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2 INTRODUCTION Positive Cloud-to-Grounc! Lightning W. DAVID RUST NOAA National Severe Storms Laboratory Of the two common types of lightning flash, cloud-to- ground (CG) and intracloud (IC), the CG flashes have historically received more attention and study. This is undoubtedly because they not only have more visible channels that lend themselves to quantitative observa- tions but also because they are responsible for most death and damage caused by lightning. There are vari- ous common names for several visually different fea- tures of CG lightning, e.g., streak, forked, and ribbon. However, the parameters often used in the scientific lit- erature to categorize CG lightning are the polarity of charge lowered, the magnitude of the current, and the direction of propagation of the initiating leader and/or ensuing return stroke. While most CG flashes transfer negative charge from the cloud to the ground, early doc- umentation of flashes that lower positive charge from cloud to ground ~ + CG) is found in Berger's classical study (1967) of lightning at Mount San Salvatore above Lake Lugano in Switzerland. Berger used instruments and photography to document strikes to towers on the mountain and found a minority of + CG flashes. All but one of the + CG flashes had an upward-propagating leader followed by a downward-moving return stroke, altogether opposite to the common negative CG flashes, consisting of downward-propagating leaders followed 41 by upward return strokes. Because these + CG flashes originated from tall towers, and apparently not even from the mountain peaks without tall structures, they have been termed "triggered" lightning, in contrast to those occurring naturally. They nearly always had only one return stroke. Berger's documentation of + CG flashes to Mount San Salvatore established their devas- tating nature; they had larger peak currents, charge transfer, action integral, and duration than most nega- tive flashes. In general the instrumentation used to observe + CG flashes is the same as that for other lightning. Typical measurements include the electrostatic-field change for the entire flash, fast electric- and magnetic-radiation- field wave forms for the return stroke, optical tran- sients, and thunder. Photographic and television re- cordings are also often made. In addition, one type of modern automatic lightning-strike locating system is be- ing used experimentally in several parts of the world to locate + CG flashes. However, its ability to distinguish between intracloud and + CG flashes and its resulting detection efficiency are still unknown. RECENT AND ONGOING STUDIES The recent and renewed interest in + CG flashes has been stimulated by observations of winter storms in Ja
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42 pan, where there is a large percentage (although low total number) of CG flashes that are positive but not triggered by tall structures. These storms have low cloud bases and cloud tops at about 5 km and are much colder in their lower region than typical summer thunder- storms. Although they produce fewer flashes, there have been several reports of strikes to aircraft. Whether the strikes to aircraft were from + CG flashes is unknown, but the prevalence of + CG flashes in these storms has led to questions concerning their correlation to storm structure and their relevance to aviation safety. The existence of + CG flashes in Japan was first re- ported by Japanese scientists, who have since conducted collaborative research programs with scientists from the United States and Sweden. The recently published results by Brook et al. (1982) include the best quantita- tive information on charge transfer for naturally occur- ring + CG flashes. From their multistation network of electrostatic field-change sensors, they calculated charge transfer and the magnitude of continuing cur- rent. As in Berger's earlier work on + CG flashes trig- gered by tall towers, the naturally occurring + CG flashes in Japan have large currents. While the largest negative flashes are comparable with + CG flashes, it appears that + CG flashes as a group tend to have signif- icantly greater charge transfer and currents. The combined data from seven Japanese winter storms show a remarkably good correlation between the percentage of + CG flashes per storm and the vertical shear in the horizontal wind in the cloud layer. This sug- gests that if a storm has its upper positive charge dis- placed horizontally from its lower negative charge, the production of + CG flashes is facilitated. This may well hold true for such storms that are relatively shallow and only mildly convective; preliminary results from other investigations indicate that the correlation may not be universal. Studies of this are in progress elsewhere. The finding of naturally occurring + CG flashes in Japan inspired the search for these flashes in the highly sheared, large, and often severe springtime storms over the Great Plains of the United States. The occurrence of + CG flashes during the mature and later stages of se- vere storms has been verified in the observational pro- gram at the National Severe Storms Laboratory (NSSL) in Oklahoma (Rust et al., 1981a). Shown in Figure 3.1 is a sketch of an isolated supercell thunderstorm, which