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T. Spangehl et al.: Intercomparing the quality of recent reanalyses for offshore wind farm planning 
Blas, 2018/01-2018/12, North Sea: 3-8.5° E/53.5-56° N Blas, 2018/01-2018/12, North Sea: 3-8.5° E/53.5-56° N 
Reference: Scatterometer and Model (e5), CMEMS Reference: HOAPS, CMSAF 
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Figure 6. As Fig. 5 (top row panels), but for year 2018. Results for COSMO-R6G2 are shown in addition. 
Table 4. Maximum and minimum bias of monthly mean near-surface wind speed [m s711 of grid boxes embedded in geographical area 
covering the German Exclusive Economic Zone of the North Sea. Biases are calculated for the various model-based products using two 
satellite based data products as reference. The time period is 2008-2017. 
Product Scatterometer and Model (e5), HOAPS, CMSAF 
CMEMS 
ERAS Amin = —3.1, Amax = 0.9 Amin = —1.5, Amax = 1.9 
COSMO-REA6 Amin = 3.7, Amax = 2.2 Amin = —1.5, Amax = 25 
CERRA-an Amin = —3.1, Amax = 2.0 Amin = —1.2, Amax = 3.0 
HoKliSim-De Amin = 3.7, Amax = 1.9 Apin = —1.4. Amax = 2.3 
slightly lower mean near-surface wind speeds when com- 
pared to the other reference data set thereby favouring more 
positive bias values (Fig. 5, Table 4). The HOAPS dataset 
does not cover the full study area due to the coarser spatial 
resolution and missing values near the coastline. 
The CMEMS dataset combines scatterometer measure- 
ments with ERAS and is therefore not purely based on mea- 
surements. HOAPS is purely satellite-based and therefore 
provides independent data, e.g. for evaluation of COSMO- 
REA6 where no satellite data is assimilated. However, 
HOAPS is subject to sampling uncertainties due to sparser 
data coverage which is clearly reflected by the larger range of 
»ases (Fig. 5, top row panels and Table 4). Maps of the spa- 
tial bias distribution facilitate the interpretation of the Box- 
Whisker plots. The multi-year average bias shows only mod- 
erate spatial variations (Fig. 5, row 2 to 5 panels). Larger 
biases are seen along the coast for the Scatterometer and 
Model (e5), CMEMS dataset. As noted earlier HOAPS does 
include missing data near the shore. From the spatiotemporal 
analysis we conclude that consistent results are found for the 
nedian bias indicating comparably small deviations of the 
near-surface wind speed for ERA5 and COSMO-REA6 from 
che satellite-based reference datasets, a slight shift of the bias 
distribution towards positive values for CERRA and a minor 
shift towards negative values for HoKliSim-De. 
Adv. Sci. Res., 20, 109-128, 202. 
3.2.2 Evaluation of bias for 2018 
Results for 2018 partly confirm findings for 2008-2017. 
For example, CERRA shows a tendency towards positive 
bias values. Moreover, a shift towards negative bias val- 
ues is clearly seen for HoKliSim-De (Fig. 6). Compared to 
the analysis for 2008-2017 the median bias of ERA5 and 
COSMO-REAG is slightly more negative when using Scat- 
terometer and Model (e5) (CMEMS) as reference (Fig. 6, 
left panel). The median bias of both ERA5 and COSMO- 
REAG is close to zero when choosing the HOAPS dataset 
as reference (Fig. 6, right panel). Note that both ERA5 and 
COSMO-REA6 show comparatively low annual mean wind 
speed values at 100 m when compared to other model-based 
products at the position of FINO1 (cf. Sect. 3.1). 
Results for COSMO-R6G2 are similar to COSMO-REA6 
for both reference datasets (Fig. 6). The median bias of 
COSMO-R6G2 is slightly negative for the Scatterometer 
and Model (e5) (CMEMS) reference and close to zero 
when choosing HOAPS as reference. Therefore, results for 
COSMO-R6G2 provide first indication of the suitability of 
the new product. 
https://doi.org/10.5194/asr-20-109-2023