PUBLICATIONSJournal of Geophysical Research: Atmospheres
RESEARCH ARTICLE
10.1002/2013JD020369
Key Points:• A special type of lightning discharge
called LBE is reported for the ﬁrst time
• LBE produces single large bipolar waveform different from any other discharges• LBE is produced in winter thunderstorms and is always located on land

Large bipolar lightning discharge eventsin winter thunderstorms in Japan
Ting Wu1, Satoru Yoshida2, Tomoo Ushio1, Zen Kawasaki1, Yuji Takayanagi3, and Daohong Wang41

Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Osaka,
Japan, 2Now at Meteorological Research Institute, Ibaraki, Japan, 3Infrastructure Systems Company, Hitachi, Ltd., Ibaraki,
Japan, 4Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu, Japan

Abstract Using a low-frequency lightning location system comprising nine stations, we have observed
Correspondence to:T. Wu,
wu.ting@comf5.comm.eng.osaka-u.ac.jp

Citation:
Wu, T., S. Yoshida, T. Ushio, Z. Kawasaki,
Y. Takayanagi, and D. Wang (2014),
Large bipolar lightning discharge
events in winter thunderstorms inJapan, J. Geophys. Res. Atmos., 119,555–566, doi:10.1002/2013JD020369.
Received 14 JUN 2013
Accepted 25 DEC 2013
Accepted article online 3 JAN 2014
Published online 29 JAN 2014

and analyzed 374 large and bipolar electric ﬁeld change waveforms that occurred during the winter of
2012–2013. Since the waveforms are different from those produced by any well-studied lightning discharge
processes, we refer to these source discharge events using a new name: large bipolar events (LBEs). LBEs canbe characterized by the following features: (1) All have the same polarity as negative return stroke. (2) All
exhibit a single bipolar pulse with a pulse width around 15 μs and similar positive and negative cycles. (3) All arelocated on the land along the Japan Sea coast, indicating they are probably associated with high grounded
objects. (4) Most LBEs produce very large electric ﬁeld changes that are even larger than that of positive andnegative return strokes. (5) Most LBEs are temporally isolated within several milliseconds but are frequently
followed by intracloud discharges after tens of milliseconds. (6) Most LBEs produce a single well-distinguishedionospheric reﬂection pulse, and the time difference between LBE pulse and the corresponding reﬂection pulse
can be used to calculate ionospheric reﬂection height. It is speculated that LBE is a type of powerful and transient
lightning discharge event produced within a compact region of strong electric ﬁeld formed when the negative
charge layer in thunderclouds is very close to the top of a tall grounded object.

1. Introduction
Lightning discharges in winter thunderstorms in Japan have many special characteristics. For example,
lightning ﬂash rates in winter thunderstorms are much lower than those in summer thunderstorms, and“single-ﬂash” thunderclouds are commonly observed [Michimoto, 1993]. Positive cloud-to-ground (CG)ﬂashes account for a high percentage, about 33%, of CG ﬂashes in winter thunderstorms, compared with
about 10% in summer thunderstorms [Suzuki, 1992]. Upward lightning and bipolar lightning are morecommon in winter thunderstorms than in summer thunderstorms [Narita et al., 1989; Miyake et al., 1990].
Fine structures of current waveform and radiation electric ﬁeld produced by lightning discharges in
winter thunderstorms also show many special features that are different from those in summer thunderstorms[Miyake et al., 1992; Ishii and Hojo, 1989].
Because of its very special characteristics, numerous studies have been devoted to studying winter lightning
over the past 30 years. We also have had many experiments near the Japan Sea coast for winter lightningobservation. During the winter of 2010–2011, we set up a small low frequency (LF) lightning location systemcomprising ﬁve stations [Takayanagi et al., 2013] in Hokuriku region and recorded a type of bipolar waveformthat seemed not have been reported before. However, because the location network was too small, most ofthese discharges could not be accurately located. In order to further study such discharge event, we set up alarger network comprising nine stations in western Japan, two of which are near the Japan Sea coast, before
the winter of 2012–2013. Such a network can effectively locate lightning discharges near the Japan Sea coast.
During the winter, we successfully recorded and located 374 such large bipolar waveforms.
Because such waveforms are different from those produced by any lightning discharge event known in theliterature and because of their large and bipolar features, we refer to their source discharge events as large
bipolar events (LBEs). LBEs seem to be only produced in winter thunderstorms in western coast of Japan. A very
special feature of LBEs is that while they are produced in the coast region, almost none of them are located on
the sea. This indicates that LBEs are probably associated with high grounded objects. LBEs seem to carry verystrong peak currents, so studying of LBE would be of great importance for lightning protection. In this paper, we
will report various characteristics of LBEs and analyze possible discharge processes associated with them.

