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Initial global extremes in lightning duration and horizontal distance were established in 2017 (Lang et al. 2017) by an international panel of atmospheric lightning scientists and engineers assembled by the WMO. The subsequent launch of NOAA’s latest GOES-16/17 satellites with their Geostationary Lightning Mappers (GLMs) enabled extreme lightning to be monitored continuously over the western hemisphere up to 55° latitude for the first time. As a result, the former lightning extremes were more than doubled in 2019 to 709 km for distance and 16.730 s for duration (Peterson et al. 2020). Continued detection and analysis of lightning “megaflashes” (Sequin, 2021) has now revealed two flashes that even exceed those 2019 records. As part of the ongoing work of the WMO in detection and documentation of global weather extremes (e.g., El Fadli et al. 2013; Merlone et al. 2010), an international WMO evaluation committee was created to critically adjudicate these two GLM megaflash cases as new records for extreme lightning. We thank S. A. Rutledge and two other reviewers for their valuable comments. M. J. Peterson was supported by the U.S. Department of Energy through the Los Alamos National Laboratory (LANL) Laboratory Directed Research and Development (LDRD) program under project number 20200529ECR. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract 89233218CNA000001). T. Logan supported by a NOAA Grant NA16OAR4320115 “Lightning Mapper Array Operation in Oklahoma and the Texas Gulf Coast Region to Aid Preparation for the GOES-R GLM.” I. Kolmasova was supported by GACR Grant 20-09671. S. D. Zhang was supported by a NOAA Grant NNH19ZDA001N-ESROGSS. The participation of J. Montanya in this work is supported by research Grant ESP2017-86263-C4-2-R funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe,” by the “European Union”; and Grants PID2019-109269RB-C42 funded by MCIN/AEI/10.13039/501100011033.

In this work we simulate lightning by means of vertical wires deployed by a multirotor drone under fair weather conditions. These wires, when isolated from ground, acquire an electric potential by virtue of electrostatic induction by the atmospheric potential. When the wires are short circuited to ground or seawater a current is measured sharing some features similar to return strokes in lightning. The results show how currents have higher amplitudes when the wires are short circuited to seawater compared to ground. This work was supported by a research grant from the Spanish Ministry of Economy EXPLORA call with reference ENE2017-91636-EXP and the Grant PID2019-109269RBC42 funded by MCIN/AEI/10.13039/501100011033. This work is also supported by the MIT MISTI Global Seed Funds MIT-Spain ‘La Caixa’ projects “Smart Lightning Protection Systems for Wind Turbines and Aircraft” and “Small Unmanned Airborne Systems for Atmospheric Electricity Research”. Also, the Industrial Doctorate grant of the AGAUR with reference to 2019 DI 039 .

It is well known that tall structures such as telecommunication towers can be exposed to numerous lightning incidence. Wind turbines are in this group, but two particular characteristics distinguish wind turbines from the rest. The first peculiarity is that a substantial part of the structure is rotating. That corresponds to the rotor blades. The second, is also related to the rotor blades and corresponds to the intensive use of composite materials. Due to these two factors, protection against lightning requests very special attention.

It is well known that tall structures such as telecommunication towers can be exposed to numerous lightning incidence. Wind turbines are in this group, but two particular characteristics distinguish wind turbines from the rest. The first peculiarity is that a substantial part of the structure is rotating. That corresponds to the rotor blades. The second, is also related to the rotor blades and corresponds to the intensive use of composite materials. Due to these two factors, protection against lightning requests very special attention.

