Comprehensive Analysis of a Coast Thunderstorm That Produced a Sprite Over the Bohai Sea

atmosphere

Article Comprehensive Analysis of a Coast Thunderstorm That Produced a Sprite over the Bohai Sea

Cong Pan 1,2 , Jing Yang 2,3,*, Kun Liu 1 and Yu Wang 4

1 College of Electronic Engineering, Chengdu University of Information Technology, Chengdu 610225, China; [email protected] (C.P.); [email protected] (K.L.) 2 Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 3 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China 4 State Grid Electric Power Research Institute, Wuhan 430074, China; [email protected] * Correspondence: [email protected]

Abstract: Sprites are transient luminous events (TLEs) that occur over thunderstorm clouds that represent the direct coupling relationship between the troposphere and the upper atmosphere. We report the evolution of a mesoscale convective system (MCS) that produced only one sprite event, and the characteristics of this thunderstorm and the related lightning activity are analyzed in detail. The results show that the parent ﬂash of the sprite was positive cloud-to-ground lightning (+CG) with a single return stroke, which was located in the trailing stratiform region of the MCS with a radar reﬂectivity of 25 to 35 dBZ. The absolute value of the negative CG (−CG) peak current for half an hour before and after the occurrence of the sprite was less than 50 kA, which was not enough to produce the sprite. Sprites tend to be produced early in the maturity-to-dissipation stage of the MCS, with an increasing percentage of +CG to total CG (POP), indicating that the sprite production was Citation: Pan, C.; Yang, J.; Liu, K.; the attenuation of the thunderstorm and the area of the stratiform region. Wang, Y. Comprehensive Analysis of a Coast Thunderstorm That Produced Keywords: sprite; mesoscale convective system; parent thunderstorm; cloud-to-ground lightning a Sprite over the Bohai Sea. Atmosphere 2021, 12, 718. https:// doi.org/10.3390/atmos12060718

Academic Editors: 1. Introduction Masashi Kamogawa and Yoav Yair Observations of TLEs have confirmed the effects of lightning in near-Earth space, such as sprites. As important members in the TLEs family, sprites are transient optical Received: 28 April 2021 emissions of vertical growth that occur between 40 to 90 km altitudes above the ground [1]. Accepted: 31 May 2021 Observations around the world have confirmed that the sprites are global discharge events Published: 2 June 2021 in the middle and upper atmosphere, and appear in various shapes, such as column, carrot, jellyfish, angel, etc. [2–7]. The sprites are mainly produced by the +CG strokes above the Publisher’s Note: MDPI stays neutral stratiform region of the MCS, but occasionally by −CG strokes [8,9]. In addition, a few with regard to jurisdictional claims in scholars believe that intracloud flashes can also produce sprites [10,11]; this may be related published maps and institutional affil- to the long-time intracloud lightning discharge of parent lightning and the long charge iations. transfer distance of the M-components [12]. The sprites usually occur between a few milliseconds to tens of milliseconds after return strokes [8,12,13], and the lateral deviation from the return strokes location can reach several kilometers to tens of kilometers [14–16]. The parent lightning of a positive sprite Copyright: © 2021 by the authors. usually occurs in the convective area of the MCS and then develops horizontally into Licensee MDPI, Basel, Switzerland. the stratiform region, and releases the positive charge to the ground [12,17,18]. When a This article is an open access article large amount of charge is transferred from the thunderstorm cloud to the ground [19], the distributed under the terms and disturbance of a quasi-electrostatic field in the middle and upper atmosphere exceeds the conditions of the Creative Commons breakdown threshold of the traditional medium, thus accelerating the lower energy in the Attribution (CC BY) license (https:// bottom space of the ionosphere at night. Electrons eventually trigger the development of creativecommons.org/licenses/by/ ionized streamers, accompanied by strong optical radiation [20,21]. 4.0/).

Atmosphere 2021, 12, 718. https://doi.org/10.3390/atmos12060718 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 17

Atmosphere 2021, 12, 718 2 of 17 in the bottom space of the ionosphere at night. Electrons eventually trigger the development of ionized streamers, accompanied by strong optical radiation [20,21]. A large number of observations have shown that if an MCS produces a sprite, it will A large number of observations have shown that if an MCS produces a sprite, it will alsoalso produceproduce manymany other sprites, sprites, and and sprites sprites us usuallyually occur occur in inclusters clusters [7,18,22–29]. [7,18,22–29 In]. this In thisstudy, study, we analyze we analyze an MCS an MCS thunderstorm thunderstorm that was that smaller was smaller and only and produced only produced one sprite, one sprite,compared compared with the with typical the typical MCS that MCS produced that produced sprites, sprites, which whichmay present may present quite unique quite uniqueproperties. properties.

