PUBLICATIONSJournal of Geophysical Research: Atmospheres
RESEARCH ARTICLE
10.1002/2015JD023546
Key Points:• Upward propagation speed in PB stage
decreases with its initiation altitude• PB pulse characteristics are related withvertical speed and initiation altitude• Step length of initial leader in PB stage
increases with initiation altitude

Correspondence to:T. Wu,
wu.ting@comf5.comm.eng.osaka-u.ac.jp

Citation:
Wu, T., S. Yoshida, Y. Akiyama, M. Stock,
T. Ushio, and Z. Kawasaki (2015),
Preliminary breakdown of intracloudlightning: Initiation altitude, propagation
speed, pulse train characteristics, andstep length estimation, J. Geophys. Res.
Atmos., 120, 9071–9086, doi:10.1002/
2015JD023546.
Received 20 APR 2015
Accepted 16 AUG 2015
Accepted article online 19 AUG 2015
Published online 17 SEP 2015
Corrected 4 DEC 2015
This article was corrected on 4 DEC
2015. See the end of the full text for
details.

Preliminary breakdown of intracloud lightning:
Initiation altitude, propagation speed, pulse
train characteristics, and step
length estimation
Ting Wu1, Satoru Yoshida2, Yasuhiro Akiyama1, Michael Stock1,3, Tomoo Ushio1, and Zen Kawasaki1,3
1

Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Osaka,
Japan, 2Meteorological Research Institute, Ibaraki, Japan, 3RAIRAN Pte. Ltd., Osaka, Japan

Abstract Using a low-frequency lightning location system comprising 11 sites, we located preliminary
breakdown (PB) processes in 662 intracloud (IC) lightning ﬂashes during the summer of 2013 in Osaka
area of Japan. On the basis of three-dimensional location results, we studied initiation altitude and upward
propagation speed of PB processes. PB in most IC ﬂashes has an initiation altitude that ranges from 5 to
10 km with an average of 7.8 km. Vertical speed ranges from 0.5 to 17.8 × 105 m/s with an average of
4.0 × 105 m/s. Vertical speed is closely related with initiation altitude, with IC ﬂashes initiated at higher
altitude having lower vertical speed during PB stage. Characteristics of PB pulse trains including pulse rate,pulse amplitude, and pulse width are also analyzed. The relationship between pulse rate and vertical speed has
the strongest correlation, suggesting that each PB pulse corresponds to one step of the initial leader during
the PB stage. Pulse rate, pulse amplitude, and pulse width all show decreasing trends with increasing initiation
altitude and increasing trends with increasing vertical speed. Using a simple model, the step length of
the initial leader during the PB stage is estimated. Most of initial leaders have step lengths that range from40 to 140 m with an average of 113 m. Estimated step length has a strong correlation with initiation altitude,
indicating that leaders initiated at higher altitude have longer steps. Based on the results of this study, we
speculate that above certain altitude (~12 km), initial leaders in PB stages of IC ﬂashes may only have horizontal
propagations. PB processes at very high altitude may also have very weak radiation, so detecting and locatingthem would be relatively difﬁcult.

1. Introduction
Lightning ﬂashes typically start with a preliminary breakdown (PB) stage which is characterized by a train of
bipolar pulses in electric ﬁeld change (E-change) records. PB is also commonly referred to as initial breakdown [e.g., Karunarathne et al., 2013; Marshall et al., 2013; Stolzenburg et al., 2013]. Due to the fact that PB
processes occur inside the cloud, analyzing far-ﬁeld E-change waveforms had been one of the few ways tounderstand the physical processes behind PB for many years [e.g., Gomes et al., 1998; Nag and Rakov,
2008; Wu et al., 2013]. Recently, channel developments during PB stages have been observed visually using
high-speed video cameras [Stolzenburg et al., 2013, 2014; Campos and Saba, 2013], and some new insights on
PB processes have been obtained. Stolzenburg et al. [2013] observed bursts of luminosity during the PB stages
in 15 cloud-to-ground (CG) and intracloud (IC) ﬂashes. They found that PB pulses in the E-change data arecoincident with light bursts in high-speed video data, and they suggested that each PB pulse “is caused bya substantial current surge that is hundreds of meters long.” As a follow-up study, Stolzenburg et al. [2014]
further analyzed the PB stage of a single-stroke CG ﬂash and found that the burst luminosity was directly
related to the amplitude of PB pulses. Campos and Saba [2013] observed the leader processes of a negative
CG ﬂash with time-correlated high-speed video camera and electric ﬁeld sensors. They showed that as theleader progressed from the initial leader in the PB stage to the stepped leader, the speed decreased fromabout 12 × 105 m/s to about 3 × 105 m/s and the channel luminosity also reduced.

©2015. American Geophysical Union.
All Rights Reserved.

WU ET AL.

High-speed video observations can show detailed development features of PB processes, but they are veryrare due to the visibility limitation of video. Another convenient way of studying PB is using an electric ﬁeld
sensor network to locate PB pulses. Three-dimensional (3-D) locating of PB pulses has recently been realized
by Bitzer et al. [2013] and Karunarathne et al. [2013]. Bitzer et al. [2013] developed a network called Huntsville

