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  • View in gallery

    Schematic diagram of the VHF lightning observation system.

  • View in gallery

    The VHF lightning observation system.

  • View in gallery

    Typical waveform of VHF broadband lightning pulses.

  • View in gallery

    Configuration of three antennas: (a) isosceles right-triangular array and (b) equilateral-triangular array.

  • View in gallery

    Schematic description of the geometry of (a) two antennas and VHF radiation source and (b) three antennas and VHF radiation source.

  • View in gallery

    Schematic diagram for 3D mapping.

  • View in gallery

    (top) Location of observation sites. (bottom) The map shows the Shonai area, and the stars indicate the lightning observation sites. The plus sign denotes the X-band Doppler radar at Shonai airport, and the crosses indicate the network of 26 surface weather stations.

  • View in gallery

    Lightning observation site L1.

  • View in gallery

    Azimuth–elevation mapping of VHF radiation sources observed at sites (top) L1 and (bottom) L4.

  • View in gallery

    3D mapping of VHF radiation sources and radar echo observed with Meteorological Research Institute (MRI) X-band Doppler radar.

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Three-Dimensional VHF Lightning Mapping System for Winter Thunderstorms

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  • 1 * Alpha-denshi Co., Ltd., and Meteorological Research Institute, Tsukuba, Ibaraki, Japan
  • 2 Meteorological Research Institute, Tsukuba, Ibaraki, Japan
  • 3 East Japan Railway Company, Saitama, Japan
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Abstract

A three-dimensional (3D) winter lightning mapping system employing very high frequency (VHF) broadband signals was developed for continuous remote observation in winter. VHF broadband pulses radiated by leader progression are received with three discone antennas arranged in a triangle (20–30 m) and recorded on a high-speed digital oscilloscope (1.25-GHz sampling) with GPS digital timing data. The two-dimensional (2D) mapping for azimuth and elevation of the VHF radiation sources was conducted by computing the arrival time differences of three pulses using a cross-correlation technique. From azimuth and elevation data from two sites extracted within a given time frame, 3D lightning mapping was performed using the triangulation scheme. An observation network for winter lightning was constructed within a comprehensive meteorological observation network in the Shonai area, which is located on the coast of the Japan Sea. This report includes the preliminary 2D and 3D mapping of winter lightning observed on 3 December 2010. The horizontal and vertical distributions of VHF radiation sources were consistent with the radar echo observed with X-band Doppler radar. These results indicate that the system can detect winter lightning discharges and perform 2D and 3D lightning mapping in detail.

Corresponding author address: Dr. Masahide Nishihashi, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail: mnishiha@mri-jma.go.jp

Abstract

A three-dimensional (3D) winter lightning mapping system employing very high frequency (VHF) broadband signals was developed for continuous remote observation in winter. VHF broadband pulses radiated by leader progression are received with three discone antennas arranged in a triangle (20–30 m) and recorded on a high-speed digital oscilloscope (1.25-GHz sampling) with GPS digital timing data. The two-dimensional (2D) mapping for azimuth and elevation of the VHF radiation sources was conducted by computing the arrival time differences of three pulses using a cross-correlation technique. From azimuth and elevation data from two sites extracted within a given time frame, 3D lightning mapping was performed using the triangulation scheme. An observation network for winter lightning was constructed within a comprehensive meteorological observation network in the Shonai area, which is located on the coast of the Japan Sea. This report includes the preliminary 2D and 3D mapping of winter lightning observed on 3 December 2010. The horizontal and vertical distributions of VHF radiation sources were consistent with the radar echo observed with X-band Doppler radar. These results indicate that the system can detect winter lightning discharges and perform 2D and 3D lightning mapping in detail.

