+
V. Maurer et al.: Evaluation of coupled and uncoupled simulations
consistent with the NEMO/SI3, which has a more detailed
treatment of thermodynamical processes within sea ice, the
sea ice albedo, thickness and surface temperature are trans-
ferred from the ocean to the atmosphere in our setup. The
ocean albedo over water is not sent, as this option is not avail-
able in the default coupling setup of NEMOv4.2.0. Only the
one over ice is available for the coupling. Over the ocean,
ICON uses a formulation by Taylor et al. (1996) for the di-
rect albedo and the value of 0.06 for diffuse albedo as in
ECMWF’s IFS model. Additionally, an albedo increase by
whitecap cover after Seferian et al. (2018) is considered.
The complete list of variables exchanged during the cou-
pling in ROAM-NBS is given in Table 1. For selected vari-
ables, a global conservation is applied, which means that
che area average is compared before and after horizontal in-
terpolation. By selecting “GLBPOS” in the OASIS3-MCT
settings, the difference between the area average on the tar-
get minus the source grid is distributed proportionally to the
value of the original field.
Because only the respective portions of fluxes for the
ocean and sea ice tiles are used, flux contributions from land
points are completely excluded when fluxes are sent from the
atmosphere to the ocean, which also ensures a consistent eXx-
change of energy and momentum along the coastlines. As
suggested by Mechoso et al. (2021) for models that incorpo-
rate the tile approach, the ocean fraction of the atmospheric
model is adapted to that of the ocean model, which is ob-
:ained by interpolating the binary land-sea mask of the ocean
model to the atmospheric grid. The land and lake fractions of
‘he atmospheric model are adapted accordingly. For a correct
treatment of coastal points, great care was put on identical
interpolation methods for the NEMO land-sea mask and the
NEMO fields to the ICON grid during coupling, as well as on
the consistency of the common land-sea mask and the masks
used by OASIS3-MCT. Thus, the flux coupling as included in
ROAM-NBS is a good compromise between simplicity and
correctness.
especially in coupled mode. To avoid a so-called “cold start”,
a spin-up for the ocean component was carried out with
NEMO-NBS for the years 1974-1978, starting with an ini-
tial field for temperature and salinity. This initial field was,
in turn, a combined product from the ORAS5S reanalysis and
the Baltic reanalysis product provided by SMHI (personal
communication) based on Hordoir et al. (2019). The restart
field for 1 September 1978 from the spin-up simulation with
NEMO-NBS was then used to start ROAM-NBS.
For a better comparison of NEMO-NBS and the ocean part
of ROAM-NBS, it was decided to use the same observation-
based dataset for the runoff for both experiments. ICON-
CLM as well as the atmospheric part of ROAM-NBS were
started from the ERA5-driven ICON-CLM-UDAG simula-
don, which had been running since 1940.
For the atmospheric part, ERAS reanalyses were used as
lateral boundary conditions, as prescribed by the CORDEX-
CMIP6 experiment protocol (CORDEX, 2025). They are
available at a 3-hourly resolution and are also used to pre-
scribe SST and sea ice in the uncoupled ICON-CLM simula-
tion. For ROAM-NBS, ERA5 SST and sea ice fields are also
used in ocean regions outside of the NEMO-NBS domain. As
lateral boundary conditions for the ocean, the temperature,
salinity, zonal and meridional ocean velocities, and sea sur-
face height (SSH) fields from the ORASS5 global reanalysis
dataset (Zuo et al., 2019) are used. The prescribed fields were
used at a monthly resolution, as daily data were not available
for the whole simulation period. To ensure that the spatial
mean of the modeled SSH fits the observed SSH, the bound-
ary conditions for the SSH from ORAS5 were bias corrected
with respect to the SSH at Helgoland from the GESLAv3.0
observational data (Haigh et al., 2023). This bias was calcu-
lated on the basis of a 5-year test simulation with uncorrected
boundary conditions.
The evaluation is generally conducted for the years 1979-
2020, which is the minimum period required for CORDEX.
However, especially for the ocean part, many reference data
are available for shorter time periods only, so that the eval-
uation period had to be adapted in these cases: SST and
sea ice data (CMEMS, 2025a) are available from Septem-
ber 1981 and surface salinity from 1993 (CMEMS, 2023).
Station data (CMEMS, 2024a) are very sparse before 1993.
Copernicus reanalyses used for the evaluation of marine heat
waves (CMEMS, 2024b, 2025b) start in 1989. For the at-
mospheric part, satellite data used for the evaluation of sur-
face radiation (CERES; NASA/LARC/SD/ASDC, 2019) are
available from 2001 only. Therefore, the evaluated time peri-
ods had to be adapted in these cases; an overview is given in
Table 2. For evaluations in which statistics were calculated
from hourly data, shorter time periods were used, partly due
to limited data availability as well, partly to reduce the com-
putational costs (see Sects. 3.2.3 and 3.3.4).
2.4 Overview of experiments and availability of
reference data
The evaluation simulation was run for the coupled
model (ROAM-NBS) and for both the atmospheric and ocean
components individually (ICON-CLM and NEMO-NBS, re-
spectively); Le., we are overall evaluating and comparing
three simulations. In Sect. 3.2.1, a basic comparison to the
evaluation simulation from the UDAG project (called ICON-
CLM-UDAG) is additionally shown. According to CORDEX
specifications (CORDEX, 2025), “the evaluation experiment
must cover at least the 1979-2020 period”. Our experiments
were started in September 1978 because of the stable thermal
mixed layer in the ocean. Starting in January would mean
starting in the middle of the winter season when thermal
conditions are unstable and the sea ice has already evolved,
which has proven disadvantageous for model initialization,
Geosci. Model Dev... 19. 543578, 2026
https://doi.org/10.5194/sgmd-19-543-2026