PRELIMINARY BREAKDOWN OF IC FLASH

9071

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Alabama Marx Meter Array comprising seven E-change sensors working
in the frequency band of 1 Hz to
400 kHz. They reported 3-D locationsof two IC ﬂashes which showed upward
propagation with vertical speed of
7.6 × 105 m/s and 1.8 × 105 m/s, respectively, during the PB stage. They also
reported a negative CG ﬂash which
propagated downward with a vertical
speed of 2.2 × 105 m/s during the PB
stage. Karunarathne et al. [2013]
analyzed 3-D locations of PB pulses
in several IC and CG ﬂashes and concluded that locations of PB pulses in
IC ﬂashes show an upward propagation trend while those in CG ﬂashes
Figure 1. Initial locations of PB processes in IC ﬂashes analyzed in this paper. show a downward propagation trend.
The blue cross sign represents the initial location of an IC ﬂash shown inMarshall et al. [2013] analyzed PB
more detail in Figure 2. The black squares show the locations of BOLT sites.
stages in 10 IC ﬂashes which may be
Numbered sites will be used speciﬁcally in following analyses.
associated with terrestrial gamma ray
ﬂashes. They demonstrated that PB
pulses are a manifestation of the initial breakdown processes and the upward motion of negative charges fromthe main negative charge region to the upper positive charge region. These recent studies established that PB
pulses in IC ﬂashes are produced by the initial negative leader which propagates upward from the main negative charge region to the upper positive charge region.
IC ﬂashes can also been initiated by narrow bipolar events (NBEs, also called compact intracloud discharges)[Rison et al., 1999; Wu et al., 2014]. The initial stages of these IC ﬂashes are very similar to PB stages of normal
IC ﬂashes, both associated with positive pulse trains whose locations show upward propagation. The only difference is that the ﬁrst pulse in the initial stage of a NBE-initiated IC ﬂash is produced by a NBE while the ﬁrst
pulse in a PB pulse train has no apparent differences with the rest of pulses. Wu et al. [2014] demonstrated
that characteristics of the leader processes initiated by NBEs have close connections with NBE altitude.
Leaders initiated by higher NBEs tend to produce weaker pulses, propagate upward for longer durations
and with slower speeds than those initiated by lower NBEs, and there are few NBE-initiated leaders above
about 10 km altitude. Considering the similarities between pulse trains observed in IC ﬂashes initiated withand without a NBE, it is expected that the properties of initial leaders associated with PB pulses in IC ﬂashes
will also vary with their initiation altitudes. However, studies on the different properties of initial leaders or PB
pulses at different altitudes are still very rare. Even 3-D location results of PB pulses are still rare, and there
have not been any statistical studies on altitude-related PB properties.
In this paper, we will analyze 3-D location results of 662 PB pulse trains in IC ﬂashes. Using this large data set,ﬁrst we will conﬁrm the altitude and propagation characteristics of initial leaders during the PB stage of IC
ﬂashes. Then we will explore the possible relations between initiation altitude, propagation characteristics,
and waveform characteristics of PB pulse trains. Finally, we will estimate the step length of the initial leader
during the PB stage using a simple model. We will demonstrate that various properties of PB processes areclosely related with their initiation altitude and upward propagation speed.

2. Experiment and Data
During the summer of 2013, we deployed a low-frequency (LF) lightning location network called Broadband
Observation network for Lightning and Thunderstorms (BOLT) in Osaka area of Japan. This system comprised
11 sites as shown by black squares in Figure 1. Distances between adjacent sites ranged from 11 to 40 km.
Each site consisted of a fast antenna with a time constant of 200 μs and effective frequency band of about
800 Hz to 500 kHz. E-change signals of lightning discharges were digitized at a sampling rate of 4 MS/s and

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9072

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 2. A typical example of an IC ﬂash with PB pulses. (a) Locations and E-change waveform of an IC ﬂash initiated with PB processes. The red triangle represents
the location of the ﬁrst pulse. The period before the red dashed line is the PB stage. The origin in the left plot (0,0) is set at the location of site 0 in Figure 1. The same
origin is used in all similar ﬁgures in this paper. (b) E-change waveforms of the PB pulses of this IC ﬂash recorded at four numbered sites in Figure 1.

with a vertical resolution of 16 bit. Each trigger had a duration of 200 ms and was time-stamped by a GPS
receiver with a timing accuracy of 50 ns. This system can be continuously triggered without any deadtime.
Time-of-arrival technique was employed to obtain 3-D locations of lightning discharges as described in
Yoshida et al. [2014].
In this study, we will analyze 3-D locations and E-change waveforms of PB pulses. Figure 2a shows 3-D locations and E-change waveform of a typical IC ﬂash, the initial location of which is shown as a blue cross inFigure 1. The portion of the ﬂash occurring before the red dashed line is PB stage. During the PB stage of thisﬂash, an initial leader propagated upward, presumably carrying negative charge from the main negative
charge region to the upper positive charge region. After the PB stage, the location results mainly showed
the development of the negative leader in the upper positive charge region. There were also a few sourcesin the main negative charge region, probably produced by negative recoil and/or positive leaders. The leaders progressed mainly horizontally, forming several clear branches spanning more than 20 km.
Figure 2b shows E-change waveforms of the PB stage recorded at the four numbered sites in Figure 1 (waveforms
recorded by other sites are not shown). All of the pulses are of positive polarity as in physics sign convention. In
practice, for the beginning part of a PB pulse train, if the pulses at one site are positive, they are also positive at
all the other sites, indicating that the initial leader starts with mainly vertical development. The PB stage usually

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9073

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

ends with pulses at several sites starting to change polarity, which is probably caused by horizontal propagationof the initial leader. The PB stage of this ﬂash lasted for about 10 ms. Based on these characteristics, we set thefollowing criteria to identify PB processes.
1. A PB pulse train should appear at the beginning of an IC ﬂash. This does not necessarily mean that there are
no E-change signals before PB pulses as these signals can be produced by discharge processes far awayfrom the PB processes, but we can exclude these signals by their locations. If two leaders develop consecutively from nearby locations such as the case discussed by Winn et al. [2011] and the one shown in
Figure 5b in Wu et al. [2014], only the ﬁrst leader is considered.
2. There should be at least ﬁve positive pulses from the start of the pulse train before any negative pulse appears
at any site. Five is a somewhat arbitrary setting, but in most of the cases, the initial leader produces many
more than ﬁve pulses. If a site is very close to a ﬂash (<10 km) and waveforms at this site are highly distorted due to the large static component, pulse polarities at this site are not considered.
3. Locations of the PB pulses should show an upward propagation trend. Not all PB processes in IC ﬂashes show
upward propagation, but calculating upward propagation speed is an essential part in this study. This criterion makes the ﬂashes in this study basically normal positive IC ﬂashes.
In order to ensure high location accuracy, only those ﬂashes that were located near the center of the BOLT
array, and occurred when all 11 sites were operating normally, were included for analysis. In total, 662 ﬂashes
with PB processes are identiﬁed, and their initial locations are shown in Figure 1. These PB processes occurred
inside or quite near the array, and the vertical error of their location results is typically less than 500 m (Figure 5
in Yoshida et al. [2014]). A large number of these ﬂashes are located in the northern region of the array, where amountainous area is conducive to the formation of thunderstorms in the summer.
In addition, PB pulses in negative CG ﬂashes are identiﬁed for a comparison in section 3.1. Negative CG
ﬂashes that occurred in the same period and region as described above are considered. If there are at least
ﬁve negative pulses at the beginning of a negative CG ﬂash before any positive pulse appears at any site,these negative pulses are identiﬁed as a PB pulse train in negative CG ﬂash. In total, 558 negative CG ﬂashes
with PB pulses are identiﬁed.