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Figure 1. Locations of sites of LF lightning location system (squares) and locations of LBEs (dots).

2. Experiment and Data
In this study, LBEs were recorded by a LF lightning location system comprising nine stations, indicatedas squares in Figure 1. Black squares represent old sites that were also used in our previous studies [Wuet al., 2013]. Two blue squares represent new sites that were installed mainly for winter lightningobservation. One of them (upper right) was installed on 31 October 2012 and the other one wasinstalled on 20 December 2012. The old sites went through an upgrade from late September to early
October of 2012, during which a new recording system based on USRPTM (Universal Software Radio
Peripheral, N2x0 series) was used to replace the old system. A major advantage of the new system isthat it has zero dead time.
A fast antenna with a decay time constant of 200 μs and frequency range of 500 Hz to 500 kHz was installed ateach site. Electric ﬁeld change signals produced by lightning discharge events were digitized by the USRP
with 4 MS/s sampling rate and time stamped by an embedded GPS disciplined oscillator with timing accuracyof 50 ns. The resolution of the analog-to-digital converter of USRP is 14 bit. The signal goes through a digitaldecimation ﬁlter, and the amplitude of the output signal is in the range of 16 bit (±215).
A total of 374 LBEs were recorded during an observation period from 23 October 2012 to 18 March 2013. The ﬁrst
LBE occurred on 1 November 2012 and the last one on 10 March 2013. Figure 2 shows the number of LBEs
observed in each day during this winter. The majority of LBEs were produced during the early and midwinter. Twodimensional locations of LBEs (latitude and longitude) were determined with time of arrival (TOA) technique.
Atmospheric sign convention is used in this paper. It is interesting to note that all of LBEs produce initial
positive electric ﬁeld changes, so they have the same polarity as negative return stroke (RS).

3. Results
3.1. Waveform Characteristics
Electric ﬁeld change waveforms of LBEs are the most straightforward evidence that LBEs are different fromany well-studied type of lightning discharge events. Figure 3 presents waveforms of a typical LBE recorded at
all nine stations. LBE pulse waveforms have following common features: (1) It is a single bipolar pulse withinitial positive polarity (the same as negative RS). (2) Its pulse width is around 15 μs. (3) Positive and negative
cycles have similar pulse widths and peak magnitudes. (4) The bipolar pulse has smooth change without anyother pulses superimposed upon. (5) It is sometimes followed by a small pulse that is inferred to be its
reﬂection signal from the ionosphere.

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Figure 2. Number of LBEs observed in each day during the winter of 2012–2013.

In order to quantitatively characterize LBE pulse features, we calculate some characteristic parameters
including pulse width of the positive cycle (tP), ratio of rise time to fall time (r1), ratio of positive to negative
pulse widths (r2), and ratio of positive to negative peak magnitudes (r3), deﬁned as
r 1 ¼ tr =tf
r 2 ¼ t P =tN
r 3 ¼ AP =AN

Figure 3. Waveform of a typical LBE recorded by all nine sites. The inset shows an enlarged waveform recorded by one of the sites. Distance of this event to each site and name of each site
are respectively shown at top left and top right of each panel.

