Propagation Effects on the Electric Field Time Derivatives Generated by the Stepped Leaders and Return Strokes in CG Flashes

In this paper, the effects of propagation over finitely conducting ground on the characteristics of the electric-field time-derivative (dE/dt) pulses, which are generated by the stepped leaders and return strokes in cloud-to-ground (CG) flashes, are investigated. First, a finite-difference time-domain (FDTD) method in two-dimensional (2-D) cylindrical coordinates is proposed. And then, the electric fields produced by different lightning processes are calculated. The current waveform parameters of the stepped leaders are derived based on the MTLE model by trial and error, given the analytical waveform and the measured data at the International Center for Lightning Research and Testing (ICLRT) in Florida, USA. In conclusion, the results demonstrate that the characteristics of the dE/dt pulses, including the peak amplitude, the half-width time, and the 10%-90% rise time, change significantly in propagating over finitely conducting ground. Furthermore, the effects of the parameters (i.e., 10%-90% rise time and half-width time) of the lightning current waveforms on the characteristics of dE/dt pulses are also discussed. The results indicate that the 10%-90% rise time, as compared with the half-width time, has a more significant influence on the characteristics of the dE/dt pulses. Lastly, average energy spectra densities of the simulated dE/dt pulses are calculated, and for comparison purposes, spectral analysis of the data obtained by the very high frequency broadband interferometer are also presented. The theoretical results are consistent with those available experimental data.