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Article

Analysis of Inverted Charge Structure and Lightning Activity during the 8.14 Local Hailstorm on the Qinghai–Tibet Plateau

by
Yajun Li
1,2,3,*,
Guangshu Zhang
1,
Weitao Lyu
2 and
Yuxiang Zhao
4
1
Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
2
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China
3
School of Physics and Electromechanical Engineering, Hexi University, Zhangye 734000, China
4
School of Electronic Information and Electrical Engineering, Tianshui Normal University, Tianshui 741001, China
*
Author to whom correspondence should be addressed.
Atmosphere 2023, 14(12), 1795; https://doi.org/10.3390/atmos14121795
Submission received: 16 November 2023 / Revised: 1 December 2023 / Accepted: 3 December 2023 / Published: 7 December 2023
(This article belongs to the Section Meteorology)
Browse Figures
Versions Notes

Abstract

:
In this paper, the charge structure and lightning activity characteristics of the thunderstorm that occurred on the Qinghai–Tibet Plateau on 14 August 2014 were analyzed using data collected from a three-dimensional (3D) lightning very-high-frequency (VHF) radiation source location system and Doppler weather radar. The analysis results showed that the charge structure of the hailstorm was maintained as an inverted dipole throughout the thunderstorm’s development process. The negatively charged region height was distributed in the 5–7 km range (above ground level (AGL)), and the positively charged region was distributed from 2 to 5 km (AGL). The lightning flash types included only cloud flashes and negative cloud–to–ground (CG) flashes in the hailstorm, and cloud flashes accounted for 93% of the total lightning flashes. Cloud flashes accounted for a high proportion of the total flashes, which may have been related to the deep lower positively charged region observed throughout the thunderstorm process. In the hailstorm development stage, the electric field was dominated by positive polarity. When the hail fell, the electric field changed negatively. When the hail ended, the electric field was dominated by negative polarity. A hail event occurred only once and lasted for a long time in the development stage, but in the mature stage, hail fell many times and every time for only a short time, and in the dissipating stage, hail events also occurred many times and each time for a long time. By comparing the radar echoes of the hailstorm cells and normal thunderstorm cells, we found that the area of the 50 dBZ echo in the hailstorm was small, the occurrence time was late, and the duration was short.

1. Introduction

The Qinghai–Tibet Plateau is the highest plateau in the world. Due to its special geographical conditions, it experiences frequent convective activities in summer, and sometimes several thunderstorm processes occur on a single thunderstorm day on the Qinhai–Tibet Plateau. A large number of observational studies performed on the thunderstorm process on the Qinghai–Tibet Plateau have found that the thunderstorm scale is small, the duration is short, and local thunderstorms are often observed [1,2,3]. The frequency of lightning is low, and the lightning discharge transmission distance is short in the overall development process of local thunderstorms on the Qinghai–Tibet Plateau. Zheng et al. [4] found that weak convection in Qinghai–Tibet thunderstorms is the main reason for the common occurrence of small flash rates and sizes observed by using satelite data. These studies indicated that the special topography of the Qinghai–Tibet Plateau makes the local thunderstorm process different from those in other areas and also causes the unique electrical structure of the local thunderstorms. Therefore, the electrical structure of thunderstorms on the Qinghai–Tibet Plateau has been a research focus studied since the last century.
Since the 1980s, many observational studies on thunderstorms occurring over the Qinghai–Tibet Plateau have been conducted. These studies have shown that during the mature stages of thunderstorms on the Qinghai–Tibet Plateau, the charge structure of most thunderstorms is tripole (the upper charge region is positive, the middle is negative and the lower is a small positive charge region), and the surface electric fields are often positive; a small number of thunderstorms have a normal dipole charge structure and negative surface electric fields [3,5,6]. By using surface field mill data and fast and slow antennae data, the charge structure of thunderstorms on the Qinghai–Tibet Plateau, which is often presented as a tripole structure, has been confirmed based on a point charge model or point dipole model fitting calculations, and the lower positive charge region on the Qinghai–Tibet Plateau has been found to be larger than the normal tripole charge structures in other areas [7,8]. Intracloud flashes often present inverted discharge and occur between the middle negative charge region and the lower positive charge region of the tripole charge structure.
Li et al. [9,10,11] used a 3D lightning VHF radiation source location system studying the charge structure during the thunderstorm process on the northeast of the Qinghai–Tibet Plateau. The research results indicated that the charge structure changed with the development of the thunderstorm. The cells usually present an inverted thunderstorm charge structure in the initial development stage, the mature stage of thunderstorms usually presents as a tripole thunderstorm charge structure, and the dissipation stage of thunderstorms has many kinds of thunderstorm charge structures, such as multiple layer charge structures or negative dipoles. Of course, the different charge structure corresponds to the different lightning activity. Some thunderstorms mainly produce intracloud flashes, while some thunderstorms mainly produce negative cloud–to–ground flashes. In some tripole–charge–structure thunderstorm cells, the number of positive intracloud flashes occurring between the upper positive charge region and the middle negative charge region is greater than the number of negative cloud flashes occurring between the middle negative charge region and the lower positive charge region.
An inverted charge structure is reversed from the normal charge structure. It means that a positive charge region temperature range is corresponding to the negative charge region temperature range of the normal dipolar structure [12]. The inverted charge structure often appears in strong storm systems, and this is one of the important reasons for the occurrence of positive CG flashes in strong storms [13,14,15,16]. The inverted charge structure is rare in the thunderstorms of the Qinghai–Tibet Plateau. Through numerical simulations, Guo et al. [17] indicated that the inverted charge structure will be formed when the concentration centers of graupel and ice crystals coincide and are mainly outside the updraft in a Qinghai–Tibet plateau storm with strong updraft. However, many studies only observed that the middle negative charge region above the lower positive charge region formed a negative dipole charge structure in the thunderstorm’s initial development stage of this area.
In this paper, a local thunderstorm process that occurred on the Qinghai–Tibet Plateau in the Datong district of Qinghai on 14 August 2014 is analyzed. This thunderstorm comprised multiple hailfall processes during its development process. It is worth mentioned that the hailstorm charge structure was maintained as an inverted dipole throughout the thunderstorm’s development process and cloud flashes accounted for 93% of the total lightning flashes in the hailstorm. The previous results indicated that hailstones are usually observed with diameters less than 1 cm and durations shorter than 10 min during thunderstorms on the Qinghai–Tibet Plateau [18]. The charge structure of thunderstorms was often tripole during the hail process and the lightning flashes mainly consisted of intracloud flashes and negative CG flashes [19]. These results are quite different from the hailstorm observations obtained in this paper. Because it is difficult to obtain multiparameter synchronous thunderstorm observation data on the Qinghai–Tibet Plateau. The results of this study will be helpful for furthering the understanding of the electric structures of hailstorms on the Qinghai–Tibetan Plateau.

