Daohong Wang

Observed leader and return-stroke propagation characteristics in the bottom 400 m of a rocket-triggered lightning channel

Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 m of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dart-stepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10-90% risetime ranging from 0.3 to 0.8 μs with a mean value of 0.5 μs and a half-peak width ranging from 0.9 to 1.9 μs with a mean of 1.3 μs. The interpulse interval ranges from 1.7 to 7.2 μs with a mean value of 4.6 μs. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9×10 7 to 1.0×10 8 m/s with a mean value of 6.7×10 7 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8×10 7 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leader-conditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 m or so of the channel are 1.3×10 8 and 1.5×10 8 m/s for the two events analyzed, with a potential error of less than 20%.

Initial stage in lightning initiated from tall objects and in rocket‐triggered lightning

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

Attachment process in rocket-triggered lightning strokes

In order to study the lightning attachment process, we have obtained highly resolved (about 100 ns time resolution and about 3.6 m spatial resolution) optical images, electric field measurements, and channel-base current recordings for two dart leader/return-stroke sequences in two lightning flashes triggered using the rocket-and-wire technique at Camp Blanding, Florida. One of these two sequences exhibited an optically discernible upward-propagating discharge that occurred in response to the approaching downward-moving dart leader and connected to this descending leader. This observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lightning strokes, these strokes being similar to subsequent strokes in natural lightning. The observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader, a length of 7–11 m, and a duration of several hundred nanoseconds. The speed of the upward connecting discharge was estimated to be about 2 × 107 m/s, which is comparable to that of the downward dart leader. In both dart leader/return-stroke sequences studied, the return stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions. This latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader/return-stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge. The length of the upward connecting discharges (observed in one case and inferred from the height of the return-stroke starting point in the other case) is greater for the event that is characterized by the larger leader electric field change and the higher return-stroke peak current. For the two dart leader/return-stroke sequences studied, the upward connecting discharge lengths are estimated to be 7–11 m and 4–7 m, with the corresponding return-stroke peak currents being 21 kA and 12 kA, and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kV/m and 43 kV/m. Additionally, we note that the downward dart leader light pulse generally exhibits little variation in its 10–90% risetime and peak value over some tens of meters above the return-stroke starting point, while the following return-stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lightning channel. Our findings regarding (1) the initially bidirectional development of return-stroke process and (2) the relatively strong attenuation of the upward moving return-stroke light (and by inference current) pulse over the first some tens of meters of the channel may have important implications for return-stroke modeling.

Characterization of the initial stage of negative rocket‐triggered lightning

We performed a statistical study on the initial stage (IS) of negative rocket-triggered lightning using 37 channel-base current recordings obtained during the summer of 1994 at Fort McClellan, Alabama, and during the summers of 1996 and 1997 at Camp Blanding, Florida. The IS can be viewed as composed of an upward positive leader (UPL) followed by an initial continuous current ( ICC ). The IS has a geometric mean (GM) duration of 279 ms and lowers a GM charge of 27 C to the ground. The average IS current in an individual lightning discharge varies from a minimum of 27 A to a maximum of 316 A with a GM value of 96 A for the entire sample of 37 discharges. We examined the current variation at the beginning of the IS in 24 flashes. In 22 out of 24 cases this initial current variation (ICV) includes a current drop, probably associated with the disintegration of the copper triggering wire and the subsequent current reestablishment. The GM time interval between the onset of the initial stage and the abrupt decrease in current is 8.6 ms, and the GM current level just prior to the current decrease is 312 A, a value about 3 times the GM value of average current for the whole IS, 96 A. Before this abrupt current decrease, a GM charge of 0.8 C has been lowered to ground with a corresponding GM action integral of 110 A2 s. The abrupt current decrease takes typically several hundred microseconds and is followed, immediately or after a time interval up to several hundred microseconds, by a pulse with a typical peak of about 1 kA and a typical risetime of less than 100 μs. The ICC usually includes impulsive processes that resemble the M processes observed during the continuing currents that follow return strokes in both natural and triggered lightning. We present statistics for the following parameters of current pulses superimposed on the ICC: magnitude, risetime, half-peak width, duration, charge transferred, preceding continuous current level, interpulse interval, and time interval between the onset of the IS and the first ICC pulse. The observed characteristics of ICC pulses varied significantly among the three data sets. For all data combined, the characteristics of the ICC pulses are similar to those of the M-component current pulses studied by Thottappillil et al [1995]. This latter finding suggests that ICC impulsive processes are of the same nature as M processes.

Spatial and temporal properties of optical radiation produced by stepped leaders

The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 × 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 m from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108 m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5–11.2×105 m/s, a step interval of 5–50 μs, and a step length of 7.9–19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14–1.7×108 m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9–5.8×105 m/s, 18–21 μs, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.

