Initial Continuous Current Pulses Research Articles

Development characteristics of triggered lightning with fast initial continuous current pulses and two grounding points

In this paper we analyze the current, electromagnetic field and optical data of a rocket-triggered lightning with two grounding points. There are 4 fast initial continuous current pulses (ICCPs) in the initial stage. Fast ICCPs have larger amplitudes and shorter rise times than those of typical ICCPs, which are mostly found in tower-initiated upward lightning. The formation of fast ICCPs are not only related to the height of the junction point, but also to the value of the initial continuous current (ICC) in the main discharge channel to ground. For the same height of the junction point, the lower the value of ICC, the more likely it is to produce fast ICCP. The lightning has three return strokes (RS1-RS3). The new grounding point forms after RS2. The location and time of the new grounding point formed is different from the previously reported “anomalous” triggered lightning and is closer to the natural lightning. The two-dimensional distance between the grounding points was 543 m. The formation of new grounding point may be associated with large leader deviation angle and low intracloud charge. The occurrence of RS3 was preceded by multiple attempted leaders (ALs). Based on the analysis of the source charge leader model, the “slow-fast-slow” trend of the electric field waveform in leader stage before RS3 might be related to the speed change of leader.

Observation of five types of leaders contained in a negative triggered lightning

A negative triggered lightning involving five types of leaders was recorded by high-speed camera using frame rate of 20,000 fps and fast antennas at different distances. Five types of leaders contained one upward positive stepped leader, one upward positive dart leader, ten downward negative dart leader, one bidirectional leader and three downward negative dart-stepped leaders were propagated successively in the same channel. The upward positive dart leader occurred after initial continuous current pulse with average 2-D speed of 1.40 × 106 m/s and started second continuous current process. The bidirectional leader was transformed from decaying unidirectional leader and showed the unique electric field changes. Faster return strokes are found to be induced by downward leaders propagating evenly and deposit more positive charge in the following residual channel. The positive charge can inhibit the potential initiation of an upward positive leader and boost the propagation of the next downward negative leader.

Peaks of return strokes and fast pulses on ICC of winter lightning initiated by upward propagating leaders in Japan

The authors have carried out measurement of the current waveforms associated with lightning discharges to wind turbines at Nikaho near the coast of the Sea of Japan since 2013. In this paper measured waveforms in winter up to 2020 are investigated and the current waveforms associated with return strokes after the extinction of the ICC (initial continuous current) and fast pulses superimposed on the ICC with the current peak of 2 kA or higher and with the risetimes of 8 µs or shorter are studied. The median current peaks of the 107 return strokes and the 123 fast pulses were 6.4 kA and 3.0 kA, respectively. The observed median current peaks of return strokes and fast pulses at the Nikaho wind turbines (6.4 kA and 3.0 kA) do not show much difference from those observed at the Gaisberg tower (9.2 kA and 4.2 kA), the Säntis tower (8 kA and 3.4 kA) and the Peissenberg tower in Germany (return stroke: 8.5 kA but no report on fast pulses) when the long sampling interval and the low upper cutoff frequency of the measuring system at the Nikaho wind turbines is taken into account.

The Damage Effects on Surge Protective Devices Caused by the Ground Potential Rise During the Initial Long Continuous Current Processes of Triggered Lightning Events

Based on the triggered lightning technology, this study carried out observational tests regarding the impulses of ground potential rise (GPR) to surge protective devices (SPD), and emphatically analyzed the impulse and damage effects of the initial long continuous current processes of triggered lightning on the SPDs. The results showed that after the triggered lightning currents were injected into grounding grids, the SPDs with rated flow rates were easily damaged under the joint actions of the initial long continuous currents of the lightning and the subsequent return strokes. It was observed that when the energy flowing through the SPDs had accumulated to a certain level, only the initial long continuous current processes had caused damages to the SPDs. The impulse effects on the SPDs were determined to be closely related to the different waveforms of the initial long continuous current processes. For example, when the long continuous current processes were superimposed by the initial continuous current pulses (ICCP), with fast rising edges and large amplitudes, then the energy passing through the SPDs tended to rapidly increase. This was believed to likely be the most critical factor related to the damages to the SPDs during the long continuous current processes.

