FDTD Computation of Ground-Level Electric Fields Generated by Compact Intracloud Discharges

Using the finite-difference time-domain (FDTD) method for solving Maxwell’s equations, we simulated ground-level electric fields produced by compact intracloud discharges (CIDs) represented by an elevated vertical low-conductivity cylinder. The cylinder had a length of about 1000 m, a radius of 100 m, the permittivity and permeability equal to those of vacuum, and the electric conductivity of 10⁻⁴–10⁻⁵ S/m. The cylinder was placed at altitudes of 14–21 km above ground, energized at its midpoint, and resultant vertical electric fields on the ground surface at horizontal distances of 14–38 km were computed. Different excitation voltage waveforms were tested to see if they can yield field waveforms matching measurements. A reasonably good match was obtained for the empirically adjusted excitation voltage waveforms with a half-peak width of about 30 μs, but not for the 5 or 10 μs ramp followed by a constant value or for a Gaussian pulse with a half-peak width of 4 or 10 μs. The results indicate that representation of CIDs by a low-conductivity cylinder, in conjunction with FDTD-enabled full-wave solution, allows one to reproduce the observed electric field signatures of CIDs.