On the Variation of Optical Return Stroke Speed Along the Bottom of Lightning Channel

In cloud-to-ground lightning flash, a varying return stroke speed during the propagation process along the lightning channel has been extensively reported. It is also reported that as the return stroke propagates upward, the waveform will be distorted, resulting in an increase in the rise time of waveform and a decrease in the magnitude. In this paper, we artificially generate the optical signal simulating the distortion along the lightning channel bottom 60 m. Using six slope-intercept methods and five percentage-of-peak methods, we analyze the influence of height-dependent distortion of the return stroke on the estimated optical return stroke speed. We show that the percentage-of-peak methods tend to underestimate the speeds, and that the slope-intercept methods, in general, but not always, first underestimate the actual speeds, and then overestimate them as the return stroke propagates upward. The deviations are more pronounced for higher actual return stroke speeds. Moreover, for an assumed constant actual return stroke speed, the estimated return stroke speeds by various methods increase with increasing heights in the channel bottom, although the increasing trends become weaker as the height increases. This artificial increase is observed from the estimated two-dimensional propagation speeds of return strokes along the lightning channel bottom 60 m in rocket-triggered lightning.