Characteristics of different charge transfer modes in upward flashes inferred from simultaneously measured currents and fields

The authors present an analysis of different charge transfer modes during upward negative flashes. The analysis includes a total number of 94 pulses that occurred during two upward negative flashes recorded at the Santis Tower. The pulses included 59 mixed-mode (MM) initial continuous current (ICC) pulses, 17 M-component-type ICC (M-ICC) pulses, 8 return-stroke pulses, and 10 classical M-component (MC) pulses. It is found that the initial stage of the flash is responsible for the largest share of the total charge transferred to the ground. Simulation results for the electric fields associated with the considered charge transfer modes are presented and discussed. Return stroke (RS) and MM pulses were simulated adopting the MTLE model, while MCs and M-ICC pulses were simulated using the guided wave model of Rakov et al. The simulated results are shown to be in good agreement with simultaneous records of electric fields measured at a distance of 15 km from the Santis Tower. The inferred velocities for MCs and M-ICC pulses range from 2.0 × 10 7 to 9.0 × 10 7 m/s, and the corresponding junction point heights range from 1.0 to 2.0 km. The inferred pulse velocities for RSs and MM pulses range from 1.3 × 10 8 to 1.65 × 10 8 m/s. The inferred current attenuation constants of the MTLE model obtained in this study range from 0.3 to 0.8 km, lower than the value of 2 km previously suggested for RSs in downward flashes. The obtained results support the assumption that the mode of charge transfer to the ground giving rise to MM pulses is similar to that of RSs. The results are also in support of the generally assumed similarity between M-ICC pulses and classical MCs.

Characteristics and mechanism of a triggered lightning with two polarity reversals of charges transferred to ground

Characteristics of a triggered bipolar lightning flash obtained in the Shandong triggering lightning experiment (SHATLE) are analyzed based on simultaneous observation results of lightning very high frequency (VHF) interferometer map, channel-base current, fast electric field change and high-speed optical images. The flash lasts about 315 ms with only the initial continuous current (ICC). As the polarity of charges transferred to the ground varies from negative to positive and then to negative, the ICC can be mainly divided into three stages including the first negative ICC stage, positive ICC stage, and second negative ICC stage, respectively, for 152.755 ms, 87.225 ms and 75.02 ms. Charges transferred to the ground during the three stages are about –40.0 C, +13.3 C and –1.0 C, respectively, with the peak current of about –3.8 kA, 1.6 kA and –2.25 kA. According to the VHF interferometer map, during the first negative ICC stage, the upward positive leader (UPL) initiated from the top of the wire as negative charges starts to be transferred to the ground. The UPL develops at a two-dimensional (2D) speed of 3.7 × 10<sup>4</sup> m/s into the cloud and multiple positive branches develop in the form of small-scale and dense recoil leaders, keeping the increase of negative charge transferred to the ground. Then a negative leader (NL) initiates on a previously ionized positive branch channel and develops into the virgin air horizontally as a floating channel at a 2D propagation speed of 1.59 × 10<sup>5</sup> m/s. About 28.816 ms later after the NL develops, a negative pulse is detected in the fast electric field change, caused by a negative polarity breakdown discharge from the grounding trunk channel to the floating channel which is observed for the first time. Then about 39 μs later, the first polarity of the channel-base current changes from negative to positive, and rapidly reaches a positive peak in 1.75 ms. Subsequently, with the positive ICC decreasing, the negative leader gradually terminates about 65.85 ms after the first polarity reversal, and then 21.38 ms later the channel-base current slowly changes to the recognizable negative polarity as the second polarity reversal. A recoil leader generating in a previously ionized positive branch channel connects to the trunk channel, resulting in the initial continuous current pulse (ICCP) during the second negative ICC stage. Then several negative recoil leaders occur, tracing back to the previous positive channels without obvious current changes until the flash ends. Based on the analysis, the positive branch channel persistently transfers negative charge to the ground in the whole discharge process by the positive breakdown on the tip or the negative recoil leaders retrograding along the previous positive channels. The trunk channel is connected to the floating channel through negative breakdown discharges, linking at the positive charge accumulation area at the tail end of the negative leader or the positive polar end of a bidirectional leader towards the trunk channel, if the negative leader develops as a bidirectional leader later. Then, the net charge transferred to the ground is dominantly positive and the reversal of first channel current polarity occurs. With the negative leader disappearing, the supply of positive charge ceases, so the current polarity reverses again (the second reversal). Connection of the negative leader to the grounding trunk channel and continuous development of the positive leader are inferred to play an important role in reversing the two current polarities. In this case, the negative leader developing in virgin air might be initiated transversely in an ionized positive channel or from the end of a decayed positive leader branch of the ionized positive channel which is small and undistinguishable from the VHF interferometer map.

