Storm- and eddy-resolving simulations with IFS-FESOM/NEMO at the kilometre scale

Global coupled simulations that can resolve atmospheric storms and mesoscale oceanic features at the kilometre-scale have recently become possible to run over short time slices, for example on a seasonal timescale. Here we give an overview of the first multi-year simulations performed with ECMWF’s Integrated Forecasting System (IFS), coupled to both the NEMO and FESOM2 ocean-sea ice models, for the H2020 Next Generation Earth Modelling Systems (nextGEMS) project. The project aims to build a new generation of eddy- and storm-resolving global coupled Earth System Models. Along with ICON, the other model participating in nextGEMS, the IFS-based models form the basis also for Digital Climate Twins of Earth as envisioned in the European Union’s ambitious Destination Earth project. nextGEMS relies on several model development cycles, in which the models are run and improved based on community feedback. In an initial set of storm-resolving coupled simulations (Cycle 1), the IFS was integrated for 75 days. For Cycle 2, IFS has been run at the operational 9 km resolution as a baseline, and at 4.4 km and 2.8 km global spatial resolution for up to 1 year of simulation (4.4 km). To our knowledge, the 8-months long 2.8 km simulation in Cycle 2 represents the first fully coupled simulation ever of this duration at this high level of spatial detail and is made available to the public. The runs at 9 km were performed with the parameterization for deep convection active as in the operational system, while at 4.4 km and 2.8 km, separate experiments with IFS were run both with and without the deep convection parameterization.

We document the model improvements made to IFS-FESOM/NEMO based on the lessons learned from the first Cycle 1 runs, which were included for the second round of Cycle 2 simulations; these mainly consist in vastly improved conservation properties of the coupled model systems in terms of water and energy balance, which are crucial for longer climate integrations, and in a more realistic representation of the snow and surface drag. Cycle 2 also targeted eddy-resolving resolution in large parts of the mid- and high-latitude ocean (better than 5km) to resolve mesoscale eddies and linear kinematic features (i.e. leads or cracks) in sea ice. For IFS-FESOM, this is made possible thanks to a recently refactored ocean model code that can be linked as an external library and that allows for efficient coupled simulations in the single-executable context with IFS, via hybrid parallelization with MPI and OpenMP.