High-speed video observations of natural cloud-to-ground lightning leaders – A statistical analysis

[1] There are only a few prior reports that detail accurate measurements of the number of strokes in natural negative cloud-to-ground (CG) lightning flashes. These are known as “accurate-stroke-count” studies, and they have been performed using various instruments and techniques. Here, we will examine the lightning characteristics produced by warm-season thunderstorms in two different climate regimes (southern Arizona, United States, and Sao Paulo, Brazil) using the same instrumentation. The lightning parameters were obtained from high-speed video recordings and time-correlated data provided by lightning locating systems (LLS). The use of these instruments has allowed us to measure the number of strokes in each flash and their polarity, the interstroke intervals, the number of different ground contacts, and the durations of the continuing currents. Altogether 209 negative flashes were recorded on video in the United States and 223 in Brazil (containing a total of 1681 strokes), and the majority of these flashes had at least one stroke reported by a LLS. Statistical analyses of these data sets as a whole did not show any significant differences in the characteristics of negative CG flashes from warm-season thunderstorms in both locations. The mean values for all data are as follows: video multiplicity, 3.9; single-stroke flashes, 19.5%; flash duration, 226.5 ms; interstroke interval, 61.5 ms; long continuing current, 17.5%; multigrounded flashes, 49.5%. Those characteristics were very similar to the well-accepted values for negative flashes in other regions. However, there were large storm-to-storm variations.

http://www.inpe.br/webelat/docs/artigos/Paper_Antonio.pdf

A comparative study of negative cloud‐to‐ground lightning characteristics in São Paulo (Brazil) and Arizona (United States) based on high‐speed video observations

High-speed video observations of the fine structure of a natural negative stepped leader at close distance

Data from the Beijing SAFIR 3000 lightning detection system and Doppler radar provided some insights into the three-dimensional lightning structure and evolution of a leading-line and trailing-stratiform (LLTS) mesoscale convective system (MCS) over Beijing on 31 July 2007. Most of the lightning in the LLTS-MCS was intracloud (IC) lightning, while the mean ratio of positive cloud-to-ground (+CG) lightning to -CG lightning was 1:4, which was higher than the average value from previous studies. The majority of CG lightning occurred in the convective region of the radar echo, particularly at the leading edge of the front. Little IC lightning and little +CG lightning occurred in the stratiform region. The distribution of the CG lightning indicated that the storm had a tilted dipole structure given the wind shear or the tripole charge structure. During the storm’s development, most of the IC lightning occurred at an altitude of ∼9.5 km; the lightning rate reached its maximum at 10.5 km, the altitude of IC lightning in the mature stage of the storm. When the thunderstorm began to dissipate, the altitude of the IC lightning decreased gradually. The spatial distribution of lightning was well correlated with the rainfall on the ground, although the peak value of rainfall appeared 75 min later than the peak lightning rate.

Evolution of the total lightning activity in a leading-line and trailing stratiform mesoscale convective system over Beijing

[1] In this paper we extend the Cooray-Rubinstein (C-R) formula into a mixed propagation path (vertically stratified conductivity) and estimate the lightning horizontal electric fields over the mixed path, and we have examined its accuracy at distances of 100 m to 1000 m from the lightning channel by using finite difference time domain (FDTD) When the ground conductivity near the observation point is less than that near the strike point, it is noted that the C-R formula predicts a satisfactory accuracy in the initial several microseconds after the beginning of the return stroke However, when the ground conductivity near the observation point is larger than that near the strike point, the initial negative excursion predicted by the C-R formula is underestimated, and the error increases with the increase of the difference between the two sections conductivity

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JD017331

Examination of the Cooray‐Rubinstein (C‐R) formula for a mixed propagation path by using FDTD

In this paper,total lightning data observed by SAFIR3000 3-D Lightning Locating System was combined with radar data to analyze characteristics of the lightning activity and electric structure of a hailstorm that occurred in Beijing on 31 May 2005The results indicated that there were two active periods for the lightning activity during the hailstorm processThe hail shooting was found in the first periodAfter the end of the hail shooting,lightning frequency decreased suddenlyHowever,more active lightning activities occurred in the second period with lots of them appearing in the cloud anvil regionThe peak of the lightning frequency came about 5 min prior to the hail shootingOnly 616%of the total lightning was cloud-to-ground(CG) lightning,among which 20%had positive polarityThis percentage was higher than that in normal thunderstormsIn addition,heavier positive CG lightning discharge occurred before rather than after the hail shootingIn the stage of the hail shooting,the electric structure of the hailstorm was inverted,with the main negative charge region located around the -40℃level and the main positive charge region around the -15℃levelIn addition,a weak negative charge region existed below the positive charge region transitorilyAfter the hail shooting,the electric structure underwent fast and persistent adjustments and became a normal tripole,with positive charge in the upper and lower levels and negative charge in the middle levelsHowever,the electric structure was tilted under the influence of the westerly wind in the middle and upper levelsThe lightning activity and electric structure were closely related to the dynamic and microphysical processes of the hailstormIt was believed that severe storms with stronger updrafts were more conducive to an inverted tripolar electric structure than normal thunderstorms,and the inverted distribution could then facilitate more positive CG lightning in the severe storms

