Journal
Decemberof the2022
Meteorological Society of Japan, 100(6),N. SAKURAI
979−993, et
2022.
al.
Special Edition on Typhoons in 2018 – 2019

doi:10.2151/jmsj.2022-051

979

Positive Cloud-to-Ground Lightning Characteristics
in the Eyewall of Typhoon Faxai (2019) Observedby Tokyo Lightning Mapping Array
Namiko SAKURAI
National Research Institute for Earth Science and Disaster Resilience (NIED), Tsukuba, Japan

Hironori FUDEYASU
Graduate School of Education, Yokohama National University, Yokohama, Japan
Typhoon Science and Technology Research Center, Yokohama National University, Yokohama, Japan

Paul R. KREHBIEL, Ronald J. THOMAS, William RISON, and Daniel RODEHEFFER
New Mexico Institute of Mining and Technology, New Mexico, USA(Manuscript received 1 August 2021, in final form 25 August 2022)

Abstract
Although a number of studies have been conducted of the lightning activity in hurricanes and typhoons, little
information has been obtained on the three-dimensional (3-D) structure of the lightning, or how it is related tothe precipitation structures within the storms. Here, we utilize observational data from the 3-D Tokyo Lightning
Mapping Array (Tokyo LMA), a Japan Meteorological Agency C-band Doppler radar, and the Japanese Lightning
Detection Network (JLDN) to conduct a study of the lightning activity during Typhoon Faxai (2019) in comparison with the storm’s precipitation structure. This is done for the dissipating stage of the typhoon, when theeyewall was well within the range of the instruments and undergoing a surge in lightning activity. Of particularinterest in the surge was the occurrence of numerous positive cloud-to-ground (+CG) lightning flashes. Detailed
study of the Tokyo LMA and JLDN data shows that, out of 52 flashes during the surge, 29 flashes or 56 % produced positive strokes to ground, an unheard of number considering that, from the lightning and 3-D radar structures, the storm appeared to be normally electrified, and under such circumstances would produce negative rather
than positive strokes to ground. It also focuses attention on the question of how +CGs are produced in tropical
cyclones in the first place. Based on a lack of −CG strokes and the LMA observations showing that the +CG
strokes are produced mid-way or toward the end of normal polarity intracloud (IC) flashes, it appears that thedissipating storm cells have a depleted or horizontally sheared mid-level negative charge, such that an IC flash
propagating into and through upper positive storm charge effectively funnels a steadily increasing amount of positive charge into the mid-level initiation region, eventually causing the positive breakdown of the IC flash to turndownward toward ground, producing a +CG stroke.
Keywords

lightning; Lightning Mapping Array; typhoon; positive cloud-to-ground lightning

Corresponding author: Namiko Sakurai, National ResearchInstitute for Earth Science and Disaster Resilience (NIED),
3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan
E-mail: sakurain@bosai.go.jp
J-stage Advance Published Date: 15 September 2022
©The Author(s) 2022. This is an open access article published by the Meteorological Society of Japan undera Creative Commons Attribution 4.0 International (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0).

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Citation Sakurai, N., H. Fudeyasu, P. R. Krehbiel, R. J. Thomas, W. Rison, and D. Rodeheffer, 2022: Positive
cloud-to-ground lightning characteristics in the eyewall of Typhoon Faxai (2019) observed by Tokyo Lightning
Mapping Array. J. Meteor. Soc. Japan, 100, 979–993, doi:10.2151/jmsj.2022-051.

1.

Introduction

Tropical cyclones (TCs) are one of the mostdestructive natural phenomena on Earth and are regularly related to the loss of life as well as extensive
property damage. Lightning has been observed inmany TCs (e.g., Black and Hallett 1999; Molinari
et al. 1999; Nakano et al. 2011; Price et al. 2009).
Price et al. (2009) found that an increase in lightning
frequency in TCs preceded the intensification of theirmaximum sustained winds and minimum pressuresapproximately 1 day before the peak winds. Theysuggested that monitoring of lightning activity within
a TC might be useful for predicting TC intensification.
Most lightning discharges in TCs have been
observed in the eyewall and outer rainband, with relatively few being observed in the inner rainband (e.g.,
Molinari et al. 1999; Nakano et al. 2011; Yokoyama
and Takayabu 2008). Zhang et al. (2012) reported
that the radial distribution of the lightning varies with
the intensity of TCs. They suggested that the lightning activity in TCs is closely related to the internal
structure of their precipitation system. However, there
are only a few observational reports on the relationship between the lightning activity and the threedimensional (3-D) structure of precipitation systems
in TCs. Squires and Businger (2008) reported casestudies of the vertical structure of precipitation in theeyewall; there was deep convection in the eyewall
region and enhanced reflectivities above the melting
layer with a gradual decrease of reflectivity withheight. Fierro et al. (2011) utilized observations oflarge-amplitude narrow bipolar events to study lightning in the eyewall of Hurricanes Rita and Katrina(2005) using the Los Alamos Sferic Array. The
observations revealed a general increase in dischargeheights during the rapid intensification period of convective elements in the eyewall.
A few studies have been conducted concerning thepolarity of cloud-to-ground (CG) lightning in TCs.
Samsury and Orville (1994) investigated the polarity
of CGs in Hurricanes Hugo and Jerry (1989) andreported that more than 20 % of CGs were positivein both hurricanes. Similar results were reported
by Zhang et al. (2012), who analyzed 18 typhoons.
Thomas et al. (2010) investigated the polarity of CGs