3. Results
3.1. Initiation Altitude of PB
One of the most important properties of PB is its initiation altitude, which is related to the origin of lightning
ﬂashes in thunderclouds. Marshall et al. [2014] reported that they detected a type of slow and small E-change
signals called initial E changes (IECs) before the beginning of PB pulses. In most PB cases in this study, we
cannot ﬁnd IECs before them probably because IECs can only be detected within small distances (around
4 km as in Marshall et al. [2014]), so we will not consider IECs in this study. Considering that the IEC is a typeof slow process with very small dipole moment and short duration, there should not be signiﬁcant charge
motions during the IEC, so we state that the initial location of PB is a good proxy of lightning initiation location.
We calculated the altitude of the ﬁrst pulses in PB pulse trains as their initiation altitude, and as a comparison,
we also calculated initiation altitude of PB pulses in 558 negative CG ﬂashes. Figure 3 shows time series(Figure 3, left) and a distribution (Figure 3, right) of the initiation altitude of PB in all IC and CG ﬂashes. The
green dashed lines divide different thunderstorm days. Altitudes of 0°C, 10°C, 20°C, and 40°C isotherms
in each day are also plotted. The isotherm altitudes are obtained by radiosonde observations by Japan
Meteorological Agency at Shionomisaki, which is about 150 km from the center of our network, and are only
available for 09:00 A.M. and 09:00 P.M. local time each day. Therefore, these data provide only an extremelyrough estimation of the temperature inside the thunderclouds.
We can see some interesting patterns in Figure 3. First, initiation altitude of PB in ICs is generally higher thanthat in negative CGs. Second, as in days 0727, 0731, 0805, and 0807, PB of most ICs is higher, and most CGs
lower, than the 10°C isotherm. It seems that the main negative charge regions of these thunderstorms residenear the 10°C level, which is a reasonable temperature level for the main negative charge region [e.g., Krehbiel,1986]. Other thunderstorms seem to be more complicated. Particularly, we can see that PB in some IC
ﬂashes is initiated below the 0°C isotherm. It is possible that these IC ﬂashes are initiated at temperatureshigher than 0°C, which may be difﬁcult to explain using the graupel-ice mechanism [e.g., Takahashi, 1978]

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9074

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 3. Initiation altitude of PB in IC ﬂashes and negative CG ﬂashes. (left) The time series of PB altitude in all thunderstorm days analyzed in this study. Different
thunderstorm days are divided by the green dashed lines. The numbers above the ﬁgure show the dates and those below the ﬁgure the durations of all PB processes
in each day. The red, black, blue, and purple lines indicate the 0°C, 10°C, 20°C, and 40°C isotherms, respectively. (right) The distribution of initiation altitude of PB
in all ﬂashes.

as it requires supercooled water for substantial charge separations. However, this may just reﬂect the factthat the actual charge structure in a thundercloud can be much more complicated than a simple tripolarstructure [Stolzenburg et al., 1998]. It is also possible that the region with strongest electric ﬁeld, wherelightning is initiated, is sometimes displaced from the active charging region where supercooled water isnecessary. Lightning ﬂash origins are probably also dependent on types and stages of thunderstorms,
and fully understanding lightning initiation locations requires detailed analysis of thunderstorm structures,
which is outside of the scope of this paper.
Figure 3 (right) shows the distribution of initiation altitude of PB for all ﬂashes. The PB for most negative CG
ﬂashes is initiated between 4 and 7 km with an average value of 5.7 km. The PB for most IC ﬂashes is initiatedbetween 5 and 10 km with an average value of 7.8 km. The overall results in Figure 3 are consistent with thenotion that IC ﬂashes are initiated between the main negative and upper positive charge regions and that theinitial leaders propagate upward. Conversely, negative CG ﬂashes are initiated below the main negative
charge region, and their initial leaders propagate downward toward the ground.
3.2. Upward Propagation Speed
PB pulses are produced by initial leaders, which propagate upward in normal positive IC ﬂashes [e.g., Marshallet al., 2013]. Speeds of initial leaders were previously observed by VHF lightning imaging systems. For example,
using a 2-D interferometer system, Shao and Krehbiel [1996] observed that initial leaders in IC ﬂashes propagated upward at a speed of 1.5 to 3 × 105 m/s. Behnke et al. [2005] reported that initial leaders in IC
ﬂashes started at a median speed of about 1.6 × 105 m/s using data obtained by lightning mapping array(LMA). Bitzer et al. [2013] compared speed values calculated from VLF/LF and VHF sources for two IC ﬂashes
and two CG ﬂashes and found that these two types of sources gave very similar results. In this section, we
will present an analysis of upward propagation speed for initial leaders during the PB stage in all 662 IC
ﬂashes. First, we will consider the vertical speed component.
Figures 4a–4d show four basic types of upward propagation during the PB stage, and they illustrate themethod used to calculate the vertical speed. In each case, the slope of the red dashed line is the estimatedvertical speed. The red dashed line represents a least squares linear ﬁt to a subset of the height and time of
PB pulses with a ﬁxed point at the height and time of the ﬁrst PB pulse. The subset includes all PB pulses
between the ﬁrst located PB pulse and a manually selected ﬁnal pulse. The selection of the ﬁnal pulse isillustrated in Figures 4a–4d. PB of type 1 (Figure 4a) propagates vertically at a relatively constant speed
and then turns horizontal; ﬁnal source is when the channel turns horizontal. PB of type 2 (Figure 4b)
propagates vertically with a speed that decreases with height (similar to the cases reported by Behnke et al.
[2005]); ﬁnal source is when the initial leader reaches about two thirds of its maximum height. The maximum
WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9075

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 4. (a–e) Illustration of the calculation of vertical speed. In each ﬁgure, the left part shows x-y results and the right part shows height-time results along with the
E-change waveform at one site. Slopes of dashed red lines are estimated vertical speed. In Figure 4b the blue line indicates the top height of the development of theinitial leader and the green line indicates the two-third top height level. The blue dashed lines in Figures 4c and 4d indicate the second stages.

and two-third maximum height are indicated by the blue and green lines, respectively. PB of type 3 (Figure 4c)
propagates upward in two stages, with the ﬁrst stage faster than the second; ﬁnal source is at the end of the ﬁrst
stage. PB of type 4 (Figure 4d) also propagates vertically in two stages, but the ﬁrst stage is slower than the
WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9076

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 5. (a) Distribution of vertical speed. (b) Scatterplot of vertical speed versus initiation altitude of PB. The black squares
represent the average speed in each kilometer bin of altitude. The PB cases represented by blue dots are shown in Figure 6,
and the one represented by a green dot is shown in Figure 8c. (c) Distribution of the ratio of horizontal speed to vertical
speed. (d) Scatterplot of 3-D speed versus initiation altitude of PB.

