Negative Flashes Research Articles

Attachment processes of negative flashes with multiple return strokes to a tall tower observed at close distances

Two downward negative flashes (F1612 & F1613) with multiple return strokes struck to the 356-m Shenzhen Tower were analyzed. The high-speed camera captured upward positive connecting leaders (UCL) for the first strokes of both flashes and two subsequent strokes of F1612. The observations also show that none of the downward negative leader (DNL) completely propagated to the tower tip before the occurrence of the return stroke (RS), so that we inferred that the UCL initiated from the tower tip might exist for each RS of the two flashes. The possible height of the UCL and DNL connecting point above the tower tip for each RS was estimated, which showed a positive correlation to the speed of corresponding DNL. The striking distances estimated for 8 subsequent RSs in F1612 were in a range from 14.6 m to 85.4 m and that for 3 subsequent RSs in F1613 were from 24.2 m to 66.2 m. There was a weak positive linear relationship between the DNL speed and striking distance for subsequent RSs in the two flashes. The DNL speed showed a positive correlation to the peak current of corresponding RS.

Direct observations of X-rays produced by upward positive lightning

X-rays have been observed in natural downward cloud-to-ground lightning for over 20 years and in rocket-triggered lightning for slightly less. In both cases, this energetic radiation has been detected during the stepped and dart leader phases of downward negative flashes. More recently, X-rays have also been reported during the dart leader phase of upward negative flashes. In this study, we present the observations of four upward positive lightning flashes from the Säntis Tower (2.5 km ASL) in Switzerland. These consist of the simultaneous records of electric current passing through the tower, and electric field strength and X-ray flux 20 m from the tower base. One of the flashes was captured by a high-speed camera operating at 24,000 frames per second, stills from which are also presented. We detected X-rays during the initial phase of upward negative leader propagation, which can be associated with the leader-stepping process from electric field and current waveforms. To the best of our knowledge, this is the first time that such measurements are reported in the literature. The obtained time-synchronised data confirm that the X-ray emissions detected are associated with the initial steps of the upward negative leader. The frequency and energy of X-ray pulses appear to decrease as functions of time, with pulses disappearing altogether within the first millisecond of the leader initiation. X-ray emission also appears to be correlated with the maximum current-derivative and the electric field change of leader steps, consistent with cold electron runaway. These observations contribute to improving our understanding of upward lightning, which is a primary source of damage to tall structures such as wind turbines and telecommunications towers, as well as aircraft during takeoff and landing.

Spatial Structure of Lightning and Precipitation Associated with Lightning-Caused Wildfires in the Central to Eastern United States

The horizontal storm structure surrounding 92,512 lightning-ignited wildfires is examined in the mid to eastern sections of the United States from 2003 to 2015 using Vaisala’s National Lightning Detection Network (NLDN), NCEP’s Stage IV gauge-corrected radar precipitation mosaic, and the US Forest Service’s Fire Occurrence Database. Though lightning flash density peaks strongly around fire ignitions on the instantaneous 1 km scale, on the hourly 10 km scale, both the lightning and precipitation peaks are typically offset from fire ignitions. Lightning density is higher, and precipitation is lower around ignition points compared to non-ignition points. The average spatial distribution of total lightning flashes around fire ignitions is symmetrical, while that of precipitation and positive flashes is not. Though regression is consistent with the claim that positive flashes have a stronger association with ignition than negative flashes, the statistical significance is ambiguous and is contradicted by an unchanging positive flash fraction in the vicinity of wildfires.

On the processes leading to different modes of charge transfer in negative flashes according to fast videos and correlated records of current and electric field

This work discusses the processes involved in different modes of charge transfer in negative lightning flashes, taking as reference video-records obtained with a fast camera and records of current and close electric-field measured at Morro do Cachimbo Station. The data of two downward and one upward negative flashes are explored for considering the features of these processes and for supporting their interpretation, based on the bidirectional and bipolar leader approach. The processes comprise three sequences (stepped leader - first return stroke, dart leader - subsequent return stroke, dart-stepped leader - subsequent return stroke), in addition to the initial continuous current in upward lightning, return strokes following it, continuing current and M-components, the latter observed superimposed on both the continuing current and the tail of return stroke currents. An unreported process of charge transfer, consisting of the reignition of the dissipating leader of an upward lightning from the ground to the cloud is analyzed as well.