2. Data and Methodology

In this paper, the 3D lightning VHF radiation source location system and field mill and Doppler weather radar synchronously observed data in the Datong district of Qinghai were analyzed. The Datong district of Qinghai Province is located in the northeastern Qinghai–Tibet Plateau. Affected by terrain and the environment, the Datong district often experiences thunderstorms accompanied by hailstones. The observation network of the 3D lightning VHF radiation source location system is located in the Datong district and consists of 7 observation stations. As Figure 1 shows, six substations are radially distributed around the central station, and the diameter of the network is approximately 15 km. The center frequency of the 3D lightning VHF radiation source location system of the 7 stations is 270 MHz, the bandwidth is 6 MHz (267–273 MHz frequency band), and the resolution of the peak value time is 50 ns (20 MHz A/D converter). Due to the adoption of a logarithmic amplifier, the measurement range reaches up to 100 dB. A wireless broadband communication system is used to form a synchronous observation network, the main station (Xibeicha station) is used to control the recording mode, and the trigger time is recorded by the high precision clock synchronized with GPS. Based on the absolute time of the arrival of the peak value determined synchronously by the seven stations, the 3D location results of the radiation source are obtained by using the time difference location technology [20]. The 3D spatial and temporal location results of the lightning radiation source can accurately describe the lightning discharge process. The horizontal error range of the location system over the center of the network is 10–48 m root mean square (rms), and the vertical error ranges from 20 m to 78 m (rms). The range and altitude errors in the outer radial direction of the network show a parabolic shape with the distance [21]. The main station of the location network is the Xibeicha station, with coordinates of E 101.47 and N 37.11 and an altitude (ground height) of 2.8 km. The distance and height used in this paper are all based on the Xibeicha station. All altitudes in this paper are given as heights above ground level (AGL). In addition, a field mill (measuring range is ±50 kV/m) was used to measure the electric field on the ground at a sampling rate of 0.1 s. The fast antenna was equipped at each observation station with a bandwidth ranging from 1.5 kHz to 10 MHz and used to determine the quantity of intracloud flashes and CG flashes in the thunderstorms. At the same time, a new-generation C-band Doppler weather radar (E 101.77, N 36.59) located 63.58 km away from the main station was used for the observations. During the whole observation period, the radar collected conventional data of volume scans every 5–6 min.
The location results of the 3D lightning VHF radiation source location system can describe each lightning discharge process in detail. Based on the bidirectional leader mode [22,23], the polarity of each lightning flash charge region at different positions within a thundercloud can be judged according to the transmission characteristics of the lightning radiation sources in positive and negative charge regions [24,25]. The method and principle of the 3D lightning VHF radiation source location system determine the polarity of the charge region and are similar to those of the Lightning Mapping Array system (LMA) [25,26,27]. In addition, because the initial breakdown of the transmission direction is related to the charge structure of the thunderstorm [28], the polarity of the pulse cluster generated by the initial preliminary breakdown process can also be used to help determine the charge structure. After the polarity and height of the charge region are determined for a period of time, the evolution process of the charge structure of the entire thunderstorm can be tracked and analyzed along with the development of the thunderstorm [9,10,11].

