The preWarmWorld project, carried out from 1 October 2021 to 30 September 2023, was funded by the German Federal Ministry of Education and Research under the FONA programme for sustainable development. It was a preparatory effort for the larger WarmWorld initiative, which seeks to build an innovative climate‑projection system based on the ICON atmospheric model. The partners involved were the German Climate Computing Center (DKRZ), the Forschungszentrum Jülich (FZ‑Jülich), the Max‑Planck Institute for Meteorology (MPI‑M), the German Weather Service (DWD), the Karlsruhe Institute of Technology (KIT) and the Center for Climate Modeling and Simulation (C2SM). The project’s lead author was Daniel Klocke of MPI‑M.
Technically, preWarmWorld focused on modernising the ICON code base to exploit next‑generation high‑performance computing (HPC) architectures and to raise the scientific quality of the model. The original ICON model, developed by MPI‑M and DWD, had been running on a 100 km grid; the WarmWorld vision is to run it on a 2 km grid, allowing direct calculation of physical processes that were previously parameterised. To achieve this, the project produced a comprehensive software blueprint that defined modularisation, libraryisation, memory‑management strategies, and a new repository structure. A dedicated test infrastructure based on GitLab and Buildbot was installed at DKRZ and made available to the ICON community, enabling continuous integration and automated regression testing on both the Jülich Supercomputing Centre (JSC) and DKRZ systems.
A key scientific outcome was the isolation and re‑implementation of critical ICON components—advection and cloud microphysics—using different programming paradigms. The team evaluated AnyDSL and eDSL, concluding that they were not yet mature enough for production use. GridTools, a library used in the Swiss EXCLAIM project, was adopted for certain grid‑related operations, while C++‑based approaches were identified as the most promising for the WarmWorld‑Faster phase. The coupled ICON model was successfully deployed on the modular supercomputing architecture at JSC, demonstrating the feasibility of the new design on large‑scale HPC resources.
Governance and licensing were also transformed. A new ICON Board (D5) was established, with senior representatives from each partner institution, to steer strategic decisions. The Board approved the transition of ICON to an open‑source BSD‑3C license, which was released in January 2024. This change, together with the new governance model, simplifies collaboration and encourages external contributions from academia and industry. The preWarmWorld blueprint and the open‑source release have already been integrated into the ICON‑Consolidated (ICON‑C) effort, providing a solid foundation for the subsequent WarmWorld‑Faster and WarmWorld‑Better phases.
In summary, preWarmWorld delivered a modern, modular, and test‑driven software foundation for the ICON model, established clear governance and open‑source licensing, and demonstrated the feasibility of running the model on a 2 km grid on contemporary HPC platforms. These achievements set the stage for the WarmWorld project to deliver higher‑resolution, more accurate climate projections while fostering a collaborative, agile development environment across its international partner network.