The Assimilation of FY3C GNOS Radio Occultation Observations within GRAPES 3D-Var: Assimilated Experiments and Forecast Impact

The GNOS (Global Navigation Satellite System Occultation Sounder), a multi-GNSS (Global Navigation Satellite System) receiver which has the ability of receiving and processing both GPS (Global Positioning System) and Chinese BDS (Beidou navigation system) satellites signals, is the first RO (Radio Occultation) instrument mounted on FY-3C satellite launched on 23 September 2013. Approximately 800 RO events are observed per day by GNOS which are very important for operational NWP (Numerical Weather Prediction) applications especially in recent years because the near real time COSMIC-1 GNSS RO observations available are decreasing in these years. In this article, an effectively quality control scheme are designed and tested for GNOS refractivity observations. And then the GNOS refractivity observations are assimilated in GRAPES (Global/Regional Analysis and PrEdiction System developed by CMA) using 3D-Var. Impacts of the GNOS refractivity observation on GRAPES analysis and forecast are evaluated and analyzed by assimilation and forecast experiments run over both a winter period (1 December 2013 – 28 February 2014) and a summer period ( 1 June 2014 – 31 August 2014) compared with the control experiment assimilated all operationally available observations. Positive impacts in SH (South Hemisphere) analysis and forecast and minor positive impact in NH (North Hemisphere) are demonstrated by experiments results. Results demonstrated here also prove once again that the GNSS RO system is not saturated.