3. Descriptions of the Thunderstorm Development and Hailfall Processes

3.1. Overview of the Thunderstorm on 14 August 2014

The radar echo change in the thunderstorm’s development process that occurred on 14 August 2014 is shown in Figure 2. As Figure 2 shows, two thunderstorm cells were generated at the beginning of the thunderstorm’s development process. Thunderstorm cell 1 was located in the northwest direction of the observation network and was a local thunderstorm. There were multiple hail events in the thunderstorm cell 1. Thus, we called thunderstorm cell 1 hailstorm cell 1. Figure 3 shows the changes in the surface electric field of hailstorm cell 1 alongside its lightning frequency and lightning discharge radiation source distribution findings. Statistics on the frequency of each lightning type that occurred during the evolution of each cell are given for each radar volume scan interval (6 min). Hailstorm cell 1 produced a lightning flash at 13:53 (All time in this paper is Beijing Time). As Figure 3 shows, the frequency of the lightning flashes increased with the development of hailstorm cell 1. After 14:16, the development of the hailstorm entered the mature stage, and the frequency of lightning flashes reached the maximum. After 14:27, hailstorm cell 1 began to dissipate, the cell split, and the frequency of lightning flashes decreased. Hailstorm cell 1 completely dissipated at 14:50. Hailstorm cell 1 lasted approximately 1 h, its horizontal scale was less than 15 km, and the echo area exceeding 40 dBZ was less than 100 km2. In the entire development process of hailstorm cell 1, a total of 212 lightning flashes were produced, comprising 199 intracloud flashes and 13 negative CG flashes. Thunderstorm cell 2, in the southeast region of the observation network, began to generate lightning flashes at 14:03. Then, thunderstorm cell 2 gradually strengthened. After 14:27, thunderstorm cell 2 began to dissipate, but at 14:44, it began to strengthen again, and at 14:50, it entered the dissipation stage. A total of 37 lightning flashes were produced by thunderstorm cell 2 until 14:50. Thunderstorm cell 3 appeared east of thunderstorm cell 2 at 14:09. The horizontal scale of thunderstorm cell 3 was very small throughout the whole thunderstorm’s development process, and only two lightning flashes were generated by this cell.
The sounding data were obtained from the Xining Sounding station, which is located 50 km away from the Xibeicha station. The sounding data at 20:00 showed that the 0 °C temperature layer of hailstorm cell 1 was located at 1.65 km (AGL), which is relatively low.

3.2. Hailfall Process of Hailstorm Cell 1

Figure 2 shows that the Xibeicha station was always located below hailstorm cell 1 during the development process of hailstorm cell 1. Therefore, the hailfall data were observed at the Xibeicha station by people. Liu et al. [29] statistically analyzed the hail characteristics on the northeast side of the Qinghai–Tibet Plateau. The results indicated that the local hailfall process on the Qinghai–Tibet Plateau mainly produces moderate hail (diameters between 5 and 20 mm), followed by small hail (diameters < 5 mm, called graupel), while the probability of large hail (diameters ≥ 20 mm) occurring is very small. Hailstorm cell 1 mainly produced moderate hail. According to the hailfall process characteristics and the development of hailstorms, the hailfall process in hailstorm cell 1 can be divided into three stages. As Figure 3a shows, the first stage was a long hailfall process (hail–1 in Figure 3a) during the development stage of hailstorm cell 1. The hailfall time was 14:07–14:16, lasting 9 min. Hailstones with a diameter of approximately 5 mm began falling at 14:07, and the sizes of the hailstones increased with the development of hailstorm cell 1. The maximum hailstone size reached approximately 8 mm at 14:13 and then lasted until 14:16. The second stage comprised multiple short-term hailfall processes (hail–2 in Figure 3a) during the mature stage of hailstorm cell 1. The period of hailfall was 14:17–14:29, and three hailfall events were observed. The first period was from 14:17–14:19, with hailstone sizes of approximately 5 mm, the quantity of which was small. The second period was from 14:21–14:25; at this time, the quantity of hailstones increased, and their diameter reached 8 mm. During the third period, from 14:26 to 14:29, the quantity of the hailstones increased rapidly, and their diameters increased to 10 mm. The third stage (hail–3 in Figure 3a) comprised multiple long–term hailfall processes in the dissipation stage of hailstorm cell 1. The period of hailfall was 14:31–14:45, and two hailfall events were observed. In the first, small hailstones began to fall at 14:31, and the hailstone diameter and quantity gradually increased with the development of hailstorm cell 1. At 14:36, the first hailfall event stopped for approximately 1 min. At 14:37, hail fell again with a diameter of approximately 10 mm. Then, the diameter of the hailstones increased to 12 mm at 14:39, and the quantity of the hailstones also increased. The hailstones stopped falling at 14:45.

