1. Introduction
Lightning is an electric discharge phenomenon that can go from cloud to cloud (CC), be intracloud, or go from cloud to ground (CG). The average peak current of CG lightning can reach up to tens of thousands of amperes (Orville et al. 2011; Enno 2011). CG lightning frequently causes damage to buildings and electronic equipment and, sometimes, loss of life. In China, with its growing population and rapid urbanization, the adverse effects of CG lightning are growing every year. Therefore, to protect and mitigate against the harmful impacts of CG lightning in China, it is important to study and understand its activity. Knowledge of the distribution of lightning discloses the patterns of regional convective activity. Therefore, it is meaningful to analyze the characteristics of the distribution of CG lightning.
There have been numerous studies on the characteristics of CG activity in North America (e.g., Orville 1991; Orville and Silver 1997; Mäkelä et al. 2011; Orville et al. 2011), Europe (e.g., Finke and Hauf 1996; Areitio et al. 2001; Rivas Soriano et al. 2005; Sonnadara et al. 2006; Antonescu and Burcea 2010; Ramos et al. 2011), and Brazil (Pinto et al. 1996, 2003). This research has revealed that the location of annual maximum CG density is not determined by the maximum CG density per thunderstorm day in the United States. The flash density maxima do not necessarily coincide with the maximum thunderstorm days (Christian et al. 2003; Pinto and Pinto 2003). The maximum CG density and largest number of thunderstorm days is in Florida (Orville 1991; Mäkelä et al. 2011), while the maximum CG density per thunderstorm day is in the Midwest (Mäkelä et al. 2011). The distribution of CG activity has a clear interannual variation (Orville and Silver 1997), which is possibly caused by the variation of weather pattern because the distribution of lightning is affected by the various combinations of synoptic and mesoscale conditions (Hodanish et al. 1997). Some studies have indicated that the percentage of positive CG (PPCG) may be affected by cloud depth, environmental conditions, and topography. Rakov (2003) indicated that the tendency for high PPCG decreases with increasing cloud depth. Carey and Buffalo (2007) found that positive CG (PCG) flashes frequently dominate in high clouds in a drier lower to midtroposphere, or for smaller warm cloud depths.
There have also been a few studies in China on thunderstorm days and lightning activity. For example, Zhang and Feng (1998) showed that the greatest number of thunderstorm days can be found in southeastern and southern China, with the second highest frequency over the Tibetan Plateau and its adjacent areas, and the smallest frequency in northwestern China. Zheng et al. (2010) indicated that thunderstorms in mountainous areas are greater in number than those over plains at the same latitude. Ma et al. (2005) indicated that the spatial distribution of lightning density (including CG and CC lightning) is markedly different between eastern and western China, and that the nature of the terrain is a significant factor impacting the climatic distribution of lightning activity. Zhang et al. (2004) showed two high lightning density centers (including CG and CC lightning) located in eastern and southern China, respectively. Ma et al. (2007) analyzed lightning data (including CG and CC lightning) from the satellite-borne Optical Transient Detector and Lightning Imaging Sensor, as well as thunderstorm day data from surface-based observation stations, and showed there to be a clear difference between the distribution of thunderstorm days and lightning density over the Tibetan Plateau and its adjacent areas. The complex topography of mountain and plateau regions in China has a great impact on the distribution of precipitation, thunderstorms, and lightning (Tao 1980; Tao and Zhao 1993; Zheng et al. 2010).
From the above, there have been some studies on lightning or thunderstorm days in China, but few papers that have examined the spatial distribution of CG lightning for the whole of China based on the lightning detection network. Thus, it is meaningful to analyze the spatial and temporal distribution of CG lightning in China and the distribution of thunderstorm days with CG lightning.
Additionally, China’s three-step topography is an important factor affecting its weather patterns, precipitation amounts, and convective activity (Sun and Zhang 2012; Tao 1980; Zheng et al. 2013). The topographic distribution is presented in Fig. 1b. This three-step variation begins with the Tibetan Plateau as the first step, followed by the high mountain ranges from the Da Hinggan Mountains over northeastern China to the Yungui Plateau over southwestern China as the second step, and finally the third step is the low-lying plains and hilly regions to the east of the second-step high mountain terrain.
