Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 91–98

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Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage: www.elsevier.com/locate/jastp

Preliminary breakdown pulses of cloud-to-ground lightning
in winter thunderstorms in Japan
Ting Wu a,n, Yuji Takayanagi a, Tsuyoshi Funaki a, Satoru Yoshida a, Tomoo Ushio a,
Zen-Ichiro Kawasaki a,b, Takeshi Morimoto c, Masahito Shimizu daDivision of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita 565-0871, Japan
Communications and Computer Engineering, Egypt–Japan University of Science and Technology, P.O. Box 179, New Borg El-Arab City,
21934, Alexandria, Egyptc
Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashiosaka City,
Osaka 577-8502, Japand
CHUBU Electric Power Co., Inc., 20-1 Kita-sekiyama Ohdaka-cho Midori-ku, Nagoya 459-8522, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 17 October 2012
Received in revised form
7 May 2013
Accepted 17 May 2013
Available online 24 May 2013

Preliminary breakdown pulses (PBP) of 26 positive and 104 negative cloud-to-ground (CG) lightning
ﬂashes observed in winter thunderstorms in Hokuriku region of Japan are analyzed. Pulses in PBP train
are mainly bipolar. Zero-crossing time of each pulse is about 7 us and total duration of each pulse train is
about 1 ms. PBPs in negative CG lightning are classiﬁed as “BIL type” (59 cases) and “BL type” (45 cases).
“BIL type” PBP contains an intermediate stage with little electric ﬁeld changes. The time interval between
PBP and the ﬁrst return stroke (PBP–RS interval) is on average 5.4 ms for “BIL type” PBP, while that for “BLtype” PBP is only 1.3 ms. Ratio of peak amplitude of PBP to the ﬁrst return stroke (PBP–RS ratio) is on
average 0.47 and 0.44, respectively, for “BIL type” and “BL type” PBPs, but the ratio for “BIL type” PBP has amuch wider distribution. It is speculated that the intermediate stage in “BIL type” PBP is caused byhorizontal propagation of leader channel. PBPs in positive CG lightning are classiﬁed as +PBP (11 cases)and −PBP (15 cases) according to their initial polarities. +PBP and −PBP have similar distributions of bothPBP–RS interval and PBP–RS ratio, but their value of PBP–RS interval is much larger and their value of
PBP–RS ratio is much smaller than corresponding values of PBPs in negative CG lightning. It is speculated
that different initial polarities of +PBP and −PBP in positive CG lightning are caused by different
directions of channel propagation.
& 2013 Elsevier Ltd. All rights reserved.

Keywords:
Preliminary breakdown pulse
Winter lightning
Initial breakdown
Cloud-to-ground lightning
LF interferometer

1. Introduction
It has long been noticed that the ﬁrst return stroke of cloud-toground (CG) lightning is sometimes preceded by a train of bipolar
pulses lasting on the order of 1 ms (e.g., Clarence and Malan, 1957)
and can be as long as 37 ms (Baharudin et al., 2012a). Such pulse
train is generally attributed to preliminary breakdown, the initiation process of a lightning ﬂash, so it is usually called preliminary
breakdown pulses (PBP) or initial breakdown pulses. The largest
amplitude of PBP is usually much smaller than that of thefollowing ﬁrst return stroke, but sometimes it can be comparable
or even larger than that of return stroke (Brook, 1992; Gomes et al.,1998). The time interval between PBP and the ﬁrst return stroke ison the order of 10 ms, but can be as long as several hundrednCorresponding author. Tel.: +81 90 9980 2428.
E-mail addresses: wu.ting@comf5.comm.eng.osaka-u.ac.jp,
wuting25@gmail.com (T. Wu).
1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jastp.2013.05.014

milliseconds (Pierce, 1955; Beasley et al., 1982) and as short as
1 ms (Brook, 1992; Ushio et al., 1998). The initial polarity of PBP innegative CG lightning is always the same as that of the ﬁrst returnstroke. However, the situation for positive CG lightning is much
more complicated. Opposite initial polarities between PBP andfollowing return stroke are occasionally observed (Ushio et al.,
1998; Nag and Rakov, 2012). According to their initial polarities,
Gomes and Cooray (2004) classiﬁed PBPs in positive CG lightning
into four types, including cases of polarity reversal and irregularpolarity in PBP train.
The percentage of CG lightning with discernable PBP varies
signiﬁcantly in different studies. As summarized by Nag and Rakov(2009), such percentage apparently varies with latitude. The
smallest percentage is 19%, recorded in Sri Lanka (71N). In regionsof higher latitudes, the percentage is much higher, such as 80% in
Austria, 90% in Finland and 100% in Sweden. However, a recent
observation in Malaysia (11N) found that 97 out of 100 ﬂashes had
detectable PBPs (Baharudin et al., 2012a). All of these observations
are for summer time lightning.

