Development of Side Bidirectional Leader and Its Effect on Channel Branching of the Progressing Positive Leader of Lightning

Fine time-resolved images of the development of lightning bidirectional leaders below the cloud base were captured by a high-speed camera in association with multiband electromagnetic field measurements. The bileaders tended to originate at a radial distance of about 200 m from a progressing positive leader channel, propagating more or less radially toward and away from the main channel. Significant asymmetrical channel extensions at opposite ends were clearly recognized. The positive ends of the bileaders propagated in single paths with average speed of 6.1 x 10(4) m/s, while the opposite negative end involved obvious branching and stepping propagation with average speed of 1.6 x 10(5) m/s. The bidirectional leaders eventually connected to the existing main positive leader, producing new branches of the flash. Another cause of positive leader branching was that the positive leader split at its head and developed into different branches synchronously, which were sometimes invisible but were revealed by recoil leaders.