Nengxing Guo

Extreme positive cloud‐to‐ground lightning over East Asian midlatitudes exhibits a spring–autumn double maximum hidden by the familiar winter climatology. Using 1,838,538 return strokes with absolute peak currents at least 200 kA during 2006–2024, we resolve its annual cycle by month and latitude. At 35–55N, positive strokes account for 86.2% in April and 77.1% in October and remain dominant from September through November; negative strokes dominate in winter and midsummer. The contrast arises primarily within latitude bands and strengthens with current: positive strokes are 28.3 and 23.7 times more likely than negative strokes to exceed 200 kA in April and October. April dominance spans cloud environments, whereas October dominance collapses above 9‐km cloud tops and accompanies freezing levels 0.3–0.4 km lower. This twice‐yearly reversal links the polarity of extreme ground lightning to seasonally changing current distributions and storm vertical structure.

The difference in ground flash density is studied under various terrains. It is found that a large error occurs when using a single ground flash density value to calculate the lightning trip-out rate of transmission lines in mountainous areas. This is because terrains have a greater effect on the ground flash density within a certain range. This range is larger than microscopic terrain ranging from tens to a few hundred meters, but smaller than macroscopic terrain ranging from tens to hundreds of kilometers or more. Therefore, the concepts of mesoscopic terrain and mesoscopic range are proposed in this paper. An approximation method for finding the probability model of the lightning distribution is proposed. It is found that the probability model of the lightning distribution changes from a uniform distribution to a non-uniform distribution during the transition from flatland to mountain. Furthermore, a method for determining the mesoscopic range is proposed using the maximum, minimum, and average values of ground flash density. The mesoscopic terrain concept proposed in this paper aids in further studies on the differences in lightning distribution and lightning performance between different areas, and offers theoretical support for improving the accuracy of lightning protection calculations in power systems.

Soil resistivity is the main factor affecting the performance of grounding system. Existing studies show that soil moisture determines soil resistivity. However, the microscopic mechanism of soil resistivity change driven by pore water content remains unclear. To solve this problem, a three-dimensional fractal model of soil was developed to study the electrical conductivity of soil in this paper. The effect of water content on soil resistivity is as follows: At high water content, pore water forms a complex, interconnected network, resulting in low resistivity. With the decrease of water content, the complexity of the network decreases and the complex network becomes a cluster of isolated paths. Below a critical water content value, the connected path suddenly breaks, resulting in a sharp increase in resistivity of several orders of magnitude. Through Monte Carlo method, the probability of forming a connected path under different pore water content is studied. It is found that for the same soil type, even if the soil porosity changes, the water content required to form a connected path is basically the same. Finally, this paper takes red soil as an example to verify the correctness of the proposed model. This study highlights that changes in pore water connectivity fundamentally govern soil resistivity variations and offers a reference for further analysis of soil electrical conductivity.

Electric field computation holds a significant position in electromagnetic field theory and its engineering applications. Particularly when addressing problems with complex boundary conditions, traditional numerical methods often encounter a trade-off between computational efficiency and accuracy. This study proposes a novel conformal mapping framework designed to efficiently compute electrostatic fields in domains featuring complex boundaries. By constructing a mapping from an arbitrary smooth domain to a standard half-plane, the original problem with intricate boundaries is transformed into a more regular and solvable form. The mapping function is developed using the Cauchy integral equation, and, in conjunction with the theory of Fredholm integral equations of the second kind, a comprehensive analysis is conducted on the singularity and continuity of the kernel function, as well as the compactness of the integral operator. To solve the integral equation, the Nyström discretization method is employed alongside high-precision numerical integration techniques. Additionally, spectral analysis and regularization techniques are utilized to effectively address the ill-conditioning issues of the discrete system. The proposed method is further validated through comparisons with the finite element method (FEM) using two-dimensional and three-dimensional electrostatic field case studies. Results demonstrate that, in two-dimensional scenarios, the relative deviation between the two methods is less than 0.5%, while in three-dimensional cases, the deviation remains within 5%, with a substantial enhancement in computational efficiency. The approach introduced in this paper offers a new theoretical and numerical tool for the high-precision, rapid solution of electrostatic fields with complex boundaries, establishing a foundation for future extensions to higher-dimensional problems and the development of adaptive algorithms.