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Studies on PBP in winter time lightning are relatively few.
Winter lightning is well known for many special features, such as
high percentage of positive CG lightning (Brook et al., 1982),
upward lightning and bipolar lightning (Narita et al., 1989). It is
expected that PBP in winter lightning may also have certain special
characteristics. Brook (1992) analyzed PBPs recorded during awinter thunderstorm in Albany N.Y., and found that PBP innegative CG lightning sometimes produced very strong electricﬁeld changes, and the interval between PBP and the following
return stroke was very short. Ushio et al. (1998) analyzed pulse
characteristics of PBP in 19 positive CG ﬂashes observed in
Hokuriku region of Japan in winter, which seemed to be similar withthose in summer lightning but distinctively different from those innegative winter CG lightning as discussed by Brook (1992).
So far it is still not completely clear what exactly happens
during preliminary breakdown pulses. Clarence and Malan (1957)
proposed “BIL” model to characterize electric ﬁeld waveform
before the ﬁrst return stroke, which included preliminary breakdown (B), intermediate stage (I) and stepped leader (L). They
attributed the “B” part to vertical discharge between the main
negative charge layer and the lower positive charge layer. Beasley
et al. (1982) examined in detail the electric ﬁeld changes preceding ﬁrst return stroke, and found that only three out of 52 records
agreed well with “BIL” model. But a recent study by Makela et al.
(2008) found that “BIL” model could successfully describe electricﬁeld waveforms of cloud-to-ground lightning ﬂash in Finland, andBaharudin et al. (2012a) reported that 47% of ﬂashes in Malaysia
were consistent with “BIL” model. However, the vertical discharge
suggested by Clarence and Malan (1957) was not conﬁrmed byfurther studies, as demonstrated by Krehbiel et al. (1979), Rhodes
and Krehbiel (1989) and Shao (1993), preliminary breakdown
involved one or more channels extending in largely horizontal
directions. Nag and Rakov (2009) proposed four types of channel
propagation starting from preliminary breakdown process according to the magnitude of low positive charge region. Their mainidea is that PBP is produced by interaction between downward
negative leader and the lower positive charge, and strong PBP canbe indicative of large lower positive charge.
Due to the complicated nature of preliminary breakdown
process, the special feature of winter lightning and the lack of
PBP observation in winter lightning, we carried out this study on
PBP of winter lightning in Hokuriku region of Japan. We will
present statistical results on various characteristics of PBP in both
positive and negative CG lightning, and based on the results, wewill compare different types of PBPs.

2. Experiment and data
Winter lightning in Hokuriku region of Japan is well known for
its unusual characteristics (Brook et al., 1982). For the study of
winter lightning, we set up an LF broadband digital interferometer(DITF) in Hokuriku region near the Japan Sea coast during thewinter of 2010–2011. This DITF system consisted of four stations,each of which was equipped with a fast antenna with a decay time
constant of 200 us. Its frequency band was from 400 Hz to 1 MHz.
An A/D converter with 4 MHz sampling rate and 12 bit resolution
was utilized to digitize the electric ﬁeld change signals producedby lightning discharges. All stations are synchronized by GPS
receivers.
Stations of this system are shown in Fig. 1, along with locationsof CG lightning associated with PBPs analyzed in this study.
Distances between these stations are from 5 km to 15 km. 3-D
locations of lightning discharges observed simultaneously by four
stations are determined by interferometry technique. Comparedwith traditional DITF in VHF band, this DITF in LF band can

Fig. 1. Locations of stations of DITF system and CG lightning associated with PBP
analyzed in this study. Locations of CG lightning are provided by LLS in Japan. Typesof PBP are explained in Section 3.