Distribution Characteristics and Formula Revision of Lightning Current Amplitude and Cumulative Probability in Zhejiang Province

The cumulative probability distribution of the lightning current amplitude can reflect the lightning activity and is important for the lightning protection engineering technology. Based on the lightning location data from 2009 to 2019, the distribution characteristics of the lightning current amplitude and cumulative probability are studied. The results show that a total of 8,634,858 lightning flashes occurred in Zhejiang province in last 11 years. Negative flashes are much more common than positive ones, accounting for 87.44% of the total. The percentage of the lightning current amplitude between 2 and 100 kA is 94.3%. The average amplitude of positive flashes is significantly higher than that of negative flashes. For the current amplitude of positive flashes, the peak occurs in March and gradually decreases with the increase of the month, reaching its lowest in December. But the monthly distribution of negative flashes is relatively concentrated, especially from February to October. When lightning current amplitude is higher than 25.5 kA, the cumulative probability of positive flashes is significantly higher than that of negative flashes and vice versa. The cumulative probability distribution of negative flashes is closer to that of total flashes and is significantly different from that of positive flashes. In order to get a more suitable cumulative probability formula of lightning current amplitude and corresponding parameters in Zhejiang province, the Levenberg–Marquardt method is used. By fitting the measured data with the formulas recommended by the Institute of Electrical and Electronics Engineers (IEEE) and Electric Power Industry Standard of China (Code for design of overvoltage protection and insulation coordination for AC electrical installations GB/T 50064-2014), respectively, the correlation coefficients of different polarities are obtained. By comparing the aforementioned two fitting results, it is found that the formula recommended by the IEEE can more objectively reflect the probability distribution characteristics in Zhejiang province, and the accuracy of the derived formula is verified by using the lightning current data in 2020. It shows that the curve based on the measured values is consistent with that fitted by the IEEE formula but quite different from that fitted by the GB/T 50064-2014 formula.

Return-stroke current measurement at the Canton Tower and preliminary analysis results

The lightning current on the 600-m high Canton Tower was measured directly for the first time, using a Rogowski coil installed at a height of 492 m above ground level. Return-stroke current measurements for three single-stroke downward negative flashes and two multiple-stroke upward negative flashes were presented and preliminarily analysed by combining the associated return-stroke luminosity and data of the lightning location system (LLS). The 10-90% rising time and the full width at half maximum (FWHM) of the first-stroke current were within a range of 6.0 to 9.3 μs and 68.1 to 96.9 μs, respectively. Subsequent-stroke currents were characterized by distinct sharp double peaks, and the typical 10-90% rising time of the initial peak was about 0.4 to 0.5 μs. The FWHM of the subsequent-stroke current ranged from 6.1 to 39.3 μs with AM (GM) value of 18.1 (15.0) μs. An approximately quadratic relation was found between the maximum peak current and the initial peak luminosity for subsequent strokes in the same upward flash. A total of 88% (14/16) of the subsequent strokes were misclassified as intracloud (IC) events by the LLS. The LLS-inferred peak current was overestimated by a larger factor for subsequent strokes than the first strokes.

Leader-chasing behavior in negative artificial triggered lightning flashes

Two special cases of dart leader propagation were observed by the high-speed camera in the leader/return stroke sequences of a classical triggered lightning flash and an altitude-triggered lightning flash, respectively. Different from most of the subsequent return strokes preceded by only one leader, the return stroke in each case was preceded by two leaders occurring successively and competing in the same channel, which herein is named leader-chasing behavior. In one case, the polarity of the latter leader was opposite to that of the former leader and these two combined together to form a new leader, which shared the same polarity with the former leader. In the other case, the latter leader shared the same polarity with the former leader and disappeared after catching up with the former leader. The propagation of the former leader in this case seems not to be significantly influenced by the existence of the latter leader.

