RESEARCH ARTICLE
10.1029/2018JD029907
Key Points:• A type of special IC ﬂash initiated athigh altitudes (>12 km) called“downward +IC ﬂash” is reported
for the ﬁrst time
• Downward +IC ﬂashes are
dominated by downward positive
leaders• Downward +IC ﬂashes areproduced in thunderstorms withdeep convective updrafts (cloud tops
higher than 14 km)
Supporting Information:• Supporting Information S1

Correspondence to:T. Wu,
tingwu@gifu‐u.ac.jp

Citation:
Wu, T., Wang, D., & Takagi, N. (2019).
Intracloud lightning ﬂashes initiated athigh altitudes and dominated by
downward positive leaders. Journal ofGeophysical Research: Atmospheres,124. https://doi.org/10.1029/
2018JD029907
Received 29 OCT 2018
Accepted 2 JUN 2019
Accepted article online 17 JUN 2019

Intracloud Lightning Flashes Initiated at High Altitudes
and Dominated by Downward Positive Leaders
Ting Wu1, Daohong Wang1, and Nobuyuki Takagi1

1

Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu, Japan

Abstract Intracloud (IC) lightning ﬂashes are normally initiated below 10 km and start with upward
negative leaders. In this paper, we report a special type of IC ﬂash called “downward positive IC(+IC) ﬂash” which is initiated at high altitudes (mainly above 12 km) and whose initial negative leaders
do not propagate upward. Three‐dimensional location results of three downward +IC ﬂashes aredescribed in detail. It is demonstrated that downward +IC ﬂashes start with positive leaders propagating
downward with speeds on the order of 104 m/s and negative leaders propagating horizontally for only ashort distance. Downward +IC ﬂashes are produced in thunderstorms with deep convective updrafts(radar echoes of cloud tops typically higher than 14 km). The charge structure responsible fordownward +IC ﬂashes is inferred to be a positive dipole including a negative charge region at a normal
altitude (near the −10 °C isotherm) and an upper positive charge region at a relatively high altitude(usually above the −50 °C isotherm), with downward +IC ﬂashes likely initiated from the upper
positive charge region. Further, lightning ﬂashes in a thunderstorm producing a large number ofdownward +IC ﬂashes are analyzed. Results show that normal IC ﬂashes in this thunderstorm are also
initiated at altitudes closer to the upper positive charge region and usually consist of downward positive
leaders propagating for longer distances than upward negative leaders. Based on these results, we
propose a relationship between the altitude of the upper positive charge region and initiation locationsof IC ﬂashes.
Plain Language Summary Intracloud (IC) lightning ﬂashes are normally initiated below 10 km.
In this paper, we report a special type of IC ﬂash called the “downward positive IC (+IC) ﬂash” which is
initiated at high altitudes (mainly above 12 km). The structure of downward +IC ﬂashes is distinctly
different from that of normal IC ﬂashes. Downward +IC ﬂashes are produced in vigorous thunderstorms
with cloud tops higher than 14 km. However, downward +IC ﬂashes tend to produce weak radiation in radio
frequencies, rendering them difﬁcult to detect. This may be the reason that downward +IC ﬂashes havenever been reported before.

1. Introduction
It is believed that the majority of lightning ﬂashes are intracloud (IC) ﬂashes, which occur inside or between
thunderclouds and do not contact the ground. Due to the fact that IC ﬂashes cannot be effectively observedby video cameras, they have been mainly studied via their radio emissions (e.g., Krider et al., 1975;
Villanueva et al., 1994; Weidman & Krider, 1979). IC ﬂashes have various morphologies (see a review by
Rakov & Uman, 2003, chapter 9). The most common morphology is the bilevel structure as ﬁrst revealed
by lightning interferometer observations (Shao & Krehbiel, 1996) and later conﬁrmed by observations with
the Lightning Mapping Array (LMA; e.g., Rison et al., 1999; Riousset et al., 2007). It is now clear that normal
polarity IC ﬂashes (i.e., positive IC ﬂashes or +IC ﬂashes) with the bilevel structure usually start with initial
negative leaders propagating upward with speeds on the order of 105 m/s (Behnke et al., 2005; Wu
et al., 2015).

©2019. American Geophysical Union.
All Rights Reserved.

WU ET AL.