effectively detect lightning discharges in a much wider range,
and it can also visualize a rough image of discharge channels, as
demonstrated by Takayanagi et al. (2011). However, due to small
area covered by this system, it can only accurately locate discharge
events close with this network. Therefore, we use the informationof lightning location system (LLS) in Japan to get the location of
return strokes in this study. LLS in Japan is operated by electriccompanies of Japan and has a location accuracy of about 0.5 km(Matsui and Takano, 2010).
There are totally 131−CGs and 36 +CGs in this analysis. Due tospecial features of winter thunderstorm, some waveforms of
return strokes are largely different from those in summer thunderstorm, and sometimes it is quite difﬁcult to determine whether
one waveform is produced by return stroke. In this study, we only
choose those waveforms that we can conﬁdently identify as returnstroke, so the actual number of CG lightning during the winter of
2010–2011 is probably larger than the value in this study. According to location information provided by LLS, most of CGs in thisstudy are within 180 km from the observation site.
Criteria for identifying PBPs are similar with previous studies(Nag and Rakov, 2008; Baharudin et al., 2012a, 2012b). Speciﬁcally,
only pulses with amplitudes larger than twice of noise level are
included and one PBP train comprises at least three such pulses
and these pulses are mainly bipolar. Due to many special characteristics of lightning ﬂashes in winter thunderstorms, we do not
impose more quantitative criteria for selecting PBPs. However, wechecked the shape of individual pulse and pulse train for every
event to make sure it was not produced by other types ofdischarges.
Atmospheric sign convention is used throughout this paper,thus a negative return stroke produces initially positive electricﬁeld change.

3. Classiﬁcation of PBP
Among the 131 −CGs, 104 are preceded by PBP train. The initialpolarity of PBP is always negative, the same as negative returnstroke. For the remaining 27 −CGs, no signiﬁcant electric ﬁeld
changes other than that of stepped leaders can be found before thereturn stroke.
For the 104 −CGs preceded by PBP train, 59 of them are similar
to the “BIL model” proposed by Clarence and Malan (1957), in

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93

Fig.2. Typical waveforms of PBP of (a) “BIL type” and (b) “BL type” associated with negative CG lightning.

which PBP is followed by an intermediate stage with little electricﬁeld changes, and then followed by leader changes and returnstroke. One typical example is shown in Fig. 2a. We will call thistype of PBP as “BIL type”. In the other 45 cases, PBP is followed
directly by small pulses possibly indicative of leader changes andthen return stroke without intermediate stage. One typical example is shown in Fig. 2b. For convenience, we will call this type of
PBP as “BL type”.
One reason of the disparity between these two types of PBP
may be different distances from observation site, as in large
distance, small signals produced by stepped leaders may be notdetected, resulting in “intermediate stage”. However, this should
not be the only reason. As shown in Fig. 1, CG lightning ﬂashes
associated with “BIL type” and “BL type” of PBP do not show
systematic difference of distance from the observation site. Thus,
differences between these two types of PBP should be due todifferent physical processes of preliminary breakdown or stepped
leader.
From another perspective, 104 −CGs that are preceded by PBP
include 73 single-stroke −CGs and 31 multiple-stroke −CGs. It isinteresting to note that PBPs of 29 multiple-stroke −CGs are “BILtype”, while those of only two multiple-stroke −CGs are “BL type”.
For single-stroke −CGs, 30 of them are associated with “BIL type”
PBP and the other 43 are associated with “BL type” PBP.
For PBP of +CGs, its initial polarity is not always the same asthat of positive return stroke. Of the 36 +CGs analyzed in thisstudy, 11 of them are preceded by PBP with positive initial polarity(+PBP), the same as positive return stroke as shown in Fig. 3a. 15+CGs are preceded by PBP with negative initial polarity (−PBP) as
shown in Fig. 3b. The rest 10 +CGs are not preceded by PBP. It

should be noted that the ratio of number of −PBPs to +PBPs ismuch higher than previous studies (Ushio et al., 1998; Gomes andCooray, 2004). For PBP of +CG, there is always an intermediatestage with little electric ﬁeld changes, similar with “BIL type” PBP
in −CG.
Numbers of CGs with different types of PBPs are summarized inTable 1.

