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Cloud-to-ground (CG) lightning days are widely used to analyze thunderstorm occurrences. According to a method of defining CG lightning days by Yang et al. (2015), at least two flashes in a 0.5° × 0.5° cell between 0000 and 2400 LST is regarded as a CG lightning day in that cell. The spatial distribution of all winter CG lightning days for 2010–20 is shown in Fig. 1a. There are three winter lightning frequent areas (WLAs), WLA1, WLA2, and WLA3, ordered from west to east. The westernmost area, WLA1, is located in southwestern Yunnan Province, in the Ailao Mountains, with its southwest edge on the southwestern boundary of the Yunnan-Guizhou Plateau. The region is mountainous, with low elevations to the southwest and high elevations to the northeast. The central area, WLA2, is located at the junction of Hunan, Guizhou, and Guangxi Provinces, near Xuefeng Mountain and Wuling Mountain on the eastern boundary of Yunnan-Guizhou Plateau. The easternmost area, WLA3, is located in the Poyang Lake Plain area in northeastern Jiangxi Province, including a small part of southern Anhui Province and western Zhejiang Province. The following mountains surround this area: the Dabie Mountains, the Nanling Mountains, the Luoxiao Mountains, and Mount Huangshan in the north, south, west, and east of WLA3, respectively. Cloud-to-ground (CG) lightning days in WLA2 are most frequent, with more than 2.6 d winter–1 in most areas. In WLA3, the thunderstorm days are fewest, with only a few grids around Jingdezhen experiencing more than 2.4 d winter–1.
Figure 1. Spatial distribution of the frequency of winter CG lightning days (d winter–1; a) and CG flash density (fl km–2 winter–1; b) within the three WLAs (red boxes) during the winters of 2010–20. The horizontal resolution of lightning days and lightning flash density is 0.5° × 0.5°. The gray shading represents topography, with lighter shading denoting a lower elevation (m). Blue curved lines represent the Yellow River and the Yangtze River.
Figure 1b shows the distribution of CG flash density in winter thunderstorms. CG flash density is defined as the average number of CG flashes per square kilometer per winter (fl km–2 winter–1). The area of each grid in Fig. 1b is calculated by treating the earth as a sphere. As shown in Fig. 1, high CG flash density roughly coincides with high numbers of lightning days. The highest CG flash density is in eastern Guizhou Province and western Hunan Province, west of WAL2. In addition, there are scattered high CG lightning density areas in southeastern Yunnan Province, northeastern Jiangxi Province. WLA2 has the highest mean CG flash density (0.023 fl km–2 winter–1), while the CG flash density of WLA1 is the lowest (0.014 fl km–2 winter–1; Table 1). Because winter lightning is rare in areas north of the Yangtze River, the mean national CG lightning density is generally an order of magnitude less (0.0023 fl km–2 winter–1).
Region Number of thunderstorm processes
(pro winter−1)*Flash density
(fl km−2 winter−1)PPCG (%) Lightning type Mean peak current (kA) China 9.6 0.0023 26.1 + 73.0 − 47.0 WLA1 5.1 0.014 35.6 + 65.0 − 34.5 WLA2 4.7 0.023 21.6 + 82.4 − 52.8 WLA3 3.1 0.015 25.2 + 69.3 − 41.4 *The number of thunderstorm processes is the number of thunderstorm processes affecting the region. A strong thunderstorm process can affect multiple WLAs, and some thunderstorm processes that occur outside of three WLAs and do not affect any WLA. Table 1. Winter CG lightning summary for China and three WLAs during the 10-yr period of 2010–20.
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Most winter thunderstorms are systematic, so we can judge whether lightning records at different moments are caused by the same thunderstorm process by analyzing atmospheric circulation patterns and the continuity of lightning records with precipitation. Combining precipitation and lightning observation, a thunderstorm process is defined as a discrete weather system featuring a continuous process having more than 50 flashes within 6 hours and significant precipitation. A thunderstorm process is considered to affect the WLA if at least one flash in a lightning cluster is located in a WLA. This implies that a strong, persistent thunderstorm process can affect two or three WLAs. As shown in Table 1, China is affected, on average, by 9.6 thunderstorm processes each winter (pro winter–1). The westernmost region, WLA1, is most susceptible to thunderstorms, with an average of 5.1 pro winter–1. In WLA1, the center of most lightning clusters is located outside China, but the edge of the lightning cluster affects the Yunnan area. Although the frequency of thunderstorm processes in WLA2 is slightly lower (4.7 pro winter–1), in contrast to WLA1, most lightning cluster centers of the thunderstorm processes that affect WLA2 are located in WLA2. The least number of thunderstorm processes occurs in WLA3, with an average of only 3.1 pro winter–1.