During the night of October 29-30, 2013, a low-light video camera at Pic du Midi (2877 m) in the French Pyrénées, recorded TLEs above a very active storm over the Mediterranean Sea. The minimum cloud top temperature reached -73 °C at ~1600 UTC while its cloud to ground (CG) flash rate reached ~30 fl min-1. Some sprite events with long duration are classified as dancing sprites. We analyze in detail the temporal evolution and estimated location of sprite elements for two cases of these events. They consist in series of sprite sequences with a duration that exceeds 1 second. By associating the cloud structure, the lightning activity, the electric field radiated in a broad range of low frequencies and the current moment waveform of the lightning strokes, some findings are highlighted: (i) In each series, successive sprite sequences reflect the occurrence time and location of individual positive lightning strokes across the stratiform region. (ii) The longer time-delayed (> 20 ms) sprite elements correspond to the lower impulsive charge moment changes (iCMC) of the parent stroke (< 200 C km) and they are shifted few tens of kilometres from their SP+CG stroke. However, both short and long time-delayed sprite elements also occur after strokes that produce a large iCMC and that are followed by a continuing current. (iii) The long time-delayed sprite elements produced during the continuing current correspond to surges in the current moment waveform. They occur sometimes at an altitude apparently lower than the previous short time-delayed sprite elements, possibly because of the lowered altitude of the ionosphere potential. (iv) The largest and brightest sprite elements produce significant current signatures, visible when their delay is not too short (~3-5 ms).

a lightning flash is defined by the initiation and multiple development of leaders. But lightning leaders are still poorly understood. Since most of my atmospheric electricity has been focused on lightning that means that it has been devoted to understand lightning leaders. In this work, I summarize my investigations related to different aspects of lightning leaders. That includes high-speed video observations of lightning leaders, investigations of bidirectional development of leaders by means of radio mapping systems, research on the high energy emissions produced by lightning leaders and the study of upward leader initiation from wind turbines and development. In the second part, the need of an improved lightning leader model is exposed and proposed as future work. Lightning leader models can be approached by different ways depending on the purpose but agreement between all the approaches is still not complete.

Electromagnetic field from simulated double-headed mid-gap streamers is computed. The effect of the applied electric field is studied by considering electric fields higher than the conventional breakdown threshold (‘overvoltage’). The obtained results show a 20 dB decay at 3.8 GHz for conventional breakdown electric field, 35 GHz for ambient electric fields five times the conventional breakdown and 170 GHz for fields ten times the conventional breakdown electric field. The limitation in nature of the intense electric fields suggest further experiments using a radio receiver at frequencies of more than 10 GHz in order to investigate the origin of the detected x-rays in laboratory sparks.

Negative downward leaders that produced lightning strokes to wind turbines are identified by means of the Lightning Mapping Array data of the Ebro Valley Laboratory (NE Spain). Four cases are analyzed together with weather radar imagery. All flashes hitting wind turbines were originated in small convective cells with moderate development. Notwithstanding the moderate convection, all cases occurred under what can be called “out of season conditions”, where the ‘‘charging zone’’ is located closer to the ground and may favour downward leaders to tall structures. Cloud charge structures showed negative region from 3 km to more than 5 km with a low positive charge below (not always detectable).

Sprites are Transient Luminous Events (TLEs) that can extend vertically from 40 to 90 km and horizontally over several tens of km to form clusters of individual or multiple column or/and carrot-shaped luminous elements. They can even extend over more than 100 km in the form of sequential luminous emissions that are called “dancing sprites”. Their optical detection and other parameters describing the storm and the lightning activity associated allow us to understand their conditions of production and their links with the lightning activity. Our observations confirm some characteristics of the sprites and put forward others: (i) the sprites are essentially produced above the stratiform region of the Mesoscale Convective Systems after positive cloud-to-ground lightning flashes that produce large Charge Moment Change (CMC), with a delay of as much shorter than the impulsive CMC (iCMC) is large. (ii) The dancing sprites reflect the timing and the location of the successive lightning strokes that generate them. (iii) The sprite elements can be shifted from the stroke location when their delay is large. (iv) Bright sprites produce current signatures in ELF radiation a few milliseconds (< 5 ms) after the positive strokes that generate them.

The 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 is included. In the analysis, each flash is simplified by a confidence ellipse fitting most of the detected sources. The major axis of the ellipse is adopted as the flash length. Flash durations are computed too. The analysis of 778 flashes results in a median flash length of ¿14 km with a median duration of ¿0.3 s. The results presented, besides characterizing the storm activity, they can be useful to define stroke grouping criteria, lightning flash density calculations and lightning warning purposes.