2.2. MaterialsMaterials andand MethodsMethods InIn 2014, 2014, a a new new observation observation station station was was built built in in Xinglong Xinglong (40.38 (40.38°◦ N, N, 117.57 117.57°◦ E; E; 960 960 m m aboveabove sea sea level), level), Hebei Hebei Province Province (see (see Figure Figure1 ),1), and and optical optical observation observation data data of of many many cases cases ofof spritessprites werewere obtained, which which achieved achieved the the purpose purpose of of preliminary preliminary research. research. In this In this po- position,sition, no no high-rise high-rise buildings buildings exist exist in in the the ca camera’smera's field field of of view, view, and and the the environment environment is isclear clear without without background background light, light, which which is very is very suitable suitable for TLEs for TLEs observations. observations. The obser- The observationvation system system installed installed at this at station this station includes includes Watec902H2 Watec902H2 Ultimate Ultimate low-light-level low-light-level black blackand white and white camera camera (equipped (equipped with with a Computar a Computar 3.6 mm/F1.4 3.6 mm/F1.4 TV lens), TV lens), with with minimum minimum illu- illuminationmination of of0.0001 0.0001 Lux, Lux, able able to to record record middle middle and and upper atmosphereatmosphere dischargedischarge eventsevents aboveabove thunderstormsthunderstorms hundredshundreds ofof kilometerskilometers awayaway atat nightnight withwith aa clearclear sky.sky. The The screen screen resolutionresolution of of the the camera camera is is 720 720× × 576576 pixels,pixels, andand thethe acquisitionacquisition softwaresoftware sets sets the the sampling sampling raterate toto 25 25 fps fps (frames (frames per per second), second), or or 50 50 fps, fps, with with 20-ms 20-ms resolution. resolution. Since Since we we use use a a fixed fixed ◦ shortshort focal-length focal-length lens, lens, the the field field of of view view of of the the observation observation system system is aboutis about 100 100°(horizontal) (horizon- ◦ ×tal)71 × 71°(vertical), (vertical), ensuring ensuring that that the the camera camera can can cover cover the the entire entire range range of of a a thunderstorm. thunderstorm. However,However, due due to to the the relatively relatively large large field field of of view, view, even even if if the the image image is is enlarged, enlarged, the the sprite sprite isis smaller. smaller.

FigureFigure 1. 1.Overview Overview ofof observationobservation points.points. TheThe locationlocation ofof the camera at the Xinglong Observatory isis indicated indicated byby aa blackblack diamond,diamond, thethe YantaiYantai radarradar station is marked by a a blue blue star, star, and and the the Beijing Beijing sounding station is marked by a green square. The black line is the sprite azimuth. The two sounding station is marked by a green square. The black line is the sprite azimuth. The two dashed dashed lines are the camera’s field of view. lines are the camera’s ﬁeld of view.

TheThe analysis analysis ofof lightninglightning characteristicscharacteristics waswas obtainedobtained fromfrom ShandongShandong ProvinceProvince light-light- ningning detection detection Network Network and and the the World World Wide Wide Lightning Lightning Location Location Network Network (WWLLN), (WWLLN), both ofboth which of which use very use very low low frequency frequency (VLF, (VLF,3–30 3–30kHz) kHz) radio radio receivers. receivers. The The VLF VLF ( 3–30(3–30 kHz kHz)) radiation produced by lightning can propagate thousands of kilometers through the reﬂec- tion between the lower ionosphere and the ground, and can penetrate environments that

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block short-wavelength radio waves. Therefore, the receiver network can geolocate and characterize lightning over large areas. WWLLN provides a global map that updates the lightning location every 10 min, which can be used to locate the lightning activity center of a thunderstorm. In most cases, WWLLN can locate the sprite-producing lightning strokes with accuracy better than 10 km, and for the sprite-producing lightning strokes in Shandong Province, WWLLN has a relatively high detection efficiency (usually >70%) [30]. The Shandong Province Lightning Detection Network is composed of 10 VLF/LF sensors and a data processing center, and adopts time-of-arrival (TOA) and magnetic direction finding (MDF) integrated positioning technologies, and detection efficiency and location error are approximately 92% and 760 m [31]. If the difference between the grounding points of the two adjacent return strokes is less than 10 km, the time interval is less than 200 ms, and has the same polarity—they are combined into one event. The CG lightning data can give the location, time (time accuracy of 0.1 µs), lightning polarity and peak current intensity of the CG lightning return strokes. The radar data used in this paper is provided by a WSR-98D S-band Fully Coherent Doppler Weather radar (37.498◦ N, 121.387◦ E) in Yantai, Shandong Province, with a scanning range of 230 km and a resolution of about 1 km, which is an upgraded version of WSR-88D and manufactured by Beijing Metstar Company, China. The data were provided every 6 min. At the same time, we also used the cloud top temperature data provided by the National Centers for Environmental Prediction (NCEP)/Climate Prediction Center (CPC) under the National Oceanic and Atmospheric Administration (NOAA) of the United States, and the sounding data of the University of Wyoming in the U.S. These data can well describe the structure and evolution of MCS, the weather background of thunderstorms and large-scale environmental conditions. The atmospheric sounding data are downloaded from http://weather.uwyo.edu/upperair/sounding.html (accessed on 16 April 2021), which is provided twice a day (0000UTC and 1200UTC). This study used Beijing sounding station data (station number 53772; (39.93◦ N, 116.28◦ E, 55 m above sea level)). Data used in this paper are in Universal Time Coordinated (UTC) unless otherwise stated.

3. Analysis and Results 3.1. Sprite Image Characteristics The sprite was captured in the southwest of the observation site at 16:47:07 UTC on 31 May 2017. The sprite azimuth, the parent CG, the location of the sounding station and the Xinglong Observatory where the camera was located are shown in Figure1; the distance between the parent lightning and the camera was calculated to be about 379 km. Figure2 shows the shape of the sprite; the image was blurred due to visibility, so its exposure was enhanced (the lower right corner in Figure2 ). Although the main optical emission body was a vertical columniform shape, compared with the column sprite discovered by Wescott [32], the image was blurred and the brightness was lower (generally, the lower part of the column sprite is the brightest). Therefore, it appears to be a column sprite. The recorded sprite lasted about 20 ms, had a 10ms delay compared with its parent CG, and was displaced horizontally about 70 km away from its parent CG. Since there is only one observation station, triangulation cannot be carried out. Atmosphere 2021, 12, 718 4 of 17 Atmosphere 2021, 12, x FOR PEER REVIEW 4 of 17

