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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 125 rms charge given by Eq. (9.3) is converted to a mean deviation (see lower inset on Figure 9.6 for the corresponding equation). We can expect that the charge distributions for large cloud drops would be skewed (no longer centered on zero charge) with higher averages as indicated by region C2 from the influence of the electric field by drift charging at cloud edges and selective ion charging. Note that charging under the influence of the electric field is depicted on Figure 9.6 by the lines labeled 1 through 6. The corresponding values of electric field for three mechanisms are found in the upper inset. For example, line 2 shows the charge magnitude for drift charging at cloud base (or top) in a field of 10 V/m and selective ion charging in a field of 60 V/m. For a small cumulus, charge separation in the cloud stage is consistent with microscale ion capture and cloud- scale convective transport of charge. This combination can account for such features in the cloud stage as the negative core and positive edges. It can also explain a positive charge in the base of a cloud very near to the ground. Charging by ion capture appears to be limited by cosmic-ray production within the cloud (Wormell, 1953) and transport from outside, and therefore additional mechanisms are required to produce the fields and charges found in the rain stage and hail stage. Rain Stage Electrification in convective clouds of less than about 3 km deep is characterized by the drop charges for the rain stage indicated in Figure 9.6 by region R 1 (Takahashi, 1973a). The mean value ( ) is approximated by the straight line proportional to R 1.3 (i.e., line r with equation shown on lower inset from Pruppacher and Klett, 1978). Since the electric field associated with these clouds is often 10 to 100 V/m, it is apparent from Figure 9.6 that drift charging (lines 2 and 3) and also selective ion charging and breakup charging can produce charges of the observed magnitude (Q) for drizzle and raindrops (R > 100 Âµm). What is not so apparent is how cloud drops and small drizzle drops acquire their relatively high charge in the rain stage. One possibility is by evaporation of drops with higher charge. Other explanations involve selective ion capture from the effects of surface potentials (Takahashi, 1973b; Wahlin, 1977). However, the details of these mechanisms are poorly understood, and consequently their role in drop electrification remains uncertain. Additional research is needed to clarify the microscale-separation mechanisms responsible for charging cloud drops and drizzle drops. Another aspect of electrification in the rain stage is the predominant sign of charge for cloud drops, drizzle, and raindrops. We can consider charge separation for a convective cloud of about 3 km deep (after Takahashi, 1982). The trajectories of drizzle and raindrops are depicted in Figure 9.7 to indicate differences for the preferred sign of charge. The drizzle drop is in a region of lower updraft speed (dashed arrow), which results in a shorter growth time within the cloud. Negative charging occurs by the Wilson effect (for a downward-directed field) and by drift current at cloud base. In addition, drizzle may be produced by breakup of raindrops, resulting in negatively charged drizzle for the field near and below cloud base. Raindrops become electrified positively by breakup charging. At an earlier time, raindrops near cloud top may also acquire a positive charge from the capture of droplets. Although this picture of drop trajectories in Figure 9.7 is greatly simplified it does illustrate some essential differences that can occur in growth histories and in the resultant charge-separation mechanisms for cloud droplets, drizzle drops, and raindrops. Our conclusions about the predominant sign of charge, based on trajectories and separation mechanisms, are consistent with extensive observation of tropical cumulus clouds (e.g., see Takahashi, 1982). These observations show a predominance of positive droplets near cloud top and negative drizzle drops and positive raindrops within and below the cloud. Figure 9.7 Rain-stage electrification based on simplified growth histories for drizzle and raindrops (modified from Takahashi, 1982). Air currents are shown by dashed arrows and ion drift currents by heavy arrows.