JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D05119, doi:10.1029/2011JD017054, 2012

Discharge height of lightning narrow bipolar events
Ting Wu,1,2 Wansheng Dong,1 Yijun Zhang,1,3 Tsuyoshi Funaki,2 Satoru Yoshida,2Takeshi Morimoto,2 Tomoo Ushio,2 and ZenIchiro Kawasaki2,4Received 25 October 2011; revised 14 December 2011; accepted 1 January 2012; published 8 March 2012.

[1] Discharge heights of thousands of narrow bipolar events (NBEs) observed in
Guangzhou and Chongqing of China are calculated using time delays between the direct
wave signals of NBEs and their ionospheric reflection pairs. The result shows that mostpositive NBEs occur between 8 and 16 km while most negative NBEs occur between 16
and 19 km. Very few negative NBEs are above 19 km or below 14 km. It is inferred that
positive NBEs are produced between main negative charge layer and upper positive
charge layer while negative NBEs are produced between upper positive charge layer andnegative screening charge layer at the cloud top. Variations of NBE discharge heights in
two thunderstorms are analyzed. It seems that NBEs can be produced at any position
between corresponding charge layers. Positive NBEs are generally higher in the periods
when negative NBEs are also occurring. For a given short time period in a single
thunderstorm, negative NBEs are always observed to occur at a higher altitude thanpositive NBEs, indicating a dividing charge layer between positive NBEs and negative
NBEs. The possibility of some NBEs as upward discharges from cloud tops mentioned byprevious studies is discussed. Supported by multiple evidences, we believe suchpossibility is very low; instead, NBEs are produced in vigorous convective surges thatdevelop to the height comparable to the discharge height of NBEs. Differences in height
distributions in Guangzhou and Chongqing are analyzed and a hypothesis is put
forward that both positive NBEs and negative NBEs can only be produced abovecertain height. The relationship between this hypothesis and the mechanism for NBE
production is discussed.
Citation: Wu, T., W. Dong, Y. Zhang, T. Funaki, S. Yoshida, T. Morimoto, T. Ushio, and Z. Kawasaki (2012), Discharge heightof lightning narrow bipolar events, J. Geophys. Res., 117, D05119, doi:10.1029/2011JD017054.

1. Introduction[2] Narrow bipolar event (NBE) is one of the most special
types of lightning discharge events. It is largely different
from regular cloud-to-ground and intracloud lightning inmany respects. The pulse width of electric field waveforms
produced by NBEs is only a few microseconds [Le Vine,
1980; Smith et al., 1999], much smaller than that of returnstrokes; NBEs produce extremely powerful radiation in the
HF and VHF radio bands, which is at least one order ofmagnitude larger than that produced by normal intraclouddischarges [Smith et al., 1999; Jacobson, 2003]; NBEs areusually found isolated with other discharge processes [LeVine, 1980; Smith et al., 1999], but positive NBEs are
1
Laboratory of Lightning Physics and Protection Engineering, Chinese
Academy of Meteorological Sciences, Beijing, China.
2
Division of Electrical, Electronic and Information Engineering,
Graduate School of Engineering, Osaka University, Osaka, Japan.
3
State Key Laboratory of Severe Weather, Chinese Academy of
Meteorological Sciences, Beijing, China.
4Electronics, Communications and Computer Science Engineering,
Egypt–Japan University of Science and Technology, Alexandria, Egypt.

Copyright 2012 by the American Geophysical Union.
0148-0227/12/2011JD017054

sometimes found to be the initiation of otherwise normal
intracloud lightning discharges [Rison et al., 1999; Wu et al.,2011]; the channel length of NBEs is inferred to be less than
1 km [Smith et al., 1999; Nag and Rakov, 2010], much
shorter than that of normal intracloud lightning discharges;
negative NBEs mostly occur above 15 km [Smith et al.,
2004; Wu et al., 2011], where few regular lightning discharge processes are observed.
[3] NBEs are also called “compact intracloud discharges”(CIDs) [Nag and Rakov, 2010; Wu et al., 2011] because ofsmall spatial extent of their discharge channels. However, it
is unclear whether such discharge events are always
accompanied by narrow bipolar electric field pulses which
are indicative of large-scale charge transfers [Jacobson,2003]. Since in this study the identification of such special
discharge events is based on electric field change records,
that is, the characteristics of narrow and bipolar pulses, wewill refer to such discharge event as NBE.
[4] Although a large amount of NBEs have been observedwith both ground-based and space-born observations andanalyzed in various ways since they were first reported in1980 [Le Vine, 1980], NBE remains one of the most mysterious lightning discharge processes. One interesting problem concerns the discharge height of NBEs. Smith et al.

