Optical characteristics of branched downward positive leader associated with recoil leader activity

The propagation characteristics of one downward positive leader with mainly four forked branches in one natural positive lightning have been investigated based on high-speed video observations. One branch develops horizontally; one branch travels downward eventually reaching the ground, and two faint branches are reilluminated by recoil leader (RL) activity multiple times. Four branches share some interesting correlation during their development. At the early phase, the horizontal branch and descending branch are inactive or propagate slowly when RL activity is more active. After the appearance of RL activity each time, the horizontal branch propagates slightly faster. After that, the two luminous branches propagate more actively and faster, while RL process becomes diminished and suppressed. Both downward and horizontal branches speed up as the downward one is approaching the ground. The two-dimensional (2-D) propagation speed for the downward branch ranges from 4.25 × 104 m/s to 3.85 × 105 m/s, with an average value of 1.01 × 105 m/s, and it ranges from 2.69 × 104 m/s to 3.61 × 105 m/s with an average speed of about 1.22 × 105 m/s for the horizontal one.