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Figure 2. Sprite static image. Figure 2. Sprite static image. 3.2. Weather Background 3.2. Weather Background Figure3 3 shows shows the the sounding sounding curve curve at 12:00at 12:00 on on 31 May31 May 2017. 2017. The The result result shows shows that thethat low-levelthe low-levelFigure wind 3 showswind rotated rotated the clockwise, sounding clockwise, andcurve theand at saturated12: th00e saturated on 31 moist May moist air 2017. changed air The changed result from shows gaseousfrom gaseousthat water moleculeswaterthe low-level molecules to wind liquid to rotated waterliquid clockwise, moleculeswater molecules and during the saturateddu thering ascending the moist ascending air process. changed process. Latent from heatLatentgaseous would heat bewouldwater released moleculesbe released during to during theliquid change thewater change and molecules the and temperature the du temperaturering the would ascending would decrease. process.decrease. At Latent anAt altitudean heataltitude of would be released during the change and the temperature would decrease. At an altitude 600of 600 hPa, hPa, the the air air was was saturated. saturated. The The constant constant accumulation accumulation of of waterwater vaporvapor broughtbrought byby of 600 hPa, the air was saturated. The constant accumulation of water vapor brought by thethe southwestsouthwest air currentcurrent belowbelow 900900 hPahPa ledled toto aa relativelyrelatively smallsmall temperaturetemperature dewdew pointpoint the southwest air current below 900 hPa led to a relatively small temperature dew point difference, relatively high high relative relative humidity, humidity, sufficient sufficient water water vapor, vapor, and and lifting lifting condensa- conden- difference, relatively high relative humidity, sufficient water vapor, and lifting condensa- sation level at height of 795 hPa; above 700 hPa, the temperature dew point difference tiontion levellevel at heightheight ofof 795 795 hPa; hPa; above above 700 700 hPa, hPa, the the temperature temperature dew dew point point difference difference inin- increased significantly, and the water vapor content was low. A typical vertical distri- creasedcreased significantly, and and the the wa waterter vapor vapor content content was was low. low. A typicalA typical vertical vertical distribution distribution bution of upper dry and lower wet was formed, which provided sufficient water vapor ofof upperupper dry andand lowerlower wet wet was was formed, formed, which which provided provided sufficient sufficient water water vapor vapor condi- condi- conditionstions for the for occurrence the occurrence of this of thunderstorm. this thunderstorm. According According to the to sounding the sounding data dataof the of Bei- the tions for the occurrence of this thunderstorm. According to the sounding data of the Bei-−1 Beijing station, the convective available potential energy (CAPE) was 1162.20 J−1 kg , the jingjing station,station, thethe convectiveconvective available available potential potential energy energy (CAPE) (CAPE) was was 1162.20 1162.20 J kg J− 1kg, the, con-the con- − −1 − ◦ convectivevective inhibition inhibition (CIN) (CIN) is − is370.80370.80 J kg− J1 kg−1, the, thelifted lifted index index (LI) (LI) is is−5.745.74 °C, C,the the K Kindex index is vective inhibition◦ (CIN) is −370.80 J kg , the lifted index (LI) is −5.74 °C, the K index is is20.5020.50 20.50 °C, C,and and thethe the precipitationprecipitation precipitation of of the the of entire theentire entire sounding sounding sounding station station station is 22.10is 22.10 is mm, 22.10 mm, indicating mm, indicating indicating that that thatthethe atmosphere theatmosphere atmosphere waswas was trulytruly truly potentially potentially potentially unstab unstab unstablele leand and and provided provided provided favorable favorable favorable environmental environmental conditionsconditions forfor MCS MCS development. development.development.

FigureFigure 3.3. Skew-T log-P log-P diagram diagram based based on onthe the weather weather balloon balloon sounding sounding in Beijing, in Beijing, at 12:00 at UTC 12:00 31 UTC 31FigureMay May 2017. 3. 2017. Skew-T The The black log-P black solid diagram solid line shows line based shows the on state thethe cu stateweatherrve,curve, the baredlloon the solid red sounding line solid shows line in the showsBeijing, atmospheric the at 12:00 atmospheric UTC 31 stratification curve, the green solid line the ambient dew shows point temperature curve, the red stratiﬁcationMay 2017. The curve, black the solid green line solidshows line the the state ambient curve, dewthe red shows solid point line shows temperature the atmospheric curve, the red stratificationdashed line represents curve, the the green dry adsolidiabatic line line, the amthebient blue dasheddew shows line represents point temperature the wet adiabatic curve, the red dashed line represents the dry adiabatic line, the blue dashed line represents the wet adiabatic line, dashedline, and line the represents green dashed the line dry represents adiabatic line,the satura the bluetion dashedmixing ratioline representsline. the wet adiabatic and the green dashed line represents the saturation mixing ratio line. line, and the green dashed line represents the saturation mixing ratio line.

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3.3. Cloud Top Brightness Temperature Figure 44 showsshows thethe cloudcloud toptop brightnessbrightness temperaturetemperature atat differentdifferent momentsmoments providedprovided by NCEP/CPC, which which could could reveal reveal the the characteristics characteristics of of the the thunderstorm from a macro perspective. The The figure figure also also shows shows the the location location of offlashes flashes within within half half an anhour hour (15 (15min min be- forebefore and and after after the thetime time in the in figure) the figure) with with the time the timeshown shown in the in figure the figure as the ascenter. the center. It can beIt canclearly be clearlyseen that seen the that thunderstorm the thunderstorm developed developed from west from to east. west During to east. the During develop- the mentdevelopment of the thunderstorm, of the thunderstorm, CG was mainly CG was conc mainlyentrated concentrated in the low-value in the low-value area of the area cloud of topthe cloudbrightness top brightness temperature temperature and was distributed and was distributed near the convection near the convection line. The line.thunder- The stormthunderstorm began to began appear to about appear 03 about:00 on 03:00 the 31st on the and 31st then and developed then developed rapidly, rapidly, and the and cloud the topcloud brightness top brightness temperature temperature decreased decreased accordingl accordinglyy and reached and reached the lowest the lowest value at value 15:00, at ◦ 15:00,which whichwas about was about−56 °C,− 56indicatingC, indicating that there that therewas a was strong a strong updraft updraft in the in cloud the cloud at this at time.this time. After After that, that,the cloud the cloud top brightness top brightness temper temperatureature continued continued to rise. to rise.At 16:00, At 16:00, it rose it ◦ torose about to about −52 °C,− 52the cloudC, the area cloud became area became larger, and larger, the and thunderstorm the thunderstorm and lightning and lightning activity beganactivity to began gradually to gradually weaken. weaken. In this paper, In this the paper, sprite the occurred sprite occurred at 16:47. at As 16:47. shown As shownin Figure in 4d,Figure at 17:004d, at (the 17:00 closest (the closestmoment moment to the occu to therrence occurrence of the ofsprite), the sprite), compared compared with the with previ- the ousprevious moments, moments, the lightning the lightning activity activity was was greatly greatly reduced reduced and and the the cloud cloud top top brightness temperature was higher, indicating the thunderstormthunderstorm was inin thethe dissipationdissipation stage.stage. The sprite was located in the region where the brightness temperature of the cloud top was ◦ ◦ approximately −−4040 °CC to to −−5050 °C.C. This This was was consistent consistent with with the the research research result result of São São Sabbas et al. [33]. [33]. São São Sabbas Sabbas et al. al. [33] [33] found found that that th thee sprite occurred in the stratiform zone of the ◦ ◦ cloud top brightness temperature of −4545 °CC to to −−5353 °C.C.

