GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 618–623, doi:10.1002/grl.50112, 2013

Spatial relationship between lightning narrow bipolar events andparent thunderstorms as revealed by phased array radar
Ting Wu,1 Yuji Takayanagi,1 Satoru Yoshida,1 Tsuyoshi Funaki,1 Tomoo Ushio,1and Zen Kawasaki1
Received 30 November 2012; revised 21 December 2012; accepted 21 December 2012; published 13 February 2013.

[1] Phased array radar with unprecedented high temporaland spatial resolution is used for the ﬁrst time to analyze
structures of thunderstorms producing lightning narrow
bipolar events (NBEs). Locations of NBEs generally
correspond well with the deepest convection, but in somethunderstorms extending higher than 15 km, positive NBEs
cluster around, rather than right at the center of the core ofdeep convection. Negative NBEs are generally higher thanpositive NBEs and are usually produced at the cloud top of
the thunderstorm. Positive NBEs, on the other hand, arealways located well inside the thundercloud. It seems thatnegative NBEs can only be produced in very vigorous
thunderstorms with cloud tops higher than about 14 km.
Numerous thunderstorms with lower height produce few
negative NBEs, indicating a height threshold for NBE
production. On the basis of these ﬁndings, it becomes very
convenient and accurate to monitor severe thunderstorms
with negative NBEs. Citation: Wu, T., Y. Takayanagi, S.
Yoshida, T. Funaki, T. Ushio, and Z. Kawasaki (2013), Spatial
relationship between lightning narrow bipolar events and parent
thunderstorms as revealed by phased array radar, Geophys. Res.
Lett., 40, 618–623, doi:10.1002/grl.50112.

1. Introduction[2] Narrow bipolar event (NBE) is a special type of intracloud lightning discharge with many distinguishing characteristics. For example, NBEs produce extremely energetic VHF
radiation that is one order of magnitude larger than that
produced by normal discharge events [Thomas et al., 2001;
Jacobson, 2003]. NBEs are usually found temporally isolated
from other discharges, but sometimes they occur as theinitiation process in otherwise normal lightning discharges[Rison et al., 1999; Wu et al., 2011]. Discharge height ofnegative NBE is close to, or even larger than, the height of
the tropopause [Smith et al., 2004; Wu et al., 2012]. The
estimated channel length of NBE is smaller than 1 km [Smithet al., 1999; Liu et al., 2012]. For this reason, NBE is also
called compact intracloud discharge (CID).
[3] Due to the fact that the strong high frequency radiation
produced by NBE can penetrate the ionosphere and can beconveniently observed by satellite, NBE is a potential
1
Division of Electrical, Electronic and Information Engineering,
Graduate School of Engineering, Osaka University, Suita, Osaka, Japan.

candidate for monitoring severe convective activities fromspace. However, such satellite-based monitoring cannot
be realized until the relationship between NBE production
and thunderstorm activities is well established. Over thelast decade, many studies have contributed to examining
such relationship [Suszcynsky and Heavner, 2003; Jacobson
and Heavner, 2005; Wiens et al., 2008]. Overall result
indicates that NBEs tend to occur in the most vigorous
thunderstorms, but statistical analysis on the relationshipbetween NBE rate and various parameters of thunderstorm
does not show a strong correlation. On the other hand, casestudies on thunderstorms producing NBEs are still rare,
and such studies mainly focus on positive NBEs [Fierro
et al., 2011; Lu et al., 2012]. As demonstrated by Wu et al.
[2011], the percentage of negative NBEs has the tendency
to increase with increasing convective strength. In otherwords, vigorous thunderstorms are more likely to produce
negative NBEs. Therefore, it is important to classify positive
and negative NBEs when analyzing the relationship between
NBE and thunderstorm, and special attention should be paid
to thunderstorms producing negative NBEs.
[4] A signiﬁcant and somehow mysterious feature ofnegative NBE is its large discharge height. Discharge heightof negative NBE is generally larger than that of positive ones
and normal intracloud discharges. Some negative NBEs are
estimated to be signiﬁcantly higher than the tropopause[Smith et al., 2004; Nag et al., 2010], which is difﬁcult tounderstand because thundercloud has little chance to develop
to such height. Wu et al. [2012] calculated discharge heightsof thousands of NBEs in two regions and demonstrated
that all NBEs, including negative NBEs, are produced inside
thundercloud, and extremely large results in previous studies
are probably due to computation errors. However, so far,there is still no direct evidence to prove that negative NBEs,
occurring higher than 15 km or even close to 20 km, areinside thundercloud.
[5] In this paper, we will show observations of NBEs by
low frequency (LF) lightning location system comprisingnine stations and simultaneous observation of thunderstorms
by a phased array radar (PAR) with the highest temporaland spatial resolutions in studies of NBE. Such coordinatedobservation provides the best chance so far to examine therelationship between NBE production and thunderstorm
structure. As an initial report of the ﬁrst observation in thesummer of 2012, this paper deals with spatial relationshipbetween NBE and parent thunderstorm.