Corresponding author address: Dr. Masahide Nishihashi, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail: mnishiha@mri-jma.go.jp

1. Introduction

Winter lightning in the coastal area of the Japan Sea exhibits various unusual characteristics that have not been observed in the summer in Japan or in any season in other geographical locations (Rakov and Uman 2003). The ratio of positive to negative cloud-to-ground (CG) lightning flashes in winter is high, while most CG lightning flashes in summer have negative polarity (Takeuti et al. 1973, 1976; Takeuti and Nakano 1977). The period of electrical activity of each thundercloud in winter is generally shorter and the frequency of lightning flashes is considerably lower compared with those of summer thunderclouds (Michimoto 1993; Kitagawa and Michimoto 1994). In addition, the percentage of upward flashes from high structures in winter is very high (Miyake et al. 1990). Upward flashes in winter have been observed from structures shorter than 100 m (Miki 2007), the waveforms of which are quite different from common return-stroke waveforms. Hence, the upward flashes are called ground-to-cloud (GC) flashes (Ishii and Saito 2009; Ishii et al. 2010). Other interesting features of winter lightning are the frequent occurrence of bipolar flashes (Narita et al. 1989), lightning flashes with multiple strike points (Takagi et al. 1991), and single-stroke lightning flashes (i.e., low multiplicity) (Takeuti and Nakano 1983).

Many scientists have reported evidence that lightning activity is associated with severe weather, such as wind gusts, tornadoes, and hail (e.g., Goodman et al. 1988; MacGorman et al. 1989; Williams et al. 1989, 1999; Kane 1991). Recent studies reported that the flash rate of total lightning [both intracloud (IC) and CG lightning] rapidly increases prior to the onset of severe weather events (Williams et al. 1999; Goodman et al. 2005; Steiger et al. 2007), which Williams et al. (1999) termed “lightning jumps.” Gatlin (2007) and Gatlin and Goodman (2010) developed an algorithm to identify impending severe weather using the trends in the total flash rate. The algorithm has been applied in other recent studies (Schultz et al. 2009, 2011; Pineda et al. 2011). However, these types of approaches have not yet been applied in winter lightning in Japan.

Total lightning data are obtained by detecting very high frequency (VHF) pulses radiated from lightning leader progressions. Observation of VHF pulses can visualize lightning channels in three dimensions (3D). There are two kinds of 3D VHF lightning mapping techniques: 1) the time of arrival (TOA) technique (e.g., Proctor 1971, 1981), which has been used in operational lightning detection networks, such as lightning mapping array (LMA) (e.g., Rison et al. 1999; Thomas et al. 2004) and lightning detection and ranging (LDAR) (e.g., Lennon 1975); and 2) the direction of arrival (DOA) technique, in which interferometry is extensively used. Previously, interferometry was utilized primarily for narrowband and two-dimensional (2D) lightning mapping (Hayenga and Warwick 1981; Rhodes et al. 1994; Shao et al. 1995), but recently it has been developed for broadband mapping (Shao et al. 1996; Ushio et al. 1997; Mardiana and Kawasaki 2000; Morimoto et al. 2004; Tantisattayakul et al. 2005; Qiu et al. 2009) and used to conduct 3D lightning mapping (Mardiana et al. 2002; Morimoto et al. 2004, 2005; Tantisattayakul et al. 2005; Akita et al. 2010a, 2011). There are, however, few reports concerning winter lightning mapping in 3D (Morimoto et al. 2004; Akita et al. 2010b), and no meteorological lightning research using 3D winter lightning mapping and meteorological radar data.

We conducted field observations, which we called “The Shonai Area Railroad Weather Project.” The Shonai area is located on the coast of the Japan Sea. The project was designed in 2007 to investigate the finescale structure of wind gusts using two X-band Doppler radars and a network of 26 surface weather stations in order to develop an automatic strong gust detection system for railroads (Kusunoki et al. 2008; Inoue et al. 2011). We focused on total lightning activity in winter to investigate the mechanism of the winter lightning discharge process and the application to the prediction of strong wind gusts. Thus motivated, we developed a 3D VHF lightning mapping system for winter thunderstorms.

In this report, we describe an overview of the new lightning observation system and the methodology for 2D and 3D lightning mapping, and we report preliminary results of 2D and 3D winter lightning mapping.

2. Instrumentation and data processing

a. Characteristics of the lightning observation system

We developed a 3D lightning mapping system using the arrival time and direction of VHF broadband electromagnetic pulses radiated by leader progression for continuous remote observation in winter. VHF broadband pulses are received with three antennas arranged in an isosceles right triangle or equilateral triangle (antenna distance d: 20–30 m) and recorded on a high-speed digital oscilloscope with GPS digital timing data. The azimuth and elevation of VHF lightning radiation sources are computed from the arrival time differences of three VHF broadband pulses. From azimuth and elevation data (2D mapping data) from two sites with baselines ranging around 10 km extracted within the given time frame, we conducted VHF lightning mapping in 3D using the triangulation scheme.

