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Narrow bipolar events (NBEs) are impulsive and powerful intracloud discharges. Recent observations indicate that some NBEs exhibit a slanted orientation rather than strictly vertical. This paper investigates the effect of the slanted NBEs using a newly developed rebounding‐wave model. The modeling results are validated against the full‐wave Finite‐ Difference Time‐Domain method and compared with measurements for both vertical and slanted NBE cases. It is found that the inclination of the NBEs affects both the waveforms and amplitudes of the electrostatic, induction and radiation components of the electric fields at close distances (≤10 km). However, it primarily influences the amplitudes of the fields for distances beyond 50 km, where the radiation component dominates, resulting in changes of ≥30% when the slant angle exceeds 30°. The slanted rebounding‐wave model improves the agreement with respect to a purely vertical channel and can be extended to any discharge geometry at arbitrary observation distances. Plain Language Summary: Narrow Bipolar Events (NBEs) are unique intracloud discharges that occur either individually or as the initiation event for lightning flashes inside thunderstorms. Knowing the physical mechanisms of NBEs will help us to better understand how lightning initiates inside thunderstorms. Recent studies indicated that NBEs could exhibit a slanted orientation rather than being strictly vertical. However, the inclination of NBEs has not been considered in previous transmission line models, leading to uncertainty when evaluating their characteristics based on electromagnetic fields. Here, in the light of recent observations, we analyze the propagation effect of the slanted NBEs by using a newly developed slanted rebounding‐wave model, and we compare the modeling results with observations. This study contributes to a better understanding of the physical mechanism of NBEs and provides a reference for accurately characterizing NBEs based on their electromagnetic fields. Key Points: The propagation effect of slanted Narrow bipolar events (NBEs) at different distance is investigated and compared with the observationsThe inclination of the NBEs could significantly affect the electromagnetic fields in the close distanceThe proposed equations will improve the quality of inferred features of slanted NBEs and can be extended to any discharge shape [ABSTRACT FROM AUTHOR]

Although a number of studies have been conducted of the lightning activity in hurricanes and typhoons, little information has been obtained on the three-dimensional (3-D) structure of the lightning, or how it is related to the precipitation structures within the storms. Here, we utilize observational data from the 3-D Tokyo Lightning Mapping Array (Tokyo LMA), a Japan Meteorological Agency C-band Doppler radar, and the Japanese Lightning Detection Network (JLDN) to conduct a study of the lightning activity during Typhoon Faxai (2019) in comparison with the storm's precipitation structure. This is done for the dissipating stage of the typhoon, when the eyewall was well within the range of the instruments and undergoing a surge in lightning activity. Of particular interest in the surge was the occurrence of numerous positive cloud-to-ground (+CG) lightning flashes. Detailed study of the Tokyo LMA and JLDN data shows that, out of 52 flashes during the surge, 29 flashes or 56 % produced positive strokes to ground, an unheard of number considering that, from the lightning and 3-D radar structures, the storm appeared to be normally electrified, and under such circumstances would produce negative rather than positive strokes to ground. It also focuses attention on the question of how +CGs are produced in tropical cyclones in the first place. Based on a lack of -CG strokes and the LMA observations showing that the +CG strokes are produced mid-way or toward the end of normal polarity intracloud (IC) flashes, it appears that the dissipating storm cells have a depleted or horizontally sheared mid-level negative charge, such that an IC flash propagating into and through upper positive storm charge effectively funnels a steadily increasing amount of positive charge into the mid-level initiation region, eventually causing the positive breakdown of the IC flash to turn downward toward ground, producing a +CG stroke. [ABSTRACT FROM AUTHOR]

Observations made in 1987 with the NCAR King Air aircraft and in 1993 with the New Mexico Institute of Mining and Technology dual-polarization radar have revealed the presence of supercooled raindrops in some New Mexico summertime cumulus clouds. In the case of the radar data, the evidence for the supercooled drops came from a column of enhanced [Z.sub.DR] that extended well above the 0 [degrees] C level. The in situ data indicated that the supercooled raindrops were observed when cloud base was warmer than about 7 [degrees] C and the depth of the cloud was greater than about 2.5 km.

