them, but it should always operate. If, on the other hand, its effectiveness is very low, as reported by Illingworth and Caranti (1984), it will not be felt in the cloud. Thus, if a charge greater than that predicted by Eq. (10.1) is found on some of the particles (Christian et al., 1980), the inductive process should have discharged them. Since such charges were observed, it must be concluded that in these cases the inductive process did not effectively operate. Most investigators seem to feel that charge separation through interactions among water drops only is not effective since most collisions result in coalescence, thus limiting the possibilities for charge separation. Nevertheless, laboratory experiments (Levin and Machnes, 1977; Beard et al., 1979) suggest that the coalescence efficiency is far from being understood, so the role of water-drop interactions should not yet be ignored completely. Laboratory measurements of the surface potentials of ice under various growth conditions (Buser and Aufdermaur, 1977; Caranti and Illingworth, 1980) reveal the complexity of the charge-transfer problem. Again, additional experiments are needed to resolve the dependence of charge separation by this process on temperature and on the strength of an external electric field. In spite of the fact that the numerical models thus far rule out convective electrification as an effective mechanism for producing strong fields by itself, it must be emphasized that these models are only quasi-static and contain parameterized dynamics. To simulate this mechanism effectively, more detailed cloud dynamics, ion convection and conduction, and precipitation processes must be included. Thus far, no such model has been developed. Such a detailed model is urgently needed, especially following the recent experiments by Vonnegut et al. (1984) that reversed the polarity of a thundercloud by emitting negative ions from a long cable electrified to 100 kV and suspended below the cloud. Their observations suggest that the negative ions penetrated the cloud, ascended to the cloud top, and attracted positive ions from the free atmosphere above and were carried down by the air currents to the cloud baseâthus reversing the previous polarity of the cloud. If the ion concentration was too small to produce this effect, it is still possible that the additional ions changed the initial conditions of the cloud electrification, which led to the reversal in the cloud polarity. With the newly available data and faster computers we can look forward to a new generation of models incorporating cloud microphysics and dynamics together with the convection and precipitation electrification mechanisms. References Al-Saed, S. M., and C. P. R. Saunders (1976). Electric charge transfer between colliding water drops, J. Geophys. 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