D051191 of 13

D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 1. Map of Chongqing and Guangzhou in China.
[2004] calculated heights of over 100,000 NBEs observedby LASA [Smith et al., 2002] and the result showed that
positive NBEs were mostly between 7 and 15 km and negative NBEs between 15 and 20 km. Wu et al. [2011] calculated heights of hundreds of both polarities of NBEs
observed in south China and had similar result. However, in
the result shown by Smith et al. [2004], a small percent ofnegative NBEs were higher than 20 km and some of themwere even as high as 30 km, which is difficult to explain
because cloud tops can rarely reach 20 km. Nag et al. [2010]
also reported several NBEs that were higher than 20 km intheir height estimation of 48 positive NBEs. They further
discussed the result and put forward the possibility that very
high NBEs could be associated with gigantic jets [Krehbiel
et al., 2008].
[5] NBEs above 20 km generally account for a very small
percent of all NBEs in previous studies; most of negative
NBEs, however, are found to occur between 15 and 20 km[Smith et al., 2004; Wu et al., 2011], which also raises
questions as the tropopause is around 17 km in tropicalregion and lower in higher latitudes, so a considerable
amount of negative NBEs seem to occur above the tropopause. Jacobson and Heavner [2005] stated that some NBEs
were truly occurring above the tropopause; they might beoccurring in clear air as blue jets or in convective surges
transiently overshooting the tropopause.
[6] In this paper, a method previously described by Wu
et al. [2011] is modified to calculate the discharge heightof thousands of NBEs that are observed in two different
regions in China. We will show that the height distributionsof both polarities of NBEs are similar to that documented bySmith et al. [2004] and Wu et al. [2011], but we will demonstrate that most of negative NBEs occur between 16 and
19 km and very few are above 19 km. Variations of NBE
discharge height in two thunderstorms will also be presented.

Further, we will try to demonstrate that NBEs are occurringinside vigorous thunderstorms rather than in clear air above
cloud tops as mentioned by Jacobson and Heavner [2005]
and Nag et al. [2010]. Finally, some inferences drawn fromdifferences in height distributions of NBEs between two
regions will be discussed.

2. Data[7] Records of NBEs used in this study were collected in
Guangzhou (23.5 N, 113.5 E) and Chongqing (29.5 N,
106.5 E), as shown in Figure 1. A very low frequency andlow frequency (VLF/LF) lightning location network consisting of 7 stations of fast electric field change meters wasestablished in Guangzhou in 2007. Six of the stations covered an area with a radius of about 10 km, and the otherstation was about 32 km from the center of the network,forming a relatively long baseline [see Wu et al., 2011,
Figure 1]. In 19 days of observation, a total of 11,876 NBEs
comprising 7,882 positive NBEs and 3,994 negative NBEs
were recorded. Classification criteria for NBEs are the sameas Wu et al. [2011] and similar with previous studies [Smithet al., 2002; Hamlin et al., 2007].
[8] A similar network consisting of 9 stations was established in Chongqing in 2010, covering an area with a radiusof about 30 km. As shown in Figure 2, locations of lightning
discharges correspond well with the area of high radarreflectivity. It should also be noted that, as the storm is
outside the network, the detection efficiency for intracloudlightning is relatively low. However, in this study, only
records of NBEs are utilized, so discrepancy in the detection
efficiency for different types of lightning discharges is not a
problem. This network had kept operating for about twomonths in the summer of 2010, recording 36,442 positive
NBEs and 7,893 negative NBEs.

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D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 2. Composite radar reflectivity with locations of lightning discharge events within 6 min of theradar scanning time. Different types of lightning discharges are represented by different symbols as
indicated by the legends. No negative NBE or +CG was detected in this time period. Red dots represent
the observation stations of the lightning location network.
[9] The electric field change systems of the two networks
were identical with each other, operating at 200 Hz to
500 kHz, having a decay time constant of 1 ms and a sampling rate of 10 MHz. Each output record had duration of
1 ms with pre-trigger time of 0.3 ms. A typical record ofnegative NBE is shown in Figure 3. Detailed description of
the system is discussed by Wu et al. [2011]. Physics sign
convention for electric field change polarity is usedthroughout this paper, thus a negative cloud-to-groundreturn stroke produces a negative polarity electric fieldchange signal and a positive NBE corresponds to discharge
between upper positive charges and lower negative charges.

Such a definition is consistent with most of previous studies
on NBEs.

3. Method for Determining Discharge Heightof NBEs[10] The method used in this study to calculate the discharge height of NBEs is similar to that used by Wu et al.
[2011] and is developed from the method used by Smithet al. [1999]. The basic principle is to use the time delays
between the original signal of an NBE and its ionosphereand ground reflection signals to determine the source height.
As in Figure 3, pulse 0 is produced when the wave signal of

Figure 3. Electric field record of a typical negative NBE.
3 of 13

D05119WU ET AL.: NBE DISCHARGE HEIGHTan NBE is directly received by the observation station, andpulses a and b correspond to the signals that are reflected by
the ionosphere and ground-ionosphere, respectively, before
they are captured by the observation station. Time delays ofpulses a and b with respect to pulse 0 is associated with the
3-D position of the NBE and ionospheric virtual height (thatis, ionospheric reflection height for VLF/LF signal of NBE),
which can be inversely retrieved based on simultaneous
observations of the NBE by multiple stations.
[11] In the study by Wu et al. [2011], the following function is constructed:
f ðH; h; x0 ; y0 Þ
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2n Xctia  ð2H  h  zi Þ2 þ ri 2 þ ðh  zi Þ2 þ ri 2¼
i¼1

