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The development of cloud‐to‐ground (CG) lightning channel is one of the main bases reflecting the discharge characteristics. Time‐resolved spectra of CG lightning with eight return strokes were recorded by a slit‐less high‐speed spectroscope. We found an unusual phenomenon in which a branch developed upward from a node following a circling structure into the cloud after the first return stroke. To investigate the propagation properties of this lightning, characteristic parameters such as the two‐dimensional speed of the leader, node temperature, and electron density are analyzed. The physical mechanism of the branch is discussed. Spectral analysis reveals that the main channel temperature and electron density are approximately 2.72–3.07 × 104 K and 0.23–2.03 × 1018 cm−3, respectively. The temperature and electron density increase as the branch approaches the cloud base, similar to the feature of the main channel approaching the ground. The temperature and electron density of the node are higher than at other locations in the circular channel, indicating that massive charge accumulated around the node. Lightning occurred in the Qilian Mountains in a complex landscape, which easily yielded an uneven distribution in the local electric field of the air, resulting in channel bending, kinking, and even circle development. The phenomenon of the branch developed from the node to the cloud may be due to the fact that the branch is formed during the return stroke process. Plain Language Summary: The lightning channel does not usually continue to develop after the lightning return stroke is grounded. However, we discover an unusual phenomenon where the lightning leader formed a circular structure around a node on the main channel during its development. Immediately after the initiation of the first return stroke, a branch developed upward from the node into the cloud. To understand the reason why the branch developed from the node of the main cloud‐to‐ground lightning channel. Based on the time resolved spectra of the channel captured by a slit‐less high‐speed spectroscope, we analyze its physical characteristics, such as the two‐dimensional speed of the lightning leader, the temperature and electron density of the node, the main channel, and the branch channel. Our study found that this phenomenon may be related to the special geographical environment. Key Points: Rare phenomenon of CG lightning branch developed from a node on main channel upwards into the cloud was examined via spectral analysisTemperature and electron density slightly increased as branch channel approaching the cloud baseTemperature and electron density of leader tip reach the maximum when node formed [ABSTRACT FROM AUTHOR]

• Positive cloud-to-ground flashes recorded at 23 locations in the Austrian Alps. • Dataset contains 107 positive Cloud-to-Ground (+CG) flashes including 121 strokes. • 11 multi-stroke +CG flashes including 3 rarely observed single-channel flashes. • 90 % single-stroke flashes, median return stroke peak current 47.4 kA. • Mean duration for 67 continuing currents of the 121 strokes 126.1 ms. Flashes with positive polarity, representing about 10 % of all cloud-to-ground flashes, were analyzed for this publication. Data from positive cloud-to-ground flashes recorded in the Austrian Alps over five years from 2015 to 2022 were used. The measurements were performed to record ground truth flash data at 23 selected locations and to compare their characteristics with values from the literature. The used measurement system consists of a high-speed Video camera and an electric Field Recording System (VFRS). Data of both systems are correlated with Lightning Location System (LLS) data by using a GPS time stamp. The overall dataset showed 107 positive flashes, comprising 121 strokes (11 multi-stroke flashes). The percentage of positive single-stroke flashes is 90 % for VFRS data and 84 % for LLS data. The mean multiplicity for VFRS data is 1.1. The LLS estimated median return stroke peak current of all analyzed positive strokes is 47.4 kA. The mean duration for 67 continuing currents of the 121 strokes is 126.1 ms. The detection efficiency of correctly detected positive flashes and strokes is 96.3 % and 89.3 %, respectively. A better understanding of positive flashes in general and in the Alpine region in particular shall be provided by the presented results. [ABSTRACT FROM AUTHOR]