Figure 4. Cloud top brightness temperature from (a‒d) 14:00-17:00 on 31 May 2017, with flashes within 15 min centered at Figure 4. Cloud top brightness temperature from (a–d) 14:00-17:00 on 31 May 2017, with flashes within 15 min centered the time shown in the figure (the white “×,” rose red “+” and red “×” mark denote −CG flashes, +CG flashes, and sprite, at the time shown in the figure (the white “×,” rose red “+” and red “×” mark denote −CG flashes, +CG flashes, and respectively). sprite, respectively).

The lower the cloud top brightness temperature and the higher the cloud top height, the more vigorousvigorous isis thethe development development of of convection convection within within the the thunderstorm thunderstorm [29 [29].]. In In order or-

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der to further study the relationship between them, Figure 5 shows the changes of differ- toent further temperature study theareas relationship of thunderstorm between cloud them, top Figure brightness5 shows temperature the changes from of different 13:00 to temperature17:00 on 31 May areas 2017. of thunderstorm The sounding cloud data topwas brightness also combined temperature in our analysis from 13:00 to toroughly 17:00 onjudge 31 Maythe development 2017. The sounding height of data thewas cloud also top, combined based on in the our vertical analysis decline to roughly rate of judge tem- theperature development (the average height temperature of the cloud drops top, based0.8 °C onper the 100 vertical m rise declinebelow the rate lifting of temperature condensa- (thetion averagelevel and temperature 0.6 °C per 100 drops m rise 0.8 above◦C per the 100 lifting m rise condensation below the lifting level). condensation According to level the andsounding 0.6 ◦C data per 100of Beijing m rise aboveStation the (ZBAA), lifting condensationthe altitude was level). about According 12.5 km towhere the sounding the tem- dataperature of Beijing was − Station55 °C. It (ZBAA), can be seen the altitude from Figure was about4 that 12.5the kmarea where where the the temperature brightness tem- was ◦ −perature55 C. Itwas can less be seenthan from−55 °C Figure (the 4height that theof the area cloud where top the was brightness above 12.5 temperature km) reached was its ◦ lessmaximum than − value55 C at (the 15:00, height indicating of the cloud that th tope thunderstorm was above 12.5 had km) a strong reached vertical its maximum develop- valuement, atand 15:00, then indicating it began to that decline the thunderstormrapidly. Moreover, had athe strong area verticalwas almost development, zero where and the thencloud it top began temperature to decline was rapidly. below Moreover, −55 °C at the13:00 area and was 14:00. almost At 17:00 zero where(the closest the cloud moment top ◦ temperatureto the occurrence was of below the sprite),−55 C the at bright 13:00 andness 14:00.temperature At 17:00 of (thethe thunderstorm closest moment was to rela- the occurrencetively high, ofand the the sprite), total area thebrightness below −30 temperature°C was the smallest, of the thunderstorm indicating that was the relatively thunder- ◦ high,storm and was the in totalthe dissipation area below stage,−30 Cwhich was thewas smallest, consistent indicating with previous that the research thunderstorm results was[7,8,29]. in the dissipation stage, which was consistent with previous research results [7,8,29].

Figure 5. EvolutionEvolution of of cloud cloud top top brightness brightness temperature temperature area area at atdifferent different moments moments of ofthe the parent parent thunderstormthunderstorm onon 3131 MayMay 2017.2017.

3.4. Thunderstorm Evolution and Characteristics According to the the Doppler Doppler radar radar image, image, at at06:00 06:00 on on May 31 31, May a thunderstorm 2017, a thunderstorm occurred occurred in the border of Inner Mongolia Autonomous Region, Hebei Province, and in the border of Inner Mongolia Autonomous Region, Hebei Province, and Liaoning Prov- Liaoning Province (on the left side of Figure6). The thunderstorm moved to the southeast ince (on the left side of Figure 6). The thunderstorm moved to the southeast along the along the border between Hebei and Liaoning provinces and was within the radar scanning border between Hebei and Liaoning provinces and was within the radar scanning range range of Yantai at 10:00. As shown in Figures6 and7, compared with other sprite-producing of Yantai at 10:00. As shown in Figures 6 and 7, compared with other sprite-producing thunderstorms [29,34], this was a small parent thunderstorm in intensity. At about 15:00, thunderstorms [29,34], this was a small parent thunderstorm in intensity. At about 15:00, the thunderstorm was composed of some small convective cells, and then these convective the thunderstorm was composed of some small convective cells, and then these convective cells gradually developed. At 15:35, strong convective cells appeared with the maximum cells gradually developed. At 15:35, strong convective cells appeared with the maximum radar reflectivity factor of 64 dBZ. At this time, the radar reflectivity factor (>55 dBZ) area radar reflectivity factor of 64 dBZ. At this time, the radar reflectivity factor (>55 dBZ) area reached the maximum, which is 87 km2. As the thunderstorm moved to the southeast, the reached the maximum, which is 87 km2. As the thunderstorm moved to the southeast, the radar reflectivity (>30 dBZ) area decreased (Figure7a) and the thunderstorm weakened at aboutradar reflectivity 15:58. After (>30 16:10, dBZ) the area intensity decreased of thunderstorms (Figure 7a) and increased the thunderstorm slightly, as shown weakened by the at reflectivity factor of 45–50 dBZ in the stratiform region, and also indicated by the increase in

