Nicolau Pineda

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.

Bidirectional leader development in sprite‐producing positive cloud‐to‐ground flashes: Origins and characteristics of positive and negative leaders

Thirty-five sprite-producing lightning flashes were recorded in nine nights in different seasons at the east coast of Spain with a 3D Lightning Mapping Array (LMA) since July 2011. A low-frequency time-of-arrival network provided data on emissions from return strokes and intracloud processes and a very-high-frequency interferometer network produced complementary lightning data. This study analyzes the bidirectional development of flashes in order to understand the positioning and timing of the positive cloud-to-ground stroke (+CG) and its consequences for charge neutralization by negative leaders, affecting sprite morphology. A summary of negative leader extents, altitudes, and speeds before and after the + CG stroke is provided, as well as positive leader origins and inferred speeds. Negative leader speeds exhibited modes at 105 and 5 × 105 m s−1. Five examples with different evolutions are discussed: (1) Slow bidirectional development with negative leader termination before the + CG stroke; (2) Fast bidirectional development with the negative leader continuing after the + CG stroke. (3) Slow-fast bidirectional development with a negative leader exhibiting a sudden lowering and speed increase; (4) Fast secondary bidirectional development from an in-cloud horizontal positive leader. Negative leaders propagated rapidly into the upper positive charge layer, continuing after the + CG stroke; (5) Slow bidirectional development with a long negative leader (50 km) subject to cutoff while the original positive leader remained trapped inside negative charge. A + CG stroke subsequently occurred under the old negative leader channel. Carrot sprites tended to be associated with fast extending leaders after the stroke, columniform/mixed sprites with slower side branches.

Lightning Detection in Spain: The Particular Case of Catalonia

This chapter reviews the lightning detection in Spain. It focuses on the region of Catalonia (NE Spain), which has the particularity of being covered by two different lightning detection systems. Since 1992, the Spanish Lightning Detection Network provides the location and characteristics of the cloud-to-ground lightning activity for the whole country. In 2003, an interferometric detection system was installed in Catalonia, providing total lightning data for this region. The performance of this system was evaluated using experimental field measurements. Afterwards, the chapter summarizes some aspects of the lightning climatology in Spain and Catalonia. Finally, in the particular case of Catalonia, other aspects are reviewed, like the convective precipitation and the lightning relationship, and the total lightning activity in hail episodes.

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

Dancing sprites: Detailed analysis of two case studies

On 29–30 October 2013, a low-light video camera installed at Pic du Midi (2877 m), recorded transient luminous events above a very active storm over the Mediterranean Sea. The minimum cloud top temperature reached −73°C, while its cloud to ground (CG) flash rate exceeded 30 fl min−1. Some sprite events have long duration and resemble to dancing sprites. We analyze in detail the temporal evolution and estimated location of two series of sprite sequences, as well as 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. (i) In each series, successive sprite sequences reflect time and location of corresponding 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 strokes (<200 C km), and they are shifted few tens of kilometers 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 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 changes in the local conductivity. (iv) The largest and brightest sprite elements produce significant current signatures, visible when their delay is not too short (~3–5 ms).

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 mapping observations of downward lightning flashes to wind turbines

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).

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.

Correction of Dual-PRF Doppler Velocity Outliers in the Presence of Aliasing

AbstractIn Doppler weather radars, the presence of unfolding errors or outliers is a well-known quality issue for radial velocity fields estimated using the dual–pulse repetition frequency (PRF) technique. Postprocessing methods have been developed to correct dual-PRF outliers, but these need prior application of a dealiasing algorithm for an adequate correction. This paper presents an alternative procedure based on circular statistics that corrects dual-PRF errors in the presence of extended Nyquist aliasing. The correction potential of the proposed method is quantitatively tested by means of velocity field simulations and is exemplified in the application to real cases, including severe storm events. The comparison with two other existing correction methods indicates an improved performance in the correction of clustered outliers. The technique proposed is well suited for real-time applications requiring high-quality Doppler radar velocity fields, such as wind shear and mesocyclone detection algorithms,...

Analysis of energetic radiation associated with thunderstorms in the Ebro delta region in Spain

The analysis of high-energy background radiation (0.1 – 2 MeV) enhancements during eight winter thunderstorms and ?ve summer storms in the Ebro delta region in the northeast of Spain is presented. For the ?rst time, high-energy radiation counts, precipitation, radar re?ectivity, and very high frequency lightning detections to infer charge regions altitude have been analyzed in order to ?nd out what produces the measured background radiation increments associated with storms. The good agreement between radar re?ectivity and precipitation with increases in background radiation counts coupled with the spectrum analysis comparing rain/no rain periods suggests that radon-ion daughters play a major role in the radiation increments reported. No evidence has been found supporting that measured background radiation enhancements can be produced by storm electric ?elds. Finally, a single case of a high-energy radiation increase was prior to a cloud-to-ground lightning stroke, which reinforces the theory that a lower positive charge layer’s existence is important for the production of Terrestrial Ground Enhancements.