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2
þ ctib  ð2H þ h  zi Þ2 þ ri 2 þ ðh  zi Þ2 þ ri 2
ð1Þ

where H is the ionospheric virtual height, and x0, y0 andh are the latitude, longitude and height of NBE, which are
four unknowns of the function. tia and tib are time differences
between pulse 0 and pulses a and b, respectively, in Figure 3.
c is the speed of light. ri is the horizontal distance between
NBE and the ith station, given by the following relationship:
ri ¼ R arccos½ sinðx0 Þ sinðxi Þ þ cosðx0 Þ cosðxi Þ cosðy0  yi Þwhere R is the radius of the earth, and xi and yi are the latitude and longitude of the ith station.
[12] With simultaneous observations by multiple stations,3-D position of NBE and ionospheric virtual height can be
determined by minimizing the function using optimization
method.
[13] In this study, as a modification of the original method,
the ionospheric virtual height H is set as a constant:
H = H0 = 90 km. Therefore the function (1) is rewritten as:f ðh; x0 ; y0 Þ
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2n Xctia  ð2H0  h  zi Þ2 þ ri 2 þ ðh  zi Þ2 þ ri 2¼
i¼1

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ2
þ ctib  ð2H0 þ h  zi Þ2 þ ri 2 þ ðh  zi Þ2 þ ri 2 ð2ÞFunction (2) has only three unknowns that determine 3-D
position of NBE.
[14] There are two reasons for such a modification. First,
with the ionospheric virtual height set as a constant, thenumber of unknowns decreases from four to three and as aresult, the computational accuracy is improved. Figures 4aand 4b show results of NBE locations computed with function (1) and function (2), respectively. In Figure 4a, most
NBEs are located close to high-reflectivity cores, but someof the location results such as the ones in the white area
obviously involve large errors. However, in Figure 4b,
almost all NBE locations are quite well corresponded withhigh-reflectivity cores, showing much higher accuracy forthe results computed with function (2). Since the latitude,
longitude and height of an NBE are determined simultaneously with function (2) using optimization method, it isbelievable that results of NBE discharge height will also

D05119have higher accuracy when computed with function (2).
Second, NBEs with ionospheric reflection pairs in this study
were all observed at night when the variation of the ionospheric virtual height is relatively small. Smith et al. [2004]
showed that variation of the ionospheric virtual height forradiation waves of NBEs is most significant during thetransition between day and night. Relevant studies also
showed that ionospheric virtual height at VLF/LF frequencies is around 90 km at night [Bracewell et al., 1951]. In
order to determine the influence of possible variation ofionospheric virtual height (H0) on the result of NBE discharge height (h), we repeated computation with function (2)with different choices of H0 from 85 km to 95 km. Figure 5
shows variations of h with different choices of H0 for 10
NBEs in different altitudes. With 5 km variation of H0,variation of the result of h ranges from 0.27 km to 0.87 km.
Therefore, setting H as a constant will not have a significant
influence on the result of h but will increase accuracy of theresult, and we believe it is a reasonable treatment.
[15] This method only utilizes time delays between different pulses in the same waveform record, so differences ofarrival times of the signal to different observation stations,which are essential in the traditional time-of-arrival technique, are irrelevant with the computation. The error of thecomputation in this method mainly comes from misidentifications of the reflection pulses (pulses a and b in
Figure 3), which have been avoided in several ways in thisstudy. First, NBEs far away from the network, which produce polarity reversal of ionospheric reflections or multiplehop reflections [Smith et al., 2004], are not utilized forcomputation. In this study, most NBEs producing clear
reflection pulses and used for computation are between
about 100 km to 300 km from the network. In this range,
reflection pulses largely resemble the one in Figure 3 and areeasy to be automatically identified. Second, all the automatic
identifications of reflection pulses are manually checked tomake sure that there is no apparent misidentification. Such
re-examination ensures that there are no large errors in thecomputation. Finally, the residual of function (2) can also
indicate error magnitude in the computation, and results withlarge-than-normal residuals will be excluded.
[16] In order to estimate errors in the results of NBE discharge height due to inaccuracy in determining reflection
pulses, we made the following test. For a typical NBE
observed by five stations, up to 4-ms error is intentionallyadded in the identification of ionospheric reflection pulse
recorded by two of the five stations. Under such condition,
2-D location (latitude and longitude) of the NBE is up to
40 km from the original position, which is a significant error.
However, the height result shows no larger than 0.5 km
variance from the original result, indicating that the height
result is much more stable than the results of latitude andlongitude. This can also be qualitatively seen from function(2), in which ri, determined by latitude (x0) and longitude(y0) of NBE, is a much larger quantity (200 km) than theheight (h, 15 km). The value of ri (and the value of x0 andy0), therefore, has the largest variation through the process ofsearching the minimum of the function, while the value ofh has relatively small variation. For the same reason, iffunction (1) is utilized, results of ionospheric virtual height(H, 90 km) will also have relatively large variations [seeWu et al., 2011, Figure 7]. Since errors in 2-D locations of

4 of 13WU ET AL.: NBE DISCHARGE HEIGHT

D05119D05119Figure 4. Composite radar reflectivity with locations of NBEs within 6 min of the radar scanning time.
Black triangles represent NBE locations computed with their ionospheric reflection pairs using (a) function 1 and (b) function 2.
NBEs rarely exceed 40 km according to comparisons of
NBE locations with radar reflectivity, we believe that errors
in discharge heights of NBEs due to inaccuracies in determining reflection pulses are generally smaller than 0.5 km.

