Characteristics of positive cloud-to-ground lightning in Da Hinggan Ling forest region at relatively high latitude, northeastern China

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 13,393 13,404, doi: /2013jd020093, 2013 Characteristics of positive cloud-to-ground lightning in Da Hinggan Ling forest region at relatively high latitude, northeastern China Xiushu Qie, 1 Zhichao Wang, 1,2 Dongfang Wang, 1 and Mingyuan Liu 1 Received 24 April 2013; revised 13 September 2013; accepted 6 November 2013; published 19 December [1] In 2009 and 2010, a multistation network of fast and slow antennas was installed on the edge of a relatively high latitude forest region in Da Hinggan Ling (50.4 N, E) of northeastern China. The documented 185 positive cloud-to-ground (CG) flashes containing 196 return strokes were analyzed in the paper. It was found that 71.89% of the positive CG flashes contained continuing current. The average duration of continuing current was short with an arithmetic mean value of ms and a geometric mean value of ms; only one continuing current lasted longer than 150 ms, probably because of the small size of the storm cell in this relatively high latitude region. The vast majority (94.59%) of positive CG flashes was characterized by a single stroke, and the average number of stroke per flash is The average charge transferred by positive stroke and continuing current, based upon the analysis of five flashes with well-documented simultaneous measurements of more than five stations, was +5.2 C from a height of 6.0 km (above ground) and C from a height of 6.4 km, respectively. The charge moment for +CG strokes ranged from C km to C km, while that for continuing current ranged from C km to C km, respectively. The preliminary breakdown process in positive CG flashes can be classified into three types, namely type S (same), type D (different), and type C (chaotic) according to the disparities in the initial polarity of bipolar pulses from the return stroke, which account for 62.92%, 23.60%, and 13.48%, respectively. According to the electric field waveforms indicative (or not indicative) of intracloud (IC) discharge, positive CG flashes are classified into four types, i.e., ordinary positive CG flash (63.78%), hybrid +CG-IC flash (21.08%), hybrid IC-+CG flash (5.41%), and hybrid IC-+CG-IC flash (9.73%). About 15.14% of the recorded positive CG flashes were byproduct of IC lightning discharge. Citation: Qie, X., Z. Wang, D. Wang, M. Liu, and Y. Xuan (2013), Characteristics of positive cloud-to-ground lightning in Da Hinggan Ling forest region at relatively high latitude, northeastern China, J. Geophys. Res. Atmos., 118, 13,393 13,404, doi: /2013jd Introduction [2] Downward cloud-to-ground (CG) lightning discharges are classified as positive or negative by their polarity of charge effectively transported from cloud to ground. Although only about 10% natural CG lightning discharges are of positive polarity [Orville and Huffines, 1999; Rakov and Uman, 2003], they tend to have high peak current, large charge transfers, and high probability of continuing current [e.g., Berger, 1967; Brook et al., 1982; Qie et al., 1991; Orville and Huffines, 1999], which may cause more severe damage than negative flashes. Recent researches also indicated that positive CG flashes, which are horizontally 1 Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 2 Institute of Atmospheric Physics, University of Chinese Academy of Sciences, Beijing, China. Corresponding author: X. Qie, Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, 40 Huayanli, Qijiahuozi, Chaoyang District, Beijing , China. (qiex@mail.iap.ac.cn) American Geophysical Union. All Rights Reserved X/13/ /2013JD extensive in the cloud and transfer large amounts of positive charge to ground, may play a crucial role in the production of red sprites [Boccippio et al., 1995; Lyons et al., 2008; Lu et al., 2009, 2013]. So the studies on positive CG flashes will contribute to the knowledge of lightning discharge characteristics and design of lightning protection, as well as the initiation mechanism of transient luminous events. [3] There are many disparities between positive and negative flashes in multiplicity, leader propagation mode, branching, and occurrence of continuing currents [Nag and Rakov, 2012]. In addition, the preliminary breakdown process in positive flashes is usually different from their negative counterparts. Generally speaking, the initial pulse polarity of preliminary breakdown process is the same as the following return stroke in most of the negative flashes [Gomes et al., 1998]. As for positive CG flashes, the initial pulse polarity of preliminary breakdown process is not always the same as the following return stroke [Ushio et al., 1998; Gomes and Cooray, 2004; Schumann et al., 2013]. Gomes and Cooray [2004] classified the trains of preliminary breakdown pulse (PBP) in positive CG flashes into four types according to the initial polarity of bipolar pulses in PBP trains. However, electric field measurements showed that positive flashes are sometimes preceded by pronounced intracloud discharge lasting ms, rather than the 13,393

