FDTD Analysis of Electric Fields at Ground Level Generated by Compact Intracloud Discharges

Using the finite-difference time-domain (FDTD) method for solving Maxwell's equations, we have studied waveguiding properties of a large low-conductivity cylinder, having a length of about 1000 m, a radius of 100 m, permittivity equal to the vacuum permittivity, permeability equal to vacuum permeability, and a conductivity of 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{-5} \mathrm{S} / \mathrm{m}$</tex>. The cylinder simulates a compact intracloud discharge (CID). It is placed at altitudes from 14 to 21 km above ground, excited by a voltage source at the center, and the resultant vertical electric fields on the ground surface at horizontal distances from 14 to 38 km from the cylinder are computed. The FDTD-computed vertical electric field waveforms roughly match the corresponding measured ones for the excitation voltage pulse with a half-peak width of 30 microseconds, but not for the 5 microsecond ramp followed by a constant value. The results indicate that the CID representation by a large lowconductivity cylinder is reasonable.