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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 104 case study has shown how the onset of lightning in a cell was correlated with the development of an updraft and precipitation within the cell (Lhermitte and Krehbiel, 1979). Particle velocity measurements provide only a part of the information needed to estimate the electrical currents of the storm; also needed is some knowledge of the charge distribution or amounts of charge carried by the particles. Determination of the charge distribution is in itself a central problem of electrification studies, for which there are unfortunately no radarlike instruments. The charge information must be determined from in-cloud measurements, which are necessarily limited in scope, or inferred from other information, such as that obtained from other storms or from lightning. Attempts to determine the charge structure of storms from remote measurements of the total electric field have given a qualitative picture of the storm charges but have not been successful at estimating their amounts or locations. There are several reasons for this, having to do with the facts that (a) the conductivity of the atmosphere increases exponentially with altitude and causes the upper charges of the storm to be masked or screened, (b) the overall charge distribution is complex and not uniquely defined by electric-field measurements, and (c) total electric-field measurements are strongly affected by local charges. These problems are alleviated somewhat by measuring the time rate of change of the electric field, which is related to the time rate of change of the charges, or to the storm currents. Such measurements have formed the basis of a new approach for estimating the storm currents, in which the displacement current associated with a time-varying electric field is added to other measurements of the local corona, conduction, and rain currents (Krider and Musser, 1982). The sum of these currents has been termed the Maxwell current after the British physicist who first described the significance of the displacement current. An example of displacement current measurements is shown in Figure 8.9. The displacement current density values can be integrated over the area affected by the storm to estimate the charging current of the storm; this gives results that are in reasonable agreement with the charging current values inferred from lightning data. The pattern of Maxwell current values, either at the ground or aloft, can in principle be used to locate and quantify the different currents of the storm, in much the same way that the lightning charges can be located. But this possibility has yet to be realized, in part because of the problems enumerated above for interpreting total electric-field measurements. A totally different approach for determining the storm currents would involve measuring the pattern of magnetic fields that they produce. This approach has not been feasible owing to the difficulty of measuring the weak fields and to the presence of the geomagnetic field, but such an approach may become practical in the future. LIGHTNING AND THE STORM ELECTRIFICATION The study of lightning is an important part of thunderstorm investigations. Lightning is of interest not only as a phenomenon in itself but as an indicator and significant modifier of the storm's electrification. Lightning generates, deposits, and redistributes substantial amounts of free charge within a storm, and this greatly complicates the storm's electrification. In the process, lightning may also enhance the electrification or the formation of precipitation within the storm. But little is known even about what lightning looks like inside a Figure 8.9 Contours of constant displacement current density at the ground beneath a thunderstorm on July 11, 1978 at Kennedy Space Center, Florida. Observations from two 5-minute time intervals are shown; contours are at 0.5 nA/m2 intervals. The heavy dashed contour shows the detectable radar echo at 7.5-km altitude; the x's mark the negative-charge centers of lightning discharges. The areal integral of the displacement current was about 0.4 A in each instance. (Krider and Blakeslee, 1985.)
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 105 storm, much less about its detailed behavior or about the possible effects that it may have. The study of lightning as a phenomenon in itself is the subject of the first five chapters in this volume. One question of interest here concerns how lightning is initiated. Measurements of the electric field inside storms give maximum (large-scale) values typically between 1 Ã 105 and 2 Ã 105 V/m (e.g., Winn et al., 1974, 1981). Winn et al. (1974) reported one measurement of 4 Ã 105 V/m. These values are 3 to 10 times smaller than the field strength required to break down clear air at the same altitude. Hydrometeors concentrate the field onto their surface by a factor of 3 or more, and this leads scientists to think that the breakdown is initiated at particle surfacesâby corona that somehow develops into a full-scale discharge (e.g., Loeb, 1953; Richards and Dawson, 1971; Crabb and Latham, 1974; Griffiths and Phelps, 1976). The manner in which lightning is initiated is an unanswered and intriguing question, but however this happens it is most likely to occur in a strong-field region of the storm. In-cloud measurements like that shown in Figure 8.4 indicate that the electric field is strongest on the periphery of the main negative-charge region. Lightning radiation studies indicate that discharges indeed tend to be initiated at these altitudes in a storm (Proctor, 1981, 1983). There is also some evidence that IC flashes begin at slightly higher altitude than CG flashes (Taylor, 1983). This suggests that discharges that are initiated above the negative charge region tend to become IC flashes, while those that are initiated below the negative charge tend to become CG flashes. Although highly variable, intracloud lightning generally outnumbers cloud-to-ground lightning in a storm, often by a factor of 5 or 10 to 1 or so, and it is of interest to ask why this happens. The charging process establishes the main negative and upper positive charges as the primary charges of the storm, and this may cause the electric field to be stronger above the main negative charge than below it. Also, the decrease in atmospheric pressure with altitude favors the occurrence of IC flashes, in that the critical field required for discharges to form and to propagate is smaller at higher altitudes. Finally, it may be that there are a greater number of initiation events above the main negative charge than below it. The occurrence of CG flashes is