YAC v3.19.0: scalable online interpolation and flexible coupling for Earth system models

Abstract. Earth system models increasingly couple many components on heterogeneous grids at rising spatial resolution, which raises requirements on coupling software for scalability, interpolation quality, workflow integration, and reliable orchestration. We present the transition to YAC 3, described here for release v3.19.0, and summarise advances that address these requirements in production-relevant settings. First, YAC 3 updates its parallel online weight-generation workflow, enabling scalable setup for very-high-resolution coupled configurations and large MPI process counts. Second, interpolation-related capabilities have been extended and refined, including improvements to conservative remapping and new methods such as Nearest-Corner-Cells, Distance-Nearest-Neighbour, and Creep Fill. Third, workflow support is broadened through Python bindings, YAML-based configuration, and tooling for weight and grid inspection. Fourth, we introduce a generic MPI handshake mechanism that makes communicator construction in multi-component, multi-library setups more reliable, and propose it as a convention for other libraries and components. Fifth, YAC is separated into a self-contained core library and a component-oriented model coupling interface, which makes its interpolation stack reusable by software outside the classical model coupling context. Use cases from coupled ICON simulations (including kilometre-scale configurations), input/output server coupling, and direct integration in other software ecosystems demonstrate that YAC 3 supports a broad range of contemporary and emerging Earth system workflows.