(a) Distribution of lightning detection sensors in China (dots), with the gray shading defining the region where lightning flashes can be detected within 300 km of a sensor. (b) The distribution of terrain (m; shading) and the Yangtze River (blue curve) are shown. The following numbers are used to represent areas of study: 1, Hainan Province; 2, Taiwan; 3, Guangdong Province; 4, Yunnan Province; 5, Sichuan basin; and 6, Jiangsu Province. The three black rectangles represent the Pearl River delta region in Guangdong Province (22°–23.5°N, 112°–115°E), the middle and lower reaches of the Yangtze River basin (28°–34°N, 111°–122.5°E), and northern China (35°–42°N, 110°–120°E).
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
In this paper, we focus on analyzing the distribution of CG lighting and CG lightning days. Section 2 introduces the data and methods used. The distributions of CG lightning frequency, CG and PCG density, PPCG, and CG days are analyzed in section 3. Finally, conclusions and further discussion are provided in section 4.
2. Data and methods
The CG lightning data during 2010–13 are derived from the China National Meteorological Center (CNMC) and China Meteorological Administration (CMA) Meteorological Observation Center. The CG lightning flashes are detected by the China Lightning Detection Network (CLDN), which comprises 357 sensors across the majority of China (Fig. 1a). The average detected radius of a sensor is approximately 300 km, and the location accuracy of the network is approximately 300 m. The CLDN uses the improved accuracy from combined technology (IMPACT) method, which combines directional information and time-of-arrival technology to determine the lightning location (Cummins et al. 1998). The detection efficiency of the network is between 80% and 90%. The CG lightning data include information on, for example, time, location, polarity, and peak current. Some regions of China cannot be effectively monitored because of the nonuniform spatial distribution of the sensors. For example, the sensors located on the mainland are unable to accurately detect the CG lightning that occurs in Taiwan, and the detection efficiency may be low in northwestern areas of the Tibetan Plateau because of the lack of sensors (Fig. 1a).
The four years of CG lightning data in this analysis cannot constitute a climatology. The results reflect the general features of CG lightning activity in China, but also contain the influence of individual lightning events. To convert strokes data into flashes, we assume that strokes in the same flash have locations within 10 km of the first stroke, the time interval between consecutive strokes is less than 500 ms, and strokes with different polarities belong to different flashes (Pinto et al. 2003). In this study, we exclude positive CG flashes with a peak current of less than 15 kA for the following reasons: 1) CG flashes with a peak current of between 10 and 20 kA can be contaminated by CC lightning (Cummins et al. 2006); 2) a flash with a peak current of less than 15 kA is difficult to completely confirm as CG lightning, and the detection efficiency of PCG with a peak current of less than 15 kA is extremely low (Cummins and Murphy 2009; Enno 2011); and 3) the peak current of most (>95%) PCG flashes is between 20 and 90 kA (Sonnadara et al. 2006; Biagi et al. 2007). In this analysis, the flashes not being within the shading shown in Fig. 1a are deleted, and no corrections for detection efficiency are made.
All geographical plots in this paper, such as for CG density, percentage of PCG, CG lightning days, etc., are calculated with a spatial resolution of 0.2°, corresponding to an approximate resolution of 20 km. Previous studies that have analyzed the characteristics of CG lightning in other parts of the world or specific regions of China [e.g., North America (Holle and Cummins 2010; Mäkelä et al. 2011; Orville et al. 2011), Europe (Rivas Soriano et al. 2005; Sonnadara et al. 2006; Antonescu and Burcea 2010; Ramos et al. 2011), and Yunnan Province in China (Xie et al. 2013)] have used a similar spatial resolution.
