Lightning Very-high-frequency (VHF) broadband interferometer has become an effective approach to map lightning channels in two dimensions with high time resolution. This paper reports an approach to mapping lightning channels in three dimensions (3D) using two simultaneous interferometers separated by about 10 km. A 3D mapping algorithm was developed based on the triangular intersection method considering the location accuracy of both interferometers and the arrival time of lightning VHF radiation. Simulation results reveal that the horizontal and vertical location errors within 10 km of the center of the two stations are less than 500 m and 700 m, respectively. The 3D development of an intra-cloud (IC) lightning flash and a negative cloud-to-ground (-CG) lightning flash with two different ground terminations in the same thunderstorm are reconstructed, and the extension direction and speed of lightning channels are estimated consequently. Both IC and CG flash discharges showed a two-layer structure in the cloud with discharges occurring in the upper positive charge region and the lower negative charge region, and two horizontally separated positive charge regions were involved in the two flashes. The average distance of the CG ground terminations between the interferometer results and the CG location system was about 448 m. Although disadvantages may still exist in 3D real-time location compared with the lightning mapping array system working with the principle of the time of arrival, interferometry with two or more stations has the advantage of lower station number and is feasible in regions with poor installation conditions, such as heavy-radio-frequency-noise regions or regions that are difficult for the long-baseline location system.

Characteristics and mechanism of initial continuous current (ICC) pulses and M components in rocket-triggered lightning were investigated in detailed. The geometric mean current peak, risetime from 10% to 90% peak, half-peak width, and charge transfer of M components were 400 A, 207 $\umu$ s, 267 $\umu$ s, and 0.19 C, respectively, exhibiting more pronounced impulsive feature than ICC pulses of which the counterpart parameters were 190 A, 462 $\umu$ s, 845 $\umu$ s, and 0.27 C, respectively. A shorter time interval between the prior return stroke (RS) and the following M component (T $_RS-M$ ) facilitated a stronger M component having larger peak while shorter pulsewidth. In two cases with extremely long lasting ICC and continuing current (CC), 20 Initial continuous current pulses occurred during a 715.3-ms duration and 15 M components occurred during a 218-ms duration. The multiple occurrences of current surges serve to keep the grounding channel active and extend the channel network inside the cloud, and hence prolong the ICC/CC duration. This situation reduced the possibility of RSs thereafter. In addition to the two traditional types of in-cloud discharges that result in M component, we recognized a new scenario that involves a short-term interruption of the upward RS wave and a reactivated breakdown promoted by the residual charge inside the main channel.

The three-dimensional (3D) low-frequency lightning mapping system (LF-LMS) in north China has been updated. The lightning electric field derivative (dE/dt) sensor and continuous acquisition mode has been newly designed to ensure a capability of entire lightning processes detection, especially weak discharges during lightning the initiation process. The twice cross-correlation delay estimation and the grid iteration nested optimization location algorithm are used to realize the 3D location of the discharge channel, and the location resolution and calculation speed are balanced consequently. The location results of the rocket-triggered lightning demonstrated that the system achieved a high-resolution mapping of lightning discharge channels, which coincided well with the optical images. The horizontal and vertical location error for rocket triggered lightning was less than 40 m in both horizontal and vertical. Intracloud (IC) lightning flashes were observed to be initiated by three different discharge processes, initial breakdown pulse (IBP), narrow bipolar event (NBE), and initial E-change (IEC). The corresponding initial height was 10.5 km, 6.9 km, and 9.2 km, respectively. The upward negative leader was initially located, followed by scatter radiation sources and negative recoil leaders in the lower negative charge region for all cases. The electric field characteristics of the IEC and subsequent IBPs indicated that they are different discharge processes with the same current direction. The IEC process might correspond to the discharge process with continuous current and less noticeable current changes.

Very-high-frequency (VHF) electromagnetic signals have been well used to image lightning channels with high temporal and spatial resolution due to their capability to penetrate clouds. A lightning broadband VHF interferometer with three VHF antennas configured in a scalene-triangle shape has been installed in Lhasa since 2019, to detect the lightning VHF signals. Using the signals from the VHF interferometer, a new hybrid algorithm, called the TDOA-EMTR technique, combining the time difference of arrival (TDOA) and the electromagnetic time reversal (EMTR) technique is introduced to image the two-dimensional lightning channels. The TDOA technique is firstly applied to calculate the initial solutions for the whole lightning flash. According to the results by the TDOA method, the domain used for the EMTR technique is predetermined, and then the EMTR technique is operated to obtain the final positioning result. Unlike the original EMTR technique, the low-power frequency points for each time window are removed based on the FFT spectrum. Metrics used to filter noise events are adjusted. Detailed imaging results of a negative cloud-to-ground (CG) lightning flash and an intra-cloud (IC) lightning flash by the TDOA method and the TDOA-EMTR are presented. Compared with the original EMTR method, the positioning efficiency can be improved by more than a factor of 3 to 4, depending on the scope of the pre-determined domain. Results show that the new algorithm can obtain much weaker radiation sources and simultaneously occurring sources, compared with the TDOA method.

Affected by solar radiation in space, the FY-4A Lightning Mapping Imager (LMI) detection array exhibits daily periodic thermal expansion and contraction, leading to deviations in lightning positioning accuracy. While LMI’s detection efficiency is higher at night, the dual edge matching algorithm, which relies on surface features for correction, does not perform well during nighttime (around 3 pixels). Analysis shows that most of the lightning data corrected by this method exhibit significant deviations from the actual lightning locations in practical applications. Therefore, this paper proposes a new correction method based on high precision ground-based lightning location data from the 2019 summer World Wide Lightning Location Network (WWLLN) and the Beijing Broadband Lightning Network (BLNET). Using these datasets as reference standards, the periodic deviation of LMI is determined, and a correction curve is derived using a weighted Gaussian fitting approach. This method further improves the nighttime lightning location accuracy of LMI on the basis of the current operational algorithm. The results demonstrate that the corrected LMI data significantly reduces the positioning errors, with an accuracy within ±1 pixel in the Beijing area, as an example.

The Doppler effect induced by the motion of lightning radiation sources, referring to electromagnetic radiation sources associated with lightning discharges (especially in the very-high-frequency (VHF) band), affects the propagation and reception of electromagnetic signals, thereby reducing the accuracy of lightning radiation source localization and identification. In this article, to address this issue, based on the Minimum Variance Distortionless Response (MVDR) beamforming algorithm and by introducing the Doppler effect into the localization and identification theory of stationary radiation sources, simulations are performed to comparatively analyze the localization and identification features of both dual-stationary and dual-moving lightning radiation sources. Then, the impact of Doppler shifts on the localization performance of moving lightning radiation sources is investigated. The results show that circular planar array localization accuracy is closely linked to the spatial resolution of the system. The higher the cloud height, the greater the elevation angle, and the higher the spatial resolution. Furthermore, the impact of the Doppler effect on localization accuracy intensifies with higher radiation source frequencies and larger frequency differences between dual radiation sources, leading to greater localization deviations, particularly for high-frequency VHF emissions. This study reveals the mechanism of how the Doppler effect affects moving lightning radiation source localization, providing theoretical support for subsequent field experimental observations and fine-structure research on lightning channel development.