4. Results
4.1. Statistical Results of Discharge Height of NBEs[17] With the method introduced in section 3, dischargeheights of thousands of NBEs were calculated. In Guangzhou, there are a total of 1318 and 625 height results for
positive NBEs and negative NBEs. The geometric means(GMs) of discharge height are 12.1 km and 17.3 km, and thearithmetic means (AMs) are 12.3 km and 17.3 km. In
Chongqing, there are a total of 5489 and 1400 height resultsfor positive NBEs and negative NBEs with GMs of 9.9 km
and 17.5 km and AMs of 10.0 km and 17.5 km, respectively.
Statistics of discharge height of NBEs observed in these tworegions are summarized in Table 1. All the results are given
as above ground level (AGL), but they should not havemuch difference with the heights above mean sea level(AMSL) because all station elevations in Guangzhou aresmaller than 100 m and the ones in Chongqing are mostly
around 300 m.
[18] Figure 6 shows distributions of discharge heights of
NBEs observed in Guangzhou and Chongqing. It clearly

5 of 13

D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 5. Variation of NBE discharge height (h) with different choices of ionospheric virtual height (H0)for ten NBEs in different altitudes:Dh1 is difference between results of h computed with H0 = 90 km and
H0 = 85 km and Dh2 is difference between results of h computed with H0 = 95 km and H0 = 90 km.
shows that positive NBEs and negative NBEs occur in two
different altitudes, with negative NBEs mostly higher thanpositive NBEs. Most positive NBEs occur between 8 and
16 km while most negative NBEs occur between 16 and
19 km in Guangzhou. The distribution in Chongqing is quite
similar, with positive NBEs converging in lower altitudes
compared with that in Guangzhou. Such distribution is also
consistent with the result of Smith et al. [2004], further
confirming that negative NBEs occur at a region higher thanpositive NBEs.
[19] An interesting result in our calculations is that very
few NBEs are above 19 km as shown in the height distribution in Figure 6. As summarized in Table 1, the highest
NBE in Guangzhou is 19.6 km, and the highest one in
Chongqing is 19.9 km. However, there are only 6 NBEs
(0.31%) in Guangzhou and 18 NBEs (0.26%) in Chongqing
above 19 km, all of which are negative polarity. The
majority of negative NBEs occur between 16 km and 19 km
with a peak between 17 and 18 km, both in Guangzhou andChongqing. The result given by Smith et al. [2004] also
showed a peak around 17 km in height distribution of negative NBEs [see Smith et al., 2004, Figure 5]. According totheir distribution curve, which shows a sudden decrease
above 20 km, it seems very likely that most NBEs above
20 km are resulted from errors probably caused by misidentifications of reflection pulses in their automatic identification routine. Nag et al. [2010] also showed several
positive NBEs that were above 20 km. In our study, themaximum heights of positive NBEs are 17.5 km and17.8 km, respectively, in Guangzhou and Chongqing, andthe majority of positive NBEs are below 15 km. Smith et al.
[2004] also showed very few positive NBEs were above
17 km. Considering their very small sample (48 positive
NBEs), it seems that results of positive NBEs above 20 km
found by Nag et al. [2010] may have some problem.

[20] The minimum height of negative NBE in Chongqingis 7.0 km, much smaller than that in Guangzhou. However,there are only 5 negative NBEs (0.073%) below 14 km in
Chongqing, among which 2 were around 12 km and 3 werefrom 7 to 7.5 km. Other than these 5 cases, the minimum
height of negative NBE in Chongqing is 14.1 km, similar tothat in Guangzhou. As for the five negative NBEs withexceptionally low height, we checked their waveforms andre-examined the computation procedure, and we believe theheight results are accurate. However, we are not surewhether they are truly NBEs, which produce not only narrow and bipolar pulses in low frequency radio bands but also
very strong radiation in high frequency radio bands. It is
possible that they are polarity-inverted intracloud discharges
produced below the main negative charge layer [Qie et al.,2005]. Such intracloud discharges may produce large bipolar electric field change waveforms that are similar to NBEs.
Since we do not have observations in VHF bands to help
identify NBEs, such misidentification is possible. Even if
they are truly NBEs, they should have some special characteristics or are produced in special conditions as they are
significantly lower than other negative NBEs and only
account for an extremely small percent (0.073%).
[21] In summary, we state that negative NBEs mostly
occur between 16 and 19 km; few are above 19 km or below
Table 1. Statistics of Discharge Height of NBEs

Guangzhou
Chongqing

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Polarity

Number

GM
(km)

AM
(km)

Maximum(km)

Minimum
(km)