2 Figure 1. Distribution of the station locations. Each station, equipped with both slow and fast antenna system, is synchronized by GPS. The C band radar is marked as a black triangle. preliminary breakdown process [Rust et al., 1981; Fuquay, 1982]. Optical records confirmed that positive CG flashes can be initiated from a channel branch of intracloud discharge [Kong et al., 2008; Saba et al., 2009], and the weak preliminarily breakdown process is replaced by IC discharge in this case. [4] With the application of high-speed video and data acquisition technology, the accumulation of data for positive flashes is more abundant than ever. Recently, Nag and Rakov [2012] examined the various conceptual cloud charge configurations and scenarios leading to production of positive lightning with a view toward an explanation of its observed properties, and they presented a thoughtful overview of positive lightning and added new important observations and inferences. However, the consensus about the discharge characteristics and the initiating mechanism of positive flashes has not been reached. Moreover, Nag and Rakov [2009a] and Lu et al. [2013] presented conceptual morphologies for negative CG flashes as well as their relevant IC discharge, but the studies on +CG flashes are relatively limited. [5] Da Hinggan Ling, located in northeastern China, is one of the largest and the most important forest regions in China. It is also a place where wildfire often occurs. In order to learn the characteristics of lightning, especially the positive flashes prone to ignite wildfire in Da Hinggan Ling forest region, a network of multistation fast and slow antennas with GPS synchronization was installed during the years of 2009 and 2010 in Da Hinggan Ling [Wang et al., 2011; Qie et al., 2013]. In this paper, the discharge characteristics of +CG flashes were analyzed using the data set from the observations in two summers. 2. Instrumentation and Data Acquisition [6] In the summer of 2009 and 2010, a VLF/LF lightning detection network consisting of seven stations of fast and slow antennas synchronized by GPS, similar to that used by Cui et al. [2009], was deployed in Da Hinggan Ling, northeast of China. Figure 1 shows the distribution of station locations. The network was located on the edge of the Da Hinggan Ling forest region (50.4 N, E), where the average ground level was 385 m above the mean sea level. Every pair of fast and slow antennas was identical to each other and the network had an overall ability to monitor lightning discharges within 20 km. The decay time constants for fast and slow antennas are 1 ms and 3 s, respectively. The frequency bandwidth of the slow and fast antenna system is 2 MHz and 1 MHz, respectively, both with sampling rates of 5 MHz. A field mill was deployed at the station of KYZ to monitor the real-time surface electrical field during the observation. Additionally, a C band weather radar, which is marked as a black triangle in Figure 1, provided the PPI (plan position indicator) and RHI (range height indicator) observation of radar echo. [7] Twenty-five thunderstorms passed over the observation site during the summer of 2009 and 2010, and 10 of which produced at least one positive CG flash. In order to reduce the possibility of misidentifying IC events as CG strokes, only the cases which were recorded by at least two stations were analyzed. A total of 185 positive CG flashes that occurred within about 30 km from the central of the network during the 10 thunderstorms was chosen in this study. Several positive CG flashes were recorded by at least four stations. Figure 2 shows one example of multistation electric field measurements. The electric fields shown in the figure are all normalized to the corresponding peak values, and the decay time constant (3 s) for slow antenna was taken into account (also for the rest of this paper referring to slow antenna). The ratio of positive CG to total CG varied from 2% to 22% in the 10 +CG-producing storms. The average ratio of positive CG to total CG was 10.2%, a value similar to that in most of the summer thunderstorms in other places [Rakov, 2003]. Characteristics of the positive CG discharges, including return strokes, continuing current, preliminary breakdown process, and morphologies, are analyzed in the following sections. 3. Return Stroke 3.1. Multiplicity of Positive CG Flashes [8] Most positive CG flashes are characterized by a single stroke, and multiple-stroke positive CG flashes occur infrequently. Out of the total 185 +CG flashes analyzed, 175 were single-stroke, 9 two-stroke, and 1 three-stroke flash, and the corresponding ratio was 94.59%, 4.86%, and 0.55%, respectively. There was a total of 196 return strokes produced by the recorded 185 +CG flashes, and the average number of stroke per flash was Figure 3 shows the overall electric field change of a three-stroke flash and time expansions for every individual stroke. As shown in Figure 3a, the time interval between R 1 and R 2 was ms and ms between R 2 and R 3. For all the multiple-stroke flashes, the interstroke time intervals varied from 6.46 ms to ms with an arithmetic mean (AM) value of ms and a geometric mean (GM) value of ms. 13,394

