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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 109 of a storm is characterized by a horizontal brightband at and just below the 0Â°C level, caused by melting snow. The horizontal discharges appear to propagate just above the level of the brightband and effectively remove positive charge at this level (Krehbiel, 1981). The lightning is observed to repeat at intervals of a few minutes or more and occasionally produces positive strokes to ground. The repetitive nature of the discharges suggests that a widespread, low-rate charging process is operating to regenerate the positive charge and that the charging process is associated with the production or fall of snow. Independent evidence for the existence of a positive-charge layer is found in electric-field soundings through dissipating storms. Only a few such soundings have been made, even though dissipating storms have a stratiform, slowly changing structure that is relatively easy to probe. One sounding obtained by researchers in France is shown in Figure 8.12 (Chauzy et al., 1980). The balloonborne instrument passed through negative charge within the radar brightband and positive charge 1 km above the brightband. Although the charge distribution was still dipolar, positive charge was found at the level where negative charge is found in the active part of a storm. Similar results were obtained by Simpson and Scrase (1937) in the dissipating parts of English storms. The sounding and lightning observations agree, but further observations are needed to check their validity. Figure 8.12 Sounding of the vertical electric field in the dissipating part of a large frontal storm in France, indicating the presence of a positive-charge layer just above the 0Â°C level and a negative-charge layer with in the radar brightband from the storm (adapted from Chauzy et al. , 1980). The above observations do not explain how the charge structure of the storm changes to produce the end-of-storm electric-field oscillation discussed in connection with Figure 8.2. Moore et al. (1958) interpreted their observations of the oscillations as being due to the subsidence that occurs as the storm dissipates, which reveals the upper positive charge and transports it downward toward the ground [see also Moore and Vonnegut (1977)]. The observations of Williams (1981) support the idea that the field reversals are associated with downward motion of charge during subsidence, but the nature and source of the charges still remain to be determined. SUMMARY AND CONCLUSIONS The ability of scientists to observe and study thunderstorms has increased greatly over the past decade or two, and this has brought their study to a particularly exciting stage. A number of ideas have been proposed over the years to explain how thunderstorms become electrified, and it is now becoming possible to test the various ideas by direct measurement. Thunderstorms provide a difficult environment for measurements, but scientists are increasingly able to probe them with instruments that reliably measure electric-field profiles and particle charges and sizes, as well as air temperature, cloud water content, and other parameters. At the same time, remote-sensing techniques are providing increasingly detailed pictures of the storm as a whole. For example, networks of Doppler radars are able to measure the three-dimensional particle motions at different locations in the storm, and the lightning channels and charges are able to be located in space and time. The major ingredients for a thunderstorm continue to be vigorous convection and the formation of precipitation at altitudes where the air temperature is colder than 0Â°C. Strong electrification does not occur until the cloud and precipitation develop above a threshold altitude that is 7-8 km above MSL in the summer months, corresponding to an air temperature of â 15 to â 20Â°C. The main negative charge resides at and below this altitude at temperatures that are remarkably similar within a given storm and in different kinds of storms. A central issue of thunderstorm studies is whether the electrification is caused by the gravitational fall of charged precipitation or whether it results primarily from the convective transport of charges by the air mo
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 110 tions of the storm. Precipitation theories predict that the main negative charge is carried by precipitation particles, and this is being tested by in-cloud measurements of the charges carried by precipitation. Laboratory studies continue to point to rebounding collisions between hail and small ice crystals as a mechanism that can charge precipitation negatively and possibly explain the electrification. This mechanism is expected to operate in storms, but it has not been shown that enough precipitation is present and involved in enough charging interactions to account for the electrification. Compounding this difficulty are observations that electrification is more widespread than strong precipitation in a storm and that precipitation below the main negative-charge region is often observed to be positively charged. In convection theories the charges reside primarily on small cloud particles, which can carry much more charge per unit volume of cloudy air than precipitation. But little is known about the amounts and motions of the cloud particle charges and whether they would combine to produce charge accumulations consistent with observations. Once a storm becomes strongly electrified and starts to produce lightning, it is likely that additional charging processes occur that complicate or possibly enhance the electrification. In particular, the lightning itself could have such a role. This could explain some of the complexity of the electrical observations and would make it more difficult to sort out the various charging processes. In order to study the initial electrification processes, it is necessary that observations be made before the onset of strong electrification and of lightning. This need has been recognized but greatly increases the logistical difficulties of studying storms. By the same reasoning, it is possible that the primary electrification mechanism changes once a storm becomes strongly electrified. For example, precipitation could initiate the electrification, and then the larger convective energies of the storm could continue the electrification. Or the electrification could be sustained and enhanced if the corona from precipitation had a systematic sign. Already there is some evidence that a different, lower-rate mechanism operates to electrify dissipating storms or parts of storms. Our understanding of the electrification processes remains limited by the need for better observations of the electrical and physical characteristics of actual thunderstorms. This need has guided thunderstorm research for a number of years and involves several parallel and interacting efforts: field programs, data analysis, and the development of observational techniques. Field programs provide the basic experimental data and allow scientists to test new instruments and observational techniques. Data analysis extracts the scientific information from the field programs and provides feedback for future studies. Instrumental development and testing can be done in limited field programs, but significant advances in understanding the electrification processes require focused, cooperative field programs that bring together the best available observational techniques. A substantial amount of data is already in hand from recent field programs of this type whose continued analysis will provide further insights into the electrification problem. But too many questions remain unaddressed in the measurements of those studies for them to hold the answers to the problems. And, as is usual in science, the results of one set of observations and experiments often raise new questions and avenues of investigation. A prime example of this is the recent experiments that have attempted to invert the electrical polarity of a storm. Much of our information about thunderstorm electrification has come from the study of relatively small, isolated storms such as those that form over the mountains of the southwestern United States or above the seabreeze convergence in southeastern coastal areas. These storms provide relatively stationary and predictable targets for study and remain attractive subjects for field programs. Although relatively small, the storms are not simple, and we have much to learn from them. As their study has demonstrated, however, it is important that different types and sizes of storms be studied and compared. In particular, it is important that electrical studies be made of severe storms, propagating squall lines, tropical storms (both ice-free and ice-containing), and winter storms. This review has concentrated primarily on the scientific observations and issues related to the problem of thunderstorm electrification. Other recent reviews on the same subject have been made by Moore and Vonnegut (1977), Illingworth and Krehbiel (1981), Latham (1981), Vonnegut (1982), Lhermitte and Williams (1983), Illingworth (1985), and Williams (1985). Another whole review could be devoted to a description of the techniques that are used to study thunderstorms and their electrification. Many of the techniques are new and are still under development and have been used in cooperative studies for only one or a few thunderstorm seasons. Other techniques have yet to be used in electrification studiesâfor example, the differential reflectivity polarization radar technique (Bringi et al., 1984). (This technique measures the difference in precipitation reflectivity for vertical and horizontal polarizations and is able to distinguish between ice and liquid water in clouds.) Much could be learned by bringing existing techniques together and applying them to the same