The VHF lightning observation system consists of discone antennas, bandpass filters (23–200 MHz), amplifiers (20 dB), a GPS antenna, a GPS receiver, a time generator, a trigger generator, a high-speed digital oscilloscope, and a personal computer (PC). Figures 1 and 2 and Table 1 show a schematic diagram, a photograph (not including the antennas), and the specifications of the observation system, respectively.

Fig. 1.
Fig. 1.

Schematic diagram of the VHF lightning observation system.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

Fig. 2.
Fig. 2.

The VHF lightning observation system.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

Table 1.

Specifications of the VHF lightning observation system.

Table 1.

1) Discone antenna

For VHF broadband observations, a circular flat-plate antenna is commonly utilized (Kawasaki et al. 2000; Morimoto et al. 2004; Tantisattayakul et al. 2005). However, snow coverage on the antenna in winter may attenuate the reception of VHF broadband wave and cannot be removed when conducting remote observation. Instead, we opted to use a nondirectional discone antenna (Diamond antenna, D1300AM) that consists of thin stainless steel elements. The discone antenna can be installed on the top of a pole (10–15 m) with a sufficient field of view, while circular flat-plate antennas are installed near the ground because they are capacitive antennas. This feature of the discone antenna is suitable for winter lightning observation because winter lightning sources are at relatively low altitudes (Yoshida et al. 2009; Akita et al. 2010b). Part of the rod antenna was removed from the discone antenna in order to reduce corona discharge (for more details, see section 3 concerning the trigger generator).

2) Digital oscilloscope

The VHF broadband waveform was recorded on a high-speed digital oscilloscope (OSC; Yokogawa, DLM2054). The OSC can record three analog signals and one digital signal with a sampling rate of 1.25 GHz and 8-bit resolution. We increased the memory in the OSC to the maximum capacity (31.25 M points per channel). As a result, the OSC could acquire VHF broadband waveforms by generating up to 2500 records. Each of the 2500 records is individually and separately triggered by the trigger generator. The sampling time per record was 10 μs, and each record contained 12.5 k points. The maximum dead time between records was 2.2 μs. VHF signals were digitized in the analog channels (CH 1–3), and the GPS time data outputted from the GPS time generator were recorded in the digital channel simultaneously. The time of skew between the analog channels is shorter than 1 ns, which makes it possible to determine the waveform coherency with high temporal resolution, thus increasing the accuracy of the 2D lightning mapping. The waveform data stored in the memory were transferred to the PC with a universal serial bus (USB) cable when the memory was full or when the OSC accepted the transfer command from the observation control software installed in the PC.

3) Trigger generator

The trigger generator (TRG) output a trigger signal to the OSC and the GPS time generator if the amplitude of three VHF broadband pulses exceeded an arbitrary threshold within given time window tw, which is expressed by
e1
where tw is the maximum transit time of electromagnetic waves between the three antennas, dmax is the maximum distance between the antennas, and c is the speed of light. When dmax is 30 m, tw is about 100 ns. Moreover, the TRG effectively reduced false triggers caused by corona discharges on the discone antennas. The corona discharges occurred in a random manner on each antenna, and the pulse signals propagated through coaxial cables into the TRG. As a result, the pulse signals derived from corona discharges were received randomly in the TRG. Because it is anticipated that the time interval between the corona pulses is longer than tw, the TRG can play a role in noise reduction.

