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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 128 discharge colliding particles by conduction, an accumulation of charge can occur in multiple collisions. In this manner an increase of 3 orders of magnitude, well into region H2, may be attained with high concentrations of ice crystals in several minutes. A key result of the above experiments is a reversal in the sign of charging at temperatures of â 10 to â 25Â°C depending on liquid-water content. Riming particles acquire negative charges if colder than the reversal temperature and positive if warmer. Thus, soft hail would be charged negatively above the reversal level in the cloud and positively below this level. Jayaratne et al. (1983) postulated that descending precipitation particles should have their maximum negative charge near the reversal level and that rebounding ice crystals carrying negative charge from below would also contribute to the negative region. The fields directed toward the reversal level would intensify during the process of charge separation by differential sedimentation. There are many features of interface charging that need clarification before we can feel comfortable with the above description of charging in the hail stage. First, the roles of temperature, liquid-water content, and solutes are poorly understood. These appear to influence contact potential through changes in rime structure. (Solutes may also affect interface charging by transient freezing potentials.) Second, the details of charge transfer have not been explained, although the charge carriers are probably associated with the contact surfaces. This concept is consistent with a rapid transfer of charge that scales with contact area. Finally, our recently acquired understanding of interface charging, even though somewhat limited, cannot be adequately assessed until it is placed within the framework of a multidimensional cloud model. CONCLUSIONS The charging mechanisms in clouds and thunderstorms are varied and numerous. Some are simple and readily appreciated, whereas others are complex. Several important mechanisms are poorly understood. Feedback readily occurs through changes in ion concentration and the electric field making it difficult to identify the primary causes for electrification. However, some simplification can result by considering the charging mechanisms in three stages of cumulus cloud development: the cloud, rain, and hail stages. The charging mechanisms discussed in this chapter are summarized in Table 9.1. Each mechanism is listed with the microscale and cloud-scale separators (with TABLE 9.1 Charge Separation in Clouds and Thunderstorms Mechanism Microscale Cloud Scale Major Roles 1. Diffusion charginga Ion capture by diffusion Removes ions within cloud 2. Drift charginga , b Ion capture in drift currents Drift currents Convection Charges particles (Sedimentation) Enhances field 3. Selective ion Ion capture by polarized Sedimentation (Convection) Charges particles chargingc , d drops Enhances fields 4. Breakup charginge Collisional breakup of Sedimentation (Convection) Charges drops polarized drops 5. Induction chargingd , f Charge transfer between Sedimentation (Convection) Charges particles polarized particles Enhances field 6. Convection chargingd , Space-charge production Convection Enhances field g , h Ion capture in drift currents (Charges particles) 7. Thermoelectric chargingi Charge transfer between Sedimentation (Convection) (Charges particles) particles of differing temperatures 8. Interface chargingj , k Charge transfer between Sedimentation (Convection) Charges particles particles involving contact Enhances field potentials (freezing potentials) a Gunn (1957); b Chiu and Klett (1976); c Wilson (1929); d Chalmers, (1967); e Matthews and Mason (1964); f Elster and Geitel (1913); g Grenet (1947); h Vonnegut (1955); i Latham and Mason (1961); j Workman and Reynolds (1948); k Buser and Aufdermaur (1977).