Fast observation operator for GNSS tropospheric gradients

Raw data collected at a single Global Navigation Satellite System (GNSS) station allow the estimation of the Zenith Total Delay (ZTD) and the tropospheric gradient. In order to make use of such data in Numerical Weather Prediction (NWP) the observation operators must be developed. Our current observation operator for tropospheric gradients is based on a linear combination of ray-traced tropospheric delays (Zus et al., 2022). Although this observation operator is tuned for high speed and precision it remains difficult to be implemented into NWP Data Assimilation (DA) systems. In this contribution we introduce a simple and fast observation operator which is based on the closed-form expression depending on the north–south and east–west horizontal gradients of radio refractivity (Davis et al., 1993). We run the Weather Research and Forecasting (WRF) model (horizontal resolution of 10km) and find that for the considered geographical region (central Europe) and time period (summer season) the root-mean-square deviation between the tropospheric gradients calculated by the fast and original approach is about 0.15 mm. In essence, the observation operator error is non negligible but acceptable for assimilation. In a first step we implemented the developed operator in our experimental DA system (Zus et al., 2019) and run a series of experiments to check the usefulness of the new approach. We present results from this assimilation experiments where we utilize both simulated and real observations. In the next step we will implement the fast observation operator in the WRF DA system in support of the research project EGMAP (Exploitation of GNSS tropospheric gradients for severe weather Monitoring And Prediction).Davis, J., Elgered, G., Niell, A., and Kuehn, K.: Ground-based measurement of gradients in the “wet” radio refractivity of air, Radio Sci., 28, 1003–1018, 1993. Zus, F.; Douša, J.; Kačmařík, M.; Václavovic, P.; Dick, G.; Wickert, J. Estimating the Impact of Global Navigation Satellite System Horizontal Delay Gradients in Variational Data Assimilation. Remote Sens. 2019, 11, 41.Zus, F., Galina, D., and Wickert, J.: Development of a cost efficient observation operator for GNSS tropospheric gradients, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1079