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In most areas of China, NCG lightning is dominant in winter, and the PPCG lightning is 26.1% nationwide. Only the coastal areas of Guangdong Province, southeastern and southwestern Yunnan Province, and the junction of southeast Tibet and India have a PPCG lightning exceeding 50% (Fig. 2). The higher PPCG lightning in the southern frontier region of Yunnan Province is consistent with the findings of Xie et al. (2013). The reason for the higher PPCG lightning in this region may be that the peak current of PCG lightning is larger than that of NCG lightning, leading to a higher detection ratio of PCG lightning (Orville and Silver, 1997). The national average peak current of PCG and NCG lightning is 73.0 kA and 47.0 kA, respectively. This difference is nearly double, consistent with the average annual peak current of positive and negative CG lightning (Li et al., 2011). Previous research has pointed out that the PPCG lightning during the cold season is higher compared to the warm season (Orville and Silver, 1997; Antonescu and Burcea, 2010; Xie et al., 2013; Yang et al., 2015). The higher PPCG lightning in winter may be attributed to an inverted charge structure [an upper negative charge and part of the positive charge below it; Wang et al. (2021)]. Owing to the weakened convection in winter, active electrification occurs in regions near or warmer than the reversal temperature, which causes the graupel particles to gain positive charges and ascending ice crystals to acquire negative charges upon collision with each other (Takahashi, 1978). The charging between snow/aggregates (which gain positive charge) and ice crystals (which gain negative charge; Williams, 2018) is supposed to be another mechanism for the inverted charge structure (Zheng et al., 2019). The inverted charge structure of winter thunderstorms has been verified in Japan (Zheng et al., 2019), but related studies are lacking in China. Considering the difference in the PPCG lightning in winter between China and Japan, the charge structure of Chinese thunderstorms is worthy of further study.
Figure 2. Spatial distribution of PPCG flashes (%) within a 0.5° × 0.5° grid during the winters of 2010–20. The horizontal resolution of lightning days and lightning flash density is 0.5° × 0.5°. The gray shading represents topography, with lighter shading denoting a lower elevation (m). Blue curved lines represent the Yellow River and the Yangtze River. Red boxes denote the three WLAs.
There is some difference in peak current and the PPCG lightning between the three WLAs. In WLA2, the average peak current of PCG and NCG lightning is 82.4 kA and 52.8 kA, respectively, which represents the strongest average peak current among the three WLAs, noting that the PPCG lightning there is 21.6%. The PPCG lightning of WLA1 is the highest (35.6%), while the mean peak currents of PCG and NCG lightning are the lowest (65.0 kA and 34.5 kA). The PPCG lightning in WLA3 is 25.2%, and the mean peak current of PCG and NCG lightning is 69.3 kA and 41.4 kA, respectively.
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The monthly and diurnal variation in accumulated winter CG flashes is shown in Fig. 3. From a national perspective (Fig. 3a), the frequency of CG lightning is the highest in February, consistent with the CG lightning distribution results calculated by Yang et al. (2015). January has only about one-third of the CG lightning than does February. The number of CG flashes in December is an order of magnitude lower than that in February. The diurnal variation shows a unimodal distribution, and the trend of the diurnal variation over the three months is the same. The frequency of CG lightning in the morning (2300−0100 LST) is three times that of the middle of the day (1100−1200 LST), which is quite different from the diurnal variation of summertime convection, which occurs mostly in the afternoon (Yang et al., 2020). This diurnal variation is similar to, although more profound than, the temporal distribution characteristics of winter CG lightning in the United States (Bentley et al., 2019).
Figure 3. Monthly and diurnal variations of accumulated winter CG lightning flashes in China (a), WLA1 (b), WLA2 (c), and WLA3 (d) from 2010 to 2020. The top histogram shows the monthly variation of all winter flashes in the region, and the right histogram shows the diurnal variation of all winter flashes in the region.
The temporal variation in CG lightning in WLA2 (Fig. 3c) is consistent with the national observation but exhibits a more profound diurnal variation due to a much larger sample size of CG lightning events.
The frequency of CG lightning is lowest in WLA3, and its temporal distribution (Fig. 3d) is substantially different from that of the entire country. In terms of monthly variation, the largest amount of CG flashes occurs in February, and the frequency of both January and December is about one-sixth that of February. The diurnal variation in WLA3 is complex. The highest number of CG flashes occurs in the early morning hours, 0000−0100 LST in December and 0500 LST in January. There are two peaks in February, close to 0000 and 0500 LST. In general, CG lightning in WLA3 is more frequent in the early morning and less frequent in the daytime.