3 Figure 2. Electric field changes for a positive CG flash detected simultaneously by seven stations (a) from fast antenna and (b) from slow antenna. Figure 3. Electric field changes for a three-stroke positive CG flash on 7 June (a) Overall waveform and the strokes are marked with R 1,R 2, and R 3, respectively. (b d) Electric field changes produced by R 1, R 2, and R 3 with expanded time scale. 13,395

4 Table 1. Comparison of Positive CG Multiplicity Among Different Authors Authors Place Method QIE ET AL.: POSITIVE CLOUD-TO-GROUND LIGHTNING +CG Flash Number Single Stroke Ratio Strokes per Flash Interstroke Intervals (ms) This study Da Hinggan Ling, China Fast and slow antennas % Qie et al. [2002] Gansu, China Fast and slow antennas 23 87% 1.13 / 88.2 Fleenor et al. [2009] America Video camera % / Saba et al. [2010] Austria, Brazil, and U.S. High-speed video % / GM AM [9] Qie et al. [2002] reported that 16% of total CG flashes were positive in Chinese inland plateau and 13.0% of positive flashes were characterized by multiple strokes. The AM value of the time interval between return strokes of positive CG flashes was 91.7 ms, which is larger than that of negative CG flashes in the same regions. Fleenor et al. [2009] examined 204 positive flashes in the Central Great Plains in the United States and found that 195 (96%) of the positive flashes produced only one single stroke and nine (4%) generated two strokes; the observed multiplicity of positive flashes was Saba et al. [2010] summarized 103 positive flashes and concluded that 81% of positive CG flashes were single-stroke flashes. The average multiplicity was 1.2 strokes per flash, and the GM of 21 interstroke time intervals was 94 ms. Table 1 compared the characteristics of positive CG flashes found in the previous literature. All the results are in good agreement, indicating that the majority of positive flashes produce a single stroke and the interstroke time intervals in multistroke positive CG flashes are relatively large Risetime of Return-Stroke Electric Field Waveforms [10] Return-stroke pulses are usually considered as the most powerful pulses in the electric field change waveform. In the data set of all the 185 positive CG flashes, the risetimes from 10% to 90% peak value for 196 return strokes varied from 2.40 μs to μs, with an AM value of 7.77 ± 2.77 μs. The AM value of risetimes from 0% to 100% peak value was ± 4.58 μs. Figure 4a shows the definitions of risetimes and histograms of the 10 90% and 0 100% risetime of return strokes. Subsequent return strokes in 11 cases of our data set tend to have smaller amplitude and shorter risetime, and the AM value of the risetime from 0% to 100% peak was 6.60 ± 3.22 μs. [11] Table 2 shows the comparisons of return stroke risetime among different authors. Hojo et al. [1985] found that the average values of the risetime from 10% to 90% peak value of field change caused by first positive return strokes in Niigata and Tokyo were 8.7 and 6.7 μs, respectively. These values were about twice the risetime from 10% to 90% peak value for negative ones over the same period in each area. Cooray [1986] reported 15 positive return strokes in Sweden and found that the average risetime from 10% to 90% peak value was 6.2 ± 1.4 μs. Qie et al. [1998] found that the risetime for positive strokes was much longer than that for negative strokes. The difference might be due to the slow front process and the charge accumulation in the downward leader Charge Transferred by Positive Return Strokes [12] To estimate the charge transferred by positive strokes, a chi-square method [Krehbiel et al., 1979; Qie et al., 2000] was used to estimate the charge magnitude and the location of the charge center neutralized by positive strokes, by using the simultaneous slow electric field change at multistations. The validity of this method is restricted to the situation where the dimension of charge removal is assumed to be much smaller compared to the distance from the observation. At the ith station, the field change caused by a charge Q removed from cloud to ground is given by ΔE i ¼ 1 2Qz 4πε h 0 ðx x i Þ 2 þ ðy y i Þ 2 þ ðz z i Þ 2 i3 ¼ 1 2Qz 2 4πε 0 R 3 i where x, y, and z are coordinates of charge Q and x i, y i, and z i are the coordinates of ith station; R i represents the distance between Q and the ith station, and ε 0 is the permittivity of free space. With x, y, z, and Q being unknown variables, more than four independent measurements are necessary to provide reasonable accuracy and quantitative error estimation. Equation (2) gives the chi-square function: (1) χ 2 ¼ 1 ½ΔE mi ΔE i ðx; y; z; QÞŠ 2 υ n i σ 2 (2) where σ 2 is the deviation of ΔE i (x,y,z,q) due to experimental error and υ refers to the number of degrees of freedom. The parameters minimize the chi-square function and will give the best estimation of (x, y, z, Q). The ground level of the seven stations were well selected due to their baseline and was relatively smooth. The local ground level was considered in charge removal calculation. [13] Five positive flashes were recorded by at least five stations on 16 July 2010, and these flashes were selected to estimate the charge amount and charge center of positive strokes and continuing current. In the calculation of charge transfer, the decay