Experiment of artificially triggering lightning in China

Triggering lightning experiment with rocket—wire technique was conducted in Gansu, northwestern China. Ten cases of triggered lightning were obtained in three summers from 1989 to 1991. They were all with positive electric field at ground (positive charge overhead) under the influence of dominant lower positive charge of Gansu thundercloud. Triggered lightnings were weak discharges initiated by upward moving negative leaders followed by continuous current processes. No dart leader—return stroke processes were observed. The discharge lasted about several tens of miliseconds with a peak current of only a few hundred amperes. Simple physical models are used to evaluate the velocity of leader propagation, magnitudes, and altitudes of the charge transferred by triggered lightnings. Results are consistent with the features of lower positive charge of Gansu thunderstorms.

Electric fields of initial breakdown in positive ground flash

Pulse trains superimposed on the initial part of the electric field change of positive ground flashes are investigated. The characteristics of these pulse trains are found to be different from those of negative ground flashes and cloud flashes. Each pulse included in a pulse train appears as a bipolar waveform with pulse width from 5 to 52 μs and a mean of 18 μs. The time intervals between successive pulses are 10 to 180 μs with a mean of 54 μs. For each bipolar pulse, the ratio of the peak amplitude of the initial polarity to the maximum overshoot amplitude ranges from 0.4 to 4.7 with a mean value of 1.3. The ratio of maximum peak amplitude of successive pulses to the amplitude of the following return-stroke peak ranges from 0.02 to 1.9 with a mean of 0.27.

Lightning‐initiator type of narrow bipolar events and their subsequent pulse trains

Previous observations show that some narrow bipolar events (NBEs) can initiate intracloud discharges, but the role of NBE as lightning initiation is still unclear. During the summer of 2013, 827 NBEs were detected with a 3-D LF lightning location system in Osaka, Japan. Out of 638 positive NBEs, 103 occurred as the initial events of lightning flashes. These initiator-type NBEs, called “INBEs” in this paper, are always followed by positive pulse trains whose locations show upward propagations probably from the main negative charge region to the upper positive charge region. Most of INBEs develop into intracloud flashes. Only two INBEs develop into positive ground flashes and five INBEs develop into negative ground flashes. Pulse widths and peak amplitudes of electric field change waveforms of INBEs are almost the same as those of normal NBEs. A major difference is that INBEs have much lower discharge heights. Most of INBEs are lower than 10 km while normal NBEs are mainly higher than 10 km. Characteristics of positive pulse trains following INBEs are closely related with discharge heights of INBEs. Higher INBEs are usually followed by weaker, fewer, and less frequent positive pulses with slower upward propagations. As the height increases to above 10 km, NBEs are usually no longer followed by such positive pulses.

Lightning attachment processes of an “anomalous” triggered lightning discharge

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the process for stepped, dart, and dart-stepped leaders in an anomalous rocket-triggered lightning flash that terminated on a 10 m grounded utility pole. The initiation of the first return stroke was found to occur at a height of 23 ± 3 m above the top of the utility pole and was associated with three “slow front” dE/dt pulses. A time of 1.5 µs later, a fast rise in luminosity at 18 ± 2 m was associated with a “fast transition” dE/dt pulse. The first return stroke propagated bidirectionally from its initiation height, as did subsequent return strokes from their initiation heights of 8 ± 1 m to 16 ± 2 m above the top of the utility pole. The initial upward speed of the first return stroke was 1.4 × 108 m/s, while its initial downward speed was 2.2 × 107 m/s. The channel bottom luminosity of the first return stroke rose more slowly to a two or more times larger amplitude than that of the subsequent stroke luminosities. In contrast, the National Lightning Detection Network-derived first-return-stroke peak current is smaller than that of the second and the third strokes, and our electric field records at 45 km show similar behavior for the initial field peaks of the first and subsequent strokes.

Luminous propagation of lightning attachment to CN tower

Using an eight-channel photodiode system, we resolved in time and space the luminous propagation of lightning attachment to the CN tower. Approximately 60 lightning discharges were recorded during 1991 and 1992. Most of these discharges demonstrated simultaneous, bidirectional propagation starting at several tens of meters above the top of the CN tower. Propagation speeds ranged from 8×10° to 287×106 m/s. The observed bidirectional propagation suggests the presence of space leaders or the reilluminations following these space leaders during the initial stage of development of upward leaders from the CN tower. A few of the bidirectional discharges were found to have the return stroke speeds and these might be deduced to be manifestations of the bidirectional propagation processes of return strokes which occurred after the connection of a downward leader and an upward connection leader. We also recorded portions of seven ordinary dart leader—return stroke sequences. The dart leader speeds ranged from 7×106 to 23×106 m/s; the velocities of the return strokes ranged from 88×106 to 143×106 m/s. These values are in excellent agreement with the previous results by other authors. Positive correlations are found among the return stroke velocity, the leader velocity, and the leader light intensity. However, it also is observed that even in the case of a very tall structure, when a dart leader approaches the CN tower, it is highly possible that no apparent connecting leader is initiated before the ensuing return stroke.