Characteristics of Impulsive Currents Superimposing on Continuous/Continuing Current of Rocket-Triggered Lightning

Characteristics and mechanism of initial continuous current (ICC) pulses and M components in rocket-triggered lightning were investigated in detailed. The geometric mean current peak, risetime from 10% to 90% peak, half-peak width, and charge transfer of M components were 400 A, 207 μs, 267 μs, and 0.19 C, respectively, exhibiting more pronounced impulsive feature than ICC pulses of which the counterpart parameters were 190 A, 462 μs, 845 μs, and 0.27 C, respectively. A shorter time interval between the prior return stroke (RS) and the following M component (TRS-M) facilitated a stronger M component having larger peak while shorter pulsewidth. In two cases with extremely long lasting ICC and continuing current (CC), 20 Initial continuous current pulses occurred during a 715.3-ms duration and 15 M components occurred during a 218-ms duration. The multiple occurrences of current surges serve to keep the grounding channel active and extend the channel network inside the cloud, and hence prolong the ICC/CC duration. This situation reduced the possibility of RSs thereafter. In addition to the two traditional types of in-cloud discharges that result in M component, we recognized a new scenario that involves a short-term interruption of the upward RS wave and a reactivated breakdown promoted by the residual charge inside the main channel.

Electromagnetic Fields Associated With the M‐Component Mode of Charge Transfer

AbstractIn upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M‐component or leader/return‐stroke‐like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC‐carrying channel at relatively short distance from the tower top, resulting in the so‐called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M‐component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Säntis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M‐component charge transfer mode (including classical M‐component pulses and M‐component‐type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2‐Dimentional Finite‐Difference Time Domain method, in which the M‐component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, https://doi.org/10.1029/95JD01924 model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Säntis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M‐component and M‐component‐type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M‐component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M‐component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M‐component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch‐to‐channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E‐field and 1.9 for the H‐field. For a junction height greater than 6 km, the E‐field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, https://doi.org/10.1109/TEMC.2015.2483018) in which an electric field propagation factor of 1.8 was inferred for return strokes and mixed‐mode pulses, considering that junction points lower than 1 km or so would result in a mixed mode of charge transfer, in which a downward leader/return‐stroke‐like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 μs or so) for which the tendency reverses. Finally, model‐predicted fields associated with different charge transfer modes, namely, return stroke, mixed‐mode, classical M‐component, and M‐component‐type ICC pulse are compared with experimental observations at the Säntis Tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed‐mode models (1.0 km for the return stroke and 0.8 km for the mixed‐mode pulse) are lower than the value of 2.0 km typically used in the literature.

Characterizing Upward Lightning With and Without a Terrestrial Gamma Ray Flash

AbstractWe compare two observations of gamma rays before, during, and after lightning flashes initiated by upward leaders from a tower during low‐altitude winter thunderstorms on the western coast of Honshu, Japan. While the two leaders appear similar, one produced a terrestrial gamma ray flash (TGF) so bright that it paralyzed the gamma ray detectors while it was occurring and could be observed only via the weaker flux of neutrons created in its wake, while the other produced no detectable TGF gamma rays at all. The ratio between the indirectly derived gamma ray fluence for the TGF and the 95% confidence gamma ray upper limit for the gamma ray quiet flash is a factor of 1.0 × 107. With the only two observations of this type providing such dramatically different results—a TGF probably as bright as those seen from space and a powerful upper limit—we recognize that weak, subluminous TGFs in this situation are probably not common, and we quantify this conclusion. While the gamma ray quiet flash appeared to have a faster leader and more powerful initial continuous current pulse than the flash that produced a TGF, the TGF‐producing flash occurred during a weak gamma ray glow, while the gamma ray quiet flash did not, implying a higher electric field aloft when the TGF was produced. We suggest that the field in the high‐field region approached by a leader may be more important for whether a TGF is produced than the characteristics of the leader itself.