Percentage of single-stroke flashes related to different thunderstorm types

This study shows an analysis of data from measurements of natural cloud-to-ground lightning performed in Austria between 2009 and 2018. The measurement system consists of a high-speed Video and electric Field Recording System (VFRS). Over the whole period, 735 negative cloud-to-ground flashes were recorded at 33 different locations on 61 individual days. The measurement locations are scattered across the Austrian territory in Alpine and pre-Alpine terrain. Data from the Austrian Lightning Location System (LLS), ALDIS, are correlated with the collected VFRS ground truth data to complete the dataset. These datasets are used to analyze possible reasons for the detected variation of single-stroke flashes in Austria. The percentage of single-stroke flashes among all negative flashes is higher in this study (27 %) than in previous studies from different countries (12 to 24 %). A classification of thunderstorms does not show any dependency of the single-stroke flash occurrence on different thunderstorm types (based on radar data) or underlying meteorological characteristics (based on vertical wind shear computed from weather stations and radiosondes). In contrast, a possible dependency of the occurrence of single-stroke flashes on underlying terrain (Alpine versus pre-Alpine) is noted.

Ground Potential Rise in Wind Farms due to Direct Lightning

This paper investigates the ground potential rise (GPR) developed in wind farms subject to direct lightning currents representative of first and subsequent negative downward strokes and return strokes associated with upward negative flashes. The wind farm grounding system is studied considering different numbers of interconnected grounding systems and for distinct values of soil resistivity taking into account frequency-dependent soil parameters. A wideband multiport model of the wind farm grounding system is obtained and implemented in a time-domain electromagnetic transient tool to include the aerial elements in simulations, namely the wind turbine blades and tower. It is shown that interconnecting more wind turbine grounding systems in parallel leads to a significant reduction of the low-frequency grounding resistance, whereas the decrease of the impulse impedance is much lower, especially for low-resistivity soils and negative downward subsequent strokes or negative upward lightning return strokes. It is also shown that the presence of the aerial components significantly affects the GPR in the response to lightning currents representative of negative downward subsequent strokes and return strokes associated with negative upward flashes, which have higher frequency content compared to negative first return strokes. The presence of the aerial components increases the GPR peak.

Cloud-to-Ground Lightning in Canada: 20 Years of CLDN Data

ABSTRACT This study presents the spatial and temporal features of more than 45 million cloud-to-ground (CG) lightning flashes recorded by the Canadian Lightning Detection Network for the years 1999–2018. Although sensor upgrades have improved the detection efficiency and location accuracy of CG lightning, the large-scale spatial patterns remain about the same as found in a previous study covering the years 1999–2008. Analyses, using equal-area squares with 10 km sides, describe the regional and seasonal characteristics of negative and positive flashes, the percentage and flash density of positive lightning, and the first-stroke peak currents of both polarities. Lightning activity over the provinces and territories is greatest in the summer, varying from 95.9% to 76.8% of the annual activity in the Northwest Territories and Ontario, respectively. Winter lightning is rare, usually occurring in extreme southern Ontario and the Atlantic Provinces, as well as over offshore regions west of Vancouver Island and the coastal waters off Nova Scotia. Preliminary analysis suggests that, compared with the 1999–2008 period, the majority of western and northern Canada has experienced more lightning days during the 2009–2018 period, whereas much of eastern Canada has experienced fewer lightning days. A statistical analysis performed on 154 stations across Canada found that the decadal increases (decreases) at 5 (31) stations were significant at the 90% confidence level or higher, and 4 (16) of these were significant at the 95% confidence level.

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.

The Spatial Evolution of Upward Positive Stepped Leaders Initiated From a 356‐m‐Tall Tower in Southern China

AbstractTwo initial upward positive leaders (UPLs) in upward negative flashes initiated from a 356‐m‐tall tower were observed with a high‐speed camera, an electric field sensor, and a magnetic field sensor. Although the waveforms of electric fields measured were saturated, both camera images and magnetic fields showed that the two UPLs had obvious stepwise characteristics in their upward moving stage. The magnetic fields associated with the UPLs were characterized by a series of fast‐changing bipolar impulses superimposed on a slowly increasing continuous component as the leaders moved upward, which were well corresponding to the light intensity changes of their high‐speed camera images. The 2‐D step length and step extension speed of the two leaders were estimated in the range of 0.5–3 m and 1.1–9.4 × 105 m/s, respectively. The radius of the leader luminous channel as a function of time and height was also estimated. The channel radius showed first a stable increasing trend in the leader upward moving stage and then 4–5 times of shrinking and expanding processes after the leader connected the cloud. The channel expanding and shrinking speeds were in the range of 2–9 × 104 m/s. Particularly, the channel radius showed an obvious increasing trend with the increase of the height all the time, which was in the range of 2.8–3.5 m when around the tower tip and of 5.6–8.4 m when at 24 m high above the tower tip. Such a feature of the channel radius is in good agreement with the concept of leader corona sheath in literature.

