RESEARCH ARTICLE
10.1029/2020JD032851
Key Points:• Three‐dimensional velocities of 24
UNLs range from 1.8 to
27.9 × 105 m/s with a mean value of
10.4 × 105 m/s• Radiation ﬁeld waveforms produced
by UNLs are distinctly different fromthose produced by other types ofnegative stepped leaders• UNLs are mainly triggered by
nearby (a few km) strong negative
strokes combined with following
in‐cloud positive leaders

Correspondence to:T. Wu,
tingwu@gifu-u.ac.jp

Citation:
Wu, T., Wang, D., & Takagi, N. (2020).
Upward negative leaders in positive
upward lightning in winter:
Propagation velocities, electric ﬁeld
change waveforms, and triggering
mechanism. Journal of GeophysicalResearch: Atmospheres, 125,e2020JD032851. https://doi.org/
10.1029/2020JD032851
Received 31 MAR 2020
Accepted 3 AUG 2020
Accepted article online 6 AUG 2020

©2020. American Geophysical Union.
All Rights Reserved.

WU ET AL.

Upward Negative Leaders in Positive Upward Lightning
in Winter: Propagation Velocities, Electric Field
Change Waveforms, and Triggering Mechanism
Ting Wu1, Daohong Wang1, and Nobuyuki Takagi1

1

Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu, Japan

Abstract Upward negative leaders (UNLs) in positive upward lightning are rarely observed. In thisstudy, 24 UNLs observed by the Fast Antenna Lightning Mapping Array during winter thunderstorms inJapan are analyzed. Three‐dimensional velocities of UNLs are calculated, and it is found that velocities
during the upward propagation stage range from 1.8 to 27.9 × 105 m/s with a mean value of 10.4 × 105 m/s,
and they are always larger than velocities during the following horizontal propagation stage. UNLs produce
distinctive electric ﬁeld change waveforms with V‐shaped overall change trend, with small pulses at the
beginning when UNLs start, large pulses during upward propagations, and small pulses again when UNLs
turn into horizontal directions. Pulses produced by UNLs are mainly unipolar but sometimes also bipolar.
Pulse interval, pulse width, rise time, and fall time are calculated for waveforms produced by ﬁve UNLs.
Pulse intervals range from 13.7 to 18.9 μs. Pulse width, rise time, and fall time are on average 5.8, 3.3, and2.4 μs, respectively. All UNLs are preceded by other discharges. Strong negative strokes including somespecial types of strokes usually precede initiations of UNLs by tens of milliseconds. These negative strokes
are usually found only a few kilometers away from UNLs. It is speculated that these negative strokes alongwith following in‐cloud positive leaders contribute to the depletion of negative charges in thunderclouds,
which result in positive electric ﬁeld changes (atmospheric electricity sign convention) large enough for
initiations of UNLs.

1. Introduction
Unlike normal lightning ﬂashes which originate inside thunderclouds, upward lightning ﬂashes start withupward leaders from tall grounded objects and pose particular threats to tall buildings, towers, and windmills. Upward lightning ﬂashes are classiﬁed into positive and negative ones according to the polarity ofcharges transferred to the ground. Positive upward ﬂashes start with upward negative leaders (UNLs) andtransfer positive charges to the ground, and negative upward ﬂashes start with upward positive leaders(UPLs) and transfer negative charges to the ground. Due to the fact that positive leaders generally requirea weaker electric ﬁeld to initiate (e.g., Williams, 2006), negative upward ﬂashes are much more frequent
than positive upward ﬂashes. For example, studies of upward ﬂashes at the Gaisberg Tower show that positive upward ﬂashes only account for about 4% (Watanabe et al., 2019; Zhou et al., 2012a). As a result, compared to upward negative ﬂashes, upward positive ﬂashes are much less well studied.
It has been clear that upward ﬂashes can occur with or without preceding lightning discharges, called“other‐triggered” and “self‐initiated” upward ﬂashes, respectively (e.g., Wang et al., 2008). Numerous studies have been contributed to the triggering mechanism of upward negative ﬂashes (Guimarães et al., 2014;
Jiang et al., 2014; Pineda et al., 2019; Qi et al., 2018; Saba et al., 2016; Sunjerga et al., 2020; Takagi et al., 2006;
Wang et al., 2008; Wang & Takagi, 2012a; Warner, 2012; Warner et al., 2012; Warner et al., 2014; Wu, Lyu,
et al., 2019; Yuan et al., 2017; Zhou et al., 2012b; Zhu et al., 2019). One interesting phenomenon is that theratio between other‐triggered and self‐initiated upward negative ﬂashes varies signiﬁcantly in different studies (e.g., Saba et al., 2016; Warner et al., 2012; Warner et al., 2014; Zhou et al., 2012b). Apparently, factors
contributing to the different ratios at least include seasons, thunderstorm characteristics, and properties oftall objects. For other‐triggered upward negative ﬂashes or UPLs, observations show that they can be triggered directly by positive return strokes (Jiang et al., 2014; Wu, Lyu, et al., 2019; Zhou et al., 2012b) or byapproaching in‐cloud negative leaders (Saba et al., 2016; Yuan et al., 2017). By contrast, triggering mechanisms of upward positive ﬂashes or UNLs are still poorly understood. So far, only a handful of upward positive
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ﬂashes or UNLs have been analyzed along with preceding lightning discharges (Heidler et al., 2015; Lu
et al., 2009; Wang & Takagi, 2012a, 2012b; Zhou et al., 2012b), and it is still not clear what kind of lightning
discharges can trigger UNLs.
Due to the rarity of upward positive ﬂashes, UNLs, the most essential component of upward positive
ﬂashes, are also poorly understood. UNLs are apparently negative stepped leaders. Other forms of negative stepped leaders include downward negative stepped leaders in −CG ﬂashes and initial negative leaders during the preliminary breakdown (PB) stage in intracloud (IC) and CG ﬂashes, both of which havebeen extensively studied using various types of devices, such as high‐speed videos (e.g., Hill et al., 2011;
Stolzenburg et al., 2013), electric ﬁeld change (E‐change) meters (e.g., Beasley et al., 1982; Krider
et al., 1977), interferometers (e.g., Shao et al., 1995; Shao & Krehbiel, 1996), and low‐frequency (LF)
lightning mapping systems (e.g., Shi et al., 2019a, 2019b; Wu et al., 2015). UNLs are similar to downward
stepped leaders in −CG ﬂashes in the respect that both propagate near the ground and largely in thevertical direction. UNLs are also similar to initial leaders in PB stages in the respect that both are theinitial leader process in a lightning ﬂash. Therefore, a natural question that arises is what are the differences and similarities between UNLs and the other two types of negative stepped leaders that we are
already familiar with.
In this paper, we will analyze 24 UNLs observed in winter in the Hokuriku region of Japan with an LF lightning mapping system called Fast Antenna Lightning Mapping Array (FALMA). Velocities, E‐change waveforms, and discharges preceding these UNLs will be analyzed to shed some light on several basic propertiesof this rare phenomenon.