contains an intense updraft, wind shear, turbulence, a large anvil, and a mesocyclone (rotation that can pro- duce a tornado). Flashes to ground in the regions of heavy precipitation have always been observed to lower negative charge, while those from the upshear and downshear anvil near the main storm tower can lower either polarity of charge. Only a relatively few CG W. DAVID RUST flashes have been observed to emerge from beneath the mesocyclone wall cloud (i.e., the visible manifestation of a mesocyclone; see Figure 3.1), and two have been documented as positive. Rust et al. (1981b) also ob- served and obtained electric-field change records for 16 + CG flashes in 30 minutes from a downshear anvil, well away from the main tower of a severe storm shortly after it produced a wall cloud. No other flashes to ground were seen from the anvil during that time. In most cases, + CG flashes appear visually to emanate from high in the storm. The + CG flashes can cluster in time and dominate CG activity for certain periods. Positive CG flashes are observed both in isolated su- percells and in squall-line storms. Preliminary analysis of two squall lines indicates that + CG flashes often oc- cur on the back side (relative to squall-line movement) after the squall line has been in existence for several hours. Acoustic recordings of thunder have been ana- lyzed for two of six + CG flashes that were detected within an 8-minute period in the back side of such a squall line. They show a significant number of acoustic sources, and thus channels, throughout a depth of 15 km (the freezing level was about 4 km above ground). Dop- pler radar data show that these + CG flashes were im- bedded in the low radar reflectivities (less than 17 dBZ) associated with very light precipitation behind the squall line. Visual observations indicate that +CG flashes may propagate horizontally through tens of kilo- meters along the back side of the squall line before com- ing to ground. Relatively large horizontal extent is also apparent in observations of a few confirmed + CG flashes during the final stage in some of the smaller thunderstorms over the Rocky Mountains where usually there are no or only a few + CG flashes (Fuquay, 1982~. -CG AND +CG FROM BACK O: MAIN TOWER ~\ ,4; ~ 1 Cam WALL CLOUD- +CG FROM DOWNSHEAR ANVIL FIGURE 3.1 Sketch of observed locations and polarities of CG flashes from severe thunderstorms. The spiral denotes the updraft re- gion and rotation (mesocyclone). Only negative CG flashes have been observed in the intense precipitation. The horizontal scale of this storm sketch is greatly compressed. After Rust et al. (1981a).
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POSITIVE CLOUD-TO-GROUND LIGHTNING The apparent correlation between + CG flashes and storm development and severity suggested above are quite tentative. This is due to the paucity of + CG flash data, caused in part by the relative infrequency of + CG flashes and in part by the need for corrobative verifica- tion of the occurrence of each + CG flash until simpler detection techniques are proven. Nowhere is this more obvious than in the large data bases recently acquired with a few of the automatic strike-locating systems that have been modified but as Vet unproved (as stated by the manufacturer, Lightning Location and Protection, Inc.) for detection of positive as well as negative ground flashes. If we had confidence in the data bases obtained with these systems, much could readily be learned, owing to the large numbers of supposed + CG flashes that have and continue to be re- corded. Samples of results from these as yet unproven systems include the following: 1. Orville et al. (1983) reported a case study of a cy- clone that produced several convective cells that moved through their East Coast strike-locating network. They found that while only 4 percent of all flashes to ground in the storm are identified as + CG flashes, the percent- age increased to 37 percent in the last hour of significant CG flash activity. The also reported observations of a higher percentage of + CG flashes in the later stages of other storms. 2. The NSSL strike-Iocating system has been used to study the diurnal variation of + CG flashes for summer- time storms in Oklahoma, which tend to be less severe than storms in the spring. The fraction of CG flashes that were positive, averaged for 1 month, peaks about 2 hours later than the total CG flash activity. 3. Attempts have been made to ascertain if the + CG flashes observed in severe storms are related to storm severity or tornado occurrence. Two tornadic storms have been analyzed using the strike-locating system in Oklahoma. The ratio of + CG to all CG flashes appears to be greater before and during tornadoes than after- ward. This result is preliminary, not only because of un- certainties in the performance of the strike locating sys- tem but also because only two tornadic storms have been analyzed. It also remains to be shown just what severe storm parameters are related to the production of + CG flashes. Some possibilities that are being examined are mesocyclone strength, updraft speed, shear, and precip- itation structure. PHYSICAL CHARACTERISTICS Although a few of the characteristics of + CG flashes have been described above, it is worth considering what 43 we know in