Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 100 storms, negatively charged precipitation and positively charged cloud particles would overlap for some distance above the main negative charge, producing a semineutral 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 observed only at and below the lower boundary of the neutral 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 reservoir almost certainly exists in some form no matter what the charging process, owing to the large distances and volumes through which charge must be transported. Such reservoirs would provide inertia to the charging process and would help to explain why lightning 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 predict 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 measurements have become possible in recent years using instruments that sense the charge on individual precipitation particles. The instruments have been used in several programs since 1978, sometimes in conjunction with particle size measurements, and show that precipitation 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 magnitudes of the individual charges are relatively large, and their signs are sometimes predominantly negative; but the fraction of charged particles is small, and the inferred volume charge densities may or may not be adequate 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 â 10Â°C and in updrafts. The particle-charge measurements 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 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 precipitation 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 strongfield regions of a storm. These particles would be expected 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 Lhermitte, 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 velocity 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 earlier 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 referred 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 precipitation 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 0Â°C temperature level (Moore, 1976b; Marshall and Winn, 1982). These observations are not
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 101 simply explained and pose an obvious problem for precipitation theories of electrification, since it is necessary to explain how the sign of the precipitation charge could be reversed within a relatively short distance of (or even inside) the main negative-charge region. Several ideas have been proposed to explain the positive charge on precipitation in the interior of a storm. (Not all of these are aimed at salvaging the precipitation hypothesis.) One is based on laboratory observations that the charge transferred during collisions between hail pellets and ice crystals changes sign at temperatures warmer than about â 10Â° C (e.g., Takahashi, 1978; Gaskell and Illingworth, 1980; Jayaratne et al., 1983). The reversed, positive charging is somewhat stronger than the negative charging observed at lower temperatures. This explanation may be correct, but it becomes more difficult or complicated for similar theories to explain the main negative charge as well. Specifically, it becomes necessary to explain why the main negative charge is laterally extensive and a major reservoir of charge in the storm, while the lower positive charge is more pointlike and a lesser reservoir of charge. Another explanation for the lower-positive-charge centers is that they are caused by lightning (Marshall and Winn, 1982; Holden et al., 1983). Lightning flashes do not neutralize the storm charge on a fine scale, as the term discharge may imply. Rather, they deposit positive charge along a finely branched structure in or near the main negative-charge region of the storm (Vonnegut and Moore, 1960). If some of the lightning tendrils terminated on precipitation particles, these particles would tend to be left with a positive charge after the lightning regardless of any initial charge that they might have carried. This explanation would account for the localized nature of the lower- positive-charge centers and could operate in storms. But recent observations have shown that positively charged precipitation is present in storms before the first lightning (T. C. Marshall, University of Mississippi, private communication). Similar findings can be inferred from the observations of Simpson and Scrase (1937) and probably from other studies of non-lightning-producing storms as well. In this event another mechanism must be operating to charge the precipitation positively, and a lightning mechanism is not necessarily required. A third possibility is that an inductive process might operate to charge the precipitation positively (e.g., Chiu, 1978). An inductive mechanism differs from the noninductive hail/ice-crystal mechanism described earlier in that the charge transferred during rebounding collisions depends on the strength and direction of the ambient electric field. In- cloud observations and laboratory studies have caused the inductive theory to lose favor (Gaskell et al., 1978; Christian et al., 1980; Gaskell, 1981), but it would be premature to rule out inductive charging altogether given our general inability to explain the various electrification processes. In evaluating observations of precipitation charge it is important to differentiate between measurements made before and those made after the onset of lightning. Lightning drastically complicates the detailed distribution of charge in a storm, to the point that the particle charges may not reflect the processes that caused the storm to become electrified. This occurs for several reasons. First, the lightning does not neutralize the storm charge on a fine scale, as discussed above, but deposits its charge along a myriad of channels and streamer paths and on particular particles. Second, the lightning subjects the cloud to large electric stresses by virtue of the fact that its channels extend across millions of volts of electric potential. The stresses undoubtedly cause transient corona discharges from particles in the vicinity of the lightning channels, which effectively erase the original charges and leave the particles with an unrelated residual charge (Dawson, 1969; Griffiths and Latham, 1972; Griffiths, 1976). Laboratory studies have shown that the residual charges are large in magnitude and variable in sign. Both the nonuniform charge deposition and corona effects would cause particles near the lightning channels to become strongly charged and would tend to leave other particles unaltered, giving rise to a mixture of particle charges, as is observed. Lightning also generates vast numbers of ion pairs, some of which will be separated in the strong electric field to increase temporarily the electrical conductivity of the cloud. This could discharge particles that are charged, or randomly charge other particles, further complicating matters. In addition to complicating the charge picture, lightning-induced or other corona could sustain or enhance the electrification of a storm if the corona from precipitation had a systematic sign (Dawson, 1969; Krehbiel, 1984). Laboratory studies have shown that corona from liquid surfaces is preferentially positive above 4-5 km altitude in the atmosphere and becomes more intense at lower pressures (higher altitudes) (Dawson, 1969). Such corona would result in positive ions and negatively charged precipitation. The positive ions would attach to nearby cloud droplets, producing a segregated distribution of negative precipitation and positive cloud particles similar to that of a collisional charging process. Systematic positive corona could occur, for example, from the liquid surfaces of wet or riming hail or from the bottom surfaces of (liquid or frozen) precipitation above the negative-charge region. It has been proposed that the strong and uneven charging that occurs in the vicinity of lightning channels could enhance the rate of precipitation formation after a