2. Data
Data were obtained during the winter observation from December 2018 to March 2019 in the Hokuriku
region of Japan, which is famous for frequent lightning ﬂashes during the winter season. During the observation, a FALMA system consisting of 14 sites were set up. The FALMA is a three‐dimensional (3‐D) LF
lightning mapping system as described in Wu et al. (2018). Locations of the 14 sites are shown in
Figure 1. Each site has a fast antenna working in the frequency band of 500 Hz to 500 kHz. Distances
between neighboring sites are from about 10 to 20 km. Triggered E‐change signals are recorded with a sampling rate of 10 MS/s. The recording length of each trigger is at least 2 s with the pretrigger length of at least
0.5 s.
A total of 24 UNLs was identiﬁed. Initiation locations of these UNLs are shown as red crosses in Figure 1.
UNLs were identiﬁed based on their 3‐D location results and multisite E‐change waveforms. Upward leaders
initiated from near the ground producing negative pulses (atmospheric electricity sign convention) at allobservation sites during their upward propagations are identiﬁed as UNLs. Further, their initiation locations
are examined in Google Earth to make sure there are tall buildings or towers within several hundred metersor they are in mountains. Information of these UNLs is summarized in Table 1 and will be described in detailin section 3. All UNLs were preceded by other discharges, and only one UNL was followed by a positive
return stroke.

3. Results
3.1. Velocities of UNLs
Velocities in 3‐D during upward propagation stages (Vu) of all 24 UNLs are estimated. Eighteen UNLs also
show horizontal propagations after upward propagations, and 3‐D velocities during horizontal propagation
stages (Vh) for these 18 UNLs are estimated as well.
Figure 2 shows an example of an UNL in an upward positive ﬂash and illustrates the method to estimate 3‐Dvelocities. Figure 2a shows the height‐time view of the whole upward positive ﬂash with preceding discharges. Colored sources indicate the UNL. Figures 2b and 2c show the E‐change waveform andheight‐time view of the UNL. We can see the upward leader produced negative pulses, the same polarity
as that produced by positive return strokes, conﬁrming that the leader carried negative charges upward.
The 3‐D mapping results of the UNL are shown in Figures 2d–2f. The 3‐D velocities are estimated using amethod similar to that described in section 3.2 in Wu, Wang, and Takagi (2019). Because the leader does
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Figure 1. Observation sites of the FALMA (black squares) and initiation locations of upward negative leaders analyzed
in this paper (red crosses). The origin (0, 0) corresponds to the latitude and longitude of (36.76°N, 136.76°E).

not propagate in a straight line, several nodes (black diamonds in Figures 2d–2f) are added to divide theleader into roughly straight segments in 3‐D. The 3‐D distance of each leader source relative to theinitiation location is calculated along the straight segments. The 3‐D distance versus the time difference
between each leader source and the ﬁrst source are plotted in Figure 2h. Sources before the verticaldashed line are identiﬁed as the ones during the upward propagation stage and those after the verticaldashed line are identiﬁed as the ones during the horizontal propagation stage. Linear regressions are
made for sources in these two stages, and velocities are calculated as the slopes of the regression lines. For
the case in Figure 2h, the red dashed line is the regression line for the upward propagation stage, and thevelocity Vu is 9.6 × 105 m/s. The blue dashed line is the regression line for the horizontal propagation
stage, and the velocity Vh is 4.5 × 105 m/s.
With this method, velocities of Vu are calculated for all 24 UNLs, and velocities of Vh are calculated for 18
UNLs showing horizontal propagations. The results are listed in Table 1, and the distributions are shownin Figures 3a and 3b. Values of Vu range from 1.8 to 27.9 × 105 m/s, the maximum being about 1 order ofmagnitude larger than the minimum. It is interesting to note that velocities of UPLs from tall objects or intriggered lightning have even larger variations, with the maximum on the order of 106 m/s (Yoshida
et al., 2010) while the minimum on the order of 104 m/s (Edens et al., 2012). The mean value of Vu is10.4 × 105 m/s and the median value is 9.6 × 105 m/s for 24 UNLs in this study. Most results are smaller than
16 × 105 m/s. Velocities of UNLs in this study are generally larger than the few cases reported outside ofJapan (Heidler et al., 2015; Pu et al., 2017; Qiu et al., 2019) but are in general agreement with observations
in winter thunderstorms in Japan (Huang et al., 2018; Miki et al., 2014). It should be noted that these previous estimations of UNL velocities were all based on high‐speed video observations, so the results were
all 2‐D estimations and were only for the initial portion of UNLs.