in the eyewall of Hurricanes Emily, Katrina, and Rita
(2005) using the World Wide Lightning Location Network and found that a high frequency of positive CGs
occurred in the decaying stage of the hurricanes. Ifhigh frequencies of positive CGs occur commonly inthe eyewall of the decaying stage of TCs, monitoringof positive CGs might be useful for predicting TC
decay. Additional case studies or statistical analysesof the relationship between the TC phase and theoccurrence of high percentage of positive CG in theeyewall would further help in the validation. Additionally, analysis of the internal structure of precipitation
systems in the eyewall has been needed to clarify themechanism by which the positive CGs are produced.
Typhoon Faxai (T1915) struck the Kanto regionof Japan during September 8 and 9, 2019, producing
many lightning discharges. 3-D observations of thetotal lightning activity in the typhoon were obtainedusing the Tokyo Lightning Mapping Array (Tokyo
LMA), which has been set up and operated in theTokyo metropolitan area by the National ResearchInstitute for Earth Science and Disaster Resilience
beginning in March 2017 (Sakurai et al. 2021). The
objective of the present study is to describe the characteristics of the lightning activity in comparison with
the internal structure of precipitation in the eyewall,using data from the Tokyo LMA, the Japan Meteorological Agency (JMA) C-band Doppler Radar (CDR),
and the Japanese Lightning Detection Network(JLDN).
2.

Data and method

2.1 Tokyo Lightning Mapping Array
The Tokyo LMA consists of 12 stations (Sakurai
et al. 2021) (Fig. 1), 8 of which monitored the lightning during the typhoon. Each sensor detects thearrival time of impulsive very-high-frequency (VHF)
radiation produced by electrical breakdown as thelightning propagates through a thunderstorm (Rison
et al. 1999; Thomas et al. 2004). Peak events aredetected in successive 80 µs time windows for signals
above a threshold value. The stations of the Tokyo
LMA are distributed over a 90-km diameter area andcan detect lightning activity as far as 200 – 300 km
distance from the center of the network (Fig. 3b). The
system typically locates tens to several hundred VHF

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detected by JLDN is approximately 3 – 5 kA (seehttps://www.franklinjapan.jp/en/jldn/).
3.

37N

Ibaraki

36N

Kashiwa

Ba

y

Chiba
To
k

yo

150km

35N

139E

140E

141E

Fig. 1. Map depicting the locations of Tokyo LMA
observation stations (square dots) and the JMA
C-band Doppler Radar (CDR; circular dot). The
circle indicates the coverage area of the JMA
CDR and the scale of the map. The dashed line
shows the path of T1915.

sources per flash, which reveals the development of
individual flashes and helps in identifying their type.
2.2 JMA C-band Doppler radar
Data from the CDR were used to determine the internal structure of the precipitation systems in T1915.
The radar was operated by JMA at the Kashiwa station and provided observations out to a radius of 150
km (Fig. 1). It scanned the typhoon in Plan Position
Indicator scan mode with 26 elevation angles between
0° and 25° and measured the horizontal reflectivity
ZH, with a volume scan time of 10 min. The ZH
values were interpolated into a cartesian grid system(Cressman 1959) having a spatial resolution of 1 km
horizontally and 250 m vertically.
2.3 Japanese Lightning Detection Network
The JLDN observations were used to investigate
lightning ground strike locations and the peak currentsof strokes. JLDN consists of 31 LF band sensors andprovides coverage over all of Japan. The reported
uncertainty of the JLDN locations is less than 500 m,
and the minimum peak current of lightning discharges