4. Analysis of the Charge Structure and Electric Field Changes

4.1. Analysis of Hailstorm Cell 1

Hailstorm cell 1 began to develop at approximately 13:50. As Figure 2a shows, hailstorm cell 1 had two strong echo centers, and the area of strong echoes larger than 40 dBZ was very small at 13:57. Figure 3b shows that there were a few lightning flashes in this period. Figure 4 shows the distribution of the lightning radiation sources during the different periods of the hailstorm cell 1 development process. The red radiation source represents the positively charged region, and the blue radiation source represents the negatively charged region. Figure 4a shows that the transmission distance of the lightning discharge was very short and less than 10 km at 13:53–13:57. The charge structure was an inverted dipole charge structure (negative above positive charge). The height of the charge region was slightly higher in the strong echo zone, the height of the positively charged region spanned from 3 to 5 km (AGL), and the height of the negatively charged region was from 5 to 8 km (AGL). The heights of these charge regions were slightly lower in the periphery of the strong echo zone, where the positively charged region was distributed at 2–4 km (AGL) and the negatively charged region was distributed at 4–6 km (AGL). At 14:03, as Figure 2c shows, the strong echo area gradually expanded. The two strong echo centers of hailstorm cell 1 were connected, and the frequency of the lightning flashes increased. The charge structure was still an inverted dipole at this time. The positively charged region was mainly distributed at 2–5 km (AGL), and the negatively charged region was mainly distributed at 5–8 km (AGL).
At 14:07, hailstones with a diameter of 5 mm were observed at the Xibeicha station, and the size of these hailstones increased with the development of hailstorm cell 1; the hailfall stopped at 14:15. By comparing Figure 2g, e, the area of the strong echo zone expanded at 14:15, and the two strong echo centers further merged. At this time, the charge structure was unchanged, maintaining an inverted dipole charge structure (Figure 4d). The charge region height of the strong echo zone was the same as the last period. The negatively charge region in the periphery of the strong echo zone center was lifted to a height of 5–7 km (AGL) and the lower positively charge region was not changed. Figure 5 shows the maximum height distribution of the different echo intensities of cells 1, 2, and 3 at different times during the thunderstorm’s development. As Figure 5a shows, the 50 dBZ echo did not appear at this stage. The heights of the 20 dBZ echo and 40 dBZ echo in hailstorm cell 1 decreased, especially the 40 dBZ echo zone. This result indicated that the updraft continuing to uplift the hydrometeors was difficult due to the drop in the hailstones at this stage. Figure 3a shows the change in the surface electric field of the Xibeicha station during the thunderstorm’s development. Figure 3a shows that the electric field was dominated by positive polarity (the positive charge on the head corresponds to the positive electric field) during the development stage. When the hailstones began to fall at 14:07, the electric field changed negatively, and the electric field value decreased. After the hailfall process stopped, the electric field began to positively recover.
Hailstorm cell 1 entered the mature stage at 14:16–14:21. As shown in Figure 2i, the two strong echo centers had completely merged at this time. Figure 5a shows that the height of the 40 dBZ echo zone rose again and reached 7 km (AGL). At the same time, a 50 dBZ echo zone appeared, and the maximum height reached 4 km (AGL). The frequency of the lightning flashes exhibited a rapid increase, as Figure 3b shows, and the type of lightning flash was mainly intracloud flashes during this stage. Figure 4e shows that the charge structure of hailstorm cell 1 still maintained the inverted dipole charge structure at this time. The strongest echo zone of hailstorm cell 1 was located in the west, where the maximum radar echo was larger than 50 dBZ. The charge region was lifted and the positively charged region was located at 3–5 km (AGL), and the negatively charged region was at 5–8 km (AGL). The intensity of the radar echo was small east of hailstorm cell 1. The height of the charge region declined, and the positively charged region was mainly distributed at heights of 2–4 km, and the negatively charged region was mainly distributed at heights of 5–7 km. At 14:22–14:27, the development of hailstorm cell 1 was the most vigorous, and the radar echo of 50 dBZ reached 5 km (AGL). As Figure 3b and Figure 4f show, the frequency of the lightning flashes reached its peak value during this period, and all lightning flashes were intracloud flashes. The charge structure of hailstorm cell 1 remained unchanged; the positively charged region was still mainly distributed at 2–5 km (AGL), and the negatively charged region was at 5–7 km (AGL). In the mature stage of hailstorm cell 1, the hailfall time was short in each event. There were three events, each lasting approximately 3 min. The height of the charge region no longer increased at this stage. After the end of each hailfall event, the height of the negatively charged region descended slightly. The electric field continued to change negatively in the mature stage of hailstorm cell 1, when the hail fell. When the last hailfall stopped, the electric field value briefly became negative, and the electric field value then became positive.
Figure 2m shows that hailstorm cell 1 entered the dissipating stage at 14:28, at which time, the area of the strong echo zone rapidly shrank. The maximum height of the echo from 20 dBZ to 50 dBZ began to descend. The lightning frequency started to decrease. At 14:31, the first hailfall process in the dissipating stage began, lasting approximately 5 min. At this stage, the electric field changed in the positive direction first. When the electric field was positive, the electric field began to change negatively, and the value of the electric field was close to 0 V. Figure 4g shows that the charge structure was still an inverted dipole in this period, with the original height of charge region distribution. The second hailfall process began at 14:38 in the dissipation stage. The diameter of the hailstones gradually increased, reaching a maximum of approximately 12 mm. As shown in Figure 5a, the 50 dBZ strong echo zone disappeared at this time, and the height of the other intensity echo zone decreased. Hailstorm cell 1 split into several cells, as shown in Figure 2o, and the area of the strong echo region shrank rapidly. When the hailfall process finished at 14:45, the lightning frequency decreased sharply, and the value of the electric field changed to –40 kV. The thunderstorm then completely dissipated, and no more lightning flashes were generated after 14:50. The charge structure of hailstorm cell 1 was maintained as an inverted dipole until the end of the hailstorm, and the height of the charge region decreased. The lower positively charged region was mainly distributed in the 2–4 km (AGL) range, and the upper negatively charged region was mainly distributed in the 5–6 km (AGL) range at 14:39–14:50 (Figure 4i,j).

