Bidirectional Leader Research Articles

Production of very high potential differences by intracloud lightning discharges in connection with terrestrial gamma ray flashes

AbstractTerrestrial gamma‐ray flashes (TGFs) have been correlated with an early development stage of high altitude positive intracloud (+IC) flashes in which the negative leader propagates up toward the upper positive charge region, while the positive leader propagates down toward the lower negative charge region. The resultant bidirectional leaders develop electrical potential differences in the vicinity of their heads with respect to the ambient potential distribution created by the thundercloud charges. These potential differences are believed to be of essential importance for the generation of TGFs. Using electrostatic calculations and a three‐dimensional Cartesian fractal model, we quantify these potential differences produced in a developing +IC lightning discharge for given thunderstorm electric configurations. We present a case of a +IC lightning discharge in a realistic thunderstorm configuration that leads to a very high (∼300 MV) potential difference and show how a delay in the development of the negative leader with respect to the positive one in a bidirectional leader system can facilitate a high potential difference in the negative leader head region.

Characteristics of lightning radiation source distribution and charge structure of squall line

Lightning information indicates the intensity and the development of severe convection. The characteristics of lightning activity and charge structure of a squall line over Beijing on 13 June 2010 are analyzed by using the SAFIR3000 lightning detection data, Doppler weather radar and precipitation data. Results show that the lightning radiation sources are the major part in the convective leading region with a high reflectivity in front of the squall line, and the number of lightning radiation sources gradually increases in the back of stratiform region only at the dissipating stage. The correlation coefficient between the total lightning and convective precipitation is found to be 0.82, and that of intra-cloud lightning and convective precipitation is 0.76. It is inferred that the lightning is closely related to dynamical and microphysical processes of the squall line. According to the lightning radiation distribution, the charge structure of squall line is also discussed. At the mature stage of the squall line, the lightning radiation sources have two layers with the upper level centered at 11 km and the lower level at 6 km. Based on the bidirectional leader of lightning propagation and by assuming VHF source density maxima being most likely associated with the positive charge region, it is concluded that the squall line is characterized by a tripole charge structure with a middle charge region between the levels of 8 km and 10 km, and the two positive charge regions at the levels of 10 to 12 km and 4 to 7 km above the ground, respectively.

The effect of ground altitude on lightning striking distance based on a bi-directional leader model

The effect of ground altitude on lightning striking distance has been investigated based on a bi-directional leader model. The model, which is a development of that proposed originally by Mazur and Ruhnke in 1998, enables the calculation of leader channel parameters, such as leader charge density, leader current, leader potential, and lightning striking distance to flat ground. In the model, the lightning striking distance is directly related to the critical electric field in the negative streamer zone in front of the leader tip and to the leader potential. The former may vary with the ground altitude above sea level and the latter may also be affected by it. Based on this thought and on the bi-directional leader model, the effect of regional ground altitude on lightning striking distance was investigated. The result shows that the striking distance increases significantly as the ground altitude increases. This is because the critical electric field necessary for sustaining the negative streamer zone decreases as the ground altitude increases. The result is useful to both physical and engineering application.

CELLS v1.0: updated and parallelized version of an electrical scheme to simulate multiple electrified clouds and flashes over large domains

Abstract. The paper describes the fully parallelized electrical scheme CELLS which is suitable to simulate explicitly electrified storm systems on parallel computers. Our motivation here is to show that a cloud electricity scheme can be developed for use on large grids with complex terrain. Large computational domains are needed to perform real case meteorological simulations with many independent convective cells. The scheme computes the bulk electric charge attached to each cloud particle and hydrometeor. Positive and negative ions are also taken into account. Several parametrizations of the dominant non-inductive charging process are included and an inductive charging process as well. The electric field is obtained by inverting the Gauss equation with an extension to terrain-following coordinates. The new feature concerns the lightning flash scheme which is a simplified version of an older detailed sequential scheme. Flashes are composed of a bidirectional leader phase (vertical extension from the triggering point) and a phase obeying a fractal law (with horizontal extension on electrically charged zones). The originality of the scheme lies in the way the branching phase is treated to get a parallel code. The complete electrification scheme is tested for the 10 July 1996 STERAO case and for the 21 July 1998 EULINOX case. Flash characteristics are analysed in detail and additional sensitivity experiments are performed for the STERAO case. Although the simulations were run for flat terrain conditions, they show that the model behaves well on multiprocessor computers. This opens a wide area of application for this electrical scheme with the next objective of running real meterological case on large domains.