4. Results
4.1. PBP waveform characteristics
Waveforms of different types of PBP are generally similar witheach other. As can be seen in Figs. 2 and 3, each pulse train usually
starts with one or two very small pulses, followed by some largest
pulses of the train, and ends with pulses of decreasing amplitudes.
Pulses in PBP train are mainly bipolar. Pulses at the beginning andmiddle of the train usually have very clear bipolar feature.
However, at the end of the train, pulses sometimes appear as
unipolar. For “BL type” PBP in negative CG lightning, the “B” partand “L” part sometimes are difﬁcult to differentiate, as pulses at theend of the train seem to gradually change from bipolar to unipolar.
Such an example is given in Fig. 4. It can be seen that before thetime of about −0.9 ms, pulses are clearly bipolar, but during thetime of −0.9 ms to −0.6 ms, pulses are more like unipolar, and it is
difﬁcult to tell whether they belong to PBP or stepped leader
pulses. For such cases, deﬁning the end of PBP train is somehow
subjective, and it may have up to 0.1 ms error in calculating theduration of PBP train.

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Fig.3. Typical waveforms of PBP of (a) positive initial polarity and (b) negative initial polarity associated with positive CG lightning.

Table 1
Classiﬁcation of PBP.
CG polarity

PBP type

Stroke multiplicity−CG 131

BIL type 59

Single stroke 30
Multiple stroke 29
Single stroke 43
Multiple stroke 2
Single stroke 10
Multiple stroke 17

BL type 45
No PBP 27+CG 36+PBP 11–PBP 15
No PBP 10

All single stroke

In order to statistically analyze PBP waveform characteristics,
for each pulse train, we calculated the average value of zerocrossing time of each pulse (Tzc), average value of time difference
between peaks of two successive pulses (Tdiff), number of pulses(N) and duration of each pulse train (Tdur). Results are summarizedin Table 2.
The zero-crossing times of different types of PBPs are quite
similar. The average zero-crossing times are on average around
7 us. In the study of PBP in positive CG lightning in Sweden
reported by Gomes and Cooray (2004), pulse widths of the ﬁrst
half cycle are mostly from 10 us to 20 us, larger than that of thisstudy. However, Ushio et al. (1998) reported that PBPs in winterthunderstorm in Hokuriku region of Japan (the same region as thisstudy) have pulse duration of on average 18 us, which is similar
with average zero-crossing time of 7 us in this study.

When comparing between PBPs of “BIL type” and “BL type”,
differences on Tdiff (time difference between successive pulses)
and N (number of pulses in each train) are quite obvious. Tdiff of“BIL type” PBP is on average twice as large as that of “BL type” PBP.
However, N of “BIL type” PBP is smaller than that of “BL type” PBP,
and its maximum and minimum values are also much smaller thanthose of “BL type” PBP. It should also be noted that Tdur (duration of
PBP train) of these two types of PBP are similar with each other. So
a possible explanation for the differences on Tdiff and N is that
pulses of “BIL type” PBP are relatively weak, and some pulses are
not detected. As a result, for “BIL type” PBP, pulse interval is larger
and pulse number is smaller while duration of pulse train is close
to that of “BL type” PBP.
For the two types of PBPs in positive CG lightning, no signiﬁcant difference is observed. It indicates that +PBP and −PBP inpositive CG lightning are generally the same. When comparing
between PBPs of positive and negative CG lightning, one noticeable difference is that PBP of positive CG lightning has relatively
larger Tdiff and smaller N. From this respect, PBP in positive CG
lightning is more similar with “BIL type” PBP in negative CG
lightning. Actually, all PBPs in positive CG lightning are “BIL type”;
that is, they always show an intermediate stage with little electricﬁeld changes.
4.2. Time interval between PBP and the ﬁrst return stroke
Time difference between the largest peak of PBP train and thefollowing ﬁrst return stroke is calculated (designated as PBP–RS
interval in following analysis), as shown in Fig. 5. Fig. 5a shows
distribution of PBP–RS interval for “BIL type” and “BL type” PBPs in

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95

Fig. 4. Example of PBP waveform showing several unipolar pulses during the transition between PBP and stepped leader.