total about their characteristics, irrespective of the storm conditions in which they occur. There are two characteristics that appear consistently in all the re- ported observations of + CG flashes: (1) the vast major- ity have only a single return stroke, and (2) the return stroke is often followed by continuing current. A repre- sentative electrostatic field change for a + CG flash is shown in Figure 3.2. Before the return stroke, there is usually lengthy preliminary activity, which averages about 0.25 see but can be as long as about 0.8 sec. If in- cloud channels for + CG flashes are primarily horizon- tal, as suggested by observations in several locations, and if progression speeds are 105 m/see as typically ob- served, then there may be large horizontal extent to many + CG flashes. Indeed, horizontal movement of + CG flashes before they come to ground has been de- termined from analysis of multistation field-change data in Florida (Brook et al., 1983) and is indicated also by visual observations of squall lines in Oklahoma. The field change for the leader to ground has not yet been extensively studied; however, both multistation analysis for several flashes in Japan and photographic evidence for a few flashes in the Rocky Mountains and Oklahoma show that the leader propagates down from the cloud to the ground, in contrast to the initially up- ward-moving, triggered flashes to Mount San Salva- tore. Recent studies in Japan indicate that + CG flashes can be preceded by either a stepped or a nonstepped leader. The return-stroke wave form (Figure 3.3) is similar in shape to that for negative flashes (Rust et al., 1981b), with a relatively slow initial ramp followed by a faster R T 2 kV/m 1: I ~C I l 0.5 - o 2115:37.27 I.Os FIGURE 3.2 Typical electric-field change for + CG flash, recorded at 2115:37.270 CST on May SO, 1982. The distance to the flash was about 4 km. Time at the center of the scale (at 0.5 see) is 2115:37.300 CST. Interval a is the preliminary breakdown and leader; b is the time from the return stroke through the end of the continuing current seen on the streak photograph in Figure 3.4: and c is a larger interval of possible continuing current or additional intracloud breakdown. The return stroke (see Figure 3.3) is labeled R.
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44 ~ x H}H ,~ 1 1 1 1 1 0 25 50 75 1 00 As FIGURE 3.3 Electric-field wave form for return stroke of the + CG flash in Figure 3. 2. The slow ramp, interval x, is followed in interval y by ~ faster transition to peak, which is typical of both negative and positive return strokes. transition to peak. Thus far, the wave forms obtained in several widely separated locations are essentially the same. For 15 visually confirmed + CG flashes in Okla- homa, the average zero-to-peak rise time is 6.9 ,usec. In Florida, the average for three visually observed + CG flashes at distances of 20-40 km is about 4 ,usec, a value comparable with negative flashes in the same storm. In both locations, some + CG flashes have been observed with fast transition portions of the wave form having rise times of less than 1 ,usec. While there was no visual or photographic documentation, apparent + CG flashes in Sweden have yielded zero-to-peak times of 5- 25 User for flashes at ranges of approximately 100 km; the mean zero-to-peak times for + CG flashes was re FIGURE 3.4 Streak-film photograph of + CG flash recorded on May SO, 1982 (see Figures 3.2 and 3.3). Continuing current is evident from the smearing of luminosity. It is visible in the photograph for about 60 msec and occurs during interval b in Figure 3. 2. W. DAVID RUST ported to be about twice that for negatives. The peak amplitude of the electrostatic-field change due to the return stroke itself averages about one tenth of the total change for the entire flash. This appears due to the large preliminary breakdown and continuing current, which dominate the field change. Analysis of multistation electric-field change mea- surements by Brook et al. (1982) reveal + CG flashes with continuing currents up to 105 A and positive charge transfer to ground of up to several hundred coulombs. Nakahori et al. (1982) made direct measurements of currents in + CG flashes and found peak stroke currents of 31 kA and total charge transfer of 164 coulombs in one flash. The largest magnitudes of charge transfer are of- ten more than 10 times greater than these for negative flashes to ground in summer storms. The duration of continuing currents in + CG flashes has been reported to vary from a few milliseconds to about 250 msec. However, the longer durations were obtained from single-station field-change measure- ments. The few streak-film and TV recordings of con- tinuing current obtained thus far indicate that later por- tions of the slow field change may not always be from continuing current in the channel to ground but may be additional intracloud activity. For example, the streak photograph in Figure 3.4 (for the field change in Figure 3. 2) indicates a continuing current duration of 60 msec, but Figure 3.2 alone could be interpreted as indicating at least 200 msec of current flow. Either the luminosity decreased below the threshold for the film, or the cur
