Abstract
<p>Global simulations with 1.45 km grid-spacing are presented that were performed with 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 including recent date initial conditions, real-world topography, and state-of-the-art physical parametrizations and diabatic forcing including shallow convection, turbulent diffusion, radiation and five categories for the water substance (vapour, liquid, ice, rain, snow). Simulations are evaluated with regard to computational efficiency and model fidelity. Scaling results are presented that were performed on the fastest supercomputer in Europe - Piz Daint (Top 500, Nov 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. </p><p>Our simulations indicate that the IFS model — based on spectral transforms with a semi-implicit, semi-Lagrangian time-stepping scheme in contrast to more local discretization techniques — can provide a meaningful baseline reference for O(1) km global simulations.</p>
Highlights
The complexity and quality of weather and climate models have improved at a remarkable speed during the last decades (Bauer et al 2015) and the steady increase in computing power has allowed for a steady increase in model resolution and complexity of forecast models
We document simulations using the Integrated Forecasting System (IFS) that are running with a horizontal grid spacing of 1.45 km from real-world initial conditions and with real-world topography on the fastest supercomputer in Europe
These simulations are generating only limited model output, are uncoupled, may require smaller timestep or non-hydrostatic adjustments, and would still be too slow to allow for operational weather and climate predictions that would require a throughput of at least 1 simulated years per day (SYPD)
Summary
The complexity and quality of weather and climate models have improved at a remarkable speed during the last decades (Bauer et al 2015) and the steady increase in computing power has allowed for a steady increase in model resolution and complexity of forecast models. The performance of the IFS is discussed, and the scalability tests on the Piz Daint supercomputer and the two supercomputers of the European Centre for Medium-Range Weather Forecasts (ECMWF) are presented These scaling results provide a good benchmark for the improvements in efficiency that would be required to allow for global, operational weather forecasts or climate projections at storm-resolving resolution. A first scientific evaluation of the IFS model fidelity for simulations at storm-resolving 1.45 km grid spacing is presented This includes a discussion of the effective resolution of atmospheric dynamics from energy spectra (Abdalla et al 2013) and a limited assessment how choices for the model configuration, for example regarding the use of non-hydrostatic equations, the parametrization of convection, or the timestep length influence model simulations.
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