3. Distribution of CG lightning frequency and density
a. Frequency
Figure 2 presents the variation of CG, PCG, and negative CG (NCG) frequency, as well as the variation of PPCG during 2010–13. The mean total CG and PCG flashes during 2010–13 are approximately 6.44 million and 0.42 million, respectively, and the mean PPCG is 6.6%. The total number of CG flashes and PPCG show no obvious change during 2010–12, with the former varying from 6.0 × 106 to 6.3 × 106, and the latter from 6.4% to 6.9%. However, the number of CG, PCG, and NCG flashes shows a small increase in 2013 (Fig. 2a). At the same time, PPCG slightly decreases to 6.5% in 2013 (Fig. 2b).
(a) Frequency distribution of CG flashes, PCG flashes (referring to the right ordinate), and NCG lightning during 2010–13. (b) PPCG during 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
The monthly average number of total flashes increases from January to August, and then sharply declines during the next four months (Fig. 3a). The peak month for CG in 2012 is July, but in the other three years it is August. Indeed, July and August are often reported as the peak months in other studies based in the Northern Hemisphere (Orville and Silver 1997; Sonnadara et al. 2006; Enno 2011). A majority (97%) of CG flashes occurs during March–September, with 69% taking place in summer (June–August) because of the prevalence of convection in that season. Some previous studies have also pointed out the tendency of CG lightning mainly appearing in the warm season (Areitio et al. 2001; Rivas Soriano et al. 2005; Antonescu and Burcea 2010; Enno 2011).
Monthly distribution of (a) CG flashes, (b) PCG flashes, and (c) PPCG in each year and the mean value during 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
From the monthly distribution of PCG during 2010–13 (Fig. 3b), the monthly variation for mean PCG flashes presents a similar statistical mode to CG except that the peak month is June, which is prior to that for CG. Antonescu and Burcea (2010) indicated a similar result; that is, positive CG flash activity peaked earlier than total activity. Also most PCG flashes are inclined to appear in the warm season, with about 97% during March–September and 58% in summer; PCG flashes rapidly decline from September to December. Additionally, the existence of severe weather events in southern China in March 2013 (Zhang and Sun 2013) is consistent with the high number of CG and PCG flashes (Figs. 3a,b).
Figure 3c shows the monthly variation of PPCG during 2010–13. As can be seen, the trend is contrary to that of CG and PCG. PPCG in the cold season is considerably greater than in the warm season; the maximum mean monthly PPCG reaches 56.2% in January, and the corresponding minimum value is 4.0% in August. A similar monthly variability of PPCG has been revealed in other regions; for example, in America (Orville and Silver 1997; Orville and Huffines 2001; Rudlosky and Fuelberg 2011), Europe (Finke and Hauf 1996; Areitio et al. 2001; Rivas Soriano et al. 2005; Sonnadara et al. 2006; Antonescu and Burcea 2010), and Yunnan Province in China (Xie et al. 2013). Furthermore, many previous studies have shown that high PPCG and thunderstorms during the cold season principally happen in synoptic-scale weather systems (Zajac and Rutledge 2001; Bentley and Stallins 2005). Additionally, some papers have concluded and explained that the cold season brings the highest PPCG. The strong shear transforms the distribution of charge in the thundercloud, which causes a tendency for PCG flashes to occur in winter thunderstorms (Williams 2001; Rakov and Uman 2003). PPCG in March 2011 is evidently greater than its counterpart values in the other three years, while the opposite is the case for the total CG flashes. This might suggest that there were fewer thunderstorms or weaker convection in March 2011. Qie et al. (2002) indicated that a higher PPCG leads to fewer total CG flashes and a weaker thunderstorm.
