Needle structure is a recently discovered lightning phenomenon, first observed by LOw Frequency ARray (Bandwidth: 30-80 MHz) radio telescope having very high spatial and temporal resolutions, which extends sideways from the positive leader channels of intracloud (IC) and negative cloud-to-ground (CG) flashes (Hare et al., 2019). Furthermore, Hare et al. (2019) conjectured that the needle-like discharge was caused by an electric field reversal, which in turn was due to positive leaders transiently disconnecting from their negative counterparts. The analysis and discussion of the characteristics and possible mechanism of needles could lead to a better understanding of the processes involved in the transverse (radial), as opposed to longitudinal motion of charge in lightning channels. The radial motion of charge during the return-stroke process was previously studied by Rakov (2006, 2009).In the following, we will briefly review the characteristics of needles reported from observations with VHF interferometers and high-speed framing cameras. Needles initiating from the main positive leader channel of an IC lightning were imaged by a short-baseline VHF interferometer with 200 MHz bandwidth (Pu & Cummer, 2019), and it was found that these needles moved forward (away from the positive leader channel) continuously with density decreasing backwards. Hare et al. (2021) showed detailed characteristics of needle propagation and flickering and observed that recoil leaders quench needle activity. Saba et al. ( 2020) observed needles appearing on upward positive leaders, and their high-speed video records showed that needles were formed at failed leader branches. It is possible that needles are a kind of common (not rare) processes occurring in lightning discharges.

The spatial distribution of charge plays a crucial role during lightning development (Iudin et al., 2017;Zheng et al., 2019). A typical thunderstorm cloud has a three-layer charge structure: a positive charge region at the top, a negative charge region in the middle and a small positive charge region at the bottom (Krehbiel, 1986;Williams et al., 1985). At present, the vertical layered structure is mainly used in research to reveal the propagation behavior of lightning (Krehbiel, 2003;Nag & Rakov, 2009;Qie, 2005). However, a growing body of research suggests that the charge structure of thunderstorm clouds is more complex (Stolzenburg et al., 1998) and possibly manifests itself as a staggering of more charge regions, and the horizontal extent of the charge regions can often be much larger than the vertical extent (

Monitoring lightning and its location is important for understanding thunderstorm activity and revealing lightning discharge mechanisms. This is often realized based on very low-frequency/low-frequency (VLF/LF) signals, very high-frequency (VHF) signals, and optical radiation signals generated during the lightning discharge process. The development of lightning monitoring and location technology worldwide has largely evolved from a single station to multiple stations, from the return strokes (RSs) of cloud-to-ground (CG) lightning flashes to total lightning flashes, from total lightning flashes to lightning discharge channels, and from ground-based lightning observations to satellite-based lightning observations, all of which have aided our understanding of atmospheric electricity. Lightning monitoring and positioning technology in China has kept up with international advances. In terms of lightning monitoring based on VLF/LF signals, single-station positioning technology has been developed, and a nationwide CG lightning detection network has been built since the end of the twentieth century. Research on total lightning flash positioning technology began at the beginning of the 21st century, and precision total lightning flash positioning technology has improved significantly over the last 10 years. In terms of positioning technology based on VHF signals, narrowband interferometers and wideband interferometers have been developed, and long-baseline radiation source positioning technology and continuous interferometers have been developed over the last ten years, significantly improving the channel characterization ability of lightning locations. In terms of lightning monitoring based on optical signals, China has for the first time developed lightning mapping imagers loaded by geosynchronous satellites, providing an important means for large-scale and all-weather lightning monitoring.

This work compares the characteristics of the first echoes of thunderstorms and nonthunderstorms retrieved from S‐band polarimetric radar observations. Observations of 57 (39) isolated thunderstorm (nonthunderstorm) cells with roughly equivalent aerosol and water vapor conditions but different convective available potential energy were obtained with a S‐band polarimetric radar and three independent lightning location systems during 2016/2017 in southern China. Storms with the first echoes were divided into three types based on echo top heights, namely, type 1 (below 0°C layer), type 2 (0°C to −10°C), and type 3 (above −10°C layer). Our observations show median values of radar reflectivity (ZH) and differential reflectivity (ZDR) of type 1 and type 2 in warm phase layer (below 0°C layer) are obviously greater in nonthunderstorms than in thunderstorms, but this feature is not significant in type 3 storms. In the mixed 1 phase layer (0°C to −10°C), median ZH in type 2 is greater in nonthunderstorms while median ZDR in type 3 is slightly smaller. In the mixed 2 phase layer (−10°C to −38°C), median ZH is greater in thunderstorms while median ZDR is smaller, and ZDR values in nonthunderstorms are closer to zero. Although results of ZDR comparisons in the mixed phase are likely affected by random errors and/or residual bias errors, these different signatures suggest different characteristics of liquid or ice particles between thunderstorms and nonthunderstorms. This study is expected to advance our understanding of physical processes responsible for the generation of the first flash.

The discharge signal in the initial stage of lightning is weak. The revelation of the discharge mechanism at this stage depends especially on close observation. In this study, a continuous interferometer (CINTF) was used to observe the initial stage of the upward positive leader (UPL) of the triggered lightning in Conghua District, Guangzhou City, Guangdong Province. The positioning error of CINTF for a close-range radiation source was analyzed, and the positioning error calibration method of CINTF for a specific close-range radiation source was studied, which improved the observation accuracy of elevation angle at the initial stage of the UPL of the triggered lightning. With the rise of the rocket, the positioning error in altitude during the initial stage of the UPL increased obviously. Under the layout condition of the Guangzhou field experiment site for lightning research, when the positioning results of the elevation angle of the initial stage of the UPL were 40°, 50°, and 60°, respectively, the calibrated altitude positioning error could be reduced by about 11 m, 14 m, and 20 m, respectively. On the basis of the calibrated observation results, the evolution characteristics of the initial stage of the UPL were studied, and its discharge mechanism was revealed. The precursor current pulse (PCP) was generated by a weak upward positive breakdown and a subsequent strong downward negative breakdown near the rising rocket tip, which was in the form of a single pulse. The precursor current pulse cluster (PCP cluster) and initial precursor current pulse cluster (IPCP) were both signs of self-sustaining development of the UPL. After the PCP cluster, self-sustaining development stopped immediately. The self-sustaining development after IPCP could be short-term or continuous.