GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 5531–5535, doi:10.1002/2013GL058060, 2013
Lightning attachment process involving connection of the downward
negative leader to the lateral surface of the upward connecting leader
Weitao Lu,1,2 Luwen Chen,3 Ying Ma,1,2 V. A. Rakov,4 Yan Gao,1,2 Yang Zhang,1,2
Qiyuan Yin,3 and Yijun Zhang1,2
Received 18 September 2013; accepted 4 October 2013; published 21 October 2013.
[1] High-speed video camera records, with a temporal
resolution of 20 μs and a spatial resolution of 2.4 m per
pixel, of a downward negative lightning flash that terminated
on a 440 m high building are examined. The attachment
process in this flash exhibited an unexpected behavior in that
the downward leader tip connected to the lateral surface of
the ~400 m upward connecting leader (UCL) below its tip. It
appears that the effect of the downward leader on the UCL is
significant, while the effect of the UCL on the downward
leader is negligible, except for the final 80 μs preceding the
beginning of the first return stroke. The ratio of speeds of the
downward leader and the UCL tends to decrease with time,
ranging from 1.8 to 0.12, although the lower 80–100 m or so
of the UCL were too faint to allow speed measurements.
Citation: Lu, W., L. Chen, Y. Ma, V. A. Rakov, Y. Gao,
Y. Zhang, Q. Yin, and Y. Zhang (2013), Lightning attachment
process involving connection of the downward negative leader to
the lateral surface of the upward connecting leader, Geophys. Res.
Lett., 40, 5531–5535, doi:10.1002/2013GL058060.
1. Introduction
[2] Understanding of the lightning attachment process is vital
for improving lightning protection techniques. Unfortunately,
due to the low sensitivity and low spatial or temporal resolution,
most of the existing optical images of natural lightning discharges are not suitable for a thorough analysis of the propagation characteristics of the downward leader and the upward
connecting leader (UCL) during the attachment process.
[3] Streak camera has been used to observe the development
of lightning discharges for many years [e.g., Uman, 1987;
Rakov and Uman, 2003]. Berger and Vogelsanger [1966]
1
Laboratory of Lightning Physics and Protection Engineering, Chinese
Academy of Meteorological Sciences, Beijing, China.
2
State Key Laboratory of Severe Weather, Chinese Academy of
Meteorological Sciences, Beijing, China.
3
Lightning Protection Center of Guangdong Province, Guangzhou,
Guangdong, China.
4
Department of Electrical and Computer Engineering, University of
Florida, Gainesville, Florida, USA.
Corresponding author: Dr. W. Lu, Laboratory of Lightning Physics and
Protection Engineering, Chinese Academy of Meteorological Sciences, No.
46 Zhongguancun South Ave., Haidian District, Beijing 100081, China.
([email protected])
©2013. The Authors. Geophysical Research Letters published by Wiley on
behalf of the American Geophysical Union.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
0094-8276/13/10.1002/2013GL058060
published a streak photograph of a lightning discharge to a
70 m tower, in which the steps of the downward leader are
seen clearly and the evidence of the UCL is given. Using an
eight-channel photodiode system, Wang et al. [1995] resolved
the luminous features of lightning attachment to the CN tower
with a temporal resolution of 0.2 μs and a spatial resolution of
29 m. In recent years, high-speed video cameras have been
used to observe the attachment process of lightning to tall
structures with relatively high spatial and temporal resolution
[e.g., Lu et al., 2010; Warner, 2010].
[4] This paper presents a negative lightning flash with
the junction point between the downward and upward
connecting leaders being below the UCL tip. To the best of
our knowledge, such behavior has never been reported
before. For this flash, high-speed video camera records with
a temporal resolution of 20 μs and a spatial resolution of
2.4 m enabled us to analyze in detail the propagation characteristics, including direction of extension, speed, and luminosity, of the downward leader and the UCL during the
attachment process preceding the first return stroke.