second; ﬁnal source is at the end of the ﬁrst stage. There are also cases in which no clear upward propagation
trend can be determined, as shown in Figure 4e. It is not clear if the lack of apparent vertical motion is due to theinitial leader propagating only horizontally or due to limitations in vertical location accuracy, so these cases are
excluded from this study.
It should be noted that the classiﬁcation of four types of PB in Figure 4 is not strict; sometimes it is difﬁcult todetermine which type a PB process belongs to. The manual determination of the ﬁnal pulse for speed calculation may also introduce certain errors, but these errors should not be signiﬁcant. Cases we had difﬁculty
making the linear ﬁt are not used such as the one in Figure 4e. The above method illustrates our purpose
to calculate the speed over the beginning period of the PB stage, during which the vertical speed does
not have large variations so that we can make the linear ﬁt. The vertical speed can change signiﬁcantly as
the initial leader develops during the PB stage, and the relatively constant vertical speed at the beginningpart of the initial leader is used. We believe that this is the best way of calculating the vertical speed for comparison of PB stages in hundreds of IC ﬂashes. Further, speed calculated in this way is most likely associatedwith the initiation altitude; the association will be analyzed later in this section.
With the above method, we calculated the vertical speed for the PB of all 662 IC ﬂashes. The results are
summarized in Figure 5. Figure 5a shows the distribution of vertical speed. The distribution shows highest
percentages between 1 and 4 × 105 m/s, which is consistent with previous studies [Shao and Krehbiel, 1996;
Behnke et al., 2005; Bitzer et al., 2013]. The average speed is 4.0 × 105 m/s, and the maximum and minimum
values are 17.8 × 105 m/s and 0.5 × 105 m/s, respectively. Note that there are many ﬂashes having quite large
speed during the PB stage, some of which are larger than 10 × 105 m/s. High speed is related to low initiation altitude as illustrated in Figure 5b, which shows a scatterplot of vertical speed versus initiation altitude.
The speed has a clear decreasing trend as the initiation altitude increases. Here we use the Spearman’s rank correlation coefﬁcient to quantitatively describe the correlation between two variables. The Spearman’s correlation

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9077

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 6. Locations and E-change waveforms of PB with the (a) fastest upward propagation, (b) slowest upward propagation, (c) lowest initiation altitude, and (d) highest
initiation altitude. Slopes of dashed red lines are estimated vertical speed.

coefﬁcient is more appropriate to describe nonlinear relations than the normal Pearson’s correlation coefﬁcient.
The Spearman’s correlation coefﬁcient is 0.68 for initiation altitude and speed in Figure 5b. The black squares
in Figure 5b represent average speeds in each kilometer bin of altitude. The average speed decreases from
11.0 × 105 m/s for PB lower than 4 km to 1.6 × 105 m/s for PB higher than 11 km. The only increase of the average
speed is in 10~11 km range, which may be due to the small number of samples in this range. This result indicates that initial leaders started at higher altitude tend to propagate upward with lower speed.
Next we will brieﬂy examine the horizontal speed component. The horizontal propagation of initial leaders ishighly varied; sometimes we cannot recognize a clear propagation direction, and sometimes there seems to
be branches. We examined locations of all PB processes in this study and calculated horizontal speed component for PB in 170 ﬂashes, all of which show clear and simple horizontal propagations. Horizontal speed
is calculated in the same period of each ﬂash as the calculation of vertical speed illustrated in Figure 4.
Figure 5c shows the distribution of the ratio of horizontal speed to vertical speed. The distribution peaks

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9078

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 7. Relations between pulse rate, initiation altitude, and vertical speed of PB. (a) Scatterplot of pulse rate versus initiation altitude with color indicating speed. (b) Scatterplot of pulse rate versus speed with color indicating initiation altitude.

around 0.5, indicating that for PB stages in IC ﬂashes, horizontal speed of the initial leader is usually only
about half of its vertical speed. For PB in most IC ﬂashes (91.2%), horizontal speed of the initial leader issmaller than its vertical speed. We also calculated 3-D speed for these cases by combining horizontal
and vertical speed components. Figure 5d shows a scatterplot of 3-D speed versus initiation altitude. Itshows a very similar trend to the relationship between vertical speed and initiation altitude (Figure 5b).
The mean value of 3-D speed is 4.3 × 105 m/s. In the following analysis, we will only consider the vertical
speed component because the vertical speed is probably more closely related with initiation altitudeand charge structures in thunderstorms. Note that when we talk about “upward propagation speed” infollowing text, we only refer to the vertical speed component.
Both initiation altitude and speed of PB vary over a wide range. It is instructive to examine the extreme cases
in more detail. Figure 6 shows 3-D locations and E-change waveforms of PB with the largest and smallest vertical speeds and initiation altitudes (blue dots in Figure 5b). Note that in Figure 5b, the largest speed is17.8 × 105 m/s occurring at 8.1 km altitude (green dot in Figure 5b). However, this case is a quite special caseand will be discussed later in section 3.3.1. Except for this case, the largest speed is 17.5 × 105 m/s, which isshown in Figure 6a. It is initiated at an altitude of 3.9 km. Figure 6b shows the PB with the smallest speed(0.5 × 105 m/s), and it is initiated at an altitude of 9.9 km. Note that there is a small pulse before PB pulses,
which is produced by a distant lightning discharge. Figures 6c and 6d show PB with the smallest (3.2 km)
and largest (11.6 km) initiation altitudes, respectively. These two cases show clear upward propagation withspeeds of 7.8 × 105 m/s and 3.2 × 105 m/s, respectively. These examples reﬂect the result in Figure 5b that initialleaders with high initiation altitude usually propagate upward more slowly while those with low initiation
altitude usually propagate upward more quickly.
3.3. Pulse Train Characteristics
PB in normal positive IC ﬂashes produces positive bipolar pulses. There have been many studies analyzing characteristics of PB pulse trains [Ushio et al., 1998; Gomes et al., 1998; Gomes and Cooray, 2004; Makela et al., 2008;
Baharudin et al., 2012]. Here we present a statistical analysis on characteristics of PB pulse trains in IC ﬂashes,
paying special attention to their association with initiation altitude and upward propagation speed.
3.3.1. Pulse Rate
From PB examples in Figure 6, we can notice an interesting feature. It seems that PB initiated at higher altitude and with slower speed (Figures 6b and 6d) produces fewer pulses than PB initiated at lower altitude andhigher speed (Figures 6a and 6c). In order to examine this feature, we will calculate pulse rate of PB pulse
trains and investigate its relations with PB initiation altitude and upward propagation speed. Pulse rate isdeﬁned as the number of PB pulses per unit time. If multiple pulses are detected within a 5 μs window, only
the largest pulse is counted as these pulses are probably multiple peaks of the same PB pulse. Every PB pulse
train is recorded by multiple sites, and in order to avoid systematic differences between waveforms recorded
at different sites, for this analysis we will only use E-change waveforms recorded by site 1 in Figure 1. In addition,pulses produced by very close discharges may be seriously distorted due to static ﬁeld component, so here we
WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9079

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 8. Locations and E-change waveforms of PB pulse trains with (a) largest pulse rate, (b) smallest pulse rate, and (c) a special case which has the highest speed.
Slopes of dashed red lines are estimated vertical speed.