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Figure 4. Illustration of characteristic parameters of LBE calculated in section 3.1.

where tr, tf, tP, tN, AP, and AN are all illustrated in Figure 4. Figure 5 shows distributions of tP, r1, r2, and r3. Here
we only use one site’s observation data to avoid possible systematic biases between different sites. This site is
located near Osaka, and all of LBEs are at least 84 km away, so signals received at this site can be treated as
only containing radiation ﬁeld component. There are 356 LBEs observed by this site.
Figure 5a shows distribution of tP (pulse width of LBE). Values of tP are mainly around 15 μs. For 76.4% of LBEs,tP values are between 10 to 20 μs. The average value is 15.1 μs. Pulse widths of LBEs are generally larger thanthose of narrow bipolar events (NBEs) but smaller than those of RSs. Pulse widths of NBEs are shorter than
10 μs [Le Vine, 1980; Wu et al., 2011], and pulse widths of RSs are generally from 30 to 90 μs, depending on
locations, seasons, and whether ﬁrst or subsequent RS [Cooray and Lundquist, 1985; Lin et al., 1979; Ishii and
Hojo, 1989].
Figure 5b shows distribution of r1 (ratio of rise time to fall time). For 92.4% of LBEs, r1 values are larger
than 1, indicating LBE pulses usually have slow rising edge and fast falling edge. Such feature is different
from RSs, which have much faster rise time than fall time.

Figure 5. Distributions of (a) pulse width (tP), (b) ratio of rise time to fall time (r1), (c) ratio of positive to negative pulse widths (r2), and (d)ratio of positive to negative peak magnitudes (r3).

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Figure 6. (a) Differences in location results of RSs detected by eight sites of LF lightning location system and LLS of Japan. (b) Density ofnegative RS recorded by LLS of Japan from 1 November 2012 to 31 January 2013.

Figures 5c and 5d show distributions of r2 (ratio of positive to negative pulse widths) and r3 (ratio of positive
to negative peak amplitudes). Distributions of r2 and r3 are quite similar and both of them are centeredaround 1. This result indicates that positive and negative cycles of LBE pulses have similar pulse width andpeak amplitude. Such features are also different from NBEs, whose radiation ﬁelds always show much larger
initial peak than overshooting peak [Smith et al., 1999]. In fact, the overall waveforms of positive and negative
cycles of LBEs are symmetrical to a certain extent as seen in examples in Figures 3 and 4. Such feature is notcommon for electric ﬁeld change waveforms produced by lightning discharges.
In conclusion, LBEs produce distinctive single bipolar pulse that is different from any other well-studied
lightning discharge events.
3.2. Locations of LBEs
Locations of LBEs are indicated by red and blue dots in Figure 1. LBEs in blue dots were recorded by both of
the two new sites represented by blue squares and are treated as having higher location accuracy. These two
new sites are essential for locating lightning discharges along the Japan Sea coast, but they have shorter timeof operation (see section 2).
A surprising result is that almost all LBEs are located in the land area. As a comparison, Figure 6b showsdensity of negative RS recorded by lightning location system (LLS) of Japan during the same winter. Thissystem records lightning return strokes and has a nominal location accuracy of 500 m [Matsui and Takano,
2010]. Note that we can only use LLS data managed by two electric power companies which cover the areafrom 134.37 to 137.5° longitude and above about 35.3° latitude, and Figure 6b only shows location results ofnegative RSs in this region. We can see that the coastal region has the highest density of RS, and most oflightning ﬂashes occur on the sea. In fact, winter thunderstorms in western coast of Japan are developed
when cold air masses from the north move over the Japan Sea with relatively warm surface temperature[Kitagawa and Michimoto, 1994]. Lightning ﬂashes start a long time before thunderstorms hit the coastal
area. Hojo et al. [1989], based on several years’ observation with a wideband magnetic direction-ﬁnding
system, also reported that winter lightning ﬂashes in Hokuriku region were mostly detected on the sea.
For LBEs, however, it seems that only after thunderstorms reach the land do they start to occur. Hojoet al. [1989] also reported that lightning ﬂashes in winter did not move inland farther than 20–30 km
from the coastline. LBEs have a similar feature. Figure 1 also shows major mountain tops in Japan, andmost LBEs do not move over the mountains. As a result, locations of LBEs form a narrow belt along thewestern coast of Japan.
Occasionally, thunderstorms in winter also develop from the south or southwest of Japan, but we did notrecord any LBE from these thunderstorms.