Initiation altitudes of most IC ﬂashes imaged by the LMA and some low frequency lightning locating systems are below 10 km (e.g., Behnke et al., 2005; Caicedo et al., 2018; Karunarathna et al., 2017; Lyu et al.,2016; van der Velde & Montanyà, 2013; Yoshida et al., 2014). Calhoun et al. (2013) reported that the majorityof lightning ﬂashes in a supercell were initiated near altitudes of 10 to 11 km and noted that such supercell
was “unusual” compared to other storms. In fact, other than some special types of lightning discharges that
do not comprise clear leader channels, such as narrow bipolar events (NBEs; e.g., Smith et al., 2004; Wu

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et al., 2012) and discharges in the overshooting tops (e.g., MacGorman
et al., 2017), we can ﬁnd few reports of full‐ﬂedged IC ﬂashes initiatedat high altitudes such as above 12 km.
On the other hand, according to some statistical distributions of the lightning initiation altitude, there is indeed a small portion of lightning ﬂashes
initiated at high altitudes of up to 15 km (Fuchs et al., 2016; Lund et al.,2009; Mecikalski & Carey, 2017). However, development characteristicsof these high‐altitude ﬂashes and the corresponding charge structures
are not clear yet.

Figure 1. Locations of three ﬂashes analyzed in this paper relative to 12 sites(black squares) of FALMA during the summer of 2017. The origin (0, 0)
corresponds to the latitude and longitude of (35.475°E, 136.960°N).

In this paper, we will report a special type of IC ﬂash initiated at highaltitudes, mainly between 12 and 15 km. We will analyze three cases ofthese ﬂashes produced in three different thunderstorms and demonstrate that these ﬂashes are fundamentally different from normal IC
ﬂashes. They always consist of initial positive leaders propagating
downward with speeds on the order of 104 m/s. At the same time, however, their initial negative leaders do not propagate upward as in normal IC ﬂashes but propagate horizontally for only a short distance.
As will be demonstrated in this paper, these special IC ﬂashes are associated with the same charge structure (the positive dipole) as normal
polarity IC ﬂashes, that is, +IC ﬂashes, but their overall development
is in a downward direction. Therefore, we will call them “downward+IC ﬂashes.”

2. Observation and Data
Data of this study were obtained during the summer of 2017 using the Fast Antenna Lightning Mapping
Array (FALMA), a lightning mapping system working in the low frequency band. FALMA can image
lightning ﬂashes with great details and high accuracy similar to the LMA. Details of FALMA and examples of lightning ﬂashes imaged by the FALMA can be found in Wu et al. (2018). Twelve sites were set up
during the summer of 2017 as shown in Figure 1. Sources of three ﬂashes analyzed in this paper are alsoplotted in Figure 1. All ﬁgures with location information in this paper use the same coordinate as
Figure 1.
In all ﬁgures showing location results of lightning ﬂashes in this paper, electric ﬁeld change (E‐change)
waveforms are also shown. As the magnitude of E‐changes has not been calibrated, it is shown in the digital
unit. The atmospheric sign convention is used in this paper, so a negative return stroke produces apositive E‐change.
In addition, radar data of a nationwide C‐band Doppler radar network operated by the Japan
Meteorological Agency are used in this study. These data have a spatial resolution of 1 km in both horizontal and vertical directions and a temporal resolution of 10 min. The resolutions are relatively low, so
we can only get a general sense of the features of thunderstorms from the radar data. Another problem isthat the data only cover altitudes up to 14 km, so cloud top heights of thunderstorms higher than 14 km
cannot be determined.
Normal IC ﬂashes start with an upward negative leader and a horizontal or downward positive leader, forming a bilevel structure. The positive leader usually cannot be directly detected but can be inferred fromsources of negative recoil leaders. Figure S1 in the supporting information shows an example of a normal
IC ﬂash imaged by the FALMA. The initial negative leader ﬁrst propagated upward and then turned in
the horizontal direction. The positive leader mainly propagated horizontally but also had a slightly downward movement. This example demonstrates that FALMA can readily detect recoil leaders and show thedevelopment of positive leaders in IC ﬂashes.
The downward +IC ﬂashes reported in this paper are fundamentally different from normal IC ﬂashes, suchas that depicted in Figure S1, in that downward +IC ﬂashes start with downward positive leaders but without any upward negative leader.

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Figure 2. Location results of the ﬁrst example of a downward +IC ﬂash (Flash One). (a) Electric ﬁeld change waveform.
(b) Height‐time view. (c) Height‐distance (from west to east) view. (d) Plan view. (e) Distance (from south to north)‐heightview. (f) Source distribution along the height. The blue triangle represents the ﬁrst located source of this ﬂash.

