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S3: Climate Model Intercomparison

Principal investigators: Dr. Friederike Pollmann (University of Hamburg), Dr. Katja Weigel (University of Bremen)

To realize the third phase motto “from accuracy to application”, the new synthesis subproject S3 will evaluate core CRC parameterizations and schemes that have been developed and/or extended in the different CRC project phases. These include, for example, the IDEMIX (Internal wave Dissipation, Energy and MIXing) closure for the parameterization of internal gravity wave effects on the ocean and atmosphere (see W4, W1) or the dynamic kinetic energy backscatter parameterization, which re-injects some of the dissipated eddy energy at larger scales (see M3, L4). This evaluation will be performed in parallel in the two national climate models ICON and IFS-FESOM such that the effect of the CRC parameterizations can be put into perspective and inter-model differences can be assessed.

Fig. 1: Mixing work, defined as the product of buoyancy frequency squared and vertical diffusivity, at 600-700 m depth in the three state-of-the-art ocean models ICON, FESOM, and MITgcm. The upper and lower rows differ in their parameterization of the vertical mixing induced by internal gravity waves: The simulations shown in the upper row applied a constant background diffusivity and the ones in the lower row the consistent IDEMIX closure. The observational reference, obtained from Argo float observations by applying the finescale parameterization as in Pollmann et al., 2017, are shown on the right. Their magnitude and spatial variability can in the models only be reproduced in the simulations with IDEMIX.

Subproject S3 will hence develop and define common diagnostics and setups for this climate model inter-comparison. We will build on the work on the first inter-model comparison with respect to the IDEMIX closure performed in phase 2 (Brüggemann et al., 2024; fig. 1). As is standard for such assessments, we will complement our analysis by a thorough evaluation against observations.

The central goal of the CRC is to build energetically consistent climate models. This is particularly crucial when climate state’s that differ substantially from today’s are simulated. Hence, S3 will apply and investigate the impact of CRC schemes and parameterizations in coupled simulations of pre-industrial and future climate conditions. We will build on selected DECK (Diagnostic, Evaluation and Characterization of Klima) experiments of CMIP (Climate Model Intercomparison Project).

By combining and evaluating in a common framework all of the CRC’s main schemes and parameterizations, subproject S3 necessarily collaborates closely with the other CRC subprojects, in particular with the other two synthesis subprojects S1 and S2. This is illustrated in the schematic (fig. 2), which involves as key CRC parameterizations: the IDEMIX closure and the kinetic backscatter parameterization mentioned above, the surface wave model ICON-Waves and the General Ocean Turbulence Model GOTM for the consistent representation of surface wave effects, and the IDEMIX closure transferred to the atmosphere, IDEMIX-A, and the Multi-Scale Gravity-Wave Model MS-GWaM for the consistent representation of atmospheric gravity wave effects.

Fig. 2: The central tasks of subproject S3 are the definition of comparable model configurations as well as common diagnostics and output for the parallel evaluation of TRR 181 parameterizations and schemes in ICON and FESOM. This will be performed in close collaboration with the other two subprojects of the synthesis area, S1 and S2, as well as with the subprojects leading the development and improvement of tools, parameterizations and numerical schemes in the CRC.