Towards higher resolution data assimilation in ECMWF IFS

The Integrated Forecast System (IFS) of ECMWF comprises an Earth-system model coupled with an advanced data assimilation system, which was recently experimentally rescaled to higher resolution. The higher resolution initial conditions were achieved through higher resolution (4.4 km) 4D-Var first guess trajectory, higher resolution (20 km) 4D-Var minimizations (utilizing tangent linear model and its adjoint) or their combination. We demonstrate significant improvements of large-scale skill scores, better use of observations in higher resolution 4D-Var minimization, and an added benefit of using higher resolution 4D-Var together with the higher resolution observations. The impact of the higher resolution DA system is demonstrated on specific test cases of extreme events where the increased spatial resolution allows to improve the fidelity of simulations. For instance, Tropical Cyclone Otis, which hit Acapulco, Mexico, in October 2023 , as a Category 5 storm, was forecasted to only reach tropical storm intensity by the majority of global (and regional) Numerical Weather Prediction models and was considered a major forecast bust. Employing higher resolution trajectory and higher resolution 4D-Var, we were able to simulate its rapid intensification into a Category 3 tropical cyclone.