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A few LBEs in Figure 1 seem to be on
the sea, but they are probably
caused by location errors. Figure 6a
is a rough estimation of location errors of our LF lightning location system. We selected some RSs near theJapan Sea coast that were detectedby eight sites of our system (exceptof the upper left one in Figure 1
which only worked from 20
December 2012) and LLS of Japan.
Figure 6a shows differences between location results provided bythese two systems. We can see thatlocation difference is quite largenear the site named BOKL, mainly
Figure 7. Distribution of normalized peak magnitudes of electric ﬁeld changes profrom 5 to 20 km. This is because thisduced by LBE, negative and positive RSs in Japan Sea coast. The number after eachregion is far away from other sites,
legend is the sample size for each type of event.
and overall propagation effects on
detected waveforms may be signiﬁcant. Location differences left of the site named NGHL are relatively small,most of which are smaller than 5 km. So LBEs at the middle and left part of Figure 1 should be relatively moreaccurate while the ones at the upper right part may contain larger location errors, which is probably thereason that some of them are located on the sea.
3.3. Peak Amplitude of Electric Field Change
Another important feature of LBEs is that they produce very large electric ﬁeld changes, which is also a reason
that we call them “large bipolar events.” In order to quantitatively evaluate this feature, peak amplitudes ofelectric ﬁeld changes produced by LBEs are compared with those produced by positive and negative RSs.
Here we also use only one site’s observation as in section 3.1. This site is located near Osaka, and all LBEs are atleast 84 km away, so their electric ﬁeld changes can be normalized to 100 km using the simple 1/R distance
relation. Positive and negative RSs are also recorded by the same site, and only those in Hokuriku region andwithin half an hour of a LBE are analyzed. There are 335 LBEs, 873 negative RSs and 475 positive RSs inthis comparison. Distributions of peak magnitudes of their normalized electric ﬁeld changes are shownin Figure 7. The scale of electric ﬁeld changes was not calibrated in our experiment, so digital unit (DU)
is used in this analysis.
First, we can see distributions of positive and negative RSs have certain differences. The average normalized
peak magnitude for negative RSs is 3251 DU, while that for positive RSs is 4697 DU. Distribution of negative
RSs shows a rapid and consistent decrease as the peak magnitude increases. There are only 7.0% of negative
RSs with normalized peak magnitude of larger than 8000 DU, while the corresponding percentage forpositive RSs is 13.6%. This result is in agreement with the well-established notion that some positive CGs
can carry extremely large peak current [e.g., Orville et al., 1987].
As for LBEs, their distribution has lower percentage below 4000 DU but higher percentage from 4000 to
20,000 DU compared with positive and negative RSs. Of the LBEs, 36.9% have peak values of larger than6000 DU, compared with 12.2% for negative RSs and 20.1% for positive RSs. The average normalized peak
amplitude for LBEs is 6086 DU, about 1.9 times that of negative RSs and 1.3 times that of positive RSs.
Therefore, LBEs have a greater tendency to produce very large electric ﬁeld changes, and they seem to beeven more powerful than positive RSs.
Because the electric ﬁeld change data are not calibrated, we cannot estimate peak currents from them.
However, we have LLS data which report peak currents of RSs. Of all the RSs analyzed above, we found 750 –RSs
and 423 + RSs that were also detected by LLS. Normalized peak magnitudes of electric ﬁeld changes in DU (alsoshown in Figure 7) are compared with peak currents reported by LLS for these RSs, and the result is shown in
Figure 8. We can see that the peak current in kiloampere is generally linearly correlated with the normalized
peak magnitude in DU, which is an expected result, and the estimated linear regression line is

