Cover image: A lightning bolt in Shanghai in 2020 (Photo: Feng Yu Sauce - SDK).
Lightning, as a common weather phenomenon in nature, is familiar to people every day, accompanied by sound and light. According to statistics, there are nearly 1.4 billion lightning strikes worldwide every year. At present, measurement techniques with high spatiotemporal resolution that can depict the morphology of three-dimensional lightning channels can give us a deeper understanding of lightning. A lightning bolt often lasts for hundreds of milliseconds, extending tens of kilometers horizontally and several kilometers vertically. Lightning extends the channel like the growth of a tree, and many branches appear. So, why does lightning take such a shape?
Fig.1 Conceptual model of the charge structure and lightning channel morphology of thunderstorm clouds. The red and blue "+" represent the positive and negative charge regions, respectively; The density represents the magnitude of the charge density; The black dots indicate the radiation source. (From Li et al.)2022 Figure 4).
Thunderstorm clouds are the main source of lightning, which generally originates in areas with strong electric fields, and can occur when the local electric field in the cloud exceeds about 400 kV m. A typical thunderstorm cloud has a three-layer charge structure, which mainly includes a positive charge region at the top, a negative charge region in the middle and a small positive charge region at the bottom. Lightning generally originates in the region of a strong electric field between the upper positive charge region and the middle negative charge region, and then travels in the upper and lower charge layers, respectively. The team of Professor Zhang Yijun from the Department of Atmospheric and Oceanic Sciences of Fudan University published in Geophysical Research Letters in 2022**,For the first time, it was proved that the charge structure in the thunderstorm cloud was not uniformly distributed in layers based on measured data (Fig. 1).。It is the inhomogeneity of the charge density in the charge layer of the thunderstorm cloud that leads to the different morphologies of lightning. In this paper, the fractal dimension is used to quantitatively describe the direct extension, bifurcation and turning of the lightning channel. The fractal dimension is known as the fractal theory of nature's geometry, which is different from the integer dimension we usually describe, and it can describe the complexity of the shape and the degree of possession of space in the form of fractions. As lightning channels develop, they branch out and turn in areas with high charge density, forming the shape of the lightning we see
Fig. 2 Turbulent dissipation rates of different lightning patterns and channel regions. (From Li et al.)2024 Figure 1).
The uneven charge structure in a thunderstorm cloud leads to the different shapes of lightning, so what kind of dynamic structure forms such a charge structure? In 2024, Prof. Yijun Zhang's team published in Geophysical Research Letters**, revealing the turbulence characteristics of different morphological regions of lightning channels. Lightning with complex morphology has a large fractal dimension and a large turbulent dissipation rate (EDR) (Fig. 2). In a single lightning, the channel that extends directly within a certain height range without obvious bifurcation and steering tends to develop in the direction of decreasing EDR (Fig. 3), while the change of channel propagation direction and bifurcation during transmission usually occur in regions with large radial velocity gradient and large EDR (Fig. 4). This study reveals the dynamic structure of thunderstorms that form the lightning channel morphologyThe microphysical structure of thunderstorms that form the morphology of lightning channels will be analyzed in future studies.
Fig.3 Turbulent dissipation rate of lightning direct extension channel and channel area. (From Li et al.)2024 Figure 2).
Fig. 4 Lightning bifurcation and steering and turbulent dissipation rate in the channel region. (From Li et al.)2024 Figure 3).
**Info:li, y., zhang, y., zhang, y., krehbiel, p. r. (2022). analysis of the configuration relationship between the morphological characteristics of lightning channels and the charge structure based on the localization of vhf radiation sources. geophysical research letters, 49, e2022gl099586. https://doi. org/10.1029/2022gl099586
li, y., zhang, y., zhang, y., krehbiel, p. r. (2024). analysis of the relationship between the morphological characteristics of lightning channels and turbulent dynamics based on the localization of vhf radiation sources. geophysical research letters, 51, e2023gl106024.