While narrow bipolar events (NBEs) could be related with lightning initiation, their intrinsic physics remains in question. Here we report on optical measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station (ISS) of blue flashes associated with NBEs. They are observed in a narrow blue band centered at 337 nm, with no simultaneous activity at 777.4 nm, considered a strong lightning emission line. From radio waves measured from the ground, we find that 7 of 10 single‐pulse blue events can be identified as positive NBEs. The source altitudes estimated from optical and radio signals agree and indicate that the sources of the blue flashes are located between∼8.5 and∼14 km, in a cloud reaching 14–15 km altitude. The observations suggest that single‐pulse blue flashes are from cold ionization waves, so‐called streamers, and that positive NBEs are corona discharges formed by many streamers.

In this paper, we analyze the location error of time of arrival (ToA)‐based lightning location systems (LLSs) caused by propagation effects over mountainous terrain around the Säntis tower located in the Swiss Alps. The study is based on a full‐wave three‐dimensional (3‐D) finite difference time domain approach using the topographic map including the Säntis tower and the nearby sensors belonging to LLSs. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity and that the location error associated with the ToA technique depends strongly on the used onset time estimation technique. The evaluated location errors associated with amplitude thresholds of 10% and 20% and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest (about 300 m or less). Finally, we assess the accuracy of two simplified methods (terrain envelope method and tight‐terrain‐fit method) to account for the location error due to propagation over mountainous terrain. These two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a nonflat terrain by using available topographic data. In addition, a possible real‐time location error compensation algorithm using the elongated propagation path method to improve the location error of the LLSs in mountainous regions is presented and discussed.

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers. We found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with streamer breakdown. The rise times of the emissions for negative NBEs are about 10 μs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~1 ms, consistent with locations deeper in the clouds. For negative NBEs, the emission strength is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause.

Narrow Bipolar Events (NBEs) are powerful radio emissions from thunderstorms, which have been recently associated with blue optical flashes on cloud tops and attributed to extensive streamer electrical discharges named fast breakdown. Combining data obtained from a thunderstorm over South China by the space‐based Atmosphere Space Interactions Monitor, the Vaisala GLD360 global lightning network and very low frequency/low frequency radio detectors, here we report and analyze for the first time the optical emissions of blue luminous events associated with negative NBEs and located at the top edge of a thundercloud. These emissions are weakly affected by scattering from cloud droplets, allowing us to estimate the source extension and optical energy involved in the process. The optical energy in the 337‐nm band emitted by fast breakdown is about 10⁴ J, which involves around 10⁹ streamer initiation events.

A three-dimensional (3-D) finite-difference time-domain (FDTD) method is developed for simulating the lightning-radiated electromagnetic field over the two-dimensional (2-D) rough ground. This 3-D FDTD method provides a new approach to analyze the propagation effect of 2-D rough ground on the lightning horizontal field. It is noted that the effect of the 2-D surface roughness on the horizontal field cannot be ignored even at a distance of 100 m from the lightning channel, and the increase of the land roughness results in a lower magnitude of the horizontal field waveform. When the root-mean-square height (RMSH) of the rough surface is 5 m, the magnitude of the horizontal field at a height of h = 10 m above the rough ground reduces to about half of its field peak value with respect to the smooth ground within distances of 100 m from the lightning channel. We also found that the horizontal field is more obviously affected by the land roughness when the land conductivity is smaller than 0.001 S/m.

We have examined the accuracy of Wait's formula for evaluating lightning vertical electric field and azimuthal magnetic field over a mixed propagation path (vertically stratified) by using the finite-difference time-domain (FDTD) technique at near (200 m) and intermediate (1000 m) distance range from the lightning channel. For the first return stroke, Wait's formula has a reasonable accuracy for predicting lightning electromagnetic fields within distances of 1000 m. However, for the subsequent return stroke, although Wait's formula predicts the nearly same field peaks as that by FDTD no matter how the ground conductivity is vertically stratified, some waveform differences can be observed at the early-time response within a few microseconds. Therefore, Wait's formula may cause some errors for predicting the field derivates within tens or hundreds of meters.