4.2. Analysis of Thunderstorm Cells 2 and 3

As Figure 2a shows, thunderstorm cell 2 developed from 13:57. The first lightning flash generated by thunderstorm cell 2 was an intracloud flash at 14:03:44, which we called intracloud flash 140344, and the second lightning flash was a negative CG flash that occurred at 14:05:51 and we called negative CG flash 140551. The lightning discharge process radiation source distributions of the intracloud flashes 140344 and negative CG flashes 140551 are shown in Figure 6. The intracloud flash discharge process occurred between the positively charged region located at 5–7 km (AGL) and the negatively charged region at 3–5 km (AGL), while the negative CG flash discharge process occurred between the negatively charged region at 3–5 km (AGL) and the lower positively charged region at 1.8–3 km. So, the charge structure of thunderstorm cell 2 was a tripole after 14:09. There were three charge regions, that is, the upper positively charged region located at 5–7 km (AGL), the middle negatively charged region at 3–5 km (AGL), and the lower positively charged region at 1.8–3 km during the initial development stage. The radar echo of cell 2 was also analyzed. The 50 dBZ echo in thunderstorm cell 2 appeared in the development stage (Figure 5b), and the horizontal scale of the echo zone larger than 40 dBZ was large, with an expanse greater than 400 km2 (Figure 2c).
With the development of thunderstorm cell 2, the charge structure was still maintained as a tripole, and the frequency of the lightning flashes increased (Figure 7). The development of thunderstorm cell 2 was the most vigorous phase during the 14:16–14:21 period, and the height of the charge region increased with the development of thunderstorm cell 2. As shown in Figure 8, the upper positively charged region was located at 6–8 km (AGL), the middle negatively charged region was located at 4–6 km (AGL), and the lower positively charged region was located at 2–4 km (AGL). The height of the 50 dBZ echo reached its highest value (Figure 5b) during this period. After 14:27, thunderstorm cell 2 began to dissipate, and the height of the 50 dBZ echo of thunderstorm cell 2 dropped sharply (Figure 5b). The charge structure changed into a negative dipole [30,31]. That is, the charge region had only two layers, the middle negatively charged layer and the lower positively charged layer, and the upper positively charged region either did not participate in the lightning discharge or did not exist in this period. The height of the charge region was the same as that of the tripole charge structure in the same polarity charge regions. At 14:44, thunderstorm cell 2 began to develop again, and the height of the 50 dBZ echo of thunderstorm cell 2 rose sharply. The charge structure changed into a tripole again, the upper positively charged region was located at 5–7 km (AGL), the middle negatively charged region was located at 4–5 km (AGL), and the lower positively charged region was located at 2–4 km (AGL). However, lightning flashes were very rare at this time. After 14:50, the thunderstorm quickly dissipated, and no more lightning flashes occurred.
Thunderstorm cell 3 was small and lasted approximately 30 min in the development process. As Figure 2 shows, the horizontal scale of the 40 dBZ echo region was only approximately 5 km. Only two lightning flashes occurred in thunderstorm cell 3. The lightning discharge process occurred between the upper positively charged region of 6–8 km (AGL) and the lower negatively charged region of 4–6 km (AGL). However, Figure 5c shows that the 50 dBZ echo zone appeared at 14:21 with a height of 6 km (AGL) and lasted approximately 24 min during the development process of thunderstorm cell 3.

5. Discussion and Analysis

5.1. Effect of Hailfall on the Heights of the Charged Regions and Electric Fields of Hailstorms

The Xibeicha station was located under hailstorm cell 1 during the entire thunderstorm’s development process. Figure 3a shows that the electric field of Xibeicha station was dominated by a positive charge throughout the thunderstorm’s initial development stage, so the lower part of the cloud was controlled by positive charges. When the hail began to fall, the electric field changed negatively. After the hail events stopped, the electric field recovered in the positive direction, and the electric field value still was kept positive at this stage. Thus, it can be inferred that hail particles fell to the ground with a positive charge during the hailfall process, thus reducing the electric field value. With the development of the hailstorms, the height of the charge region rose gradually.
In the mature stage of hailstorm cell 1, hailfall processes lasting approximately 3 min appeared three times. The electric field continued to change negatively during these hailfall processes, and the electric field value positively changed when hailfall stopped. However, the electric field value became negative after the last hailfall event occurred. The height of the charge region no longer increased during the mature stage of cell 1. After the hailfall stopped, the height of the negatively charged region decreased by approximately 1 km. This indicated that the hail particles continued to carry positive charges to the ground and decrease the electric field value.
When hailstorm cell 1 entered the dissipation stage, the electric field changed positively and was finally restored to a positive value. Then, the first hailfall event of the dissipation stage began, and the electric field changed negatively. The hailfall event lasted approximately 6 min. When the hail briefly stopped, the electric field value was near 0 V. The duration of the second hailfall event at this stage was 8 min, during which time the electric field continued to change negatively. Finally, the electric field value dropped to −40 kV after the hailfall events concluded. The lightning frequency decreased sharply in the dissipating stage. The heights of both the positively and negatively charged regions decreased. The long-term hail process in the dissipating stage accelerated the dissipation of thunderstorms, sharply decreased the lightning flash frequency, caused the heights of both the positive and negative charge regions to decrease, and eventually turned the electric field value negative.
During the initial development stage and mature stage of hailstorm cell 1, when the hailfall process stopped for a short time, the electric field positively recovered. This result indicated that the positive charge was supplemented after the hailfall. At the end of the last hailfall event, the electric field no longer changed in the positive direction. This result indicated that no additional positive charge was added to the hailstorm. After hailstorm cell 1 was concluded, no end of thunderstorm oscillation (EOSO) phenomenon occurred in the electric field [32]. This indicated that a large number of hydrometeors with positive charge were brought to the ground during the hailfall process, weakening the positively charged region. Hailstorm cell 1 was inclined in the dissipation stage, causing the declined negatively charged region to be exposed to the outside of the positively charged region, and the electric field at this time was dominated by the declined negatively charged region, so the value of the electric field became negative. When hailstorm cell 1 dissipated and moved out of the space above the Xibeicha station at all, no EOSO phenomenon occurred. This indicated that hailstorm cell 1 had no upper positively charged region.