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Table 1
Information of 24 UNLs Analyzed in This Paper
Date and time

Vu (×10 m/s)

5

Vh (×10 m/s)

Horizontal propagation
altitude (km)Horizontal propagation
temperature (°C)

12

17/12/2018 13:50:40
17/12/2018 19:03:56

9.0
7.3

—
—

—
—

—
—

3

17/12/2018 20:47:04

4.9

2.2

4.8

−28.4

4

17/12/2018 20:51:54

9.6

4.5

4.5

−25.9

5

17/12/2018 20:54:01

9.9

1.6

4.0

−23.1

6
7

18/12/2018 14:51:40
18/12/2018 23:48:02

9.6
27.9

5.7
5.9

3.0
3.9

−16.4
−21.8

8

18/12/2018 23:49:40

14.5

5.8

4.1

−23.1

9

19/12/2018 08:47:04

6.9

2.5

4.1

−21.6

10

19/12/2018 13:38:05

10.9

0.9

3.7

−18.6

11
12
13
14

19/12/2018 13:41:36
19/12/2018 17:50:44
19/12/2018 18:03:22
19/12/2018 18:06:24

20.5
9.2
2.6
15.3

2.8
7.3
—
2.0

2.8
1.6
—
3.7

−11.3
−4.5
—
−18.6

15

17/1/2019 17:19:09

14.4

—

—

—

16

17/1/2019 20:26:14

3.6

1.8

3.3

−19.1

17

20/1/2019 14:54:12

11.6

3.9

3.1

−10.7

18

25/1/2019 20:46:08

14.8

4.7

2.3

−12.3

19

25/1/2019 20:46:34

4.3

1.9

2.8

−15.8

20

25/1/2019 22:51:37

7.5

—

—

—

21

25/1/2019 22:54:03

10.5

6.6

2.6

−14.5

22

28/1/2019 18:05:36

1.8

—

—

—

23

28/1/2019 18:16:09

9.1

4.7

3.7

−23.7

24

28/1/2019 18:20:55

14.2

4.2

3.5

−22.9

Event

5

Preceding discharges
IC (negative leader propagating away)
Special negative stroke(Δd ¼ 1.0 km, Δt ¼ 50.0 ms)−RS (short predischarge)(Δd ¼ 0.8 km, Δt ¼ 19.9 ms)
Special negative stroke(Δd ¼ 0.7 km, Δt ¼ 58.8 ms)−RS (short predischarges)(Δd ¼ 2.0 km, Δt ¼ 22.9 ms)
IC−RS (short predischarges)(Δd ¼ 2.7 km, Δt ¼ 53.7 ms)−RS (short predischarges)(Δd ¼ 3.1 km, Δt ¼ 43.0 ms)−RS (short predischarges)(Δd ¼ 2.4 km, Δt ¼ 13.3 ms)
IC (−RS 500 ms ago)(negative leader propagating away)−RS (Δd ¼ 5.9 km, Δt ¼ 148.3 ms)
IC−RS (Δd ¼ 1.7 km, Δt ¼ 23.3 ms)
Special negative stroke(Δd ¼ 0.8 km, Δt ¼ 68.6 ms)+RS (downward negative
leader before) (Δd ¼ 11.0 km,
Δt ¼ 93.4 ms)−RS (short predischarges)(Δd ¼ 2.6 km, Δt ¼ 90.6 ms)+RS (IC discharges after)(Δd ¼ 22.1 km, Δt ¼ 243.8 ms)
LBE‐like pulse(Δd ¼ 1.5 km, Δt ¼ 15.5 ms)
LBE‐like pulse(Δd ¼ 0.7 km, Δt ¼ 49.2 ms)
LBE‐like pulse(Δd ¼ 4.3 km, Δt ¼ 8.9 ms)
LBE‐like pulse(Δd ¼ 14.4 km, Δt ¼ 397.9 ms)−RS (short predischarges)(Δd ¼ 3.2 km, Δt ¼ 30.8 ms)
LBE‐like pulse(Δd ¼ 0.8 km, Δt ¼ 109.9 ms)
LBE‐like pulse(Δd ¼ 2.0 km, Δt ¼ 60.6 ms)

Note. Vu ¼ velocity during the upward propagation stage; Vh ¼ velocity during the horizontal propagation stage.