Total Lightning Characteristics and Electric Structure Evolution in a Hailstorm

Using 2 high-speed cameras, we have recorded 14 negative cloud-to-ground (CG) lightning flashes, half of which are natural and the others are artificially triggered. The two-dimensional (2D) propagation speed of different type leaders and the luminosity of lightning channel are analyzed in detail. Bidirectional leader processes are observed during the initial processes of two altitude triggered negative lightning (ATNL) flashes. The analysis shows: the propagation speed of the upward positive leader (UPL) before the initiation of the downward negative leader (DNL) is at the order of 104-105 m s-1; the UPL can be intensified by the initiation and development of the DNL in the way that the luminosity is enhanced and the speed is sped up; after initiation, the DNL in one ATNL flash propagates downward three times intermittently with interval of about 1 ms, while that in the other ATNL flash propagates downward continuously with a speed at the order of 105 m s-1. In the five classical triggered negative lightning (CTNL) flashes, the propagation speeds of the UPLs vary between 0.35 105 and 7.71×105 m s-1, and the variations of their luminosities and speeds are quite complex during the development processes. Among the four observed natural negative lightning flashes occurred on the land, three have only one return stoke (RS) each and all of their DNLs have many branches with an average speed at the order of 105 m s-1; while the another one has 13 RSs. In the CG flash with 13 RSs, the DNL before the first RS has no obvious branch below 1.4 km above the ground, and its speed ranges from 2.2×105 to 2.3×106 m s-1 between the heights of 0.7 and 1.4 km and exceeds 3.9×106 m s-1 below 0.7 km; preceding the 4th RS, an attempted leader is observed with a speed ranging from 1.1×105 to 1.1×106 m s-1 between 0.8 and 1.5 km. As for the three observed natural negative lightning flashes occurred on the sea, each has only one RS, and each DNL preceding the RS has a few branches, two of which have an average propagation speed at the order of 105 m s-1, and the other of 106 m s-1, respectively. All the DNLs contained in the observed natural negative lightning flashes, except the attempted leader, propagate with gradually increasing luminosity and increasing speed in whole.

Optical Observations on Propagation Characteristics of Leaders in Cloud-to-Ground Lightning Flashes

Review on Inverted Charge Structure of Severe Storms

A new lightning locating technology, called Lightning Mapping Array (LMA), has been developed. The system takes the advantage of GPS technology to measure the times of arrival (TOA) of lightning impulsive VHF radiation events at each remote location. The temporal and 3-D spatial development processes of lightning are described by measurements of the system with high time resolution (50 ns) and space precision (50-100 m). The charge structures in thunderstorms and the relationship between the charge structure and lightning discharge processes are revealed. The temporal and spatial characteristics of preliminary breakdown process in negative cloud-to-ground(CG) lightning discharges are analyzed based on the data of lightning VHF radiation events. The effect of the positive charge region in the lower part of thunderstorms on the occurrence of negative CG lightning discharge is discussed. The results indicate that the preliminary breakdown process with longer lasting time of negative CG lightning discharges is an intracloud discharge process between the negative and positive charge regions located in the lower and middle parts of thunderstorms respectively. It initiated from the negative charge region in the middle part and propagated downward. After entered the positive charge region in the lower part, the lightning channel developed horizontally. The characteristics of preliminary breakdown process are consistent with that of inverted intracloud lightning discharges. The stepped leaders were initiated by the K type breakdown which occurred in the last stage of the preliminary breakdown process and penetrated downward through the positive charge region. The existence of positive charge region in the lower part of thunderstorms is the reason why there is a preliminary breakdown process with longer lasting time in thunderstorms before the first return stroke of negative CG lightning discharges take place.

Positive Charge Region in Lower Part of Thunderstorm and Preliminary Breakdown Process of Negative Cloud-to-Ground Lightning

On the basis of“leader model”and“return stroke model”about cloud-to-ground lightning stroke channel,“leader-return stroke model”was established.This model was used to calculate the slow electric field change data to explore the characteristics of twelve dart leaders and subsequent return strokes in three artificial triggered lightning on August 2,2005. The results indicate that the height of three triggered lightning is respectively 8.68,4.48 and 10.59 km,the line charge density along the leader channel is from 49.31 to 130.05μC/m,the average dart leader speed is from 0.23×10~7 to 1.48×10~7m/s,the average dart leader current is from 0.14 to 1.87kA,the electric potential of the dart leader channel is from 7.94 to 20.33MV, the charge neutralized by subsequent return stroke is from 0.16 to 1.21C,the average subsequent return stroke speed is from 2,61×10~7 to 11.86×10~7m/s and the average return stroke current is from 1.56 to 12.59kA.The analysis also indicates the application of“leader-return stroke model”not only reduce the demand for the number of observation station and observation environment but also provided with preferable calculation capability for the lightning stroke channel.

Zheng Dong

Papers

Total Lightning Characteristics and Electric Structure Evolution in a Hailstorm

Optical Observations on Propagation Characteristics of Leaders in Cloud-to-Ground Lightning Flashes

Review on Inverted Charge Structure of Severe Storms

Positive Charge Region in Lower Part of Thunderstorm and Preliminary Breakdown Process of Negative Cloud-to-Ground Lightning

Leader-return Stroke Model and Calculation of Artificial Triggered Lightning Stroke's Characteristics