## 1 Introduction  
Atmospheric temperature, pressure and humidity profiles are essential for meteorological research. Notably, accurate and stable long-term observations of the vertical structure of atmospheric temperature are crucial for climate change monitoring. Radiosondes are the only operational instruments that have provided these atmospheric profiles in the troposphere and lower stratosphere for more than three decades, and they have been widely used in climate studies [1; 2; 3; 4]. However, radiosondes are launched only twice (00 and 12 UT) per day during operation, and their spatial distribution is inhomogeneous with a relatively low density, particularly over plateaus, deserts, oceans, and polar regions. Additionally, the atmospheric temperature trends estimated from radiosonde observations are sensitive to the objective selection of radiosonde samples [5; 6; 7; 8]. The selection of radiosonde samples can result in differences in temperature trends, as radiosonde observations are obtained with various types of radiosondes manufactured by different countries. Therefore, it is critical to have accurate, consistent, and stable observations from different satellite missions to improve climate change monitoring. The Global Positioning System (GPS) radio occultation (RO) limb-sounding technique uses radio signals from the GPS satellites propagating through the Earth's atmosphere. The GPS RO technique has the advantages of high accuracy, high precision and high vertical resolution, with no contamination from clouds, no system calibration requirement, and no instrument drift [9; 10; 11; 12; 13].These features make GPS RO particularly useful for climate studies [14; 15; 16; 17; 18; 19]. Early studies have shown that GPS RO soundings from the first RO mission, GPS Meteorology (GPS/MET), agree with those from radiosondes and large-scale analyses to better than 1.5 K from 530 km of altitude [10; 20; 21; 22; 23]. Furthermore, GPS RO datasets are relatively new and are valuable sources of observational information on temperature and humidity [24; 25; 26; 27; 28; 29; 30]. The successful launch of the six satellites in the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC, also called FORMOSAT-3 in Taiwan) project in April 2006 began a new era of GPS atmospheric remote sensing [31; 32; 33; 34; 35; 36; 37]. COSMIC is a joint mission between Taiwan and the United States (U.S.) with the goal of demonstrating the use of constellation GPS RO data in numerical weather prediction (NWP), climate monitoring, and space weather forecasting. With the ability to perform both rising and setting occultation, COSMIC has produced approximately 2500 soundings per day with global coverage since mid-2007. However, the number of COSMIC soundings has been gradually decreasing since about 2010 as the COSMIC satellites reach the end of their designed 5-year lifetime. The median values of COSMIC-derived dry temperature differences are estimated to be better than 0.05 K from 8 km to 30 km [32; 38]. According to previous studies, the temperature profiles of different radiosonde types generally agree well with those of COSMIC, with mean differences of less than 0.5 K and standard deviations of less than 2.0 K [4; 11]. Because COSMIC RO can provide atmospheric temperature, pressure, and humidity profiles with a 100-m vertical resolution in all weather conditions, it makes up for the lack of operational radiosonde soundings with high spatiotemporal distributions, especially over deserts, oceans, and polar regions [4; 11; 29; 38; 39; 40]. Eastern China is mostly a plains region, but plateaus and deserts occupy most areas of western China, notably, the Qinghai-Tibet Plateau (QTP), which occupies approximately one fourth of China. The QTP is a source of dynamic and thermodynamic turbulence and has a profound influence on atmospheric circulation patterns at various temporal and spatial scales [41; 42; 43; 44; 45].