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 develop- ment 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 Atmosphere 2021, 12, 718 3 of 17 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.