4) GPS antenna, receiver, and time generator

We used a GPS timing signal because time synchronization with high accuracy is required for 3D lightning mapping. A GPS antenna (Furuno, GPA-014B) that does not suffer from snow coverage was utilized. We developed the GPS time generator (GTG), which received 1 pulse-per-second (pps) signal and date/time data from the GPS receiver (Furuno, GT-80) and output digital timing data with a 100-ns resolution generated with a 10-MHz oven-controlled crystal oscillator (OCXO). The frequency–temperature characteristic of the OCXO was 0.15 ppm (peak to peak in 0°–60°C). The maximum error of the digital timing data was ±1 digital unit (±100 ns). When the GTG received a trigger signal from the TRG, the digital timing data (h, min, s, ms, 0.1 μs) were generated and transferred to the digital channel of the OSC. The timing data were recorded into the same record as the VHF broadband waves.

b. 2D lightning mapping

1) Processing of arrival time difference

We describe the methodology used to compute azimuth α and elevation β using VHF broadband waveforms observed at one field site. The typical waveform of VHF broadband lightning pulses is shown in Fig. 3. The arrival time differences of VHF broadband pulses between antennas 1 and 2 (Δt21) and antennas 2 and 3 (Δt23) are given by
e2
and
e3
respectively. Here, t1, t2, and t3 are the arrival times of VHF broadband pulses at antennas 1, 2, and 3, respectively. A cross-correlation method was applied to compute Δt21 and Δt23. The VHF signals ri(t) received at antenna i were digitized at a constant time interval Δt = 0.8 ns and expressed as a discrete time series, given by
e4
First, we searched for an integer na such that r2(n) exceeds an arbitrary threshold A. Then, in order to focus on the VHF broadband pulses radiated from a common radiation source, we extracted r1(n), r2(n), and r3(n) within the range nanwnna + nw, where nw is the index range corresponding to the transit time tw. On the basis of the extracted r1(n), r2(n), and r3(n) values, the cross-correlation coefficient R between r2(n) and r1(n), r2(n), and r3(n) was obtained by increments of n. Denoting r2(n) and r3(n) as x(n) and y(n), respectively, the cross-correlation coefficient Rxy(k) is given by
e5
where k is lag. We can obtain t1, t2, t3, Δt21, and Δt23 when Rxy(k) is at its maximum value.
Fig. 3.
Fig. 3.

Typical waveform of VHF broadband lightning pulses.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

2) Methodology for 2D lightning mapping

Let us consider three antennas, which form two baselines of distance d perpendicular to each other, as shown in Fig. 4a. Denoting the incident angles of the VHF broadband waves radiated from the VHF radiation source for the baseline of antennas 2 and 3 as α (Fig. 5a), Δt23 is expressed by
e6
In the same manner, Δt21 is provided by
e7
The xy coordinate of the VHF radiation source (xs, ys) shown in Fig. 5b is given by
e8
and
e9
where the position of the origin (0, 0, 0) is antenna 2 and r is the distance from antenna 2 to the VHF radiation source. Note that r cosα and r cosβ are the direction cosines of the VHF radiation source. Meanwhile, xs and ys are expressed in terms of azimuth φ and elevation θ as follows:
e10
e11
From Eqs. (6)(11), φ and θ are given by
e12
and
e13
Furthermore, Δt21 and Δt23 in an equilateral-triangular array (Fig. 4b) are provided by
e14
and
e15
From Eqs. (14) and (15), Δt21t23 is given by
e16
Hence, φ and θ are given by
e17
and
e18
Fig. 4.
Fig. 4.

Configuration of three antennas: (a) isosceles right-triangular array and (b) equilateral-triangular array.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

Fig. 5.
Fig. 5.

Schematic description of the geometry of (a) two antennas and VHF radiation source and (b) three antennas and VHF radiation source.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

