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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 91 steadily improved our ability to observe the electrical characteristics of thunderclouds both from the ground and inside the storm. Most electrification studies have lacked particle observations inside the storm, a shortcoming that has been partially addressed in recent years. The study of thunderstorms and their electrification is important not only because of their instrinsic scientific interest but for other reasons as well. A significant fraction of the Earth's rainfall in temperate climates comes from electrified clouds, and it is possible or likely that the precipitation processes in these storms are influenced by their electrification. Also, electrified and lightning-producing storms may play an important role in the chemical reactions responsible for the production of acid rain. Finally, vigorous downbursts, which recently have been identified as dangerous to aircraft in the vicinity of airports, may be accompanied by an electrical signature that could aid in their detection and the warning of hazardous conditions. In this chapter we discuss what is currently known about the electrification of thunderstorms and what is not known and indicate some directions for research efforts over the next decade or so. ELECTRICAL STRUCTURE AND DEVELOPMENT The distribution and motion of electric charge in and around a thunderstorm is complex and changes continuously as the storm evolves. Nevertheless, we have a rudimentary picture of how charge is distributed in an already-electrified storm. This is depicted in Figure 8.1 on a photograph of a small thunderstorm over Langmuir Laboratory. The interior of the storm contains a dipolar charge distribution consisting of positive charge in the upper part of the cloud and negative charge below the positive. These are the dominant accumulations of charge in the storm and are called the ''upper positive" and "main Figure 8.1 An isolated thundercloud over Langmuir Laboratory in central New Mexico and a rudimentary picture of how electric charge appears to be distributed inside and around the thundercloud, as inferred from in-cloud and remote observations.
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 92 negative" charges, respectively. The upper positive charge attracts negative ions to the top of the cloud from the electrically conducting clear air around the storm. The ions, which are produced by cosmic radiation, attach to small cloud particles at the edge of the cloud, forming a negative screening layer that partially cancels or screens the interior positive charge from an outside observer. The main negative charge causes point discharge or corona from trees, vegetation, and other pointed or exposed objects on the ground below the storm, which leaves positive charge in the air above the Earth's surface. Positive charge is also found beneath and inside the base of the cloud below the main negative charge; this is called the "lower positive charge." Two sources for the lower positive charge are the corona from the ground, which may be carried upward into the cloud by the updraft, and positive ions generated by cosmic rays below and around the cloud base, which are attracted to the cloud by the main negative charge. Additional lower positive charge is carried by descending precipitation and occurs in localized regions known as "lower positive charge centers" (LPCCs). Several hypothetical LPCCs are depicted in Figure 8.1; they may be caused by a subsidiary charging process in the cloud. The above description presents a simplified picture of how charge is distributed in a thunderstorm; the actual charge distribution is more complicated than this and needs to be better understood before we can answer the question of how thunderstorms become electrified. For example, it is necessary to know what types of particles carry the charges and how these particles move. Information on these and other questions is being obtained both from in-cloud and remote observations, as we discuss in this review. Charge accumulates in the upper positive and main negative regions as a result of the charging mechanisms until the electric stresses are such that a lightning discharge occurs. Two primary forms of discharges are cloud-to-ground and intracloud lightning. Cloud-to-ground (CG) is the most familiar and spectacular form of lightning; it usually occurs between the main negative charge and ground and lowers negative charge to ground along one or more distinct and highly luminous channels. Some CG flashes lower positive charge to ground (see Chapter 3, this volume); these are called positive CG flashes and have been difficult to distinguish from normal CG discharges until recent years. They are of interest both because they are different from normal-polarity CG flashes and because they are often more damaging to objects that they strike. Intracloud (IC) lightning is usually confined to the cloud interior and diffusely illuminates the cloud, being visible primarily at night. Intracloud lightning often occurs as a primarily vertical discharge between the main negative and upper positive charge regions of the storm. Horizontal IC lightning is also common, particularly in large storm systems where the lightning may propagate over distances of 100 km or more. These extensive discharges may have CG components or may be initiated by a CG discharge. For studying the processes of electrification and electrical breakdown, the most interesting parts of a lightning discharge are inside the cloud where they are obscured from direct optical observation. Clouds and precipitation are transparent at microwave and longer wavelengths and to other kinds of signals, however, and several techniques that sense these signals are beginning to provide us with important information on what lightning looks like inside a storm. The techniques locate the lightning channels and charges and are discussed later in this chapter. The charges of the storm itself can be