time constant (3 s) was taken into account and a dedroop process was implemented to compensate the relaxation associated with the feedback capacitor in the charge amplifier circuit [e.g., Sonnenfeld et al., 2006]. All of these five +CG flashes occurred in the dissipating stage of the thunderstorm, with the fifth almost at the end. Table 3 provides the fitting results for the five +CG flashes. The AM value of the transferred charge was +5.2 C, and the charge height for return stroke was 6.0 km. Of all these charge centers, the fifth one located the lowest, which indicated that the positive charge center lowered due to the decreasing of the upward drafts in the dissipating stage. Zhang et al. [2009] analyzed two positive flashes in Chinese inland plateau by using the same method and the same instruments and found that the neutralized charge was C and C and the corresponding height of charge center was 5 and 6 km, respectively. This is much smaller than that in Da Hinggan Ling. In the Chinese inland plateau, the thunderstorm usually develops a triple-charge structure 13,396

5 E field change /kv/m Percentage /% Percentage /% (a) ΔT1 ΔT Time /ms (b) N=196 GM=7.27μs AM=7.77μs SD=2.77μs Time /μs (c) N=196 GM=13.18μs AM=13.96μs SD=4.58μs Time /μs Figure 4. Distribution of risetime of electric field change for positive CG return strokes. (a) Definition of risetime ΔT 1 and ΔT 2. ΔT1 denotes the time from 10 to 90% peak, and ΔT 2 from 0 to 100% peak, (b) the distribution of 10 90% risetime ΔT 1, and (c) the distribution of 0 100% risetime ΔT 2. with a lower positive charge center (LPCC) larger than usual [Qie et al., 2009]. The positive CG flashes may be initiated from the LPCC in the Chinese inland plateau. The comparison with radar echoes shows that the five CG flashes examined in this paper occurred during the dissipation stage of thunderstorm and initiated in the upper part of the storm (refer to the following section). 4. Continuing Current 4.1. Duration of Continuing Current [14] Positive lightning flashes are usually followed by continuing current. Shindo and Uman [1989] once grouped the continuing current into three categories according to their duration: long continuing current with duration exceeding 40 ms, short continuing current with duration between 10 ms and 40 ms, and questionable continuing current with duration between 1 ms and 10 ms. For easy comparison, the same definition is followed in the following statistical analysis. Of all the 185 positive flashes, 133 (71.89%) contained continuing current with duration longer than 1 ms, including 36 long, 47 short, and 50 questionable continuing currents. The longest continuing current lasted ms. Figure 5a shows its corresponding electrical field change waveform from slow antenna system. The distribution of the continuing current duration for positive flashes is shown in Figure 5b. The AM value of continuing current duration was ms, while the corresponding GM value was ms. [15] Fuquay [1982] reported an AM duration of 73 ms for continuing current in 13 positive CG flashes. Zhang et al. [2010] found that 69.2% contained continuing current for 81 positive CG flashes and the corresponding AM value of continuing current duration was 113 ms in Beijing. The continuing current duration in Da Hinggan Ling forest region is much shorter than others. It can be seen from Figure 5b that very small samples of longer continuing current were recorded in the research region and just one exceeded 150 ms. The reason for this is possibly the relatively small charge region inside the thunderstorm with small convective cell in such a higher-latitude forest region [Wu et al., 2013] Charge Transferred by Continuing Current [16] The magnitude and height of neutralized charge by positive continuing current were estimated by using the same method as described in section 3.3. Table 3 shows the fitting results of five positive continuing currents. The AM value of neutralized charge was C, and the charge height for continuing current was 6.5 km. For a single positive flash, the charge transferred by continuing current was larger than that by return stroke (almost twice as much as that by return stroke). The total charge transferred by a single positive flash was ± 8.0 C. Nag and Rakov [2012] provided singlestation estimation of charge transfer for positive flashes with assumed charge height. The charge transfer for return strokes in our results is corresponding to those of Nag and Rakov [2012] in 1 ms and 2 ms, and the charge transfer for continuing currents in ours are corresponding to those of Nag and Rakov [2012] in 40 ms. Comparing with the results of Nag and Rakov [2012] which the charge height was assumed to be 7 km, the charge transfer both for return strokes and continuing current was relatively small. 13,397

