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Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-Ray Flashes

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Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-Ray Flashes remote sensing Article Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-ray Flashes Tao Xian 1,2 , Gaopeng Lu 1,3,4,*, Hongbo Zhang 5, Yongping Wang 1, Shaolin Xiong 6, Qibin Yi 6,7, Jing Yang 1 and Fanchao Lyu 8 1 School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China; [email protected] (T.X.); [email protected] (Y.W.); [email protected] (J.Y.) 2 Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology, Shenzhen 518055, China 3 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Hefei 210044, China 4 Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China 5 Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] 6 Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; [email protected] (S.X.); [email protected] (Q.Y.) 7 School of Physics and Optoelectronics, Xiangtan University, Yuhu District, Xiangtan 411105, China 8 Nanjing Joint Institute for Atmospheric Sciences, Chinese Academy of Meteorological Sciences, Nanjing 210000, China; [email protected] * Correspondence: [email protected] Citation: Xian, T.; Lu, G.; Zhang, H.; Abstract: The thermal structure of the environmental atmosphere associated with Terrestrial Gamma- Wang, Y.; Xiong, S.; Yi, Q.; Yang, J.; ray Flashes (TGFs) is investigated with the combined observations from several detectors (FERMI, Lyu, F. Implications of GNSS-Inferred RHESSI, and Insight-HXMT) and GNSS-RO (SAC-C, COSMIC, GRACE, TerraSAR-X, and MetOp-A). Tropopause Altitude Associated with The geographic distributions of TGF-related tropopause altitude and climatology are similar. The Terrestrial Gamma-ray Flashes. regional TGF-related tropopause altitude in Africa and the Caribbean Sea is 0.1–0.4 km lower than Remote Sens. 2021, 13, 1939. https:// the climatology, whereas that in Asia is 0.1–0.2 km higher. Most of the TGF-related tropopause doi.org/10.3390/rs13101939 altitudes are slightly higher than the climatology, while some of them have a slightly negative bias. The subtropical TGF-producing thunderstorms are warmer in the troposphere and have a colder Academic Editor: and higher tropopause over land than the ocean. There is no significant land–ocean difference in the Guillermo Gonzalez-Casado thermal structure for the tropical TGF-producing thunderstorms. The TGF-producing thunderstorms Received: 14 March 2021 have a cold anomaly in the middle and upper troposphere and have stronger anomalies than the Accepted: 7 May 2021 deep convection found in previous studies. Published: 16 May 2021 Keywords: Terrestrial Gamma-ray Flashes (TGFs); tropopause; deep convection; GNSS-radio Publisher’s Note: MDPI stays neutral occultation with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Terrestrial Gamma-ray Flashes (TGFs) are sub-millisecond bursts of energetic MeV gamma-ray photons produced in the troposphere that can be detected on the low-Earth Copyright: © 2021 by the authors. orbit [1–5] or even at ground level [6,7]. It is known that TGFs are forged inside thunder- Licensee MDPI, Basel, Switzerland. storms and related to the initial upward leader development of intra-cloud (IC) lightning This article is an open access article flashes [8–11], or downward propagating negative leaders [12–14]. However, the charac- distributed under the terms and teristic charge structure of TGF-producing thunderstorms and the associated convection conditions of the Creative Commons properties remain an open question to be investigated. The authors of [15] found a prefer- Attribution (CC BY) license (https:// ence for TGF-producing thunderstorms for larger values of surface convective available creativecommons.org/licenses/by/ potential energy (CAPE), whereas the authors of [16] recently found no difference in CAPE 4.0/). Remote Sens. 2021, 13, 1939. https://doi.org/10.3390/rs13101939 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 1939 2 of 12 between tropical TGF-producing thunderstorms and local monthly climatology. The au- thors of [17] showed that TGF-producing thunderstorms contain higher concentrations of both cloud water and ice and precipitation water and ice, which results in more vigor- ous lightning activity. Using the observations from the World-Wide Lightning Location Network (WWLLN) and Next Generation Weather Radars (NEXRAD), the authors of [18] found that 24 oceanic TGF-producing thunderstorms had a variety of convective strengths, ranging from relatively