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POSITIVE CLOUD-TO-GROUND LIGHTNING 43 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 verification 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 yet 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 recorded. Samples of results from these as yet unproven systems include the following: 1. Orville et al. (1983) reported a case study of a cyclone 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 percentage 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-locating system has been used to study the diurnal variation of + CG flashes for summertime 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 afterward. This result is preliminary, not only because of uncertainties in the performance of the strike locating system 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 precipitation structure. PHYSICAL CHARACTERISTICS Although a few of the characteristics of + CG flashes have been described above, it is worth considering what 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 reported observations of + CG flashes: (1) the vast majority have only a single return stroke, and (2) the return stroke is often followed by continuing current. A representative 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 sec but can be as long as about 0.8 sec. If incloud channels for + CG flashes are primarily horizontal, as suggested by observations in several locations, and if progression speeds are 105 m/sec as typically observed, then there may be large horizontal extent to many + CG flashes. Indeed, horizontal movement of + CG flashes before they come to ground has been determined 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 upward- moving, triggered flashes to Mount San Salvatore. 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 Figure 3.2 Typical electric-field change for + CG flash, recorded at 2115:37.270 CST on May 30, 1982. The distance to the flash was about 4 km. Time at the center of the scale (at 0.5 sec) 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.
POSITIVE CLOUD-TO-GROUND LIGHTNING 44 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 Oklahoma, the average zero-to-peak rise time is 6.9 Âµsec. In Florida, the average for three visually observed + CG flashes at distances of 20-40 km is about 4 Âµsec, 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 Âµsec. While there was no visual or photographic documentation, apparent + CG flashes in Sweden have yielded zero-to-peak times of 5-25 Âµsec for flashes at ranges of approximately 100 km; the mean zero-to-peak times for + CG flashes was reported 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. 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 a faster transition to peak, which is typical of both negative and positive return strokes. Analysis of multistation electric-field change measurements 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 often 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 measurements. The few streak-film and TV recordings of continuing current obtained thus far indicate that later portions 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 Figure 3.4 Streak-film photograph of + CG flash recorded on May 30, 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.