Values of Vh also have a large range, from 0.9 to 7.3 × 105 m/s with a mean value of 3.8 × 105 m/s and a median value of 4.1 × 105 m/s. For all the 18 UNLs showing horizontal propagations, Vu is larger than Vh(Table 1).
The horizontal propagation altitudes and corresponding ambient air temperatures are also analyzed. For
each UNL, its horizontal propagation altitude is calculated as the median value of all source altitudes during
the horizontal propagation, and the corresponding temperature is calculated based on the radiosonde observation by the Japan Meteorological Agency. The radiosonde observation site was about 80 km away from thecenter of our observation network, and the data at 9:00 and 21:00 local time were available. The distributions

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Figure 2. An example of an upward positive lightning ﬂash. (a) Height‐time view and E‐change waveform of the
upward positive lightning ﬂash with preceding discharges. Colored sources correspond to the UNL. (b) E‐change
waveform and (c) height‐time view of the UNL. The value of d indicates the distance between the site recording the
waveform and the UNL. (d) Height‐distance (from west to east) view, (e) plan view, and (f) distance (from south tonorth)‐height view of the UNL. Black diamonds represent nodes to divide the leader channel into several straightsegments. (g) Source distribution along the height. (h) Distance versus time difference of UNL sources relative to theﬁrst source. The value of vu represents the 3‐D velocity during the upward propagation (slope of the red dashed line),
and the value of vh represents the 3‐D velocity during the horizontal propagation (slope of the blue dashed line).

of propagation altitudes and ambient temperatures are shown in Figures 3c and 3d. Because negative leaders
normally propagate horizontally in the positive charge region, these distributions can be treated as
distributions for the positive charge region. From Figure 3d, we can see ambient temperatures are mainly
from −10°C to −25°C. With such a temperature range for the positive charge region, the negative charge
region likely resides below the positive charge region, at the temperature level of about −10°C. Such anormal dipolar charge structure seems to be common for winter thunderstorms in Hokuriku region(Brook et al., 1982).
Values of Vh are found to be correlated with the horizontal propagation altitudes as shown in Figure 3e.
Values of Vh show a decreasing trend with increasing altitudes with a correlation coefﬁcient of −0.41.
Similar correlation was also observed for initial negative leaders during the PB stage (Wu et al., 2015;
Zheng, Shi, et al., 2019) and positive leaders in IC ﬂashes (Wu, Wang, & Takagi, 2019), indicating that there
may be a universal relationship between leader velocities and altitudes.
3.2. E‐Change WaveformE‐change waveforms of UNLs have been reported before but were mostly measured at close distances(Azadifar et al., 2018; Heidler et al., 2015; Wang & Takagi, 2012b). In fact, the majority of studies on
UNLs so far is based on current measurements and/or high‐speed video observations on some speciﬁc tallobjects; remote observations of UNLs are very rare. In this section, we will analyze E‐change waveformsof UNLs recorded at tens of kilometers away. This analysis will provide some useful information on the possibility of remotely identifying UNLs. The results can also be used for the comparison with similar leader
processes such as PB and stepped leaders, whose E‐change waveforms have been extensively measured for
decades (e.g., Beasley et al., 1982; Krider et al., 1977).
Figure 4 shows four examples of UNLs with E‐change waveforms. Another example can be found in
Figure 2. From these examples, we can see that E‐change waveforms produced by UNLs have the same
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Figure 3. Velocity distributions during the (a) upward and (b) horizontal propagation stages. Distributions of (c) thealtitude and (d) the corresponding ambient air temperature of leader horizontal propagations. (e) Altitude versus
velocity during the horizontal propagation stage. The value of r represents the correlation coefﬁcient.

overall shape, with small pulses at the beginning when UNLs start, large pulses during upward propagationsof UNLs, and small pulses again when UNLs turn to horizontal directions. This V‐shaped overall change
trend is distinctly different from that produced by initial negative leaders in PB stage or negative stepped
leaders in −CG ﬂashes.
Figure 5 further shows waveforms on 1‐ms time scale produced by these UNLs during their upward propagations. Figures 5a–5d correspond to Figures 4a–4d, and Figure 5e corresponds to Figure 2b. For each UNL,
waveforms recorded by two sites with different distances are shown. One site has a distance of 40 to 50 km
and the other 10 to 25 km. It is clear that waveforms recorded at these two distances are generally the same.
From Figure 5, we can see that UNLs mainly produce unipolar pulses like negative stepped leaders, but there
are also some bipolar pulses. For some pulses, it is difﬁcult to determine whether they are unipolar or bipolar. Pulses seem to be produced continuously, and in many occasions, multiple pulses seem to overlap witheach other. Because it is very difﬁcult to set an appropriate criterion to automatically identify all pulses, we
manually select pulses for the ﬁve examples in Figure 5. The selections are somewhat subjective, and it isindeed difﬁcult to determine what kind of variations should be classiﬁed as “pulses.” Therefore, all selected
pulses are indicated as red and blue crosses in Figure 5, and readers can determine by themselves if too manyor too few pulses are selected. The number of selected pulses is indicated by the value of Np in Figure 5.
Numbers of pulses range from 53 to 73, corresponding to average pulse intervals of 13.7 to 18.9 μs. This result
is in general agreement with an observation by Huang et al. (2018), who reported that step intervals of four
branches in an UNL ranged from 6.6 to 42.9 μs with an average of 19.3 μs.
If we assume that these pulses are produced by stepping processes of UNLs, we can estimate the average step
length (l) from the leader velocity (Vu) and the pulse frequency (ρ) with the following equation.
l ¼ V u =ρ(1)

where ρ ¼ Np/10−3(s‐1).

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Figure 4. Example of UNLs with E‐change waveforms. In each subplot, the upper part shows the E‐change waveform,
and the lower part shows the height‐time view of location results. The value of d represents the distance between
the site recording the waveform and the UNL. (a)‐(d) show four different UNLs.