Analysis of the lightning impulse and low-frequency performance of wind farm grounding systems

Previous analysis of the lightning response of wind turbine grounding systems has often considered only one unit. This paper investigates the impulse and low-frequency performance of isolated and interconnected grounding systems of wind turbines, subjected to lightning currents representative of first and subsequent negative downward strokes and return strokes associated with upward negative flashes. The system behavior is studied for different lengths of interconnecting conductor between adjacent grounding systems and for distinct values of soil resistivity considering frequency-dependent soil parameters. It is shown that in the case of lightning currents with shorter front-times, the interconnection improves the impulse response of grounding, reducing the GPR peak, only for soils of higher resistivity, whereas in the case of currents with longer front-times, the interconnecting improves the impulse response in all cases. The relative improvement of the impulse response when the adjacent grounding systems are interconnected becomes more significant with the increase of the soil resistivity. In all cases, interconnecting the grounding systems strongly reduces the low-frequency grounding resistance seen by each individual wind turbine earth termination.

Current Observation Results of Downward Negative Flashes at Tokyo Skytree From 2012 to 2018

Direct observation of lightning current is a fundamental approach to evaluating lightning parameters, which are among the most basic factors for the lightning protection design of various grounded objects. However, for a tall grounded structure, upward lightning flashes initiated from the top of the structure have usually been observed, and the direct observation results of downward flashes have been limited. We developed and installed a pair of Rogowski coils at Tokyo Skytree, which is a 634-m-high freestanding broadcasting tower in Japan (35.71°N, 139.81°E), and have conducted direct observation of lightning flashes striking the tower. As of November 2018, 70 lightning flashes striking the tower, 26 downward and 44 upward, have been observed since the end of February 2012. In this paper, we focus on the downward negative flashes that occurred at the tower for seven years from 2012 to 2018 and report on the observed results of typical waveform parameters of lightning stroke currents in comparison with other observed results on downward negative flashes.

Characteristics of Negative Flashes With Multiple Ground Strike Points Located by the Japanese Lightning Detection Network

Approximately 50% of natural lightning flashes make multiple contacts on the ground. Lightning-induced voltage by nearby lightning stroke is the major cause of overvoltage. The electromagnetic waves emitted by lightning flashes with multiple ground strike points (GSPs) increase new risks to damage electronics information devices located in the vicinity of the GSPs. Therefore, it is very important for electromagnetic compatibility engineering to clarify the characteristics of lightning flashes with multiple GSPs. We have observed lightning discharges over the whole area of Japan detected by the Japanese Lightning Detection Network (JLDN). Specifically, we analyzed negative cloud-to-ground (CG) flashes observed by JLDN within 80 km of Sagamihara in summer. We investigated the median location accuracy of the JLDN in the observation area to evaluate characteristics of spatial distribution of subsequent strokes hitting the GSPs. The median location accuracy varied between 0.2 and 0.4 km in the observed area. The mean multiplicity of negative CG flash observed by JLDN was 3.5. The average inter-stroke interval of new ground contact (NGC) strokes was longer than that of pre-existing channel (PEC) strokes. The ratio of peak current to the first return stroke was higher in the NGC than that in the PECs. Some of flashes hit the ground at several points. The mean number of GSPs was 2.0 per flash, with the mean number of 1.6 subsequent strokes per a GSP. Furthermore, the number of strokes per a GSP has not been previously reported. We also observed that the subsequent strokes altered between different GSPs.

Variation of Atmospheric Temperature with Height in the Phenomena of Lightning Waveforms

The temperature at different height at different places is different, it may depend on various factors. The variation of atmospheric temperature varies as a function of height taken from the sea level. The temperature decreases with height in the troposphere. The location of the charge centre appears to be determined by temperature and not by the height of the cloud above the ground. The main negative charge centre is generally located between the -10°C and -25°C isotherms with the main positive charges some km higher up. Due to the variation of temperature, the ice crystal, super cooled water droplet etc. move updraft and downdraft, forms the charge in the cloud due to which the lightning phenomena occurs. In the cloud, the negative charge lies in the central part of the cloud, positive charge remains on above the negative charge centre in the cloud and small amount of positive charge as a pocket charge remains on the bottom part of the cloud. The positive ground flashes are initiated from the upper positive charge region and the negative ground flashes are initiated from the negative charge region in the thunder-cloud. The lower positive charge also play an important role for determining the different types of lightning.