Table 2
Statistics on pulse characteristics of different types of PBP.
PBP type

–
CG
+
CG

BIL
BL
+PBP
−PBP

Tzc (us)Tdiff (us)N

Tdur (ms)

max

min

mean

max

min

mean

max

min

mean

max

min

mean

12
15
15
14

2.8
2.8
4.3
3.5

6.1
7.7
7.3
8.4

265
76
100
143

8.9
4.7
17
23

43
20
44
47

156
207
35
48

3
7
7
7

31
39
19
22

2.6
2.8
1.8
3.8

0.13
0.13
0.26
0.25

0.80
0.67
0.81
1.1

negative CG lightning. “BL type” PBPs mostly have PBP–RS intervalof smaller than 2 ms, with an average value of 1.3 ms. By
comparison, only 7 “BIL type” PBPs (12%) have PBP–RS interval ofsmaller than 2 ms, and ten of them are larger than 10 ms. The
average PBP–RS interval for “BIL type” PBP is 5.4 ms, more thanfour times of that of “BL type”. Median values of PBP–RS interval
are 3.6 ms and 1.2 ms, respectively, for “BIL type” and “BL type”
PBPs. So PBP–RS interval for “BIL type” PBP is generally larger thanthat for “BL type” PBP. The average value of PBP–RS interval for all
PBPs in negative CG lightning is 3.6 ms.
PBP–RS interval for both “BIL type” and “BL type” PBPs isgenerally smaller than those reported from summer thunderstorm(Gomes et al., 1998; Makela et al., 2008). However, PBP–RS interval
for “BL type” PBP is exceptionally small. Brook (1992) reported that
the PBP–RS interval for −CGs in winter thunderstorm in Albany, N.
Y. had a mean value of 2.75 ms, which is comparable to 3.6 ms, themean value for all PBPs in −CGs in our study. However, Brook(1992) reported that the minimum value of PBP–RS interval was
1.5 ms, while in our study, there are 18 PBPs in −CGs with PBP–RS
interval below 1 ms, which has never been reported before. Suchshort time interval should be a manifestation of special characteristics of winter lightning in Hokuriku region of Japan. Indeed, toour best knowledge, there has been no study on PBP of negative
CG lightning in winter thunderstorm of Japan. The minimum
PBP–RS interval in our study is 0.48 ms, shown in Fig. 6.
PBP–RS interval for 26 PBPs in positive CG lightning has a meanvalue of 17 ms and maximum and minimum values of 97 ms and2.7 ms. This is in agreement with that reported by Ushio et al.
(1998) in the same region, but it is generally smaller than thatreported by Nag and Rakov (2012) in Florida. As shown in Fig. 5b,
distributions of PBP–RS interval for +PBP and −PBP have certain
differences, with values of +PBP being slightly smaller. But thismay be due to small sample. The minimum values of PBP–RS
interval for +PBP and −PBP are 2.7 ms and 5.0 ms. Their meanvalues are 11.6 ms and 21.0 ms, respectively. Such large difference
in mean values is mainly because one −PBP has a value of 97.4 ms,

much larger than the rest of cases. Their median values, though,are closer, which are 9.1 ms and 14.2 ms.
It is interesting to note that large values of PBP–RS interval in
negative CG lightning seem to be associated with certain thunderstorm types. Fig. 7 shows time series of PBP–RS interval on
December 7, 2010. Large values of PBP–RS interval tend to cluster
together. In our analysis, there are totally 13 cases with PBP–RS
interval larger than 6 ms, while 11 of them are produced within
40 min indicated in Fig. 7. So certain type of thunderstorm orcertain stage of thunderstorm may be favorable for production ofnegative CG lightning with larger time difference between PBP and
the ﬁrst return stroke.
4.3. Ratio of peak amplitude of PBP to ﬁrst return stroke
Brook (1992) noted that in winter thunderstorm, negative CG
lightning could have very intense PBP that was sometimes evenstronger than the return stroke, but positive CG lightning had very
weak PBP either in winter or summer. Here we further analyze theratio of peak amplitude of PBP to the ﬁrst return stroke (indicatedas PBP–RS ratio in following analysis) of both positive and negative
CG lightning in winter.
Fig. 8a shows distribution of PBP–RS ratio for “BIL type” and “BLtype” PBP in −CGs. For “BIL type” PBPs, 28 of them (47%) have PBP–RS ratio of lower than 0.2, showing quite weak PBP compared withthe ﬁrst return stroke. By comparison, only four “BL type” PBPs
(9%) have PBP–RS ratio of lower than 0.2. On the other hand, nine
“BIL type” PBPs (15%) have PBP–RS ratio of higher than 1, while thecorresponding number for “BL type” PBP is only two (4%). As aresult, the mean values of PBP–RS ratio for “BIL type” and “BL type”
PBPs are quite close, which are 0.47 and 0.44, respectively.
Fig. 8b shows distribution of PBP–RS ratio for +PBP and −PBP in+CG. The distributions for +PBP and −PBP are quite similar. The
mean values are 0.17 and 0.18, respectively, for +PBP and −PBP.
It seems that the two types of PBPs in +CG have almost thesame intensity. It should also be noted that the maximum value of