6 Table 2. Comparison of Return Stroke Risetimes Among Different Authors Sample 10 90% Risetime (μs) 0 100% Risetime (μs) Authors Place Season Size GM AM SD GM AM SD This study Da Hinggan Ling Summer Hojo et al. [1985] Tokyo Summer 44 / / / / Niigata Winter 32 / / 22.3 / Cooray [1986] Sweden Summer 15 / / / / 20 / / / / Ushio et al. [1998] Japan Winter 19 / / / / 18 / Schumann et al. [2013] Brazil Summer [17] The radar echo was used to examine the charge center of both return stroke and continuing current in the thunderstorm. Figure 6 shows one of the PPI radar echoes at 15:15 CST around the first four positive CG flashes which occurred at 14:56, 14:59, 15:10, and 15:28 on 16 July Figure 6a shows the 2-D positions of all charge centers superimposed on the PPI radar echo at 3.4 elevation angles. The return strokes were marked as solid black diamonds, and the continuing current was marked as white circles. The relative magnitude of the charge neutralized is represented by the size of the symbols. All these flashes occurred in the early dissipating stage of the thunderstorm. Most of the charge centers were well fallen into the areas with reflectivity of more than 45 dbz, and only one charge center was on the edge of areas with reflectivity of more than 30 dbz. [18] The RHI display from direction from point A to point B in Figure 6a is shown in Figure 6b, and the charge centers were also projected on the RHI display. It is indicated that the vertical reflectivity reached as high as 8 km for the convective region, and the charge centers were usually located at the upper part, even the top of the radar echo with a reflectivity lower than 35 dbz and on the leading edge of the echo centers, indicating that the upper part of the thunderstorm is dominated by positive charge. The height of charge center neutralized by continuing current is a little higher than the return stroke, indicating a development of discharge toward higher altitude. [19] The charge moment (QH) of +CG stroke and continuing current is also shown in Table 3. For most of the strokes, the charge moment for +CG stroke ranges from C km to C km, while that for continuing current ranges from C km to C km. The continuing current usually demonstrates large charge moment than return stroke. It is believed that the higher charge moment will increase the probability of red sprite, and the sprite-producing +CG usually induces a charge moment change of hundreds, even thousands of C km [Cummer and Lyons, 2005; Lu et al., 2009, 2013]. If this conclusion is true worldwide, the +CG flash in the Da Hinggan Ling forest region seems not sufficient for producing sprites. Moreover, the existing observed sprites are typically initiated by +CG strokes in the stratiform region of mesoscale convective systems (MCSs) [Boccippio et al., 1995; Lyons, 1996; Lu et al., 2013]. However, the thunderstorms producing +CG flashes examined in this study are not characterized as MCS based on the radar echo. 5. Preliminary Breakdown Pulse Trains [20] PBP trains are typically bipolar and comparable to the following return strokes in amplitude, and these pulse trains may be tens of milliseconds prior to the return strokes. As for intracloud (IC) pulse trains that also originate inside the cloud, they are usually not so intense in amplitude in electric field waveforms and may be more unipolar. Electrical waveforms for preliminary breakdown pulse (PBP) trains in positive CG flashes are bipolar in the initial period with relatively larger PBP amplitudes and shorter inter pulse intervals (referred to as highly active period hereafter), similar to that in negative CG flashes. Some of the pulses are unipolar at the end of the PBP trains. Usually, the initial polarity of bipolar pulses is dominated by the polarity of the initial half circle of the biggest pulse in the PBP train. For the PBP trains in positive flashes, three types of PBP trains are identified: (1) type S (same), with the dominating polarity of initial half circle of the pulses in PBP trains being the same as the polarity of the following return stroke; (2) type D (different), with the dominating polarity of initial half circle of the pulses in PBP trains being different to the polarity of the following return stroke; and (3) type C (chaotic), with the PBP trains having pulses with both initial half-circle polarities in irregular orders. Type C is some kind of a combination of type S and type D, but more like type S. Of all the positive flashes observed, 89 (48.11%) exhibiting obvious PBP trains were analyzed. Among them, 56 (62.92%) are classified as type S, 21(23.60%) as type D, and 12 (13.48%) as type C. Figure 7 shows typical waveforms of type-s and type-d PBP trains. We did not discern the so-called type b PBP trains by Gomes and Cooray [2004], which contain two parts of PBP trains with different initial polarities. [21] Since positive PBP trains were dominated by type S, some statistical analyses for this type of PBP trains were Table 3. Location and Magnitude of Charge Sources Neutralized by Positive CG Flashes a Flash Number Type Height (m) Q (C) QH (C km) ΣQ (C) RS ± CC ± RS ± CC ± RS ± CC ± RS ± CC ± RS ± CC ± GM RS CC AM RS CC SD RS CC a QH denotes the charge moment of +CG stroke or continuing current. 13,398

