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.