3. Results
3.1. Three‐Dimensional Structures of Downward +IC Flashes
3.1.1. Flash One
The ﬁrst example of a downward +IC ﬂash is shown in Figure 2. It had a duration of about 961 ms. The ﬁrst
source of this ﬂash was located at an altitude of 13.6 km, indicated by the blue triangle in Figure 2. From theinitiation location, we can see that some sources gradually moved downward as indicated by the red arrowand some sources moved mainly horizontally as indicated by the blue arrow. In the following analysis, wewill demonstrate that the downward‐moving sources indicate the development of a downward positive leader, and according to the bidirectional leader concept (e.g., Mazur & Ruhnke, 1993), the sources moving in
the horizontal direction indicate the development of a negative leader.
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Figure 3. Location results of a recoil leader in Figure 2. Subplots are arranged as in Figure 2. Sources in black are thoseoccurred before the recoil leader.

At the time of about 854 ms, a leader was clearly imaged propagating upward (labeled as RL1 in Figure 2b).
Details of this leader are plotted in Figure 3. Sources before this leader are plotted in black. We can see that
this leader retraced the old channel and progressed back to the origin of this ﬂash. This leader produced predominantly negative pulses (using the atmospheric sign convention), indicating that it carried negative
charges upward. Based on these characteristics, it is clear that this was a negative recoil leader similar to thatin normal IC ﬂashes. Accordingly, the old channel it retraced was produced by a positive leader. Note thatthe speed of this recoil leader was about 5.9 × 105 m/s, which is relatively slow for a recoil leader. It seems
that recoil leaders well located by the FALMA are typically those with low speeds. In Figure 2b, we can seethat at the end of this ﬂash, these was another recoil leader (labeled as RL2), which retraced the same channel as RL1. The speed of RL2 is estimated to be about 4.9 × 106 m/s, typical for a recoil leader, but only a few
sources were located.
Therefore, we conclude that this ﬂash was initiated at an altitude of 13.6 km and started with a downward
positive leader and a horizontal negative leader. The downward‐moving sources were likely produced bynegative recoil leaders retracing the positive leader channel. We can roughly estimate the downward
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Figure 4. Distance versus time of sources above the black dashed line in Figure 2 (sources of the initial negative leader).
Distance is relative to the origin of the ﬂash (the blue triangle in Figure 2). Sources are color coded with the same timescale as those in Figure 2b.

Figure 5. Sources of Flash One superimposed on the radar echo. (a) and (b) show radar reﬂectivity at the time of 18:20 (about 7 min before the ﬂash). (c) and (d)show radar reﬂectivity at the time of 18:30 (about 3 min after the ﬂash). (a) and (c) show radar reﬂectivity at a constant height of 2 km. (b) and (d) show range‐heightindicator of radar reﬂectivity at the white lines.

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Figure 6. Location results of the second example of a downward +IC ﬂash (Flash Two). Subplots are arranged as in
Figure 2. The blue triangle represents the ﬁrst located source of this ﬂash. RL = recoil leader.

speed of the positive leader from the descending speed of these sources. The speed of the positive leader isestimated to be 1.7 × 104 m/s, represented by the slope of the purple dashed line in Figure 2b.
The initial negative leader, which developed horizontally, apparently had a much lower speed than normal negative leaders. Figure 4 shows the distance‐time view of the sources of the negative leader relative
to the origin of this ﬂash. Sources above the black dashed line in Figure 2b are selected as those associated with the negative leader. Sources in Figure 4 are color coded with the same time scale as those in
Figure 2b. The speed of the negative leader is roughly estimated to be 3.0 × 104 m/s, represented by theslope of the purple dashed line in Figure 4. This value is an order of magnitude smaller than the typical
speed of negative stepped leaders. Note that the initial variation of the distance near time zero was due tothe random height error of a few sources at the beginning of this ﬂash and was not associated with the
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Figure 7. Sources of Flash Two superimposed on the radar echo. (a) Radar reﬂectivity at a constant height of 2 km. (b) Range‐height indicator of the radar reﬂectivity at the white line.

