Three-dimensional propagation characteristics of the leaders in the attachment process of a downward negative lightning flash

• 3-D channels of leaders during the attachment process were reconstructed.
• Variations of the 3-D and 2-D speeds of the leaders versus time were obtained.
• The 3-D speeds of downward leader and upward connecting leader were compared.
• The upward leaders can propagate significantly faster than the downward leader.

A downward negative lightning flash that contained three return strokes and terminated on a 440-m-high building in Guangzhou in 2012 was analyzed. The three-dimensional (3-D) channels of the return stroke and an unconnected upward leader (UUL) were reconstructed using the dual-station optical observation data. The 3-D propagation characteristics of the downward leader, the upward connecting leader (UCL), and the UUL during the attachment process prior to the first return stroke were obtained. For the 3-D propagation speed, both the UCL and the UUL exhibit an increasing trend after their inception, whereas the downward leader shows no clear trend, except for the final 200 μs preceding the first return stroke onset. The speed of the UCL can reach five times that of the downward leader. The two-dimensional propagation characteristics of these leaders during the attachment process were also analyzed using single-station high-speed video recordings and compared with the 3-D results.