b. Density
1) Annual CG density
The spatial distribution of annual CG density is shown in Fig. 4. In general, the CG density in the high elevations and arid regions of western China is less than that in the low elevations and coastal regions of southeastern China (Figs. 1b and 4). From the average value during 2010–13 (Fig. 4e), the maximum CG density (approximately 9.2 flashes km−2 yr−1) is in Guangdong Province. The minimum CG density (<0.05 flashes km−2 yr−1) is located in northwestern China and the Tibetan Plateau. Other areas of frequent lightning (>4 flashes km−2 yr−1) are the Sichuan basin, the region south of Jiangsu Province, and north of Hainan Island. The densities in most parts of eastern China range from 0.5 to 2.5 flashes km−2 yr−1. The geographic distribution of CG density in China is similar to the result found by Christian et al. (2003), which show a global satellite-based lightning climatology. Ma et al. (2007), utilizing satellite lightning data (CC and CG lightning), indicated that the high lightning density areas tend to be located in southern China and across the Yungui Plateau, and the low lightning density areas are in western China. Zhang et al. (2004) used TRMM lightning data to conclude that the high-prevalence areas are in east-central China (28°–36°N), followed by the coastal areas of southern China (21°–24°N).
Spatial distribution of annual CG density (flashes km−2 yr−1) within 300 km during 2010–13; the gray shading is as in Fig. 1a. Shown are results for (a) 2010, (b) 2011, (c) 2012, (d) 2013, and (e) the mean value for 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
The greatest flash densities occur in coastal areas, mountainous regions, regions frequented by migrating synoptic-scale cyclones, and convergence zones (Christian et al. 2003). Meanwhile, the high CG density regions in Guangdong and Jiangsu Provinces belong to coastal regions. CG densities (>2 flashes km−2 yr−1) around the Wuyi Mountains are relatively great. Sichuan basin is the only high-value center of CG density in western China. The reason may be that the Sichuan basin is a region significantly affected by systems that originate from the Tibetan Plateau (Yu et al. 2007a,b; Zhang et al. 2014). Previous studies have found that convective systems from the Tibetan Plateau produced heavy rainfall directly or triggered southwest vortices associated with the majority of the heavy rainfall events in the Sichuan basin (Chen et al. 2007; Zhang et al. 2014).
The pattern of the CG spatial distribution is likely affected by the fact that eastern China is influenced by the East Asian monsoon and that western China is perennially controlled by westerly flow (Tao and Chen 1988; Ding 1992). Furthermore, tropical weather systems affect southeastern China, which may help explain why a large number of CG flashes occur in that region. The regions where the density is greater than 0.5 flashes km−2 yr−1 are mainly located in the third-step terrain and Yungui Plateau (Fig. 4), suggesting that the distribution of CG density is affected by the terrain (Ma et al. 2005).
The CG activity has interannual variability during the four years. The density in the middle and lower reaches of the Yangtze River basin is greater in 2010 than in the other three years (Figs. 4a–d). More heavy rainfall processes occurred over the middle and lower reaches of the Yangtze River basin in 2010 (Wang et al. 2011), and large, deep convective systems contribute the majority of the precipitation totals over eastern China (Luo et al. 2013; Xu 2013), which would have caused the CG density to be greater. The CG density in Jiangsu Province decreases in 2013 (Fig. 4d) and there is a high value there during 2010–12 (Figs. 4a–c). The reason may be that the typical mei-yu period did not occur in 2013 over the middle and lower reaches of the Yangtze River basin (Lin et al. 2013; Yang and Fu 2013; Zhang and He 2013). The mei-yu front is a quasi-stationary, east–west-oriented frontal zone over East Asia during summer that often causes severe flooding along the middle and lower reaches of the Yangtze River basin in eastern China. It is characterized by weak temperature gradients and strong moisture gradients in the lower troposphere (Tao 1980; Ninomiya 2000). CG densities in northeastern China and the Sichuan basin are much greater in 2013 than in the other three years (Figs. 4a–d). That may contribute to a slight overall increase in CG flashes during 2013 (Fig. 2a).
