Assimilation of GNSS Tropospheric Gradients into the Weather Research and Forecasting Model Version 4.4.1

Abstract. In this study, we have incorporated tropospheric gradient observations from a Global Navigation Satellite Systems (GNSS) ground station network into the Weather Research and Forecasting (WRF) model through a newly developed observation operator. The experiments are aimed to test the functionality of the developed observation operator and to analyze the impact of tropospheric gradients in the sophisticated Data Assimilation (DA) system. The model was configured for a 0.1-degree mesh over Germany with 50 vertical levels up to 50 hPa. Our initial conditions were obtained from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) data at 0.25-degree resolution, and conventional observations were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF), restricted to surface stations and radiosondes. We selected approximately 100 GNSS stations with high data quality and availability covering Germany. We performed DA every 6 hours for June and July 2021. Three experiments were conducted: 1) The control run assimilating only conventional observations; 2) the impact run assimilating Zenith Total Delays (ZTDs) on top of the Control run; and 3) the Impact-Gradient run assimilating ZTDs and gradients on top of the Control run. The error for the Impact run was reduced by 32 % and 10 % for ZTDs and gradients, whereas the error for the Impact-Gradient run was reduced by 35 % and 18 %, respectively. Overall, the newly developed operator for the WRF DA system works as intended. In particular, the combined assimilation of gradients and the ZTDs led to a notable improvement in the humidity field at altitudes above 2.5 km. With the source codes developed and freely available to the WRF users, we aim to trigger further GNSS tropospheric gradient assimilation studies to refine the technique and improve its performance.