development of the negative leader. It should also be noted that after the time of about 80 ms, thenegative leader had little growth, while the positive leader continued growing. However, it is possiblethat the sources of the presumed initial negative leader were actually produced by recoil leaders. It
seems likely from Figures 2 and 4 that the sources of the initial negative leader did not develop
forward spontaneously but was pushed forward by recoil leaders. Currently, we are uncertain of thetrue physical process of initial negative leaders in downward +IC ﬂashes, but they are apparently
different from initial negative leaders in normal IC ﬂashes.
After the time of about 600 ms, the positive leader turned horizontally at the altitude of 6 to 7 km, andseveral branches can be recognized in the plan view in Figure 2d. The horizontal propagation was likely
in a negative charge layer, and it indicates that the negative charge layer was at the altitude of 6 to 7 km.
Figure 2c also shows altitudes of 0 to −60 °C isotherms. The isotherm altitudes were obtained by radiosonde observations by the Japan Meteorological Agency at an observation site about 200 km from ourobservation area, so they were extremely rough estimations. However, we can see that the inferred negative charge layer was close to the −10 °C isotherm, as expected from the noninductive charging mechanism (e.g., Takahashi, 1978). Similarly, the horizontal negative leader can also indicate the positive charge
region. It seems that the positive charge region was at an altitude of about 14 km, near the −60 °C isotherm. This is generally higher than the upper positive charge region normally found in thunderstorms(e.g., Stolzenburg & Marshall, 2008).
Figure 5 shows the radar reﬂectivity of the thunderstorm producing this ﬂash. Figures 5a and 5c show thereﬂectivity at the altitude of 2 km, and Figures 5b and 5d show the range height indicator (RHI) plot at
the white lines in Figures 5a and 5c. As the temporal resolution of the radar data is 10 min, the data before
the ﬂash (about 7 min before the ﬂash, Figures 5a and 5b) and after the ﬂash (about 3 min after the ﬂash,
Figures 5c and 5d) are both plotted to show the movement of the thunderstorm. We can see that the storm
moved in the southeast direction. From the RHI plots (Figures 5b and 5d), we can see that the storm contained a small region of deep convective updrafts extending above 14 km. By comparing Figures 5b and5d, it seems very likely that the ﬂash was initiated right at the deep convective region. Note that the radar
data only cover the altitude below 14 km, so we have no way to know the altitude of the storm top.
However, the reﬂectivity at 14 km altitude in Figures 5b and 5d was larger than 30 dBZ, so it is very likelythat this ﬂash occurred inside a very vigorous thunderstorm.
3.1.2. Flash Two
The 3‐D location result of the second example is shown in Figure 6. This downward +IC ﬂash had a durationof about 1,198 ms. The ﬁrst source was located at an altitude of 14.3 km as indicated by the blue triangle in
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Figure 8. Location results of the third example of a downward +IC ﬂash (Flash Three) along with a −CG ﬂash following
it. Subplots are arranged as in Figure 2. The blue triangle represents the ﬁrst located source of thedownward +IC ﬂash. The square represents the ﬁrst located source of the −CG ﬂash. The cross sign represents a returnstroke of the −CG ﬂash. –CG = negative cloud‐to‐ground.

Figure 6. Note that the sources at the beginning of this ﬂash were scattered, so we are uncertain of the exact
initiation altitude of this ﬂash. This seems to be a common problem when locating downward +IC ﬂashes. It
seems that pulses at the beginning of downward +IC ﬂashes are especially small and sparse, making it
difﬁcult to determine the exact beginning of a ﬂash.
From Figure 6b, we can see that sources gradually moved downward before the time of 600 ms, indicating
the downward development of a positive leader similar to that in Flash One. The downward speed of thepositive leader is estimated to be 1.6 × 104 m/s, represented by the slope of the purple dashed line in
Figure 6b. After the time of about 600 ms, the positive leader mainly developed in the horizontal direction,
as can be seen from Figures 6d and 6e. Meanwhile, at least four recoil leaders were well located and are
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Figure 9. Sources of Flash Three (black) and the subsequent negative cloud‐to‐ground ﬂash (blue) superimposed on the radar echo. (a) Radar reﬂectivity at a constant height of 2 km. (b) Range‐height indicator of the radar reﬂectivity at the white line.

