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The Electrical Structure of Thunderstorms 8 PAUL R. KREHBIEL New Mexico Institute of Mining arid Technology Thunderstorms and the lightning that they produce are inherently interesting phenomena that have in- trigued scientists and mankind in general for many years. A number of theories have been proposed to ex- plain how thunderstorms become electrified, and many field and laboratory experiments have been conducted to determine the electrical nature of storms and to test the electrification theories. Through this effort we are beginning to understand how electric charge is distrib- uted in storms, but the mechanisms that cause their elec- trification continue to elude scientists and remain the subject of considerable inquiry and debate. The basic difficulty in determining how thunder- clouds become electrified lies in the fact that they are large, complex, and short-lived phenomena that need to be examined both as a whole and in detail to understand how they function. The electrical processes are inti- mately related to the cloud dynamics or motions and to the microphysics of the cloud, namely, to the popula- tions and interactions of the precipitation, cloud drop- lets, ice crystals, and other particles that make up the cloud. These important aspects of storms are themselves incompletely known or understood, yet a detailed com- prehension of them is necessary to understand the elec- trification processes. Attempts to simulate possible electrification processes in the laboratory or by theoretical modeling have been 90 helpful in evaluating some theories but have not demon- strated the efficacy of any particular mechanism. This is because thunderstorm conditions are inherently diffi- cult to simulate and are insufficiently understood for us to be confident that we are simulating them properly. At present, further progress in understanding the electrification of thunderstorms, and indeed in under- standing their dynamics and precipitation processes as well, requires simultaneous observations of their dy- namical, microphysical, and electrical properties. This need has been increasingly recognized in recent years and has given rise to a number of cooperative studies of storms. The cooperative studies employ the latest tech- niques for internally and remotely probing storms and rely on the combined expertise of university and na- tional laboratory researchers to conduct and analyze the observations. The studies typically use instrumented aircraft and balloons to penetrate the storms, multiple radar systems to measure precipitation strengths and ve- locities, and ground-based instrument networks for measuring meteorological and electrical quantities. A few research programs have focused on the electri- fication question, including the ongoing studies at the Langmuir Laboratory for Atmospheric Research in the mountains of central New Mexico and the Thunder- storm Research International Program (TRIP) in Flor- ida and New Mexico. These and other studies have

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 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 short- coming 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 frac- tion 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 influ- enced by their electrification. Also, electrified and light- ning-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 vi- cinity of airports, may be accompanied by an electrical signature that could aid in their detection and the warn- ing of hazardous conditions. 91 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 ef- forts 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 continu- ously as the storm evolves. Nevertheless, we have a rudi- mentary picture of how charge is distributed in an al- ready-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 posi- tive. These are the dominant accumulations of charge in the storm and are called the "upper positive" and "main .~ s_ : _ . :s :~ ~ it. ;_ ~.sS ~.~:~:~ :~ ~ ::. ~.~.~:~;~ ~: : _ ~'~:"'~'~s~s ~ ~ ~ IS , ~' ~-'~s~ '~ ~: ~ _ ~s:s: ::s.ss OCR for page 90
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, at- tach 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 dis- charge 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 up- draft, 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 sub- sidiary 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. Infor- mation on these and other questions is being obtained both from in-cloud and remote observations, as we dis- cuss 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 dis- charge 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 nega- tive 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 distin- guish 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 PAUL R. KREHBIEL visible primarily at night. Intracloud lightning often oc- curs 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 dis- charges may have CG components or may be initiated by a CG discharge. For studying the processes of electrification and elec- trical breakdown, the most interesting parts of a light- ning discharge are inside the cloud where they are ob- scured 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 light- ning 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 mea- suring 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 atmo- spheric 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 thunder- storm the electric field at the ground is often substan- tially 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 ver- sus time measured on the ground beneath a thunder- storm over central New Mexico. The storm went through its complete life cycle over the recording instru- ment, and the electric field record illustrates different stages in its electrical activity. As the storm became elec- trified, 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-produc- ing stage. The active stage can last from a few minutes to an hour or more depending on the size and convective vigor

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS ~con - y - J LD LO CE (O - 1 0 UJ L1J O > ~ (A cL, Z Z CE - ~O AH ,( Lightning - mm/hr I- End-of-storm oscillation \ I ~ Field excursion associated with precipitation 6.5 mm of rain fell _~ ~_ _ ~1 1 2:00 1 3:00 1 4:00 1 5:00 TIME, MST FIGURE 8.2 The atmospheric electric field and precipitation inten- sity on the ground beneath an isolated, stationary thunderstorm in central New Mexico (adapted from Moore and Vonnegut, 1977). See text for description. 7 ~ of the storm. During this time lightning discharges sud- denly decrease the electric field while the charging pro- cess steadily increases the field. As observed at the ground, the electric field jumps from positive to nega- tive values and then grows back to positive values. The sign reversal indicates the presence of positive corona charge above the measuring instrument, which domi- nates 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 hun- dred meters above the ground do not show field rever- sals during lightning and usually do not show limiting field values (Standler and Winn, 1979; Golden 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 "blan- ket" close to the Earth's surface. The pronounced excursion of the electric field to neg 93 alive values in the middle of the active stage was associ- ated 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 Toured was measured to be weak and neg- ative, i.e., of the wrong sign (and insufficient in magni- tude) 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 dis- charge 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 subse- quent 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 dis- charge 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 exhib- ited 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 pe- riods 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 dis- tribution is during an EOSO or how it changes to pro- duce 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.