Length estimations of presumed upward connecting leaders in lightning flashes to flat water and flat ground

Using video data recorded at 50,000 frames per second for nearby negative lightning flashes, estimates are derived for the length of positive upward connecting leaders (UCLs) that presumably formed prior to new ground attachments. Return strokes were 1.7 to 7.8 km distant, yielding image resolutions of 4.25 to 19.5 m. No UCLs are imaged in these data, indicating those features were too transient or too dim compared to other lightning processes that are imaged at these resolutions. Upper bound lengths for 17 presumed UCLs are determined from the height above flat ground or water of the successful stepped leader tip in the image immediately prior to (within 20 μs before) the return stroke. Better estimates of maximum UCL lengths are determined using the downward stepped leader tip's speed of advance and the estimated return stroke time within its first frame. For 17 strokes, the upper bound length of the possible UCL averages 31.6 m and ranges from 11.3 to 50.3 m. Among the close strokes (those with spatial resolution <8 m per pixel), the five which connected to water (salt water lagoon) have UCL upper bound estimates averaging significantly shorter (24.1 m) than the average for the three close strokes which connected to land (36.9 m). The better estimates of maximum UCL lengths for the eight close strokes average 20.2 m, with slightly shorter average of 18.3 m for the five that connected to water. All the better estimates of UCL maximum lengths are <38 m in this dataset

Current Observation Results of Upward Negative Flashes Initiated from Tokyo Skytree during 2012 to 2016

Current Observation Results of Upward Negative Flashes Initiated from Tokyo Skytree during 2012 to 2016

Cloud-to-ground lightning activity in Colombia: A 14-year study using lightning location system data

This paper presents the analysis of 14years of cloud-to-ground lightning activity observation in Colombia using lightning location systems (LLS) data. The first Colombian LLS operated from 1997 to 2001. After a few years, this system was upgraded and a new LLS has been operating since 2007. Data obtained from these two systems was analyzed in order to obtain lightning parameters used in designing lightning protection systems. The flash detection efficiency was estimated using average peak current maps and some theoretical results previously published. Lightning flash multiplicity was evaluated using a stroke grouping algorithm resulting in average values of about 1.0 and 1.6 for positive and negative flashes respectively and for both LLS. The time variation of this parameter changes slightly for the years considered in this study. The first stroke peak current for negative and positive flashes shows median values close to 29kA and 17kA respectively for both networks showing a great dependence on the flash detection efficiency. The average percentage of negative and positive flashes shows a 74.04% and 25.95% of occurrence respectively. The daily variation shows a peak between 23 and 02h. The monthly variation of this parameter exhibits a bimodal behavior typical of the regions located near The Equator. The lightning flash density was obtained dividing the study area in 3×3km cells and resulting in maximum average values of 25 and 35flasheskm-2year-1 for each network respectively. A comparison of these results with global lightning activity hotspots was performed showing good correlation. Besides, the lightning flash density variation with altitude shows an inverse relation between these two variables.

Striking Distance Determined From High‐Speed Videos and Measured Currents in Negative Cloud‐to‐Ground Lightning

AbstractFirst and subsequent return strokes' striking distances (SDs) were determined for negative cloud‐to‐ground flashes from high‐speed videos exhibiting the development of positive and negative leaders and the pre–return stroke phase of currents measured along a short tower. In order to improve the results, a new criterion was used for the initiation and propagation of the sustained upward connecting leader, consisting of a 4 A continuous current threshold. An advanced approach developed from the combined use of this criterion and a reverse propagation procedure, which considers the calculated propagation speeds of the leaders, was applied and revealed that SDs determined solely from the first video frame showing the upward leader can be significantly underestimated. An original approach was proposed for a rough estimate of first strokes' SD using solely records of current. This approach combines the 4 A criterion and a representative composite three‐dimensional propagation speed of 0.34 × 106 m/s for the leaders in the last 300 m propagated distance. SDs determined under this approach showed to be consistent with those of the advanced procedure. This approach was applied to determine the SD of 17 first return strokes of negative flashes measured at MCS, covering a wide peak‐current range, from 18 to 153 kA. The estimated SDs exhibit very high dispersion and reveal great differences in relation to the SDs estimated for subsequent return strokes and strokes in triggered lightning.