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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 94 THE MAIN NEGATIVE AND UPPER POSITIVE CHARGES: SOME OBSERVATIONAL EVIDENCE The dipolar structure of the storm interior was first inferred in England during the 1920s by Wilson (1920, 1929). Wilson observed that the electric-field change produced by intracloud lightning reversed sign with increasing distance from storms, as if the lightning were discharging an upper positive and lower negative charge. (Earlier work by Wilson on the properties of atmospheric ions led him to develop the cloud chamber for studying high-energy particles, for which he was awarded a Nobel Prize in 1927.) Subsequent field studies in England and New Mexico between about 1935 and 1955 confirmed this basic picture of the storm charges and indicated that the main negative charge resided at altitudes where the ambient temperature is less than 0Â°C (Simpson and Scrase, 1937; Workman et al., 1942; Reynolds and Neill, 1955). The observations by Simpson and Scrase in England also revealed the presence of lower positive charge below the main negative-charge region. Field studies during the past 15 yr have further confirmed and refined these early results, as discussed below. In particular, the studies have indicated that the main negative charge is found in a relatively narrow range of altitudes at temperatures that vary between about 0 and â 25Â°C. Figure 8.3 presents a modern-day equivalent of Wilson's result. It shows the centers or sources of charge for the first, intracloud lightning discharge in a small Florida storm, superimposed on a vertical cross section of the radar echo from the storm's precipitation. The charge centers were determined from simultaneous measurements of the electric- field change produced by the lightning at eight locations on the ground beneath and around the storm. The flash effectively removed negative charge from within the precipitation echo between about 6- and 7.5-km altitude and transported the charge upward in the cloud, to above the detectable precipitation. Data from a higher-power, Doppler radar observing the same storm showed that a weaker radar echo extended up to and above the upper charge centers, indicating that the lightning remained within the cloud. The air temperature outside the cloud at the level of the negative-charge centers was between â 10 and â 15Â°C. Figure 8.3 Vertical cross section of the radar echo from a small Florida thunderstorm at the time of the first lightning flash in the storm and the centers of charge transferred by the first lightning flash. The lightning was an intracloud discharge that effectively transported negative charge from within the precipitation echo centered at about 7-km altitude (â 15Â°C) to above the detectable precipitation (from Krehbiel et al., 1984b). Comparison of the lightning and radar data in three dimensions has shown that the lightning occurred in the part of the storm having the greatest vertical extent of precipitation. Additional comparison with the Doppler observations of precipitation velocities has indicated that the negative charge sources of the lightning were located adjacent to and in the updraft of the storm. The initial charge centers coincided with a localized region of stronger precipitation that was falling toward earth on one side of the updraft, and the subsequent charge centers were displaced through the updraft toward its opposite side. Figure 8.4 shows a vertical sounding of the electric field in a small New Mexico storm, obtained with a balloonborne instrument that measured the corona current from a 1-m-long vertical wire. The corona current reversed sign as the instrument ascended through the negative-charge region between 6- and 7-km altitude [above mean sea level (MSL)] and reversed sign again as it ascended through the upper positive charge, above 9-km MSL. The temperature at the altitude of the negative-charge region was between about â 5 and â 10Â°C. No lightning was produced by the storm. Soundings through lightning-producing storms also indicate a dipolar charge structure but are complicated by the large-amplitude field changes of the lightning discharges. The soundings can be made with more sophisticated instruments that sense the electric field directly and in three dimensions (e.g., Winn et al., 1981). This can be done from balloons or on aircraft, and the observations show that the fields and charges have a more detailed structure than suggested by Figure 8.4. The electric-field measurements indicate that the volume density of electric charge is on the order of 1-10 coulombs/km3 inside storms. This results in total charge amounts of a few coulombs to a few hundred coulombs
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 95 or more depending on the size and age of the storm. Greater charge densities may exist in localized regions of a storm (e.g., Winn et al., 1974). Figure 8.4 Vertical sounding of the corona current from a 1-m-long wire carried by balloon through a storm over Langmuir Laboratory in New Mexico (Byrne et al., 1983). The wire was vertically oriented, and the corona current record is indicative of the vertical component of the electric field in the storm. The above results indicate the dipolar nature of thunderstorms and illustrate two techniques for studying their charge structure. Of particular importance from these and similar types of observations are the findings (1) that net negative charge is distributed horizontally within a storm [rather than in vertical columns as had been inferred by Malan