labeled as “RL1” to “RL4” in Figure 6b. They were all upward negative recoil leaders. They all traversed thepositive leader channel and propagated from the main negative charge region to the origin of this ﬂash.
Their estimated vertical speeds are 3.4, 7.3, 2.5, 1.9 × 106 m/s.
The initial negative leader in this ﬂash was not very clear. In Figures 6c and 6e, some sources in green
extending outward may indicate the negative leader channel. We did not intend to calculate the speed of
the negative leader as only a few sources were detected and the channel was not very clear. However, thespeed of the negative leader was apparently much lower than that of the positive leader in this ﬂash.
Figure 6c also shows altitudes of 0 to −60 °C isotherms. Similar to Flash One, the main negative charge layer
indicated by the horizontal propagation of the positive leader was near the −10 °C isotherm. If we assume
that the positive charge region was near the origin of this ﬂash, it was higher than the −60 °C isotherm.
Figure 7 shows sources of this ﬂash superimposed on the radar echo. From the RHI plot in Figure 7b, we
can see that this ﬂash was indeed produced in deep updrafts extending above 14 km. The maximum reﬂectivity at the altitude of 14 km was about 24 dBZ. Similar to the ﬁrst example, it seems that the charge structure of this thunderstorm included a negative charge layer at a normal altitude (−10 °C isotherm) and asmaller positive charge region at a relatively high altitude (−60 °C isotherm).
3.1.3. Flash Three
In this example, a downward +IC ﬂash was followed by a negative cloud‐to‐ground (−CG) ﬂash. Location
results of the downward +IC ﬂash and the following −CG ﬂash are shown in Figure 8. The downward+IC ﬂash was initiated at an altitude of 13.3 km and lasted for about 899 ms. The −CG ﬂash was initiatedabout 11 ms after the end of the downward +IC ﬂash. The horizontal distance between initiation locationsof these two ﬂashes was about 5.6 km. Examination of Figure 8 reveals that although these two ﬂashes werevery near each other, they had no direct contact, and there is no evidence that the −CG ﬂash was initiated by
the downward +IC ﬂash. Both of these two ﬂashes indicated a negative charge layer at the altitude of 6 to 8km, corresponding to the −10 °C isotherm. The occurrence of the −CG ﬂash further conﬁrms that the downward +IC ﬂash consists of a positive leader ﬁrst going downward and then moving horizontally in the main
negative charge layer.
Another special feature of this ﬂash is that, during the ﬁrst 200 ms, sources were located at almost the same
location and did not show any clear development. It seems neither the positive nor negative leaders progressed forward during this period. After about 200 ms, some descending sources were detected, indicating
the development of a downward positive leader. The speed of the positive leader is estimated to be 2.3 × 104
m/s represented by the slope of the purple dashed line in Figure 8b. Currently, it is not clear what kind ofdischarges occurred during the ﬁrst 200 ms of this ﬂash. As mentioned in section 3.1.2, pulses at the
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Figure 10. Initiation locations of all lightning ﬂashes during 18:22:30–18:27:30 superimposed on the radar echo. Downward triangles (▽) represent downward+IC ﬂashes. Upward triangles (△) represent IC ﬂashes starting with upward negative leaders. Diamonds (⋄) represent IC ﬂashes starting with both upward
negative leaders and downward positive leaders. Cross signs (×) represent negative cloud‐to‐ground ﬂashes. Plus signs (+) represent isolated positive narrow bipolar
events. Circles (○) represent ﬂashes with no clear types. Blue vertical lines represent upward negative leaders, and red vertical lines represent downward positive
leaders. Flashes in (c) are those in the dashed boxes in (a) and (b). IC = intracloud.

beginning of downward +IC ﬂashes are especially small and infrequent and are thus difﬁcult to locate. It is
possible that the stationary discharges in this example are a common phenomenon for downward +IC
ﬂashes but were not well‐located in most cases.
The radar echo along with locations of this downward +IC ﬂash and the following −CG ﬂash are shown in
Figure 9. The cloud top was higher than 14 km, and the maximum reﬂectivity at 14 km was about 17 dBZ.
Similar to the ﬁrst two examples, we conclude that Flash Three was also produced inside a thunderstorm
with deep convective updrafts.
3.2. Lightning and Storm Characteristics When Downward +IC Flashes Occur
In order to further investigate the charge structure for the production of downward +IC ﬂashes and theirrelationship with other lightning ﬂashes, we examined lightning ﬂashes in a short period from 18:22:30 to
18:27:30 on 22 August in a thunderstorm with frequent downward +IC ﬂashes. Initiation locations of alllightning ﬂashes during this period are plotted over radar reﬂectivity at 18:20 and 18:30 as shown in
Figures 10a and 10b. Note that the radar reﬂectivity data in Figures 10a and 10b are the same as those in
Figures 5a and 5c. There are 48 downward +IC ﬂashes (represented by “▽”), 120 normal IC ﬂashes startingwith both upward negative leaders and downward positive leaders (represented by “⋄”), 26 normal IC
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Figure 11. Location results of lightning ﬂashes during about 3 s from 18:23:21.7 in the storm in Figure 10. Subplots arearranged as in Figure 2. At least seven ﬂashes can be identiﬁed. Initiation locations of these seven ﬂashes are labeled bynumbers. Location results of these seven ﬂashes are shown in Figures S2–S8.