+NBE
-NBE
+NBE
-NBE

1318
625
5489
1400

12.1
17.3
9.9
17.5

12.3
17.3
10.0
17.5

17.5
19.6
17.8
19.9

7.0
14.6
6.4
7.0

D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 6. Distributions of discharge heights of NBEs observed in (a) Guangzhou and (b) Chongqing.
14 km. Positive NBEs occur in a wider range, mostly
between 7 and 16 km. On the basis of discharge height
distributions and discharge polarities, we conclude that
positive NBEs are probably produced between the main
negative charge layer and the upper positive charge layer
while negative NBEs are probably produced between theupper positive charge layer and the screening negative
charge layer at the cloud top as roughly illustrated by
Figure 7. Such a charge structure is consistent with typicalcharge distributions in updraft regions of thunderstorms[Stolzenburg et al., 1998].
4.2. Variations of NBE Discharge Height inThunderstorms[22] Fierro et al. [2011] discussed variation of NBE discharge heights in Hurricanes Rita and Katrina, but NBEs intheir study were all positive polarity. Here we present thefirst analysis of height variations of both polarities of NBEs
in two thunderstorms. Both of these two thunderstorms
occurred at night with a distance of about 200 km from thelightning location network, so most of NBEs were recordedwith well-defined ionospheric reflection pairs and could belocated with the method introduced in section 3. Althoughradar observations of these two thunderstorms are available,
which can be used to relate NBEs with the thunderstorm
producing them, the radar stations were too far away in mostperiods of the thunderstorms and therefore could not giveaccurate information on the structure of the thunderstorms.
So this section only discusses variations of NBE dischargeheight and next section will include a discussion on the
structure of NBE-producing thunderstorms using the ones atmore appropriate distances with radar stations.

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D05119WU ET AL.: NBE DISCHARGE HEIGHT

Figure 7. Illustration of discharge heights of positive NBE
and negative NBE with respect to the charge layers in athunderstorm.
[23] The first thunderstorm was observed in Chongqing,which was the only one in the observation in Chongqing that
produced more negative NBEs than positive NBEs. As in
Figure 8, there are 615 positive NBEs and 1108 negative
NBEs, most of which are produced in the period from 23:30
to 02:30.
[24] Similar to the height distribution in Figure 6b, heightof positive NBE shows a much larger variation than that ofnegative NBE. In the period from 00:00 to 02:00, heightvariations of positive NBEs are as large as 8 km in several
minutes. Fierro et al. [2011] shows that in each NBE burst
in Hurricanes Rita and Katrina, height variations of positive
NBEs are only 2 to 3 km. However, at the end of the Hurricane Rita, the height variation is as large as 12 km [seeD05119Fierro et al., 2011, Figure 3], very similar to our result. The
reason for such large height variation is currently unknown.
It may be simply because NBEs can be produced in any
place between corresponding charge layers as illustrated in
Figure 7. Relatively small height variations of negative
NBEs indicate narrow space between upper positive charge
layer and the screening charge layer. It is also reported thatthese two charge layers are sometimes partially mixed witheach other [Riousset et al., 2010].
[25] Each 15 successive positive NBEs and negative
NBEs, respectively, are grouped together (the last groupmay contain less than 15 positive NBEs or negative NBEs),
and the average height of each group is computed andrepresented by blue and green triangles in Figure 8. The
density of triangles can also indicate NBE rate. Rate of
positive NBE and negative NBE generally correspond witheach other, both of which reach very large values in the
period from about 00:00 to 02:00. In this period, a considerable amount of positive NBEs are occurring at relatively
large heights (above 13 km). At the end of this thunderstorm(after about 03:00), few negative NBEs are produced, and all
positive NBEs are occurring at lower heights (no positive
NBE is above 10 km after 03:00). In the periods when no
negative NBE is produced (indicated by shaded rectangles in
Figure 8), there are 87 positive NBEs with an average heightof 10.1 km, while in the periods with negative NBEs, there
are 528 positive NBEs with an average height of 11.0 km.
So positive NBEs are generally higher when negative NBEs
are produced at the same time.
[26] The black curve in Figure 8 divides positive NBEs
and negative NBEs into two sides, representing possibleheights of the upper positive charge layer (see charge
structure in Figure 7). It is interesting to note that all negative NBEs are above the curve and all but two positive NBEs
are below the curve, indicating that negative NBEs arealways above positive NBEs at the same moment, consistent
with the charge structure for NBE productions in Figure 7. It
also indicates that height results in Figure 8 do not havesignificant errors, otherwise there should be at least some