only include PB processes at least 50 km away from site 1. These standards limit this analysis to the PB in
365 ﬂashes. The pulse rate is calculated over the same period used for the upward speed calculation(red dashed lines in Figure 4).
Figure 7 shows the relations between pulse rate of PB pulse trains and their initiation altitude and upward
propagation speed. Figure 7a is a scatterplot of pulse rate versus initiation altitude with color indicating
the speed, and Figure 7b is a scatterplot of pulse rate versus speed with color indicating the initiation altitude.
In Figure 7a we can see as the initiation altitude increases, the pulse rate decreases, and for PB cases with thesame initiation altitude, those having higher pulse rate generally have higher speed. Figure 7b shows similar
results. As speed increases, pulse rate also increases, and for PB cases with the same speed, those having
higher pulse rate generally have lower initial height. The Spearman’s correlation coefﬁcient is 0.86 for pulse
rate and initiation altitude and 0.88 for pulse rate and speed. In conclusion, PB initiated at lower altitude or
propagating with higher vertical speed generally produces pulses more frequently.
To further illustrate the above results, some PB examples are shown in Figure 8. The PB in Figure 8a has thehighest pulse rate, quite a large upward propagation speed, and a relatively low initiation altitude. On theother hand, the PB in Figure 8b has the lowest pulse rate, a relatively low propagation speed, and a high initiation altitude. The PB in Figure 8c is a special case. It has the highest propagation speed but also a relatively
high initiation altitude, which seems to contradict our conclusion in section 3.2 (PB with high initiation altitude tends to have low propagation speed). The reason is that this PB starts with many pulses within a shortperiod, and in this period, it propagates upward very rapidly. After this period, the pulses are much lessfrequent and the speed also decreases. This is type 3 PB as shown in Figure 4c, and its speed is calculated
for the ﬁrst upward propagation stage as indicated by the red dashed line. This example further illustrates
WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9080

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 9. Relations between estimated peak current, initiation altitude, and upward propagation speed of PB pulse trains.
(a) Scatterplot of peak current versus initiation altitude. (b) Scatterplot of peak current versus speed. The black squares
represent the average values.

the result that more frequent pulses correspond to higher upward propagation speed. A similar feature canalso be seen in the type 4 PB in Figure 4d. Here the ﬁrst stage has a relatively small speed and sparse pulses,
and the second stage has a larger speed and more frequent pulses. It seems that pulse rate is more closely
associated with upward propagation speed than with initiation altitude.
3.3.2. Pulse Amplitude
In this section, we will investigate characteristics of PB pulse amplitude for PB pulse trains with different initiation altitudes and upward propagation speeds. In order to compare the amplitude of PB pulses at different
distances, we convert the E-change records to estimated peak current using the method presented in Wu
et al. [2014]. This method is based on the transmission line model mainly applicable to return strokes, so
the estimated current values of PB pulses may not be accurate. However, the method will be correct for
the amplitude falloff with distance of the radiation component of the electric ﬁeld. For this analysis, only
PB processes located more than 50 km from site 2 or site 3 are considered, the same as the treatment in
Wu et al. [2014]. This limits the sample size to the PB pulse trains in 443 ﬂashes. For each PB pulse train,
the peak current is found from the PB pulse with the largest amplitude.
Figure 9 shows relations between estimated peak current of PB pulse trains, initiation altitude, and upward
propagation speed. The black squares represent average values of peak current in (a) 1 km bin of altitude
and (b) 2 × 105 m/s bin of speed. We can see that as the initiation altitude increases, the peak current showsa decreasing trend, although the scatter is very large. The Spearman’s correlation coefﬁcient is 0.46. The
relation between peak current and speed is stronger, with the Spearman’s correlation coefﬁcient of 0.64.
PB initiated at lower altitude and with larger upward propagation speed tends to produce larger PB pulses.
3.3.3. Pulse Width
Many studies on PB pulse trains analyzed PB pulse widths [e.g., Ushio et al., 1998; Gomes and Cooray, 2004].
We will also make a statistical analysis on pulse width of PB in IC ﬂashes and investigate if it has any association with initiation altitude or upward propagation speed. Data used in this analysis are the same as that used
for the analysis of pulse rate in section 3.3.1. For each of 365 pulse trains, the width of all pulses is calculated(from 10% peak at the leading edge to zero crossing) and then averaged. The distribution of the averagepulse width is shown in Figure 10a, showing highest percentages between 5 and 20 μs with an average valueof 15.3 μs. This is generally longer than pulse widths in positive and negative CG ﬂashes reported in previous
studies [Gomes and Cooray, 2004; Baharudin et al., 2012; Wu et al., 2013]. It is also consistent with the report by
Bitzer et al. [2013] that PB pulses in IC and hybrid ﬂashes have longer pulse durations than those in negative
CG ﬂashes.
Figures 10b and 10c explore associations of pulse width with initiation altitude and upward propagation
speed of PB. The black squares represent average value of pulse width in (a) 1 km bin in altitude and (b)2 × 105 m/s bin in speed. The relation between pulse width and initiation altitude is not strong with theSpearman’s correlation coefﬁcient of 0.32, although the average values show a general decreasing trend.
The relation between pulse width and speed is stronger with the Spearman’s correlation coefﬁcient of
0.49. It is evident that PB with high speed usually produces pulses with long widths and PB producing pulses
with short widths mostly has slow speed.

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9081

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 10. Pulse width of PB pulse trains in IC ﬂashes. (a) Distribution of average pulse width. (b) Scatterplot of pulse width versus initiation altitude. (c) Scatterplot of
pulse width versus upward propagation speed. Y axes in Figures 10b and 10c are logarithmic scales.

3.4. Step Length Estimation
Section 3.3.1 demonstrated a very strong association between the pulse rate of PB pulse trains and upward propagation speed, which is consistent with the hypothesis that each PB pulse is produced by a stepping of theinitial leader. Stolzenburg et al. [2014] made a detailed discussion on whether the initial leader is a stepped leader. They presented evidence both supporting and contradicting the idea. They also proposed a hypothesisthat initial leaders may step in an unusual way; their steps “are driven primarily by large ambient electric ﬁeld
and occur in the absence of a hot channel.” Stolzenburg et al. [2013] observed that each PB pulse was coincident
with light burst from the initial leader, and they suggested that there are repeated initial leaders during the PB
stage with each initial leader ending with an impulsive breakdown event, which produces a PB pulse.
If we assume that the initial leader producing PB pulses is indeed stepped (even though its physical mechanism may be different from normal stepped leaders), we can estimate the average step length (l) of an initialleader from PB pulse rate (ρ; analyzed in section 3.3.1) and upward propagation speed (v; analyzed in
section 3.2) with the following relationship:
l ¼ v=ρ

(1)

Note that equation (1) also assumes that the initial leader does not have any branch and the estimated step
length l is only in the vertical direction. Also note that v and ρ should be calculated using the same period ineach PB pulse train; otherwise, equation (1) may not be true.
With the relationship in equation (1), we calculated step length using data of PB pulse rate and upward propagation speed shown in Figure 7. Figure 11a shows a distribution of the estimated step length. Note that thebins are not equally distributed. Estimated step length varies over a large range, from 20 to 426 m, with anaverage value of 113 m. We can see from Figure 11a that the highest percentages are around 40 to 100 m
and the majority (81.9%) is smaller than 160 m.