The estimated average step lengths of these ﬁve cases are 15.5, 20.5, 18.0, 25.3, and 13.1 m. These values are
somewhat larger than the two UNLs observed by high‐speed video cameras (Pu et al., 2017; Qiu et al., 2019).
One reason may be that some pulses with small amplitude could not be identiﬁed, resulting in an underestimated ρ. Another possible reason is that these two studies only observed the initial ~100 m of the UNLs
while our calculations correspond to the stage when UNLs already propagated upward for 1 or 2 km, and
it is possible that the step length increases as an UNL propagates upward. Interestingly, three UNLs in winter thunderstorms analyzed by Miki et al. (2014) had average step lengths ranging from 51 to 71 m during
propagations in the initial ~500 m.
Pulse rise time, fall time, pulse width, and pulse width at half maximum are also statistically analyzed.
Because many pulses overlap with each other, it is difﬁcult to get an accurate result for pulse characteristics.
Here we manually select some relatively large pulses without overlapping pulses and calculate the parameters. These selected pulses are indicated by blue crosses in Figure 5. The second selected pulse in
Figure 5c is shown in Figure 6a to illustrate deﬁnitions of pulse parameters. As many pulses are unipolar
pulses, we deﬁne rise time as the time from 10% peak to the peak and fall time as the time from the peakto 10% peak as illustrated in Figure 6a, and pulse width is deﬁned as the sum of rise time and fall time.
Figures 6b–6d show distributions of rise time, fall time, pulse width, and pulse width at half maximum.
We can see the pulse width is mainly from 2 to 8 μs, with the rise time slightly larger than the fall time.
The mean values of pulse width, pulse width at half maximum, rise time, and fall time are 5.8, 2.7, 3.3,
and 2.4 μs, respectively.
3.3. Triggering Mechanisms
All 24 UNLs are preceded by other discharges, that is, they occur as the initial stage of “other‐triggered”
upward positive lightning ﬂashes. Information of preceding discharges is summarized in Table 1. It seems
most UNLs are preceded by negative strokes, contrary to UPLs, which are usually preceded by positive
strokes (e.g., Warner et al., 2012; Zhou et al., 2012b).
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Figure 5. E‐change waveforms of UNLs in Figures 4 and 2 in 1‐ms time scale. Waveforms recorded by two sites withdifferent distances (represented by the value of d) are shown for each event. The value of Np represents the numberof selected pulses (red and blue crosses). Pulses with blue crosses are further analyzed for pulse parameters
shown in Figure 6. (a)‐(e) show waveforms in ﬁve different UNLs.

The ﬁrst type of triggering event is called “special negative stroke” (Events 2, 4, and 14 in Table 1). Two
examples are shown in Figures 7a and 7b. In each case, we can see a large pulse with positive initial polarity(the same as negative return stroke) occurred about 50 ms before the UNL. The pulse was so large that inevery case, it saturated all of our observation sites. The polarity of the pulse indicates that it is produced
by a negative stroke, but the stroke may be different from normal negative return strokes because the waveforms show similar rise time and fall time. It is also different from return strokes in that there are only a few
pulses lasting for ~200 μs before the main pulse, and no typical PB pulses can be identiﬁed. This special typeof event has recently been reported by Wada et al. (2020) to be associated with downward terrestrial gamma
ray ﬂashes. Two events reported by Wada et al. (2020) had estimated peak currents of −260 and −197 kA.
Although we cannot estimate currents of the events we observed, the fact that these events saturated allof the observation sites indicates that they also carried extremely large current. Note that Wada et al. (2020)
interpreted this special type of discharge as negative polarity “energetic in‐cloud pulse” (Lyu et al., 2015), butaccording to our recent analysis, it is more likely a type of return stroke occurring in certain special
conditions, which is why we call it “special negative stroke” in this study. Detailed analysis will be reported
in a follow‐up paper.

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Figure 6. (a) Deﬁnitions of pulse parameters. Distributions of (b) rise time and fall time, (c) pulse width, and (d) pulse
width at half maximum for those pulses marked by blue crosses in Figure 5.

The second type of triggering event is negative return strokes (Events 3, 5, 7, 8, 9, 11, 13, 16, and 22). It isinteresting to note that most of these return strokes have extremely short time differences from the beginning of ﬂashes (denoted as “short predischarges” in Table 1). Two examples are given in Figures 7c and
7d. Discharges before the return strokes in Figures 7c and 7d lasted for only about 300 and 800 μs, respectively. Negative return strokes with such short predischarges were not observed in summer. Even assuming
the initiation altitudes of these −CG ﬂashes were only 1 km, we would get the overall velocities of stepped
leaders on the order of 106 m/s. The high velocity of stepped leaders is usually associated with the large peakcurrent of return strokes (Shi et al., 2019a), indicating that negative return strokes triggering UNLs usually
have high peak currents.
The third type of triggering event is bipolar pulses similar to the so‐called “large bipolar event” (LBE)
reported by Wu et al. (2014) (Events 18, 19, 20, 21, 23 and 24). As analyzed by Wu et al. (2014), LBEs always
have the same polarity as negative return strokes and are on average stronger than return strokes. In two of
the events (Events 20 and 21), another large pulse occurred before the LBE‐like pulse, and these two examples are shown in Figures 7e and 7f. In each example, we can see at least two large pulses before the start of
the UNL. The second pulse is similar to the LBE, while the ﬁrst pulse has an initial small negative change,
and it is not clear what kind of discharges produced this pulse. For the case in Figure 7f, discharges before the
UNL are not so strong and have a long time different (~400 ms) from the UNL, so it is also possible that the
UNL was not associated with the pulses at the beginning.
The fourth type of triggering event is IC discharges (Events 1, 6, 10, and 12). In two of the cases, leader channels are well imaged and are shown in Figure 8. Although some sources were very close to the ground in
Figure 8, we conﬁrmed from E‐change waveforms that there were no return strokes before the initiationof UNLs. Initiation locations of UNLs are indicated by red diamonds. We can see in each example, leaders
passed by the initiation locations with a distance of about 3–4 km and propagated away. About 50 ms after
the leaders stopped propagating, UNLs were initiated. It has been reported that approaching negative