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POSITIVE CLOUD-TO-GROUND LIGHTNING rent in the channel ceased and the remaining activity was intracloud. PRACTICAL IMPLICATIONS The destructive nature and practical importance of + CG flashes to the electrical power industry are at least partially documented. The multiline grounding and wire-strand fusing in high-voltage transmission lines cannot be explained by normal negative flashes to ground in Japan. Because of the usual occurrence of continuing cur- rent, + CG flashes may ignite a disproportionate num- ber of fires, especially in grasslands and forests. Of the 75 + CG flashes reported in the Rocky Mountain study, all had field changes indicative of continuing current. The apparent pattern is for + CG flashes to strike out- side the rainfall, further enhancing the likelihood of their starting a fire. One third of all storm days in a 3- year period had + CG flashes within a 30-km radius of the U.S. Forest Service observing site at Missoula, Mon- tana. Thus on any given day the fire-starting probability from + CG flashes in mountain thunderstorms appears significant. The results to date of the observations in severe storms suggest that + CG flashes may be correlated with storm severity and tornado occurrence. If this hypothesis were shown to be true, it would enhance existing severe- storm detection and warning capabilities. Additional observations to test this hypothesis are in progress. An area of increasing importance and study is the ef- fect of lightning on new-generation aircraft (see Chap- ter 5, this volume). The relevance of + CG flashes to this problem is currently unknown; however, the reports of strikes to aircraft in the winter storms in Japan suggest that + CG flashes were involved. There are several as- pects of + CG flashes needing additional clarification to ascertain whether these flashes pose an unusual threat to aircraft. They include the presence of fast return-stroke rise times and large peak currents; the frequent occur- rence of continuing current; and the apparent ten- dency, especially in squall lines, for + CG flashes to propagate through large horizontal and vertical extents and to be in low radar reflectivity regions, which can appear "innocent" on radar. Because + CG flashes are a small percentage of the total flashes that most storms produce, appear to have large spatial extent, and do not seem to have a unique electric-field change, it is difficult to verify + CG flashes. However, because of their potentially devastat- ing nature and their possible link to storm severitv~ + CG flashes have become an important research topic in several parts of the world. 45 AREAS NEEDING ADDITIONAL RESEARCH It is worth noting those general areas of research that are needed to increase our understanding of + CG flashes and our ability to determine and cope with their effects on important technologies. These research areas are listed below but not in order of priority. 1. Measure electric-field changes and wave forms for a large number of confirmed + CG flashes to establish their typical characteristics with greater certainty. 2. Determine what storm types and environmental conditions are conducive to + CG flashes for storms throughout the year. 3. Relate the production of + CG flashes to storm evolution and structure, including also flash initiation and propagation characteristics. 4. Determine typical and extreme peak currents. 5. Evaluate the capabilities of automatic strike-lo- cating systems in identifying + CG flashes, especially their detection efficiency and false identification rate. 6. Determine the significance of + CG flashes to avi- ation, especially to new-generation aircraft (typified by composite structures and computer-controlled flight). 7. Determine the importance of the threat posed by + CG flashes to power distribution systems, including whether they are the cause of the unexpected large num- ber of faults on power lines in various parts of the United States. REFERENCES Berger, K. (1967). Novelobservationsoflightning discharges: Results of research on Mount San Salvatore, J. Franklin Inst. 283, 478-525. Brook, M., M. Nikano, P. Krehbiel, and T. Takeuti (1982~. The elec- trical structure of the Hokuriku winter thunderstorms, J. Geophys. Res. 87, 1207-1215. Brook, M., P. Krehbiel, D. MacLaughlan, T. Takeuti, and M. Na- kano (1983). Positive ground stroke observations in Japanese and Florida storms, in Proceedings in Atmospheric Electricity, L. H. Ruhnke and J. Latham, eds., A. Deepak Publishing, Hampton, Va., pp. 365-369. Fuquay, D. M. (1982). Positive cloud-to-ground lightning in summer thunderstorms, J. Geophys. Res. 87, 7131-7140. Nakahori, K., T. Egawa, and H. Mitani (1982). Characteristics of winter lightning currents in Hokuriku district, manuscript 82WM205-3, IEEE Power Engineering Society 1982 Winter Meet- ing (Jan. 31-Feb. 5, 1982, New York). Orville, R. E., R. W. Henderson, and L. F. Bosart (1983). An East Coast lightning detection network, Bull. Am. Meteorol. Soc. 64, 1029-1037. Rust, W. D., W. L. Taylor, D. R. MacGorman, and R. T. Arnold (1981a). Research on electrical properties of severe thunderstorms in the Great Plains, Bull. Am. Meteorol. Soc. 62, 1286-1293. Rust, W. D., D. R. MacGorman, and R. T. Arnold (1981b). Positive cloud-to-ground lightning flashes in severe storms, Geophys. Res. Lett. 8, 791-794.
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