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PBP–RS ratio is only 0.48 for all +CGs in this study. Therefore, forwinter lightning, PBPs in +CGs are much weaker than those in−CGs, and +CGs are unlikely to be associated with very strong
PBPs. This conclusion is in agreement with the observation by
Brook (1992).
When putting all results of PBP–RS interval and PBP–RS ratio
together, as shown in Fig. 9, we have some interesting observation.
Fig. 9 shows a scatterplot of PBP–RS interval versus PBP–RS ratio

for different types of PBPs. Each point in Fig. 9 represents one CG
with PBP. Their distributions are the same as shown in Figs. 5 and8. However, it is interesting to note that no point in Fig. 9 has bothlarge value of peak ratio and large value of time difference. In
other words, if PBP is very strong, the following ﬁrst return strokealways comes in a short time; on the other hand, if there is a longwait between PBP and the ﬁrst return stroke, the PBP is always
very weak compared with the RS.

5. Discussions
5.1. Two polarities of PBPs in +CG

Fig. 5. Distribution of PBP–RS interval for (a) −CG and (b) +CG.

It has been reported by many researchers that PBPs with both
positive and negative initial polarities are observed (Gomes andCooray, 2004; Ushio et al., 1998). There are mainly two hypotheses
for this phenomenon. One hypothesis is that +CGs with PBPs ofdifferent initial polarities are initiated in different charge layers.
For example, those initiated above and below the main negative
charge layer would have opposite polarities. For a thunderstorm
with normal tripolar charge structure, +CGs with +PBPs would beinitiated between the upper positive and the main negative charge
layers while +CGs with −PBPs would be initiated between themain negative and the lower positive charge layers. However, insuch scenario, PBP–RS interval probably would be shorter for−PBPs than for +PBPs, and their intensities should also havecertain differences. As shown by our analysis, time differencesfor −PBPs are almost the same as, if not larger than, those of +PBPs.
In fact, based on our analysis, it seems that +PBPs and −PBPs of+CG have almost no difference except of their initial polarities. So
our observation does not support this hypothesis.
The second hypothesis is that PBPs of different polarities are
due to charges moving in different directions. For example,
charges moving toward and away from the observer would
produce opposite polarities. In this scenario, PBPs of opposite
polarities are produced by the same discharge process and theirstatistical characteristics should be the same. So this hypothesis isconsistent with our observation. This hypothesis can also conveniently explain the observation by Gomes and Cooray (2004)
that some PBP trains contain two parts of pulses with opposite
initial polarities, as direction of PBP channel is changed during its
propagation. Under this hypothesis, PBPs of opposite polarities
should account for almost the same percentage, as in our study.
However, we are not able to explain why −PBPs are usually much
fewer than +PBPs as reported by previous studies (Ushio et al.,
1999; Gomes and Cooray, 2004; Nag and Rakov, 2012).

Fig. 6. Waveform of PBP train with a time difference of only 0.48 ms between the peak of PBP and return stroke, the smallest one in this study.

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97

Fig. 7. Time series of PBP–RS interval on December 7, 2010.

Fig. 9. Scatterplot of PBP–RS interval versus PBP–RS ratio for all types of PBP.

open question, and more observations are needed to further
investigate this issue.
5.2. “BIL type” and “BL type” PBPs in −CG

Fig. 8. Distribution of PBP-RS ratio for (a) –CG and (b) +CG.