5.2. Relationship between the Charge Structures of Hailstorms and Lightning Activity

The charge structure often presents as a negative dipole in the initial development stage of a thunderstorm cell [11], with the development of thunderstorms, the charge structure of the thunderstorm cell changes into a tripole in the local area. That is, the tripole charge structure has often been observed on the Qinghai–Tibet Plateau [10,33]. MacGorman et al. [34] pointed out that the inverted charge structure observed in the cloud was due to the thunderstorm being dominated by positively charged graupel, while the main negatively charged region was replaced by the positively charged region and negatively charged ice crystal particles existed in the upper part of the storm. The hailstorm cell 1 showed that the upper negatively charged region was mainly concentrated at 5–7 km (AGL), the lower positively charged region was mainly distributed at 2–5 km (AGL), and no positively charged region existed above the negatively charged region to participate in lightning discharge. The charge structure of thunderstorm cell 2 occurring at the same time was tripole, with heights of 5–7 km (AGL) of the upper positively charged region, heights of 3–5 km (AGL) of the middle negatively charged region, and heights of 1.8–3 km (AGL) of the lower positively charged region. By comparing hailstorm cell 1 and thunderstorm cell 2, the height of the upper negatively charged region of hailstorm cell 1 corresponded to the height of the upper positively charged region of thunderstorm cell 2, and the height of the lower positively charged region of hailstorm cell 1 corresponded to the heights of the middle negatively charged region and lower positively charged region of cell 2. This result indicated that the charge structure of hailstorm cell 1 was an inverted dipole charge structure, differing from the negative dipole structure in the normal tripole charge structure. Moreover, the inverted dipole charge structure was maintained throughout the whole development process of hailstorm cell 1. This result differed from the findings of a previous study on the charge structure of the thunderstorms observed on the Qinghai–Tibet Plateau.
Many researchers have proposed that the lower positively charged region enhances the electric field at the bottom of the middle negatively charged region in thunderstorms, making it easier for the negative leader to reach the ground and generate negative CG flashes. However, a large lower positively charged region will hinder the transmission of the negative leader to the ground and generate negative ground flashes, so that lightning will occur between the middle negatively charged region and the lower positively charged region and generate cloud flashes [2,6,30,35,36,37,38].
The sounding data Indicated that the lower positively charged region at a height of 2–5 km (AGL) in hailstorm cell 1 corresponded to the temperature layer of −2.2 °C~−23.9 °C. In the studies of Qinghai–Tibetan Plateau thunderstorms, researchers have found that thunderstorms in this region usually have the characteristics of weak development, a relatively low height of the 0 °C layer, a relatively humid lower part of the thunderstorm clouds but a relatively dry upper part of the thunderstorm clouds [19,39,40]. The positively charged region in the lower part of hailstorm cell 1 corresponds to the lower temperature than the lower positive charge region of the normal thunderstorm. Therefore, the relatively low temperatures and abundant liquid water in the lower part of hailstorms cause the positively charged hydrometeor particles in the positively charged region to collide and grow under the action of updrafts, rapidly growing into large graupel particles or hailstones. Thus, an amount of large positively charged hydrometeors and a small amount of small positively charged hydrometeors together formed a deep, low positively charged region in hailstorm cell 1 at 2–5 km (AGL), approximately 3 km thick.
Hailstorm cell 1 had a deep positively charged region that made it difficult for the negative leader to pass through the positively charged region and produce a negative CG flash. The discharge of most of the lightning flashes occurred only between the deep lower positively charged region and the upper negatively charged region in hailstorm cell 1. Therefore, a large number of intracloud flashes were produced in hailstorm cell 1, while few negative CG flashes occurred. The lightning flash types observed in hailstorm cell 1 included only intracloud flashes and negative CG flashes. The intracloud flashes accounted for 93% of the total lightning flashes in hailstorm cell 1.