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Figure 7. Lightning discharges preceding UNLs. (a and b) Special negative strokes. (c and d) Negative return strokes
with extremely short predischarges. (e and f) At least two large pulses including one special pulse with initial small
negative change and one LBE‐like pulse. In each subplot, the ﬁrst ﬁgure shows E‐change waveform and height‐time viewof location results from the beginning of discharges preceding the UNL to the UNL. Red points represent sources ofUNLs. Figures below show expanded views of large pulses before UNLs.

leaders can trigger UPLs (Saba et al., 2016; Yuan et al., 2017), consistent with our observation that negative
leaders propagating away preceded initiations of UNLs.
In the remaining two cases (Events 15 and 17), positive return strokes were observed before UNLs. However,
it is unlikely that the positive return strokes triggered UNLs. In Event 15, a downward negative leader wasobserved right before the positive return stroke, and in Event 17, IC discharges were observed after the positive return stroke. It is possible that some in‐cloud discharges triggered these UNLs. Triggering mechanisms
will be further discussed in section 4.3.
In the case of upward negative lightning, there have been reports that one positive return stroke triggered
several UPLs (Warner, 2012; Wu, Lyu, et al., 2019). In this study, although we did not observe multiple
UNLs triggered together, it seems not rare that some scattered sources are located near the ground rightWU ET AL.

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Figure 8. Two examples of UNLs preceded by IC discharges. The left part shows the plan view, and the right part shows
the height‐time view. Colored points represent sources of IC discharges. Red diamonds represent initiation
locations of UNLs. (a) and (b) show two different ﬂashes.

before the onset of UNLs. One example is shown in Figure 9. The UNL started at the time of about 583 ms,
and sources of discharges right before along with E‐change waveform are shown in Figure 9. In
Figure 9a, the blue cross indicates the location of the triggering event (also shown in Figure 7a), thered diamond indicates the initiation location of the UNL, and black triangles indicate locations oftransmission towers identiﬁed from Google Earth. Interestingly, sources right before the UNL seem tobe distributed near transmission towers, and their source altitudes are all lower than 1 km. It seems
very likely that these sources are produced by discharges on transmission towers triggered by thesame stroke, but these discharges failed to develop into long negative leaders. They may be aborted
UNLs as reported by Wang et al. (2008). They can also be corona discharges. Wu et al. (2017)
reported that increased sources of corona discharges from tall objects were detected following nearby
lightning discharges.

Figure 9. Discharges right before the onset of an UNL. (a) Plan view. The blue cross represents the negative stroke before
the UNL (also shown in Figure 7a). The red diamond represents the initiation location of the UNL. Black triangles
represent transmission towers. (b) E‐change waveform. (c) Height‐time view.

