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

摘要

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 503m(number of samples: 45), <0.1 to 1.32ms(38), 20m to 1.3km(38), 99 to 578m(21), 0.48 to 399m(45), and 5.79 to 33.8 × 10 ~4m s ~(-1)(22), respectively. 86%(19/22)of the velocities are smaller than 1.7 × 10 ~5m s ~(-1). No UUL with an inception time prior to RS greater than 0.5ms is initiated from a structure lower than 300m. Those UULs with inception heights lower than 300m seldom exhibit lengths longer than 50m and only can be initiated by flashes within approximately 600m, while those higher than 400m can even reach several hundred meters and be initiated by flashes over 1km away. The maximum distances for the downward leaders to attract the UULs with inception heights from 100 to 200m, 200 to 300m, and over 400m are approximately 350m, 450m, and 600m, respectively.