Corresponding author: Ting Wu, Division of Electrical, Electronic andInformation Engineering, Graduate School of Engineering, Osaka University,
Suita, Osaka, Japan. (wu.ting@comf5.comm.eng.osaka-u.ac.jp)

2. Experiment©2012. American Geophysical Union. All Rights Reserved.
0094-8276/13/10.1002/grl.50112[6] LF lightning location system in Osaka region of Japan
has been set up since the summer of 2011. During the618

WU ET AL.: NBE LOCATION IN THUNDERSTORM

summer of 2012 (period for this study), there were 9 stations,shown as red squares in Figure 1. Each station was equippedwith a fast antenna with frequency range of 400 Hz to
1 MHz and decay time constant of 200 ms. Electric ﬁeld
change signals were digitized with 4-megasample/sec sampling rate and 12-bit resolution. All stations are synchronized
by GPS receivers. Details of this system were described by
Takayanagi et al. [2011].
[7] 3-D locations of lightning discharges are automaticallydetermined by interferometry technique, and real-time monitoring of lightning activities in the Osaka region has beenrealized since the summer of 2012. Locations in Figure 1
are from results of real-time monitoring. However, NBEs
have very clear peaks, so it is better to use time-of-arrival(TOA) technique to locate them. In this study, locations of
NBEs are all determined with TOA technique. Because of
relatively small sample of NBEs in this study, we manuallychecked waveforms and location results of all NBEs to makesure there is no apparent error. 3-D location error for NBE
should be smaller than 1 km, as Takayanagi et al. [2011]
compared location results determined by this system, which
only contained four stations at that time, with those determined by a VHF interferometry system, and the difference
was no larger than 1 km.
[8] The PAR started working since July 2012. It is asingle-faced phased-array system located in Suita campus,
Osaka University (34.82˚N, 135.52˚E; origin in Figure 1).
In our system, the beam is electronically steered in elevation
direction by transmitting a fan beam and receiving scatteredsignals as multibeam using digital beam–forming technology[Yoshikawa et al., 2013]. In this way, one rotation of theantenna can cover a whole volumetric scan, and the time forvolumetric updates can be signiﬁcantly reduced. The current
system can work in two scanning modes: fast-observation
mode and wide-observation mode. Currently, the PAR isoperated under the wide-observation mode, which takes
30 seconds for a whole volumetric scan, and the largest

Figure 1. Height of 20 dBZ calculated from PAR reﬂectivity data. The PAR is represented by blue “+”. Nine stationsof LF lightning location system are represented by red squares.
Black points represent lightning locations within 15 secondsof the radar time.