Results

3.1 Overview
Figures 2 and 3 provide an overview of the lightning activity that occurred as the typhoon passed
over the Kanto area, and how the activity was related
to the typhoon’s life cycle. T1915 developed from a
tropical depression into a typhoon in the sea south of
Minamitorishima at 0300 JST on September 5, 2019,and its central pressure decreased to 955 hPa at 0300
JST on September 8, 2019 (Figs. 2b, 3a). Simultaneously, the maximum wind speed around the center of
the typhoon increased to 45 m s−1. The typhoon began
to decay at 0900 JST on September 9 (Fig. 2b) andsubsided into an extratropical cyclone at 0900 JST on
September 10.
Lightning started to be detected in the typhoon by
the Tokyo LMA at 1524 JST on September 8 (initial
burst of sources in Fig. 2a), while the typhoon was~ 300 km south of the Tokyo LMA (southernmost
dark blue sources in Fig. 3b). Figure 3c shows thetyphoon’s radar-observed precipitation structure at1901 JST on September 8, along with the location of
the lightning sources at that time (black “x” symbols
in Fig. 3c and some of the light green sources in Fig.
3b). A close examination of Fig. 3c shows that thelightning was occurring primarily in the developing
eyewall of the typhoon, with a lesser amount in an
inner rainband. At this point, the typhoon was centered~ 230 km south of the Tokyo LMA.
After a 4-hour quiescent period or “lull” in the LMA
observations, the lightning activity started up again,
this time primarily in the easternmost outer rainband(second burst of activity in Fig. 2a and orange sourcesin Fig. 3b). During this time (0200 JST – 0500 JST
on September 9), the typhoon had an axisymmetriceyewall structure (Fig. 3d) and propagated over Tokyo
Bay and the Kanto region. Although a small amountof lightning occurred in the eyewall during this time,the main activity was in the eastern outer rainband.
Four hours later, as the typhoon weakened and moved
offshore east of Ibaraki Prefecture (Fig. 3e), a third
burst of lightning occurred in the expanded eyewall
structure (red sources in Fig. 3b and the final activity
in Fig. 2a). As can be seen from Fig. 2b, the final
lightning activity occurred as the typhoon transitioned
from its mature stage to the decaying stage.
In summary, the observations show that the lightning was episodic in nature, with each episode lasting
approximately 4 h (Fig. 2a) and occurring at different

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Fig. 2. a) Time series of the number of VHF lightning sources observed by Tokyo LMA from 0900 JST on September 8 to 0000 JST on September 10, 2019, illustrating the episodic nature of the lightning activity and the increasein detection efficiency as the typhoon approached the Tokyo LMA. b) Evolution of the central pressure and maximum wind speed of T1915 from 0000 JST on September 7 to 0000 JST on September 10, 2019, showing that the
final episode of lightning occurred as the typhoon began its final decay.

stages in the typhoon’s life cycle. A total of 7152 VHF
sources were detected by 7 or more stations during
the episodes, with the detection efficiency increasing
as the typhoon approached and passed by the Tokyo
LMA. There were fewer lightning sources in the inner
rainband than in the eyewall or the outer rainband,in agreement with the radial distribution of lightning
activity in TCs reported by earlier studies (e.g.,
Molinari et al. 1999; Nakano et al. 2011; Yokoyama
and Takayabu 2008).
3.2 Lightning observations
Figure 4 shows an overview of the total lightning
activity during the final, decaying stage of the typhoon, as determined by the Tokyo LMA and the
JLDN. The Tokyo LMA observations are colored bothas a function of time (Fig. 4a) and by the logarithmic
density of sources (Fig. 4b). A total of 55 flashes
occurred between 0643 and 0858 JST (2:15 h) on
September 9, corresponding to an hourly flash rate of~ 24 h−1. During this time, the JLDN detected 52 of
the flashes, with 36 of the flashes reported as being
CG discharges and 16 reported as being intracloud (IC)flashes. The total number of JLDN sferic events was
278, corresponding to an average of ~ 5 events per
flash. Of the 36 CGs, 32 were reported by the JLDN

as being +CG flashes, and 4 were reported as −CGs.
Comparing the JLDN events on a flash-by-flash basiswith the more detailed observations by the Tokyo
LMA (e.g., Fig. 6) shows that in actuality there were
29 +CG flashes and only 1 −CG flash, with 3 of the
JLDN-indicated +CG flashes and 3 of the −CG flashes
being misidentifiied as IC flashes (Table 1). Thus,
29/52 or 56 % of the total number of flashes were+CGs, with the +CGs being approximately 1.3 times
more than IC flashes.
The JLDN-detected sferic events are presented inhistogram form in Fig. 5. Sferic counts for positive
and negative CG events are shown in the upper rightand left quadrants, respectively, as a function of theirpeak currents. IC events of positive and negative
polarity are shown in the lower quadrants and occur
in much larger numbers, as indicated by the different
vertical scales. Overall, the histograms graphicallyillustrate the dominance of +CG events in the eyewallof the decaying stage of the typhoon and the wide
range of their peak currents (~ 30 – 130 kA). Events
having peak currents less than +20 kA are known as“weak positive” events (in pink) and are misidentified+IC events, as evidenced by their incidence rate versus
peak current being the same as that of the +IC events,
and constituting a small fraction of the much larger

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Fig. 3. a) Trajectory of T1915 over the 5-day time period of its overall development. b) Zoomed in view of the lightning activity as the typhoon approached and passed by the Tokyo LMA. (c – e) Radar observations of the typhoon’s
structure and the lightning activity (“x” marks) around the time of (c) initial lightning detection by the Tokyo LMA(dark blue and light green eyewall sources in b), (d) as the eyewall of T1915 moved over Tokyo Bay (yellow andorange outer rainband sources in b), and e) during the decaying stage showing its dense and widespread lightning
activity (red sources in b). Rainfall intensity near the ground was observed by eXtended Radar Information Network (XRAIN) operated by Japan’s Ministry of Land, Infrastructure, Transport and Tourism. Dark blue sources
near the Tokyo LMA stations are unrelated local lightning activity prior to the approaching typhoon.