Figure 11. (a) Distribution of estimated step length. Note that the bins are not equally distributed. (b) Scatterplot of
estimated step length versus initiation altitude. The Y axis is a logarithmic scale.

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9082

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Table 1. Spearman’s Correlation Coefﬁcient Between Different Pairs of Variables

Initiation altitude
Vertical speed

Vertical Speed

Pulse Rate

Peak Amplitude

Pulse Width

Step Length

0.68
-

0.86
0.88

0.46
0.64

0.32
0.49

0.77
0.48

One major source of error in our calculation is the determination of pulse rate (ρ). It is possible that some relatively small pulses are buried in the background noise and are not detected. In this situation, our results
would be overestimations. However, all ﬂashes analyzed in this study are inside or very near the network(Figure 1), and we should be able to detect most of PB pulses in these ﬂashes.
To the best of our knowledge, step length of initial leaders in IC ﬂashes has not been reported in the literature. Edens et al. [2014] observed cloud-to-air leaders in two IC ﬂashes with a digital still camera and theLMA. They reported that the step lengths were at least 200 m and 264 m, respectively, in two leaders.
They also noted that steps of negative leaders at 10 km altitude are an order of magnitude longer thanthose near sea level. Stolzenburg et al. [2014] analyzed high-speed video observations of a negative CG
ﬂash and reported that step lengths of the initial leader ranged from 30 to 183 m with an average of
89 m. Petersen and Beasley [2014] made a similar observation of a negative CG ﬂash and suggested thatthe initial leader in the PB stage extends with step lengths often exceeding 200 m. The above studies are
all 2-D estimations of step lengths so these results should be underestimations. Our results in Figure 11a
are consistent with these observations. However, in our results there are only 45 initial leaders (12.3%) withaverage step lengths longer than 200 m. This may be because that we are calculating average step lengthfor each initial leader, so extremely long or short steps may not show up. Besides, leader processes analyzed in
the above studies are not initial leaders in IC ﬂashes, so it may have some problems when comparing our results
directly to the above results.
Figure 11b shows a scatterplot of estimated step length versus initiation altitude of initial leaders. Step lengthis calculated from pulse rate and upward propagation speed (equation (1)), both of which decrease withincreasing initiation altitude (Figures 5b and 7a). Conversely, step length shows a clear increasing trend withincreasing initiation altitude with the Spearman’s correlation coefﬁcient of 0.77. We can see that as the initiation altitude increases, an initial leader develops upward with longer (on average) but less frequent steps(lower pulse rate) and the resultant upward propagation speed decreases.

4. Discussion
We have demonstrated that PB properties have close connections with initiation altitude and upward propagation speed. Table 1 summarizes Spearman’s correlation coefﬁcients between different pairs of variables of
PB properties. The coefﬁcients indicate how strong two variables are associated with each other.
Edens et al. [2014] gave an explanation for the correlation between step length and initiation altitude. Lower
pressure and air density at higher altitude result in a longer mean free path of free electrons and larger streamer zones at the leader tip and are conducive for the production of longer steps. Therefore, step lengths ofleaders at higher altitude tend to be longer. Based on the assumption that propagation speeds of negative
leaders of IC and negative CG ﬂashes are similar, Edens et al. [2014] explained why the rate of radio emissions
from IC ﬂashes are lower than that in CG ﬂashes, as at lower altitude, more frequent steps (with shorter step
lengths) are required to achieve a similar speed. Our results further demonstrate that for PB stages in IC
ﬂashes, radio emissions also have a lower rate at higher altitudes (correlation between pulse rate and initiation altitude). However, the propagation speed is not the same at different altitudes. Upward propagation
speed decreases with initiation altitude with a correlation coefﬁcient of 0.68. The speed of an initial leader
initiated at 5 km altitude can be 1 order of magnitude higher than those initiated at 10 km altitude. It is alsoquite possible that initial leader speeds in IC and CG ﬂashes are different. If this is true, the difference in therate of radio emissions from IC and CG ﬂashes cannot be simply attributed to the difference in step length indifferent altitudes as in Edens et al. [2014]. Instead, we think that the ambient electric ﬁeld is a more importantfactor, which will be discussed below along with other parameters in Table 1.
Pulse rate, peak amplitude, and pulse width all have higher correlations with vertical speed than with initiation altitude. One essential factor behind these correlations may be the ambient electric ﬁeld. A stronger
WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9083