The CG temporal distribution of WLA1 (Fig. 3b) is substantially different from that of other WLAs. From the monthly variation perspective, the number of lightning flashes in January and February is nearly equal; and the frequency of lightning in December is the least, about a quarter of that observed in January. The diurnal variation in winter shows a bimodal distribution (0100 LST and 1800 LST), with the highest peak in the afternoon, similar to the annual diurnal variation in Yunnan (Xie et al., 2013). The cause for the afternoon peak may be attributed to the blocking effect of the Yunnan-Guizhou Plateau, which makes it difficult for cold air to reach WLA1. Combined with the lower latitude, higher temperature, and larger relief, afternoon, local convection occurs more easily in WLA1.
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By analyzing the weather patterns of all winter thunderstorms, CG lightning mostly occurs during convective weather when southward-moving cold air meets warmer air. Some weather patterns can generate flashes without a front, such as CG lightning in southwest Yunnan. Therefore, according to the weather situation at 1000 hPa, the weather patterns of winter CG lightning in China are manually divided into two types: frontal and non-frontal. In the frontal weather patterns, the location of the frontal zone is manually determined according to the convergence of cold and warm currents. Combined with the most frequent CG lightning location, the frontal patterns can be further manually divided into three types: pre-frontal, near-frontal, and post-frontal. Following Hunter et al. (2001), to make lightning data and reanalysis data coincide in time, a period is defined based on the temporal resolution of reanalysis data. If there are at least 50 CG lightning flashes within three hours before and after this moment, this six-hour interval is defined as a period. Different weather patterns are divided according to the 1000 hPa synoptic pattern for the middle time of the period. If most of the lightning occurs in the presence of a northerly wind, the pattern is post-frontal. If the predominant CG flashes occur during a southerly wind, the pattern is pre-frontal. If there is no significant difference or the number of lightning records occurs in the presence of both southerly and northerly winds, the pattern is classified as near-frontal. Figure 4 describes the spatial relationship between the lightning zone and the front. Only the location of lightning is considered, and the movement of the front is ignored. In addition, most winter lightning is related to the cold frontal process in China. Thus, the difference between warm and cold fronts is ignored, and we assume that all lightning occurs in the vicinity of the cold front.
Figure 4. Schematic diagram of the method for judging the relative locations of lightning zone and front. Geopotential height (blue lines), horizontal winds (brown vectors), and warm moist flow (green vector) at 1000 hPa are shown. Winter lightning zones of pre-frontal, post-frontal, and near frontal weather patterns are shown in red, light blue, and yellow, respectively.
Figure 5 shows the proportion of CG lightning in the various weather patterns across China and the three WLAs. About 69% of CG flashes occur in the frontal weather pattern nationwide. Among these CG flashes, about 8% are pre-frontal, 38% are post-frontal, and about 23% are classified as the near-frontal type (Fig. 5a).
Figure 5. The proportion of winter CG lightning flashes in China (a) and three WLAs (b–d) occurring in post-frontal (blue), pre-frontal (brown), near frontal (battle green), and non-frontal (light green) weather conditions.
The proportion of various weather patterns in the three WLAs is quite different. In WLA1 (the furthest south of the three WLAs), 83% of CG flashes occur in the non-frontal pattern (Fig. 5b). Theoretically, all flashes in WLA1 should be non-frontal, but the classification method resulted in 17% of lightning with front. The classification method is when the vast majority of lightning at a given moment is of a frontal type, all lightning occurring at this moment is classified as a frontal pattern. WLA2 is located in latitudes between the other two WLAs and has the highest CG lightning frequency. The proportion of CG flashes in non-frontal weather patterns is 13%, and the amount of CG flashes occurring in the post-frontal type is largest in WLA2 (46%) (Fig. 5c). In WLA3 (the furthest north among the three WLAs), CG flashes mainly occur in the post-frontal weather pattern (70%), while the CG flashes in non-frontal patterns account for only 15% of the wintertime total (Fig. 5d).
