Characteristics of unconnected upward leaders initiated from tall structures observed in Guangzhou

Forty-five unconnected upward leaders (UULs) occurred in 19 downward negative flashes are analyzed. Each observed UUL is initiated by a downward stepped leader before a new strike point is struck. For each UUL, several parameters are determined when possible mainly by using high-speed images: inception height, inception time prior to return stroke (RS), horizontal distance from the flash's strike point, two-dimensional (2D) distance between the nearest downward leader branch tip and the UUL's inception point at its inception time, 2D length, and 2D average propagation velocity. Their values range from 40 to 503 m (number of samples: 45), <0.1 to 1.32 ms (38), 20 m to 1.3 km (38), 99 to 578 m (21), 0.48 to 399 m (45), and 5.79 to 33.8 x 10(4) m s(-1) (22), respectively. 86% (19/22) of the velocities are smaller than 1.7 x 10(5) m s(-1). No UUL with an inception time prior to RS greater than 0.5 ms is initiated from a structure lower than 300 m. Those UULs with inception heights lower than 300 m seldom exhibit lengths longer than 50 m and only can be initiated by flashes within approximately 600 m, while those higher than 400 m can even reach several hundred meters and be initiated by flashes over 1 km away. The maximum distances for the downward leaders to attract the UULs with inception heights from 100 to 200 m, 200 to 300 m, and over 400 m are approximately 350 m, 450 m, and 600 m, respectively.