Physical processes during development of upward leaders from tall structures

The objective of our study was to identify and interpret the various processes during development of upward positive leaders from tall structures. We provide a physical interpretation for the varying luminosity of the leader channel during its initial ascent and the pulsing luminosity during much later stages of the established leader. Our analysis confirms that pulsed luminosity is a result of recoil leaders intercepting the current-carrying channel connected to ground. This interception produces an M-component. Applying the bidirectional leader concept and a simplified electrostatic approximation, we introduce physical models of the dart leader-return stroke sequence and the M-component.

ＶＨＦ波帯広帯域干渉計で観測された夏季正極性落雷の特性

The VHF broadband digital interferometer (DITF) observed two- (2D) and three-dimensional (3D) developments of summer positive cloud-to-ground (CG) lightning flashes with high time resolution. The characteristics of summer positive CG lightning flashes attract a great deal of interest. In the first stage, the in-cloud negative breakdown progresses about 15 km horizontally between 6 and 10 km high. In the next stage, the lightning location and protection system (LLS) detects the positive lightning stroke near the initiation point of the preceding negative breakdown. In the last stage, the negative breakdown runs through the channel of the first negative breakdown. For the positive return stroke to run through the first negative channel, the positive leader, which is not visualized by the DITF, progresses from the initiation point of the first negative channel and touches to the ground. The return stroke current propagates and penetrates in the opposite direction as visualized subsequent negative breakdown. This suggests the negative and positive breakdown progress together. To realize this situation, we conclude the necessity to apply the bi-directional leader progression concept. In a word, we observed bi-directional leaders in summer nature positive CG lightning flashes.

Simultaneous optical and electrical observations on the initial processes of altitude-triggered negative lightning

Using a high-speed camera system and two electric field antenna systems, we have documented the initial processes of an altitude-triggered negative lightning (ATNL). The optical records clearly show that ATNL begins with the inception and propagation of an upward positive leader (UPL) and then a simultaneous propagation of UPL and downward negative leader (DNL), known as the bidirectional leader process, follows. Based on the optical records, it is inferred that (1) the triggering height is about 371 m; (2) the two-dimensional (2D) propagation speed of the UPL in its inception phase is about 3.8–5.5 × 10 4 m s − 1 during its propagation from about 393 to 452 m above the ground; (3) the grey levels of the DNL are about one order of magnitude higher than that of the UPL in their inception phase; (4) a discharge phenomenon propagating along the elevated triggering wire part of the lightning channel occurs after the mini-return stroke (MRS), with a 2D propagation speed of about 1.6–2.0 × 10 5 m s − 1 . Combined with the simultaneous electric field change records, it is further inferred that (1) the UPL incepts about 932 μs earlier than the unstable DNL and about 4.1 ms earlier than the stable DNL; (2) the unstable DNL propagates downward intermittently three times with a time interval of about 1 ms, and each propagation contains a different number of steps with an average step length of about 7 m; (3) the stable DNL incepts at the tip of the unstable one, with a 2D propagation speed of about 1.9 × 10 5 m s − 1 , an average step length of about 3 m, and a stepping time interval varying from 6 to 31 µs with a mean value of 15 µs.

Blue Jets: Upward Lightning

Blue jets are beams of blue light propagating from the tops of active thunderclouds up to altitudes of ∼50 km. They resemble tall trees with quasi-vertical trunk and filamentary branches. Their apparent speeds are in the range of 10 s to 100 s km/s. Other events, having essentially lower terminal altitudes (<26 km), are named blue starters. These phenomena represent the first documented class of upward electrical discharges in the stratosphere. Some of upward discharges, termed gigantic jets, propagate into the lower ionosphere at much higher speeds in the final phase. We describe salient features of the upward discharges in the atmosphere, give an assessment of the theories of their development, and discuss the consequences for the electrodynamics and chemistry of the stratosphere. We argue that this upward lightning phenomenon can be understood in terms of the bi-directional leader, emerging from the anvil.

Aircraft Triggered Lightning Caused by Winter Thunderclouds in the Hokuriku Coast, Japan -A Case Study of a Lightning Strike to Aircraft below the Cloud Base-

A case of aircraft triggered lightning, which occurred below the cloud base, was observed at Komatsu Airbase along the coast of the Sea of Japan during the intensive observation period using Doppler radar. The aircraft triggered lightning was accompanied by winter thunderclouds with graupel precipitation in the dissipating stage. The aircraft triggered lightning was initiated by a bi-directional leader. Lightning channels from the aircraft propagated simultaneously both upward and downward with return strokes. The surface electrical field was relatively weak (10 kV/m) compared with that of natural CG lightning.

