Figure 4. Measured current profile of an UCL from B and an UUL from A, and the electric field acquired during the event. The electric field was saturated before time -400 µs.
4. Analysis and discussion
4.1 Leader speed
Based on the video images of both high-speed cameras it was possible to measure the 2D speed of leaders A and B, but also of other four UULs that are described in Table 1. The fastest upward leaders were the UCL (leader B) and leader A, the closest UUL to the attachment point. All upward leaders propagated in a constant speed (Figure 5). The downward leader that connected to leader B had also a constant speed but 2.0 times higher (Vd = 28.5 × 104 m/s) than the speed of leader B. This confirms what was found for other cases of attachment process in the same buildings by Saba et al. (2017), i.e., that contrary to what is observed in taller structures (Lu et al., 2013 and 2015) and assumed in some leader propagation models (Rizk, 1990 and 1994), speeds are approximately constant, and the speed of the downward leader is higher than the speed of the upward leaders (speed ratio between 2.0 and 5.5). Table S1 in Supplementary Information compares the speeds, speed ratios and other important characteristics for the case analyzed in this work with three other attachment processes in the same building as reported by Saba et al. (2017). We haven’t found any published report of these characteristics for common structures (height under 60 m).
There is a significant increase in the speed of leader B (UCL) immediately before connection that suggests the final jump condition of the attachment process (also observed by Saba et al., 2017). Leader B (UCL) propagated at a constant speed of 14.3 × 104 m/s and, during the last frame before connection, the speed increased to 51.4 × 104 m/s, 3.3 times higher than the previous average speed. Combining the time and length information given in the images from both cameras, the final jump speed (a combination of the downward and upward leader propagation, see e.g. discussion done by Saba et al. 2017) was estimated to be higher than 640 × 104m/s, that is, at least 45 times faster than the average speed of the upward leader.