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THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 97 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 electrification 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 growing 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 â 20Â°C. Kasemir and Cobb (see Cobb, 1975; Illingworth and Latham, 1977) reported a similar threshold effect from aircraft measurements near the tops of Florida clouds; electric- field values of 1 kV/m were not detected until the radar echo top grew above about â 5Â°C. A recent set of observations that illustrate the onset of electrification in a storm is presented in Figure 8.7 [J. 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 instrumented aircraft penetrating the cloud at 7-km altitude (â 15Â°C) 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 indicated weak electrification starting at about 12:31, probably associated with the earlier convective surge at 12:25. The sailplane spiraled upward in the storm updraft and measured 1 kV/m maximum fields by 11:40 at 6-km altitude. The field appeared to originate from negative charge in nearby precipitation of 40-dBZ reflectivity (Dye et al., 1985). The first lightning discharge occurred at 12:44 when the echo top had reached 10-km altitude. By this time, moderately strong (40-dBZ) echoes had developed up to 8-km altitude and were beginning 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 importance 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 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 atmosphere and is usually preceded by a succession of convective surges or turrets before one or more of these succeed in penetrating the stable layer. The example also indicates that moderately strong precipitation had developed 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 â 10Â°C. 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. Historically, observations have led many scientists to assume or favor the precipitation explanation, and the recent radar and electrical observations described above continue to fuel this idea. The temperature values at which electrification is observed have caused many researchers to focus on frozen precipitation as a primary agent in the electrification process. Other observations, discussed below, have raised questions about precipitation 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 explain how thunderstorms become electrified. Precipitation theories hypothesize that the relatively large precipitation particles acquire negative charge, in most cases by colliding with or shedding smaller cloud particles. The cloud particles acquire a corresponding positive 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,
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 98 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 either small or of the same sign as that on the cloud particles. 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 â 20Â°C) 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).] In-cloud observations at the level of the main negative 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
THE ELECTRICAL STRUCTURE OF THUNDERSTORMS 99 hail. The precipitation particles coexist with a large number of small, unfrozen cloud droplets that are carried above the 0Â°C 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 sufficiently low temperature.) Riming is the dominant growth process of graupel and dry hail and entraps significant amounts of air, giving the particles a milky appearance. The riming process is also responsible for the dangerous ice loads that aircraft develop in flying through convective clouds above the 0Â°C 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 extents, 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., Reynolds et al., 1957; Takahashi, 1978; Gaskell and Illingworth, 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., Latham, 1981; Illingworth, 1985). But the laboratory-observed charging is able to account for the observed electrification only when the precipitation rates are high, on the order of 30 mm/h, and when the ice crystals are relatively abundant, 10-50 per liter or more (e.g., Illingworth, 1985; Williams, 1985). A precipitation rate of 30 mm/h 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 electrification 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 observations of New Mexican storms have indicated that they can become electrified when their radar echoes are weakerâ33 to 35 dBZ or perhaps less (Moore, 1963; Holmes et al., 1977). These echo strengths correspond to frozen precipitation rates of about 10 mm/h or less. While precipitation rates can be estimated remotely using 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. 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 locations of interest. The above discussion points to a central issue of thunderstorm studies, namely, whether sufficient precipitation is present and involved in enough charging interactions to account for the initial electrification. There has been much discussion of this issue in the scientific literature (e.g., Moore, 1976a, 1976b, 1977; Mason, 1976; Illingworth and Latham, 1977; Illingworth, 1985; Williams, 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 precipitation charging mechanism. (These are cited at the end of the preceding section.) Recent results from these studies indicate that the electric fields inside the cloud appear to originate from regions of stronger radar reflectivity at the negative-charge level and indicate negative charge in those regions (Dye et al., 1986). But observations in already-electrified storms show that the electrification is more widespread than the strong precipitation echoes (Krehbiel, 1981; Winn et al., 1981; Weber et 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 electrification 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 convection theories of electrification. The convective energy 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 0Â°C 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. Warmcloud 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 0Â°C level, leads many researchers to consider that warm-cloud electrification is an anomaly that is explained by a different mechanism than that which electrifies colder clouds. If a precipitation mechanism operates to electrify