7 E field change /kv/m ΔE CC RS ΔE RS CC (a) Time /ms (b) N=133 GM=16.74ms AM=33.29ms SD=38.44ms [22] All the parameters of type-s pulse trains are shown in Figure 8. T 1 (10.29 ± 5.98 μs) and T 2 (10.82 ± 6.87 μs) were almost equal to each other and the average total pulse width was ± μs. The AM value of pulse separation was ± μs. Pulse train duration and PBP-RS separation were 2.97 ± 1.61 ms and ± ms, respectively. Some of the statistical parameters are with pronounced standard deviations, which might imply large variance between different subtypes in this type. [23] Ushio et al. [1998] analyzed 19 positive flashes of winter thunderstorm in Japan and also found the bipolar characteristics of PBPs. The average pulse width and pulse separation were 18 μs and 54 μs, respectively. Their pulse train usually lasted 1 ms, and AM value of PBP-RS interval was 12 ms. Qie et al. [2002] analyzed the PBP trains of positive flashes in Chinese inland plateau and found that the time interval for 50 pulses was ± μs, pulse widths were 27.0 ± 19.0 μs, and PBP-RS separation was Percentage /% >160 Time /ms Figure 5. Electric field change produced by positive CG flash and distribution of continuing current duration. (a) An example of electric field change by a positive CG flash with one stroke (R) and a long continuing current (CC) recorded by slow antenna on 7 June (b) Distribution of continuing current duration for all the positive CG flashes. given. Hereafter, the parameters are defined similarly to Gomes and Cooray [2004]. For the statistical analysis of parameters related to the pulses in PBP trains, only five consecutive pulses in highly active period were taken into account in order to identify the pulses precisely and ignore unipolar pulses far from the highly active period. T 1 and T 2 mean the duration of the first and second half cycle in individual bipolar pulses, respectively. So (T 1 +T 2 ) stands for the total pulse width of a bipolar pulse. Pulse separation is defined as the time interval between two consecutive pulses in highly active period of PBP trains. The pulse train duration and PBP-RS (return stroke) separation were also analyzed. Pulse train duration is defined as time interval between the first pulse in PBP trains and the first pulse that has amplitudes twice as background noise in the same PBP train, and PBP-RS separation is defined as the time interval between the pulse with the biggest amplitude in the pulse train and the following return stroke. Figure 6. Radar echoes at 15:15, on 16 July (a) PPI display superimposed by estimated charge centers. (b) RHI display from direction from point A to point B in Figure 6a and projections of charge centers. The return strokes were marked as solid black diamonds, and the continuing current was marked as white circles. The relative magnitude of the charge neutralized is represented by the size of the symbols. 13,399

8 Figure 7. Typical waveforms of preliminary breakdown pulses in positive CG flashes. (a) Overall waveform of a positive CG flash with type-s preliminary breakdown pulses. (b) Pulse train of the PBP for type S on an expanded time scale. (c) Overall waveform of a positive CG flash with type-d preliminary breakdown pulses. (d) Pulse train of the PBP for type D on an expanded time scale ms. Nag and Rakov [2012] found that the AM value of total pulse widths in PBP trains were 25 μs with a mean pulse separation of 157 μs in eight PBP trains. Schumann et al. [2012] and Zhang et al. [2013] also gave their statistical results for PBP trains in positive CG flashes. Table 4 shows the comparison of PBP characteristics for positive CG flashes between different studies. All of these studies were focused on PBPs of positive flashes in warm-season thunderstorms except that by Ushio et al. [1998], whose studies were on winter thunderstorms in Japan. The pulse widths in our data set are the shortest ones except those of Ushio et al. [1998], the pulse separation is moderate, which is longer than that of Gomes and Cooray [2004] and Schumann et al. [2013], but slightly shorter than that of Qie et al. [2002] and Nag and Rakov [2012]. For the duration of PBP trains, the results are almost the same with each group of researchers. The differences may relate to regional disparity as well as differences of storm structures, observation latitudes, and so on. [24] In the case of vertical discharge channel, as Gomes and Cooray [2004] discussed, type-s PBP trains may indicate a lowering of positive charge, and type-d PBP trains indicates an uplifting of positive charge in breakdown processes. While in cases of nonvertical geometry, positive charges moving toward the observer will also produce type-s PBP trains. Usually, a typical thundercloud can be viewed as a vertical tripole structure containing three primary charge centers: main positive in the upper of thundercloud, main negative in the middle, and a lower small positive charge center below. The positive CG flashes usually occur from the upper main positive charge based on the height of charge center estimated in sections 3 and 4. Positive CGs also seemed to occur when the thunderstorm started to dissipate and the main negative charge center was weakened. So for most of the cases, negative charge center under the main positive charge center may trigger type-s PBP and eventually form a positive CG flash, which also explains why type S dominates in positive PBP trains. Type-D PBP trains might take place between main negative and lower positive charge center, as Cooray and Scuka [1996] assumed, similar to the preliminary breakdown processes