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Ip ¼ α E pwhere Ip is the peak current inkiloampere, Ep is the peak magnitudeof electric ﬁeld change in DU, and
α = 0.0113 is the coefﬁcient ofproportionality.
Note that the time accuracy of LLS
data is only 1 ms, so there are maybe
some mismatches between LLS dataand LF data, which are probably thereason for some points in Figure 8that are apparently not ﬁtted with thelinear relationship.
According to the above relationship,
negative and positive RSs analyzed in
Figure 8. Correlation between normalized peak magnitudes of electric ﬁeld
this section have mean peak currentschanges and peak currents reported by LLS of Japan for 750 – RSs and 423 + RSs.
of 36.7 kA and 53.1 kA, respectively.
If we assume peak currents of LBEs follow the same rule as RSs, we can have a very rough estimation of mean
peak current of 68.8 kA for LBEs.
3.4. Temporal Relationship With Other Discharge Processes
LBEs appear to be temporally isolated from other discharge processes within at least several milliseconds,
such as examples in Figures 3 and 4. However, more careful examinations of the records show that LBEs are
frequently associated with other discharge processes within tens of milliseconds. Results are summarized inTable 1. In this analysis, events within 100 ms before or after LBE and located within 15 km of the LBE aretreated as associated with the LBE. Of all 374 LBEs, 117 LBEs are isolated, 124 LBEs precede other discharge
processes, and 99 LBEs belong to multistroke events (multiple LBEs occurring within 100 ms and within
15 km). The remaining LBEs have other discharge processes before them.
If LBEs belonging to the same multistroke event are grouped together and treated as a single event (similar tomultiple RSs being grouped into one ﬂash), 99 LBEs can be grouped into 44 multistroke events, and temporal
relationships of these multistroke events with other discharge processes are shown in the parentheses inTable 1. Seventeen of them are isolated and 24 precede other discharge events.
The most common situations are LBEs occurring in isolation or before other discharge processes. However, thatsome LBEs appear to be isolated may be because they are too far away and other discharge events associated withthem are not detected or cannot be accurately located. As a result, the number of LBEs occurring in isolation in
Table 1 is an overestimation. Another typical pattern is LBEs occurring tens of milliseconds before other discharge
processes or multiple LBEs occurring within tens of milliseconds. An example is shown in Figure 9. Two LBEs
occurred within 10 milliseconds, followed by regular intracloud processes after about 50 milliseconds.
The second LBE is much weaker than the ﬁrst one. Horizontal distances between two LBEs are only 0.22 km,
indicating that there is probably no horizontal development between them. The following intracloud processes
are about 6 km from the second LBE, so they seem to be induced by the preceding LBEs but do not occur at theexact same location as the LBEs. No processes are discernable before the ﬁrst LBE.
Table 1. Statistics of Temporal Relationship of LBEs With Other
Discharge Processes
374 (319a)

Total Number
Isolated
Following others
Preceding others
Both following and preceding others
Belong to multistroke event

117 (+17)
16 (+1)124 (+24)
18 (+2)99 (44 events)a

See text for the meaning of the values in parentheses.

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The majority of discharge processes following
LBEs are similar to the example in Figure 9,
showing multiple negative pulses. If the following
process has initial positive-going electric ﬁeld
change, it usually only contains one or two
irregular pulses. We have only one case in which
LBE is followed by RS. In this case, a positive RS
occurred about 90 ms after a LBE. Their horizontal
distance was about 4 km.

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Figure 9. (a) An example of two LBEs occurring within 10 ms followed by regular intracloud processes. D1 indicates the horizontal distance between the ﬁrst and the second LBEs. D2 indicatesthe horizontal distance between the second LBE and the largest pulse in the following intracloud processes. (b–d) Expanded views of pulses labeled as “A,” “B,” and “C” in Figure 9a.

