The Tuning Strategy of IPSL‐CM6A‐LR

Abstract

The tuning process of IPSL-CM6A-LR under present-day control conditions is described • The associated continuous atmospheric energetics adjustment is presented• The parameter tuning was based on the use of a few large scale metrics.• Successes, lessons and prospects of the IPSL-CM6A-LR tuning strategy are discussed

Semi-automatic tuning of coupled climate models with multiple intrinsic timescales: lessons learned from the Lorenz96 model

“…As noted in studies of model calibration or "tuning" (e.g. Hourdin et al, 2017), this is generally done by first separately calibrating individual components of the climate system to conform to theoretical and observational constraints, and then in a separate step, applying global constraints on emergent properties of the coupled climate system, such as top-of-atmosphere radiative balance, or ENSO behavior (Held et al, 2019;Mignot et al, 2021). The ini-tial stages are done with fixed surface exchanges: for example in AMIP experiment, Zhao et al (2018a) calibrates the atmosphere under prescribed sea surface temperatures.…”

“…This carries the risk, of course, of compensating for atmospheric model biases by adjusting ocean parameters. When tuning the IPSL climate model for CMIP6, it was actually intentional to compensate for biases in the atmosphere, that initially limited dense water formation, by tuning the sea-ice leads fraction and finally enhance ventilation of the deep ocean (Mignot et al, 2021).…”

“…Notwithstanding, our conclusion that ocean-only experiments cannot be used to calibrate ocean parameters if the emergent-coupled metrics are included in the HM procedure, seems to be robust. For example, tuning sea-ice quantities in IPSL-CM6A-LR had to be done in coupled mode (Mignot et al, 2021). This advocates for running OMIP experiments to calibrate ocean quantities related to the ocean interior dynamics, and calibrating upper-ocean variables, including sea-ice, in coupled mode.…”

“…(Other components of the climate, for example changes in the biosphere are also "slow" relative to the atmosphere: in this paper, we use the ocean to stand in as the canonical "slow" component.) While the ocean component can be validated to a certain extent following the Ocean Model Intercomparison Project (OMIP: Griffies et al, 2016), several key phenomena of interest such as the El-Niño Southern Oscillation (ENSO), or simply sea-ice cover in northern and southern hemispheres, are emergent properties of the coupled ocean-atmosphere system, as noted by Held et al (2019) and Adcroft et al (2019) for GFDL, and Mignot et al (2021) for IPSL.…”

“…We draw attention to the appearance of the word "tuning" in the title of several of the articles cited above (Zhao et al, 2018b;Mignot et al, 2021). The tuning, or calibration as termed by statisticians, of ESMs is itself an object of study in its own right (Hourdin et al, 2017;Schmidt et al, 2017).…”