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Analysis of the CG lightning field in winter shows that the circulation depiction without fronts is relatively simple. Most lightning in the non-frontal type is located in Yunnan Province. Elsewhere, the counts of flashes in the non-frontal type are usually less than five records per period. Therefore, only the non-frontal types in Yunnan Province are discussed. Since the elevation of the Yunnan-Guizhou Plateau is between 400 and 3500 m (Zhao and Chen, 2004), only the circulation field above 700 hPa is analyzed. Fourteen typical cases belonging to the non-frontal pattern in southwestern Yunnan Province are selected for composite analysis, and the results are shown in Fig. 6. Due to the influence of the South Branch Trough (some cases have closed low-pressure circulation), the lightning zone is controlled by the southwesterly jet downstream of the trough. Water vapor from the Indian Ocean is constantly transported into the vicinity of the Yunnan-Guizhou Plateau, causing the relative humidity in the CG lightning zone to exceed 85%. In addition, the West Pacific Subtropical High (WPSH) is strong, and the lightning zone is located on the northwestern side of the WPSH, which further strengthens the southwesterly jet. The terrain of the lightning zone displays a spatial gradient that is low in the southwest and high in the northeast, aligning with the Yunnan-Guizhou Plateau, which leads to the accumulation of warm moist air southwest of the Ailao Mountains. The lightning zone is also downstream of the trough at 500 hPa, which is conducive to the generation of convective weather.
Figure 6. Composite field of typical non-frontal weather pattern with winter CG lightning in southwest Yunnan. Geopotential height (blue lines), horizontal winds (vectors; green denotes a wind speed ≥12 m s–1), and high humidity region ( ≥85%, area filled with dots) at 700 hPa are shown. The rainbow color indicates the number of winter CG lightning flashes within a 0.5° × 0.5° cell. The small image in the upper left is a conceptual map of the weather pattern. The green vectors represent the strong warm moist flow, the green area represents the Qinghai-Tibet Plateau, and the red area is where lightning occurs.
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The weather situation of the frontal patterns is much more complicated and is divided into the following types, low vortex, confrontational convergence, asymptotic convergence, transversal trough after high pressure, and other. Other types include situations with small amounts of lightning, inconspicuous weather patterns, and topographic fronts in the northern part of the Yunnan-Guizhou Plateau. The frequency and proportion of CG flashes and periods are shown in Table 2. From the perspective of the periods, the transversal trough after high pressure type is the most frequent, accounting for 38.3% of the frontal pattern total. The periods for the low vortex type are the least at 10.4%. The confrontational convergence and asymptotic convergence types account for 19.9% and 15.2% of the periods, respectively. In addition to these four typical patterns, there are 16.1% of other types. From the perspective of CG flash frequency, the transversal trough after high pressure has the highest proportion of flashes at 62.1%. The low vortex, confrontational convergence, and asymptotic convergence types constitute 5.9%, 13.2%, and 12.2% CG flashes, respectively. Other types only account for 6.6% of the flashes. Although other types contain a large number of periods (16.1%), the number of CG flashes is relatively small, averaging only 228.3 CG flashes per period. Among the four typical weather patterns, the transversal trough after high pressure has the most CG flashes per period [906.9 fl (6 h)–1], followed by the asymptotic convergence [450.2 fl (6 h)–1], and the low vortex and confrontational convergence have fewer CG flashes [315.2 and 368.9 fl (6 h)–1, respectively].
Type Number of periods* Percentage of period Number of flashes Percentage of flashes Number of flashes per period
[fl (6 h)−1]Low vortex 33 10.4% 10402 5.9% 315.2 Confrontational convergence 63 19.9% 23242 13.2% 368.9 Asymptotic convergence 48 15.2% 21611 12.2% 450.2 Transversal trough after high pressure 121 38.3% 109731 62.1% 906.9 Other types 51 16.1% 11644 6.6% 228.3 Total 316 176630 559.0 *A period is defined based on the temporal resolution of reanalysis data. If at least 50 CG lightning flashes occur within three hours before and after this moment, these six hours are considered a period. The 850 hPa synoptic pattern for the middle time of the period is used to determine the weather pattern. Table 2. Summary of the number and proportion of periods and flashes in various types of weather patterns in China.
We then search for periods when the circulation pattern is fairly obvious and when lightning concentrates in a certain area to make a composite analysis among the four typical weather patterns. Five to ten cases are selected for each pattern, and the results are shown in Fig. 7.
Figure 7. Composite field of four typical frontal weather patterns, transversal trough after high pressure (a), low vortex (b), confrontational convergence (c), and asymptotic convergence (d), with winter CG lightning. Geopotential height (blue lines), horizontal winds (vectors; green denotes a wind speed ≥12 m s−1), and high relative humidity regions (≥90%, area filled with dots) at 850 hPa are shown. The rainbow color indicates the number of winter CG lightning flashes within a 0.5° × 0.5° cell. The small image in the upper left corner is a conceptual map of the weather pattern. The green and brown vectors represent the strong warm-moist and cold-dry flows, respectively, and the green area represents the Qinghai-Tibet Plateau, and the red area is where lightning occurs.
