A New FDTD Model for Lightning Return Stroke Channel Above Lossy Ground and Its Validation With Rocket-Triggered Lightning Data

FDTD method has been widely used in calculations of electromagnetic fields produced by a lightning return stroke and its effects on other systems. However, the traditional Pi-type distributed resistance (R) - inductance (L) - capacitance (C) model representing a lightning return stroke is no longer suitable for FDTD method. In this study, we propose a new type RLC model that consists of a current source at the lightning channel base feeding a series of passive RLC loads above. The lightning return stroke channel is divided into small segments; each segment is presented by a height-dependent R in series with an L and in parallel with a C. With a current waveform measured for a return stroke in a rocket-triggered lightning discharge as the current source, impacts of different RLC settings on the channel current distribution properties were studied. Results show that the value of R has a significant impact on the attenuation pattern of both the current amplitude and propagation speed along the channel. Values of L and C have more impacts on the rising and falling edge of the current waveform and current propagation speed along the channel. The model was then applied to two sets of channel base currents and multi-station electric field measurements from two return strokes in rocket-triggered lightning experiments. By fitting the simulated electric field with the measured electrical field at each station, an optimal set of lightning channel RLC values for each of the two return strokes were determined, and hence the current propagation speed and amplitude as a function of the channel height were successfully estimated. The results show that both the current speed and amplitude decreased exponentially with the increase of the channel height, which are well consistent with existing optical observations and physical predictions in literature.