Measurement of continuing charge transfer in rocket-triggered lightning with low-frequency magnetic sensor at close range

Based on the magnetic fields recorded with a compact low-frequency (LF) magnetic sensor deployed at 78 m distance from the channel base, we reconstruct the time-resolved current waveform for the continuously discharging processes in classical rocket-and-wire triggered lightning flashes, including the initial continuous current (ICC) and long continuing current. Both the overall feature and the millisecond-scale slow variations (e.g., initial current variation, ICC pulses and M-components) embedded in the channel-base current as measured with the conventional methods (such as the shunt or Pearson coil) can be retrieved through the numerical integral of close LF magnetic signals. Despite the artifact caused by the magnetic fields radiated by the fast in-cloud processes, the new approach has the advantage of significantly reduced noise in comparison with the measurements of conventional methods, and it is likely applicable to remotely measure the initial continuous current in upward lightning from high objects and altitude-triggered lightning, as well as long continuing current in natural cloud-to-ground (CG) lightning strokes that occur at sufficiently close range (e.g., within 100 m).

An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Säntis Tower

AbstractWe present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Säntis Tower in Switzerland. The electric field was measured 15 km from the tower. Upward flashes always begin with the initial stage composed of the upward‐leader phase and the initial‐continuous‐current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return‐stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no‐current time intervals; (2) mixed‐mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively small junction heights; (3) “classical” M‐component pulses superimposed on the continuing current following some return strokes; and (4) M‐component‐type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively large junction heights. We consider a data set consisting of 9 return‐stroke pulses, 70 mixed‐mode ICC pulses, 11 classical M‐component pulses, and 19 M‐component‐type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed‐mode and M‐component‐type pulses, which is based on the current waveform features. The characteristics of M‐component‐type pulses during the initial stage are found to be similar to those of classical M‐component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed‐mode ICC pulses were preceded by microsecond‐scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond‐scale electric field pulses observed for M‐component‐type ICC pulses and which can be attributed to the junction of an in‐cloud leader channel to the current‐carrying channel to ground. Classical M‐component pulses and M‐component‐type ICC pulses tend to have larger risetimes ranging from 6.3 to 430 μs. On the other hand, return‐stroke pulses and mixed‐mode ICC pulses have current risetimes ranging from 0.5 to 28 μs. Finally, our data suggest that the 8‐μs criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M‐components. However, mixed‐mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28 μs, as observed in this study.

Ground‐level observation of a terrestrial gamma ray flash initiated by a triggered lightning

AbstractWe report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude‐triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a ground‐level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.

Fast initial continuous current pulses versus return stroke pulses in tower‐initiated lightning

AbstractWe present a study focused on pulses superimposed on the initial continuous current of upward negative discharges. The study is based on experimental data consisting of correlated lightning current waveforms recorded at the instrumented Säntis Tower in Switzerland and electric fields recorded at a distance of 14.7 km from the tower. Two different types of pulses superimposed on the initial continuous current were identified: (1) M‐component‐type pulses, for which the microsecond‐scale electric field pulse occurs significantly earlier than the onset of the current pulse, and (2) fast pulses, for which the onset of the field matches that of the current pulse. We analyze the currents and fields associated with these fast pulses (return‐stroke type (RS‐type) initial continuous current (ICC) pulses) and compare their characteristics with those of return strokes. A total of nine flashes containing 44 RS‐type ICC pulses and 24 return strokes were analyzed. The median current peaks associated with RS‐type ICC pulses and return strokes are, respectively, 3.4 kA and 8 kA. The associated median E‐field peaks normalized to 100 km are 1.5 V/m and 4.4 V/m, respectively. On the other hand, the electric field peaks versus current peaks for the two data sets (RS‐type ICC pulses and return strokes) are characterized by very similar linear regression slopes, namely, 3.67 V/(m kA) for the ICC pulses and 3.77 V/(m kA) for the return strokes. Assuming the field‐current relation based on the transmission line model, we estimated the apparent speed of both the RS‐type ICC pulses and return strokes to be about 1.4 × 108 m/s. A strong linear correlation is observed between the E‐field risetime and the current risetime for the ICC pulses, similar to the relation observed between the E‐field risetime and current risetime for return strokes. The similarity of the RS‐type ICC pulses with return strokes suggests that these pulses are associated with the mixed mode of charge transfer to ground.