weak to deep convection, without distinct features. Although African thunderstorms have the highest lightning density in the world [19], they produce a relatively low TGF-to-lightning ratio in comparison with the TGF-producing thunder- storms in America and Southeast Asia, attributed by [20] to the relatively small spacing between layered charge regions in the parent thunderstorms. Since the tropopause imposes a constraint on the vertical development of convection in thunderstorms [21], the authors of [22] found that TGFs are usually associated with a relatively high tropopause using the climatology monthly NCEP/NCAR reanalysis of tropopause altitudes for the TGF observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Based on radiosonde observations, the authors of [23] recently showed that there is no considerable difference in tropopause altitudes between TGF-producing equatorial thunderstorms and those in South China (with latitude between 20◦N and 26◦N). Considering the inter-annual variability and the significant bias in tropopause altitude from reanalysis [24,25], using tropopauses derived from high vertical resolution temperature observations associated with TGF-producing thunderstorms from Global Navigation Satellite System–Radio Occultation (GNSS–RO) would improve our understanding of the thermal structures of the environmental atmosphere associated with TGF generation. The GNSS–RO technique provides global-scale high-resolution (≤0.1 km) accurate (with error less than 1 ◦C) temperature profiles under all weather conditions [26], which is ideal for the studies of tropopause and thermal structure associated with TGF- producing thunderstorms. To evaluate the relationship between atmospheric thermal structure and TGF genesis on a global scale, we will address the following questions in this paper: Is the tropopause associated with TGFs substantially higher than the climatology? How does the TGF- related tropopause compare over land and over ocean, and over different regions? What are the thermal structure patterns of the environmental atmosphere of TGF-producing thunderstorms? How does their intensity compare to the deep convection mentioned in previous studies [27,28] on a statistical basis? 2. Data and Methodology In this work, we adopt the TGF observations from three space-borne missions: the RHESSI project from April 2006 to November 2013 [2], Fermi Gamma-ray Burst Monitor (GBM) from August 2008 to December 2019 [4,5], and Insight Hard X-ray Modulation Telescope (Insight-HXMT) from July 2017 to September 2019 [29]. We use the quality-controlled temperature profiles from the following four missions and time periods: SAC-C (Scientific Applications Satellite C, [30]) from March 2006 to August 2011, COSMIC (Formosa Satellite Mission 3/Constellation Observing System for Meteorology, Ionosphere, and Climate, [26]) from April 2006 to present, GRACE (Gravity Recovery and Climate Experiment, [31]) from March 2007 to November 2017, TerraSAR- X[32] from February 2008 to present, MetOp-A (October 2007–December 2019), and MetOp-B (February 2013–December 2019) [33–35]. All data are available from the Uni- versity Corporation for Atmospheric Research (UCAR) Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Data Analysis and Archive Center (CDAAC) and are provided at 0.1 km vertical resolution from the surface to about 40 km altitude. SAC-C and TerraSAR-X acquire up to 250 daily occultations (temperature profiles), MetOp-A and B provide about 630–680 daily occultations, while GRACE retrieves about 150–200 occultations and COSMIC provides up to 2500 occultations per day, respectively. Remote Sens. 2021, 13, 1939 3 of 12 The observations from the TGF detectors and GNSS-RO are required to be within 300 km and ±3 h of each other [28], resulting in 1917 TGF-related GNSS-RO temperature profiles. The spatial relationship between TGFs and associated lightning has been confined typically within 300 km based on the space-borne TGF detection and ground-based broad- band spheric measurements [36,37]. The authors of [36] found that the majority (about 75% for TGF-associated lightning discharges located with three or more stations) of TGFs are detected within 300 km of the associated lightning, and the authors of [38] attributed half of 2188 TGFs located within 300 km range from the associated lightning discharge. Tropopause altitudes are calculated for each GNSS-RO profile by employing the WMO temperature lapse rate tropopause definition [39], which defines the primary tropopause as the lowest altitude
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