number of +ICs. It is well known that weak positive
events are misidentified +IC events produced bynormal polarity IC flashes, due to their sferics being
similar and difficult to distinguish between (Cummins
et al. 1998; Cummins and Murphy 2009).
Similarly considered to be in the IC category are therelatively weak −CG events (light blue, Fig. 5). This isdetermined in part by their peak currents being weak(less than 10 kA) and/or by the LMA observations
showing that the events occurred within the first millisecond or so of the flashes, with insufficient time for adownward negative leader to propagate to ground. (AnJournal of the Meteorological Society of JapanTOKYO LMA on 09 Sept 201910
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140.5

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10
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Altitude (km)Fig. 4. Overview of the lightning activity in the eyewall of the decaying stage of T1915, as determined by the Tokyo
LMA. Fifty-five lightning flashes occurred over a 2-h and 15-min time period. a) Temporal progression of the VHF
sources. b) Logarithmic density of the sources. Triangle, cross, diamond, and star marks in the plan views indicate
the location of −CG, +CG, −IC, and +IC events detected by JLDN, respectively (shown as gray bars at the base of
the vertical projection panels). Square green symbols at the left edge of the plan panels indicate two easternmost
Tokyo LMA stations. (Plots utilize solutions from six or more stations).

Table 1. The number of flashes observed in the eyewall
between 0643 JST and 0858 JST on September 9. a) The
number of flashes detected by both the Tokyo LMA and
the JLDN. b) The number of IC and CG flashes determined by JLDN. c) The number of CG flashes of eachpolarity determined by JLDN. d) The number of flashes
as determined utilizing the Tokyo LMA data (see text). e)
The corrected total number of IC and CG flashes.

example of an incorrectly identified weak negative
CG event is seen at the beginning of the example flashof Fig. 6, where it is also substantially mislocated.)
Instead, there was only 1 actual −CG flash (the 37th
flash at 08:16:49 JST in Fig. 4). The flash produced
multiple strokes to ground, and the second was doubly
detected as a 38/39-kA −CG stroke. From the LMA
observations, and as discussed further below, the −CG
flash was immediately preceded by a strong, doubly
detected +CG flash, which would have diminished thestorm’s positive charge, possibly allowing (or causing)
the next flash to be a −CG. The dearth of −CG flashes
in the eyewall of the decaying stage of the typhoon isadditionally indicated by the lack of LMA sources thatare produced between the IC flashes and ground. That
downward leaders of −CG strokes are readily detectedby the Tokyo LMA has been documented in the studyof lightning by Sakurai et al. (2021).
It should be noted that six of the high-peak current

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Fig. 5. Histograms of JLDN-detected sferic events versus peak current in the eyewall of the decaying stage ofT1915. Sferics identified as CG events are shown in the upward histograms, with positive events on the right andnegative events on the left. Much more numerous IC events are shown in the downward filled gray bars (note vertical scale difference).

+CG counts in Fig. 5 were doubly detected +CG
strokes. Such doubly detected events occur within a
few microseconds of each other and are readily recognized as corresponding to a single stroke. The double
detections are caused by the strong sferic of the +CG
stroke being located by two independent sets of JLDN
stations and are often seen in the U.S. National Lightning Detection Network (NLDN) data. However, they
are readily identified and affect only the stroke/event
counts, but not the number of flashes. Finally, if the
weak −CG events in the JLDN data were taken at facevalue, 8 out of the 29 +CG flashes would have included a negative ground stroke, making them appear to be
hybrid +CG/−CG flashes (Table 1). Thus, determining
flash statistics from sferics data alone would be
misleading for understanding the typhoon lightning.
At the same time, the VHF LMA observations alone
would not have been able to distinguish between IC
and CG events in the storm.
The combination of the LMA and JLDN shows thatthe positive strokes typically occurred partway throughthe flashes. Their occurrence was further confirmed

by characteristic reinvigoration or “blooming” of the
LMA activity radially away from the extremities ofbranches in different directions around the plan location of the positive stroke. Figure 6 shows an example
of such blooming following a 117-kA +CG stroke.
This was the third flash in the decaying sequence ofFig. 4 and occurred at 06:55:38 JST on September
9. As with all the flashes in the decaying system, the06:55:38 flash was a normal polarity IC discharge,
beginning with upward negative breakdown into andhorizontally through upper positive charge in thestorm. The JLDN located three events within the firstmillisecond of the flash, the first and third of whichwere 17 kA and 14 kA +IC events, respectively, with
their positive polarity consistent with being produced
by upward negative breakdown. The second event wasclassified as a 14-kA −CG stroke, but is an example
of a misclassified −CG. This is indicated by occurringat the very beginning of the flash, too early for adownward leader to reach ground and, in this case,being substantially mislocated, ~ 30 km to the NW of
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Fig. 6. VHF lightning sources for a +CG flash at 06:55:38 JST on September 9 that produced a 117-kA +CG discharge 200 ms into the flash (black X symbol). Arrival of ground potential within the cloud initiated characteristic“blooming” of the breakdown in different directions away from the extremities of the preceding activity, including
an entirely new branch to the east and north (dashed areas). Triangle, cross, and star marks indicate the locationand time of −CG, +CG, and +IC events detected by JLDN, respectively. (Plot shows sources located by six ormore stations with chi-square values ≤ 5).