2) Seasonal distribution
The seasonal distribution of CG density is shown in Fig. 5. In spring, the dominant CG flash area is south of 30°N and east of 105°E (Fig. 5a). The maximum CG density occurs in the Pearl River delta in Guangdong Province, followed by the center of Hainan Province. Summer is the most vigorous season for CG activity (Fig. 5b) with peak CG flashes in July and August (Fig. 3a). The three high-value areas have similar results to the annual CG density (Fig. 4e), and the sub-high-value regions are scattered over northern China and the middle and lower reaches of the Yangtze River basin (Fig. 5b). CG flashes mainly occur during the summer in regions north of 30°N. The CG activity in eastern China rapidly weakens in autumn (Fig. 5c), but it is still active in September (Fig. 3a). The maximum density areas are located in the Pearl River delta in Guangdong Province, and two high CG density bands are distributed in southwestern China and coastal southeastern China. In winter, CG lightning activity is extremely weak (Fig. 5d), especially in December and January (Fig. 3a). The CG density is less than 0.025 flashes km−2 season−1 in northwestern China and across the Tibetan Plateau for each of the four seasons.
Spatial distribution of average CG density (flashes km−2 season−1) within 300 km for each season; the gray shading is as in Fig. 1a. Shown are results for (a) spring (March–May), (b) summer (June–August), (c) autumn (September–November), and (d) winter (December–February).
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
c. Spatial distribution of CG days and daily CG density
1) CG days
For the purposes of this paper, a CG lightning day is a day during which at least one CG flash is detected within a 0.2° grid box, and a thunderstorm day is counted when thunder is heard or lightning is seen at a reporting station, but the lightning may be CC or intracloud—not necessarily CG lightning. The spatial distribution of CG lightning days is shown in Fig. 6. The number of CG lightning days in southeastern China is greater than that in northwestern China, which is similar to the distribution of the annual CG lightning density (Fig. 4e). But the spatial distribution of lightning days is much smoother than that of lightning density. From the spatial distribution of the 4-yr mean of CG lightning days (Fig. 6e), the regions where CG lightning days are greater than 30 days are predominantly located south of 30°N, and areas experiencing more than 50 CG lightning days per year are mainly distributed in southern China. The areas where CG lightning days are between 10 and 30 days per year are mainly in northern and northeastern China. It is approximately 10–20 days per year over the central Tibetan Plateau. The number of CG lightning days is obviously greater in 2010 than in the other three years for the middle and lower reaches of the Yangtze River basin (Fig. 6a), which is consistent with CG density (Fig. 4a). The number of CG lightning days over the southwestern Sichuan basin is much greater in 2013 (Fig. 6d) than in 2010–12, which is also consistent with the CG density (Fig. 4d). The geographic distribution of CG lightning days is similar to that for thunderstorm days (Ma et al. 2007; Zheng et al. 2010). But the number of CG lightning days is frequently less than the number of thunderstorm days in most parts of China, especially in the Tibetan Plateau, suggesting that CG flashes do not occur or are not observed during some thunderstorm days.
Spatial distribution of annual frequency of CG lightning (days) during 2010–13, the gray shading is as in Fig. 1a. Shown are results for (a) 2010, (b) 2011, (c) 2012, (d) 2013, and (e) the mean value for 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
2) Distribution of daily CG density
Daily CG density is defined as a value equal to the number of CG flashes over a square kilometer divided by the number of CG lightning days. The spatial distribution of mean daily CG density is shown in Fig. 7e. The daily CG density in eastern China is also greater than that in western China and the positions of three high-value centers are similar to that for the annual CG density (Fig. 4), but the contrast in daily lightning density between southern China and northern China is less than that for the mean annual CG density (Fig. 4e). The maximum mean value reaches 0.2 flashes km−2 day−1 in the Sichuan basin. The high-value centers (>0.1 flashes km−2 day−1) of daily CG density are located in the Sichuan basin, the Pearl River delta in Guangdong Province, and in Jiangsu Province, followed by secondary high-value centers (0.06–0.1 flashes km−2 day−1) scattered throughout central and northern China (Fig. 7e). Areas in which the daily CG density ranges from 0.02 to 0.06 flashes km−2 day−1 are located in the remaining parts of eastern China, and values of less than 0.01 flashes km−2 day−1 are distributed throughout northwestern China and across the Tibetan Plateau.