6. Conclusions

In this paper, 3D lightning VHF radiation source location data and radar echo data were used to analyze a long–term multiple hailstorm event on the Qinghai–Tibetan Plateau. The analysis results of the charge structure, lightning activity, and radar echo of the analyzed thunderstorm are described as follows.
  • Hailstorm cell 1 comprised several hailfall processes. The charge structure of this hailstorm showed an inverted dipole throughout the whole thunderstorm’s development process. The lower positively charged region was deep and was mainly distributed at heights of 2–5 km (AGL), while the upper negatively charged region was mainly distributed at heights of 5–7 km (AGL). The normal thunderstorm cell 2 exhibited a normal tripole charge structure. The upper positively charged region spanned from 5 to 7 km (AGL), the middle negatively charged region spanned from 3 to 5 km (AGL), and the lower positively charged region spanned from 1.8 to 3 km (AGL).
  • The lightning flash types observed in hailstorm cell 1 included only intracloud flashes and negative CG flashes. The intracloud flashes accounted for 93% of the total lightning flashes in hailstorm cell 1; this may have been related to the deep position of the lower positively charged region in the thunderstorm.
  • The electric field value of hailstorm cell 1 was positive during the development and mature stages. When the hail was falling, the electric field value decreased, and when hail stopped falling, the electric field positively recovered. In the dissipating stage of the hailstorm, the value of the electric field became negative after the hail concluded, and no EOSO phenomenon occurred in this hailstorm.
  • The 50 dBZ echo zone in hailstorm cell 1 appeared approximately half an hour after the development of the thunderstorm, with a short duration of approximately 15 min and a low height. In contrast, the 50 dBZ echo zone in normal thunderstorm cell 2 appeared early and accompanied almost the whole thunderstorm process, with a relatively high height.
  • The hailfall process differed in each stage of the thunderstorm’s development process. In the development stage, hailfall occurred only once and lasted for a long time. In the mature stage, hail fell many times and lasted for only a short time. In the dissipating stage, hail fell many times and lasted for a long time.
In the studies of the inverted polarity charge structure, it has been found that the inverted structure does not always exist in severe thunderstorms in the Great Central Plains of the United States, but gradually changes from the normal polarity charge structure in thunderstorms [39,41,42]. In addition, the inverted tripolar charge structure easily produces positive cloud–to–ground flash [34]. In this paper, the observation results indicated that the charge structure always maintained the inverted charge structure during the whole thunderstorm’s development process, and the intracloud flash dominated. This is different from the inverted charge structure in severe thunderstorms. The difference in the location of thunderstorms and the intensity of thunderstorms may be the main reason for the difference in the results. The Qinghai–Tibet Plateau is a mountainous area, so most thunderstorm cells are caused by topographic uplift [19]. A thunderstorm occurring in different locations will have different hydrometeors particles and liquid water content in thunderstorms, and this will result in different charge structures. From the analysis in this paper, the distance between hailstorm cell 1 and thunderstorm cell 2 was approximately 15 km, which is very close, but their charge structures, lightning frequencies, and radar echoes were completely different. Of course, this paper only studies an individual case of the Qinghai–Tibet Plateau thunderstorm, and has only shown that the thunderstorm process on the Qinghai–Tibetan Plateau has the inverted polarity charge structure, so the research results have certain limitations. The dynamics and microphysical processes of hailstorms on the Qinghai–Tibet Plateau are complex and related to the topography, underlying surface, and environmental factors during the formation of thunderstorms on the plateau. Therefore, more synchronous meteorological data and dual polarization Doppler radar observation data are needed for further research.

Author Contributions

Conceptualization, Y.L. and G.Z.; methodology, Y.L. and G.Z.; software, G.Z.; validation, Y.L., G.Z., W.L. and Y.Z.; formal analysis, Y.L. and G.Z.; investigation, Y.L.; resources, G.Z.; data curation, Y.L., G.Z., W.L. and Y.Z.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L.; visualization, Y.L. and G.Z.; supervision, G.Z. and W.L.; project administration, Y.L., W.L. and Y.Z.; funding acquisition, Y.L., W.L. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 42165006), the Open Research Program of the State Key Laboratory of Severe Weather (Grant No. 2019LASW-B10), and Science and Technology Supporting Program Project of Tianshui City (2020-FZJHK-9757).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Readers can obtain the data from the corresponding author ([email protected]).

