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.
POSITIVE CLOUD-TO-GROUND LIGHTNING 41 3 Positive Cloud-to-Ground Lightning W. David Rust NOAA National Severe Storms Laboratory INTRODUCTION 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 observations but also because they are responsible for most death and damage caused by lightning. There are various common names for several visually different features of CG lightning, e.g., streak, forked, and ribbon. However, the parameters often used in the scientific literature 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 documentation 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 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 devastating nature; they had larger peak currents, charge transfer, action integral, and duration than most negative 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 transients, and thunder. Photographic and television recordings are also often made. In addition, one type of modern automatic lightning-strike locating system is being 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
POSITIVE CLOUD-TO-GROUND LIGHTNING 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 thunderstorms. 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 reported 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 quantitative information on charge transfer for naturally occurring + CG flashes. From their multistation network of electrostatic field-change sensors, they calculated charge transfer and the magnitude of continuing current. As in Berger's earlier work on + CG flashes triggered 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 significantly 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 suggests that if a storm has its upper positive charge displaced 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 severe storms has been verified in the observational program 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 produce 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 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 observed 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 supercells and in squall-line storms. Preliminary analysis of two squall lines indicates that + CG flashes often occur 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 analyzed 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). Doppler radar data show that these + CG flashes were imbedded 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 kilometers along the back side of the squall line before coming 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). Figure 3.1 Sketch of observed locations and polarities of CG flashes from severe thunderstorms. The spiral denotes the updraft region 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).