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4. Discussion
4.1. Comparison of UNLs With Initial Negative Leaders in PB Stage
Both UNLs and initial negative leaders in PB stages of IC and CG ﬂashes serve as the initial leader processes
in lightning ﬂashes. However, there are several apparent differences between these two types of leaders.
First, they produce distinctly different E‐change waveforms. As shown in Figure 5, pulses produced by UNLs
are mainly unipolar pulses. As for pulses produced by initial negative leaders, that is, PB pulses, although
they also include narrow unipolar pulses, bipolar pulses with pulse widths of tens of microseconds, called“classic PB pulses,” are the primary feature of PB pulses (Marshall et al., 2014; Nag et al., 2009).
According to Figure 6, pulse widths of UNL pulses are mostly smaller than 8 μs with a maximum of
13.0 μs. In other words, no classic PB pulses can be found in pulses produced by UNLs. Pulse intervals of
UNL pulses are also much shorter than those of PB pulses. For example, pulse intervals of PB pulses hada mean value of 73 μs reported by Nag and Rakov (2008) and 152 μs reported by Baharudin et al. (2012).
By contrast, E‐change waveforms of the ﬁve UNLs in Figure 5 had average pulse intervals smaller than 20 μs.
High‐speed video observations of initial negative leaders in PB stages have established that each step of theinitial leader corresponds to a classic PB pulse (Campos & Saba, 2013; Stolzenburg et al., 2013, 2014; Wilkes
et al., 2016). For UNL pulses, however, it seems that all the narrow and small pulses are produced by leader
steps according to the analysis in section 3.2.
Step lengths of UNLs seem to be much shorter than those of initial negative leaders. Leader steps in the PB
stage of a −CG ﬂash reported by Stolzenburg et al. (2014) had step lengths ranging from 30 to 183 m with amean value of 89 m. Step lengths of initial leaders in hundreds of IC ﬂashes estimated by Wu et al. (2015)
ranged from 20 to 426 m with a mean value of 113 m. By contrast, step lengths of ﬁve UNLs estimated in thisstudy range from 13.1 to 25.3 m, and step lengths of two UNLs observed by high speed videos reported by Pu
et al. (2017) and Qiu et al. (2019) were smaller than 10 m.
Velocities of UNLs analyzed in this study have large variations, from 1.8 to 27.9 × 105 m/s. Velocities of
initial negative leaders in both −CG and IC ﬂashes also have large variations. Shi et al. (2019a) reported thatvertical velocities of initial negative leaders in 279 −CG ﬂashes ranged from 1.6 to 12.4 × 105 m/s. Wu
et al. (2015) reported that vertical velocities of initial negative leaders in 662 IC ﬂashes ranged from 0.5 to
17.8 × 105 m/s. Initial leader velocities larger than 106 m/s were also reported by Heavner et al. (2002).
Based on these results, it seems velocities of UNLs are generally similar to those of initial negative leaders.
Differences between UNLs and initial negative leaders can be partly explained by their different altitudes ofoccurrences. As demonstrated by Wu et al. (2015), with decreasing initiation altitudes, initial negative
leaders in IC ﬂashes produce more frequent pulses and have shorter step lengths. If this altitude‐related correlation can be applied to UNLs, which occur near the ground, small pulse intervals and short step lengths
are expected results.
4.2. Comparison of UNLs With Stepped Leaders in −CG Flashes
Both UNLs and stepped leaders in −CG ﬂashes propagate near the ground, and it seems that they have moresimilarities than differences. First, it seems that they have similar step lengths. Step lengths of stepped leaders in −CG ﬂashes generally range from several meters to a few tens of meters (e.g., Chen et al., 1999; Hillet al., 2011; Qi et al., 2016), consistent with our estimations and previous observations (Miki et al., 2014; Pu
et al., 2017; Qiu et al., 2019). Second, although velocities of stepped leaders in −CG ﬂashes are thought tohave a typical value of 2 × 105 m/s, they have signiﬁcant variations in different studies. For example, according to the review by Rakov and Uman (2003, p. 123), velocities of stepped leaders range from 0.3 to
39 × 105 m/s, in general agreement with velocities of UNLs.
One apparent difference between UNLs and stepped leaders in −CG ﬂashes is their E‐change waveforms. As
shown in Figure 4, E‐change waveforms produced by UNLs are very small at the beginning of the leader, andthen the amplitude increases until the leader turns into a horizontal direction. Such V‐shaped overall change
trend was not observed in E‐change waveforms produced by stepped leaders in −CG ﬂash. This difference
may lie in the fact that formations of UNLs include an initiation process while stepped leaders in −CG
ﬂashes do not. Stepped leaders, when they are observed, already have a self‐propagating hot channel, which
may result in similar peak currents in different steps. On the other hand, for UNLs, no leader channels exist
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at the time of their initiations. The formation of leader channels
likely consists of different types of processes that have different currents, resulting in obvious variations of E‐change pulses.

4.3. Triggering Mechanism of UNLs
Opposite to UPLs, which are usually preceded by positive returnstrokes (e.g., Warner et al., 2012; Zhou et al., 2012b), UNLs reported
in this paper are usually preceded by negative strokes, including “special negative strokes,” negative return strokes, and LBE‐like pulses aslisted in Table 1. All of these strokes produce positive E‐changes(atmospheric electricity sign convention), which are necessary forthe initiation of UNLs.
However, time differences between these strokes and initiations of
UNLs are generally tens of milliseconds, implying that these strokes
did not directly trigger UNLs. As for the triggering of UPLs, some
UPLs are reported to be preceded by positive return strokes within
Figure 10. Scatterplot of horizontal distance and time difference between 1 ms (Jiang et al., 2014; Wu, Lyu, et al., 2019; Zhou et al., 2012b), indicating that these UPLs were likely triggered directly by the preceding
initiation locations of UNLs and preceding negative strokes. Slopes of two
blue dashed lines represent two velocity values indicated above.
positive return strokes. However, many more UPLs were preceded bypositive return strokes for a much longer time. For example, according to Figure 6 in Saba et al. (2016), many UPLs were preceded bytriggering events for more than 50 ms. Therefore, a more common
triggering component for UPLs seems to be approaching negative leaders, which can also create negative
E‐changes necessary for the initiation of UPLs. This has been conﬁrmed by high‐speed video observations(Saba et al., 2016; Yuan et al., 2017). In fact, if we look at the left half of Figure 6 in Saba et al. (2016) anddraw a regression line, the slope (corresponding to the ratio between distance and time difference fromthe triggering event) would be close to 2 × 105 m/s, typical for the velocity of negative leaders propagatingin virgin air. This result supports the theory that negative leaders in +CG ﬂashes propagating toward the tallobject are responsible for the triggering of UPLs.
Accordingly, we examine a similar mechanism that approaching positive leaders trigger UNLs. Positive
leaders are basically invisible to LF systems, so we do not have direct evidence. However, there seem tobe some difﬁculties for this mechanism. First, distances and time differences between triggering eventsand UNLs do not show any correlation. Figure 10 shows a scatterplot of distances versus time differencesfor UNLs preceded by negative strokes, and apparently, there is no correlation between these twoparameters. We also analyzed distances and time differences between the initiation of preceding ﬂashes
and UNLs, and they did not show any correlation either. Second, according to Wu, Wang, andTakagi (2019), positive leaders in −CG and IC ﬂashes propagate with a stable velocity of 1 to
3 × 104 m/s. Two dashed lines representing these two velocities are also plotted in Figure 10, and we
can see for many UNLs, distances between preceding strokes and UNLs are too large for positive leaders
to traverse within the time differences. For example, Event 9 in Table 1 is preceded by a negative returnstroke with a distance of 2.4 km and a time difference of 13.3 ms. A positive leader needs to have a velocity of at least 1.8 × 105 m/s to travel to the tall object before the initiation of the UNL, which isapparently too fast according to the result of Wu, Wang, and Takagi (2019).
One possible explanation for these discrepancies is that positive leaders do not have to propagate to a location right above a tall object to trigger UNLs. Triggering events listed in Table 1 are generally less than 3 km
away from UNLs, close enough to have signiﬁcant inﬂuences on electric ﬁelds at initiation locations of
UNLs. The relatively large time differences between triggering events and UNLs may be due to the fact that
UNLs are relatively difﬁcult to be initiated, and a negative stroke alone is usually not able to produce
E‐changes large enough for the initiation of UNLs. After the negative stroke, in‐cloud positive leaders,
although being a few kilometers away from the tall object, keep draining negative charges in thunderclouds
and contribute to further increases of positive E‐changes at the tall object until the initiation of UNLs.