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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 in- creasing distance from storms, as if the lightning were discharging an upper positive and lower negative charge. (Earlier work by Wilson on the properties of at- mospheric 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 stud- ies 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 0C (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 O and - 25C. Figure 8.3 presents a modern-day equivalent of Wil DAY 220 8/08/77 18:14:16.400 10. O. RlIMUTH 270.2 DEGREES D ~ STRNCE FROM RRDRR (KM) _c -40- - a~c~ =00 .................... i=,, - ........... ...... 1~ - 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 (-15C) to above the detectable precipitation (from Krehbiel et al., 1984b). PAUL R. KREHBIEL son's result. It shows the centers or sources of charge for the first, intracloud lightning discharge in a small Flor- ida storm, superimposed on a vertical cross section of the radar echo from the storm's precipitation. The charge centers were determined from simultaneous measure- ments of the electric-field change produced by the light- ning at eight locations on the ground beneath and around the storm. The flash effectively removed nega- tive 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 precipita- tion. Data from a higher-power, Doppler radar observ- ing the same storm showed that a weaker radar echo extended up to and above the upper charge centers, in- dicating that the lightning remained within the cloud. The air temperature outside the cloud at the level of the negative-charge centers was between - 10 and - 15C. 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 bal- loonborne instrument that measured the corona current from a 1-m-long vertical wire. The corona current re- versed sign as the instrument ascended through the neg- ative-charge region between 6- and 7-km altitude t 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 nega- tive-charge region was between about - 5 and - 10C. No lightning was produced by the storm. Soundings through lightning-producing storms also indicate a di- polar charge structure but are complicated by the large- amplitude field changes of the lightning discharges. The soundings can be made with more sophisticated instru- ments 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 struc- ture than suggested by Figure 8.4. The electric-field measurements indicate that the vol- ume 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

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 10 9 s 13:05 =00 \ r l 12:58 1 -50 KV AUG.~6, 8]/ LAUNCH 2:531 ~ / / I+~+., _ ~ 1 1 1 _~__ ~ 1 1 1 1 1 1 - 1 1 1 _ 1 _ ~1 -2s ~ 2s~ sow _ 1 ' ' ' ' ' ' ~ -4 -2 0 2 ICOQONA (~) -10 1 FIGURE 8.4 Vertical sounding of the corona current from a 1-m- long wire carried by balloon through a storm over Langmuir Labora- tory in New Mexico (Byrne et al., 1983~. The wire was vertically ori- ented, and the corona current record is indicative of the vertical component of the electric field in the storm. 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) . The above results indicate the dipolar nature of thun- derstorms 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 bather 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 light- ning 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 dis- placed 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 0C 95 level, and often above it as well. New Mexico storms are drier and generally smaller than Florida storms, having cloud bases just below the 0C level. The results suggest that the charging processes are the same in the two types of storm and operate at temperatures less than O or -10C. They also suggest that the part of a storm warmer than 0C is not directly involved in the electrifi- cation. The lightning-charge observations are supported by electric-field soundings through storms, which indicate that the main negative-charge region is relatively shal- low, 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 sound- ing observations tends to be lower than that inferred from lightning-charge observations, and there is some indication that the negative-charge region may be sys- tematically higher in Florida storms than in New Mex- ico 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 dis- tributed horizontally in a storm but to remain at approx- imately 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 (- 30 to - 60C), but the negative-charge centers remained at about 7-km altitude (-15C). Sequences of radar pic- tures like the one shown in Figure 8.3 confirm the up- ward growth of the storm and show that it occurred at the same rate as for the positive-charge centers of the lightning (8 miser). 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 down- ward 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 observa- lions become available such charge motions will un- doubtedly 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 hori- zontal and stable nature of the main negative charge are that the charging process operates only at certain tem

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96 I PAUL R. KREHBIEL -20 S u m m e r S t o r m s 18 _~\ 16 ~ ~ 2 (~] 6 ~ ~ _ _ _1(: -4 ~0C^\ ) _ ~_ - _ ,, me ~% 1 ~/~. ~ + ~Storms Florida Florida New Mexico Plain Winter 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 0C 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. 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 ver- tical extent indicates that other processes operate to change or mask the particle charges as they emerge from the negative-charge region. Preliminary studies of lightning in Japanese winter storms have indicated that these shallow but vigorously FIGURE 8.6 The altitude of the lightning charge centers for the first 15 discharges in the 1 4 small Florida storm of Figure 8.3. The upper positive-charge centers of the intracloud ~ flashes increased in altitude as the storm ~ 12 grew, Awhile the negative-charge centers re- ~ mained at constant altitude. Two cloud-to- ~ 1 C around discharges occurred toward the end of ~ the sequence. y 8 C) LL I convective and strongly sheared storms also have a dipo- lar 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. 6 4 T T e m p , T ITT T~i- I_ i-ii-l-l l l (+)_|-i | | | i I T I ~ i ~i i ~i ~ c Hi, Discharges 1 1 1 1 1 1 0 100 200 300 400 500 TIME, SECONDS c - -60 - - 5 0 4o - -30 -20 - 1 0 o