Spatial distribution of daily CG density (flashes km−2 day−1) within 300 km during 2010–13; the gray shading is as in Fig. 1a. Shown are results for (a) 2010, (b) 2011, (c) 2012, (d) 2013, and (e) the mean value for 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
As mentioned, the number of CG lightning days is greater over the southwestern Sichuan basin in 2013 (Fig. 6d), which is consistent with the increase in daily CG density (Fig. 7d). A high value of CG density exists in the Sichuan basin (Fig. 4e), while the 4-yr mean for CG lightning days there is less than the value in the eastern region at the same latitude (Fig. 6e). Therefore, it is shown that there is more daily CG lightning activity in the Sichuan basin (Fig. 7e). Additional studies are needed to find out why the daily CG density is greatest in the Sichuan basin.
d. Spatial distribution of PCG density and PPCG
1) PCG density
The spatial patterns of PCG are similar to those for CG density, but the values are very different. The high-value centers do not completely match those for CG density. The 4-yr mean PCG density (Fig. 8e) shows a more homogeneous distribution than that of CG. The maximum value (0.63 flashes km−2 yr−1) occurs in the Pearl River delta, followed by the region north of the Wuyi Mountains. The PCG densities in most parts of eastern China mainly range from 0.05 to 0.15 flashes km−2 yr−1, and the value is less than 0.005 flashes km−2 yr−1 in northwestern China and across the Tibetan Plateau. The spatial distribution of average annual PCG density changes from a low value (0.06 flashes km−2 yr−1) to the highest value (0.15 flashes km−2 yr−1) in Yunnan Province (Xie et al. 2013). The mean maximum PCG density was found to be less than 0.3 flashes km−2 yr−1 in Canada, and the mean maximum value was found to lie within the range of 0.5–0.7 flashes km−2 yr−1 in the central United States (Orville et al. 2011).
Spatial distribution of annual PCG density (flashes km−2 yr−1) within 300 km during 2010–13; the gray shading is as in Fig. 1a. Shown are results for (a) 2010, (b) 2011, (c) 2012, (d) 2013, and (e) the mean value for 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
The spatial distribution of PCG density, like CG density, also presents obvious interannual variation (Figs. 8a–d). The PCG density over the Yangtze River basin in 2010 is greater than during the other three years, and the PCG density increased in 2013 over Sichuan basin and northern China (Fig. 8e). Densities of greater than 0.05 flashes km−2 yr−1 are mainly distributed in the third-step terrain and the Yungui Plateau (Fig. 8e). It is suggested that the spatial distribution of PCG flashes may be affected by the terrain.
2) PPCG
The values of PPCG in western China are apparently greater than those in eastern China (Fig. 9), which is different from the pattern found for CG density and PCG density. Furthermore, PPCG in southern China is smaller than in northern China. The average PPCG in eastern China is less than 8%, except in northeastern China (Fig. 9e). The CG and PCG flashes mainly occur over eastern China during March–September (Figs. 3a,b and 5), when PPCG is a relatively small value (Fig. 3c). Mean PPCG during that period varies from 14.2% to 4.0% (Fig. 3c), and the mean value is about 6.4% during March–September. The PPCG in northwestern China and most parts of the Tibetan Plateau is greater than 18%. PPCG in the Tibetan Plateau is considerably greater than the value in southeastern China; the mean PPCG reaches 16% and the value ranges from 6.5% to 100% over the Tibetan Plateau (Qie et al. 2002). Qie et al. (2002) further indicated that weak convective storms are more likely to generate PCG flashes than are strong storms, because the CC lightning generally happens at the bottom of thunderstorms in the Tibetan Plateau and the negative charge in the middle of storms neutralizes the positive charge at the bottom. PPCG has been reported to reach around 10% in southern Sweden, while in northern Sweden the value ranges from 20% to 100% (Sonnadara et al. 2006). Orville et al. (2011) showed that PPCG ranged from less than 2% in Florida to greater than 25% along the western coast of the United States and that most flashes over the western coast occurred in October–March and were associated with winter convection, which accorded with the PPCG during October–March being greater (Fig. 3c). Orville and Silver (1997) explained that instrumentation effects can contribute to high PPCG being recorded along the western coast, or, to the east of the Continental Divide, with the frequent occurrence of stratiform precipitation in mesoscale convective systems being characteristic of high PPCG.
