The Earth's Electrical Environment (1986)

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CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 126 A fascinating aspect of electrification in the rain stage is the reported occurrence of lightning for clouds warmer than 0°C (e.g., see Moore et al., 1960). The evidence is incomplete because this phenomenon has not been verified by in situ measurements of cloud temperature. Such lightning is apparently restricted to the tropics and probably occurs only in clouds that are deeper than discussed above. For warm clouds of about 5 km depth average charges are typified by region R 2 shown on Figure 9.6 (Takahashi, 1978). These charges were measured at the ground with associated fields of less than 1 kV/m. Drift charging, selective ion capture, and breakup charging can probably account for such high charges providing the fields in or around the cloud reach about 1 kV/m. (Fields as large as 3 kV/m were measured from an aircraft in the vicinity of warm cloud lightning by Moore et al., 1960.) There is also the possibility that induction charging could contribute to the field intensification within the cloud, but the enhancement of drop coalescence in fields of 30 kV/m suggests that lightning cannot be achieved by induction alone. We should keep in mind, however, that induction charging has been investigated for only a narrow range of drop sizes and that lightning in deep (warm) convection has not been studied in detail. Therefore, additional research on charge separation mechanisms is required to understand the strong electrification that occurs in deep warm clouds. Hail Stage There are three aspects of charging in the hail stage that must be explained: (1) the observed region of negative charge, (2) the buildup of fields, and (3) the average values of charge. Regions of negative charge lie generally between the – 10 and – 25°C level even in winter thunderstorms, as illustrated by the location of lightning sources shown on Figure 9.8 (from Krehbiel et al., 1983). The space-charge densities associated with the region of negative charge average about 1 C/km3 from estimates based on lightning currents and particle charges (Latham, 1981). The second aspect of charging in the hail stage is the development of large electric fields. The maximum fields measured in thunderstorms are consistent with estimates of the requirement for lightning initiation (about 400 kV/m). The average magnitude of charge on individual particles ( ) in the hail stage is shown on Figure 9.6 by region H1 for cloud droplets and region H2 for precipitation particles, with line h giving an approximate from Grover and Beard (1975). The charges on raindrops are in the lower portion of H2 (after Takahashi, 1937a), whereas charges on solid precipitation reside in the upper portion of H2 (Latham, 1981). In both cases the average for the negative charges usually exceeded the positive charges by a significant amount, with charges on individual particles sometimes in excess of 100 pC. As a result of the high fields found in the hail stage, breakup charging would be highly efficient (Figure 9.6, line 6), with the sign depending on the orientation of the local field. (Note that a variety of field orientations must occur within thunderstorms around charge centers, see Figure 9.8.) Drift charging would also be efficient in strong fields if ion concentrations are enhanced by corona from ice crystals (Griffiths and Latham, 1974). Thus the generally high charges found on cloud and precipitation particles are probably an indication of field-driven mechanisms that separate charge on the microscale. For the causes of high fields we will consider the Figure 9.8 Schematic diagram illustrating the levels and distribution of charge sources for ground-flash lightning observed for summertime in Florida and New Mexico and for wintertime in Japan (from Krehbiel et al., 1983). 

CHARGING MECHANISMS IN CLOUDS AND THUNDERSTORMS 127 microscale and cloud-scale mechanisms that lead to the negative-charge region observed to occur between – 10 and – 25°C. We first evaluate convection charging, which has held a controversial position among theories of thunderstorm electrification (e.g., see arguments in Mason, 1976; Moore, 1977). These theories, proposed by Grenet (see Chalmers, 1967) and Vonnegut (1955), rely on the transport by updrafts and downdrafts of space charges and screening layers. In the hail stage, with a highly electrified cloud, the charge from the positive corona at the ground is carried into the cloud base and by the updraft, to the cloud top where it attracts a negative screening layer. The downdrafts carry the negative charge back toward the cloud base to strengthen the negative field between the cloud base and the ground, thus enhancing the positive corona. The notion of negative charge in downdrafts has been reintroduced in the form of nonrandom mixing from the cloud top by Telford and Wagner (1979) to provide a qualitative explanation for a negative-charge region near the – 10°C level. This picture of descending negative charges appears to be at odds with the early stages of convective electrification modeled by Chiu and Klett (1976). They found a positive screening layer at a cloud top and positive descending charges. Convection and mixing were found to weaken the field within the cloud. Since Chiu and Klett did not expect their model outcome to change appreciably for clouds deeper than 5 km, it is difficult to envision how convective (single-cell) transport could be the source of strong electrification. However, in the hail stage with multicell convection and with corona from precipitation, the ion concentrations would differ considerably from the model of Chiu and Klett. Thus, a better model is required to evaluate the importance of convection in highly electrified clouds. Another criticism of convection charging is that updrafts and downdrafts may disorganize their associated charges through mixing (e.g., see Chalmers, 1967). The study of Chiu and Klett clearly shows that single-cell convection with eddy diffusion diminishes the electric field within the cloud. If we picture cloud turrets as convective cells (similar to Figure 9.2) with interspersed updrafts and downdrafts, then the possibility for disorganization by mixing between adjacent charge regions becomes rapidly apparent. Questions regarding the importance of mixing in cloud electrification probably will not have a satisfactory answer until we have more quantitative models of turret scale motions and entrainment. In addition, the common occurrence of a negative charge center near – 15°C suggests that transport of ions and charged particles by convection is not so important as microscale charge separation involving ice particles. In evaluating induction charging in the hail stage, we consider that the most efficient microscale interactions are collisions between wet hail and ice particles. The maximum charge according to Eq. (9.8) is about 1 pC in the limiting field of 400 KV/m for a particle of 100-µm radius and 10 pC for a 300-µm particle. Since the wet growth of hail requires sizes larger than about 10 mm diameter, the induction limit of 1 to 10 pC even for R = 10 mm is well below the charge expected from ion capture in high fields (see line 6, Figure 9.6). Thus, induction charging cannot be directly responsible for the average charges (1 to 100 pC) found on precipitation particles (0.5 to 2 mm radius) in the hail stage. Another aspect of the induction charging of wet hail is its importance to the region of negative charge near the – 15°C level. For hailstones with a maximum charge of 10 pC at a maximum concentration of 1 m–3, the resulting space- charge density is 0.01 C/km3. This estimate of the maximum charge density is several orders of magnitude smaller than a charge of about 1 C/km3 found from measurements of particles and from estimates based on lightning currents. Although the induction mechanism provides negative charge on precipitation particles and field intensification through feedback, it appears to be too weak to account for the charge densities associated with the hail stage. Its shortcomings are twofold: the maximum charge is limited by the effects of size and contact angle given in Eq. (9.8), and the charge density is limited by the instrinisically low concentration of hailstones. The microscale separation mechanism in the negative-charge region is probably associated with smaller precipitation particles (e.g., soft hail), because their higher concentration could lead more easily to a sufficient charge density. For example, particles at a concentration of 100 m– 3 carrying 10 pC would result in a more realistic 1 C/km3. The charging of soft hail has been simulated in the laboratory by collisions between ice particles and an ice electrode in the process of riming (Gaskill and Illingworth, 1980; Jayaratne et al., 1983). Although our understanding of the separation mechanism is incomplete, the evidence points to interface charging from contact potentials with freezing potentials having a secondary role. Investigations of temperature effects in the above studies have ruled out thermoelectric charging. The formula applicable to these results scales with contact area and is the same as line r on Figure 9.6 when evaluated for an ice particle of r = 60 µm. Thus charge transfer for individual collisions between soft hail and small ice particles is around 0.01 pC. Since contact time is relatively short compared with the time required to