The radiated electric field produced by lightning plays a vital role in lightning location systems (LLSs). The presence of the elevated objects, i.e. transmission lines in the presented study, result in an increase of amplitude and multiple sub‐peaks in the observed electric fields. On the basis of the electric fields obtained from multiple sensors of Foshan total LLS, electric fields from nine negative lightning strokes associated with transmission‐line faults at a distance of 10–86 km in Foshan area were examined in detail, which were adopted to make comparison with the radiated electric fields from natural negative cloud‐to‐ground lightning detected nearby within 1 min. It can be found that the arithmetic means (AMs) of initial electric‐field peak values of waveforms associated with transmission‐line faults are about twice those of natural negative lightning and the second and third sub‐peaks can be identified obviously. In addition, the correlation between electric‐field waveform characteristics and propagation distance was validated. The AM zero‐crossing time, fast‐transition 10–90% rise time and the time interval between sub‐peaks slightly increase with distance. [ABSTRACT FROM AUTHOR]

We analyzed currents associated with upward connecting leaders (UCLs) initiated from the Kennedy Space Center Industrial Area Tower in two negative cloud-to-ground strokes that struck the tower. One stroke was also recorded using a high-speed video camera located 760 m from the KSC IAT. The NLDN-reported peak currents for the two strokes were − 31.7 and − 98.5 kA. During the UCL development phase the current waveforms exhibited a monotonically (quasi-exponentially) increasing "background" current overlaid with 10-μs scale pulses with a median amplitude of 51.1 A. The UCL current durations for the two strokes were 1039 and 449 μs, respectively. During the pre-attachment processes (UCL and slow front) the total negative charge effectively transferred to ground were 70.2 and 55 mC, respectively. For the stroke captured on high-speed video, the average line-charge-density for the 109-m long UCL was found to be 0.5 mC/m. The average UCL 2-D speed was 2.4 × 105 m/s, and it was observed to accelerate toward the downward leader prior to attachment. We observed that UCL-pulse amplitudes are larger, background currents are higher, and interpulse intervals are shorter at later times during UCLs, which can be attributed to the intensification of the local electric field due to the approaching downward negative leader. The median positive charge injected into the UCL by a pulse was 297 μC. The UCL associated with the higher peak-current stroke produced the highest injected pulse-charge values about three times sooner during its development, likely due to the 2–2.7 times faster average downward leader vertical speeds. • We measured upward connecting leader (UCL) current durations of 1039 and 449 μs. • We computed an average line-charge-density of 0.5 mC/m for a 109-m long UCL. • UCL was observed to accelerate toward the downward leader prior to attachment. • Median pulse amplitude and pulse-charge were larger for UCLs than those for UULs. [ABSTRACT FROM AUTHOR]

• Model of lightning-channel formation in negative cloud-to-ground flashes. • Simulation of the lightning channel formation using bidirectional leader theory. • Estimating the Striking Distance using the bipolar leader model. • Estimating the Striking Distance for ground objects over non-uniform ground. • Threshold electric field for initiation of upward leader from ground structures. A model for simulating the process of formation of the lighting channel in negative cloud-to-ground lightning, based on the bidirectional leader theory that allows calculating the evolution of the involved electrical quantities is presented. In particular, a new approach for modeling the effect of the ground by means of surface charges was included and substantially improved the quality of the model results. The tool that implemented the model was applied for developing certain results, including the threshold electric-field intensity required for initiation of the upward connecting leader from ground structures and estimates of striking distances. The striking-distance results tend to be consistent with those provided by expressions based on observations of the real leaders with fast cameras, when their three-dimensional and inclined trajectories are considered. [ABSTRACT FROM AUTHOR]