The fact that PBPs of −CG only have one initial polarity
indicates that the discharge always starts with charge movementin vertical direction, which may be due to smaller electric potential
in the region of −CG initiation compared with that in the region of+CG initiation as further discussed below. However, this is still an

Fig. 2 shows that locations of −CGs with “BIL type” and “BL type”
PBPs do not show any systematic difference. So what causes the “I”
part (intermediate stage) of “BIL type” PBP? In other words, what isthe difference between “BIL type” and “BL type” PBPs?
Fig. 5a shows that PBP–RS interval of “BIL type” PBP is generally
larger than that of “BL type” PBP. This is an expected result, as “BILtype” PBP contains one more stage than “BL type” PBP. Further,
Fig. 7 shows that PBPs with large PBP–RS interval (all of them are“BIL type”) tend to converge in a short period. Such a period is alsocharacterized by larger-than-normal ﬂash rate. It seems that asthundercloud develops more vigorous, it is more likely to produce
PBPs with quite large PBP–RS interval.
Coleman et al. (2008) demonstrated that when there is apotential well between the lightning initiation altitude and theground, channels propagate horizontally for some time at thealtitude of potential well before initiating the ﬁrst return stroke,
resulting in considerably longer time between initiation andreturn stroke. When there is no potential well, the channel just98

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goes to the ground almost immediately, and the time difference ismuch smaller. Based on this conclusion, we speculate that theintermediate stage in “BIL type” PBP is caused by horizontal
propagation of leader channel. For the same current and velocity
and the same distance to the observer, horizontal discharge
usually produces a much smaller vertical electric ﬁeld change
than vertical discharge does, so there is little electric ﬁeld changes
during intermediate stage. And because formation of strong
potential well requires strong updraft for charge separation, “BILtype” PBPs naturally tend to converge in a short period probably
corresponding to intense updraft in the thunderstorm.
Therefore, “BL type” PBP probably does not contain such ahorizontal propagation; it just goes right toward the ground,resulting in generally smaller PBP–RS interval. And the fact thatvery small value of PBP–RS interval (around 1 ms) is much morecommon in winter lightning may indicate that the lower positive
charge in winter thunderstorm is weaker compared with that insummer thunderstorm.
6. Conclusions
104 PBP trains in −CG and 26 PBP trains in +CG are analyzed inthis study. Initial polarity of PBP in −CG is always the same as
negative return stroke. 59 PBPs in −CG are classiﬁed as “BIL type”
as they agree well with the “BIL model” proposed by Clarence andMalan (1957) while the rest 45 PBPs are classiﬁed as “BL type”
because they do not contain the intermediate stage. For PBPs in+CG, 11 of them have the same initial polarity as positive returnstroke and the rest 15 PBPs have opposite initial polarity.
Waveform characteristics of different types of PBPs are generally the same. Pulses in PBP train are mainly bipolar. The zerocrossing time is on average around 7 us. Each pulse train has
duration of about 1 ms.
Time difference between PBP and the ﬁrst return stroke (PBP–
RS interval) is calculated for each type of PBP. For −CG, “BL type”
PBP generally has smaller PBP–RS interval than “BIL type” PBP
does. Most of them are smaller than 2 ms, with a minimum valueof 0.48 ms and a mean value of 1.3 ms, much smaller than those insummer lightning. The mean value of PBP–RS interval for “BILtype” PBP is 5.4 ms. It seems that “BIL type” PBPs tend to converge
in a short period when the thunderstorm is more vigorous. For+CG, PBP–RS intervals for +PBP and −PBP are similar with eachother, with median values of 9.1 ms and 14.2 ms.
Ratio of the peak of PBP to the ﬁrst return stroke (PBP–RS ratio)
is also calculated for each type of PBP. PBP–RS ratio of PBPs in −CG
is generally larger than that of PBPs in +CG, indicating very strong
PBP in −CG in winter. The mean values of PBP–RS ratio for “BILtype” and “BL type” PBPs in −CG are 0.47 and 0.44, while the meanvalues for +PBP and −PBP in +CG are only 0.17 and 0.18.
+PBP and −PBP in +CG have almost the same characteristics
except of their initial polarity, and it is speculated that different
initial polarities are caused by different directions of channel
propagation. The intermediate stage in “BIL type” PBP is thought
to be caused by horizontal propagation of leader channel. Suchhorizontal propagation produces little vertical electric ﬁeld
changes at the ground and makes the time interval between PBP
and the ﬁrst return stroke longer.

Acknowledgment
This work was supported by Japanese Ministry of Education,
Science, Sports and Culture, a Japanese Grant-in-Aid for ScientiﬁcResearch.

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