Zhuling Sun

Burst of intracloud current pulses during the initial continuous current in a rocket‐triggered lightning flash

For a rocket-triggered lightning flash on 2 August 2013, the measurement with one pair of broadband low-frequency (20–400 kHz) magnetic induction coils registered a long sequence of over 600 intermittent pulses during the initial continuous current. The timescale of these pulses is about 3-8 µs, and the typical interpulse interval is about 30 µs. The source discharges of these pulses, which are not readily detectable in the frequency range (140–300 MHz) of a short-baseline very high frequency (VHF) lightning imaging system, are attributed to the stepping processes when the positive leader propagated into the cloud region. The reversals in the polarity of magnetic pulses are related to the variation in the vertical direction of positive leader propagation as resolved by the VHF imaging system.

Characteristics of a rocket-triggered lightning flash with large stroke number and the associated leader propagation

A negative lightning flash with 16 leader-return stroke sequences, triggered in the summer of 2013 using the classical rocket-and-wire triggering technique, was examined with simultaneous two-dimensional (2D) imaging of very high-frequency (VHF) radiation sources, channel-base current measurement, broadband electric field waveforms and high-speed video images. A total of 28.0 C negative charge was transferred to ground during the whole flash, and the charge transferred during the initial stage was 4.9 C, which is the weakest among the triggered lightning flashes at the SHandong Artificially Triggering Lightning Experiment (SHATLE). The peak current of 16 return strokes ranged from 5.8 to 32.5 kA with a geometric mean of 14.1 kA. The progression of upward positive leader and downward negative (dart or dart-stepped) leaders was reproduced visually by using an improved short-baseline VHF lightning location system with continuous data recording capability. The upward positive leader was mapped immediately from the tip of the metal wire during the initial stage, developing at a speed of about 104 m/s without branches. The upward positive leader and all the 14 negative leaders captured by the 2D imaging system propagated along the same channel with few branches inside the cloud, which might be the reason for the relatively small charge transfer. The 2D imaging results also show that dart leaders may transform into dart-stepped leaders after a long time interval between successive strokes.

Characteristics of lightning leader propagation and ground attachment

The grounding process and the associated leader behavior were analyzed by using high-speed video record and time-correlated electric field change for 37 natural negative cloud-to-ground flashes. Weak luminous grounded channel was recognized below the downward leader tip in the frame preceding the return stroke, which is inferred as upward connecting leader considering the physical process of lightning attachment, though not directly confirmed by sequential frames. For stepped leader-first return strokes, the upward connecting leaders tend to be induced by those downward leader branches with brighter luminosity and lower channel tip above ground, and they may accomplish the attachment with great possibility. The upward connecting leaders for 2 out of 61 leader-subsequent stroke sequences were captured in the frame prior to the return stroke, exhibiting relatively long channel lengths of 340 m and 105 m, respectively. The inducing downward subsequent leaders were of the chaotic type characterized by irregular electric field pulse train with duration of 0.2–0.3 ms. The transient drop of the high potential difference between stepped leader system and ground when the attachment occurred would macroscopically terminate the propagation of those ungrounded branches while would not effectively prevent the development of the existing space stem systems in the low-conductivity streamer zone apart from the leader tip. When the ungrounded branches are of poor connection with the main stroke channel, their further propagation toward ground would be feasible. These two factors may contribute to the occurrence of multiple grounding within the same leader-return stroke sequence.

Recent advances in research of lightning meteorology

Lightning meteorology focuses on investigating the lightning activities in different types of convective weather systems and the relationship of lightning to the dynamic and microphysical processes in thunderstorms. With the development and application of advanced lightning detection and location technologies, lightning meteorology has been developed into an important interdiscipline between atmospheric electricity and meteorology. This paper mainly reviews the advances of lightning meteorology research in recent years in China from the following five aspects: 1) development of advanced lightning location technology, 2) characteristics of lightning activity in different convective systems, 3) relationship of lightning to the dynamic and microphysical processes in thunderstorms, 4) charge structure of thunderstorms, and 5) lightning data assimilation techniques and application to severe weather forecasting. In addition, some important aspects on future research of the lightning meteorology are proposed.

High‐speed video observation of stepwise propagation of a natural upward positive leader

Using a high-speed video camera operated at 150,000 frames per second, we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in Beijing. The stepwise development of the upward positive leader was clearly revealed during its self-sustained propagation toward the cloud base, providing the first optical evidence for the stepping processes of a positive leader. The leader developed with definitive intermittent pauses and re-establishment with abrupt jump of the leader top. Obvious brush-like corona zone emitting outward from the leader top was identified in the frame of stepping, and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping, indicating that a current wave was generated at the leader top and subsequently traveled down the channel. The channel luminosity decreased during the leader pause stage, with the corona zone dimmed correspondingly. The positive leader experienced an average interstep interval of 61.7 µs (ranging between 30 µs and 120 µs) and an average 2-D speed of 8.1 × 104 m/s. The transient speed of the step jump was estimated to be larger than 7.3 × 105 m/s with an average step length of 4.9 m. The need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics.