in negative CG flashes, but the negative charge is largely expended in the positive CG cases and repolarized in the main positive charge center, initiating a positive leader to ground. The type-c PBP trains might contain breakdown discharges between both upper and lower two charge layers. Nag and Rakov [2009a] stated the importance of low positive charge center (LPCC) in the production of PBPs in negative CG flashes, indicating that a significant LPCC at relatively high latitudes may have a better chance to produce detectable PBP trains. This might be in line with our hypothesis. [25] The ratio between the biggest amplitude in PBP and the return stroke was also analyzed. It varied from 0.03 to 0.99 with an AM value of 0.32 ± The cases with the biggest amplitude of the PBP train exceeded the following return stroke were not found. Ushio et al. [1998] reported the cases with the ratio exceeded 1 in winter thunderstorms in Japan. The AM value was 0.27 and in some cases the ratio reached 1.9 for their cases. Nag and Rakov [2009b] examined the ratios of the biggest amplitude in PBP and their corresponding first return-stroke pulses in 59 negative CG flashes and found that the GM and AM of the ratio was 0.45 and 13,400

9 Figure 8. Distribution of characteristic parameters of PBP for positive CG flashes. (a) Width of the initial half cycle of pulses (T 1 ), (b) width of the second half cycle of pulses (T 2 ), (c) pulse width (T 1 +T 2 ), (d) pulse separation, (e) pulse train duration, and (f) PBP-RS separation. 0.62, respectively. In 19% of the cases, this ratio exceeded 1, implying that this ratio is much bigger in negative CG flashes than that in positive CG flashes. Zhang et al. [2013] found that the average ratio of PBP to RS in Beijing and Guangzhou were and 0.195, respectively. Even though the example of the ratio bigger than 1 was not found in our data set, the results still show that preliminary breakdown process radiates strongly in positive CG flashes. 6. Morphologies of Positive CG Flashes Accompanied by IC Discharge [26] Positive CG flashes sometimes are accompanied by IC discharge. Table 5 illustrates four types of positive CG morphologies accompanied by IC discharge. The electric field waveforms of these four morphologies are shown in Figure 9. The first type corresponds to the ordinary +CG Table 4. Comparison of PBP Characteristics for Positive CG Flashes Among Different Authors This Study Ushio et al. [1998] Qie et al. [2002] Gomes and Cooray [2004] Schumann et al. [2013] Zhang et al. [2013] Beijing Guangzhou T 1 (μs) / / / / T 2 (μs) / / / / Pulse width (μs) Pulse separation (μs) Pulse train duration(ms) / PBP-RS separation (ms) Percentage of dominating type 62.92% / / 80.3% / 55% 81% 13,401

10 Table 5. Four Types of Morphologies of Positive CG Lightning Accompanied With IC Discharge +CG Flash Type Flash Description Flash Number (Percent) Ordinary +CG flash Hybrid +CG-IC flash Hybrid IC-+CG flash Hybrid IC-+CG-IC flash The flash develops downward positive leaders from the positive charge region in the cloud and results in positive stroke The flash first produces positive strokes following downward positive leaders and then develops an IC discharge The flash begins as intracloud discharge and then generates positive strokes associated with downward positive leader descending from the main positive cloud region The flash begins as intracloud discharge and then generates positive stroke associated with downward positive leaders descending from the main positive cloud region, and then develops an IC discharge 118 (63.78%) 39 (21.08%) 10 (5.41%) 18 (9.73%) flash, and there was no IC discharge other than the PBPs. This positive CG lighting usually develops downward positive leaders from the positive charge region in the cloud and results in a positive return stroke. The second type corresponds to the hybrid +CG-IC flash, which usually first produces positive strokes following downward positive leaders and then develops an IC discharge. The third type corresponds to the hybrid IC-+CG flash, which usually begins as intracloud lightning and then generates positive strokes associated with downward positive leader descending from the main positive cloud region. The fourth type is a positive CG accompanied by pronounced IC discharge both before and after the stroke, corresponding to the hybrid IC-+CG-IC flash, which begins as intracloud lightning and then generates positive stroke associated with downward positive leaders descending from the main positive cloud region, and then develops an IC discharge. [27] Of all the positive CGs in our data set, 118 (63.78%) of the positive CG flashes were ordinary positive CG flash, 39 (21.08%) exhibited IC discharge only after the strokes, 10 (5.41%) exhibited IC discharge only before the strokes, and 18 (9.73%) had IC discharge both before and after the strokes (see Table 5). If the IC discharge was recorded during the 200 ms pre-trigger time and no obvious preliminary breakdown and leader processes can be identified, we attribute this kind of positive CG flashes to be initiated by intracloud discharges, as found by Kong et al. [2008]and Saba et al. [2008] using high-speed video observations. The preliminary breakdown and leader processes are not considered as intracloud discharge in our analysis. Hereafter, if there was no IC discharge recorded during this pre-trigger time except for the PBPs and leader processes, it gives evidence of no intracloud discharge leading to the initiation of the positive stroke. There were totally 28 (15.14%) positive flashes generated by intracloud discharges. Figure 9. Four morphologies of positive CG flashes. (a) A +CG flash with no IC discharge other than the PBPs, (b) a two-stroke positive CG flash with IC discharge only after the second stroke, (c) a positive CG flash with IC discharge only before the stroke, and (d) a positive CG flash with pronounced IC discharge both before and after the stroke. 13,402