plan view of Fig. 6). The ensuing +CG stroke occurred
approximately 200 ms into the flash (black X marker
in each of the panels) and was correctly located by the
JLDN. Significantly, and as will be discussed in moredetail later, the +CG strike point was in close proximity to the plan flash initiation region. The post-stroke
blooming is outlined by dashed areas in the planview, which contain the yellow-orange-red sources
following the positive stroke in the height–time panel.
The blooming is caused by ground potential being introduced into the storm by the energetic return strokeof +CG flashes, which substantially reinvigorates thebreakdown, extending the channels in space and time.

In this case, a new and extensive branch was initiatedclose to the flash initiation point that developed over alarge area to the east and north.
Finally, an interesting feature of the observations
concerns the lone −CG flash which, as discussed
above, occurred at 08:16:49 JST as the 37th flash in
the storm. In particular, the flash occurred one-halfsecond after and in the exact same location and altitude as flash 36, which produced a strong, doubly detected 32/26 kA +CG stroke to ground. This preceding
stroke would have deposited negative charge in thestorm’s upper positive charge region which, combinedwith any other negative charge, might have caused orDecember 2022

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Altitude [km]Fig. 7. Horizontal cross-section of the radar reflectivity at 1 km altitude (b) and east–west and north–south verticalcross-sections (a, c) at 0710 JST on September 9, 2019. Overlaid on the cross-sections are the VHF sources (□)
for three horizontally extensive (20 – 25 km) lightning flashes between 0700 and 0710 JST. The VHF sources were
produced by negative breakdown propagating through upper positive charge, which formed immediately above theconvective core and was advected northward and eastward by the eyewall winds, descending somewhat in altitude
away from the core (black arrow in c). Downward descent rate of the arrow is overly steep due to the vertical scale
being greatly exaggerated relative to the horizontal scale (see Fig. 9).

assisted in the 37th flash being a −CG. Four minutes
later, flash 38 occurred, again in the same location
as the two previous flashes. This was a long-lasting~ 600 ms duration flash that produced a prolificnumber (12) of weak positive and negative IC events,
in the middle of which a single, high current (+63kA) event occurred. Instead of being classified as a+CG, it was classified as a +IC event. It is seen in thebottom right quadrant of the histogram plot of Fig. 5,
but almost certainly a misclassified +CG whose sferic
waveform may have been made more complex by theunusual preceding activity. In any case, the complexity of the overall three-flash sequence is indicative of
the substantial effect of lightning charge deposition
altering the electrical charge structure and lightning

activity in storms (e.g., Coleman et al. 2003; Brothers
et al. 2018).
3.3 Comparison with radar observations
The internal structure of precipitation systems in
the eyewall of T1915 was investigated using the JMA
CDR installed at Kashiwa (Fig. 1). Figure 7 shows
how the lightning activity was related to the radar-derived reflectivity and precipitation structure during a
10-min time interval between 0700 JST and 0710 JST
on September 9. The comparison shows that the lightning was occurring on the eastern side of the partial
eyewall structure, where 30 dBZ reflectivity extended
up to approximately 10 km altitude (Figs. 7a, c).
Thirty-five dBZ reflectivity extended up to approx-

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imately 7 km altitude in the convective core, which
implies that not only ice crystals and super-cooled
droplets but also graupel existed in the deep convection. Three lightning flashes occurred during the
10-min time interval, in particular the fifth, sixth, andseventh flashes of the overview plots of Fig. 4. In eachcase, the flashes began by discharging upper positive
charge at ~ 8 – 10 km altitude above the convective
core, where the reflectivity was between 30 and 20
dBZ, then progressed northward and downward to
between ~ 6 km and 8 km altitude (black arrow in
Fig. 7c), following the 30 – 20 dBZ contours. Similar
downward development was observed during the flashof Fig. 6, which occurred in the same location ~ 4 min
before the three-flash eyewall sequence. The altitude
descent with northward propagation is consistent withthe upper positive charge being advected northward
by the typhoon’s counterclockwise rotation anddown­ward by the 20 – 30 dBZ contours decreasing inaltitude off to the side of the core. In this altitude andreflectivity regime, the temperature would have been
−30°C, and the hydrometeors would have consistedof crystalline ice particles and frozen cloud droplets,
which are the carrier of upper positive charge in electrified storms.
To investigate the relationship between the verticaldevelopment of the precipitation and the lightning
activity, a statistical analysis was conducted of thecorrelation between the 30 dBZ echo top height andthe lightning activity versus time and location. The
analysis period was the duration in which the centerof T1915 was within range of the CDR (Fig. 1), andthe analysis area was within 100 km of the typhoon’s
center. The Critical Success Index (CSI) of a positive
correlation between 30 dBZ echo top height and theoccurrence of lightning detected by the Tokyo LMA
was 0.63 when echo top height of 30 dBZ exceeded
11 km. In addition, the CSI was 0.46 (0.59) when theheight of 30 dBZ exceeded 9 km (10 km). Overall, theresults support the premise that lightning developedin convective cells where ice crystals, graupel, andsuper-cooled cloud droplets were present to produce
electrification by the non-inductive graupel–ice mechanism (Takahashi 1978).
3.4 Flash extents
The spatial extent of the VHF lightning sources
can be regarded as a proxy of the extent of the charge
region in storms having low flashing rates, since theflashes tend to propagate through the full extent of thecharge regions. Figure 8 shows the distribution of flash
extents in the eyewall from 0000 JST to 0900 JST on

