Lightning Strike to a Tall-Grounded Object: Part 2—LEMP Calculation With a Channel Approximation

Lightning electromagnetic pulse (LEMP) waveforms associated with lightning strikes to the 634-m high Tokyo Skytree are calculated by the finite-difference time-domain (FDTD) method in the two-dimensional (2-D) cylindrical coordinate. The three-dimensional (3-D) tortuous lightning channel and the tall strike object are represented by an “equivalent vertical” phased-current-source array. The current propagation speed along the equivalent vertical lighting channel is determined in two steps: first, the channel is projected on a plane including the strike object and a field observation point, and then it is converted into the straight vertical channel right above the strike object. Thus, the height-dependent lightning-current propagation speed along the equivalent channel depends on the direction of the observation point. The proposed method greatly reduces computational resources required for calculating LEMPs without deteriorating accuracy since it allows the use of the 2-D FDTD method. LEMP waveforms measured at a horizontal distance of 27, 57, and 101 km (in different directions) from the strike object are accurately reproduced by the proposed method. The results clarified that the lower region of the tortuous lightning has a large influence on the wavefront of the LEMPs at a horizontal distance ranging from 20 to 100 km from the strike point.