Effects of channel length on calculation accuracy of lightning return stroke electromagnetic fields

Finite-difference time-domain (FDTD) method has been widely used in the calculation of lightning generated electromagnetic fields, and the length of lightning discharge channel is usually set to be 2500 similar to 3000m according to the height of cloud layer, this is rather time consuming. In order to save the computation time, we shorten the channel length to 1000m, calculate the electric and magnetic fields at flat ground due to lightning return stroke. To compare the results with those of H=2500m, we found that, the error caused by the shortening only occurs in the decreasing period of the lightning waveforms, and the relative error of Ez is about 3% for near field, while the computation time is only 1/3 of that calculated with H=2500m. This analysis of relative error and calculation accuracy will provide more comprehensive criterions on computational parameters for lightning electromagnetic pulse.