# Implications of GNSS-Inferred Tropopause Altitude Associated with Terrestrial Gamma-ray Flashes
## 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 orbit [1, 2, 3, 4, 5] or even at ground level [6, 7]. It is known that TGFs are forged inside thunderstorms and related to the initial upward leader development of intra-cloud (IC) lightning flashes [8, 9, 10, 11], or downward propagating negative leaders [12, 13, 14]. However, the characteristic charge structure of TGF-producing thunderstorms and the associated convection properties remain an open question to be investigated. The authors of [15] found a preference for TGF-producing thunderstorms for larger values of surface convective available potential energy (CAPE), whereas the authors of [16] recently found no difference in CAPEbetween tropical TGF-producing thunderstorms and local monthly climatology. The authors 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 vigorous 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 thunderstorms 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\({}^{\circ}\)N and 26\({}^{\circ}\)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 Occulation (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 (\(\leq\)0.1 km) accurate (with error less than 1 \({}^{\circ}\)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?