Vol. 100, No. 6

Fig. 8. Distribution of horizontal flash extents inthe eyewall of T1915 between 0000 JST and
0900 JST on September 9, 2019.

September 9 when the typhoon center passed overTokyo Bay, the north part of Chiba, Ibaraki Prefecture,and offshore of the Ibaraki Prefecture coast. The flash
extent is defined as the square root of the horizontal
convex hull (polygon) of the source locations, whichin this study is the longitude–latitude plane. The
definition is the same as the flash extent in previous
studies (Bruning and MacGorman 2013; Mecikalski
et al. 2015; Yoshida et al. 2019). In the present case,the analysis was applied to 50 of the lightning flashes
in the decaying system between 0600 JST and 0900
JST and to two small lightning flashes that occurred in
the eyewall as the center of the typhoon was passing
near Tokyo Bay between 0000 JST and 0200 JST. The
arithmetic mean of the flash extents was 20.6 km, thegeometric mean was 19.3 km, and the median extent
was 21.1 km. The maximum flash extent was 36.3 km,
and the minimum extent was 4.8 km. The latter value
corresponded to that of the two early flashes, bothof which had extents less than 5 km. Otherwise, themininum flash extents in the decaying sequence weretypically ~ 10 – 15 km.
Table 2 compares the flash extents of the present
study with those obtained by other investigators andstorm types in Japan. Yoshida et al. (2019) investigated flash extents for lightning in the summer andwinter seasons in Japan, which were observed by
the Broadband Observation network for Lightning
and Thunderstorms. They reported arithmetic mean
and median flash extents of 6.5 km and 5.1 km in thesummer season and 16.9 km and 14.8 km for winterstorms. Zheng et al. (2019), using LMA observationsof winter storms, reported arithmetic mean andmedian flash extents of 19.8 km and 18.3 km. Bruning

December 2022

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N. SAKURAI et al.
Table 2. Comparison of flash extents for four different storm types.
T1915

arithmetic mean
medianYoshida et al. (2018)

typhoon FAXAI

summer lightning

winter lightning

winter lightning

20.6 km21.1 km

6.5 km
5.1 km16.9 km14.8 km19.8 km18.3 kmand MacGorman (2013), using LMA observations oflarge Great Plains storms in the central U.S., reported
flash widths/extents up to 20 – 30 km. The basic conclusion is that the flash extents depend on the size ofthe storms.
4.

Zheng et al. (2019)

Discussion and conclusions

4.1 Storm charge structure and the cause of +CGs
From the radar and lightning comparisons of Fig. 7,
the storm cells appear to have been normally electrified, with IC flashes occurring between mid-level negative and upper positive storm charges, produced by
the standard non-inductive graupel–ice electification
mechanism. This is supported by the VHF sourcesindicating upward negative development at the very
beginning of the flashes, followed by horizontal propagation through upper positive charge. However, there
is no evidence of opposite polarity positive breakdown
propagating bidirectionally into and through mid-level
negative charge, as is usually seen during normal polarity IC flashes, albeit on a weaker and delayed basis.
In addition, storms having a normal polarity electricalstructure invariably produce negative CG discharges(e.g., Krehbiel 1986; MacGorman and Rust 1998), forwhich the downward negative leaders to ground arereadily detected at VHF (e.g., Shao et al. 1995; Sakurai
et al. 2021). Instead, negative CG strokes were conspicuously absent, replaced instead by a large percentage of positive strokes to ground. This is unheard of
in normally electrified storms in their convective
stage of development. Such lightning activity is not
explained by the storms being anomalously electrified,
as in Great Plains storms studied during the SevereThunderstorm Electrification and Precipitation Study(STEPS 2000; Lang et al. 2004a). In such storms, the+CG flashes are infrequent and vastly outnumbered
by IC flashes, with the IC flashes being inverted inpolarity between mid-level positive and upper-level
negative charges (e.g., MacGorman et al. 2005). And
not necessarily by winter storms, whose +CG discharges originate in horizontally distributed separate
layers of positive and negative charges, at relatively
low altitudes above sea level (e.g., Wang et al. 2021;
Wu et al. 2021).