Spatial distribution of annual PPCG (%) within 300 km during 2010–13; the gray shading is as in Fig. 1a. Shown are results for (a) 2010, (b) 2011, (c) 2012, (d) 2013, and (e) the mean value for 2010–13.
Citation: Weather and Forecasting 30, 6; 10.1175/WAF-D-14-00132.1
4. Summary and discussion
The CG lightning data during 2010–13 being detected by the CLDN are used to reveal the distribution of CG lightning across China. The spatial and temporal distributions of CG and PCG lighting are investigated. The key findings are summarized below.
The yearly mean total numbers of CG and PCG flashes during 2010–13 are approximately 6.44 million and 0.42 million, respectively, and the mean PPCG is 6.6%. CG flashes predominately (97%) occur between March and September, with approximately 69% happening during summer. The mean peak month for CG is August, while it is June for PCG. However, PPCG in the cold season is considerably greater than in the warm season. PPCG reaches a maximum (56.2%) in January and a minimum (4.0%) in August.
The spatial distribution of CG density is affected by the geography, the land–sea distribution, and weather systems. CG and PCG lightning densities have obvious interannual and seasonal variations. Generally, the CG density in the high elevations and arid regions of western China is less than that in the low elevations and coastal regions of southeastern China. Three high-value centers of CG density can be found in Guangdong Province, across the Sichuan basin, and in Jiangsu Province. The spatial distribution of PCG density is approximately similar to that of the CG density. However, the spatial distribution of PPCG is different from the pattern found for CG and PCG density. PPCG in western China is greater than in eastern China. The PPCG is greater than 18% in northwestern China and most parts of the Tibetan Plateau.
CG lightning days and daily CG density also show interannual and regional variations. CG lightning days in southeastern China are more common than in northwestern China. The regions experiencing more than 30 CG lightning days per year are primarily located south of 30°N. The frequency of CG lightning days varies from 10 to 30 days per year in northern and northeastern China, while the range is approximately 10–20 days per year over the central Tibetan Plateau. Three high daily CG density centers are scattered throughout the Sichuan basin, the Pearl River delta, and south of Jiangsu Province.
Thunderstorms frequently occur over the central Tibetan Plateau (Zhang and Feng 1998; Ma et al. 2007; Zheng et al. 2010), but the CG density there is extremely small (Fig. 4) and the number of CG lightning days is also small (Fig. 6)—a pattern that was also pointed out by Ma et al. (2005). In other words, though thunderstorms frequently occur over the central Tibetan Plateau, CG lightning activity tends to be weak or there are many thunderstorms without CG lightning or not being detected over there. Some previous studies have indicated that the density of lightning (CC and CG) located over the Tibetan Plateau is low (Ma et al. 2005, 2007) because the majority of thunderstorms have weak convection, are short lived, and therefore generate a small number of CG flashes. Indeed, the convective activity of the Tibetan Plateau is frequently found to be weak (e.g., Zhang et al. 1998; Qie et al. 2002). PPCG is greater than 18% in the area at the edge of the CLDN. The reasons may be that PCG has a greater peak current than NCG, which contributes to higher detection efficiency for PCG than NCG there (Orville and Silver 1997).
Acknowledgments
The cloud-to-ground lightning data were provided by the National Meteorological Center (NMC) and the China Meteorological Administration Meteorological Observation Center. This research was supported by the China Meteorological Administration (Grant GYHY201406002), the Key Program of the Chinese Academy of Sciences (Grant 2013CB430100), and the National Natural Science Foundation of China (Grant 41405007). We appreciate the helpful comments provided by three anonymous reviewers.
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