Electromagnetic field from simulated double-headed mid-gap streamers is computed. The effect of the applied electric field is studied by considering electric fields higher than the conventional breakdown threshold (‘overvoltage’). The obtained results show a 20 dB decay at 3.8 GHz for conventional breakdown electric field, 35 GHz for ambient electric fields five times the conventional breakdown and 170 GHz for fields ten times the conventional breakdown electric field. The limitation in nature of the intense electric fields suggest further experiments using a radio receiver at frequencies of more than 10 GHz in order to investigate the origin of the detected x-rays in laboratory sparks.

Multi-megawatt wind turbines are a special type of tall structures. Two characteristics stand out from other tall objects such as buildings and telecommunication towers. The first particularity is that a large part of the structure is rotating, and the second, is the extensive use of composite materials in the rotor blades. Due to these two factors, the protection against lightning requests very special attention. Wind turbines installed all over the world have shown how a particular design of their lightning protection system (LPS) can perform very different in areas with different thunderstorm climatology. Up to now, the most severe damages are reported in those areas affected by winter thunderstorms such as Japan and Europe (e.g. Honjo, 2015). These type of storms pose the most favorable conditions for upward lightning initiation from tall structures. That is because the heights of charge regions of winter thunderstorms are significantly low (e.g. Montanyà et al., 2007). On the other hand, most wind turbine blades are built wind composite materials such as glass fibers (GF) and carbon reinforced plastics (CFRP). Blades are typically composed of two shells enclosing a spar. The lighting protection system consists of one or several air terminals connected to a down conductor that brings lightning currents to ground (e.g. IEC 61400-24, 2010). However, the insulation provided by the shells is not perfect and it is possible to experience lightning attachments to non-desired locations. Some blades are equipped with electrically conductive carbon fiber reinforced plastics (CFRP), these components shall be bonded to the lightning protection system (IEC 61400-24, 2010). CFRP laminates are poor conductive presenting high anisotropy. Moreover, electrical connections to CFRP can be very complex because in most of the cases it is not possible to use bolted connections. In the first part of this work lightning interactions to wind turbines are described according to the types of thunderstorms. The effects of the different type of interactions to the calculation of risk assessment is discussed. In the second part, lightning damages are treated. This include a review of the mechanisms of damages and statistics. The case of incidences to blades and protection to CFRP components are highlighted.

Carbon Fiber Reinforced Plastic (CFRP) components are commonly present in new generation of wind turbine blades and aircraft. CFRP components are preferred for its good mechanical strength and low weight. However, CFRP is more vulnerable to lightning currents and arcs. In addition, CFRP can present very anisotropic electric properties that can difficult its lightning protection. In this paper we present two types of tests intended for improving the understanding of electrical properties of CFRP useful for lightning protection. The experimental and simulation study of the potential distribution along a CFRP coupon suggest how the currents are distributed and some defects in the electrical connections. A new method in order to investigate connection effectiveness in full scale wind turbine blades is proposed. The method is minimal invasive. The second part of the paper discusses the possible current paths in different arrangements of connections by means of thermal imaging

In this paper we present the evolution of a bidirectional leader which appeared in ‘virgin’ air. In the observations we did not find any delay between the onsets of the two leader ends. The positive leader end presented a single channel, brighter and slower than the negative end. On the other side, the negative end was branched, less bright and twice as fast as the positive end. The attachment of one of the negative channels with a pre-existing intra-cloud channel showed that the polarity reversal point in the bidirectional leader remained fixed. Recoil leaders appeared along old negative channels of the bidirectional leader. Recoil activity started after the first branching of the positive leader. The discussion is aimed at improving current bidirectional leader models.

This paper presents thunderstorm electrostatic field measurements carried out by electric field mills located at mountainous regions in Spain and Colombia. The profile of electric field changes ¿E due to lightning vs distance is computed by using data from a lightning location system. The effect of the topography on the ¿E vs distance profile is studied in order to identify necessary adjustments for field mills used as lightning warning systems affects by topographycal factors.