scanning range is 60 km. Its temporal resolution of 30 seconds
is far better than conventional weather radar with parabolic
antenna. Its spatial resolution is 100 m in radial direction,
1.2˚ in azimuthal direction and 0.9˚ in elevation direction.
Such spatial resolution is also much better than conventional
weather radar.
[9] Data quality has not been fully controlled. The reﬂectivity data commonly contain some contaminations fromground clutter, such as the dark blue region in the lower
part of Figure 1. Echoes from such contamination havestrong intensity (larger than 50 dBZ), but they only occur
at a low altitude (lower than 2 km). Therefore, we use heightof 20 dBZ to indicate evolution of thunderstorms as shownin Figure 1. Another problem is that observation within
about 10 km of the PAR is not very accurate due to someunsolved problems on pulse compression in this newly built
system. So data within 10 km of the PAR are not shown as in
Figures 1 and 2.

3. Results[10] There are more than ten thunderstorms observed by
the PAR during the summer of 2012. In Osaka region ofJapan, each thunderstorm usually lasts for less than 1 hourwith cloud top height of around 12 km. A total of 232
positive NBEs and 22 negative NBEs were observed during
the summer of 2012, which had simultaneous observationby the PAR. Physics sign convention is used throughout
this paper, so positive NBE corresponds to the discharge
between upper positive charge and lower negative charge.
3.1. Horizontal Structure[11] Locations of NBEs usually correspond well with theregion of strongest convection (indicated by the largest 20
dBZ height) as shown in Figures 2a and 2b. In Figure 2a,
the 20 dBZ height is 9.0 km, and NBE height is 5.9 km. In
Figure 2b, the 20 dBZ height is 13.6 km, and NBE heightis 10.7 km. The region where NBE is produced has thelargest cloud top height at that moment. It appears that NBE
is selective for the deepest convection. Such corresponding
relationship has been suggested and reported by many studies[Jacobson and Heavner, 2005; Wiens et al., 2008].
[12] However, the actual situation is more complicated.
Figures 2c–2f are from two independent thunderstorms on
14 August 2012. Cloud tops of these two thunderstorms
stayed higher than 15 km for a long time, which is notcommon in the Osaka region. These two thunderstorms
produced 150 positive NBEs (65%) and 11 negative NBEs(50%). Positive NBEs in these two thunderstorms are apparently not selective for the deepest convection; instead, they
converge around, and outside of, the region with deepest
convection. In Figures 2c and 2d, locations of several
positive NBEs form a circle right around the region withthe highest cloud top, and all NBEs are located in regionswith 20 dBZ height lower than 15 km. In Figure 2e, four
positive NBEs are located at one side of the 15 km contour.
In Figure 2f, two negative NBEs (with the same location,
also shown in Figure 3d) are located almost right at thecenter of the thunderstorm, but seven positive NBEs arescattered near the edge of the thunderstorm. Such feature
can also be found in other thunderstorms, but it seems mostpronounced in very deep convections.

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WU ET AL.: NBE LOCATION IN THUNDERSTORM

Figure 2. Height of 20 dBZ calculated from PAR reﬂectivity data. The PAR is located at (0, 0). Black pluses represent
positive NBEs and black points represent negative NBEs within 15 seconds of the radar time.

[13] One possible explanation for such phenomenon isthat under intense updraft, ice crystals with positive charges
are elevated to a higher altitude while graupels (larger andheavier) carrying negative charges still stay in the mixedphase region, so the distance between the upper positive
charge layer and the main negative charge layer increases
and discharges in this region are less likely to occur. Anotherreason may be that as cloud top extends into higher altitude,
it becomes easier to form the screening negative charge layer
due to increase of atmospheric conductivity [Riousset et al.,2010]. Such screening negative charge layer can reduce thenet electric ﬁeld between the main negative charge layer
and the upper positive charge layer and reduce the possibility of positive NBE production in this region. As a result,positive NBEs are more likely to occur outside of the
deepest convective region.
3.2. Vertical Structure[14] According to the results of NBE discharge heightcalculated by Smith et al. [2004] and Wu et al. [2012], it isspeculated that positive NBEs are produced between themain negative charge layer and the upper positive charge
layer, while negative NBEs are produced between the upper
positive charge layer and the screening charge layer at thecloud top. However, due to some results of NBEs that aremuch higher than normal cloud top [Smith et al., 2004;
Nag et al., 2010], such speculation is still questionable. Herewe compare locations of NBEs, especially negative NBEs,
with vertical structures of thunderstorms revealed by the
PAR to examine this issue.