Figure 8. NBE discharge heights in a thunderstorm in Chongqing. Blue and green triangles represent
average heights for each 15 successive positive NBEs and negative NBEs, respectively. Shaded rectangles
indicate the periods when no negative NBE is produced. Black curve represents the possible location of
the upper positive charge layer, dividing almost all positive NBEs and negative NBEs into two sides.
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D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 9. As in Figure 8, but for a thunderstorm in Guangzhou. The black ovals labeled 1 to 3 highlight
NBE bursts.
positive NBEs and negative NBEs mixed together and it
would be impossible to draw such a dividing curve.
[27] The second thunderstorm was observed in Guangzhou, which was predominated by positive NBEs. As shownin Figure 9, negative NBEs are only produced in a shortperiod. There are a total of 262 positive NBEs and 69 negative NBEs in Figure 9. Similar to the case in Figure 8,
height variations of positive NBEs are very large, about
6 km in several minutes. The NBE production shows 3
bursts in this case in which relatively large number of NBEs
are produced in short intervals, similar to that shown by
Fierro et al. [2011]. In the burst 2, negative NBEs were also
produced, and the overall height of positive NBEs in thisperiod is larger than that in other periods. In the periods
when no negative NBE is produced (indicated by shaded
rectangles in Figure 9), the average height of positive NBEs
is 12.7 km, much smaller than that in the periods with negative NBEs, which is 15.0 km. The black curve has the samefunction as in Figure 8. Note that all positive NBEs are
below the curve and all but 1 negative NBE are above thecurve.
[28] Summarizing the above two cases, we have the following conclusions. First, NBEs seem to be able to occur inany position between corresponding charge layers, making
height variations very large in most periods of a thunderstorm, especially for positive NBEs. However, at the end of
a thunderstorm, when elevations of charge layers are probably decreasing, heights of NBEs are also decreasing andusually have small variations. Second, discharge heights ofpositive NBEs are generally higher when negative NBEs are
occurring in the same period. This indicates that when negative NBEs are occurring, updraft is stronger, lifting charge
layers responsible for positive NBE production to a larger
altitude. Third, for a given short time period in a single
storm, negative NBEs are always observed to occur at ahigher altitude than positive NBEs, supporting the conclusion that positive NBEs are below the upper positive charge

layer while negative NBEs are above the upper positive
charge layer.

5. Discussions
5.1. Possibility of NBEs Occurring Above
Thunderclouds[29] Jacobson and Heavner [2005] and Nag et al. [2010]
both mentioned the possibility that some NBEs may be
produced above the cloud top because of their high discharge heights. However, we think that NBEs are all produced inside thunderclouds as indicated by Figure 7, at least
for those observed in our study. We have the following
evidences to support our opinion.
[30] First, upward discharges from thundercloud tops
including blue jets and gigantic jets are different from NBEs
in many respects. For example, both of blue jets and gigantic
jets are optically visible [Wescott et al., 1995; Su et al.,2003], and their structural features have been well characterized; on the contrary, the “appearance” of NBEs is still
unclear although it is commonly recorded by electric fieldmeasurements in various radio bands. The vertical extents ofblue jets and gigantic jets are tens of kilometers, bridging thethundercloud top and lower ionosphere [Pasko et al., 2002;
Su et al., 2003], but the spatial extent of NBEs is inferred tobe less than 1 km. Except that some NBEs occur at an altitude comparable to the initiation height of blue jets orgigantic jets, they generally have no similarities.
[31] Second, as discussed by Krehbiel et al. [2008], blue
jets and gigantic jets are produced as a result of certain
special charge structures in the thundercloud. For example,
large positive charge at the upper level and mixing of thescreening negative charge with the upper positive charge areconducive to gigantic jets, while upper positive charge larger
than the main negative charge added with screening negative
charge is favorable for the production of blue jets. Krehbiel
et al. [2008] also showed a jet produced in a decaying
storm system. Here we will show that NBEs, different from

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jets, are closely related with deep convections. The relationship between NBEs and thunderstorms has been discussed by many studies [Suszcynsky and Heavner, 2003;
Jacobson and Heavner, 2005; Wiens et al., 2008]. Here wewill mainly demonstrate that when negative NBEs are produced, thundercloud tops are usually higher than 15 km,
comparable to the discharge height of negative NBEs.
[32] Observations of a conventional S-band Doppler
weather radar are employed here. Since radar observationsof targets at high altitudes (roughly above 15 km in our dataset) have low resolution reflectivity, making it seriously
inaccurate to determine cloud top of higher than 15 km fromradar echo top, here we define two quantities to roughly
infer storm structures: total volume of reflectivity larger
than 30 dBZ (V30) and total volume of reflectivity larger than
30 dBZ above 15 km (V3015). Two adjacent thunderstorms
producing more negative NBEs than positive NBEs
observed in Guangzhou in August 8, 2007 are selected.
These two thunderstorms are selected because they are inappropriate distances (between 60 and 100 km) from theradar station; they are close to the lightning location network(less than 120 km), so the location accuracy is high; and theyboth have clear boundaries, making the analysis easy.
Figure 10 shows the time series of positive NBE rate, negative NBE rate and the value of V30 and V3015. Bursts ofnegative NBEs are clearly associated with abrupt increasesof V3015, but V30 does not show obvious variation accordingly, indicating that changes of the storms mainly occur
above 15 km; that is, the storms are developing higher during the bursts of negative NBEs. Although it is so far
unclear why variations of V3015 are always several minutes
ahead of variations of negative NBE rate, the connection
between negative NBE production and deep convection
is obvious, which is also consistent with the finding of Wu
et al. [2011] that a thunderstorm has higher tendency to
produce negative NBEs as it is more vigorous. It is important to note that in these two cases, 30-dBZ heights arehigher than 15 km when V3015 > 0, so thundercloud tops are probably comparable to the discharge height of negative NBEs (16 to 19 km) during bursts of negative NBEs.
This phenomenon is also consistent with charge structurein Figure 7; when negative NBEs are produced, thunderclouds are developing higher, and charge layers are liftedto the height comparable to the discharge height ofNBEs, supporting the conclusion that NBEs are produced
inside thunderclouds.
[33] Finally, Jacobson and Heavner [2005] and Nag et al.
[2010] mentioned the possibility of some NBEs occurring
above thunderclouds mainly because some NBEs were
found to occur at very large heights, which thundercloud
tops could rarely reach. However, in this study, we havedemonstrated that almost all NBEs are below 19 km. It
seems that very high NBEs in previous studies are probably
due to computation errors. Under such circumstances, theassumption of NBEs being upward discharges from cloud
tops is unnecessary, because thundercloud tops of 17 to
18 km high do not seem to be rare, and that up to 19 km highis also possible. Zipser et al. [2006], summarizing over
7 years of TRMM observation, found that maximum heightof 40 dBZ of about 0.01% events can reach above 14 km,
and some can even reach up to 19 km. Ushio et al. [2001]
also showed that cloud top heights up to 18 km are not