11 [28] Rakov [2003] suggested that +CG flashes might be generated by intracloud lightning that extends horizontally. The optical observation and lightning mapping results with very high frequency (VHF) imaging also proved that some channels of intracloud lightning can produce +CG strokes [Kong et al., 2008; Saba et al., 2009; Lu et al., 2013]. Saba et al. [2009] found that leaders with both polarities might trigger positive CG strokes. Nag and Rakov [2012] analyzed 52 cases of +CG strokes and found 10 (19%) of them were produced by intracloud discharge. These results are all in good agreement with ours. The analysis of ground-based electrical field is consistent with that some +CG flashes are byproduct of intracloud discharges. 7. Conclusions [29] The characteristics of 185 +CG flashes containing 196 strokes were analyzed, based on multistation measurements of electric field changes in a higher latitude region, Da Hinggan Ling forest area, northeastern China. The results are summarized as follows: [30] Most of the +CG flashes (94.59%) produced a single return stroke, and the average number of strokes per flash was For the 10 multistroke +CG flashes, the interstroke intervals varied from 6.46 ms to ms, with an AM value of ms and a GM value of ms. [31] For the 196 positive strokes, the AM values of risetime from 10% to 90% peak value and from 0% to 100% peak value in the electric field change waveforms were 7.77 ± 2.77 μs and ± 4.58 μs, respectively. [32] About 71.89% of positive CG flashes contained continuing current longer than 1 ms. The average duration of continuing current was relatively short with an AM value of ms and a GM value of ms in comparison with results found in previous literature. Only one continuing current exceeded 150 ms with duration of ms. [33] For five +CG strokes selected for the quantitative analysis, the total charge transferred by a single stroke was ± 8.0 C. The AM value of transferred charge for five strokes was +5.2 C with a height of about 6.0 km and C with an average height of 6.5 km for continuing current, respectively. For a specific positive flash, the continuing current usually brought more charge to ground than the return stroke primarily due to a sufficiently long duration. The estimated charge moment of positive stroke and continuing current in the Da Hinggan Ling forest area range from C km to C km and from C km to C km, respectively, which usually is not sufficient for initiating sprites based on existing observations of sprite-producing +CG strokes. [34] About 48.11% of the +CG flashes exhibited clear preliminary breakdown processes. According to their disparities in initial polarities of bipolar pulses, the preliminary breakdown processes in +CG flashes can be divided into three categories: type S with the dominating polarity of initial half circle in PBP trains being the same as the polarity of following return stroke, type D with the dominating polarity of initial half circle in PBP trains being different from the polarity of following return stroke, and type C with the PBP trains having pulses with both initial half-circle polarities in irregular orders. There were 56 (62.92%) belonging to type S, 21 (23.60%) for type D, and 12 (13.48%) for type C. [35] The durations of the first and second half cycle in individual bipolar pulses in PBP trains (T 1 and T 2 ) were ± 5.98 μs and ± 6.87 μs, respectively. The total pulse width (T 1 +T 2 ) was ± μs, the pulse separation was ± μs, the pulse train duration was 2.97 ± 1.61 ms, and the PBP-RS separation was ± ms. The average ratio of PBP to RS was 0.32 ± [36] Four types of morphologies of +CG lightning accompanied by IC discharge are summarized, including ordinary +CG flash (63.78%), hybrid +CG-IC flash (21.08%), hybrid IC-+CG flash (5.41%), and hybrid IC-+CG-IC flash (9.73%). The relation between +CG flashes and IC discharge indicates that 15.14% of the +CG flashes were byproduct of intracloud discharges. The result is similar to that of Nag and Rakov [2012]. Both of the results confirm that there are indeed some positive flashes caused by intracloud discharges, as demonstrated by Kong et al. [2008] and Saba et al. [2009] with high-speed video observations. [37] Although the statistical characteristics of positive CG flashes are presented and some progresses are made through the work in this paper, the origin and propagation of positive CG lightning in the research region remain unclear. 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