Characteristics of a negative lightning with multiple‐ground terminations observed by a VHF lightning location system

Propagation characteristics of a negative cloud-to-ground lightning with four leader-return stroke sequences exhibiting four ground terminations, referred to as multiple termination flash, are analyzed using a two-dimensional mapping system of lightning radiation sources in VHF band along with broadband electric field change records. The four stepped leader-stroke sequences were produced from two different branches of the preliminary breakdown process inside the cloud, and the time intervals between each adjacent two leader-stroke sequences were 82.9 ms, 81.4 ms, and 152.2 ms, respectively. The first two stroke sequences were multiple termination strokes (MTSs), and each MTS showed two different ground terminations induced by two leader branches. The corresponding electric field changes exhibited two fast-rising waveforms separated by 1.5 ms and 2.7 ms, respectively, and the second peak was 3–4 times smaller than the first. The second grounding branch in each MTS occurred when its channel was in poor connection conditions to the main stroke channel. The third and fourth leader-stroke sequences connected to the ground with one of the different grounded channels of the second MTS. The dart leaders before these two strokes exhibited chaotic pulse trains (CPTs) with significant slow electric field variations accompanying strong and continuous VHF radiations. The CPTs only occurred in the dart leader stage which propagated with a speed in an order of 107 m/s. Different from dart leader, attempted leader was associated with regular pulse trains and propagated at a speed order of 106 m/s along the previous channel.

Channel branching and zigzagging in negative cloud-to-ground lightning

A fundamental question in lightning flash concerns why the discharge channel propagates in a zig-zag manner and produces extensive branches. Here we report the optical observation of two negative cloud-to-ground lightning discharges with very high temporal resolution of 180,000 frames per second, which shows in detail the dependence of channel branching and tortuous behavior on the stepping process of the leader development. It is found that the clustered space leaders formed in parallel ahead of the channel tip during an individual step process. The leader branching is due to the multiple connection of the clustered space leaders with the same root channel tip, which occur almost simultaneously, or successively as some space leaders/stems resurrect after interruption. Meanwhile, the irregularity of angles between the clustered space leaders and the advancing direction of leader tip is the origin of channel tortuosity. The statistical analysis on 96 steps shows a geometric-mean value of 4.4 m for the step length, ranging between 1.3 and 8.6 m, while the distance from the center of space leader to the channel is 3.6 m, ranging between 2.1 and 6.9 m. More than 50% steps occurred within an angle range of ±30° from the advancing direction of the leader.

Characterization of initial current pulses in negative rocket-triggered lightning with sensitive magnetic sensor†

We report the new measurement of initial current pulses in rocket-triggered lightning with a broadband magnetic sensor at 78 m distance. The high sensitivity of our sensor makes it possible to detect weak ripple deflections (as low as 0.4 A) that are not readily resolved in the typical measurements of channel-base current in rocket-triggered lightning experiments. The discernible magnetic pulses within 1 ms after the inception of a sustained upward positive leader from the triggering wire can be classified into impulsive pulses and ripple pulses according to the discernibility of separation between individual pulses. The timescale (usually >20 µs) of ripple pulses is substantially longer than the leading impulsive pulses (with timescales typically <10 µs), and the amplitude is significantly reduced, whereas there is no considerable difference in the inter-pulse pulse. Along with our previous finding on the burst of magnetic pulses during the initial continuous current (ICC) in rocket-triggered lightning, the new measurements suggest that the stepwise propagation might be a persistent feature for the upward positive leader in rocket-triggered lightning, and the stepping of positive leader early in triggered lightning could be characterized with the observation of ripple pulses. The precedence of impulsive magnetic pulse measured at 78 m range relative to the arrival of corresponding current pulse at the channel base indicates that the ionization wave launched by individual stepping of positive leader propagates downward along the triggering wire at a mean velocity of 1.23 × 108 m/s to 2.25 × 108 m/s.

Development of short-baseline time-difference of arrival location system and observations on Lightning Discharge

A new short-baseline TDOA VHF lightning location system is developed. The results indicate that the location system could effectively map the lightning radiation sources, and visually reproduced the development of lightning occurring inside the cloud. The technique provides an effective approach to describe the temporal and spatial development of lightning discharge in two dimensions with high temporal and spatial resolution.

Bidirectional leader development in a preexisting channel as observed in rocket‐triggered lightning flashes

Two cases of bidirectional leader, starting almost immediately below a decaying dart or dart-stepped leader which terminated before reaching the ground, were identified, for the first time, in rocket-triggered lightning flashes based on high-speed video, electric field changes, and channel base current, indicating that a bidirectional leader can be excited by a decayed dart leader and propagate in a preexisting discharge channel. The positive end of the bidirectional leader moved upward along the terminated downward dart leader path, and the negative end extended downward along the decayed initial continuous current or preceding stroke path to the ground and culminated in a return stroke. The positive leader propagated with an average speed of 1.3 × 106 m/s and 2.2 × 106 m/s in the two cases, roughly twice as fast as its negative counterpart with speed of 7.8 × 105 m/s and 1.0 × 106 m/s, respectively. The positive end started earlier than the negative end with a preceding time less than 40.3 µs. The bidirectional leader can be regarded as a recoil leader with the positive end retrogressing along a negative leader channel, whose polarity is contrary to the traditional recoil leader with negative leader end retrogressing along an existing positive leader channel. Weak luminosity was continuously observed during the terminated dart leader and bidirectional leader stage, but the channel current was less than the minimum detection capability of 9.3 A.