D05119rare according to the data from TRMM. In a recent study on
the relationship between cloud top and the tropopause, Pan
and Munchak [2011] showed that in the region of ourstudy, tropopause height is close to 17 km [see Pan andMunchak, 2011, Figure 8] and cloud tops can be 0.5 to
2.5 km higher than the tropopause in summer [see Pan andMunchak, 2011, Figure 7].
[34] In conclusion, NBEs with discharge heights up to
19 km are totally possible to be produced inside thunderclouds. The fact that cloud tops have relatively small chance
to reach the height of 16 to 19 km where negative NBEs
occur results in the phenomenon that negative NBEs areusually much fewer than positive NBEs. The possibility of
NBEs occurring above thunderclouds is very low.
5.2. Comparison of Height Distributions in Two
Regions[35] When comparing distributions of NBE dischargeheight in Guangzhou and Chongqing (Figure 6), it is interesting to note that height distributions of negative NBEs arealmost the same, while that of positive NBEs are largely
different in these two regions. Average heights of negative
NBEs in these two regions are also very close, while that of
positive NBEs differ by more than 2 km (Table 1). It seems
that large number of positive NBEs cluster below 10 km in
Chongqing, while positive NBEs in Guangzhou distribute
relatively evenly between 9 and 15 km.
[36] Another difference worth noting is that the percentageof negative NBE is much larger in Guangzhou than inChongqing. The number of negative NBEs in Guangzhou is
about half of the number of positive NBEs, while that in
Chongqing is only about one fifth of positive NBEs.
According to the conclusion of Wu et al. [2011] that percentage of negative NBE increases as convective strength of
the thunderstorm increases, it seems that thunderstorms in
Guangzhou have stronger convection than that in Chongqing. This can also be inferred from frequencies of negative
return stroke (–RS) in these two regions. In Guangzhou,
138,148 –RSs were detected in 19 days, equivalent to 7271
–RSs per day. In Chongqing, 174,756 –RSs were detected inabout two months, equivalent to about 2913 –RSs per day,much smaller than that in Guangzhou. From another perspective, we compared 50 largest hourly rates of total discharge events and –RSs in Guangzhou and Chongqing. In
Guangzhou, the 50 largest hourly rates of total discharge
events range from 22215 to 3203 counts per hour, muchlarger than that in Chongqing, which range from 2410 to1288 counts per hour. The 50 largest hourly rates of –RSs in
Guangzhou range from 5826 to 853 counts per hour, also
larger than that in Chongqing, which range from 1437 to
776 counts per hour. Given the fact that the lightning detection network in Chongqing has more observation stations andcovers larger area than that in Guangzhou (see section 2), it
should be concluded that thunderstorms are generally morevigorous in Guangzhou than in Chongqing.
[37] Combining these two differences, we speculate on a
possible explanation. It is likely that negative NBEs can only
be produced above certain height. When the charge layers
responsible for negative NBE production are lifted abovecertain height, negative NBEs can be produced, otherwise
negative NBEs can rarely occur. Such “critical height” isaround 15 km according to the result of this study. Since

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D05119WU ET AL.: NBE DISCHARGE HEIGHT

D05119Figure 10. Time series of positive NBE rate, negative NBE rate and the value of V30 and V3015 in two
thunderstorms observed in Guangzhou: V30 and V3015 share a scale, but the real value of V30 is the scale
multiplied by 20.
thunderstorms in Chongqing are generally less vigorous thanthat in Guangzhou, thunderclouds in Chongqing are generally lower and the upper charge layers have less chance todevelop above the critical height, resulting in much smaller
percentage of negative NBEs in Chongqing. Normally the
middle charge layers responsible for positive NBE production are also lower in Chongqing than that in Guangzhou, so
positive NBE discharge heights in Chongqing are also
lower. However, because of the limitation imposed by the“critical height,” discharge heights of negative NBEs arealmost the same in Chongqing and Guangzhou; the difference is that negative NBE percentage is much smaller inChongqing because thunderclouds in Chongqing have lesschance to develop to the critical height.
[38] The higher the upper charge layers are lifted above
the critical height, the larger chance negative NBEs can beproduced. However, it is difficult for thundercloud tops topenetrate the tropopause. Competition of such two limitations results in distribution peak between 17 and 18 km fornegative NBE discharge height (Figure 6). Positive NBE

