The ICON-A model for direct QBO simulations on GPUs (version icon-cscs:baf28a514)

Abstract. Classical numerical models for the global atmosphere, as used for numerical weather forecasting or climate research, have been developed for conventional central processing unit (CPU) architectures. This now hinders the employment of such models on current top performing supercomputers, which achieve their computing power with hybrid architectures, mostly using graphics processing units (GPUs). Thus also scientific applications of such models are restricted to the lesser computer power of CPUs. Here we present the development of a GPU enabled version of the ICON atmosphere model (ICON-A) motivated by a research project on the quasi-biennial oscillation (QBO), a global scale wind oscillation in the equatorial stratosphere that depends on a broad spectrum of atmospheric waves, which origins from tropical deep convection. Resolving the relevant scales, from a few km to the size of the globe, is a formidable computational problem, which can only be realized now on top performing supercomputers. This motivated porting ICON-A, in the specific configuration needed for the research project, in a first step to the GPU architecture of the Piz Daint computer at the Swiss National Supercomputing Centre, and in a second step to the Juwels-Booster computer at the Forschungszentrum Jülich. On Piz Daint the ported code achieves a single node GPU vs. CPU speed-up factor of 6.3, and now allows global experiments at a horizontal resolution of 5 km on 1024 computing nodes with 1 GPU per node with a turnover of 48 simulated days per day. On Juwels-Booster the more modern hardware in combination with an upgraded code base allows for simulations at the same resolution on 128 computing nodes with 4 GPUs per node and a turnover of 133 simulated days per day. Additionally, the code still remains functional on CPUs as it is demonstrated by additional experiments on the Levante compute system at the German Climate Computing Center. While the application shows good weak scaling making also higher resolved global simulations possible, the strong scaling on GPUs is relatively weak, which limits the options to increase turnover with more nodes. Initial experiments demonstrate that the ICON-A model can simulate downward propagating QBO jets, which are driven by wave meanflow interaction.