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.
Schematic diagram of the VHF lightning observation system.
Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1
The VHF lightning observation system.
Citation: Journal of Atmospheric and Oceanic Technology 30, 2; 10.1175/JTECH-D-12-00084.1
Specifications of the VHF lightning observation system.
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
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
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
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
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
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.
(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
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.
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
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.
The authors wish to thank the reviewers for their useful comments and suggestions.
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