The numerical modeling of electrical discharges occurring in atmospheric air has been in continuous development during the past decades in different fields, such as high-voltage techniques and lightning protection. Different methodologies have been proposed to represent the physical phenomena taking place at a single full discharge event, departing both from experimental and theoretical approaches. The implementation of these methodologies in numerical routines combined with the use of numerical methods to determine the electric potential distribution permits the creation of models whose predictions closely agree with the real case situations, where electrode arrangements might have nonsymmetric geometries. In this paper, we present an improved version of a simulation methodology for representing electrical discharges in long air gaps. This simulation methodology includes new elements like: 1) the 3-D leader channel tortuosity based on laboratory experimental measurements and 2) two new methods for the estimation of the electric charge contained in the so-called leader-corona region based on the electrostatic potential of fictitious potential rings representing the active region in front of the leader tip. Results from the simulation were compared with experimental records and a reasonably good agreement is found between them.

Temporal variation of the luminosity of seven natural cloud-to-cloud lightning channels were studied, and results were presented. They were recorded by using a high-speed video camera with the speed of 5000 fps (frames per second) and the pixel resolution of 512 x 512 in three locations in Sri Lanka in the tropics. Luminosity variation of the channel with time was obtained by analyzing the image sequences. Recorded video frames together with the luminosity variation were studied to understand the cloud discharge process. Image analysis techniques also used to understand the characteristics of channels. Cloud flashes show more luminosity variability than ground flashes. Most of the time it starts with a leader which do not have stepping process. Channel width and standard deviation of intensity variation across the channel for each cloud flashes was obtained. Brightness variation across the channel shows a Gaussian distribution. The average time duration of the cloud flashes which start with non stepped leader was 180.83 ms. Identified characteristics are matched with the existing models to understand the process of cloud flashes. The fact that cloud discharges are not confined to a single process have been further confirmed from this study. The observations show that cloud flash is a basic lightning discharge which transfers charge between two charge centers without using one specific mechanism.

We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of upward initiated lightning from Gaisberg Tower in Austria which is located at a 1300 m altitude.  This is the first time that the X-rays from upward lightning from a tower top located in high altitude is observed. Measurement was carried out using scintillation detectors installed close to the tower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of three flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to the observations from downward natural or triggered lightning, X-rays were observed only within 10 µs before the subsequent return stroke. This shows that X-rays were emitted when the dart leader is in the vicinity of the tower top and hence during the most intense phase of the dart leader. Both the detected energy and the fluence of X-rays are far lower compared to X-rays from downward natural or rocket-triggered lightning. In addition to above 108 flashes, an interesting observation of X-rays produced by a nearby downward flash is also presented. The shorter dart-leader channels length in Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.

Modeling the lightning attachment process is required on any method to design the air-termination elements of a lightning protection system. An attachment model that adopts the leader progression concept is used to evaluate the three-dimensional attractive zone of a lightning rod on a common 54-m tall building in Sao Paulo, Brazil. Electric field and scalar potential distributions are calculated numerically with a finite element method. The result is compared with the interception volume predicted by the electro-geometric model, as applied by the rolling sphere method. The results show that the electrogeometric theory underestimates the striking distance and the attractive radius. Moreover, in the presence of upward connecting leaders, the striking distance varies according to the field enhancement on the geometry of the structure and the lateral displacement of the stepped leader. The simulated propagated distances and speeds of the downward and upward leaders are compared with a recently published high-speed video analysis of a natural lightning attachment case observed on the evaluated lightning rod. A reasonable agreement between the simulated and measured leader characteristics has been found.

This paper presents the characteristics of the electric field pulses observed during leader propagation in positive ground flashes. We analysed in detail the electric field changes occurring just before the first return stroke in 51 positive ground flashes during 2014 summer thunderstorms in Uppsala, Sweden. Pronounced leader pulses (having the same polarity as the return stroke) were observed in 22% of the cases. They were observed to occur within 1.4ms before the first return stroke. Interpulse duration ranged from 13.3 to 50.3 mu s with a mean value of 24.7 mu s The peak amplitude of the leader pulses relative to the return stroke peak ranged from 2.7 to 17.8%. The presence of these pulses shows that the leaders propagate in a stepped manner. Based on the leader pulses' time of initiation and average speed of the leader, the distance travelled by the leader was also estimated. One case of positive ground flash preceded by opposite polarity leader pulses just before the return stroke is also reported. To the best of our knowledge, this is the first time that such a case in positive ground flashes is reported. We suggest that these opposite polarity leader pulses are due to the negatively-charged leader branch of a bi-directional leader inside the cloud that propagates towards observation point.

Electromagnetic fields generated by accelerating charges can be utilized to evaluate the electromagnetic fields generated by systems where moving charges and/or propagating currents are present. The technique can be used easily to evaluate the electromagnetic fields generated by systems in which propagating currents are present. This is illustrated by utilizing the equations to derive expressions for the electromagnetic fields generated by systems in which current pulses injected by lightning flashes are propagating.

Emission heights for narrow bipolar events (NBEs) have been reported mostly from observations at mid latitudes but none have been reported from tropical regions. In this paper, we are reporting for the first time the heights of NBE emissions from a tropical storm over the Malacca Strait, a narrow water passage between the Malay Peninsula and Sumatra Island. A total of 49 positive NBEs (+NBEs) were detected from the storm. The NBE activity can be divided into two stages according to the emission heights and radar reflectivity data. The first stage (or S1) lasted for only 6 minutes, started with the first detected NBE, and produced 20 NBEs (41%). The emission heights ranged between 12.0 and 16.7 km. Radar reflectivity data showed that the storm reached maximum values at 55 dBZ within the period S1. In contrast, the second stage (S2) lasted longer (32 minutes) and produced 29 NBEs (59%). The emission heights were lower and ranged from 8.5 to 13.7 km. Radar reflectivity data showed that the storm reached maximum values at 50 dBZ within the period S2.

The energy deposited from X-rays generated by 1 m long laboratory sparks in air created by 950 kV negative lightning impulses on scintillated detectors was measured. Assuming the X-ray energy detected in such sparks results from the accumulation of multiple photons at the detector having a certain energy distribution, an experiment was designed in such a way to characterize their distribution parameters. The detector was screened by a copper shield, and eight series of fifteen impulses were applied by stepwise increasing the copper shield thickness. The average deposited energy was calculated in each series and compared with the results from a model consisting of the attenuation of photons along their path and probable photon distributions. The results show that the energy distribution of X-ray bursts can be approximated by a bremsstrahlung spectrum of photons, having a maximum energy of 200 keV to 250 keV and a mean photon energy around 52 keV to 55 keV.