Abstract: Preliminary breakdown pulses (PBP) of 26 positive and 104 negative cloud-to-ground (CG) lightning flashes observed in winter thunderstorms in Hokuriku region of Japan are analyzed. Pulses in PBP train are mainly bipolar. Zero-crossing time of each pulse is about 7us and total duration of each pulse train is about 1ms. PBPs in negative CG lightning are classified as “BIL type” (59 cases) and “BL type” (45 cases). “BIL type” PBP contains an intermediate stage with little electric field changes. The time interval between PBP and the first return stroke (PBP–RS interval) is on average 5.4ms for “BIL type” PBP, while that for “BL type” PBP is only 1.3ms. Ratio of peak amplitude of PBP to the first return stroke (PBP–RS ratio) is on average 0.47 and 0.44, respectively, for “BIL type” and “BL type” PBPs, but the ratio for “BIL type” PBP has a much wider distribution. It is speculated that the intermediate stage in “BIL type” PBP is caused by horizontal propagation of leader channel. PBPs in positive CG lightning are classified as +PBP (11 cases) and −PBP (15 cases) according to their initial polarities. +PBP and −PBP have similar distributions of both PBP–RS interval and PBP–RS ratio, but their value of PBP–RS interval is much larger and their value of PBP–RS ratio is much smaller than corresponding values of PBPs in negative CG lightning. It is speculated that different initial polarities of +PBP and −PBP in positive CG lightning are caused by different directions of channel propagation. [Copyright &y& Elsevier]

Abstract: In this investigation, time series consisting of electric field waveforms of 15 positive return strokes and 10 negative return strokes were analyzed. The data came from a summer thunderstorm in March in a range of about 200km around São José dos Campos, São Paulo. The electric field recording system consisted of a flat plate antenna with a decay time constant of 260μs and a sample rate of 800,000 samples per second. The bandwidth observed was up to 100kHz and the recording system was synchronized with GPS time and located at São José dos Campos. Wavelet analysis of the electric field waveforms was done in order to investigate the behavior of the return stroke spectrum in time. The return stroke was suggested to be divided into two portions: initial stage and overshoot (for far return stroke) and initial stage and ramp (for close return stroke). The return stroke power spectrum was observed to be distributed in a frequency range with the peak value also distributed in a fraction of this range. Power peaks for ramps are stronger than power peaks for initial stage and overshoot. Finally, it was observed how powerful the wavelet is in the analysis of lightning. [Copyright &y& Elsevier]

• Flashes observed at 33 measurement locations using a VFRS, between 2009 and 2018. • 735 negative cloud-to-ground flashes recorded in Austria during 61 individual days. • Single-stroke flash percentage of present analysis higher than in previous studies. • Investigation of possible influences of thunderstorm characteristics and terrain. • Analyzed dataset shows uniform distribution between 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. [ABSTRACT FROM AUTHOR]

Apparent ionospheric reflection heights estimated using electric field waveforms of negative and positive cloud-to-ground lightning return strokes (-CGs and +CGs) that occurred during the same thunderstorms have been compared. We analyzed electric field waveforms of 101 -CGs and 74 +CGs that were recorded in August 2016 in Florida. For daytime conditions, the mean ionospheric reflection height for lightning electromagnetic pulses (LEMPs) produced by + CGs is larger than that for LEMPs produced by -CGs for either first or subsequent strokes, although the difference is statistically significant only for subsequent strokes. For nighttime conditions, the trend is the same, but the sample sizes are too small to draw meaningful conclusions. For daytime conditions, the mean ionospheric reflection height for subsequent strokes in -CGs is smaller than that for first strokes, while for + CGs the mean reflection heights for first and subsequent strokes are essentially the same. This disparity is related to the fact that subsequent strokes in –CGs usually develop in the channel of the first stroke, while subsequent strokes in +CGs usually form new terminations on ground and, hence, are similar to first strokes. In our dataset, NLDN-reported peak currents for + CGs tend to be higher than for –CGs for both daytime and nighttime conditions and for either first or subsequent strokes. • In daytime, the mean ionospheric reflection height for +CGs is larger than that for -CGs. • In nighttime, the trend is the same as in daytime, but the sample sizes are too small to draw meaningful conclusions. • Peak currents of +CGs tend to be higher than those of -CGs, which could be the reason for larger reflection heights for +CGs. • In daytime, the mean ionospheric reflection height for subsequent strokes in -CGs is smaller than that for first strokes. • For +CGs, the mean reflection heights for first and subsequent strokes are essentially the same. [ABSTRACT FROM AUTHOR]