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the area of the 30–40 dBZ in Figure7b. After 17:02, the thunderstorm began to weaken, and almost disappeared at 19:00. The sprite occurred at 16:47 during the stage of thunderstorm maturity–dissipation, which was consistent with previous studies [4,18,26,27]. We used the composite radar reflectivity to comprehensively analyze the evolution of radar reflectivity area from 14:00 to 19:00, and the results are shown in Figure7. As shown in Figure7a, the thunderstorm area (radar reflectivity > 40 dBZ) and the area (<30 dBZ) changed more consistently during the period when the sprite was not produced, and the area (>40 dBZ) during the occurrence of the sprite changed relatively smoothly— the area (>25 dBZ) increased. The maximum area (>10 dBZ) of the thunderstorm was approximately 21,830 km2 1 hour before the sprite occurred (Figure7a), which is smaller than the results reported by Lyons [8] and Soula et al. [29](∼105 km2), indicating that the thunderstorm was relatively small. At about 16:45 (∼2 min before the sprite occurred), the area (>10 dBZ) of the thunderstorm was about 20,766 km2, which is consistent with the research results of Lyons et al. [4] (1.5 × 104 km2 ∼ 2 × 104 km2). Lyons [34] pointed out that the sprite-producing summer MCS in North America should not only meet the condition that the area (>10 dBZ) must reach 2 × 104 km2 [4], but the maximum radar reflectivity must be greater than 55 dBZ. Figure7a shows that the maximum radar reflectivity of MCS reached 55 dBZ and the area was about 30 km2 when the sprite occurred. Both the whole thunderstorm and the convective core on the southwest side of the thunderstorm became weaker at 16:45; the thunderstorm area also became smaller, and the sprite occurred only 2 minutes after 16:45. After the sprite occurred, neither the intensity of the thunderstorm nor the stratiform area changed much (the evolution of the reflectivity area shown in Figure7a,b, plus the radar reflectivity at 16:45 shown in Figure6d). After 18:29, the area of thunderstorm decreased rapidly, indicating that the thunderstorm was dissipating rapidly at this time, and almost completely disappeared at 19:00. Soula et al. [29] showed that the radar reflectivity area of 35–40 dBZ and 30–35 dBZ increased during the time when the sprite occurred. Figure7b shows that the area of reflectivity with 30–35 dBZ and 35–40 dBZ did not change much before and after the sprite occurred. Therefore, no significant relationship between the sprite and the 30–35 dBZ and 35–40 dBZ ranges was found in this study. This thunderstorm was an MCS and in its entire life cycle, only one sprite was captured. Therefore, the radar reflectivity characteristics before and after the sprite event are worthy of our careful study. Most sprites occurred during periods of rapid development of stratiform regions [4,8,28–30]. As shown in Figure7c, during the period 14:00–19:00, the radar reflectivity area of 10–40 dBZ appeared in three large values, which were named M 1–3. Further analysis showed that the radar reflectivity area of 10–40 dBZ jumped from 18,131 km2 at 15:30 to 21,311 km2 at 15:53 (namely M1), but no sprites were produced. The sprite did not occur between the peaks M1, M2, and M3, but after M3. The sprite occurred when the area (10–40 dBZ) decreased, which means that the thunderstorm was dissipating. During the time when the sprite occurred, the area (>40 dBZ) decreased, which was in good agreement with the weakening of the strong echo area. From the above analysis, it can be seen that the sprite occurred when the thunderstorm was in the maturity–dissipation stage, and the sprite production was related to the decay of convection and the development of the stratiform zone. Generally, during the later period of thunderstorm development, the stratiform cloud rainfall area gradually expands, which is conducive to the horizontal expansion of positive charge area, and the increasing of the charge transferred from the +CG to the ground. Atmosphere 2021, 12, x FOR PEER REVIEW 8 of 17 Atmosphere 2021, 12, 718 8 of 17

Figure 6. Evolution of composite radar reﬂectivity of the sprite-producing thunderstorm at (a) 15:01, (b) 15:35, (c) 1604, Figure(d 6.) 1645,Evolution (e) 1720, of composite (f) 1829, (g radar) 1702, reflectivity (h) 1829 31 of May the sprite-producing 2017. the “ ” stands thunderstorm for the location at (a) of15:01, sprite-producing (b) 15:35, (c) 1604, +CG. The (d) 1645,upper (e) left1720, corner (f) 1829 indicates May 31, that 2017. a thunderstorm the “◇” stands occurred for the inlocation the3 border of sprite-producing of Inner Mongolia +CG. Autonomous The upper left Region, corner Hebei indicatesProvince, that a andthunderstorm Liaoning Province. occurred in the border of Inner Mongolia Autonomous Region, Hebei Province, and Liao- ning Province.

