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Atmosperic profiles derived from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements make up for the lack of operational radiosonde soundings with a high spatiotemporal distribution, and their performance over China is assessed in this paper. COSMIC-retrieved atmospheric wet profiles from 2014 to 2015 are compared to the contemporaneous radiosonde profiles from 120 stations, and the vertical mean differences are used. The results show that the vertical mean biases of temperature, pressure and vapor pressure are -0.10 K, 0.69 hPa and -0.01 hPa, respectively, and that for refractivity is 0.17 N. Moreover, the temperature differences are positively correlated with station altitude, yet both the pressure and vapor pressure differences are negatively correlated with station latitude, as is the refractivity difference. The large temperature difference arising from the Qinghai-Tibet Plateau (QTP) region may be associated with the complex topography of the area and the limitations in the background model used in the COSMIC profile retrieval. Furthermore, negative refractivity bias between COSMIC and radiosonde data occurs below 5 km and is large in wet southern China, with a value of less than 1%. This result may be related to more humid conditions and super-refraction. [ABSTRACT FROM AUTHOR]

In this paper, a global ionospheric data assimilation model is constructed based on the empirical international-reference-ionosphere model and the Kalman filter. A sparse matrix method is used to militate the huge computation and storage problems. A series of observing system simulation experiments has been performed based on the existing global ground-based global navigation satellite system (GNSS) network, the planned Constellation Observing System for Meteorology, Ionosphere, and Climate #2/Formosa Satellite Mission #7 (COSMIC-2/FORMOSAT-7) orbits, and the real global position system and GLObal NAvigation Satellite System (GLONASS) orbits. Specifically, the COSMIC-2 will have six 24° inclination satellites in 500-km altitude and six 72° inclination satellites in 800-km altitude. The slant total electron content of ground-based GNSS, radio occultation and ocean reflection (OR) of 12 low-Earth-orbit satellites, and cross-link between COSMIC-2 low and high inclination satellites are simulated by the NeQuick model. The ORs show great impacts in specifying the ionosphere except over the inland area. It complements the existing ground-based GNSS network, which mainly observes the ionosphere over the land area. The 24° and 72° satellites can complement each other to optimize the global ionospheric specification. The COSMIC-2 mission is expected to contribute significantly to the accurate ionospheric nowcast. Its potential ability in ionospheric short-term forecast is also discussed. [ABSTRACT FROM PUBLISHER]

Launched in 2006, the Formosa Satellite Mission 3-Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) was the first constellation of microsatellites carrying global positioning system (GPS) radio occultation (RO) receivers. Radio occultation is an active remote sensing technique that provides valuable information on the vertical variations of electron density in the ionosphere, and temperature, pressure, and water vapor in the stratosphere and troposphere. COSMIC has demonstrated the great value of RO data in ionosphere, climate, and meteorological research and operational weather forecasting. However, there are still challenges using RO data, particularly in the moist lower troposphere and upper stratosphere. A COSMIC follow-on constellation, COSMIC-2, was launched into equatorial orbit in 2019. With increased signal-to-noise ratio (SNR) from improved receivers and digital beam steering antennas, COSMIC-2 will produce at least 5,000 high-quality RO profiles daily in the tropics and subtropics. In this paper, we summarize 1) recent (since 2011 when the last review was published) contributions of COSMIC and other RO observations to weather, climate, and space weather science; 2) the remaining challenges in RO applications; and 3) potential contributions to research and operations of COSMIC-2. [ABSTRACT FROM AUTHOR]