ﬂashes starting with upward negative leaders but without downward positive leaders (represented by “△”),64 −CG ﬂashes (represented by “×”), 30 isolated positive NBEs (represented by “+”), and 33 ﬂashes with noclear types (represented by “○”). Excluding positive NBEs and ﬂashes with no clear types, a total of 258
ﬂashes occurred during the 5‐min period, indicating a relatively high ﬂash rate (e.g., compared to Liu
et al., 2012). According to the analysis in section 3.1, it has been clear that downward +IC ﬂashes areproduced in thunderstorms with high cloud tops. Therefore, the high ﬂash rate is an expected result
considering the theory that the ﬂash rate is correlated with the storm top height (e.g., Ushio et al., 2001).
Location results of lightning ﬂashes during about 3 s from 18:23:21.7 are further shown in Figure 11 for additional details. Close scrutiny of the 3‐D structure in Figure 11 reveals that there were at least seven independent ﬂashes, including four −CG ﬂashes, two downward +IC ﬂashes, and two normal IC ﬂashes with bothWU ET AL.

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upward negative and downward positive leaders. Initiation locations of these ﬂashes are labeled by numbers
in Figure 11 in accordance with the time sequence. Location results of these seven ﬂashes are shown in
Figures S2–S8. Figure 11 illustrates the feature that when downward +IC ﬂashes occur (and when thestorm top is very high), multiple lightning ﬂashes can occur simultaneously in different regions of thestorm, resulting in a high ﬂash rate. It should also be noted that without detailed location results, itwould be extremely difﬁcult to accurately group sources to ﬂashes when multiple ﬂashes occurred withina short period and within a small area, such as the case in Figure 11.
Lightning ﬂashes within dashed boxes in Figures 10a and 10b are further shown in Figure 10c with extensions of upward negative leaders (blue lines) and/or downward positive leaders (red lines). Negative leadersof −CG ﬂashes always moved downward to the ground and are not shown in Figure 10c. From Figure 10c, it
is clear that the main negative charge region was at the altitude of about 6 km (corresponding to the temperature of −10 to 0 °C according to Figure 2c) where −CG ﬂashes started. The upper positive charge region
was at the altitude of about 15 km but had some variations. Altitudes of positive NBEs were also in agreement with an upper positive charge region at about 15 km (Wu et al., 2012). This result further conﬁrms thata negative charge region at a normal altitude and an elevated upper positive charge region are responsible forthe production of downward +IC ﬂashes.
It is also interesting to note that normal IC ﬂashes with only upward negative leaders (such as the example in Figure 3 in Rison et al., 1999) are rare in Figure 10. Most IC ﬂashes started with both upward negative leaders and downward positive leaders, and the downward positive leaders usually extended for alarger distance (red lines are generally longer than blue lines in Figure 10c). A typical example is given in
Figure 12. Two more examples can be found in Figures S2 and S7. It seems that in thunderstorms withfrequent downward +IC ﬂashes, normal IC ﬂashes also tend to start at an altitude closer to the upper
positive charge region. In other words, as the upper positive charge region is lifted by strong updrafts,
initiation locations of IC ﬂashes are also lifted, even though the main negative charge region remains
unchanged. On the other hand, we did not notice any special characteristics of −CG ﬂashes in thunderstorms with frequent downward +IC ﬂashes, probably because the production of −CG ﬂashes is not closely related with the upper positive charge region.