Figure 9 shows the lightning-inferred charge structure during a 15-min time interval at the beginning of
the decaying stage of T1915, when the lightning wasclosest to the Tokyo LMA. Seven flashes occurred
during the time interval, beginning with the example
flash of Fig. 6 and including the three flashes used
for the radar comparsion in Fig. 7. Unlike the radar
overlay, where the exaggerated vertical scale makes
the lightning sources appear to be compressed horizontally and expanded vertically, the sources in thevertical projections of Fig. 9 are plotted with a 1:1
aspect ratio, showing that in actuality they were highly
layered vertically and horizontally extensive. The redsources correspond to negative breakdown propagating through upper positive charge. (The scatteredadditional sources are the same, but are slightly mis­located in altitude.) Consistent with the flash extent
determinations, the flashes developed over areal
extents up to ~ 40 km north to south and ~ 20 km east
to west. The large extents are indicative of northwardand eastward advection of the upper positive charge
away from the convective core, as seen in the planview of Fig. 7. Unusually, no sources could be identified as being due to positive breakdown throughmid-level negative charge.
To help understand how the positive strokes toground occur, it is instructive to look at how othertypes of +CG discharges are produced. Particularly
useful in this regard are high-peak current +CG
strokes that occur in the trailing stratiform regions oflarge mesoscale convective systems (MCCs), which
are sufficiently energetic to produce luminous sprites
in the upper atmosphere. From lightning charge centerstudies of similar, horizontally extensive stratiformdischarges in Florida, energetic +CGs were found tooccur as a result of negative breakdown propagating
through positive charge just above the radar-detected
bright band at the 0°C melting level (Krehbiel 1981;
Section 3.3). The continuosly propagating breakdown
was shown to effectively transfer positive charge backto the opposite end of the conducting leader channels,
where it builts up to the point of eventually initiating adownward positive leader to ground and a +CG stroke.
That the same mechanism produces +CG discharges

990

Journal of the Meteorological Society of Japan

Vol. 100, No. 6

Altitude (km)TOKYO LMA on 09 Sept 2019
20

10

0
06:57:20

07:00:40

07:04:00

07:07:20

07:10:40

North-South distance (km)Altitude (km)

20
15

15

10

10

5

5

0
90

0
100

110

120

130

770 pts140

alt-histogram

50

50

40

40

30

30

20

20

10

10
90

100

110
120
130
East-West distance (km)

140

0

5
10 15 20
Altitude (km)Fig. 9. LMA-inferred distribution of upper positive charge (red sources) corresponding to seven flashes at the beginning of the decaying stage of T1915, showing the layered nature and large horizontal extent of the charge causedby downwind advection away from the convective core. Black dots show the flash initiation locations above thecore. Scattered green sources are slightly mislocated events. (Sources are six or more station solutions with chisquare ≤ 1.)

in trailing stratiform regions of MCCs is readily seenin LMA observations of trailing stratiform lightning,such as those studied during STEPS 2000 (Lang et al.
2004b). In particular, NLDN-detected high-peakcurrent +CG strokes are found to occur several kilometers behind the leading edge of the flash’s negative
breakdown, often at successive times and locations as
the negative breakdown continues propagating.
In the context of the present study, rather than propagating through positive charge above a low-altitude
radar bright band, as in stratiform situations, the negative breakdown of the IC flashes propagates throughupper positive storm charge. However, similar tothe lower altitude stratiform discharges, the negative

breakdown would be transferring an increasing
amount of positive charge back into the flash initiation
region. Normally, the positive charge would be deposited within the storm’s mid-level negative charge
by means of bidirectional positive leader activity.
However, if the negative charge were to be depleted
or displaced in comparison to the upper positive (e.g.,
by vertical wind shear), the accumulated or excess
positive charge would end up producing a positive
leader to ground, as in the stratiform +CG discharges.
Evidence that this is how the +CG strokes are produced in the typhoon flashes is provided by the +CG
strokes occurring relatively late in the flash’s development (approximately half to 2/3 of the way throughDecember 2022