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Further, we speculate that unlike the triggering of UPLs, which favorites an approaching negative leader, thetriggering of UNLs does not require an approaching positive leader but a nearby positive leader due to thefact that positive leaders travel much more slowly than negative leaders. Assuming a positive leader velocityof 2 × 104 m/s, the leader only moves for 2 km in 100 ms, so even if it moves away from a tall object, it still has
signiﬁcant inﬂuences on E‐changes on the tall object after 100 ms, as long as the leader starts at a location
close enough to the tall object. Therefore, it is likely that the direction of positive leaders, whether approaching or not, is not important. The more essential factor for the triggering of UNLs is that the triggering ﬂash
starts close to the tall object from which UNLs are to be triggered. This is why in our study, distances
between triggering events and UNLs are generally only a few kilometers, much smaller than those for
UPLs (mostly from 10 to 40 km as reported by Saba et al., 2016), while time differences are normally tensof milliseconds, comparable to those for UPLs.
Apart from those UNLs preceded by negative strokes, as analyzed in section 3.3, four UNLs were preceded by
IC discharges, and in two of them, negative leaders propagating away from the initiation locations wereobserved. Negative leaders propagating away can also produce positive E‐changes which are necessary forthe initiation of UNLs. There are also two UNLs preceded by positive return strokes. However, it is unlikelythat positive return strokes can trigger UNLs; some not‐well‐resolved leader processes may be the triggering
component.
It is also interesting to note that different types of preceding discharges appear to cluster in certain periods.
For example, according to Table 1, all LBE‐type discharges occurred in 25 and 28 January. This feature indicates that different types of triggering events are likely associated with different charge structures. Indeed,
winter thunderstorms in Japan sometimes have quite complicated charge structures (e.g., Wang et al., 2018;
Zheng, Wang, Zhang, Wu, & Takagi, 2019), which may result in the special types of triggering events such as
the special negative strokes and LBE‐like discharges, and the triggering mechanism of UNLs may well bemuch more complicated than what we have proposed.
In summary, lightning discharges efﬁcient in triggering UNLs seem to be a strong negative stroke combinedwith following in‐cloud positive leaders. These positive leaders need to be within a few kilometers from tallobjects, but they do not have to propagate toward tall objects.

5. Conclusions
With the observation of the FALMA in winter thunderstorms in Japan, 24 UNLs were recorded, and theirvelocities, E‐change waveforms, and triggering mechanism are analyzed. Major ﬁndings are summarized
as follows.
The 3‐D velocity during the upward propagation stage of UNLs ranges from 1.8 to 27.9 × 105 m/s with amean value of 10.4 × 105 m/s. When an UNL turns into a horizontal direction after the upward propagation
stage, the velocity always decreases. Velocities during the horizontal propagation stage range from 0.9 to
7.3 × 105 m/s with a mean value of 3.8 × 105 m/s. Velocities of UNLs seem to be generally similar to velocitiesof downward stepped leaders in −CG ﬂashes and initial negative leaders in PB stages.
The E‐change waveforms of UNLs measured at tens of kilometers show a distinctive V‐shaped overallchange trend. The waveforms include small pulses at the beginning when UNLs start, large pulses during
the upward propagations of UNLs, and small pulses again when UNLs turn into horizontal directions.
UNLs produce mainly unipolar pulses but also some bipolar pulses. Pulses produced by ﬁve UNLs are statistically analyzed. Pulse intervals range from 13.7 to 18.9 μs. Pulse width, pulse width at half maximum, risetime, and fall time are on average 5.8, 2.7, 3.3, and 2.4 μs, respectively. E‐change waveforms of UNLs are distinctly different from those produced by other types of negative leaders, such as downward stepped leaders in−CG ﬂashes and initial negative leaders in PB stages.
Negative strokes are usually found tens of milliseconds before initiations of UNLs. Probably due to special
conditions in winter, these negative strokes include some special types of discharges that are different fromnormal negative return strokes, but they all have the same polarity as negative return strokes. Further
analysis indicates that these negative strokes did not directly trigger UNLs; nearby in‐cloud positive leaders
following these strokes further deplete negative charges in thunderclouds and contribute to further positive
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E‐changes (atmospheric electricity sign convention) on tall objects, which seems to be essential for initiations of UNLs.

Data Availability Statement
E‐change waveform data of UNLs analyzed in this study can be downloaded from the link (http://doi.org/
10.5281/zenodo.3733900).
Acknowledgments
This work was supported by the
Ministry of Education, Culture, Sports,
Science, and Technology of Japan(Grants 16H04315 and 18K13618).

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