3.5. Ionospheric Reﬂection Pulse
As in LBE examples in Figures 3 and 4, sometimes there is a small pulse following LBE pulse. This small pulse iscaused by reﬂection signal from the ionosphere. Similar reﬂection signals of NBEs and RSs are commonly
observed [Smith et al., 2004; Wu et al., 2012; Lay and Shao, 2011] and have been used to infer the electron
density proﬁle of the ionosphere [Shao et al., 2013]. Reﬂection signal of NBEs always forms two pulses, ﬁrst ofwhich is associated with the signal reﬂected from the ionosphere and the second with the signal ﬁrst
reﬂected from the ground and then reﬂected from the ionosphere. However, there is always only one reﬂection pulse for LBEs, indicating that sources of LBEs are close to the ground.
Because the 2-D location of every LBE is determined by the LF lightning location system, its distance to eachsite is readily known, and we can calculate ionospheric reﬂection height using the following relation:
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ð2HÞ2 þ D2  D ¼ cΔtwhere H is the ionospheric reﬂection height, D is the horizontal distance between LBE and observation site,t is the time difference between LBE pulse and its reﬂection pulse, and c is the speed of light. And we haveqﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1ðcΔt Þ2 þ 2DcΔt
H¼
2△

So observation of LBE by each site can give a result on the ionospheric reﬂection height.
Such relation is based on two assumptions. First, source heights of LBEs are assumed to be zero. This is areasonable assumption because sources of LBEs are inferred to be quite close to the ground as analyzed
above, which is also a common situation for lightning discharges in winter thunderstorms. Second, the Earthis assumed to be a ﬂat plane. This is reasonable only when the distance between LBE and observation site issmall. Therefore, here we only use records of LBEs within 250 km of the observation site. There are 584
records of LBEs with reﬂection pulse within this range (the same LBE observed by different sites are counted

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as different records) and ionosphericreﬂection heights calculated fromthese records are shown in Figure 10.
Red points indicate average height ineach hour. Note that there are no records from 12:00 to 13:00. Ionosphericreﬂection height shows a clear diurnal
variation. Nighttime height is around
90 km, and daytime height decreases
to around 75 km. This result agrees wellwith estimation of ionospheric reﬂection height using NBE reﬂection pulses[Smith et al., 2004]. Such diurnal variation also validates 2-D location resultsof LBEs with TOA technique. It also
Figure 10. Diurnal variation of ionospheric reﬂection height calculated from thedemonstrates that LBE is a potential
time difference between LBE pulse and its reﬂection pulse. Red points indicate
candidate for ionospheric sounding
hourly average heights.
[Jacobson et al., 2007]. LBEs have much
simpler waveforms than RSs andshould be more convenient to be modeled and employed for inferring ionospheric electron properties[Shao et al., 2013].

4. Comparison of LBEs and Other Types of Lightning Discharges
LBEs are basically different from all types of well-studied lightning discharge processes such as RSs, NBEs, andregular intracloud discharges, and now the key question is what kind of process produces LBEs. Based on
various characteristics of LBEs presented above, we will compare LBEs with other types of lightning
discharges that share certain similarities and analyze physical processes possibly associated with them.
4.1. LBEs and Lightning Discharges on Tall Grounded Objects
The most distinguishing feature of LBEs is that almost none is produced on the sea, and they are very close to
the ground, indicating that LBEs are probably associated with tall grounded objects. One possibility is that
LBEs are RS-like processes. Multiple LBEs occurring within tens of milliseconds as analyzed in section 3.4 issimilar to multiple RSs in a CG. However, normal RS produces different radiation ﬁeld from that of LBE and isusually preceded by preliminary breakdown pulses (PBPs) and leader pulses. Considering the large andbipolar radiation waveform and the polarity of LBE, it is speculated that LBEs are produced when the negative
charge layer in thunderclouds is very close to the top of a tall grounded object as illustrated in Figure 11.
In such circumstance, the distance between the negative charge layer and the grounded object is small
and the electric ﬁeld in this region can be exceptionally strong. A relatively long upward positive leader canbe initiated by the downward negative leader. Similar amount of charges with opposite polarities can beconcentrated on the heads of these two approaching leaders, and a RS-like discharge can occur between theapproaching leaders. Such discharge process is conﬁned within a compact region of strong electric ﬁeld
and lasts a very short time, so it is possible to produce a large single bipolar pulse like LBE.
Such special circumstance as in Figure 11 is rare in summer thunderstorms, but it should be much more common
in winter thunderstorms. Winter thunderstorms in western coast of Japan have quite low vertical extent, typically
only half of that of summer thunderstorms [Kitagawa and Michimoto, 1994]. With very low vertical extent, negative
charge layer in thunderclouds can be close to high grounded objects such as towers and wind mills, forming asmall gap with strong electric ﬁeld.
That LBEs are associated with such small gap is also supported by the fact that only LBEs of negative polarity
have been observed. Winter thunderstorms are found to exhibit a positive dipole or a normal tripole[Kitagawa and Michimoto, 1994], so LBEs are associated with the discharge between the main negative
charge layer and the grounded object. The discharge between the main positive charge layer and thegrounded object is not likely to happen (or does not result in LBE even if it happens) because the gap
between them is much larger.