[15] Figure 3 shows some examples of range-height indicator (RHI) of reﬂectivity with NBE locations. Figure 3ashows a positive NBE of 5.7 km inside a thundercloudwith top height of about 9 km. Figure 3b shows a much morevigorous thunderstorm with top height of about 14 km.
Three positive NBEs are produced at the upper part of thethundercloud. In fact, all positive NBEs analyzed in thisstudy are found to be well inside thunderclouds regardlessof storm-top heights.
[16] The situation for negative NBEs is quite different.
Figure 3c shows two negative NBEs right at the upperboundary of the thundercloud. Because the PAR is designed
for observation of up to 15 km altitude, only fragments ofreﬂectivity above 15 km can be captured by the end of somebeams as shown in Figure 3. However, the upper cloudboundary can still be roughly ﬁgured out. In Figure 3c, one
negative NBE is at the inner side and the other at the outer sideof the upper boundary. A positive NBE is produced outside of
the reﬂectivity core similar with those analyzed in section 3.1.
Figure 3d is only 30 seconds from Figure 3c, and two negative
NBEs occurred again at almost the same location. Figure 3eshows another two negative NBEs. The cloud top is higher
than 15 km and difﬁcult to discern, but the two NBEs seemto be quite close to the cloud top. Figure 3f shows similar
situation with Figure 3e. It also shows a positive NBE at theedge of the reﬂectivity core. The negative NBE in Figure 3g
is well inside, and the one in Figure 3h is well outside of thethundercloud, somehow different from other cases. The one
inside the thundercloud may be due to lower altitude of theupper positive charge layer, and the one outside thundercloud
may be due to transient overshooting that is not captured by620

WU ET AL.: NBE LOCATION IN THUNDERSTORM

Figure 3. RHI of reﬂectivity observed by the PAR. The PAR is located at (0, 0). Black pluses represent positive NBEs andblack points represent negative NBEs within 15 seconds of the radar time.
the PAR. Figure 3i shows a very low negative NBE (13.3 km),which is also right at the top of the thundercloud.
[17] In summary, positive NBEs are always well inside
thundercloud, while negative NBEs are usually at the upperboundary of the thundercloud. Sometimes negative NBEs
are at the inner side of the boundary while sometimes they
are at the outer side, which may be due to different situations
analyzed above. Figure 4 shows a scatterplot of dischargeheight of all NBEs versus 20 dBZ height of parent thunderstorms. It should be noted that the actual storm-top height isa little higher than 20 dBZ height. Negative NBEs are
generally higher than positive NBEs. One negative NBE is
exceptionally low (8.1 km), which may be similar with threenegative NBEs around 7 km reported by Wu et al. [2012].
It may indicate that negative NBEs can also be produced
below the main negative charge layer, but it is more likelythat such low negative NBEs are different from othernegative NBEs in certain respect such as VHF radiation
characteristics. However, such cases are very rare (0.073%
in the study by Wu et al. [2012] and 1 in 22 in this study),
and we will not further discuss them in this study.

Figure 4. Height of all NBEs in this study versus 20 dBZ
height of their parent thunderclouds. At the black line,
NBE height equals 20 dBZ height.