Spatiotemporal characteristics of positive cloud-to-ground lightning discharges and bidirectional leader of the lightning

The three-dimension spatiotemporal development characteristics of positive cloud-to- ground (CG) lightning discharges have been analyzed by using the data measured by the lightning mapping array system with high time and space resolution. The results indicate that a positive CG lightning discharge can be divided into three stages based on the characteristics of its development. The first stage is discharge process in cloud with a long duration preceding the return stroke. This process with an average of 370 ms propagated at velocity of 10 5 m/s and produced intensive radiation with a magnitude equal to that of the negative leader. During this stage, the lightning channels developed horizontally in the positive charge region with few branches as the negative polarity breakdown. During the stage after the return stroke of the positive CG lightning, the lightning channels propagated at velocity of 2 times faster than that before the return stroke. This stage involved lots of positive fast impulses and corresponded to the continuing current process producing less and dispersed radiation points and more intensive radiation powers. During the final stage of the positive CG lightning, the lightning channels developed at velocity equal to that before the return stroke and the radiation points appeared mainly at the ends of channel. The spatiotemporal development characteristics of the positive CG lightning are very different from that of the negative CG lightning. All of the radiation points of the positive CG lightning appeared in the positive charge region of cloud. Little or no radiation was detected during the positive leader just before the return stroke. The duration of the positive CG lightning was an average of 730 ms. The positive CG lightning discharges with lasting time of 500―600 ms were 43%. 90% of the positive CG lightning discharges involved one return stroke, the most return stroke number being four. The current of the return stroke was an average of 36.5 kA with maximum and minimum values of 70 kA and 11.5 kA respectively. 40% of the return stroke currents were more than 40 kA.

Initiation and development of first lightning leader: The effects of coronae and position of lightning origin

This paper addresses, by means of numerical simulations, the issues of lightning initiation near tall, grounded objects and lightning development in the cloud-to-ground gap. Taken into account is the effect of coronae on the initiation of an upward leader from the top of a fixed grounded structure that is exposed to the electric field produced by the storm-cloud and approaching downward leader. It is shown that the potential carried by a downward leader to the ground and, consequently, the return stroke current depend greatly on the position of a bi-directional leader origin inside a cloud and on the shape of the leader path.

Characteristics of Leader Processes in Altitude‐Triggered Lightning

AbstractThe physical processes of leader in one triggered lightning flash using the altitude‐triggering technique are studied. The main characteristics involved in the triggered lightning are inferred from data of simultaneous electric field change measurements at two stations and highly time‐resolved optical signals. The results reveal the characteristics of leader propagation in triggered lightning. When rocket attached thin steel wire is ascending to the height of several hundred meters, a bi‐directional leader is initiated at top and bottom end of the wire. When the negative leader propagating downward approaches to the ground, a positive connecting leader is initiated from the ground and propagates upwardly. The mean speed of the negative and connecting positive leader is around 0.86×105 m/s. A mini‐return stroke is produced with the connection to the ground of the downward negative leader. The upward positive leader initiated from the top end of wire propagates at a speed of about 1.1×105 m/s.

Simultaneous observations of optical and electrical signals in altitude‐triggered negative lightning flashes

This paper presents an analysis of the experimental data on five negative lightning flashes initiated using the altitude‐triggering technique in China. The data include highly time‐resolved optical images and electric fields measured 60 m and 1300 m from the lightning channel. The triggering technique involves the launching upward of a small rocket trailing a wire electrically floating. The data show that these 5 flashes have a similar chronological sequence of events, including a bidirectional leader system followed by a mini‐return stroke and a bidirectional discharge process. The bidirectional leader system consists of an upward positive leader initiated from the top of the wire and a downward negative stepped leader from the bottom, with the onset of the former prior to the latter by 3 to 8.3 ms. The downward negative stepped leader, having a step interval of 12–30 μs, appears to pause and resume several times while the upward positive leader extends forward continuously. With the downward negative stepped leader close to ground, a mini‐return stroke occurs between the ground and the bottom of the wire. The mini‐return stroke propagates upward with a speed of 1–2 × 108 m/s and emits intense light signals similar to a normal return stroke below the bottom of the wire. It becomes invisible after entering the bottom of the wire and appears again as a bright upward discharge from the top of the wire several microseconds later. This upper bright discharge ceases after propagating forward several hundred meters at a speed of 1.5–5.4 × 107 m/s. The cessation of the upper bright discharge is obviously associated with the disintegration of the wire at that moment. Right after the cessation of the upper bright discharge, a bidirectional discharge process starts from the bottom of the wire with its positively charged part having an upward speed of 3–10 × 105 m/s and its negatively charged part a downward speed of 2–2.6 × 105 m/s. Reflection of current waves at the bottom of the wire due to the explosion of the wire at that moment may be a major reason for the occurrence of this lower bidirectional discharge.