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FigureFigure 7. 7.The The evolution evolution of of different different radar radarreflectivity reflectivity areasareas ofof thethe sprite-producingsprite-producing thunderstorm. (a) The evolution of radar reflectivity larger than a certain value. (b) the evolution of different ra- (a) The evolution of radar reflectivity larger than a certain value. (b) the evolution of different radar dar reflectivity. (c) the evolution of radar reflectivity with 10–40 dBZ (blue line) and larger than 40 reflectivity. (c) the evolution of radar reflectivity with 10–40 dBZ (blue line) and larger than 40 dBZ dBZ (red line). The dashed line in the figure shows the occurrence time of the sprite. (red line). The dashed line in the figure shows the occurrence time of the sprite.

3.5.3.5. Thunderstorm Thunderstorm Lightning Lightning Activity Activity LightningLightning activity activity characteristics characteristics are are important important information information in in the the development development of of thunderstorms.thunderstorms. We We used used the the lightning lightning location location data data provided provided by the by localthe local lightning lightning location loca- networktion network to analyze to analyze the lightning the lightning activity activity characteristics characteristics of the of sprite. the sprite. Figure Figure8 shows 8 shows the evolutionthe evolution of the of lightning the lightning ﬂash rateflash obtained rate obtained by the by local the lightning local lightning detection detection network network from 14:00from to 14:00 19:00 to on 19:00 31 May on May 2017. 31. Figure Figure8a 8a shows shows the the changes changes in in the the number number of of +CGs +CGs and and −−CGsCGs and and the the ratio ratio of of POP. POP. The The results results show show that that the numberthe number of +CGs of +CGs was farwas less far than less that than ofthat−CGs, of − theCGs, large the ratelarge of rate−CGs of corresponded−CGs corresponded to the smallto the rate small of +CGs,rate of and+CGs,−CGs and were −CGs were dominant in the development of thunderstorm. From 14:00 to 14:20 (the early stage

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dominantof the thunderstorm), in the development there were of thunderstorm. only 3 positive From flashes 14:00 and to 14:20 fewer (the negative early stage flashes, of the POP thunderstorm),reached the first there peak were (about only 3 50%), positive and flashes the rates and fewerof +CGs negative and −CGs flashes, did POP not reachedreach the thehighest. first peak The (about POP at 50%), the time and of the the rates sprite of +CGsoccurred and was−CGs about did 40%, not reachwhich the was highest. smaller Thethan POP the at maximum the time of value the sprite of 64% occurred during was the aboutevolution 40%, of which the thunderstorm, was smaller than and the also maximumsmaller than value previous of 64% during studies the [6,29] evolution but greater of the thunderstorm,than Yang et al. and [35] also results smaller (approxi- than previousmately studies20%). During [6,29] but 15:30–16:45, greater than the Yang number et al. [of35 ]− resultsCGs decreased (approximately and the 20%). +CGs During rate in- − 15:30–16:45,creased, indicating the number that of the CGsoverall decreased POP showed and the an +CGsupward rate trend. increased, At 16:47, indicating a case of that a red thesprite overall was POP captured. showed After an upwardthat, the trend.rate of At+CGs 16:47, and a − caseCGs of showed a red sprite an overall was captured.downward After that, the rate of +CGs and −CGs showed an overall downward trend. At the end of trend. At the end of the thunderstorm, POP reached 100%, which meant that only +CGs the thunderstorm, POP reached 100%, which meant that only +CGs occurred at that time. occurred at that time. As shown in Figure8b, the rate of +CGs was higher and the rate of −CGs was lower As shown in Figure 8b, the rate of +CGs was higher and the rate of −CGs was lower during the occurrence of the sprite, and the +CGs rate was slightly larger than the −CGs during the occurrence of the sprite, and the +CGs rate was slightly larger than the −CGs rate, which was consistent with the result of Soula et al. [29]—most sprites occurred during rate, which was consistent with the result of Soula et al. [29]—most sprites occurred dur- the period of low −CGs rate and high +CGs rate. The increase of POP might be a good ing the period of low −CGs rate and high +CGs rate. The increase of POP might be a good indicator of the sprite produced by the thunderstorm [36]. indicator of the sprite produced by the thunderstorm [36].

FigureFigure 8. (8.a )(a Evolution) Evolution of of CG CG ﬂashes flashes per per 5 minutes 5 minutes in thein the parent parent thunderstorm. thunderstorm. (b) ( CGb) CG ﬂashes flashes per per 3 3 minutes about 2 hours before and after the sprite. The black dashed line in (a) and (b) shows the minutes about 2 hours before and after the sprite. The black dashed line in (a,b) shows the sprite sprite occurrence time, respectively. occurrence time, respectively.

FigureFigure9a shows9a shows the the return return stroke stroke peak peak currents currents of CG of half CG an half hour an beforehour before and after and the after timethe of time the of sprite the sprite produced produced (within (within 50 km 50 from km the from center the center of the parentof the parent CG location). CG location). The resultsThe results show thatshow there that werethere 43 were positive 43 positive return return strokes strokes with an with average an average value value of 86.34 of kA86.34 inkA the in 30 the min 30 centered min centered on the occurrenceon the occurrence time of thetime sprite, of the but sprite, the peakbut the current peak of current parent of CGparent was 76.5 CG was kA. Negative76.5 kA. Negative return strokes, return whichstrokes, could which also could produce also produce sprites, usuallysprites, usually have largehave peak large currents peak currents [9,37], but [9,37], the absolutebut the absolute value of value the peak of the currents peak currents of all negative of all negative return strokesreturn shown strokes in shown Figure9 inawas Figure less 9a than was 50 less kA, sothan it was50 kA, unlikely so it towas produce unlikely sprites. to produce sprites.