and Schonland (1951)] and (2) that the negative charge is found at similar temperature values in different sizes and types of storms. These results have been inferred from a combination of lightning and in-cloud measurements and are illustrated in Figure 8.5. The negative-charge sources of both cloud-to-ground and intracloud lightning are found to be displaced horizontally within a storm and are found to be at similar temperature levels in Florida storms and New Mexico storms. The similarity of the lightning-charge heights and temperatures is particularly significant in view of the fact that Florida storms have substantially greater depth of cloud and precipitation below the 0Â°C level, and often above it as well. New Mexico storms are drier and generally smaller than Florida storms, having cloud bases just below the 0Â°C level. The results suggest that the charging processes are the same in the two types of storm and operate at temperatures less than 0 or â 10Â°C. They also suggest that the part of a storm warmer than 0Â°C is not directly involved in the electrification. The lightning-charge observations are supported by electric-field soundings through storms, which indicate that the main negative-charge region is relatively shallow, on the order of a kilometer deep, and is laterally extensive (e.g., Winn et al., 1981; Byrne et al., 1983). The altitude of the negative-charge region from sounding observations tends to be lower than that inferred from lightning-charge observations, and there is some indication that the negative-charge region may be systematically higher in Florida storms than in New Mexico storms (Williams, 1985). The latter difference could result from the greater water content or size of Florida storms. But any such differences in electrical structure need to be substantiated by more direct observational comparisons. The main negative charge appears not only to be distributed horizontally in a storm but to remain at approximately constant altitude or temperature as the storm grows. This is indicated by the results of Figure 8.6, which shows the heights of the charge centers for the first 15 lightning discharges in the small Florida storm of Figure 8.3. As the storm grew vertically, the positive (upper) charge centers of the intracloud lightning flashes increased from 10- to 14-km altitude (â 30 to â 60Â°C), but the negative-charge centers remained at about 7-km altitude (â 15Â°C). Sequences of radar pictures like the one shown in Figure 8.3 confirm the upward growth of the storm and show that it occurred at the same rate as for the positive-charge centers of the lightning (8 m/sec). This agrees with the idea that the upper positive charge resides on small particles that are carried by the updraft into the upper part of the storm. The apparent altitude stability of the negative charge is remarkable in view of the fact that convective storms are characterized by substantial upward and downward motions of both air and precipitation. The storm charges are carried on cloud and precipitation particles and must follow the motions of the particles until their charge somehow changes. As time-resolved observations become available such charge motions will undoubtedly be found; indeed there is some evidence for them in the variability of electric-field data from storm to storm. Possible explanations for the otherwise horizontal and stable nature of the main negative charge are that the charging process operates only at certain tem
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 96 peratures (or pressures) or that the dynamics of the storm causes negatively charged particles to accumulate at the observed altitude. In any event the fact that net negative charge tends to be observed over a limited vertical extent indicates that other processes operate to change or mask the particle charges as they emerge from the negative-charge region. Figure 8.5 Illustration of how the negative-charge centers of cloud-to-ground lightning are at similar temperature levels in New Mexico and Florida storms, even though the latter have much greater extent of cloud and precipitation below the 0Â°C level and often above this level as well (adapted from the original by M. Brook, expressing the results of Jacobson and Krider, 1976; Krehbiel et al., 1979; Krehbiel, 1981; Brook et al., 1982). The negative charge centers of intracloud lightning are also at similar altitudes and temperatures even though intracloud discharges extend upward in the cloud rather than downward. Preliminary studies of lightning in Japanese winter storms suggest that the negative charge is at lower altitude but similar temperature values as in the summer storms. Preliminary studies of lightning in Japanese winter storms have indicated that these shallow but vigorously convective and strongly sheared storms also have a dipolar charge structure in which negative charge is at a lower altitude (but at similar temperature values) as in summer thunderstorms (Brook et al., 1982). These results are also illustrated in Figure 8.5; if confirmed by additional observations they suggest that temperature or the storm dynamics, rather than absolute altitude or pressure, are the important factors in the electrification. Figure 8.6 The altitude of the lightning charge centers for the first 15 discharges in the small Florida storm of Figure 8.3. The upper positive-charge centers of the intracloud flashes increased in altitude as the storm grew, while the negative-charge centers remained at constant altitude. Two cloud-to-ground discharges occurred toward the end of the sequence.