On Lightning Electromagnetic Field Propagation Along an Irregular Terrain

In this paper, we present a theoretical analysis of the propagation effects of lightning electromagnetic fields over a mountainous terrain. The analysis is supported by experimental observations consisting of simultaneous records of lightning currents and electric fields associated with upward negative lightning flashes to the instrumented Santis tower in Switzerland. The propagation of lightning electromagnetic fields along the mountainous region around the Santis tower is simulated using a full-wave approach based on the finite-difference time-domain method and using the two-dimensional topographic map along the direct path between the tower and the field measurement station located at about 15 km from the tower. We show that, considering the real irregular terrain between the Santis tower and the field measurement station, both the waveshape and amplitude of the simulated electric fields associated with return strokes and fast initial continuous current pulses are in excellent agreement with the measured waveforms. On the other hand, the assumption of a flat ground results in a significant underestimation of the peak electric field. Finally, we discuss the sensitivity of the obtained results to the assumed values for the return stroke speed and the ground conductivity, the adopted return stroke model, as well as the presence of the building on which the sensors were located.

Lightning current and luminosity at and above channel bottom for return strokes and M‐components

We measured current and luminosity at the channel bottom of 12 triggered lightning discharges including 44 return strokes, 23 M-components, and 1 initial continuous current pulse. Combined current and luminosity data for impulse currents span a 10–90% risetime range from 0.15 to 192 µs. Current risetime and luminosity risetime at the channel bottom are roughly linearly correlated (τr,I = 0.71τr,L1.08). We observed a time delay between current and the resultant luminosity at the channel bottom, both measured at 20% of peak amplitude, that is approximately linearly related to both the luminosity 10–90% risetime (Δt20,b = 0.24τr,L1.12) and the current 10–90% risetime (Δt20,b = 0.35τr,I1.03). At the channel bottom, the peak current is roughly proportional to the square root of the peak luminosity (IP = 21.89LP0.57) over the full range of current and luminosity risetimes. For two return strokes we provide measurements of stroke luminosity vs. time for 11 increasing heights to 115 m altitude. We assume that measurements above the channel bottom behave similarly to those at the bottom and find that (1) one return stroke current peak decayed at 115 m to about 47% of its peak value at channel bottom, while the luminosity peak at 115 m decayed to about 20%, and for the second stroke 38% and 12%, respectively; and (2) measured upward return stroke luminosity speeds of the two strokes of 1.10 × 108 and 9.7 × 107 ms−1 correspond to current speeds about 30% faster. These results represent the first determination of return stroke current speed and current peak value above ground derived from measured return stroke luminosity data.

On the field‐to‐current conversion factors for large bipolar lightning discharge events in winter thunderstorms in Japan

AbstractIn this paper we have simulated the far‐field waveform characteristic of large bipolar events (LBEs) occurred in winter thunderstorms in Japan and compared the field‐to‐current conversion factors (FCCFs) of LBEs with that of the lightning cloud‐to‐ground (CG) return stroke (RS) in summer thunderstorm. As for the physical process of LBEs, Wu et al. (2014) considered that LBEs may be very similar to the typical lightning RS (RS‐like process) or caused by an initial continuous current pulse (ICC‐like process) in upward lightning flashes. We assume that the lightning channel length of LBEs ranges from 500 m to 1000 m, and the height of tall object struck by LBEs is from 100 m to 300 m. By using the bouncing wave model, we found that only when the injected current waveform of LBEs is characterized with a symmetric Gaussian pulse, the simulated far‐field waveform of LBEs both for RS‐like process and ICC‐like process is similar to that observed by Wu et al. (2014). For striking tall objects with heights from 100 m and 300 m, the FCCFs of LBEs are positively correlated with its channel length and derivatives of injected current waveform, and the FCCF for RS‐like process is about similar to that for ICC‐like process. However, the FCCFs of LBEs are very different from lightning RS in summer thunderstorm; that is to say, the FCCFs developed for the well‐known lightning RS in summer thunderstorm are not suitable for LBEs.