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The statistical analysis in Figure9b shows the return stroke peak currents distribution fromThe 14:00 statistical to 19:00. analysis The results in Figure indicate 9b thatshow thes the local return lightning stroke detection peak currents network distribu- detected ation total from of 821 14:00 CG to flashes, 19:00. ofThe which results +CGs indica accountedte that the for local 31.4%, lightning and the detection peak current network ranged fromdetected +15.9 a total kA toof +346.4821 CG kA; flashes,−CGs of accountedwhich +CGs for accounted 68.6%, and for the31.4%, peak and current the peak range cur- was betweenrent ranged−135 from kA +15.9 and − kA5.7 to kA; +346.4kA; the average −CGs values accounted of the for peak 68.6%, currents and the of +CGspeak current and −CGs wererange +85.4 was between kA and −−13518.1 kA kA, and respectively. −5.7 kA; the average The number values ofof positivethe peak strokescurrents withof +CGs a peak currentand −CGs of were greater +85.4 than kA orand equal −18.1 to kA, 100 respectively. kA was 72 The and numb accounteder of positive for 27.9% strokes of thewith total +CGa peak return current strokes, of greater while than the or number equal to of 100 negative kA was strokes 72 and with accounted an absolute for 27.9% value of ofthe peak currenttotal +CG greater return than strokes, or equal while tothe 100 number kA was of negative only 2 and strokes accounted with an for absolute only 0.36% value ofof the totalpeak negativecurrent greater strokes. than or equal to 100 kA was only 2 and accounted for only 0.36% of the totalThe negative results show strokes. that the sprite was produced during the maturity–dissipation stage of theThe thunderstorm results show and that the the number sprite was of flashesproduced rapidly during decreased the maturity–dissipation from 18:00 to 19:00, stage indi- catingof the thunderstorm that the thunderstorm and the nu wasmber dying of flashes rapidly rapidly at this decreased time. Although from 18:00 the to thunderstorm 19:00, in- wasdicating dominated that the bythunderstorm negative return was dying strokes, rapi mostdly at ofthis the time. high-peak Although current the thunderstorm return strokes werewas dominated positive strokes. by negative Moreover, return during strokes, the most period of the from high-peak 14:00 to current 16:00, return there werestrokes many were positive strokes. Moreover, during the period from 14:00 to 16:00, there were many positive return strokes with large peak currents (the maximum more than 300 kA), but no positive return strokes with large peak currents (the maximum more than 300 kA), but no sprites were recorded. Unfortunately, due to the lack of corresponding magnetic field data, sprites were recorded. Unfortunately, due to the lack of corresponding magnetic field iCMC cannot be calculated as a criterion for whether the sprite occurred (iCMC, impulse data, iCMC cannot be calculated as a criterion for whether the sprite occurred (iCMC, charge moment change, was defined as the product of the charge transferred by the +CG impulse charge moment change, was defined as the product of the charge transferred by stroke within 2 ms after stroke onset and the original height of the transferred positive the +CG stroke within 2 ms after stroke onset and the original height of the transferred charge)positive [charge)12,19,38 [12,19,38].].

FigureFigure 9.9. (a) The return return stroke stroke peak peak currents currents (within (within 50 50 km km centered centered at at the the parent parent CG CG location) location) of of CG CG lightning flashes within half an hour centered at the sprite. The red dots and black circles indi- lightning ﬂashes within half an hour centered at the sprite. The red dots and black circles indicate cate positive and negative return strokes, respectively, and also mark the parent CG, with a peak positivecurrent of and 76.5 negative kA. (b) returnPeak current strokes, of respectively,CG flashes inand the pa alsorent mark thunderstorm the parent from CG, with14:00 ato peak 19:00. current ofThe 76.5 black kA. "+" (b )is Peak the parent current CG of of CG the ﬂashes sprite, inand the th parente red circle thunderstorm and blue circ fromle represent 14:00 to 19:00.the positive The black “+”strokes is the and parent negative CG ofstrokes, the sprite, respectively. and the red circle and blue circle represent the positive strokes and negative strokes, respectively.

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3.6.3.6. Parent Parent CG CG and and Thunderstorm Thunderstorm Structure Structure FigureFigure 10 10 showsshows the location of of the the lightning lightning distribution distribution with with time, time, which which was was de- detectedtected by by WWLLN. WWLLN. It It can can be be seen seen that that the overall thunderstorm waswas developingdeveloping towards towards thethe southeast. southeast. Both Both the the local local lightning lightning detection detection network network and and WWLLN WWLLN had had detected detected the the locationlocation of of the the parent parent CG. CG. Two Two different different lightning lightning location location systems systems had had obtained obtained almost almost the samethe same location location and time, and indicatingtime, indicating that the that parent the parent CG was CG a +CG was witha +CG single with return single stroke. return stroke.

FigureFigure 10. 10.WWLLN WWLLN lightning lightning distribution, distribution, colored colored dots dots show show WWLLN WWLLN location location results, results, red red diamond diamond indicates parent CG. indicates parent CG.

InIn order order to to further further explore explore the the characteristics characteristics of of thunderstorms thunderstorms that that produced produced the the sprite,sprite, Figure Figure 11 11 shows shows the the superposition superposition of of the the radar radar reflectivity reflectivity at at different different elevation elevation anglesangles at at 16:45 16:45 (2 (2 min min before before the the sprite sprite event) event) and and CG CG strokes strokes at 16:39–16:51 at 16:39–16:51 (the (the radar radar re- flectivityreflectivity did did not not change change much much during during this this period). period). The The red red “+” "+" and and black black “× "×"” represented represented positivepositive and and negative negative strokes, strokes, respectively, respectively, and and the the parent parent CG CG was was represented represented by by a a red red diamond.diamond. The The results results show show that that there there were were 22 22 positive positive strokes strokes during during this this period, period, one one of of whichwhich was was the the parent parent lightning lightning of theof the sprite sprite and and the parentthe parent CG was CG far was away far fromaway the from radar the stationradar station (distance (distance of approximately of approximately 172.26 km).172.26 With km). increasing With increasing the radar the scanning radar scanning angle, theangle, radar the reflectivity radar reflectivity where thewhere parent the CGparent was CG located was located reduced reduced from 30–40 from dBZ 30–40 to aboutdBZ to 20–25about dBZ 20–25 (at dBZ an elevation (at an elevation angle of angle 2.40◦ of), and2.40°), then and the then thunderstorm the thunderstorm disappeared disappeared when thewhen elevation the elevation angle was angle 4.30 was◦ and 4.30° the and height the he ofight the thunderstormof the thunderstorm top was top about was about 15.7 km. 15.7 km.

