## 1 Introduction  
Gravity waves (GWs), the fluctuations generated by buoyancy and gravity under stable stratification in the atmosphere, transfer energy, momentum, atmospheric constituents and water vapor in fluid environments, depositing vertical mixing of heat [1, 2, 3]. The propagation of GWs in the atmosphere is affected by various dissipation processes, and the energy and momentum carried by GWs are stored in the background atmosphere, which further affects the local and global atmospheric thermodynamic and dynamic processes [4, 5]. As a key driving mechanism in the middle and lower atmosphere through drag and diffusion processes, GW activity plays an important role in the global circulation and atmospheric system [2, 6, 7] and must be considered in global circulation models (GCMs) to reproduce the real atmospheric structure [5, 8, 9]. At present, it is still difficult to explicitly solve GWs and their associated eddy diffusion in GCMs due to the computational power and required cost [10, 11, 12, 13]. Therefore, obtaining the GW parameters through observations is of great importance, which will improve the accuracy and reliability of the predictions of GCMs. Satellite observations, which are valuable complements to traditional ground-basedand field observations, can provide global information about the spatial and temporal distributions of GWs and make it possible to provide some constraints on the parameterization of GW activity in GCMs [13]. Global navigation satellite system (GNSS) radio occultation (RO), which is a satellite-based observation technology developed rapidly in the last two decades, can provide atmospheric and ionospheric products with global coverage, all-weather capability, and long-term stability. The vertical resolution of RO temperature profiles ranges from 100 m in the lower troposphere to 1.4 km in the upper stratosphere. The RO temperature profiles generally have errors of less than 0.4 K in the upper troposphere and lower stratosphere (UTLS) [14; 15; 16], which are ideal data sources for the study of GWs in this altitude region [2; 17; 18; 19]. The Formosa Satellite-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (FORMOSAT-7/COSMIC-2, hereafter COSMIC-2) mission, a Taiwan-United States GNSS RO mission, was launched on 25 June 2019 [20]. The six satellites of COSMIC-2 were deployed in 24\({}^{\circ}\) inclination parking orbits at an altitude of about 550 km [21; 22], each of which carries an advanced Tri-GNSS RO System instrument (TGRS) developed by the National Aeronautics and Space Administration's Jet Propulsion Laboratory [23]. COSMIC-2 is the successor to the highly successful COSMIC mission, whose products have improved global weather forecasts and supported hundreds of scientific studies of weather, climate, and space weather [2; 16; 24; 25; 26]. After the COSMIC-2 mission is fully operational, it will observe about 5000 RO events per day at middle and low latitudes of 40\({}^{\circ}\)S-40\({}^{\circ}\)N. Compared to COSMIC, which can only observe the global positioning system (GPS) RO events, COSMIC-2 can observe both the GPS and the global navigation satellite system (GLONASS) RO events, and the performance of the RO receivers has been improved substantially [20; 21; 27]. The large volume of high-quality temperature profiles retrieved from the COSMIC-2 RO atmospheric observations are valuable data sources for extracting reliable GW parameters in the lower stratosphere (LS) of the subtopic and tropical regions, based on which the sources of GWs over these regions can be further investigated. GWs can be generated from various sources, such as flow over topography, wind shear, jet stream, and atmospheric convection, and can propagate from the unstable airflow in the lower atmosphere to the middle and upper atmosphere under the influence of the background wind [7; 28; 29]. In previous studies, some scholars have studied the sources of GWs and the influencing factors in some local regions.