# COSMIC-2 RO Profile Ending at PBL Top with Strong Vertical Gradient of Refractivity
## 1 Introduction  
The application of the Global Navigation Satellite System (GNSS) radio occultation (RO) sounding technique in meteorology began with the GPS/Meteorology (GPS/MET) experiment in 1995 [1]. Many GNSS RO missions have successfully been launched since then, including the Challenging Mini-Satellite Payload (CHAMP) [2], the Satelite de Aplicaciones Cientificas-C (SAC-C) [3], the Gravity Recovery and Climate Experiment (GRACE) [4], the Formosa Satellite-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) [5], and the Formosa Satellite-7/COSMIC-2 [6]. The RO sounding technique measures atmospheric and humidity information over the globe with high accuracy, precision, and vertical resolution in all weather conditions [7]. GNSS RO observations complement the satellite microwave and infrared measurements for numerical weather prediction (NWP). Many studies showed the values of GPS RO data for research on tropical cyclones [8; 9; 10], planetary boundary layer detection [11; 12], and NWP [13; 14; 15; 16]. The application of RO observations in the lower troposphere has always been challenging. Considering the large biases of RO observations in the lower troposphere, particularly in regions with large refractivity gradients, the European Met Office only operationally assimilated the RO data above 4-km altitude at the early stages of RO data assimilation to avoid biases. Since April 2008, all RO observations were assimilated by assuming large observation errors in the lower troposphere, and the corresponding experiments showed a small positive impact as a result of the inclusion of low-levelRO data [17]. NCEP has also operationally assimilated COSMIC RO data since 1 May 2007, in which the RO observations below 4 km were assimilated if they passed quality control checks and obtained a significant improvement on the anomaly correlation scores and tropical wind forecasts [18; 19]. The COSMIC was the first GNSS RO observing constellation system and delivered near-real-time RO data to operational weather centers all over the world since 2006 [5; 20]. Different from its previous GNSS RO missions, COSMIC consists of six low-earth orbiting (LEO) satellites that move in six different orbits with the same inclination of 72\({}^{\circ}\). COSMIC can provide about 3500 RO profiles globally. COSMIC-2 is a follow-on mission of COSMIC, and it was successfully launched on 25 June 2019. Like COSMIC, COSMIC-2 also contains six LEO satellites, whose orbital inclination is twice smaller than that of COSMIC (i.e., 24\({}^{\circ}\)). More than 5000 RO profiles are provided daily by COSMIC-2 within the latitudinal band (45\({}^{\circ}\)S, 45\({}^{\circ}\)N). Each COSMIC-2 LEO satellite carries an advanced RO receiver, which is named the Tri-GNSS Radio Occultation System (TGRS), with a high-gain beam-forming RO antenna that can produce a higher signal-to-ratio (SNR) than any RO receiver of previous RO missions. Therefore, COSMIC-2 delivers RO data of better precision and lower penetration into the troposphere [6]. About 50% of COSMIC-2 RO profiles can detect below 200 m above the surface. These features make COSMIC-2 data of unique value for the investigation of weather systems in the tropics and subtropics, especially in deep oceans. RO observations can provide high-resolution vertical distributions of atmospheric refractivity, temperature, and moisture under almost all atmospheric weather conditions. The near-surface atmospheric profiling information is precious for studying tropical convection and hurricane genesis and development [21; 22; 23]. The sea-surface warm anomalies and the wet atmosphere in the lower troposphere are critical conditions for triggering strong convection [24; 25; 26]. However, in some specific cases, such as when the refractivity gradient is strong, the amplitude of RO signals is reduced because of its defocusing or undergoing strong fluctuation such that the RO signals cannot be distinguished from noise.