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FigureFigure 11. Overlaps 11. Overlaps of radar of radar reflectivity reflectivity at different at different elevation elevation angles angles (shown (shown in the infigure) the figure) at 16:45 at 16:45 with withCGs CGsduring during the time the timeperiod period from from 16:39 16:39 to 16:51. to 16:51. The The"+" stands “+” stands for +CGs for +CGs and "×" and for “× negative” for negative ones.ones. The " The◇" means “ ” means that the that parent the parent flash flashfor the for sprite the sprite is marked is marked as “ asCG” “CG” in the in thefigure. figure. 3 FigureFigure 12 shows 12 shows the radar the radar echo echo top height top height and andvertical vertical section. section. Figure Figure 12a shows12a shows the the parentparent CG was CG wasabout about 6–7 km 6–7 on km top on of top the of radar the radar echo echo at 16:45. at 16:45. The Thevertical vertical section section can can showshow the structure the structure of the of thunderstorm the thunderstorm more more clearly. clearly. Figure Figure 12a,b 12 isa,b the is vertical the vertical sections sections takentaken along along the AB the line AB (through line (through the parent the parent lightning) lightning) and the and CD the line CD (the line azimuth (the azimuth of the of sprite) in the radar reflectivity at the 0.5° elevation angle in Figure 10. As shown in the

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the sprite) in the radar reflectivity at the 0.5◦ elevation angle in Figure 10. As shown in thefigure, figure, CG CGwas was located located in the in thunderstorm the thunderstorm area areawith with the echo the echotop height top height of about of about 6 km. 6There km. There was relatively was relatively strong strong updraft updraft in the inco thenvective convective area (as area show (asn shown by the by radar the radarreflec- reflectivitytivity of > 35 of >dBZ). 35 dBZ). The maximum The maximum echo echotop height top height of the of thunderstorm the thunderstorm was wasgreater greater than than12 km. 12 km.The Thevertical vertical section section along along the theCD CDline line shows shows a similar a similar thunderstorm thunderstorm structure. structure.

Figure 12. (a)The radar echo top height at 16:45, the parent CG is labeled by “+” and marked as Figure 12. (a)The radar echo top height at 16:45, the parent CG is labeled by “+” and marked as CG. CG. (b) and (c) are the vertical sections along lines AB and CD (in Figure 11). (b,c) are the vertical sections along lines AB and CD (in Figure 11).

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4. Conclusions This paper presents a detailed analysis of an MCS that produced only one sprite during its life span. Comprehensive analysis of the characteristics of the thunderstorm and lightning activity before and after the sprite occurred was carried out using multiple data, including a low-light level sprite image, S-band radar, lightning detection network, cloud top brightness temperature provided by NECP/CPC and sounding data. The sprite event lasted about 20 ms and had a 10 ms delay compared with its parent CG. The sprite was displaced horizontally about 70 km away from its parent CG. The vertical structure of moisture on the top of the water vapor and the accumulation of unstable energy triggered the thunderstorm. The parent flash occurred in the stratiform area of 25−35 dBZ of radar echo. At 15:35, MCS developed to its strongest, with a maximum reflectance factor of 64 dBZ. At this time, the radar reflectance factor (>55 dBZ) area reached the maximum (about 87 km2). No significant relationship between the sprite and the 30–35 dBZ and 35–40 dBZ ranges was found in this study. The maximum radar reflectivity factor reached 55 dBZ and the area was about 30 km2 when the sprite occurred; the area (radar reflectivity > 40 dBZ) decreased, indicating that the sprite was in the maturity–dissipation stage of the thunderstorm. The sprite production was related to the decay of the thunderstorm convection and the development of the stratiform area. The parent CG was positive with single return stroke (peak current of 76.5 kA). The average values of the peak currents of +CGs and −CGs were +85.4 kA and −18.1 kA, respectively. The absolute values of the peak currents of −CGs were less than 50 kA, within half an hour before and after the time the sprite occurred, which was not enough to produce sprites. In the life cycle of thunderstorm, the number of −CGs was dominant, and the number of +CGs was relatively small. The −CG activity gradually decreased and the +CG increased significantly when MCS began to weaken. The sprite occurred in a time interval during which the POP was about 40%, and both +CG and −CG flashes rate were at a low level.

Author Contributions: Conceptualization, C.P. and J.Y.; methodology, C.P., J.Y., K.L. and Y.W.; investigation, C.P. and J.Y.; formal analysis, C.P. and J.Y.; writing original draft preparation, C.P.; writing—review and editing, C.P. and J.Y.; supervision, J.Y.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by National Natural Science Foundation of China (Grant Nos. 41574141 and 41374153), Key Program for International Science and Technology Innovation Coop- eration Projects of China (2018YFE0101200) and the International Partnership Program of Chinese Academy of Sciences (Grant No. 183311KYSB20200003). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available as consent was not obtained to hold them in a public repository. Acknowledgments: This work was supported by National Natural Science Foundation of China (Grant Nos. 41574141 and 41374153), Key Program for International Science and Technology Innova- tion Cooperation Projects of China (2018YFE0101200) and the International Partnership Program of Chinese Academy of Sciences (Grant No. 183311KYSB20200003). Authors would like to thank Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, and Chengdu University of Information Technology for their great support. Conﬂicts of Interest: The authors declare no conﬂict of interest. Atmosphere 2021, 12, 718 16 of 17

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