thought to be aided by the presence of the lower positive charge, which increases the electrical energy below the negative-charge region, and by the tendency (mentioned earlier) for a storm to acquire a net negative charge with time. The past 15 years have seen major advances in techniques for remotely sensing lightning inside a storm. In particular, radio-frequency radiation from the lightning may be located using one of several direction-finding or time- of-arrival techniques (Proctor, 1971, 1983; Taylor, 1978; Warwick et al., 1979; Hayenga and Warwick, 1981; Taylor et al., 1984; Richard et al., 1986). The charge centers of the lightning can be located from simultaneous measurements of the lighting electric-field change at a number of ground locations (Figure 8.3; Jacobson and Krider, 1976; Krehbiel et al., 1979). The hot lightning channels are readily detected by radar at 10-cm wavelength or longer (e.g., Holmes et al., 1980; Mazur et al., 1985), and the main channels can be reconstructed from recordings of the thunder that they produce (e.g., Teer and Few, 1974; Winn et al., 1978; Christian et al., 1980; MacGorman et al., 1981; Chapter 3, this volume). Finally, changes in the electrical forces on charged cloud particles during lightning cause low-frequency changes in the atmospheric pressure, called infrasound, which can be detected and used to estimate the charge heights (e.g., Wilson, 1920; Bohannon et al., 1977; Balachandran, 1983; Few, 1985). Figure 8.10 shows two examples of lightning data that complement the electrical observations discussed earlier. Figure 8.10(a) shows the height of the radiation sources from lightning as a function of time in a Florida storm. Although not resolved in the figure, the radiation occurred in distinct bursts from individual discharge events. Only a few radiation sources were located during each discharge, but the results give a useful picture of the overall lightning activity in the storm. Events with sources located below 7-8-km altitude were usually CG discharges; the large number of remaining events were IC discharges. Of particular interest in the figure are the sequences of increased lightning activity whose sources moved upward with time. These were associated with the electrification of new convective cells in the storm and provide another indication that the electrification is associated with vertical growth. The fact that the sequences start above 8-km altitude reflects the existence of an altitude threshold for the electrification. The discharge rate during the most intense sequence reached 37 per minute. Similar observations have been reported by Lhermitte and Krehbiel (1979), who found a discharge rate of 60 per minute in a relatively small cell of a storm. Such high discharge rates are not unusual for large storms, but their occurrence in small, individual cells of normal-sized storms is a new finding. The high-rate discharges have been shown to transfer relatively small amounts of charge (Krehbiel et al., 1948b), indicating that the high-rate sequences result from a large number of initiating events rather than from superelectrification
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 106 of the cell. High-rate sequences have been observed only in subsequent cells of already-electrified Florida storms, but it appears that they are a common feature of such storms. This suggests that initiation events are somehow enhanced in subsequent cells. It would not be surprising if high-rate sequences of small discharges are found in storms at other locations as well. For instance, Taylor et al. (1983) reported the occurrence of minor discharges in large Oklahoma storms. Figure 8.10 (a) The height of VHF radiation sources from lightning versus time in a Florida storm. The upward- moving sequences of enhanced lightning activity were associated with the electrification of new, growing cells, while the increase in the number of sources above 8 km reflects the altitude dependence of the electrification (Krehbiel et al., 1984b). (b) A vertical cross section of the radar reflections from precipitation (solid contours) and from lightning (hatched areas, dashed contours) during a 5-minute time interval in a squall line near Wallops Island, Virginia (Mazur et al., 1984). The greatest number of lightning echoes were observed at altitudes that correspond with the charge centers of intracloud and cloud-to-ground discharges (Figures 8.3 and 8.6) and may indicate the locations of the main negative and upper positive charges in the storm. Figure 8.10(b) shows observations of radar echoes from lightning during a 5-minute time interval in a squall line over the East Coast of the United States. The echo locations are superimposed on measurements of the precipitation reflectivity in the storm. The lightning echoes were detected by a UHF radar operating at 70-cm wavelength; the precipitation reflectivities were determined using a separate radar at 10-cm wavelength. The lightning echoes were located most often in strong precipitation on the leading edge of a well-developed cell at 30- to 40-km range from the radars. The largest number of echoes were observed between 5- and 8-km altitude and vertically above this from 10- up to 14-km altitude. These altitudes correspond to the heights of the positive-and negative-charge centers of lightning in other storms (e.g., Figures 8.3 and 8.6), suggesting that the echoes are strongest in the vicinity of the lightning charge centers. This is where the discharges are expected to be most highly branched. The increasing ability of researchers to sense lightning inside thunderclouds has raised questions about the extent to which lightning indicates or reflects the electrification of a storm (Vonnegut, 1983b). Once initiated, the lightning channels and charges themselves influence the continued propagation of a discharge, enabling the discharge to develop in a manner that can be unrelated to the storm charges and fields. While it is necessary to be cautious in making inferences about the electrification from lightning observations, some evidence exists that suggests that lightning can be a reasonable indicator of the storm charges. For example, it has been found that the negative charge sources of the CG lightning in the storm of Figure 8.6 coincided with those of the IC lightning that immediately preceded and followed the CG discharges, even though the two types of discharge developed in opposite vertical directions. This result, which is illustrated in Figure 8.5, suggests that the negative-charge sources for the lightning coincided with main negative charge in the storm. The question of how the lightning and storm charges are related has also been investigated by studying the