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TTIE ELECTRICAL STRUCTURE OF THUNDERSTORMS INITIAL ELECTRIFICATION Observations of the onset of electrification in storms are consistent with the above picture of thunderstorm charges and provide additional insight into the electrifi- cation problem. In particular, it is found that a storm does not become strongly electrified until its radar echo extends above a certain altitude threshold and is grow- ing vertically. The threshold altitude depends some- what on the sensitivity of the radar but is about 8 km above MSL in the summer months, corresponding to an air temperature of about - 20C. Kasemir and Cobb (see Cobb, 1975; Illingworth and Latham, 1977) re- ported a similar threshold effect from aircraft measure- ments near the tops of Florida clouds; electric-field val- ues of 1 kV/m were not detected until the radar echo top grew above about - 5C. A recent set of observations that illustrate the onset of electrification in a storm is presented in Figure 8.7 t]. E. Dye (National Center for Atmospheric Research) and W. P. Winn and C. B. Moore (New Mexico Institute of Mining and Technology), private communications]. The figure shows the height of the precipitation echo versus time in a small storm near Langmuir Laboratory and an electric-field record from a ground station 5 km distant from the storm. Electrification was not detected at the ground until about 12:40 MST, shortly after the radar echo began a sustained period of growth above 8- km altitude. More sensitive measurements from an in- strumented aircraft penetrating the cloud at 7-km alti- tude (- 15C) showed weak electrification (on the order of 100 V/m electric-field perturbations) during a pass between 12:34 and 12:36 and strong electrification on re-entering the cloud at 12:45. Other measurements from a sailplane at 4-km altitude inside the cloud indi- cated weak electrification starting at about 12:31, prob- ably associated with the earlier convective surge at 12:25. The sailplane spiraled upward in the storm up- draft and measured 1 kV/m maximum fields by 11:40 at 6-km altitude. The field appeared to originate from neg- ative charge in nearby precipitation of 40-dBZ reflectiv- ity (Dye et al., 1985~. The first lightning discharge oc- curred at 12:44 when the echo top had reached 10-km altitude. By this time, moderately strong (40-dBZ) ech- oes had developed up to 8-km altitude and were begin- ning to subside. Equally strong precipitation developed during the earlier convective surge, but the earlier cell had less convective energy and did not become strongly electrified. The above example illustrates graphically the impor- tance of convective growth in the electrification of a storm. This fact has been recognized for a number of years and is generally accepted (e.g., Workman and 97 Reynolds, 1949; Reynolds and Brook, 1956; Moore et al., 1958~. The convective growth is often retarded by stable air or by strong winds at mid-altitudes in the at- mosphere and is usually preceded by a succession of con- vective surges or turrets before one or more of these suc- ceed in penetrating the stable layer. The example also indicates that moderately strong precipitation had de- veloped in the storm before to its electrification. That precipitation must be present and must develop above a certain altitude or temperature threshold is a consistent feature of field observations that is being documented for an increasing number of storms in New Mexico, Florida, and Montana (Reynolds and Brook, 1956; Holmes et al., 1977; Lhermitte and Krehbiel, 1979; Krehbiel et al., 1984a; Dye et al., 1985, 1986~. DISCUSSION The above results and others like them indicate that the electrification process operates at temperatures of less than 0 or - 10C. In addition, they indicate that convection and precipitation somehow combine to cause the electrification. One of the biggest questions and sources of debate among thunderstorm researchers has been whether the kinds of precipitation and cloud particles that grow in convective storms cause their electrification or whether the convective motions themselves directly electrify the storm without involving or requiring precipitation. His- torically, observations have led many scientists to as- sume or favor the precipitation explanation, and the re- cent radar and electrical observations described above continue to fuel this idea. The temperature values at which electrification is observed have caused many re- searchers to focus on frozen precipitation as a primary agent in the electrification process. Other observations, discussed below, have raised questions about precipita- tion theories and cause some researchers to look toward a convective explanation. Chapters 9 and 10 (this volume) discuss the various theories and mechanisms that have been proposed to ex- plain how thunderstorms become electrified. Precipita- tion theories hypothesize that the relatively large pre- cipitation particles acquire negative charge, in most cases by colliding with or shedding smaller cloud parti- cles. The cloud particles acquire a corresponding posi- tive charge and are carried by the updraft into the upper part of the storm, whereas the precipitation may rise or fall with respect to the ground depending on the relative magnitudes of its fall speed and the updraft. Negative and positive charges are segregated onto large and small particles, respectively, and are separated by the action of gravity to electrify the storm. In convection theories,

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98 PAUL R. KREHBIEL 12t 10t 6 I 5 - RADAR REFLECTIVITY AUGUST 3. 1984 8~ 1 0 dBZ ~ an\ 20 dBZ //~/ \- An dB2 - - ~ l | 4 O d B Z \ r 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 \1 1 1 2:05 1 2:30 1 3:00 1 3:30 - - 12:05 12:30 13:00 13:30 TIME, MST FIGURE 8.7 The radar reflectivity of precipitation versus height and time in a small storm near Langmuir Laboratory on August 3, 1984, and a record of the electric field at the ground 5 km from the storm. The electrification was associated with convective growth above 8-km altitude (about - 20C) and with the development of moderately strong precipitation up to this altitude. An initial convective surge between 12:20 and 12:25 produced only weak electrification, as measured by instrumented aircraft inside the storm. [Unpublished data from J. E. Dye (National Center for Atmospheric Research) and C. B. Moore and W. P. Winn (New Mexico Institute of Mining and Technology) . positive and negative charges are spatially segregated and the energy of electrification is derived directly from the convective motions of the storm, which transport charges of opposite sign away from each other. The charges are expected to reside primarily on small cloud particles, with the net charge on precipitation being ei ther small or of the same sign as that on the cloud parti- cles. In-cloud observations at the level of the main nega- tive charge show that the cloud contains a mixture of particle sizes and types. All or most of the precipitation particles are frozen and are in the form of graupel or