c. 3D lightning mapping

We have conducted VHF lightning mapping in 3D using a triangulation scheme. Figure 6 shows the principle of 3D lightning mapping. Azimuth and elevation datasets from two sites were extracted from within a given time frame tAB. This tAB is equal to the maximum transit time of electromagnetic waves between sites A (xA, yA, 0) and B (xB, yB, 0), and is expressed by
e19
where dAB is the horizontal distance between sites A and B. When dAB is 10 km, tAB is about 33 μs. Defining the x and y coordinates of the VHF radiation source as xS and yS, respectively, yield
e20
and
e21
where φA and φB are azimuth values of the VHF radiation source observed at sites A and B, respectively. From Eqs. (20) and (21), the xy coordinate of the VHF radiation source can be determined by the following equations:
e22
e23
From each elevation value observed at sites A (θA) and B (θB), the possible z coordinate of the VHF radiation source can be obtained as zSA and zSB, defined as
e24
e25
Here, dA and dB are the horizontal distances from sites A and B to the VHF radiation source, respectively. In general, each line of sight from sites A and B to the VHF radiation source, rA and rB, respectively, will not intersect in 3D space precisely, and thus zSA and zSB values will be different because of the error associated with this imprecision. The possible errors in the observation include antenna positioning error, digitizing error in the OSC, and GPS timing error (~100 ns). Meanwhile, in the data processing, there is the possibility of cross-correlation error in the processing of arrival time difference as well as estimation error for low elevation data in the triangulation scheme. Hence, in order to exclude large estimation errors, we extracted only the result of < 2 km and determined the z coordinate of the VHF radiation source, zS, as the z-coordinate value computed from a shorter horizontal distance of dA or dB. Moreover, we compared the possible origin times tSA and tSB estimated from each radio wave propagation time between the determined VHF radiation source and sites A and B, rA/c and rB/c, and the observed arrival times tA and tB, respectively, defined as
e26
e27
If the time difference > 10 μs, the determined position of the VHF radiation source will suffer from a large estimation error, and so such results were excluded from our analysis. In the same manner as the z-coordinate estimation, we determined the origin time of the VHF radiation source tS as the origin time derived from shorter horizontal distance of dA or dB. If azimuth and elevation datasets were obtained at either or both sites at intervals shorter than tAB, then all possible combinations were processed. After coordinate estimation of the possible VHF radiation sources, the combination producing the minimum ΔtS was chosen. However, when large numbers of VHF broadband pulses were radiated in proximity, it was difficult to conduct a precise positioning of the VHF radiation sources.
Fig. 6.
Fig. 6.

Schematic diagram for 3D mapping.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

3. Observations and preliminary results

We installed the lightning observation system at four sites in the Shonai area in 2009–10 (Fig. 7). To satisfactorily observe lightning discharges over the Shonai area, we arranged a diamond-like array of about 10 km on each side. For continuous remote operation and high-speed data transfer, each site was connected to the Internet through broadband access service [fiber to the home (FTTH)]. The peak speed of the FTTH is 100 Mbps (Shinohara 2005). A photograph of observation site L1 is shown in Fig. 8. We installed not only the VHF lightning observation system but network cameras (Panasonic, BB-HCM735) as well to monitor lightning activity, current weather, and antenna condition. Since October 2009 at site L1 and September 2010 at sites L2, L3, and L4, we have continuously observed winter lightning.

Fig. 7.
Fig. 7.

(top) Location of observation sites. (bottom) The map shows the Shonai area, and the stars indicate the lightning observation sites. The plus sign denotes the X-band Doppler radar at Shonai airport, and the crosses indicate the network of 26 surface weather stations.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

Fig. 8.
Fig. 8.

Lightning observation site L1.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

