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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 107 behavior of electrical discharges in plexiglass (Williams et al., 1985). These laboratory discharges appear to simulate the large-scale behavior of lightning in clouds. High-energy electrons are deposited in a controlled manner within the plexiglass and the resulting space charge is discharged by mechanical disruption at some point on the surface. The dendritic or finely branched structure of the discharges follows the pattern of space charge within the plexiglass, suggesting that real lightning may do the same in storms. Assuming that lightning tells us something about the electrification, one question of interest has been how the lightning channels and charges are related to precipitation in the storm, as revealed by radar. If a precipitation mechanism were operating to electrify the storm, one would expect the lightning and precipitation to be correlated in some manner. Not surprisingly in phenomena as complex as thunderstorms and lightning (and as complicated to study), a wide variety of observations have been found. These range from observations that lightning and precipitation are correlated (e.g., Larsen and Stansbury, 1974; Krehbiel et al., 1979; Taylor et al., 1983; Figures 8.3 and 8.10b), to observations that lighting avoids regions of strong precipitation (MacGorman, 1978; Williams, 1985), to other observations of precipitation echoes that develop after nearby lightning (Moore et al., 1964; Szymanski et al., 1980). There is some uncertainty and debate as to what the various observations mean. In the author's opinion, one of the most striking results has been the degree to which the lightning charges correlate with radar echoes from precipitation. LIGHTNING EVOLUTION We finish this review with a brief and simple description of how lightning appears to evolve with time in a thunderstorm. This is depicted in Figure 8.11 and provides a framework for understanding some of the wide variety of lightning observations. In addition it gives further insight into the electrical nature of storms. The description is based on a number of different studies and observations of lightning (e.g., Ligda, 1956; Teer and Few, 1974; Krehbiel et al., 1979, 1984a; Krehbiel, 1981; Proctor, 1981, 1983; Rust et al., 1981; Fuquay, 1982; Taylor, 1983). In response to the dipolar structure of the storm, the initial lightning discharges are usually intracloud flashes that transport charge vertically between the main negative-and upper positive-charge regions (Figures 8.11a and 8.3). The first cloud-to-ground discharge usually follows an initial sequence of intracloud flashes (Figures 8.11b and 8.6), but simple CG flashes sometimes begin the lightning activity. The latter situation occurs presumably because conditions somehow favor the initiation of CG discharges. CG flashes consist of a number of discrete strokes down the channel to ground; the early CG flashes are simple in that they produce one or only a few strokes. The initial lightning activity is associated with the cell having the greatest vertical development in the storm. Other cells do not generate lightning until they develop vertically above 7-8-km altitude MSL (in summertime), even though the subsequent cells may have stronger precipitation echoes within them than within the initial, lightning- producing cell. As additional cells become electrified, the IC flashes remain basically vertical but become broader in horizontal extent and can exhibit a pattern of cross-discharging between cells (Figure 8.11c). The CG flashes produce a larger number of discrete strokes whose negative-charge sources progress horizontally through the precipitating part of the storm (Figure 8.11d). For some still-unknown reason, the CG flashes can initiate a continuing current or arc-type discharge down the channel to ground from within the horizontally extensive negative charge. The continuing currents can last for a few tenths of a second and produce a persistent luminosity that is sometimes detectable visually. In large storm complexes having a number of cells, the intracloud and cloud-to-ground discharges can have large horizontal extents, corresponding to the horizontal dimensions of the storm system. Because the horizontal dimensions can be much greater than a storm's vertical dimension, the discharges become primarily horizontal in nature. As the storm grows, its top reaches the base of the stratosphere or is sheared off by high-level winds to form an anvil cloud (Figure 8.1). The anvil cloud is composed of small ice crystals that carry part of the upper positive charge and is penetrated by intracloud discharges from the active region of the storm (Figure 8.11e). The anvil clouds commonly extend tens or hundreds of kilometers downwind from the parent storm. Cloud-to-ground discharges have also been observed to emanate from anvil clouds, well away from the active region of the storm. As older cells dissipate, predominantly horizontal intracloud lightning occurs between negative charge in still- active cells and apparent positive charge at about the same level in the dissipating part of the storm (Figure 8.11f). In propagating storms the dissipating part trails the active part and can have substantial horizontal extent. The horizontal discharges within them are correspondingly extensive and are observed to propagate over distances of 50 to 100 km (Ligda, 1956; Proctor, 1983). The radar echo from within the dissipating part
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS Figure 8.11 The apparent evolution of lightning with time in a thunderstorm, based on a variety of observations in different storms. See text for explanation. The dendritic structure of the lightning has been guessed in all cases except for the multicellular intracloud discharge of part c. The dotted region in the dissipating part of the storm in parts e and f represents the radar brightband from melting snowflakes. 108