资源环境科技发展态势分析平台

The characteristics of two natural positive cloud-to-ground flashes are analyzed in detail with the broadband very high frequency (VHF) mapping system. The bidirectional leader processes at the lightning initiation are observed, with negative leaders propagating upward and positive leaders propagating downward. The downward positive leaders can be divided into two stages, with scattered and slowly extending sources in the first stage and more concentrated and faster sources in the second stage. The overall extension speeds of the first stages are 1.8 x 10(4)and 2.7 x 10(4) m/s for the two flashes, and the speeds of the second stages are 7.8 x 10(5)and 1.0 x 10(6) m/s, respectively. The received peak power of the positive leaders is substantially lower than that for the negative leaders. The frequency characteristics show that the second stage emits more high-frequency radiation than the first stage, while there is no significant difference in low-frequency radiation content.