G. Solà

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.

Registration of X-rays at 2500 m altitude in association with lightning flashes and thunderstorms

Electric fields and high-energy radiation of natural lightning measured at close range from a mountaintop tower are discussed. In none of the 12 negative cloud-to-ground upward flashes were X-rays observed. Also no energetic radiation was found in one negative upward leader at close range (20?m). In the first of two consecutive negative cloud-to-ground flashes, X-rays were detected during the last ~1.75?ms of the leader. During the time of energetic radiation in the flash an intense burst of intracloud VHF sources was located by the interferometers. The X-ray production is attributed to the high electric field runaway electron mechanism during leader stepping. Even though the second flash struck closer than the previous one, no X-rays were detected. The absence of energetic radiation is attributed to being outside of the beam of X-ray photons from the leader tip or to the stepping process not allowing sufficiently intense electric fields ahead of the leader tip. High-speed video of downward negative leaders at the time when X-rays are commonly detected on the ground revealed the increase of speed and luminosity of the leader. Both phenomena allow higher electric fields at the leader front favoring energetic radiation. Background radiation was also measured during thunderstorms. The count rate of a particular day is presented and discussed. The increases in the radiation count rate are more coincident with radar reflectivity levels above ~30 dBZ than with the total lightning activity close to the site. The increases of dose are attributed to radon daughter-ion precipitation

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.

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.

Lightning flash properties derived from Lightning Mapping Array data

In this paper size and duration of lightning flashes are examined. Data from the Ebro Valley Laboratory Lightning Mapping Array is used as reference. Additional data from the VLF/LF LINET network and meteorological radar is included. In order to simplify the complex geometry of a lightning flash, each flash is described by a confidence ellipse fitting most of the detected sources. The flash length is represented by the major axis of the ellipse. The analysis of 1047 flashes in a set of different type of thunderstorms results in a median flash length of ~20 km with a median duration of ~0.4 s. The results presented here, besides characterizing the storm activity, provide valuable information in order to improve the stroke grouping criteria in the flash algorithm, lightning flash density calculations and lightning warning procedures.

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.

Lightning attachment test on a scale aircraft model as subject for educational purposes

A new subject on lightning protection of aircraft has been developed at the Technical University of Catalonia. This elective subject is implemented according to project-based learning (PBL). In this way, during the subject, students face up to a design and validation of a lightning protection system of an aircraft.