2. Experiment
[5] A field experiment, mainly focusing on the observation
of lightning flashes terminating on tall structures, has been
conducted since 2009 in Guangzhou, Guangdong, China
[Lu et al., 2010]. We established the Tall-Object Lightning
Observatory in Guangzhou (TOLOG), which is located on
the top of a building with a height of approximately 100 m that
belongs to the Guangdong Meteorological Bureau. Several
types of instruments are installed at the TOLOG to
simultaneously measure the acoustic, optical, electric field,
and magnetic field signals produced by lightning discharges
[Lu et al., 2012].
[6] In this paper, we analyze a downward negative lightning flash that occurred at 07:13:32 (UT) on 18 July 2011
and struck the 440 m high Guangzhou International
Finance Center (GIFC). This flash is labeled F1111 in our database. F1111 is captured by two Photron FASTCAM SA5
cameras (HC-1 and HC-2) and one Photron FASTCAM
SA3 camera (HC-3). The three cameras are operated at sampling rates of 10,000 frames per second (fps) (100 μs per
frame), 50,000 fps (20 μs per frame), and 1000 fps (1 ms
per frame), respectively. Their recording lengths are set at
0.2 s, 24 ms, and 1.2 s with pretrigger lengths of 0.1 s,
20 ms, and 0.1 s, respectively. HC-1 and HC-2 images are
mainly analyzed here. The focal length of the lens mounted
on HC-1 is 24 mm and 20 mm on HC-2. The distance from
the TOLOG to the GIFC is approximately 2.4 km, and the
corresponding spatial resolutions of the HC-1 and HC-2
images are 2.0 and 2.4 m per pixel, respectively.
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LU ET AL.: UNEXPECTED LIGHTNING ATTACHMENT PROCESS
downward leader (PBDL) that facilitates connection to the
building and all the three upward leaders, reconstructed by
using the HC-2 images, are shown in Figure 3.
Height AGL (m)
1240
1040
840
640
440
(a) -0.06 ms
440 m AGL
(b) 1.54 ms
Figure 1. (a and b) High-speed video camera (HC-1) images
of flash F1111 captured with a sampling rate of 10,000 fps.
Figure 1a is cropped vertically to save space. Time 0 is set at
the beginning of the first return stroke, which occurs during
the frame (not shown here) immediately following Figure 1a
with most pixels saturated. Figure 1b shows the frame
1.54 ms after the beginning of the first return stroke.
3. Results
[7] From analysis of the electric field change record and
HC-3 images (not shown here), we determined that F1111
was a downward negative cloud-to-ground lightning flash that
lasted for approximately 330 ms and contained eight return
strokes (along a single path) with interstroke intervals ranging
from 11 ms to 117 ms. Four return strokes (first, third, fifth,
and sixth) were recorded by the Lightning Location System
(LLS) of the Guangdong Power Grid Corporation, whose detection efficiency and location accuracy have been evaluated
by Chen et al. [2012]. The LLS-reported peak current values
are 102, 29, 63, and 26 kA, respectively.
[8] Selected high-speed images of F1111 are shown in
Figures 1 and 2. Time 0 is set at the beginning of the first
return stroke (hereinafter, the “first” is omitted for brevity).
The time stamps of the HC-1 and HC-2 images are synchronized by using Figures 1a and 2b with the same end time of
exposure duration because the UCL lengths in those images
are almost the same, namely, 333 m and 336 m, respectively.
The downward leader of F1111 induces three upward leaders
(one UCL and two unconnected leaders) from the top of the
GIFC. The two unconnected upward leaders are very faint,
so they cannot be distinguished easily from the background
in high-speed camera images. The primary branch of the
3.1. Two-Dimensional Extension Directions of the
PBDL and the UCL
[9] Like most natural downward lightning flashes, F1111
exhibits a downward leader with many branches. The
branches of the downward leader extend in a large spatial
region and even beyond the HC-1 field of view, as shown
in the top left and top right corners in Figure 1a. The
extension directions of the individual branches of the
downward leader appear to be random (although a downward
trend on the whole is clear), except for the PBDL during the
final 80 μs prior to the beginning of the return stoke
(Figures 2a–2d). From the HC-1 images, it appears that the
PBDL and the UCL bend toward each other during the final
60 μs, and finally, their tips make contact, which is a generally
expected lightning behavior during the attachment process.