4. Discussion
4.1. Relationship Between Charge Layer Altitude and IC Flash Morphology
Proctor (1997) compared lightning ﬂashes with low origins (1 to 7.4 km) and those with high origins (7.4 to
12 km) and found that channel extension speed was generally lower for high origin ﬂashes. Wu et al. (2015)
analyzed initial negative leaders in 662 IC ﬂashes and demonstrated that the upward speed of initial negative
leaders decreased as the initiation altitude increased. These results made us speculate that, if the initiation
altitude is high enough, the initial negative leader of IC ﬂashes would not even develop upward, and resultsof the current study conﬁrm this speculation. Combining the results of previous studies and the current
study, we have a complete picture of IC ﬂashes in which characteristics of IC ﬂashes gradually change as
the initiation altitude increases, and we suspect that the increase of the initiation altitude is a direct resultof the elevation of the upper positive charge region as illustrated in Figure 13 and described in thefollowing paragraphs.
In relatively weak thunderstorms, IC ﬂashes are usually dominated by an upward negative leader. The
positive leader usually propagates horizontally (scenario A in Figure 13) or has a slightly downward
movement (scenario B in Figure 13). The IC ﬂash in Figure 3 of Rison et al. (1999) and the example in
Figure S1 are typical examples. In fact, we believe that most normal‐polarity IC ﬂashes reported in theliterature belong to these two types. In relatively strong thunderstorms, the upper positive charge region
is lifted to higher altitudes due to strong updrafts, and initiation altitudes of IC ﬂashes also increase. IC
ﬂashes initiated at relatively high altitudes tend to be dominated by downward positive leaders. Usually
upward negative leaders still exist, but the upward speed is relatively low (scenario C in Figure 13). The
ﬂash in Figure 12 is a typical example; the speed of the upward negative leader was only about 1.9 ×
104 m/s. Occasionally, downward +IC ﬂashes as reported in this paper are produced. They are likely
initiated from the upper positive charge region. In downward +IC ﬂashes, negative leaders no long
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Figure 12. Location results of an intracloud ﬂash starting with both upward negative leaders and downward positive leaders. Subplots are arranged as in Figure 2. The diamond represents the ﬁrst located source of the ﬂash.

travel upward; they usually move horizontally for a short distance with a low speed (scenario D in
Figure 13).
It is important to note that in the illustration of Figure 13, while the upper positive charge region is lifted due
to strong updrafts, the main negative charge region generally remains at the same altitude. This is based on
the results of the present study and is also consistent with previous observations (Byrne et al., 1983; Krehbiel
et al., 1984).
We also speculate that the upper positive charge region has a relatively small horizontal extent when downward +IC ﬂashes are produced (scenario D in Figure 13). This is based on the observation that negative leaders (propagate in positive charge regions) in downward +IC ﬂashes propagate for short distances. It can
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Figure 13. Illustration of different structures of intracloud ﬂashes with different altitudes of the upper positive charge
region. Blue curves represent negative leaders. Red curves represent positive leaders. Diamond signs represent initiation
points.

also be inferred from Figure 5 that downward +IC ﬂashes are likely associated with particularly strong
updrafts in a narrow region, which may result in a lifted upper positive charge region with a limited
horizontal extent.
The illustration in Figure 13 also has limitations. First, the thickness of charge layers is not considered in
Figure 13. It is possible that the upper positive charge region may extend downward or the main negative
charge region may extend upward. Second, Figure 13 only shows charge regions that are responsible forthe detected lightning ﬂashes; other charge regions such as the screening charge layer and the lower positive
charge region may also exist. The evidence that the upper positive charge region is relatively small in scenario D is also not conclusive.
4.2. Remaining Questions
As this is the ﬁrst study of the downward +IC ﬂash, there are still many questions remaining to be answered.
We will put forward three questions to which we currently do not have clear answers.
1. Why do IC ﬂashes rarely initiate near the main negative charge region when downward +IC ﬂashes occur?
From Figure 10c, it seems that most IC ﬂashes are initiated closer to the upper positive charge region.
This implies that as the upper positive charge region is elevated, initiation locations of IC ﬂashes are alsobrought upward. The main negative charge region remains at the same altitude, and −CG ﬂashes occur
as normal. It is an unexpected result that IC ﬂashes are no longer initiated near the main negative charge
region.
2. Does the net charge of positive and negative leaders in the downward +IC ﬂash equal to zero? A basic
assumption in modeling lightning channels is the overall neutrality, that is, positive and negative leaders
originating from the neutral point carry equal amount of charges with opposite polarities (e.g., Kasemir,
1960; Riousset et al., 2007). According to examples of downward +IC ﬂashes described in this paper, it
seems obvious that positive leaders usually propagate for much larger distances and have many morebranches than negative leaders. If we apply a simple electrostatic model assuming a stationary neutral
point such as that described by Tran and Rakov (2017), we would get the result that the charge density
in negative leader channels are many times larger than that in positive leader channels and keeps
increasing as the ﬂash develops.
3. Do downward +IC ﬂashes start with preliminary breakdown (PB)? Although it has not been examined indetail, E‐change pulses at the beginning of downward +IC ﬂashes usually seem to be small, narrow, andsparse and are distinctly different from PB pulses reported in the literature (e.g., Ma, 2017; Marshall et al.,2014; Wu et al., 2013). (E‐change waveforms of the three downward +IC ﬂashes analyzed in section 3.1
are provided in Figures S9–S11.) This is in fact an expected result according to Wu et al. (2015), whichshowed that the PB of IC ﬂashes initiating at higher altitudes generally produced less frequent PB pulses
with smaller amplitudes and smaller pulse widths. Similar results have also been reported by Proctor(1997). If normal PB pulses are no longer detected, can we say that the lightning ﬂash does not start
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with PB? Or is it possible that the same physical process of PB still exists, but PB at high altitudes does notproduce normal PB pulses?