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

electric ﬁeld can make electrons at the leader tip move forward more rapidly, so the speed of leader extension is larger. If each extension (or step as assumed in section 3.4) produces one PB pulse, then the PB pulse
rate is also larger. Fast moving electrons also correspond to larger currents and thus larger peak amplitude of
PB pulses. (As a similar process, growth of corona current was discussed by Bazelyan and Raizer [1998, p. 6].) A
stronger electric ﬁeld may also generate more electrons at the leader tip, and the current produced by leader
extension may have longer duration, so the PB pulse produced by the current also has larger width. These
associations may explain correlations of vertical speed with pulse rate, peak amplitude, and pulse width.
The correlation between pulse rate and vertical speed has the largest coefﬁcient (0.88). This is consistent withthe observation by Stolzenburg et al. [2013] that each PB pulse is coincident with light burst from the initialleader, suggesting that each PB pulse corresponds to one step of the initial leader, although the initial leader
steps may be physically different from normal leader steps.
Pulse rate, peak amplitude, and pulse width are all negatively correlated with initiation altitude. At higher
altitude, the electric ﬁeld required for initiation and leader propagation is smaller, and with the relations
analyzed above, weaker electric ﬁeld is associated with lower pulse rate, smaller peak amplitude, andsmaller pulse width. However, the electric ﬁeld does not necessarily decrease with the altitude. As theinitial leader propagates upward, it may encounter regions with electric ﬁeld stronger than where it isinitiated. This may be one reason why these coefﬁcients are relatively small. The correlation for pulse
width is especially weak. A possible reason is that the current duration is more closely related with theamount of charge that is available for the extension of the initial leader than with ambient electric ﬁeld
strength. A strong electric ﬁeld can also produce discharges associated with very narrow pulses suchas NBEs.
Wu et al. [2014] reported the so-called “initiator-type NBEs (INBEs),” which occur at the very beginning of IC
ﬂashes. INBEs are followed by upward propagating leaders which produce positive pulse trains very similar
to PB pulse trains. INBEs with subsequent upward propagating leaders are also similar to initial leaders in
PB stages. Characteristics of positive pulse trains and corresponding leader processes following INBEs alsochange with initiation altitude. As shown by Figures 8 and 11 in Wu et al. [2014], pulse amplitude, pulse
rate, and upward propagation speed all decrease with increasing initiation altitude, the same as PB processes in IC ﬂashes. Therefore, we conclude that leader processes following INBEs are the same as initialleaders in PB stages of IC ﬂashes. The only difference is that the INBE produces a distinguishing large bipolar pulse as the start of a ﬂash while the ﬁrst pulse of a PB pulse train is usually very small and does not haveany apparent difference with other pulses. It is not clear whether the presence of an INBE has any inﬂuenceon the development of the ﬂash.
As reported by Wu et al. [2014], INBEs are mostly lower than 10 km. At higher altitude, there are only normal
NBEs which are relatively isolated and are not followed by positive pulse trains. This feature makes us wonder
whether PB processes also only exist below certain altitude. The highest initiation altitude for PB processes inthis study is 11.6 km (the example in Figure 6d). It is very difﬁcult to ﬁnd ﬂashes showing clear initiations at
higher altitude. One reason may be the height limitation for thunderstorm development and charge layers,limiting the possible initiation height of lightning. Another reason may be reﬂected from one result of thisstudy: PB processes at higher altitude tend to produce less frequent and smaller pulses. Fewer and smaller
pulses make it difﬁcult to trigger our system and to locate them. In fact, in many cases, we cannot determine
the beginning of ﬂashes at very high altitude.
As an example with relatively good location results, Figure 12 shows an IC ﬂash initiated at 14.5 km. Even
this ﬂash only has one site (the closest) triggered continuously during the whole ﬂash. E-change waveformat this site is shown in the ﬁgure. This ﬂash starts with leader processes mainly propagating horizontally,
presumably in the upper positive charge region. Later this leader continues propagating horizontally, while
another leader starts to propagate downward. In the left plot, it is difﬁcult to recognize any clear channels
due to limited number of sources. E-change pulses before the red dashed line are all positive polarity. This
ﬂash would have been included in the 662 ﬂash samples of this study, except the initial leader shows noclear upward propagation (section 2). This example is also consistent with the result that upward propagation speed decreases with increasing initiation altitude. We speculate that above certain altitude, initialleaders mainly propagate only horizontally. However, these initial leaders produce relatively few and small
pulses, so detecting and locating them is difﬁcult.

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9084

Journal of Geophysical Research: Atmospheres

10.1002/2015JD023546

Figure 12. An IC ﬂash initiated at a very high altitude (14.5 km). The red triangle represents the initial source of this ﬂash. E-change pulses before the red dashed line
are all positive polarity.

5. Conclusions
This study analyzed E-change records and 3-D locations of PB processes in 662 IC ﬂashes. The large sample
size enabled us to investigate possible associations among various characteristics of PB. The major conclusions of this study are summarized as follows.
The initiation altitude of PB in IC ﬂashes predominantly lies in the 5 to 10 km range, with an average of 7.8 km.
In comparison, PB in negative CG ﬂashes lies between 4 and 7 km with an average of 5.7 km. Initial leadersduring the PB stages in IC ﬂashes are probably initiated between the main negative and upper positive
charge regions and propagate upward. Initial leaders in negative CG ﬂashes are probably initiated below
the negative charge region and propagate downward.
Upward propagation speed of initial leaders during the PB stage in IC ﬂashes ranges from 0.5 to 17.8 × 105 m/swith an average of 4.0 × 105 m/s. The speed clearly decreases with increasing initiation altitude.
Pulse rate, peak amplitude, and pulse width of PB pulses all show negative correlation with altitude and positive
correlation with vertical speed. PB initiated at higher altitude tends to produce less frequent and smaller pulses
with shorter pulse widths. The relationship between pulse rate and vertical speed has the strongest correlation,suggesting that each pulse in a PB pulse train probably corresponds to one step of the initial leader.
The average step length of each initial leader is estimated from pulse rate and vertical speed. The resultshows that most of initial leaders have step length that ranged from 40 to 140 m with an average of
113 m. The step length has a strong connection with initiation altitude; leaders initiated at higher altitudetend to have longer steps.
The physical mechanisms behind the associations between different properties of PB are not fully understood. Longer steps at higher altitude are probably due to lower pressure and a longer mean free path of freeelectrons. PB pulse train characteristics including pulse rate, peak amplitude, and pulse width may be relatedwith ambient electric ﬁeld strength.
Because the upward propagation speed decreases with initiation altitude, it follows that ﬂashes initiated highenough will have no vertical motion at all and only propagate horizontally. One such example is presented.
The ﬂash is initiated at an altitude of 14.5 km, and the initial leader propagates horizontally. There is also a
train of positive pulses at the beginning of this ﬂash, similar to a PB pulse train. However, because PB initiatedat higher altitude tends to produce less frequent and smaller pulses, it is relatively difﬁcult to detect andlocate PB at very high altitude.
Future studies will analyze characteristics of PB in CG ﬂashes and investigate whether its characteristics also
vary with initiation altitude and propagation speed.

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9085

Journal of Geophysical Research: Atmospheres
Acknowledgments
The data for this paper were obtained
during the summer campaign in 2013.
Readers can request the data from thecorresponding author (wu.ting@comf5.
comm.eng.osaka-u.ac.jp). This work was
supported by Japanese Ministry of
Education, Science, Sports and Culture,a Japanese Grant-in-Aid for ScientiﬁcResearch.

