## 2 Data and Methodology  
The GPS RO limb-sounding technique for sounding the atmosphere of the Earth was first demonstrated by the proof-of-concept GPS/MET experiment in 1995-1997 [21]. Preliminary results showed that accurate vertical temperature profiles from an altitude of approximately 40 km to about 5 km can be obtained using the GPS limb-sounding technique. The COSMIC mission placed six small microsatellites into six different orbits at 700-800 km above the Earth's surface. These satellites formed a unique low Earth orbit (LEO) constellation that receives signals from U.S. GPS satellites. By measuring the phase delay of radio waves from GPS satellites as they are occulted by the Earth's atmosphere, accurate and precise vertical profiles of the bending angles of radio-wave trajectories are obtained for the lower atmosphere. Under the assumption of the spherical symmetry of the refractive index in the atmosphere, vertical profiles of the bending angle, and therefore the refractivity, can be derived from the raw RO measurements of the excess Doppler shift of the radio signals transmitted by GPS satellites [9; 12; 54]. The profiles of refractivity can then be used to generate the so-called dry and wet atmospheric profiles. For dry air, the density profiles are first calculated using the relationship between density and refractivity. The dry temperature profiles are then derived from the density profiles based on the hydrostatic equation and the ideal gas law under the assumption that the partial pressure of water vapor is zero. The wet retrieval algorithm estimates both the temperature and water vapor profiles using the 1 DVar (one-dimentional variation) algorithm [39; 55; 56]. 1 DVar seeks the optimal solution that reproduces the observed refractivity most closely while satisfying other imposed constraints, amongst many physically admissible combinations of state variables. The COSMIC data used in this study include the atmospheric profiles of temperature, pressure, vapor pressure, and analyzed refractivity in the wetPrf (wet profile) suite of products [51]. These profiles are based on 1D variational analysis using low-resolution analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Gridded analysis or short-term forecasting is used to separate the pressure, temperature, and moisture contributions to refractivity. Finally, these wet profiles are interpolated to 100-m height intervals from the geometric height of sea level to 40 km. Because operational radiosonde data are reported only at mandatoryand significant pressure levels and at a lower vertical resolution than COSMIC RO data [4], COSMIC atmospheric profiles  
Figure 1: The distribution and elevations of radiosonde stations in China. are generally interpolated to the vertical levels of radiosonde profiles. This process results in fewer available data for COSMIC atmospheric profiles. In this study, high vertical-resolution profiles of temperature, pressure, and humidity are obtained from raw radiosonde soundings (launched twice daily at 00 and 12 UTC) with a vertical resolution of \(\sim\)10 m or a 1-s temporal resolution [57]. This spatial resolution is higher than the 100-m vertical resolution of the COSMIC wetPrf profiles and can improve the analysis. Figure 1 shows the distribution and altitudes of the 120 radiosonde stations used in this study. The radiosonde profiles from 2014 to 2015 were collected and used to validate the contemporaneous COSMIC-retrieved profiles of temperature, pressure, vapor pressure, and analyzed refractivity. These radiosonde profiles were observed with the same GTS1 radiosondes manufactured by China, which came into use as the new generation upper-air sounding sysytem in operation in China since about 2010. The accuracies of temperature, humidity, and pressure for the GTS1 radiosondes are 0.2 K, 5% and 2 hPa, respectively, with corresponding resolutions of 0.1 K, 1% and 0.1 hPa.