Forecast Impact of FORMOSAT-7/COSMIC-2 GNSS Radio Occultation Measurements

The FORMOSAT-7/COSMIC-2 GNSS-RO mission was launched on June 25, 2019, and it has provided a large increase in the number of GNSS-RO observations available for operational numerical weather prediction (NWP) in the latitude band between ±40°. A key aim of this mission has been to improve the GNSS-RO measurement quality in the lower and middle troposphere. In this study, we summarize the impact of the FORMOSAT-7/COSMIC-2 measurements in two independent NWP systems, which are now assimilating these measurements operationally. These are the United States Navy Global Environmental Model (NAVGEM) and the European Center for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). Both systems employ a 4-dimensional variational system (4D-Var), and assimilate GNSS-RO bending angles. The experiments cover the period January to March 2020. The impact of the FORMOSAT-7/COSMIC-2 measurements is assessed using improvements in short-range forecast departures to other observations such as radiosonde and radiances, forecast error statistics against a verifying analysis, and adjoint based Forecast Sensitivity to Observation Impact (FSOI) estimates. The FORMOSAT-7/COSMIC-2 measurement has a clear impact on stratospheric temperatures and winds in the tropics. A novel finding is that the measurements also improve the tropical tropospheric humidity fit to radiosondes, and the fit to tropospheric radiances sensitive to humidity. To date, the impact of GNSS-RO on humidity has been difficult to demonstrate in well constrained, operational NWP systems assimilating the full suite of observations. The results are achieved with a conservative assimilation approach which extended the quality control and observation error assignments used for the previous COSMIC receivers; further, possible improvements to the assimilation strategy are noted.