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS hail. The precipitation particles coexist with a large number of small, unfrozen cloud droplets that are car- ried above the 0C level by the updraft. The droplets remain in a supercooled liquid state until they contact an ice surface, whereupon they freeze and stick to the surface in a process called riming. (Alternatively, the supercooled droplets freeze spontaneously at suffi- ciently low temperature.) Riming is the dominant growth process of graupel and dry hail and entraps sig- nificant amounts of air, giving the particles a milky ap- pearance. The riming process is also responsible for the dangerous ice loads that aircraft develop in flying through convective clouds above the 0C level. Storms that produce snow, such as winter storms and the dissipating parts of summer storms, can be strongly electrified but tend to produce only occasional lightning or none at all. This suggests that snow, whose crystals grow directly from water vapor in the air, does not in itself cause the electrification of active thunderclouds. The primary differences between winter or dissipating storms and active thunderclouds is that the latter are more strongly convective, develop greater vertical ex- tents, and produce graupel or hail rather than snow. A number of laboratory studies since the 1950s have shown that rebounding collisions between hail pellets and small ice particles cause charge of the appropriate sign to be transferred between the particles (e.g., Rey- nolds et al., 1957; Takahashi, 1978; Gaskell and I1- lingworth, 1980; Jayaratne et al., 1983~. This charging process operates in the correct temperature range and is considered by some researchers to be the most promising of the precipitation mechanisms at present (e.g., La- tham, 1981; Illingworth, 1985~. But the laboratory-ob- served charging is able to account for the observed elec- trification only when the precipitation rates are high, on the order of 30 mm/in, and when the ice crystals are rela- tively abundant, 10-SO per liter or more (e.g., I1- lingworth, 1985; Williams, 1985~. A precipitation rate of 30 mm/in corresponds to a radar echo of about 40 dBZ if the precipitation is frozen. Radar echoes of this strength have been observed during the initial electrifi- cation of Florida storms (Lhermitte and Krehbiel, 1979; Krehbiel et al., 1984a) and recently in New Mexico storms (Figure 8.7; Dye et al., 1986~. Earlier observa- tions of New Mexican storms have indicated that they can become electrified when their radar echoes are weaker- 33 to 35 dBZ or perhaps less (Moore, 1963; Holmes et al., 1977) . These echo strengths correspond to frozen precipitation rates of about 10 mm/in or less. While precipitation rates can be estimated remotely us- ing radar, the populations of small ice crystals can be determined only from in-cloud measurements and vary greatly with the particular conditions and with altitude. 99 Concentrations of 10-50 per liter are large but have been observed. As noted by Dye et al. (1986), however, few measurements have been made in the conditions and lo- cations of interest. The above discussion points to a central issue of thun- derstorm studies, namely, whether sufficient precipita- tion is present and involved in enough charging interac- tions to account for the initial electrification. There has been much discussion of this issue in the scientific litera- ture (e.g., Moore, 1976a, 1976b, 1977; Mason, 1976; Illingworth and Latham, 1977; Illingworth, 1985; Wil- liams, 1985~. An increasing number of field studies are indicating that the initial electrification occurs during the growth of precipitation in an updraft, where the conditions would be conducive to an ice-based precipi- tation charging mechanism. (These are cited at the end of the preceding section. ~ Recent results from these stud- ies indicate that the electric fields inside the cloud ap- pear to originate from regions of stronger radar reflec- tivity at the negative-charge level and indicate negative charge in those regions (Dye et al., 1986~. But observa- tions in already-electrified storms show that the electri- fication is more widespread than the strong precipita- tionechoes (Krehbiel, 1981; Winnet all, 1981; Weberet al., 1982~. In addition, estimates of the energy available from falling precipitation indicate that the energy may only be comparable with the electrical energy of some storms, particularly at altitudes where the electrifica- tion occurs. In this case a precipitation mechanism would have to be highly efficient if it were to cause the electrification (Williams and Lhermitte, 1983~. Similar issues and questions exist with regard to con- vection theories of electrification. The convective en- ergy of a storm is easily sufficient to account for the storm's electrical energy, but it has not been shown that the convective motions transport charge in a manner and in amounts required to explain the electrification. There are some reports of lightning in clouds whose tops have not reached the 0C level and that therefore cannot contain frozen precipitation (see Moore, 1976a, for a summary). These are called warm clouds, and the occurrence of lightning within them is a phenomenon that needs to be better documented and studied. Warm- cloud lightning appears to be uncommon. however even though warm clouds in tropical climates can be strongly convective and can produce heavy rainfall. This, coupled with the observation that thunderstorms in temperate climates become electrified only when they grow above the 0C level, leads many researchers to consider that warm-cloud electrification is an anom- aly that is explained by a different mechanism than that which electrifies colder clouds. If a precipitation mechanism operates to electrify

