Modeling the Earth System on Modular Supercomputing Architectures: coupled atmosphere-ocean simulations with ICON
Authors: Stein Olaf , Bishnoi Abhiraj , Kornblueh Luis , Hoffmann Lars , Eicker Norbert , Suarez Estela , Meyer Catrin I. 
Published: February 2023

Significant progress has been made in recent years to develop km-scale versions of global Earth System Models (ESM), combining the chance of replacing uncertain model parameterizations by direct treatment and the improved representation of orographic and land surface features (Sch&#228;r et al., 2020, Hohenegger et al., 2022). However, adapting climate codes to new hardware and at the same time keeping the performance portability, still remains a major issue. Given the long development cycles, the various maturity of ESM modules and their large code bases, it is not expected that all code parts can be brought to the same level of exascale readiness in the near future. Instead, short term model adaptation strategies need to focus on software abilities as well as hardware availability. Moreover, energy use efficiency is of growing importance on both sides, supercomputer providers and scientific projects employing climate simulations.Here, we present results from first simulations of the coupled atmosphere-ocean modelling system ICON-v2.6.6-rc on the supercomputing system JUWELS at the J&#252;lich Supercomputing Centre (JSC) with a global resolution of 5 km, using significant parts of the HPC system. While the atmosphere part of ICON (ICON-A) is capable of running on GPUs, model I/O currently performs better on a CPU cluster and the ocean module (ICON-O) has not been ported to modern accelerators yet. Thus, we make use of the modular supercomputing architecture (MSA) of JUWELS and its novel batch job options for the coupled ICON model with ICON-A running on the NVIDIA A100 GPUs of JUWELS Booster, while ICON-O and the model I/O are running simultaneously on the CPUs of the JUWELS Cluster partition. As expected, ICON performance is limited by ICON-A. Thus we chose the performance-optimal Booster-node configuration for ICON-A considering also memory requirements (84 nodes) and adapted ICON-O configuration to achieve minimum waiting times for simultaneous time step execution and data exchange (63 cluster nodes).&#160; We compared runtime and energy efficiency to cluster-only simulations (on up to 760 cluster nodes) and found only small improvements in runtime for the MSA case, but energy consumption is already reduced by 26% without further improvements in vector length applied with ICON. When switching to even higher ICON resolutions, cluster-only simulations are not fitting to most of current HPC systems and upcoming exascale systems will rely to a large extent on GPU acceleration. Thus exploiting MSA capabilities is an important step towards performance portable and energy efficient use of km-scale climate models.References:Hohenegger et al., ICON-Sapphire: simulating the components of the Earth System and their interactions at kilometer and subkilometer scales, https://doi.org/10.5194/gmd-2022-171, in review, 2022.Sch&#228;r et al., Kilometer-Scale Climate Models: Prospects and Challenges, https://doi.org/10.1175/BAMS-D-18-0167.1, 2020.&#160;