N. SAKURAI et al.

the flash), providing time for positive charge to build
up in the flash start region and producing a positive
leader to ground. The resulting return stroke introduces ground potential into the storm, reinvigorating andextending the flash development and overall duration,
as seen in the blooming of Fig. 6. Not all IC flashes
would necessarily reach this threshold, but apparently
a large fraction reach this threshold (29/52 or 56 %of the flashes in the present study). The strength of
the resulting +CG stroke would be variable from one
flash to the next, as seen in the broad range of peakcurrents for different flashes in Fig. 5. The resulting
positive strokes are highly energetic, as evidenced
by their peak currents and a notable fraction being
doubly detected by the JLDN. Another indication of
the mechanism is that when a positive stroke does
occur, its JLDN ground strike location is relatively
close to the plan location of flash initiation – i.e.,
where the negative breakdown started and where thepositive charge would tend to be accumulated. This isseen, for example, in Fig. 6, where the JLDN source
for the +CG stroke in the plan view panel is near thelocation of the flash’s initial sources. In the Fig. 6case, the post-stroke breakdown propagated in multi­
ple directions away from its center initiation point.
Even more striking are flashes where the breakdown
propagated unidirectionally away from its initiation
point, yet the +CG stroke is back at the starting point.
An example of this was the 35th flash at 08:15:24 JST
on September 9 (not shown), which developed 10 km
unidirectionally away from its initiation point, thenproduced a doubly detected 116 kA +CG stroke backat and directly below the starting point, causing the
flash to extend its unidirectional breakdown for an
additional 10 km.
A possible cause for the main negative storm charge
appearing to be depleted is the negative and upper
positive charges being partially displaced horizontallyfrom each other by vertical wind shear. Evidence thatvertical shear has an effect on lightning in typhoons
has been reported by Corbosiero and Molinari (2002).
Also, Levin et al. (1996) showed that a good correlation existed between the fraction of positive CG flashes and wind shear in other types of thunderstorms.
4.2 Range of +CG peak currents
Another issue that is raised in the typhoon observations concerns the peak current values above which
sferics identified as +CG events are actual strokes
to ground, and below which they are weak positive
events that are misidentified +IC sferics. Using
blooming as an indicator of an actual stroke to ground,991

and benefiting from the fortuitous occurrence of a
relatively large number of +CGs, the peak transition
current appears to be approximately 25 kA. A related
question is whether non-return stroke IC events canoccur at or above the transition value. The rapid
decrease in the numbers of both positive and negative
ICs at magnitudes above 15 kA in Fig. 5 indicatesthat IC peak currents above 25 kA are rare. Four such
events were identified to occur in the JLDN data:
flash 7 at 07:08:55 JST (a +51-kA event), flash 13 at
07:25:35 JST (a +42-kA event), flash 25 at 07:48:44
JST (a +51-kA event), and flash 38 at 08:20:11 JST(a +63-kA event) on September 9, all of which wereclassified as +ICs. However, due to producing strong
blooming like that of +CGs of similar current magnitude, they were almost certainly misidentified +CGs,
supporting the idea that high-peak current IC events(comparable to return stroke currents) do not occur.
4.3 Summary
The results of this study provide the first good understanding of eyewall lightning over a large area inthe decaying phase of a typhoon, obtained from joint
observations provided by the Tokyo LMA, the JLDN
lightning detection network, and the Tokyo-based
CDR of the JMA. The storm was highly unusual inthat it produced a high percentage of +CG flashes, despite appearing to be normally electrified. The results
are summarized as follows:• Even in the decaying stage of the typhoon, thestorms were strongly convective and normallyelectrified, in the same manner as other convective
storms, namely by the non-inductive graupel–ice
mechanism.
• That the otherwise normally electrified storm
produced positive instead of negative CG flashes
indicates that the convective core of the storm had adepleted mid-level negative charge.
• A possible explanation for the depletion is thenegative and upper positive charges being displaced
horizontally from each other by vertical wind shear.
• The storm’s upper positive charge was advected
downwind from above the convective core bystrong upper-level winds, giving rise to horizontallyextensive (up to 30 – 40 km) flash extents.
• The relative lack of mid-level negative charge resulted in individual IC flashes appearing to produce
an increasing amount of positive charge back in the
flash initiation region during the flash development,
eventually causing positive breakdown downward
to ground and producing the +CG strokes.
• The high percentage of energetic +CG strokes to992

Journal of the Meteorological Society of Japanground can be explained by the flashes all being IClike, with every flash having a chance of continuing
to ground (29/52 or 56 % in the present study).
• The resulting +CGs have a wide range of peakcurrents due to the positive charge accumulation
changing from flash to flash, and not occurring in
all flashes.
• The above explanation is the same basic processthat produces sprite-producing +CG discharges inthe trailing stratiform region of mesoscale convective complexes.
• A similar explanation may be applicable to +CG
flashes in winter storms, whose +CG discharges
originate in horizontally extensive regions of positive charge at relatively low altitudes above meansea level.
• The peak current value above and below which
actual +CGs and misclassified weak +CG eventsoccur appears to be in the 20 – 25 kA range.
Data Availability Statement
The XRAIN data are available for download from
DIAS at https://diasjp.net/service/xrain-data/. The
JLDN data are not publicly available due to the management policy of Franklin Japan Co. Ltd. The otherdatasets generated and/or analyzed in this study areavailable from the corresponding author on reasonableusage upon request.
Acknowledgments
The authors are grateful to the editor and two anonymous reviewers for their comments that valuably
improved the paper. We also thank the Innovation HubConstruction Support Project of the Japan Science andTechnology Agency (JST) for supporting the acquisition of the Tokyo LMA. We are grateful to Dr. Kyle
C. Wiens for supporting in the analysis using radar
data and LMA data. The eXtended Radar Information
Network (XRAIN) data sets used for this study areprovided by the Ministry of Land, Infrastructure,
Transport and Tourism, Japan. This dataset was alsocollected and provided under the Data Integration andAnalysis System (DIAS), which was developed andoperated by a project supported by the Ministry of
Education, Culture, Sports, Science and Technology.
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