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100 storms, negatively charged precipitation and positively charged cloud particles would overlap for some distance above the main negative charge, producing a semineu- tral but segregated reservoir of charge between the main negative- and upper positive-charge regions. This is the situation depicted in Figure 8.1. The existence of such a reservoir was first postulated by Wilson (1920, 1929) to explain the large apparent separation of the positive and negative charges. Net negative charge would be ob- served only at and below the lower boundary of the neu- tral region, and this would partly explain why the main negative charge appears to be distributed horizontally in a storm. The presence of such a reservoir has not been demonstrated by direct observations. But a charge res- ervoir almost certainly exists in some form no matter what the charging process, owing to the large distances and volumes through which charge must be trans- ported. Such reservoirs would provide inertia to the charging process and would help to explain why light- ning often occurs at nearly regular time intervals in a storm. Observations of the nature of the reservoirs would greatly aid our understanding of the charging processes. PARTICLE-CHARGE OBSERVATIONS To sort out how the electrification occurs it is essential to know the charge carried by the different types and sizes of particles in the cloud. Precipitation theories pre- dict that the main negative charge of the storm resides on precipitation particles, and it has been of interest to test this prediction by direct measurement. Such mea- surements have become possible in recent years using instruments that sense the charge on individual precipi- tation particles. The instruments have been used in sev- eral programs since 1978, sometimes in conjunction with particle size measurements, and show that precipi- tation carries a mixture of positive and negative charges (Gaskell et al., 1978; Christian et al., 1980; Marshall and Winn, 1982; Gardiner et al., 1984~. The magni- tudes of the individual charges are relatively large, and their signs are sometimes predominantly negative; but the fraction of charged particles is small, and the in- ferred volume charge densities may or may not be ade- quate to account for the observed electrification. Few measurements have been obtained in the interesting parts of a storm, i.e., at temperatures of less than - 10C and in updrafts. The particle-charge measure- ments are made from aircraft or below balloons and are difficult to obtain. First there are the logistical problems of being in the right place at the right time; then there are experimental problems of measuring weak charges in an icing and strongly electrified environment. As further observations are obtained we can expect PAUL R. KREHBIEL better answers to the question concerning precipitation charge. Marshall and Marsh (1985) recently reported measurements of precipitation charges within the main negative-charge region of a storm in which all the pre- cipitation particles whose charge was great enough to be detected by their instrument were negatively charged, in amounts that appeared to be sufficient to account for the field gradient in the negative-charge region.] Still unknown, however, will be the amounts and sign of charge carried by the large number of smaller particles that coexist with the precipitation but that are below the detection limit of present instruments. Cloud particles have a much greater charge-carrying capacity per unit volume of cloudy air than precipitation particles, and it is important to know how much charge they carry. No good technique exists for doing this in the uncontrolled and hostile environment of an active thunderstorm. The in-cloud observations show that millimeter-size precipitation particles sometimes carry sufficient charge so that the electrical force on them would be comparable with the gravitational force in the strong- field regions of a storm. These particles would be ex- pected to exhibit measurable velocity changes after nearby lightning. But attempts to detect such velocity changes using Doppler radars have been unsuccessful in most instances (Zrnic et al., 1982; Williams and Lher- mitte, 1983~. These results indicate that only a fraction of the precipitation particles are highly charged, in agreement with the in-cloud observations. If the energy considerations mentioned earlier were to require that the precipitation be efficiently charged, these results would indicate that convective motions are important in charging a storm (Williams, 1985~. The fact that ve- locity changes are observed occasionally indicates that precipitation is strongly and efficiently charged at some locations and times. Measurements of the charge on precipitation arriving at the Earth's surface show that it often has the same polarity as the point discharge being given off from the ground. This is the mirror-image effect mentioned ear- lier and indicates that the precipitation charges have been modified by the capture of point discharge ions as the precipitation falls to earth. Below cloud base or in the bases of clouds, precipitation is often observed to be positively charged and occurs in localized regions re- ferred to as lower-positive-charge centers (Simpson and Scrase, 1937; Rust and Moore, 1974; Winn et al., 1981; Marshall and Winn, 1982; Holden et al., 1983~. One explanation for these observations has been that the pre- cipitation captures positively charged cloud droplets while falling through cloud base. However, positively charged precipitation is found well inside the cloud, up to and above the 0C temperature level (Moore, 1976b; Marshall and Winn, 1982~. These observations are not

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 103 draws on a reservoir of charge as discussed earlier. In with the charging current at the beginning of a storm this case the charging current and field buildup would but is less than the average current of cloud-to-round 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 convec tive motions and turbulence the flow of negative screen ing 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 comple tion. 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 con vection 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, in creasing the electrical energy of the storm. An alterna tive possibility is that turbulent mixing folds the screen ing 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 light ning 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 co rona from the ground. The convection hypothesis postu lates 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 pri mary source of the upper positive charge. Other possi bilities 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 elec tric-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 - lightning in the active stage of a storm. Recent experiments designed to test electrification ideas have attempted to influence or alter the electrifi- cation 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 in- fluenced by the direction of the initial electric field in- side the storm (as in the case of an inductive precipita- tion mechanism), such priming should - invert the polarity of the electrification, i.e., produce a storm hav ~ng an upper negative- and main positive-charge struc- ture. The results of the experiments are that storms de- veloping above negative-charge releases are anomalous in that the field at the ground is often dominated by positive charge overhead, which lightning acts to re- move. 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 de- tected 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 ele- ment in furthering our understanding of thunderstorms. A continuing problem in their determination is the rap- idly changing nature of convective storms, which re- quires 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

