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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 102 lightning discharge (Vonnegut and Moore, 1960) and that this may be responsible for the sudden bursts or gushes of rain that are commonly observed from thunderstorms. Some observations have been reported in support of this idea (e.g., Moore et al., 1964; Syzmanski et al ., 1980), but the possible effects that lightning may have on the cloud microphysics and precipitation formation remain unclear and are in need of continued study. At the very least, the fact that the storm produces strong electric fields makes it easier for precipitation to form, in that the electric forces increase the probability that particles will coalesce after coming in contact (Rayleigh, 1879; Goyer et al., 1960; Moore et al., 1958). CHARGE MOTIONS AND CURRENTS The charging mechanism constitutes a source of current that continually increases the electrical energy of the storm, while lightning intermittently reduces the storm's electrical energy. The charging current is on the order of 1 A and ranges from inferred values of about 0.1 A during the initial stages of small storms (Krehbiel, 1981) to values greater than 1 A for large storm complexes. Currents of comparable average magnitude flow between the cloud and ground and between the upper atmosphere and cloud top. The current at the cloud top results from the flow of negative screening charge to the upper cloud boundary. The current between the cloud and ground is due to the action of CG lightning and point discharge from the Earth's surface and, like each of the other currents, predominantly lowers negative charge (or raises positive charge). The various currents are illustrated in Figure 8.8. Their net effect is to transfer negative charge from the upper atmosphere to the Earth's surface. This transfer, from approximately 1000 storms that are estimated to be in progress at any time over the entire planet, is thought to be the reason that the ionosphere is maintained at a potential of several hundred thousand volts with respect to the Earth's surface. The vast majority of the electrical charges inside a storm reside on cloud or precipitation particles. Free charge or ions, such as are generated by cosmic radiation, corona, or lightning, collide with and attach to cloud particles within about a second after being produced. This immobilizes the charge and makes the cloud a good electrical insulator. The currents that charge the storm should not be weakened significantly by leakage currents between the charge regions, except those involving the motion of charged cloud particles, which are expected to have small effect. The storm charges and electric fields are therefore expected to build up in a manner that reflects the strength of the charging process, until lightning occurs, after which the buildup repeats. In-cloud electric-field measurements with slowly moving balloonborne instruments have shown that the electric field increases in an approximately linear manner with time between lightning discharges (Winn and Byerley, 1975). This indicates that the current source that charges the storm is relatively constant between discharges and is independent of the magnitude of the field. Theories that employ positive feedback to electrify a storm, such as an inductive theory, predict that the charging current would increase exponentially with time and thus would not appear to be operating. This may not be a reliable prediction or test of the mechanism type, however, if the charging process establishes and Figure 8.8 Diagrammatic illustration of the electric currents for precipitation and convection scenarios of electrification. In both cases the thunderstorm transfers negative charge from the upper atmosphere to the Earth's surface. The arrows point in the direction of positive current flow; the short half-arrows indicate when this current is caused by negative charge flowing in the opposite direction. Intracloud lightning opposes the charging process in the precipitation scenario but provides a parallel charging path to the upper atmosphere in the convection scenario.
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 103 draws on a reservoir of charge as discussed earlier. In this case the charging current and field buildup would depend on the rate at which charge emerges from the reservoir, which will tend to be field independent. A major unresolved question in understanding the electrical behavior of storms concerns the role and fate of the upper screening current in the electrical budget of the storm (Vonnegut, 1982). In the absence of convective motions and turbulence the flow of negative screening charge would completely shield the interior positive charge in a few tens of seconds. Airborne electric-field observations outside the tops of growing clouds show that a screening charge does form, but not to completion. Negative charge continues to be attracted to the cloud surface at rates that are comparable with the charging current of the storm (Gish and Wait, 1950). The question is what happens to this charge. The convection hypothesis of electrification postulates that the screening charge is carried downward by convective overturning to the level of the main negative charge and that this is the primary source of the main negative charge (Vonnegut, 1953). This charge transport would be generative, i.e., negative charge would be carried downward away from the upper positive charge, increasing the electrical energy of the storm. An alternative possibility is that turbulent mixing folds the screening charge into the upper positive charge of the storm, which would be a dissipative process. Regardless of its eventual fate, the substantial flow of negative screening charge to the upper part of the cloud appears not to be matched by the flow of positive charge to cloud base, causing the storm as a whole to build up a net negative charge with time. The buildup is alleviated intermittently by negative cloud-to-ground lightning, and this is undoubtedly the reason why most CG lightning has a negative polarity. The buildup also increases the dominant effect of negative charge on the electric field at the ground, which is alleviated by positive corona from the ground. The convection hypothesis postulates that the positive corona charge is carried into the upper part of the cloud by the updraft, which feeds the cloud with low-level moisture, and that this is the primary source of the upper positive charge. Other possibilities are that much of the corona charge is carried into the main negative-charge region and is dissipated there or that most of it remains near the ground. The strength of the point discharge current beneath storms has been estimated both from ground-based electric- field observations and from measurements of the corona current given off by vegetation beneath storms. Over a typical area of 10 km2 the total corona current is estimated to be about 0.1 A (Livingston and Krider, 1978; Standler and Winn, 1979). This is comparable with the charging current at the beginning of a storm but is less than the average current of cloud-to-ground lightning in the active stage of a storm. Recent experiments designed to test electrification ideas have attempted to influence or alter the electrification of a storm by releasing charge into the bases of growing clouds prior to their electrification (Vonnegut et al., 1984; Moore et al., 1985). In these experiments, several kilometers of cable and fine wire are strung over mountainous terrain and maintained at a high positive or negative potential. Natural clouds grow over a fair-weather supply of positive space charge near the Earth's surface, which tends to be ingested into the cloud along with surface moisture. By maintaining the wires at a high negative potential the researchers hope to give off sufficient negative corona charge to override the natural supply of positive charge and to prime the cloud with negative charge. If a convective mechanism operates to initiate the electrification, or if the electrification is influenced by the direction of the initial electric field inside the storm (as in the case of an inductive precipitation mechanism), such priming should invert the polarity of the electrification, i.e., produce a storm having an upper negative-and main positive-charge structure. The results of the experiments are that storms developing above negative-charge releases are anomalous in that the field at the ground is often dominated by positive charge overhead, which lightning acts to remove. There is incomplete and conflicting information on the question of whether the polarity of the main storm charges was inverted. One alternative possibility is that the experiment modifies primarily the subcloud and cloud-base charges. The success of the experiments in at least partially altering the electrical structure of storms makes them intriguing subjects for continued field programs. Because the interior storm charges reside on cloud or precipitation particles, their motion is the same as the particle motions and can be investigated using Doppler radars when the particles are large enough to be detected by radar. A single Doppler radar measures the component of the particle velocity along the direction of the radar beam; a network of three or more Doppler radars is needed to determine the particle velocities in three dimensions. Three- dimensional measurements of particle velocites have become possible only recently (e.g., Lhermitte and Williams, 1985b) but are a key element in furthering our understanding of thunderstorms. A continuing problem in their determination is the rapidly changing nature of convective storms, which requires that the storm be scanned as rapidly as possible. Multiple Doppler radars have been used to study the electrification of storms in Florida and New Mexico; one