10.1002/2015JD023546

References
Baharudin, Z. A., N. A. Ahmad, M. Fernando, V. Cooray, and J. S. Makela (2012), Comparative study on preliminary breakdown pulse trainsobserved in Johor Malaysia and Florida, USA, Atmos. Res., 117, 111–121.
Bazelyan, E. M., and Y. P. Raizer (1998), Spark Discharge, Chem. Rubber Co., New York.
Behnke, S., R. Thomas, P. Krehbiel, and W. Rison (2005), Initial leader velocities during intracloud lightning: Possible evidence for a runaway
breakdown effect, J. Geophys. Res., 110, D10207, doi:10.1029/2004JD005312.
Bitzer, P. M., H. J. Christian, M. Stewart, J. Burchﬁeld, S. Podgorny, D. Corredor, J. Hall, E. Kuznetsov, and V. Franklin (2013), Characterization
and applications of VLF/LF source locations from lightning using the Huntsville Alabama Marx Meter Array, J. Geophys. Res. Atmos., 118,
3120–3138, doi:10.1002/jgrd.50271.
Campos, L. Z. S., and M. M. F. Saba (2013), Visible channel development during the initial breakdown of a natural negative cloud-to-groundﬂash, Geophys. Res. Lett., 40, 4756–4761, doi:10.1002/grl.50904.
Edens, H. E., K. B. Eack, W. Rison, and S. J. Hunyady (2014), Photographic observations of streamers and steps in a cloud-to-air negative leader,
Geophys. Res. Lett., 41, 1336–1342, doi:10.1002/2013GL059180.
Gomes, C., and V. Cooray (2004), Radiation ﬁeld pulses associated with the initiation of positive cloud to ground lightning ﬂashes, J. Atmos.
Sol. Terr. Phys., 66(12), 1047–1055, doi:10.1016/j.jastp.2004.03.015.
Gomes, C., V. Cooray, and C. Jayaratne (1998), Comparison of preliminary breakdown pulses observed in Sweden and in Sri Lanka, J. Atmos.
Sol. Terr. Phys., 60, 975–979, doi:10.1016/S1364-6826(98)00007-8.
Karunarathne, S., T. C. Marshall, M. Stolzenburg, N. Karunarathna, L. E. Vickers, T. A. Warner, and R. E. Orville (2013), Locating initial breakdown
pulses using electric ﬁeld change network, J. Geophys. Res. Atmos., 118, 7129–7141, doi:10.1002/jgrd.50441.
Krehbiel, P. R. (1986), The electrical structure of thunderstorms, in The Earth’s Electrical Environment, pp. 90–113, National Academy Press,
Washington, D. C.
Makela, J. S., N. Porjo, A. Makela, T. Tuomi, and V. Cooray (2008), Properties of preliminary breakdown processes in Scandinavian lightning,
J. Atmos. Sol. Terr. Phys., 70(16), 2041–2052, doi:10.1016/j.jastp.2008.08.013.
Marshall, T., M. Stolzenburg, S. Karunarathne, S. Cummer, G. Lu, H.-D. Betz, M. Briggs, V. Connaughton, and S. Xiong (2013), Initial breakdown
pulses in intracloud lightning ﬂashes and their relation to terrestrial gamma ray ﬂashes, J. Geophys. Res. Atmos., 118, 10,907–10,925,
doi:10.1002/jgrd.50866.
Marshall, T., M. Stolzenburg, N. Karunarathna, and S. Karunarathne (2014), Electromagnetic activity before initial breakdown pulses oflightning, J. Geophys. Res. Atmos., 119, 12,558–12,574, doi:10.1002/2014JD022155.
Nag, A., and V. A. Rakov (2008), Pulse trains that are characteristic of preliminary breakdown in cloud-to-ground lightning but are notfollowed by return stroke pulses, J. Geophys. Res., 113, D01102, doi:10.1029/2007JD008489.
Petersen, D., and W. Beasley (2014), High-speed video observations of the preliminary breakdown phase of a negative cloud-to-groundlightning ﬂash, paper presented at XV International Conference on Atmospheric Electricity, Norman, Okla., 15–20 June.
Rison, W., R. J. Thomas, P. R. Krehbiel, T. Hamlin, and J. Harlin (1999), A GPS-based three-dimensional lightning mapping system: Initialobservations in central New Mexico, J. Geophys. Res., 26(23), 3573–3576, doi:10.1029/1999GL010856.
Shao, X. M., and P. R. Krehbiel (1996), The spatial and temporal development of intracloud lightning, J. Geophys. Res., 101(D21), 26,641–26,668,
doi:10.1029/96JD01803.
Stolzenburg, M., W. D. Rust, and T. C. Marshall (1998), Electrical structure in thunderstorm convective regions: 3. Synthesis, J. Geophys. Res.,
103(D12), 14,097–14,108, doi:10.1029/97JD03545.
Stolzenburg, M., T. C. Marshall, S. Karunarathne, N. Karunarathna, L. E. Vickers, T. A. Warner, R. E. Orville, and H.-D. Betz (2013), Luminosity of
initial breakdown in lightning, J. Geophys. Res. Atmos., 118, 2918–2937, doi:10.1029/2012JD018675.
Stolzenburg, M., T. C. Marshall, S. Karunarathne, N. Karunarathna, and R. E. Orville (2014), Leader observations during the initial breakdown
stage of a lightning ﬂash, J. Geophys. Res. Atmos., 119, 12,198–12,221, doi:10.1002/2014JD021994.
Takahashi, T. (1978), Riming electriﬁcation as a charge generation mechanism in thunderstorms, J. Atmos. Sci., 35, 1536–1548, doi:10.1175/
1520-0469(1978)035<1536:REAACG>2.0.CO;2.
Ushio, T., Z.-I. Kawasaki, K. Matsu-ura, and D. Wang (1998), Electric ﬁelds of initial breakdown in positive ground ﬂash, J. Geophys. Res.,
103(D12), 14,135–14,139, doi:10.1029/97JD01975.
Winn, W. P., G. D. Aulich, S. J. Hunyady, K. B. Eack, H. E. Edens, P. R. Krehbiel, W. Rison, and R. G. Sonnenfeld (2011), Lightning leader
stepping, K changes, and other observations near an intracloud ﬂash, J. Geophys. Res., 116, D23115, doi:10.1029/2011JD015998.
Wu, T., Y. Takayanagi, T. Funaki, S. Yoshida, T. Ushio, Z. Kawasaki, T. Morimoto, and M. Shimizu (2013), Preliminary breakdown pulses ofcloud-to-ground lightning in winter thunderstorms in Japan, J. Atmos. Sol. Terr. Phys., 102, 91–98, doi:10.1016/j.jastp.2013.05.014.
Wu, T., S. Yoshida, T. Ushio, Z. Kawasaki, and D. Wang (2014), Lightning-initiator type of narrow bipolar events and their subsequent pulse
trains, J. Geophys. Res. Atmos., 119, 7425–7438, doi:10.1002/2014JD021842.
Yoshida, S., T. Wu, T. Ushio, K. Kusunoki, and Y. Nakamura (2014), Initial results of LF sensor network for lightning observation andcharacteristics of lightning emission in LF band, J. Geophys. Res. Atmos., 119, 12,034–12,051, doi:10.1002/2014JD022065.

Erratum
In the originally published version of this article, an error in the ﬁrst key point was discovered. The error has
since been corrected and this version may be considered the authoritative version of record.

WU ET AL.

PRELIMINARY BREAKDOWN OF IC FLASH

9086