However, the HC-2 images reveal more details and give us
an additional insight into the attachment process of F1111.
[10] From Figures 2 and 3, it can be clearly seen that the
PBDL bends sharply toward the UCL after the two-dimensional
(2-D) distance between them decreases to less than approximately 60 m, then extends toward the UCL and finally makes
contact with the UCL lateral surface. The connection occurs between TD (the tip of the PBDL in Figure 2d) and a point, HU,
located on the UCL, but below its tip (TU) in Figure 2d. It is
known that some flashes strike tall, stationary structures below
their tops [e.g., Gorin et al., 1976; Hussein et al., 2007]. In
our case, however, the downward leader connects to the UCL
(plasma channel) below its tip when its extension is in progress.
[11] The 2-D length of the UCL in Figure 2d is 403 m and
that of the UCL part higher than HU is 67 m. It is impossible
to detect propagation of the PBDL and the UCL after the end
of the exposure time of Figure 2d because the return stroke
causes most pixels in the following frame (not shown here)
to be saturated. It can be inferred that the final 2-D length
of the UCL is longer than 403 m and that of the UCL part
above HU is over 67 m. As shown in Figures 2d, 2e, 2f,
and 3, the UCL at heights above HU propagates predominantly upward, not toward the PBDL tip, although with a little slant toward the PBDL. We infer that the effect of the
PBDL on the UCL is limited (even though the 2-D distance
between them decreases to less than 60 m) and that the integrated effect of all branches of the downward leader produces
a stronger attractive effect to the UCL than the PBDL alone.
3.2. Two-Dimensional Propagation Speeds of the PBDL
and the UCL
[12] The 2-D propagation speeds (hereinafter “speeds”) of
the PBDL and the UCL, Vd and Vu, respectively, calculated
by using high-speed images, are plotted versus time in
Figure 4a. It should be noted that not all frames were suitable
for unambiguous identification of the progressing leader tip.
A total of 21 Vd values were obtained from the HC-1 images
after the PBDL entered the HC-1 field of view and 37 Vd
values from the HC-2 images. The initial (before 0.7 ms)
83 m of the UCL channel cannot be discerned from the
HC-1 images due to the low luminosity of the UCL. Seven
HC-1 frames can be used to analyze the propagation characteristics of the UCL, and six Vu values can be obtained. By
using the HC-2 images, the initial (before 640 μs) 96 m of
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LU ET AL.: UNEXPECTED LIGHTNING ATTACHMENT PROCESS
(a) -80 µs
(b) -60 µs
TU
TD
(c) -40 µs
(d) -20 µs
TU
HU
HU
TD
Height AGL (m)
1040
840
640
440
(e) 0.5 ms
(f) Composite image
Figure 2. Frames of flash F1111 acquired using HC-2 operating with a sampling rate of 50,000 fps. (a–e) Individual frames.
(f) A 76-frame composite image. The 76 frames include 75 consecutive frames preceding the return stroke and the frame
shown in Figure 2e. All the images are inverted for a better view. TD is the tip of the primary branch of downward leader (that
facilitated connection to the building) in Figure 2d, the last frame preceding the return stroke. TU is the tip of the UCL in
Figure 2d. HU is the highest UCL point that is discernible in the return stroke channel in Figure 2e.
the UCL channel cannot be discerned. A total of 20 HC-2
frames can be used to analyze the propagation characteristics
of the UCL, and 19 Vu values can be obtained.
