V. Maurer et al.: Evaluation of coupled and uncoupled simulations 
85: 
(a) 
(b) 
nm TO 
mean ROAM-NBS 
mean obs 
mean ICON-CLM 
rmse ROAM-NBS 
mean ICON-CLM 
40 
nA 
?0- 
KM! 
AS 
A 
$ 
S$ 
& 
Na 
Fe 
KK 
PS 
N 
Figure 8. Map with an overview of the evaluated station observations (a); mean station observations of hourly wind speed for 2011-2020, 
mean values and root-mean squared errors (rmse) of 9 closest grid points for each station for ROAM-NBS and ICON-CLM (b); violinplot 
displaying median, minimum and maximum values of hourly wind data at 100 m height at FINO1 and at the closest grid points in ROAM- 
NBS and ICON-CLM. for the vears 2004-2010 (6). 
Fehmarn, which is a larger island compared to those in the 
North Sea, the mean wind speed is well represented by both 
ROAM-NBS and ICON-CLM, and the RMSE values are also 
small compared to other stations. 
3.3 Oceanographic conditions 
3.3.1 Sea ice concentration and extent 
The bias of ROAM-NBS and NEMO-NBS against the 
Copernicus observations for the winter season (DJF) is 
small (Fig. 9a), and the difference between ROAM-NBS 
and NEMO-NBS is negligible (Fig. 9b). Nevertheless, in the 
spring season (MAM), the amplification of sea ice extent is 
prominent in the Gulf of Bothnia. While the start of the ice 
growth season and the growth rate in the end of December 
are well captured, the sea ice extent is overestimated during 
che peak later in winter (February-March) by both simula- 
tions (mean annual time series of sea ice extent not shown). 
Furthermore, the sea ice season is prolonged towards May 
https:/doi.o0rg/10.5194/smd-19-543-2026 
compared to the observations. The overestimation of sea ice 
concentration as well as extent in MAM might originate from 
the prevailing negative bias in SST in the Bothnian Bay and 
Sea during winter and spring (Fig. 2 and the temperature pro- 
file for DJF at SMHISR5C4 station in Fig. 10) and an un- 
derestimation of salinity, especially in the Gulf of Bothnia 
(see the next subsection). Further, in Nie et al. (2023), the 
NEMO-SI3 model for version 4.0 was found to be most sen- 
sitive to the ice-ocean and atmosphere—-ice drag coefficients 
and, therefore, to ice dynamics. Since only thermodynami- 
cal processes are modeled within ROAM-NBS and NEMO- 
NBS, further enhancements in results could be obtained by 
also considering ice dynamics. First tests with ice dynamics 
did not improve the overestimation of sea ice in spring. They 
will need to be carried out in the future in combination with 
further parameter tuning and recalibration to eliminate the 
temperature and salinity bias in the Northern Baltic. 
Geosci. Model Derv.. 19. 543-578, 2026