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WU ET AL.: NBE LOCATION IN THUNDERSTORM[18] Apart from the exceptionally low one, the lowest
negative NBE is 13.3 km, and the highest one is 16.0 km.
Most of negative NBEs are clustered around the line ofequal NBE height and 20 dBZ height, indicating negative
NBEs occur near the upper boundary of thunderclouds.
Positive NBEs, on the other hand, are usually much lower
than the 20 dBZ height. As the 20 dBZ height increases,
some positive NBEs also reach quite close to the cloud
top, but all of them are still inside the thundercloud. Such
result is consistent with the notion that positive NBEs areproduced below and negative NBEs above the upper
positive charge layer.

4. Discussion
4.1. Height Threshold for NBE[19] Wu et al. [2012] proposed a hypothesis that NBEs canonly be produced above a certain height (“critical height”).
Our observation is consistent with such hypothesis, especially for negative NBE. Discharge heights of negative
NBEs in this study range from 13.3 to 16.0 km (except for
the especially low one), and most of them are between 14
and 16 km. Such distribution resembles the lower tail ofthe distributions in Guangzhou and Chongqing of China(see Figure 6 in Wu et al. [2012]). This hypothesis indicatesthat only when the charge layers responsible for productionof NBEs are lifted above the critical height can NBEs occur.
Since negative NBE is produced near the upper boundary ofthundercloud, the thundercloud top has to develop to thecritical height, which is probably around 14 km, before
negative NBE can occur. However, thunderstorm with cloud
top of higher than 14 km is very rare in Osaka region (thePAR is only designed for observation of up to 15 km). Most
thunderstorms in Osaka region only developed to around
12 km or even lower, and no negative NBEs were produced.
Cloud tops of some rare cases analyzed in section 3.2
exceeded the critical height and produced several negative
NBEs. The highest negative NBE is only 16.0 km in thisstudy, compared with larger than 16 km for majority ofnegative NBEs in south China [Wu et al., 2012], which isalso a manifestation of much lower storm top in Osaka
region than that in south China.
4.2. Signiﬁcance for Monitoring Severe Thunderstorm
with NBE[20] When analyzing the relationship between NBE andthunderstorm, previous studies usually make no discrimination between positive and negative NBEs [Jacobson andHeavner, 2005; Wiens et al., 2008]. We suggest thatnegative NBE is more useful and accurate in indicating thedeepest convection.
[21] As analyzed in this study, negative NBEs are
only produced in the most vigorous thunderstorms (usuallyhigher than 14 km), and negative NBEs always occur near
the upper boundary of the thundercloud. Therefore, if we
can detect negative NBE and determine its height, we canestimate the thunderstorm top height right from the negative
NBE height. Even if we cannot determine NBE height,
we can roughly decide the severity of thunderstorm by thepresence of negative NBE. As long as negative NBE is
produced, the thunderstorm has probably developed aboveat least 14 km, which is quite severe. In this way, severe

thunderstorms can be conveniently monitored by detecting
negative NBEs.

5. Conclusions[22] Locations of NBEs detected by LF lightning location
system are compared with thunderstorm structure observedby the PAR. It is found that NBEs usually correspond wellwith the deepest convection. However, in some thunderstorms with intense updraft extending above 15 km, positive
NBEs are produced at the periphery, instead of the center, ofthe deepest convection. This may be because of decrease ofelectric ﬁeld due to elevation of the upper positive charge
layer or formation of screening negative charge layer in theregion of the deepest convection.
[23] Positive NBEs are usually lower than negative NBEs.
Positive NBEs are always produced well inside thundercloud,while negative NBEs are usually close to the upper boundaryof the thundercloud. Negative NBEs were only produced in a
few thunderstorms with the deepest convection, while numerous thunderstorms with shallower convection did not produce
any negative NBE, a phenomenon consistent with “criticalheight” hypothesis proposed by a previous study. Such feature
also makes negative NBE a perfect proxy for monitoring
severe thunderstorm.
[24] Acknowledgments. This work was supported by the projectfor development of phased array radar from the National Institute ofInformation and Communications Technology. The authors thank Abram
Jacobson and one anonyous reviewer for their valuable comments.

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