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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, 19794. Particle velocity measurements provide only a part of the information needed to estimate the electrical cur- rents 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 atmo- sphere 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 af- fected 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 measure- ments have formed the basis of a new approach for esti- mating the storm currents, in which the displacement current associated with a time-varying electric field is added to other measurements of the local corona, con- duction, and rain currents (Krider and Musser, 1982~. The sum of these currents has been termed the Maxwell FIGURE 8.9 Contours of constant dis- placement current density at the ground be- neath a thunderstorm on July 11, 1978 at Kennedy Space Center, Florida. Observa- tions from two 5-minute time intervals are shown; contours are at 0.5 nA/m- intervals. The heavy dashed contour shows the detect- able radar echo at 7.5-km altitude; the x's mark the negative-charge centers of lightning discharges. The areal integral of the displace- ment current was about 0.4 A in each in- stance. (Krider and Blakeslee, 1985.) PAUL R. KREHBIEL 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 in- tegrated 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 dif- ferent 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 thun- derstorm investigations. Lightning is of interest not only as a phenomenon in itself but as an indicator and sig- nificant modifier of the storm's electrification. Light- ning 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 for- mation of precipitation within the storm. But little is known even about what lightning looks like inside a RS~ ~ its 19:28:09 v .. . 19:25:49 N

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THE ELECTRICAL STR UCTURE OF THUNDERSTORMS 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 initi- ated. Measurements of the electric field inside storms give maximum (large-scale) values typically between 1 X 105 and 2 x 105 V/m (e.g., Winn et al., 1974, 1981~. Winn et al. (1974) reported one measurement of 4 x 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 par- ticle surfaces by corona that somehow develops into a full-scale discharge (e. g., Loeb, 1953; Richards and Dawson, 1971; Crabb and Latham, 1974; Griffiths and Phelps, 19764. The manner in which lightning is initiated is an unan- swered and intriguing question, but however this hap- pens 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 in- deed 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 gener- ally 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 estab- lishes 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 in- creases 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 tech- niques for remotely sensing lightning inside a storm. In 105 particular, radio-frequency radiation from the light- ning may be located using one of several direction-find- ing 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 elec- tric-field change at a number of ground locations (Fig ure 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 light- ning 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 dur- ing 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 associ- ated with the electrification of new convective cells in the storm and provide another indication that the elec- trification is associated with vertical growth. The fact that the sequences start above about 8-km altitude re- flects the existence of an altitude threshold for the elec- trification. The discharge rate during the most intense sequence reached 37 per minute. Similar observations have been reported by Lher- mitte 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 dis- charges have been shown to transfer relatively small amounts of charge (Krehbiel et al., 1984b), indicating that the high-rate sequences result from a large number of initiating events rather than from superelectrification

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106 1-2Q. -15. 5, : )IQlt~lllllIll~llItItItItItItItItItItIlll 30. 40. 50. 60. 70. 8Q. 90. 1QU. HEIGHT [KM) (a) PAUL R. KREHBIEL REFLECTIVITY (dBZ) FLASH DENSITY (min~l km~l) _->? O>S =>12 ~>14 - >16 / `, _ _\ ~- 16r 14: 12 10 Q ~ 8 Ill 6 TINE tMINUTES) 313111llIllll1lltIttt~t~tttl /~, - 10 ~ 204~ ' - ~: 30~: ,,~" ~ 10 20 30 40 RANGE (km) (b) I! TIC 50 60 70 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. 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(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 de- termined 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 ex- pected to be most highly branched. The increasing ability of researchers to sense lightning inside thunderclouds has raised questions about the ex- tent to which lightning indicates or reflects the electrifi- cation 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 electrifica- tion from lightning observations, some evidence exists that suggests that lightning can be a reasonable indica- tor 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 nega- tive-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