Verification of Bi-directional Leader Concept by Interferometer Observations

Verification of Bi-directional Leader Concept by Interferometer Observations

Bidirectional propagation of lightning leader

The characteristics of initial leader of triggered lightning flashes are studied by use of a high-speed digital camera system with the rate of 1 000 frame per second and simultaneous measurements of current and electric field change with 0.1 μs time resolution. The results show that the altitude-triggered lightning was initiated by bidirectional propagation leader because of “current interruption” by air gap, whereas the grounded-triggered one was not. The onset of downward-moving positive leader was prior to that of the upward-going negative leader for 843 ms in an altitude-triggered lightning under the positive ambient electric field. The downward-moving leader, regardless of its polarity, would produce inhomogeneous distribution of luminosity (current) along the channel on the moment of connection to the ground.

New insights into lightning processes gained from triggered‐lightning experiments in Florida and Alabama

Analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan, Alabama, in conjunction with currents measured at the lightning channel base and with optical observations, allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published “two‐wave” mechanism of the lightning M component. At very close ranges (a few tens of meters or less) the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel. There is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base. The formation of each step of a dart‐stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude‐triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low‐impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely being similar to those in classical triggered lightning. Lightning appears to be able to reduce, via breakdown processes in the soil and on the ground surface, the grounding impedance which it initially encounters at the strike point, so at the time of channel‐base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close ranges the measured M‐component magnetic fields have waveshapes that are similar to those of the channel‐base currents, whereas the measured M‐component electric fields have waveforms that appear to be the time derivatives of the channel‐base current waveforms, in further confirmation of the “two‐wave” M‐component mechanism.

Aircraft lightning initiation and interception from in situ electric measurements and fast video observations

Multipoint electric field measurements on the French C‐160 research airplane presented additional verification for a physical model of aircraft‐triggered lightning as a bidirectional leader initiated on the aircraft. For the few lightning strikes to aircraft that were not aircraft‐triggered, the multipoint electric field measurements proved that these strikes resulted from interception of the aircraft by natural lightning flashes. Video images of lightning channels at 200 frames per second provided the first and only physical evidence of the presence of a positive leader exiting from the positive pole of the airplane's electrical dipole. The existence of a small (&lt;1 A) positive leader current had been predicted in the physical model but had not yet been measured. Weak pulse processes found in magnetic field variations during positive leader development confirm the presence of small amplitude streamers in addition to a continuous leader process.

COMMON PHYSICAL PROCESSES IN NATURAL AND TRIGGERED LIGHTNING IN WINTER STORMS IN JAPAN

An analysis of measurements of electric, magnetic, and radiation field variations produced by rocket-triggered discharges in winter storms in Japan shows that a discharge initiated by a rocket with trailing and grounded wire is a negative leader that consists of continuous current and a pulse series associated with streamer development at the tip. The negative leader is not followed by a process analogous to a return stroke in cloud-to-ground flashes. Application of the electrostatic model of lightning as a bidirectional and uncharged leader in an ambient electric field to analysis of rocket-initiated discharges and positive cloud-to-ground flashes uncovers the commonality of processes occurring in both types of discharges.

Multistroke cloud‐to‐ground strike to the NASA F‐106B airplane

An analysis of electromagnetic waveform records and video images of a multistroke cloud‐to‐ground (CG) strike to the NASA F‐106B instrumented airplane is presented. The CG flash started as a lightning strike triggered by the airplane and later produced multiple return strokes to the ground (the ground strike network registered six return strokes). Although there are some uncertainties in data interpretation resulting from lack of independent measurements with other than the airborne instruments, recoil streamers and eight sequences of dart leaders followed by return strokes (two more than were indicated by the ground network) were identified in the airborne data. At least three of the subsequent return strokes were attached to the airplane. The analysis provides evidence that formation of recoil streamers and dart leaders is accompanied by a surge in continuous current. This feature is similar to that observed in the bidirectional leader development during lightning initiation on the airplane.