The Tuning Strategy of IPSL-CM6A-LR

The assessment of current and future risks for natural and human systems associated with climate change largely relies on numerical simulations performed with state-of-the-art climate models. Various steps are involved in the development of such models, from development of individual components of the climate system up to free parameter calibration of the fully coupled model. Here, we describe the final tuning phase for the IPSL-CM6A-LR climate model. This phase alone lasted more than 3 years and relied on several pillars: (i) the tuning against present-day conditions given a small adjustment of the ocean surface albedo to compensate for the current oceanic heat uptake, (ii) the release of successive versions after adjustments of the individual components, implying a systematic and recurrent adjustment of the atmospheric energetics, and (iii) the use of a few metrics based on large scale variables such as near-global mean temperature, summer Arctic sea-ice extent, as targets for the tuning. Successes, lessons and prospects of this tuning strategy are discussed.