Carina Schumann

Upward lightning flashes characteristics from high‐speed videos

One hundred high-speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high-speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud-to-ground flashes.

Continuing current intensity in positive ground flashes

The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. Brook at al. [1] for one positive lightning in a winter storm in Japan, inferred a charge transfer in excess of 300 coulomb (C) during de first 4ms. But charge transfers to beyond 3000C were reported from direct current measurements, by Miyake et al.[1] for positive winter lightning in Japan. Positive strokes may have high peak currents followed by long continuing current (CC), and thus combine these two threatening features for lightning protection. Although positive flashes are usually less frequent than negative lightning, the special characteristics of their CC make the understanding of positive lightning an important issue. Positive lightning were recorded in southeastern Brazil during the summers of 2009-2011. This study presents some CC intensity estimates obtained from an electric field capacitive antenna. Most CC intensities were much higher than the usual intensity values of CC in negative flashes.

Observations of bidirectional lightning leader initiation and development near positive leader channels

Based on the analysis of high-speed optical and electric field change data, we present three observed cases in which a naturally occurring bidirectional lightning leader initiated and developed in virgin air near a previous established positive leader channel. Twice a new leader formed near an upward propagating positive leader that had initiated from a tower during an upward flash and once a new leader formed near a downward propagating positive leader prior to a positive cloud-to-ground return stroke. There were clear and consistent behavioral differences between the positive and negative leader ends of the newly formed bidirectional leader, and the positive end grew more slowly than the negative end in each case. In all three cases, the negative end of the bipolar leader connected with the previously formed positive leader channel creating a new positive leader branch. These rare observations show the bidirectional nature of naturally occurring lightning and suggest that positive leaders can gain branches by connection with newly formed bipolar leaders.

Detection of upward lightning by lightning location systems

We report upward lightning observations in São Paulo city, SP, Brazil and compare them to data from Lightning Location System (LLS). These data are provided by 4 different networks from different technologies: BrasilDAT, RINDAT, Worldwide Lightning Location Network (WWLLN), and Earth Networks Global Lightning Network (ENGLN). Several upward flashes were observed from 2012-2014 using GPS time-stamped optical sensors and electric field measurements. These upward flashes were initiated from tall towers located at Jaragua Peak (70 and 130 m) and along Paulista Avenue (23 towers with heights between 50 and 220m). Time-correlated analyzes allowed to evaluate the network detection efficiency, intracloud (IC) / cloud-to-ground (CG) misclassification percentage, and location accuracy of the 4 different LLS. We will also show how different upward flash physical processes (leader initiation, recoil leaders, return strokes) are detected and classified by the distinct LLS technologies. Preliminary results show that in general, recoil leaders were not detected; on the other hand, several cases of return strokes and some cases of M-component were detected by one or more LLS networks.

High-speed video observations of positive lightning flashes

Although positive lightning flashes to ground are not as frequent as negative flashes, their large amplitudes and destructive characteristics make understanding their parameters an important issue. This study summarizes the characteristics of 103 positive cloud-to-ground (+CG) flashes that have been recorded using high-speed video cameras (up to 8000 frames per second) in three countries together with time-correlated data provided by lightning location systems (LLS). A large fraction of the +CG flashes (81%) produced just a single-stroke, and the average multiplicity was 1.2 strokes per flash. All the subsequent strokes in multiple-stroke +CG flashes created a new ground termination except one. 75% of the +CG flashes contained at least one long continuing current (LCC) ≥ 40 ms, and this percentage is significantly larger than in the negative flashes that produce LCCs (approximately 30%). The median estimated peak current, (Ip) for 116 positive strokes that created new ground terminations was 39.4 kA. Positive strokes with a large Ip were usually followed by a LCC, and both of these parameters are threats in lightning protection. The characteristics presented here include the multiplicities of strokes and ground contacts, the percentage of single-stroke flashes, the durations of the continuing current, and the distributions of Ip.