5. Conclusions
In this paper, we reported a special type of IC ﬂash called downward +IC ﬂash. Downward +IC ﬂashes arefundamentally different from normal IC ﬂashes in that they do not contain upward initial negative leaders asfound in normal IC ﬂashes. Instead, they are dominated by downward positive leaders with speeds on theorder of 104 m/s. Initial negative leaders still exist, but they only have horizontal propagations with speeds
1 order of magnitude smaller than normal negative stepped leaders, and they usually develop for a muchsmaller distance than initial positive leaders do.
Another important feature of downward +IC ﬂashes is that they are usually initiated at higher altitudes thannormal IC ﬂashes. The majority of downward +IC ﬂashes are initiated above 12 km, while most normal IC
ﬂashes reported in the literature are initiated below 10 km.
The charge structure responsible for downward +IC ﬂashes is a positive dipole including a negative charge
region at a normal altitude (near the −10 °C isotherm) and a positive charge region at a relatively high altitude (usually above the −50 °C isotherm), and downward +IC ﬂashes are likely initiated from the upper
positive charge region.
A short period in a thunderstorm producing a large number of downward +IC ﬂashes is analyzed. When
downward +IC ﬂashes frequently occurred, −CG ﬂashes occurred as normal and did not show any special
characteristics. Other IC ﬂashes, however, were somewhat special in that they were usually initiated at altitudes closer to the upper positive charge region and consisted of downward positive leaders more prominent
than upward negative leaders.
The occurrence of downward +IC ﬂashes is an expected result according to the ﬁnding of Wu et al.
(2015) that the upward speed of initial negative leaders in IC ﬂashes decreases as the initiation altitude
increases. We further propose a possible relationship between the altitude of the upper positive charge
region and the morphology of IC ﬂashes illustrated in Figure 13. In relatively weak thunderstorms, IC
ﬂashes are usually initiated near the main negative charge region and are dominated by upward
negative leaders. As the convective strength increases, the upper positive charge region is lifted to higher
altitudes, and initiation altitudes of IC ﬂashes also increase. In relatively strong thunderstorms, IC
ﬂashes are initiated near the upper positive charge region and are dominated by downward
positive leaders.
Further studies based on a large sample of downward +IC ﬂashes are needed. However, there are difﬁculties
in conducting a statistical study. As can be seen in the three examples described in section 3.1, location
results at the beginning of downward +IC ﬂashes are usually not very good. This is due to the fact that
E‐change pulses at the beginning of downward +IC ﬂashes are usually very small and sparse. As shownin Figures S9–S11, there were only a few pulses during the beginning 100 ms of the three downward +IC
ﬂashes; at many sites, these initial pulses were buried in the background noise and could hardly be recognized. This feature makes it very difﬁcult to accurately locate the beginning part of downward +IC ﬂashes.
It is particularly difﬁcult to accurately determine the initiation altitude, which is essential for a statistical
study. We also speculate that the difﬁculty in locating downward +IC ﬂashes is the reason that they havenever been reported before.

Acknowledgments
This work was supported by the
Ministry of Education, Culture, Sports,
Science, and Technology of Japan(Grants 15H02597, 16H04315, and
18K13618). Location results of downward +IC ﬂashes analyzed in this paper
can be downloaded online (http://doi.
org/10.5281/zenodo.1473170).

WU ET AL.

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