In this paper, we report the preliminary results of 2D and 3D lightning mapping observed at sites L1 and L4 at 1613:32 UTC 3 December 2010. The layouts of the three antennas at sites L1 and L4 comprise an isosceles right-triangular array (d = 20 m, Fig. 4a) and an equilateral-triangular array (d = 30 m, Fig. 4b), respectively. To detect VHF lightning signals from low elevation angles, we installed the antennas at heights of 10.5 and 15 m at sites L1 and L4, respectively. There are no large buildings higher than the antennas within at least a 500-m radius of site L1 and a 1.8-km radius of site L4. Figure 9 shows the temporal variation of azimuth and elevation of the VHF lightning radiation sources observed at sites L1 and L4. We observed a lightning flash that initiated at 1613:32.231 UTC and continued for 55 ms. The leaders progressed near site L4 because the range of elevation was 0°–90° at site L4, while the range was 0°–25° at site L1. The variation of azimuth and elevation at site L4 demonstrated that the observation system is capable of detecting the detail of leader branching. Meanwhile, the number of the VHF radiation sources observed at site L1 was low compared with site L4 because the amplitudes of received pulses were small because of the distance of the sources, thus decreasing the number of triggered signals. Using these datasets for azimuth and elevation, we conducted 3D lightning mapping (Fig. 10). Most of the VHF radiation sources were distributed about 7 km east–west and 10 km north–south. Figure 10 also shows the radar reflectivity observed with X-band Doppler radar at the Shonai airport (Kusunoki et al. 2008). The radar echo was observed by plan position indicator (PPI) scan (elevation: 8.4°) at 1613:49 UTC. The horizontal distribution of the VHF radiation sources was consistent with the area of radar echo. The vertical distribution of the VHF sources was 0–4.5 km in altitude, corresponding to the echo-top height (about 5 km) observed by range–height indicator (RHI) scan. In particular, 32% of the source was located at 2–2.5 km in altitude. This lightning flash initiated in a strong echo region (>40 dBZ; 38.83°N, 139.94°E) at ~2.2 km in altitude and progressed toward the north-northwest for 4 ms. After that, the area of the VHF radiation sources moved to 38.81°–83°N, 139.92°–93°E and 38.81°–82°N, 139.88°E with reflectivity 25–35 dBZ during 1613:32.239–254 UTC. Then the leader progressed from 38.86°N, 139.96°E, about 5 km in altitude from north to south of site L4 with descending altitude from 1613:32.257 to 1613:32.285 UTC. Note that this leader propagated along the edge of a weak echo region with reflectivity 10–25 dBZ. These results indicate that our system detected winter lightning discharges and conducted 2D and 3D lightning mapping in detail.

Fig. 9.
Fig. 9.

Azimuth–elevation mapping of VHF radiation sources observed at sites (top) L1 and (bottom) L4.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

Fig. 10.
Fig. 10.

3D mapping of VHF radiation sources and radar echo observed with Meteorological Research Institute (MRI) X-band Doppler radar.

Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1

4. Summary and future work

We developed a 3D winter lightning mapping system utilizing VHF broadband observation. The system is well suited for continuous remote observation in winter—in particular, the VHF and GPS antennas and the trigger generator. VHF broadband electromagnetic pulses (23–200 MHz) radiated by leader progression were received with three discone antennas arranged in a triangle and recorded on a high-speed digital oscilloscope (1.25-GHz sampling) with GPS digital timing data (100-ns resolution). The 2D mapping for azimuth and elevation of the VHF lightning radiation sources was conducted by computing the arrival time differences of three pulses using the cross-correlation method. From azimuth and elevation datasets from two sites extracted within a given time frame, we conducted VHF lightning mapping in 3D using a triangulation scheme. An observation network for winter lightning was constructed within a comprehensive meteorological observation network in the Shonai area, which is located on the coast of the Japan Sea. We showed the initial results of 2D and 3D lightning mapping observed at sites L1 and L4 at 1613:32 UTC 3 December 2010. The horizontal distribution of the VHF radiation sources was consistent with the radar echo observed with X-band Doppler radar. The lightning flash initiated in a strong echo region (>40 dBZ), and the leader propagated along the edge of a weak echo region with reflectivity 10–25 dBZ. The vertical distribution of the VHF sources was 0–4.5 km in altitude, corresponding to the echo-top height (~5 km). These results indicate that our system can detect winter lightning discharges and perform 2D and 3D lightning mapping in detail.

We need to evaluate the accuracy of the 2D–3D lightning mapping data. We also need to clarify the relationship between lightning activity and severe weather (e.g., wind gust) and the mechanism of winter lightning by combining 2D–3D lightning mapping data, X-band radar data, and meteorological data observed with the high-density surface stations in the Shonai area. Moreover, our 2D–3D broadband lightning mapping data will clarify the discharge processes for various channel lengths as well as the various progression speeds of channels associated with the charge distribution in thunderclouds. In particular, our observation system compared with a narrowband TOA system has the advantage of detecting fast-propagating processes, such as recoil streamers associated with K changes (e.g., Kitagawa 1957; Ogawa and Brook 1964; Akita et al. 2010a), by higher coherency with high temporal resolution in 2D mapping. Statistical and case studies of the channel structure and radiated frequency will reveal valuable information needed to clarify the lightning discharge process and the cloud electrification mechanism.

Acknowledgments

The authors wish to thank the reviewers for their useful comments and suggestions.

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