Optical Characteristics of Recoil Leader Activity in one Positive Cloud-to-ground Lightning Flash

The initiation and propagation characteristics of recoil leader (RL) activity during the long continuing current (CC) stage of one positive cloud-to-ground (CG) lightning flash have been investigated based on high-speed video observations. The main findings are as follows: Abundant RL activity mainly occurs along three previously formed positive leader channels. Some RL events show apparent bidirectional propagation, and the two-dimensional (2-D) average propagation speed ranges from 8.10x10(5) m/s to 5.99x10(6) m/s for both ends, mainly in the order of 10(6) m/s. RL activity shows some interesting characteristics that the branched leader tips formed on its positive end can develop alternately during one development process and RL initiates next time can follow either path of these branches; after RL channel has been decayed for some time, RL can further develop and reilluminate on its positive end with the branched leader tips; RL propagation upon connection to the active positive discharge leader can cause an M-component on the background CC current, with great luminosity and a rather long duration.