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It is also possible that LBE is a componentof an upward lightning ﬂash. Ishii and Saito[2009] presented some waveforms of upward lightning discharges associated withtransmission line faults. Some of theirwaveforms have certain similarities with
LBEs. As pointed out by Wang and Takagi[2012], a grounded object even with amodest height in the western coast ofJapan could experience dozens of upward
lightning discharges each winter season. An
upward lightning discharge starts with an
initial continuous current (ICC) stage that
may include many ICC pulses [Rakov andFigure 11. Illustration of possible scenario for LBE production.
Uman, 2003], no matter it is self-initiated or
triggered by nearby lightning discharges[Wang et al., 2008; Warner et al., 2012]. Some of the ICC pulses may also occur like the scenario shownin Figure 11 and thus radiate LBE pulse waveforms.
4.2. LBE and Preliminary Breakdown
Although LBE pulses have no similarity with PBPs and may not have any association with normal preliminary
breakdown processes, a considerable percentage of LBEs in this study are found to be preceding other discharge
processes (Table 1) and may serve as a type of special preliminary breakdown process in winter lightning.
Lightning ﬂash rate in winter thunderstorms is known to be much lower than that in summer thunderstorms. Some thunderstorms are reported to produce only a “single ﬂash” [Michimoto, 1993]. The
main reason for such low ﬂash rate is that winter thunderstorms have small vertical extent and weak
convective strength, which hinders effective charge separation and forming of strong electric ﬁeld.
However, as a thunderstorm moves over a high grounded object, although its microphysical properties are not changed, the electric ﬁeld strength between charge layer and the grounded object
increases due to decrease of distance, and as analyzed above, LBEs may occur in such circumstance.
After the occurrence of a LBE, a breakdown channel forms between the charge layer and the grounded object.
There are possibly other less powerful streamers creating conducting channels inside the cloud as well. Charge
structures in the thundercloud are also reorganized due to the discharge. These effects create an environment
where lightning ﬂashes are much more easily triggered than in common situations in winter. As a result, LBEs
are commonly followed by more discharge processes. In such scenario, LBEs are similar to preliminary
breakdown, serving as the initiation process in winter lightning.
However, currently, we are not able to explain why discharges following LBEs are mostly intracloud processes
rather than CGs.
4.3. LBE and NBE
LBE and NBE surely have considerable differences. First of all, their electric ﬁeld change waveforms aredifferent as analyzed in section 3.1. A more fundamental difference is that NBEs are high-altitude intraclouddischarges produced in vigorous thunderstorms in summer, [Jacobson and Heavner, 2005; Wiens et al., 2008;
Wu et al., 2012, 2013] while LBEs are produced in winter thunderstorms and probably have connections withground objects.
However, pulse waveforms of LBEs are most similar with those of NBEs. In fact, the original purpose of thiswinter campaign was to investigate whether winter thunderstorms produce NBEs. We failed to detect any
NBE but recorded a type of similar pulses which turned out to be LBEs. Both of LBE and NBE produce isolated,bipolar, and very strong pulses, which distinguish them from the rest of lightning discharges. Another possible
similarity is the channel length. Channel lengths of NBEs are inferred to be shorter than 1 km [Liu et al., 2012].
LBEs are speculated to be produced in a small gap between grounded objects and negative charge layers inthunderclouds (section 4.1), so they should have much shorter channel lengths compared with RSs, possibly
close to those of NBEs.