Lightning involves high energy processes that still are not well understood. In the laboratory voltages pulses are used to produce long sparks allowing the production of energetic radiation. In this paper X-rays emitted by long sparks in air are simultaneously measured with the RF power radiation at ¿2.4 GHz. The experiment showed that RF power systematically peaks at the time of the X-rays generation (in the measurement time scale). Sparks presents peaks of RF radiation before the breakdown of the gap. The peaks are related to the applied voltage to the gap. RF peaks are also detected in discharges without breakdown. Cases where X-rays are detected present higher RF peak power. The results indicate that at some stage of the discharge before the breakdown electrons are very fast accelerating letting in some cases to produce X-rays. Microwave radiation and X-rays may come from the same process.

El delta de l’Ebre presenta les característiques idònies per ser un gran laboratori a l’aire lliure per a l’observació de llamps i d’altres fenòmens elèctrics, que tenen lloc per sobre dels núvols de tempesta. Aquests cosins dels llamps, de formes sorprenents, no es van descobrir fins a finals dels 90 i han despertat un gran interès en la comunitat científica.

Sprites and Elves are commonly observed in Europe, especially also in winter maritime storms. It is well known that sprites are related to large electric charge positive flashes and elves are related with high amplitude in either negative and positive flashes. The analysis of the occurrence of sprites and elves revealed that sprites can occur inland and over the sea whereas elves prefer to occur over the sea. The distribution of positive and negative flashes during the warm period (April-September) and the cold period (October-March) depicts that flashes with intense amplitudes present a particular pattern. Intense negative flashes occur mostly over the sea and coastal areas during summer and winter whereas intense positive tend to occur in the mountains during the warm season and over sea during the cold period. Also, sprite and elves can indicate regions where energetic flashes occur. This can be a valuable information related to the risk assesment of wind turbines.

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.

Lightning detections of the Catalan Lightning Location Network (XDDE) are compared with high speed video recordings carried out in Spain during summer 2009. At that time the XDDE was composed by four sensors: two SAFIR 3000 type and two LS8000 type. The comparison showed good agreement in the observations at the center and at the south of the network. However, the observations recorded at the north of the network showed a poorer detection efficiency and location accuracy. On the other hand, the fine comparison between frame by frame of video recordings and network detections reveals that the network often detects mostly intra-cloud (IC) sources which probably belong to the preliminary breakdown in downward cloud-to-ground flashes. In some cases few sources are detected during steeped leaders toward to the ground. In the case of our observed IC flashes, the detected sources never corresponded to observations of propagation leaders, if not, small burst of detections were linked to permanent illuminated channels or permanent visible luminosity from the cloud.

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.

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 cloudto- 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.

Wind turbine blades are the most exposed parts of the turbine, and would experience the full impact from the electric fields as associated with the lightning attachment process, the lightning currents, and the magnetic field associated with lightning currents.At some point in time hopes were high that lightning would not strike blades made of non-conducting material only, but practical experiences have clearly demonstrated that this is not the case. Lightning does in fact strike blades without any metallic components, and whenever a lightning arc is formed inside the blade damage is severe.

Wind turbine blades are the most exposed parts of the turbine, and would experience the full impact from the electric fields as associated with the lightning attachment process, the lightning currents, and the magnetic field associated with lightning currents.At some point in time hopes were high that lightning would not strike blades made of non-conducting material only, but practical experiences have clearly demonstrated that this is not the case. Lightning does in fact strike blades without any metallic components, and whenever a lightning arc is formed inside the blade damage is severe.

This study presents an analysis of total lightning and radar data of a severe weather event that took place in the early morning of 2nd November 2008 in Catalonia (NE Spain). A general overview of the synoptic framework and damage observed, including local flash floods, strong wind gusts and an F2-tornado, is given in a companion study

El passat 28 de Juliol del 2007 es va inaugurar oficialment la torre instrumentalitzada per a la mesura de corrents de llamps del Niu de l’Àliga (Alp, 2537 m). Aquesta torre és actualment la cinquena torre actualment operativa en el món, fet que la fa especialment interessant. A més de la mesura de corrents de llamps, també s’hi realitzen observacions de fenòmens lluminosos a l’alta atmosfera i, per altre banda, mesures de tensions induïdes per els llamps en línies aèries. El present article presenta la evolució i un recorregut per aquesta estació de mesura.