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS behavior of electrical discharges in plexiglass (Williams et al., 1985~. These laboratory discharges appear to sim- ulate the large-scale behavior of lightning in clouds. High-energy electrons are deposited in a controlled manner within the plexiglass and the resulting space charge is discharged by mechanical disruption at some point on the surface. The dendritic or finely branched structure of the discharges follows the pattern of space charge within the plexiglass, suggesting that real light- ning may do the same in storms. Assuming that lightning tells us something about the electrification, one question of interest has been how the lightning channels and charges are related to precipita- tion in the storm, as revealed by radar. If a precipitation mechanism were operating to electrify the storm, one would expect the lightning and precipitation to be cor- related in some manner. Not surprisingly in phenomena as complex as thunderstorms and lightning (and as com- plicated to study), a wide variety of observations have been found. These range from observations that li~ht- ning and precipitation are correlated (e.g., Larsen and Stansbury, 1974, Krehbiel et al., 1979; Taylor et al., 1983; Figures 8.3 and 8.10b), to observations that light- ing avoids regions of strong precipitation (MacGorman, 1978; Williams, 1985), to other observations of precipi- tation echoes that develop after nearby lightning (Moore et al., 1964, Szymanski et al., 1980~. There is some uncertainty and debate as to what the various ob- servations mean. In the author's opinion, one of the most striking results has been the degree to which the lightning charges correlate with radar echoes from pre- cipitation. LIGHTNING EVOLUTION We finish this review with a brief and simple descrip- tion of how lightning appears to evolve with time in a thunderstorm. This is depicted in Figure 8.11 and pro- vides a framework for understanding some of the wide variety of lightning observations. In addition it gives further insight into the electrical nature of storms. The description is based on a number of different studies and observations of lightning (e.g., Ligda, 1956; Teer and Few, 1974; Krehbiel et al., 1979, 1984a; Krehbiel, 1981; Proctor, 1981, 1983: Rust et al.. 1981 Fuouav 1982; Taylor, 1983~. In response to the dipolar structure of the storm, the initial lightning discharges are usually intracloud flashes that transport charge vertically between the main negative- and upper positive-charge regions (Fig- ures 8.1 la and 8.3) . The first cloud-to-ground discharge usually follows an initial sequence of intracloud flashes (Figures 8.11b and 8.6), but simple CG flashes some 107 times begin the lightning activity. The latter situation occurs presumably because conditions somehow favor the initiation of CG discharges. CG flashes consist of a number of discrete strokes down the channel to ground; the early CG flashes are simple in that they produce one or only a few strokes. The initial lightning activity is associated with the cell having the greatest vertical development in the storm. Other cells do not generate lightning until they develop vertically above 7-8-km altitude MSL (in summertime), even though the subsequent cells may have stronger pre- cipitation echoes within them than within the initial, lightning-producing cell. As additional cells become electrified, the IC flashes remain basically vertical but become broader in hori- zontal extent and can exhibit a pattern of cross-discharg- ing between cells (Figure 8.11c). The CG flashes pro- duce a larger number of discrete strokes whose negative-charge sources progress horizontally through the precipitating part of the storm (Figure 8.11d). For some still-unknown reason, the CG flashes can initiate a continuing current or arc-type discharge down the channel to ground from within the horizontally exten- sive negative charge. The continuing currents can last for a few tenths of a second and produce a persistent luminosity that is sometimes detectable visually. In large storm complexes having a number of cells, the intracloud and cloud-to-ground discharges can have large horizontal extents, corresponding to the horizon- tal dimensions of the storm system. Because the horizon- tat dimensions can be much greater than a storm's vertical dimension, the discharges become primarily horizontal in nature. As the storm grows, its top reaches the base of the stratosphere or is sheared off by high-level winds to form an anvil cloud (Figure 8.1). The anvil cloud is composed of small ice crystals that carry part of the upper positive charge and is penetrated by intracloud discharges from the active region of the storm (Figure 8.11e). The anvil clouds commonly extend tens or hundreds of kilometers downwind from the parent storm. Cloud-to-ground dis- charges have also been observed to emanate from anvil clouds, well away from the active region of the storm. As older ceils dissipate, predominantly horizontal in- tracloud lightning occurs between negative charge in still-active cells and apparent positive charge at about the same level in the dissipating part of the storm (Fig- ure 8.110. In propagating storms the dissipating part trails the active part and can have substantial horizontal extent. The horizontal discharges within them are cor- respondingly extensive and are observed to propagate over distances of 50 to 100 km (Ligda, 1956; Proctor, 1983~. The radar echo from within the dissipating part

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108 If++ '++~ ++ <~_ J l \ \ . \ \ ~ \ . .. . . n ~Y~ _k ' :'++ i. If+ +\\ ~ + +I+t, 1 + \ +\ \\_ \ + \ + + [, + it i.. + . + + + it'd A. 1... a) o SO 5 - V: C) . ,~ 4 - ~:5 ~ ~ V: ~ Ct \ 0,= + :=S if\ ~ ~ \ XO \ . \ \ Cot ~ == \ .=,0 l \ ~ \ >~ ,~ 0 ,5: o En C) C) . - Ct > O O to ~ Ct v: US C~ C) rQ V' o o ~ .= ct ~- ~ ~ ~- ~ - ~ ~ ~o ) o ~ ~ , 'e ~ - o 'e =) ct ~ ~ ~ ~ o 5 D s~ E~ c~i~ ~ - ~ ,.~) ~ b4 ~ oo .= ~ ._ .= ,~ _ ~ V ~ ~

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS of a storm is characterized by a horizontal bright band at and just below the 0C 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 light- ning 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 operat- ing to regenerate the positive charge and that the charg- ing 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 bal- loonborne 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 Height ~ km 7 6 -1 5Oc 4 1 '--/~/////] ///////2 Radar Bright band it+) T- o Oc A//////// (_) //~/ ~ A) Ev . ~_ .L -4G -2C O 20 4C Electric Field, kV/m FIGURE 8.12 Sounding of the vertical electric field in the dissipat- ing part of a large frontal storm in France, indicating the presence of a positive-charge layer just above the 0C level and a negative-charge laxer within the radar brightband from the storm (adapted from Chauzy et al., 1980). 109 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 fur- ther observations are needed to check their validity. 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 con- nection 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 re- veals the upper positive charge and transports it down- ward toward the ground tsee 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 deter- mined. SUMMARY AND CONCLUSIONS The ability of scientists to observe and study thunder- storms has increased greatly over the past decade or two, and this has brought their study to a particularly excit- ing stage. A number of ideas have been proposed over the years to explain how thunderstorms become electri- fied, 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 wa- ter 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 loca- tions 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 precipita- tion at altitudes where the air temperature is colder than 0C. Strong electrification does not occur until the cloud and precipitation develop above a threshold alti- tude that is 7-8 km above MSL in the summer months, corresponding to an air temperature of - 15 to - 20C. The main negative charge resides at and below this alti- tude 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

