The Community Earth System Model (CESM2) is a fully-coupled global climate model developed at the US National Center for Atmospheric Research (NCAR) [1] that allows state-of-the-art simulations of the Earth's past, present, and future climate system. The availability of this model will provide scientists with tools to study these topics, and also to make comparisons with other models, which is an essential part of climate science. Beyond the scientific advantages, the model can be more easily configured compared to other models currently available on ARCHER2, such as the Unified Model [2], as all of the infrastructure can be maintained and run in a self-contained manner on one system. The most current scientifically validated CESM2 version was released in June 2020 (CESM2.1.3), and this report details its porting, optimisation, testing and validation to ARCHER2. The key choices made during the model porting, specifically with regards to the software and hardware available on the system, are described. The model has been run in several configurations, which are the typical set-ups used by the climate research community, and the performance is characterised. These configurations have also been scientifically tested, and results are shown to satisfy validation requirements. [1] https://www.cesm.ucar.edu/ [2] https://www.metoffice.gov.uk/research/approach/modelling-systems/unified-model

This work was funded under the embedded CSE programme of the ARCHER2 UK National Supercomputing Service (http://www.archer2.ac.uk).