Evaluation of the GLD360 performance characteristics using rocket‐and‐wire triggered lightning data

AbstractWe estimated the performance characteristics of the Global Lightning Dataset (GLD360) using rocket‐and‐wire triggered lightning data acquired at Camp Blanding, Florida, in 2011–2013. The data set consisted of 201 return strokes and 84 kiloampere‐scale (≥1 kA) superimposed pulses (initial continuous current pulses and M components) in 43 flashes. All the events transported negative charge to ground. The GLD360 detected 75 strokes and 4 superimposed pulses in 29 flashes. The resultant detection efficiencies were 67% for flashes, 37% for strokes, and 4.8% for superimposed pulses. Out of 75 detected strokes, one (1.3%) was reported with incorrect polarity. The median location error was 2.0 km, and the median absolute current estimation error was 27%. This is the first comprehensive evaluation of GLD360 performance characteristics relative to absolute ground truth, with all previous evaluations being at least in part relative to other locating systems. The results presented in this work may be applicable to regions in and around Florida.

Performance characteristics of the NLDN for return strokes and pulses superimposed on steady currents, based on rocket‐triggered lightning data acquired in Florida in 2004–2012

AbstractWe present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground truth, Florida rocket‐triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return‐stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly measured channel‐base currents are available for 290 of them) and 173 kiloampere‐scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M components. All these events transported negative charge to the ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return‐stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event‐time mismatch (the difference in times at which the event is reported to occur by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 µs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a reilluminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.

Rocket‐and‐wire triggered lightning in 2012 tropical storm Debby in the absence of natural lightning

AbstractLightning Mapping Array source locations, channel base currents, and electric field waveforms are presented for a lightning flash triggered in the rainbands of 2012 tropical storm Debby. The National Lightning Detection Network reported no natural cloud‐to‐ground discharges within 60 km of the North Florida triggering site for at least 20 h before and 8 h after the triggered flash. Additionally, local electric field mill and wideband antenna networks show no close cloud or cloud‐to‐ground flashes. The triggering rocket was launched with negative charge overhead producing an electric field at the ground of 5 kV m−1 and in coordination with X‐band, dual‐polarimetric radar observations of streamers of enhanced precipitation descending from the melting level as they approached the site. The Debby flash consisted of an initial stage (IS) followed by eleven leader/return stroke sequences. The flash exhibited all the processes of normal triggered and natural cloud‐to‐ground lightning: leader/return stroke sequences, continuing currents, K events, and M components. Additionally, the flash exhibited several exceptional characteristics: three return stroke peak currents greater than 25 kA, one very long, 352 ms, continuing current that transferred about 35 C of charge to ground, and a relatively short, 202 ms, IS containing no initial continuous current pulses. Following a near‐vertical upward positive leader attaining 2.8 km height, the IS branched and propagated horizontally at 3.5 km altitude. The flash, exhibiting strokes and continuing current, then ascended to and propagated horizontally at 5.5 km, extending about 25 km south and 15 km east. The 0°C level was near 4.5 km above sea level. It follows from the above that clouds that are not producing natural lightning can represent a triggered lightning hazard to launch vehicles and aircraft.

Characteristics of current pulses in rocket-triggered lightning

The waveform parameters of current pulses in negative rocket-triggered lightning are analyzed statistically using recorded channel-base current during Shandong Artificially Triggered Lightning Experiment (SHATLE) from 2005 to 2011. The current pulses are divided into 3 categories, including return strokes, ICC pulses in initial continuous current (ICC) stage and M-components superimposed on the continuing current following the return stroke. The geometric mean values (GM) of peak current, risetime from 10% to 90% peak, half peak width and charge transfer for return strokes are 12.1kA, 1.0μs, 14.8μs, and 0.86C, respectively. The corresponding parameters for ICC pulses are 0.09kA, 437μs, 712μs, and 0.10C, respectively, while they are 0.28kA, 251μs, 242μs and 0.10C for M-components, respectively. There is significant fraction of larger M-components (about 10% of 63 samples) demonstrating peak current of several kilo-amperes, comparable to weak return strokes. The ICC pulses show somewhat lower current peak and slower risetime than the M-components with a difference of roughly 2–3 times, resulting from the lower charge source of ICC pulses. In general, the ICC pulses and M-components show smaller peak current and charge transfer while longer risetime and half peak width than return strokes, differing with 1–2 orders of magnitude. The charge neutralized by an individual triggered lightning flash ranged from 6.3C to 68.1C, while the initial stage (IS), lasted about 245ms (GM) and lowered a GM charge of 21.2C to the ground. The average IS current in an individual lightning discharge varies from 45.8A to 140.6A with a GM value of 86.7A.