The characteristics of 45 unconnected upward leaders (UULs) occurred in 19 downward negative flashes observed in Guangzhou during 2009-2011 are analyzed. Each of the 45 UULs is an upward positive leader initiated by a downward negative 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 between the strike point and the UUL's inception point, two-dimensional (2D) distance between the nearest tip of the downward leader branches and the UUL's inception point when the UUL is initiated, 2D length, and 2D average propagation velocity, whose values range from 40 m to 503 m (number of samples: 45), less than 0.1 ms to 1.32 ms (38), 20 m to 1.3 km (38), 99 m to 578 m (21), 0.48 m to 399 m (45), and 5.79 to 33.8 x 10' ms respectively. Statistical analysis of the UULs' characteristics shows the following: No UUL with an inception time to RS larger than 0.5 ms is initiated at a height lower than 300 m.
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