Acknowledgments

We are indebted to all those who participated in our lightning observation experiments in Qinghai. We also thank the Qinghai Provincial Meteorological Bureau for their support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Map of the observation sites.
Figure 1. Map of the observation sites.
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Figure 2. Radar composite reflectivity from 13:53 to 14:50 and corresponding lightning radiation sources from 13:53 to 14:50 during the thunderstorm that occurred on 14 August 2014, overlain with the radar composite reflectivity and charge structure. The 0 point is Xibeicha station location. The pink “.” Symbols are the radiation sources on radar composite reflectivity and the “o” symbols are the observation stations.
Figure 2. Radar composite reflectivity from 13:53 to 14:50 and corresponding lightning radiation sources from 13:53 to 14:50 during the thunderstorm that occurred on 14 August 2014, overlain with the radar composite reflectivity and charge structure. The 0 point is Xibeicha station location. The pink “.” Symbols are the radiation sources on radar composite reflectivity and the “o” symbols are the observation stations.
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Figure 3. Changes in the electric field at Xibeicha station, the lightning frequency change in hailstorm cell 1, and the lightning discharge radiation source height over time in the hailstorm development process on 14 August 2014. (a) The change in the surface electric field and hailfall stage over time, (b) the lightning flash frequency change in hailstorm cell 1 over time, and (c) the height distribution of the lightning discharge radiation source in hailstorm cell 1 over time. The hail–1, –2, –3 represent the hailfall period in development stage, mature stage, and dissipation stage of thunderstorm. The IC, −CG, +CG and total stand for the number of intracloud flashes, positive cloud–to–ground flashes, negative cloud-to-ground flashes, and total number of flashes. The color represent time in Figure 3c.
Figure 3. Changes in the electric field at Xibeicha station, the lightning frequency change in hailstorm cell 1, and the lightning discharge radiation source height over time in the hailstorm development process on 14 August 2014. (a) The change in the surface electric field and hailfall stage over time, (b) the lightning flash frequency change in hailstorm cell 1 over time, and (c) the height distribution of the lightning discharge radiation source in hailstorm cell 1 over time. The hail–1, –2, –3 represent the hailfall period in development stage, mature stage, and dissipation stage of thunderstorm. The IC, −CG, +CG and total stand for the number of intracloud flashes, positive cloud–to–ground flashes, negative cloud-to-ground flashes, and total number of flashes. The color represent time in Figure 3c.
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Figure 4. Charge structure distributions of the lightning radiation sources of hailstorm cell 1 in different directions on 14 August 2014. S–N indicates a south–north vertical projection, and W–E indicates a west–east vertical projection. Red radiation sources indicate a positively charged region, and blue radiation sources indicate a negatively charged region.
Figure 4. Charge structure distributions of the lightning radiation sources of hailstorm cell 1 in different directions on 14 August 2014. S–N indicates a south–north vertical projection, and W–E indicates a west–east vertical projection. Red radiation sources indicate a positively charged region, and blue radiation sources indicate a negatively charged region.
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Figure 5. Maximum height changes in the radar reflectivity with time in different thunderstorm cells on 14 August 2014. (a) The maximum radar reflectivity height changes in thunderstorm cell 1; (b) the maximum radar reflectivity height changes in thunderstorm cell 2; (c) the maximum radar reflectivity height changes in thunderstorm cell 3.
Figure 5. Maximum height changes in the radar reflectivity with time in different thunderstorm cells on 14 August 2014. (a) The maximum radar reflectivity height changes in thunderstorm cell 1; (b) the maximum radar reflectivity height changes in thunderstorm cell 2; (c) the maximum radar reflectivity height changes in thunderstorm cell 3.
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Figure 6. Radiation source distributions of the intracloud flash 140344 and negative cloud–to–ground flash 140551 that occurred at 14:03:44 and 14:05:51, respectively, on 14 August 2014. (a) Height–time plots, (b) east–westward vertical projection, (c) height distribution of the number of radiation events, (d) plan view, and (e) north–southward vertical projection of the lightning radiation sources. Red radiation sources indicate a positively charged region; blue radiation sources indicate a negatively charged region; green radiation sources indicate the negative leader.
Figure 6. Radiation source distributions of the intracloud flash 140344 and negative cloud–to–ground flash 140551 that occurred at 14:03:44 and 14:05:51, respectively, on 14 August 2014. (a) Height–time plots, (b) east–westward vertical projection, (c) height distribution of the number of radiation events, (d) plan view, and (e) north–southward vertical projection of the lightning radiation sources. Red radiation sources indicate a positively charged region; blue radiation sources indicate a negatively charged region; green radiation sources indicate the negative leader.
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Figure 7. Lightning flash frequency changes in thunderstorm cell 2 over time.
Figure 7. Lightning flash frequency changes in thunderstorm cell 2 over time.
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Figure 8. Charge structure distribution of lightning radiation sources in thunderstorm cell 2 in the 14:16–14:21 period on 14 August 2014. (a) Height–time plots, (b) east–westward vertical projection, (c) height distribution of the number of radiation events, (d) plan view, and (e) north–southward vertical projection of the lightning radiation sources. Red radiation sources indicate a positively charged region; blue radiation sources indicate a negatively charged region.
Figure 8. Charge structure distribution of lightning radiation sources in thunderstorm cell 2 in the 14:16–14:21 period on 14 August 2014. (a) Height–time plots, (b) east–westward vertical projection, (c) height distribution of the number of radiation events, (d) plan view, and (e) north–southward vertical projection of the lightning radiation sources. Red radiation sources indicate a positively charged region; blue radiation sources indicate a negatively charged region.
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Li, Y.; Zhang, G.; Lyu, W.; Zhao, Y. Analysis of Inverted Charge Structure and Lightning Activity during the 8.14 Local Hailstorm on the Qinghai–Tibet Plateau. Atmosphere 2023, 14, 1795. https://doi.org/10.3390/atmos14121795

AMA Style

Li Y, Zhang G, Lyu W, Zhao Y. Analysis of Inverted Charge Structure and Lightning Activity during the 8.14 Local Hailstorm on the Qinghai–Tibet Plateau. Atmosphere. 2023; 14(12):1795. https://doi.org/10.3390/atmos14121795

Chicago/Turabian Style

Li, Yajun, Guangshu Zhang, Weitao Lyu, and Yuxiang Zhao. 2023. "Analysis of Inverted Charge Structure and Lightning Activity during the 8.14 Local Hailstorm on the Qinghai–Tibet Plateau" Atmosphere 14, no. 12: 1795. https://doi.org/10.3390/atmos14121795

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