Ten typical CG lightning periods occurring in the eastern part of the Yunnan-Guizhou Plateau are used to describe the transversal trough after high pressure pattern (Fig. 7a). At 850 hPa, the WPSH is stronger, and the ridge on its northwest side extends westward to North China. There is an area of low pressure to the west of the WPSH, and the low-pressure center is located in the western Sichuan Basin, oriented southeast and northwest, extending to the Yunnan-Guizhou Plateau. The low interacts with the WPSH and forms a trough over the Yangtze River Basin, while the ridge of high pressure controls South China. Under the combined effect of the high- and low-pressure systems, there is a strong southwesterly low-level jet on the east side of the Yunnan-Guizhou Plateau, causing the relative humidity to exceed 95% over a widespread area. Influenced by the low-pressure trough and the positive vorticity downstream of the 500 hPa trough, there is dynamically-forced upward motion and low-level convergence, which is conducive to the production of convective weather. This weather pattern mainly occurs in areas south of the Yangtze River.
Figure 7b shows the low vortex pattern, which assumes the composite field of five typical periods in the Yangtze River and Huaihe River Valley. At 850 hPa, a cyclonic vortex in northern Jiangsu Province occurs in the low-pressure zone between the cold high pressure in the northwest and the WPSH to the southeast. Under the influence of the strong WPSH, a low-level jet transports warm and moist air toward the lightning zone. The cold high on the northwest side continuously transports dry and cold air to the Yangtze River and Huaihe River Valley, resulting in the convergence of cold and warm air. Moreover, the lightning zone is located downstream of the 500 hPa trough, favorable for low-level convergence and ascent. The synoptic pattern produces convective weather with large amounts of lightning. This type of weather situation occurs mostly in the Yangtze River and Huaihe River Valley.
Nine typical CG lightning periods on the east side of the Yunnan-Guizhou Plateau are taken as examples to show the circulation pattern of confrontational convergence (Fig. 7c). This type is similar to the low vortex pattern except that the low-pressure zone between the two high-pressure centers does not form a closed low pressure. The confrontational convergence of the warm moist air transported from the southwest (the northwest flank of the WPSH) and the cold air intrusion from the northeast (southeast flank of the cold Mongolian high), working in conjunction with the positive vorticity advection downstream of the 500 hPa trough, results in convective weather, which is located on the east side of Yunnan-Guizhou Plateau.
Figure 7d shows the asymptotic convergence pattern, using a composite field of seven typical CG lightning periods in the southern lower Yangtze River valley. There are two main influencing systems at 850 hPa—the powerful WPSH and a low-pressure trough extending from the Sea of Okhotsk to the Huanghe-Huaihe area—and the lightning zone is located at their junction. The southwesterly low-level jet on the northwest side of the WPSH continuously transports warm and moist air to the lightning zone, while the low-pressure trough guides the dry and cold air from the north to the lower Yangtze River. These contrasting airflows produce low-level convergence downstream of the 500 hPa trough, where positive vorticity advection tends to force ascent. In this way, thunderstorms are generated.
The lightning zones of four typical patterns are all located downstream of the 500 hPa trough, and a wide expanse of ascending air is located mostly downstream of the 700 hPa trough. The differences in the synoptic situation, all ultimately resulting in convergence at 850 hPa, are quite large. The common features are that the WPSH is stronger, and the lightning zone is located along the northwestern flank of the WPSH, accompanied by a southwesterly, moisture-laden low-level jet so that the relative humidity in the lightning zone exceeds 95%.
Region | Number of thunderstorm processes (pro winter−1)* | Flash density (fl km−2 winter−1) | PPCG (%) | Lightning type | Mean peak current (kA) |
China | 9.6 | 0.0023 | 26.1 | + | 73.0 |
− | 47.0 | ||||
WLA1 | 5.1 | 0.014 | 35.6 | + | 65.0 |
− | 34.5 | ||||
WLA2 | 4.7 | 0.023 | 21.6 | + | 82.4 |
− | 52.8 | ||||
WLA3 | 3.1 | 0.015 | 25.2 | + | 69.3 |
− | 41.4 | ||||
*The number of thunderstorm processes is the number of thunderstorm processes affecting the region. A strong thunderstorm process can affect multiple WLAs, and some thunderstorm processes that occur outside of three WLAs and do not affect any WLA. |