El passat 28 de Juliol del 2007 es va inaugurar oficialment la torre instrumentalitzada per a la mesura de corrents de llamps del Niu de l’Àliga (Alp, 2537 m). Aquesta torre és actualment la cinquena torre actualment operativa en el món, fet que la fa especialment interessant. A més de la mesura de corrents de llamps, també s’hi realitzen observacions de fenòmens lluminosos a l’alta atmosfera i, per altre banda, mesures de tensions induïdes per els llamps en línies aèries. El present article presenta la evolució i un recorregut per aquesta estació de mesura.

The features of lightning flashes occurring on tall structures can give some valuable information about their initiation, especially the cases of upward flashes for lightning protection of tall structures such as wind-turbines. More than 300 flashes which struck the Gaisberg tower (Austria) have been analyzed in order to investigate differences in occurrence conditions between winter and summer. More flashes struck the tower in winter unlike the Austrian lightning activity is much lower during this season. These winter flashes deposit to ground more electrical charge than summer ones. The median values of the -10 ºC isotherm altitudes during winter are 2-km lower than during summer. This difference would produce stronger electric fields at ground level which could favor the triggering of upward flashes.

This paper presents the analysis of a hailstorm episode occurred in the city of Terrassa (Barcelona) Spain on the 10th July of 2005 (Figure 1.a)

During last three summers a thunderstorm and lightning related electromagnetic fields measuring campaign were carried out over Catalonia (N-E of Spain). In these campaigns electric and magnetic fields were recorded under and in the vicinities of thunderstorms. The first part of this manuscript presents the measurement campaigns, the equipment employed, available data for posterior analysis such as lightning location data and meteorological radar; and results. Then radar and lightning data of the August 22th of 2003 episode were five sprites were observed is presented and discussed.

During last three summers a thunderstorm and lightning related electromagnetic fields measuring campaign were carried out over Catalonia (N-E of Spain). In these campaigns electric and magnetic fields were recorded under and in the vicinities of thunderstorms. The first part of this manuscript presents the measurement campaigns, the equipment employed, available data for posterior analysis such as lightning location data and meteorological radar; and results. Then radar and lightning data of the August 22th of 2003 episode were five sprites were observed is presented and discussed.

Knowledge of lightning parameters related to electrical atmospheric discharges are essential to predict the severity such as over voltages in power lines or other damages. Today the availability of statistical data related to lightning is still important in order to infer the severity of particular situations such as an engineering design. One of the lightning parameters useful for engineering is the total electric charge related to the entire flash. This paper presents estimations of the total charge deposited by lightning as a result of the analysis of the electric field changes due to lightning.

Els mecanismes d'electrificació d'un núvol, basats en l'intercanvi de càrregues elèctriques entre partícules, avui en dia encara no són perfectament coneguts. Les teories formulades fins ara s’ocupen d’alguns dels processos que intervenen en l’electrificació, però manca una teoria general capaç d’explicar l'elevada electrificació que hi ha en un núvol de tempesta.

Els mecanismes d'electrificació d'un núvol, basats en l'intercanvi de càrregues elèctriques entre partícules, avui en dia encara no són perfectament coneguts. Les teories formulades fins ara s’ocupen d’alguns dels processos que intervenen en l’electrificació, però manca una teoria general capaç d’explicar l'elevada electrificació que hi ha en un núvol de tempesta.

The grounding behaviour is a critical aspect in a windmill lightning protection system because can appear sparks due to a higher surge and damage parts of the windmill [1][2]. This paper proposes a model to a lightning current path across the windmill up to earth including non linear components of the grounding system. Moreover, different lightning current waveforms are employed in the simulations as well as the effect of first return stroke and subsequent return strokes [3]. This performance allow study the effects of the subsequent return strokes that normally have less current amplitude but faster rise times than the first return stroke.

Durante el verano del 2003 se realizó en Cataluña una campaña de medidas de campo electrostático y eléctrico en situaciones de tormenta. La finalidad de esta campaña fue la recolección de datos de campo eléctrico característicos de las tormentas y especialmente de los rayos.