[13] From Figure 4a, it can be seen that both the PBDL and
the UCL have speeds mainly on the order of 105 m s 1. From
the HC-1 images, Vd ranges from 1.7 × 105 to 4.2 × 105 m s 1
(with an average value of 3.0 × 105 m s 1) between the
heights of 700 m and 1320 m AGL, and Vu ranges from
1.8 × 105 to 11.3 × 105 m s 1 (average: 4.2 × 105 m s 1) between 490 m and 720 m. From the HC-2 images, Vd ranges
from 1.7 × 105 to 8.6 × 105 m s 1 (average: 3.0 × 105 m s 1)
between the heights of 700 and 1070 m, and Vu ranges from
1.4 × 105 to 17.1 × 105 m s 1 (average: 5.0 × 105 m s 1) between 500 and 790 m.
[14] The ratio of the speeds of the PBDL and the UCL
(Vd/Vu), the 2-D distance between their tips (Ddt-ut), and the
2-D distance between the PBDL tip and the closest point on
the UCL channel (Ddt-u), each versus time, are shown in
Figure 4b. The Vd/Vu calculated by using the HC-1 and
HC-2 images range from 1.2 to 0.28 and from 1.8 to 0.12,
respectively. Overall, Vd/Vu exhibits a decreasing trend because Vd shows no clear change, except for the final 80 μs prior
to the beginning of the return stroke, while Vu tends to generally increase, as shown in Figure 4a. During the final 160 μs,
as Dut-dt decreases to less than 150 m, Vu sharply increases
from 3.6 × 105 to 17.1 × 105 m s 1. During the final 60 μs, although the predominantly upward direction of the UCL extension above point HU results in the increase of the Dut-dt, Ddt-u
(always equal to Dut-dt before 60 μs) decreases continuously.
From Figure 2a to 2d, after the PBDL bends toward the UCL,
Ddt-u decreases from approximately 60 m to 30 m, and Vd
increases from 1.7 × 105 to 8.6 × 105 m s 1. During the final
80 μs, although both Vd and Vu increase sharply, the former
increases faster, so that Vd/Vu increases from 0.12 to 0.50.
3.3. Luminosities of PBDL and UCL Tips
[15] In this paper, the average grey level of the five
brightest pixels near the leader tip in the high-speed image
is used to represent the luminosity of the leader tip. The variations of the leader tip luminosity versus time for the PBDL
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LU ET AL.: UNEXPECTED LIGHTNING ATTACHMENT PROCESS
the downward leader branches appears to be negligible,
except for the final 80 μs, which is evidenced by the lack of
changes in the extension direction, speed, and luminosity of
the downward leader until the distance between the PBDL
and the UCL decreases to less than approximately 60 m
(see Figures 2, 3, 4a, and 4c). The speeds of the PBDL and
the UCL, Vd and Vu, are both mainly on the order of 105 m
s 1. Vd exhibits no clear trend before 80 μs and sharply
Figure 3. The primary branch of the downward leader that
facilitated connection to the building and the three upward
leaders reconstructed from the HC-2 images. The channel
between TD and HU can be only reconstructed from unsaturated post-return-stroke images (e.g., Figure 2e).
and the UCL obtained from the HC-2 images are shown in
Figure 4c. Similar to the variation of speed, the variation of
luminosity of the PBDL tip exhibits no clear trend until the
final 80 μs prior to the beginning of the return stroke, while
the luminosity of the UCL tip exhibits an overall increasing
trend, although the trend before 300 μs is not clear.