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10.1002/2013JD020369

LBEs and NBEs, respectively, seem to be unique to winter and summer thunderstorms. To our best knowledge,
no other type of lightning discharge is unique to either winter or summer thunderstorms. It is possible thatthe processes responsible for NBEs in summer thunderstorms produce LBEs under different meteorologicalconditions in winter thunderstorms.

5. Conclusions
A type of special discharge event called LBE is reported. LBEs are observed in winter thunderstorms in JapanSea coast. Electric ﬁeld change waveform produced by LBEs is a single bipolar pulse with pulse width around
15 μs and initial polarity the same as that of negative RS. LBE pulses have slow rising edge and fast fallingedge, and their positive and negative cycles have similar pulse widths and peak amplitudes. These
characteristics make LBE pulses different from all the other electric ﬁeld change waveforms produced bylightning discharge processes.
Almost all LBEs are located on the land despite the fact that winter lightning ﬂashes in Japan Sea coast are
mostly produced on the sea. This indicates that LBEs are probably associated with high grounded objects.
LBEs produce very large electric ﬁeld changes that are even larger than that produced by some positive andnegative RSs. The average normalized peak amplitude of electric ﬁeld changes produced by LBEs is 1.9 times thatof negative RSs and 1.3 times that of positive RSs. The estimated peak currents of LBEs are on average 68.8 kA.
LBEs produce a single pulse that is usually isolated within several milliseconds, but multiple LBEs can occur
within tens of milliseconds and intracloud discharge processes are commonly observed tens of milliseconds
after LBEs. Only one LBE is recorded as followed by a RS.
LBE signals reﬂected by the ionosphere produce a single reﬂection pulse. Ionospheric reﬂection height is calculatedusing the time difference between LBE pulse and the reﬂection pulse and shows an expected diurnal variation.
It is likely that LBEs are produced in a small gap between the main negative charge layer in winter thunderclouds
and the top of high grounded objects. Small distance and thus strong electric ﬁeld between the negative charge
layer and the top of high grounded objects are necessary for production of LBEs. This also explains why LBEs are
only produced in winter and only one polarity of LBE (lowering negative charges) is observed.
The occurrence of LBEs may be able to create an electrical environment that is favorable for production of
intracloud discharges, which are commonly observed to follow LBEs. In this sense, LBEs may serve as a special
type of preliminary breakdown in winter lightning.
LBEs and NBEs also share some similarities such as simple bipolar pulse, very large current, short duration,and possibly short channel length. LBEs and NBEs seem to be unique to winter and summer
thunderstorms, respectively.
If further studies can provide conclusive evidence that LBEs are associated with tall grounded objects,
studying LBEs will have great practical signiﬁcance. First, LBEs carry strong peak currents that are even larger
than those of some positive RSs, so they are probably the most powerful lightning discharges in winterthunderstorms. Second, if LBEs are a component of upward lightning discharges, they may be useful fordifferentiating upward lightning discharges in winter thunderstorms, which are quite difﬁcult to distinguishby lightning location systems or VHF lightning mapping systems because of their irregular radiation waveforms and low source altitudes in winter.

Acknowledgments
This work was supported by the Japanese
Ministry of Education, Science, Sports andCulture, a Japanese grant-in-aid for
Scientiﬁc Research. The authors thankHokuriku Electric Power Company andKansai Electric Power Company forproviding LLS data. Very comprehensive and valuable comments fromthree anonymous reviewers are
greatly appreciated.

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