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D05119WU ET AL.: NBE DISCHARGE HEIGHT

production should also have a critical height, which is possibly around 7 km. The upper limit for the height of middle
charge layers responsible for positive NBE production
seems to be around 17 km, so positive NBEs can occur
between 7 km and 17 km, much larger a range than that ofnegative NBEs. Therefore positive NBEs have the “space”
to show large difference in height distributions in two
regions. However, although average discharge heights ofpositive NBEs in two regions differ by more than 2 km, theminimum values only differ by 0.6 km, indicating a lower
height limit for positive NBEs. The minimum dischargeheights of negative NBEs in these two regions are also very
close (14.6 km and 14.1 km, see discussion in section 4.1),
supporting our hypothesis.
[39] Under this hypothesis, negative NBE will rarely occur
between the lower positive charge layer and the main negative charge layer (Figure 7), because these charge layers
have very little opportunity to be lifted above the criticalheight of negative NBE (around 15 km). This also results inthe fact that negative NBEs are usually much fewer thanpositive NBEs, because it is much easier for charge layers
responsible for positive NBE production to reach the criticalheight.
[40] Such a hypothesis indicates an initiation mechanismof NBE in which there is energy injecting into thunderclouds
from space. The higher a thundercloud develops, the larger
energy it receives, and the larger chance there is that NBEs
can be produced. Such notion is also consistent with thephenomenon that negative NBEs are usually more powerfulthan positive NBEs [Wu et al., 2011]. A possible candidate
for such mechanism is runaway breakdown mechanism[e.g., Roussel-Dupré and Gurevich, 1996] and likely
included with relativistic feedback to increase multiplicationof energetic electrons [Dwyer, 2003]. In such mechanism,
cosmic ray particles play a critical role. There are also somestudies trying to relate runaway breakdown mechanism to
NBE phenomenology [Gurevich and Zybin, 2004; Gurevich
and Zybin, 2005], but details are still missing to account fordifferences between positive NBE and negative NBE.
[41] If our hypothesis is true, a direct result is that NBEs
will not be produced in thunderclouds with small verticalextent. Sharma et al. [2011] reported that no NBE wasobserved in Sweden (59.8 N, 17.6 E). According to ourhypothesis, this result is likely due to lower tropopause athigh latitude, limiting cloud tops below small altitude. It isalso expected that in some regions with appropriate height of
tropopause, only positive NBEs can be produced.

6. Summary[42] Discharge heights of thousands of NBEs in two
regions have been calculated employing time delays
between direct radiation waves of NBEs and their ionospheric reflection pairs. The result shows that most positive
NBEs occur between 8 and 16 km while most negative
NBEs occur between 16 and 19 km. Few negative NBEs areabove 19 km or below 14 km. It is likely that NBEs above
20 km reported by previous studies are due to errors incomputations. According to discharge height distributions
and discharge polarities, positive NBEs are probably produced between main negative charge layer and upper positive charge layer while negative NBEs are probably

D05119produced between upper positive charge layer and negative
screening charge layer at the cloud top.
[43] Variations of NBE discharge heights in two thunderstorms are presented. It seems that NBEs can be produced at
any position between corresponding charge layers, resulting
in large variations of discharge heights. Positive NBEs are
generally higher when negative NBEs are occurring in thesame period, and for a given short time period in a single
thunderstorm, negative NBEs are always higher than positive NBEs, supporting the charge layer structures for NBE
production illustrated in Figure 7.
[44] The possibility of some NBEs as upward discharges
from cloud tops mentioned by previous studies is discussed
in many respects. Two thunderstorms producing more negative NBEs than positive NBEs are analyzed to demonstratethat negative NBE productions are closely associated withdeep convections higher than 15 km. Supported by someother evidences, we concluded that such possibility is verylow.
[45] Height distributions of NBEs in Guangzhou andChongqing are compared, and it is found that height distributions of negative NBEs are almost the same in these tworegions, but height distributions of positive NBEs have large
difference. A hypothesis is put forward that there are “critical heights” for both positive NBEs and negative NBEs, andpositive NBEs and negative NBEs can only be produced
above their respective critical heights. Such hypothesis isconsistent with many phenomena, and it also has somepredictions. To further test this hypothesis, observations of
NBEs of both polarities should be carried out in different
regions over the world. Special attention should be paid tothe discharge height of negative NBEs, studies of which arestill scarce.
[46] Acknowledgments. This work was supported in part by the
National Natural Science Foundation of China under grant 41075022 and41030960 and the Ministry of Science and Technology under grant
2008EG137292. The authors thank Abram Jacobson and two anonymous
reviewers for their valuable comments.

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