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110 lions of the storm. Precipitation theories predict that the main negative charge is carried by precipitation parti- cles, and this is being tested by in-cloud measurements of the charges carried by precipitation. Laboratory studies continue to point to rebounding collisions be- tween hail and small ice crystals as a mechanism that can charge precipitation negatively and possibly explain the electrification. This mechanism is expected to oper- ate in storms, but it has not been shown that enough precipitation is present and involved in enough charging interactions to account for the electrification. Com- pounding this difficulty are observations that electrifi- cation 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 obser- vations. 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 com- plexity 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 diffi- culties of studying storms. By the same reasoning, it is possible that the primary electrification mechanism changes once a storm be- comes strongly electrified. For example, precipitation could initiate the electrification, and then the larger convective energies of the storm could continue the elec- trification. Or the electrification could be sustained and enhanced if the corona from precipitation had a system- atic sign. Already there is some evidence that a differ- ent, lower-rate mechanism operates to electrify dissi- pating storms or parts of storms. Our understanding of the electrification processes re- mains limited by the need for better observations of the electrical and physical characteristics of actual thunder- storms. This need has guided thunderstorm research for a number of years and involves several parallel and in- teracting efforts: field programs, data analysis, and the development of observational techniques. Field pro- grams provide the basic experimental data and allow scientists to test new instruments and observational PAUL R. KREHBIEL techniques. Data analysis extracts the scientific infor- mation 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 un- derstanding the electrification processes require fo- cused, cooperative field programs that bring together the best available observational techniques. A substan- tial amount of data is already in hand from recent field programs of this type whose continued analysis will pro- vide further insights into the electrification problem. But too many questions remain unaddressed in the mea- surements 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 exam- ple of this is the recent experiments that have attempted to invert the electrical polarity of a storm. Much of our information about thunderstorm electri- fication has come from the study of relatively small, iso- lated storms such as those that form over the mountains of the southwestern United States or above the sea- breeze convergence in southeastern coastal areas. These storms provide relatively stationary and predictable tar- gets 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 com- pared. In particular, it is important that electrical stud- ies 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 scien- tific observations and issues related to the problem of thunderstorm electrification. Other recent reviews on the same subject have been made by Moore and Vonne- gut (1977), Illingworth and Krehbiel (1981), Latham (1981), Vonnegut (1982), Lhermitte and Williams (1983), Illingworth (1985), and Williams (1985~. An- other 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 electri- fication studies for example, the differential reflectiv- ity polarization radar technique (Bring) 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

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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS storms. In some instances a brute-force approach would also be helpful, for example, in obtaining successive or simultaneous balloon soundings of the electric-field pro- file in a storm. Still other observational needs provide challenging problems for development for example, measurement of the net charge density in storms or of the charge car- ried by small clouc] particles. Even after comprehensive observations have been ob- tainecl on storms, it is entirely possible that the specific mechanism or mechanisms that caused their electrifica- tion will continue to elude precise definition. In the case of a precipitation mechanism, it would be extremely dif- ficult to catch particular charging events "in the act." This would need to be clone in the controlled environ- ment of the laboratory, simulating cloud conditions as closely as possible. Specific areas of interest in present and future laboratory studies are (1) contact electrifica- tion processes at ice surfaces and (2) corona discharges from precipitation. Computational models, both simple and complex, will continue to be useful in interpreting field and laboratory observations and in predicting the ability of particular mechanisms to electrify a storm. Such modeling will rely heavily on parameterizations of observational clata, however, since the electrification results from a cascade of physical processes each of which are inherently difficult to simulate in their own right. In conclusion we note that, although the problems of thunderstorm electrification are difficult and complex, their solution is becoming possible and is a prized goal of scientists. ACKNOWLEDGMENTS The comments and reviews by William Winn, Earle Williams, Don MacGorman, Charles Moore, and Marx Brook significantly improved this review. 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Moore, and J. J. Jones (1985~. The rela- tionship between precipitation and electrical development in New Mexico thunderstorms, EOS 66, 840. Dye, J. E., J. J. Jones, W. P. Winn, T. A. Cerni, B. Gardiner, D. Lamb, R. L. Pitter, J. Hallett, and C. P. R. Saunders (1986~. Early electrification and precipitation development in a small, isolated Montana cumulonimbus, J. Geophys. Res. 91, 1231-1247. Few, A. A. (1985~. The production of lightning-associated infrasonic sources in thunderclouds, J. Geophys. Res. 90, 6175-6180. Fuquay, D. M. (1982~. Positive cloud-to-ground lightning in summer thunderstorms, J. Geophys. Res. 87, 7131-7140. Gardiner, B., D. Lamb, R. Pitter, and J. Hallett (1984~. Measure- ments of initial electric field and ice particle charges in Montana summer thunderstorms, J. Geophys. Res. 90, 6079-6086. Gaskell, W. (1981~. A laboratory study of the inductive theory of thun- derstorm electrification, Q. J. R. Meteorol. Soc. 107, 955-966. Gaskell, W., and A. J. 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