Based on the ensemble empirical mode decomposition (EEMD) method, a DAF method for signal construction is proposed that repeatedly decomposes (D) the signal, amplifies (A) the local signal characteristics, and then filters (F) the signal. This method is used to decompose and reconstruct the electric field waveform (called a sferic) of an energetic in-cloud pulse (EIP) with a 247-kA peak current detected by a fast antenna (FA). Based on synchronous sub-microsecond very high-frequency (VHF, 14–88 MHz) radio interferometer (INTF) observations and observed downward fast positive and upward fast negative breakdowns, which occurred simultaneously with the EIP, the EIP sferic is decomposed by the DAF method in 11 steps into two independent sferics: a smoother filtered EIP sferic and an embedded narrow bipolar-like event (NBE). It is verified that strong VHF radiation is generated by the NBE-like event, rather than being associated with the smooth EIP sferic. The analysis, decomposition, and reconstruction of the correlated signals by the EEMD-based DAF method proposed in this article support the idea that the large-amplitude EIP sferic was generated by relativistic discharge responsible for an accompanying terrestrial gamma-ray flash (TGF) rather than by streamer or leader activity. [ABSTRACT FROM AUTHOR]

The National Research Institute for Earth Science and Disaster Resilience deployed a lightning mapping array (LMA) in the Tokyo metropolitan area in March 2017. Called the "Tokyo LMA," it obtains detailed three-dimensional observations of the total lightning activity (cloud-to-ground and intracloud flashes) in storms. The network initially consisted of 8 receiving stations, expanded to 12 stations in March 2018. Real-time total lightning images were first opened on the webpage in Japan. Real-time observations from the Tokyo LMA will be used in nowcasting lightning hazards and mitigating lightning disasters. Archived data will be used to develop lightning prediction techniques and a lightning climatology for the Tokyo metropolitan area. [ABSTRACT FROM AUTHOR]

Deployed in the mountainous island of Corsica for thunderstorm monitoring purpose in the Mediterranean Basin, SAETTA is a network of 12 LMA stations (Lightning Mapping Array, designed by New Mexico Tech, USA) that allows the 3-D mapping of VHF radiations emitted by cloud discharges in the 60-66 MHz band. It works at high temporal (80 µs) and spatial (tens of meters at best) resolutions within a range of about 350 km. Originally deployed in May 2014, SAETTA was commissioned during the summer and fall seasons and is now permanently operational since April 2016 until at least the end of 2020. We first evaluate the performances of SAETTA through the radial, azimuthal, and altitude errors of VHF sources localization with the theoretical model of Thomas et al. (2004). We also compute on a 240 km x 240 km domain the minimum altitude at which a VHF source can be detected by at least 6 stations by taking into account the mask effect of the relief. We then report the 3-year observations on the same domain in terms of number of lightning days per square kilometer and in terms of lightning days integrated on the whole domain. The lightning activity is first maximum in June because of daytime convection driven by solar energy input, but concentrates on a specific hot spot in July just above the crossroad of the three main valleys. This hot spot is probably due to the low-level convergence of moist air fluxes from sea breezes channeled by the three valleys. Lighting activity increases again in September due to numerous small thunderstorms above the sea and to some high precipitating events. Finally we report lightning observations of unusual high altitude discharges associated with the mesoscale convective system of June 8, 2015. Most of them are small discharges on top of an intense convective core during convective surges. They are considered in the flash classification of Thomas et al. (2003) as small-isolated and short-isolated flashes. The other high altitude discharges, much less numerous, are long range flashes that develop through the stratiform region and suddenly undergo upward propagations towards an uppermost thin layer of charge. This observation supports the recent conceptual model of Dye and Bansemer (2019) that explains such upper level layer of charge in the stratiform region by the development of a non-riming ice collisional charging in a mesoscale updraft. [ABSTRACT FROM AUTHOR]