sensed by measuring the electric field that they produce. The electric field indicates the strength and direction with which the storm charges attract or repel other charges and can be measured at the ground or in the air outside or inside the cloud. In clear-sky (fair-weather) conditions the atmospheric electric field at the Earth's surface has a negative value of about 100 to 200 V/m. This is caused by the fact that the ionosphere is charged positively to a potential of about 300,000 V with respect to the Earth's surface. (In turn the ionospheric potential is believed to result from the global thunderstorm activity.) Beneath a thunderstorm the electric field at the ground is often substantially larger, up to 10,000 V/m or more, and tends to be reversed in sign from fair-weather conditions. Figure 8.2 shows a recording of the electric field versus time measured on the ground beneath a thunderstorm over central New Mexico. The storm went through its complete life cycle over the recording instrument, and the electric field record illustrates different stages in its electrical activity. As the storm became electrified, the buildup of negative charge in its base caused the electric field at the ground to reverse sign from the fair-weather (negative) value and to increase rapidly in magnitude. This is called the initial electrification of the storm. In-cloud measurements have indicated that the initial electrification can occur in a relatively short time, on the order of 5 to 10 minutes or perhaps less. The initial electrification is usually considered to end with the occurrence of the first lightning discharge, which marks the beginning of the active or lightning-producing stage. The active stage can last from a few minutes to an hour or more depending on the size and convective vigor
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 93 of the storm. During this time lightning discharges suddenly decrease the electric field while the charging process steadily increases the field. As observed at the ground, the electric field jumps from positive to negative values and then grows back to positive values. The sign reversal indicates the presence of positive corona charge above the measuring instrument, which dominates the field for a short time after the discharge. As the storm charges build back up to the next lightning flash, point discharge limits the electric field at the ground to some maximum value indicated by the upper envelope of the record (about 8000 V/m in the case of Figure 8.2). Electric-field meters flown from balloons several hundred meters above the ground do not show field reversals during lightning and usually do not show limiting field values (Standler and Winn, 1979; Holden et al., 1983). This confirms that both effects are associated with corona from the ground and indicates that most of the corona charge resides in a relatively shallow "blanket" close to the Earth's surface. Figure 8.2 The atmospheric electric field and precipitation intensity on the ground beneath an isolated, stationary thunderstorm in central New Mexico (adapted from Moore and Vonnegut, 1977). See text for description. The pronounced excursion of the electric field to negative values in the middle of the active stage was associated with the arrival of a downdraft and a transient burst of precipitation at the observing location. This is a common feature of thunderstorm observations and is called a field excursion associated with precipitation. In this example the charge carried by the precipitation as it arrived at the ground was measured to be weak and negative, i.e., of the wrong sign (and insufficient in magnitude) to have caused the field excursion. This is often the case and is called the mirror-image effect (Chalmers, 1967). The precipitation is believed to capture point discharge ions produced during the field excursion and to return them to earth. On the other hand, balloonborne measurements of lower positive charge centers carried by precipitation have been correlated with the subsequent occurrence of a field excursion at the ground, of the right sign to be explained by the precipitation charge (Marshall and Winn, 1982; Holden et al., 1983). It is uncertain whether field excursions are usually caused by descending, charged precipitation (whose sign may be reversed close to the earth by the capture of point discharge ions) or whether the downdrafts that accompany the precipitation carry or reveal other charge that causes the excursion. In any event, it has been suggested that the field excursions could help to detect downbursts in storms that are responsible for aircraft accidents on takeoff and landing (C. B. Moore, New Mexico Institute of Mining and Technology, private communication; Lhermitte and Williams, 1985a). During the final or dissipating stage of the storm the lightning activity died out and the electric field exhibited a large, sustained swing to negative values and back, called an end of storm oscillation (EOSO). EOSOs are observed directly beneath a dissipating storm and are associated with the storm's physical collapse. The field at the ground is dominated for relatively long periods of time by positive charge overhead, and this is found to be a favored time for the occurrence of positive CG lightning. (None occurred in the storm of Figure 8.2, however.) It is not understood what the charge distribution is during an EOSO or how it changes to produce the field reversals; what little information we have is discussed at the end of this chapter. The electrification of a storm is cellular in nature, i.e., it is associated with the development of individual convective cells within the overall storm. All but the simplest of storms are multicellular, with the lifetime of an individual cell being about 10 or 15 minutes. Some severe storms of the Great Plains develop into large, highly organized systems called supercell storms. These and other large storms appear also to have a dipolar charge structure, but little is known about the details of their electrification.