Abstract
Abstract. The evaluation and model element description of the second version of the unstructured-mesh Finite-volumE Sea ice-Ocean Model (FESOM2.0) are presented. The new version of the model takes advantage of the finite-volume approach, whereas its predecessor version, FESOM1.4 was based on the finite-element approach. The model sensitivity to arbitrary Lagrangian–Eulerian (ALE) linear and nonlinear free-surface formulation, Gent–McWilliams eddy parameterization, isoneutral Redi diffusion and different vertical mixing schemes is documented. The hydrographic biases, large-scale circulation, numerical performance and scalability of FESOM2.0 are compared with its predecessor, FESOM1.4. FESOM2.0 shows biases with a magnitude comparable to FESOM1.4 and simulates a more realistic Atlantic meridional overturning circulation (AMOC). Compared to its predecessor, FESOM2.0 provides clearly defined fluxes and a 3 times higher throughput in terms of simulated years per day (SYPD). It is thus the first mature global unstructured-mesh ocean model with computational efficiency comparable to state-of-the-art structured-mesh ocean models. Other key elements of the model and new development will be described in follow-up papers.
Highlights
Ocean general circulation models that work on unstructured meshes were established in the coastal ocean modeling community a long time ago, offering the multi-resolution functionality without grid-nesting techniques required by regular-grid models
The same mesh has already been used in a variety of studies carried out with FESOM1.4, such as in the model intercomparison project of the Coordinated Ocean Ice Reference Experiment – phase II (CORE2), which proved that FESOM1.4 performs well compared to structured-mesh ocean models
The computational performance and scaling estimates of FESOM2.0 and FESOM1.4 in Sect. 4 are conducted on a medium-sized mesh (Fig. 1b, 0.64 M surface vertices) that shares the same resolution with the reference mesh, except for the Arctic Ocean and Bering Sea, where the resolution is refined to ∼ 4.5 and ∼ 10 km, respectively
Summary
Ocean general circulation models that work on unstructured meshes were established in the coastal ocean modeling community a long time ago, offering the multi-resolution functionality without grid-nesting techniques required by regular-grid models. With global mesoscale eddy-resolving configurations becoming a focus of climate research, the limits of FESOM1.4 set by its high demand of computational resources became more and more obvious (Sein et al, 2017, 2018) This motivated the development of the new model version, FESOM2.0 (Danilov et al, 2017). FESOM2.0 builds on the framework of its predecessor, FESOM1.4, using its sea ice component Finite-Element Sea Ice Model (FESIM; Danilov et al, 2015), general user interface and code structure Both model versions work on unstructured triangular meshes, the horizontal location of quantities and vertical discretization are different.
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