Global Simulations of the Atmosphere at 1.45 km Grid-Spacing with the Integrated Forecasting System

Global simulations with 1.45 km grid spacing arc presented that were performed using the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Simulations are uncoupled (without ocean, sea ice, or wave model), using 62 or 137 vertical levels and the full complexity of weather forecast simulations is presented, including recent date initial conditions, real-world topography, and state-of-the-art physical parametrizations, as well as diabatic forcing including shallow convection, turbulent diffusion, radiation and five categories for the water substance (vapor, liquid, ice, rain, and snow). Simulations are evaluated with regard to computational efficiency and model fidelity. Scaling results are presented, which were performed on the fastest supercomputer in Europe, Piz Daint (Top 500, November 2018). Important choices for the model configuration at this unprecedented resolution for the IFS are discussed such as the use of hydrostatic and non-hydrostatic equations or the time resolution of physical phenomena which is defined by the length of the time step. Our simulations indicate that the IFS model-based on spectral transforms with a semi-implicit, semiLagrangian time stepping scheme in contrast to more local discretization techniques-can provide a meaningful baseline reference for O(1) km global simulations.