4. Summary and Discussion
[16] High-speed video camera records showing the details of
the connection between the downward leader and the UCL in a
natural downward negative lightning flash that terminated on
the 440 m high Guangzhou International Finance Center,
China, are analyzed. In this flash, the presence of multiple
branches of the downward leader appears to be causing the occurrence of multiple upward leaders from the top of the building and the propagation of the UCL without clear bending
toward the PBDL (the primary branch of the downward leader
that facilitated connection to the building). The latter behavior
apparently resulted in the connection of the PBDL to the UCL
over 67 m below the UCL tip. There is no information on how
the PBDL and the UCL propagated after Figure 2d. Vd was
increasing sharply prior to the return stroke and reached
8.6 × 105 m s 1. It is possible that the PBDL propagated all
the 42 m between TD and HU (see Figure 3) and finally made
contact with the UCL at HU, i.e., the junction point may be
HU. It is also possible that a branch of the UCL initiated from
HU and made contact with the PBDL, i.e., the junction point
may be located somewhere between TD and HU. It should
be mentioned that the flash presented here is not the only event
in which we observed the connection occurring below the UCL
tip, although there are cases that do show connections between
the tips of the downward leader and the UCL. Further analysis
of the phenomenon presented in this paper is in progress.
[17] The predominantly upward direction of the UCL extension (see Figures 2 and 3) and the overall increase of both
speed and luminosity of the UCL (Figures 4a and 4c) suggest
that the integrated effect of (electric field produced by) all
branches of the downward leader on the UCL is significant,
especially during the final 160 μs preceding the beginning
of the return stroke. In contrast, the effect of the UCL on
Figure 4. (a) Variations of 2-D speeds of the PBDL and the
UCL, Vd and Vu. (b) Variations of Vd/Vu, the 2-D distance
between the tips of the PBDL and the UCL (Ddt-ut), and the
2-D distance between the PBDL tip and the closest point on
the UCL channel (Ddt-u). (c) Variations of the leader tip luminosity for the PBDL and the UCL.
5534
LU ET AL.: UNEXPECTED LIGHTNING ATTACHMENT PROCESS
increases from 1.7 × 105 to 8.6 × 105 m s 1 during the final
80 μs, while Vu tends to increase in general and sharply increases from 3.6 × 105 to 17.1 × 105 m s 1 during the final
160 μs. The ratio of Vd and Vu exhibits an overall trend to
decrease, with the range of variation being from 1.8 to
0.12, although the lower 80–100 m or so of the UCL were
too faint to allow speed measurements.
[18] The optical properties of the downward negative
stepped leader have been studied by many researchers [e.g.,
Krider, 1974; Orville and Idone, 1982; Chen et al., 1999; Lu
et al., 2008; Hill et al., 2011]. Observations of downward
and upward connecting leaders during the attachment process
preceding the first return stroke in natural lightning flashes are
limited [e.g., Lu et al., 2010; Warner, 2010]. The ranges and
average values of the speeds of the downward leader and the
UCL presented in this paper are consistent with those reported
previously. Detailed information on variations of the ratio of
speeds of downward and upward connecting leaders and the
luminosities of leaders during the attachment process is not
found in the previous literature. The observed characteristics
of leaders are important for understanding the lightning attachment process, which is one of the least understood lightning
processes. The results presented in this paper also provide
information needed in developing leader progression models
[e.g., Dellera and Garbagnati, 1990; Rizk, 1994; Mazur
et al., 2000]. In particular, there is no consensus on the time
variation of Vd/Vu. For example, Dellera and Garbagnati
[1990] assumed that the ratio of speeds of the negative
downward leader and the positive UCL changes from 4 to 1
during the attachment process, while Rizk [1994] assumed that
Vd/Vu = 1, and Mazur et al. [2000] assumed that Vd/Vu = 2. Our
data show that the Vd/Vu exhibits an overall trend to decrease.
We also found that Vu can significantly exceed Vd, and hence,
Vd/Vu can be significantly less than 1. To the best of our
knowledge, this latter possibility has never been considered
in the models.
[19] Acknowledgments. This work was supported in part by the
National Natural Science Foundations of China under grant 41075003,
41175003, and 41030960; by the Basic Research Fund of Chinese Academy
of Meteorological Sciences under grant 2010Z004; and by the U.S. National
Science Foundation.
[20] The Editor thanks two anonymous reviewers for their assistance in
evaluating this paper.
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