V. March

High-speed intensified video recordings of sprites and elves over the western Mediterranean Sea during winter thunderstorms.

We report the first intensified high‐speed video images of elves, sprites, and halos observed in Europe. All the events corresponded to winter season thunderstorms over the Mediterranean Sea. The observations comprise many elves generated by both cloud‐to‐ground lightning current polarities. In 8 of the 14 sprite observations we observed an elve previous to the sprite. In three cases we observed also an elve quickly followed by a halo and a sprite. In several observations we observed lightning light before the mesospheric transient luminous event. We present a case where the lightning from cloud tops was visible during the entire event. Thanks to the high‐speed videos and their resolution and low intensifier phosphor persistence we analyzed the timing distribution of the development phase of sprite elements, the persisting luminosity phase, and the total duration. Finally, we summarize one particular observation where a streamer collides and bounces with a previous formed column; it may be a new phenomenon of collision between an existing column body that interacts with a later streamer point‐like tip which is not merged and attached.

X-rays from laboratory sparks in air: The relationship between runaway electrons and the electric field

This article presents an experiment in which the gap distance was varied between 46 and 84 cm. Series of 15 voltage impulses were applied for each distance and different peak voltages with a rise time of 600 ns. The results of a total of 20 series of impulse voltages show that emissions depend on the electric field derivative within the gap.

Winter lightning activity in specific global regions and implications to wind turbines and tall structures

The paper presents winter lightning maps on specific regions in the northern hemisphere. Four different degrees of winter lightning activity are defined based on information derived from Japanese case. Based on this reference case it is possible to determine regions where winter lightning can be a threat to specific structures. Guidance on risk assessment to tall structures and wind turbines are described as well.

Lightning risk assessment to wind turbines: Methodology and guidelines

The paper deeps into the existing problem in industry for Lightning Risk Assessment on wind turbines. The method is based on current IEC standard but addresses the influence of lightning data quality, terrain influence, winter lightning activity and multiple ground strike terminations for downward and upward lightning. Guidelines and Recommendations for the assessment are deeply described in the paper. A validation of the method is described as well.

High-speed video recordings of lightning, electric fields and high-energy detections during thunderstorms in Catalonia Spain

High-speed video recordings of natural lightning flashes show amazing details of this phenomenon. The paper summarizes the results of the 2009 measurement campaign where the combination of electric fields, total lightning, high-energy detections and high-speed videos provided a valuable data. The paper describes the visible differences between the recorded negative, positive and upward cloud-to-ground flashes. Thanks to the precise time synchronization of each video frame we investigated which processes are detected by the lightning location networks (VHF and VLF). We suggest that the VHF interferometer network detects mostly those processes of intra-cloud flashes where substantial charge is transferred during few milliseconds. The study of the detections of intra-cloud events provided by the VLF network were related to high and fast (less than a millisecond) charge transfers within the cloud that saturated the camera in the same manner as return strokes in cloud-to-ground flashes. In other hand, the paper discusses the high-energy detections related to a very close cloud-to-ground lightning flash.

High-speed videos of laboratory leaders emerging from wind turbine blade tips

This paper presents some observation results of the attachment test of a wind turbine blade as described in the upgrade of the IEC-61400-24. The optical observations were performed by means of high-speed video and UV-sensitive photo camera. This study investigates the effects on the leaders emerging from the blade comparing three different air-terminals: discrete receptors, discrete receptors with diverter strips and metallic tip. In the first two types, salt mist pollution was applied to the blade. For non polluted tips positive leaders appeared from receptors towards the ground avoiding the blade surface. When the blade was polluted additional leaders emerged moving horizontally. Moreover, in this situation the leader progressed on the blade surface. Pollution also produced the inception of more leaders when the blade was equipped with discrete receptors and diverter strips.

Total lightning observations to wind turbines

In summer 2011 a new VHF Lightning Mapping Array was installed at the northeast of Spain. In that area a VHF interferometer and a VLF/LF lightning detection networks are also operative. The close presence of wind farms in the area of the Lightning Mapping Array showed some lightning activity not reported before. This paper describes the observations and discusses the possible effects to the wind turbine lightning protection.

Two upward lightning at the Eagle Nest tower

A new instrument composed by a high speed camera, two high energy detectors, a E-field antenna and a VHF antenna were installed at the Eagle Nest tower (northeast of Spain) during summer 2011. With this equipment several lightning flashes to the tower and its vicinity have been observed. This paper presents two examples: the first was an upward negative leader triggered by a close cloud-to-ground flash and the second was an upward negative flash not associated with previous lightning activity. The paper also presents 3D mapping of leaders of two upward lightning.

A propagative model for simulations of electric fields produced by downward leaders

This paper presents simulations of a downward negative leaders performed by means of 3D finite element method (FEM). The leaders are considered do progress vertically leaders in which different charge distributions and leader speeds can be adjusted. The output of the model is the electric field at ground level for different leader heights and time. By comparison with experimental measurements of natural cloud-to-ground stepped leaders we find consistent results by adjusting the speed of the leader.

Measurement of high energy particles in a lightning impulse voltage test in air

Detections of high energy particles in a high voltage laboratory during the application of lightning impulse voltages of both polarities with a rod-rod configuration has been developed. The detections of high energy radiation in the spark disagree with conventional electrical breakdown mechanism. The high electric fields during the ionization process could be related with the appearance of these particles.