The lightning locations of tall buildings are relatively predictable with high occurrence probability, and the lightning of tall buildings does not need a larger cost compared with artificially triggered lightning. Therefore, tall buildings can provide a good observation platform for lightning research. Additionally, with the rapid development of urbanization, the probability of lightning striking tall buildings is increasing. Thus, the study on the lightning of tall buildings can provide practical references for the lightning protection design of tall buildings. With deepening research on the physical characteristics of lightning discharge, the spectral diagnosis of lightning plasma has become an important tool for measuring lightning properties. At present, the observations and research of lightning spectra mainly focus on natural lightning and artificially triggered lightning, but there are few studies on lightning spectral observations of tall buildings. In addition, the optical thickness of the lightning channel is an important prerequisite for quantitative analysis of the lightning spectrum. Due to the lack of spectral resolution in previous experimental systems, the experimental verification of the optical thickness of lightning NI and OI radiation in the near-infrared spectrum is rare. This paper employs the spectra of one lightning with three return strokes on the 600-meter-high Canton Tower obtained at the Tall Object Lightning Observatory in Guangzhou (TOLOG) to analyze the evolution and variation characteristics of the spectra with the time and the channel height in detail. Experimental verification of the optical thickness of lightning near-infrared radiation is also presented by comparing the measured intensities of spectral lines of NI [856.8 nm, 859.4 nm, 862.9 nm] multiplet with the theoretical values. This study hopes to deepen the scientific understanding of the microcosmic physical process of lightning discharge and provide an experimental basis for the quantitative analysis of the near-infrared lightning spectrum.

The TOLOG with six stations is established by Chinese Academy of Meteorological Sciences and Guangdong Meteorological Service. Spectral observations are set up at Station 1 and recorded by a slitless spectrograph with a high-speed camera. The splitting system of the spectrograph is a plane transmission grating, which is placed tightly in front of the objective lens of the camera. Based on the spectra of one lightning with three return strokes on the 600-meter-high Canton Tower, the evolution and variation characteristics of the spectra with the time and the channel height are analyzed. In addition, the influence of opacity on the spectral line intensity of lightning plasma can be determined with the intensity ratio of the spectral lines, and one way to determine the optical thickness is to compare the intensities of several lines with the same upper energy level within the same multiplet. Thus, after comparing the measured intensities of spectral lines of NI [856.8 nm, 859.4 nm, 862.9 nm] multiplet with the theoretical values, this study presents the experimental verification of the optical thickness of lightning near-infrared radiation.

The results show that the discharge channels of three return strokes on the Canton Tower have stronger luminescence below 200 m (Fig. 6). In the initial discharge stage of the return stroke, when the upward current wave does not reach the top of the channel, the radial spectral radiation at the bottom of the channel is composed of stronger ionized lines and weaker neutral lines. Meanwhile, the radial spectral radiation at the top of the channel mainly depends on the downward leader and is composed of weaker ionized lines and stronger neutral lines (Figs. 4-5). When the current wave is transmitted to the top of the channel, the whole channel radially radiates strong ionized lines and strong neutral lines, and the total intensities of ionized lines and neutral lines all decrease with the increasing channel height (Figs. 4-5). After 70 μs discharge, the total intensities of ionized lines and neutral lines remain basically unchanged with the channel height above 200 m (Figs. 5-6). This observation directly confirms that the lightning channel consists of a hot core radiating ionized lines and a cold peripheral corona radiating neutral lines. Additionally, intensity ratios of the spectral lines and the theoretical optically thin limit within the NI [856.8 nm, 859.4 nm, 862.9 nm] multiplet show that the measured ratios of NI lines within this multiplet are basically unchanged with the time (Fig. 7), which means that the near-infrared spectrum of the lightning channel meets the optically thin condition.

Based on the spectra of one lightning with three return strokes on the 600-meter-high Canton Tower obtained at the TOLOG, the evolution and variation characteristics of the spectra with the time and the channel height are analyzed first in detail. Experimental verification of the optical thickness of lightning near-infrared radiation is also presented by comparing the measured intensities of spectral lines of NI [856.8 nm, 859.4 nm, 862.9 nm] multiplet with the theoretical values. The results show that the discharge channels of three return strokes on the Canton Tower have stronger luminescence below 200 m. In the initial discharge stage of the return stroke, when the upward current wave does not reach the top of the channel, weak neutral lines in the near-infrared band are radiated by the channel when the ionized lines in the visible band just appear in spectrum at the bottom of the channel. When the intensity of ionized lines in the visible band peaks, the intensity of neutral lines in the near-infrared band also peaks. This is different from previously reported observations, which directly confirms that the lightning channel consists of a hot core radiating ionized lines and a cold peripheral corona radiating neutral lines. In the initial discharge stage of the return stroke, the total intensities of ionized lines and neutral lines all decrease with the increase in the channel height. After 70 μs discharge, the total intensities of ionized lines and neutral lines remain basically unchanged with the channel height above 200 m.