Detection of M-components and return strokes in upward and downward lightning flashes

This work aimed to investigate which pulses (M-components and/or subsequent return stroke) of the upward lightning are detected by BrasilDAT lightning location network. It was found that, these pulses are similar to downward lightning pulses and therefore, they were well detected by the network. This report is based on a set of 29 upward lightning and 16 downward lightning whose strike points are known. All lightning used were recorded in the Jaraguá Peak region. High-speed cameras and electric field sensors were used in this analysis to find out the rise time of the waveform and the peak value of the electric field. The comparison with the pulses of the downward lightning recorded by cameras allowed us to validate the BrasilDAT network, determining the detection efficiency and location error. We found a detection efficiency of 37-56%, and a location error of approximately 600 meters. It was observed that the M components of the upward lightning resemble the subsequent return strokes of the downward lightning because the luminosity prior to the pulse is low and the rise time of the electric field waveform is short. The current peaks of the pulses of the upward and downward lightning were intense. These facts explain why BrasilDAT captured these pulses, since the networks tend to discard pulses with low intensity and long rise times.

Triggered upward flashes: Analysis of positive cloud-to-ground waveforms

Upward flashes are a subject of study and research since 1939 [1]. The recent increasing number of tall buildings, wind turbines and telecommunication towers has also increased the need of a better understanding of these lightning flashes, particularly, how they are triggered. Upward flashes may be self-triggered or triggered by some previous electrical activity (intracloud or cloud-to-ground flashes). In Brazil, upward flashes are a recent study subject. The first upward flash was observed in January 2012. All cases observed since then were preceded by some lightning flash. The same situation was observed in upward flashes recorded in Rapid City during 2011-2013 which are included in our dataset. To investigate upward flashes we use high-speed cameras (1,000 up to 100,000 frames per second), electric field sensors and lightning location system data. In this paper, waveforms of fast electric field sensors of positive cloud-to-ground flashes which triggered upward lightning flashes were analyzed and compared with high speed cameras observations.

On the occurrence of recoil leaders in negative upward flashes in Brazil

The processes that give origin to the current pulses during the initial continuous current (ICC), M-components and return strokes in upward flashes are subject of recent studies and require further understanding. Apparently, the common aspect behind these processes relies on the physics that govern the recoil leaders (RL), which is presently not well understood. This study uses high-speed cameras to record images of 80 upward lightning flashes in Sao Paulo, Brazil. From these videos some characteristics of the recoil leaders were analyzed: time interval between the initiation of the positive upward leader and the occurrence of the first RL; the height of the tip of the upward positive leader when the RLs occur; where in the channel the RL initiates; the relationship between branching and presence of RLs, and the initiation of ICC pulses, M-components and dart leaders by RLs.

Features of lightning flashes obtained from high-speed video recordings

Most of what is known about the structure and time evolution of lightning was determined by high-speed photography. The first measurements were obtained using a two-lens streak camera, named Boys camera after its inventor. In a streak camera, a relative movement between the lens and the film is used to record the phases of a lightning discharge. Subsequent improvements of Boys camera allowed measurements of several lightning parameters.

Waveform analysis of cloud-to-ground flashes as detected by fast e-field antennas and lightning location systems: On the way to precisely estimate the stroke peak current

BrasilDAT network is an EarthNetworks Total Lightning System (ENTLS) that detects both intra-cloud and cloud-to-ground discharges. Multiple time-of-arrival (TOA) sensors send to central processor the whole waveform of every event detected. The EarthNetworks Lightning Sensor (ENLS) operates in a wide range (from 1Hz to 12MHz) and was designed to reduce system noise and to broaden the frequency range. At present, BrasilDAT is composed of 56 sensors covering 10 States of Brazil: RS, SC, PR, SP, RJ, ES, MG, MS, GO, and BA. We expect to conclude the full network deployment by the end of 2013 reaching up to 75 sensors covering almost all Brazilian regions (except the Amazon basin). This work intends to compare the fast e-field (E-fast) waveforms recorded during CHUVA project campaign for CG strokes that struck at a known location with the waveform recorded by several sensors at different distances from the flash. We know that waveform shapes recorded by different sensors at different distances from the flash have not just different amplitudes, but also different shapes. We know that even waveform shapes recorded by different sensors at the same distance from a flash have different shapes. To better understand these differences, we will predict electric fields at one location from measurements at many other locations and compare our predictions to measurements. We expect this to improve our propagation model and our site calibrations, and we expect it to provide a measurement of one part of the BrasilDAT peak current error.