资源环境科技发展态势分析平台

Original simultaneous records of currents, close electric field, and high-speed videos of natural negative cloud-to-ground lightning striking the tower of Morro do Cachimbo Station are used to reveal typical features of upward positive leaders before the attachment, including their initiation and mode of propagation. According to the results, upward positive leaders initiate some hundreds of microseconds prior to the return stroke, while a continuous uprising current of about 4A and superimposed pulses of a few tens amperes flow along the tower. Upon leader initiation, the electric field measured 50m away from the tower at ground level is about 60kV/m. The corresponding average field roughly estimated 0.5m above the tower top is higher than 0.55MV/m. As in laboratory experiments, the common propagation mode of upward positive leaders is developing continuously, without steps, from their initiation. Unlike downward negative leaders, upward positive leaders typically do not branch off, though they can bifurcate under the effect of a downward negative leader's secondary branch approaching their lateral surface. The upward positive leader's estimated average two-dimensional propagation speed, in the range of 0.06x10(6) to 0.16x10(6)m/s, has the same order of magnitude as that of downward negative leaders. Apparently, the speed tends to increase just before attachment.