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of the higher grid resolutions of the RCMs compared to ERA-40, it should be ex 
pected that a lee effect is smoothed out in the ERA-40 fields. Near coastal areas have 
therefore been excluded from the following calculation of differences in wind fields. 
Five of the RCMs produce yearly means of wind speed which differ less than ± 0.5 
m/s from ERA-40 wind speeds in the entire North Sea area (see Fig. 3.3.2). Seven of 
the RCM give results that deviate at most ±1.5 m/s from ERA-40 means. Except 
from the above mentioned coastal grid points the wind fields of six RCMs show 
smaller wind speeds in the whole North Sea area compared to those of ERA-40. 
These RCMs also produce less frequent wind speeds larger than 10 m/s, as frequency 
distributions in Fig. 3.3.15 indicate. Only one model (DMI-HIRLAM) produces high 
er mean wind speeds than ERA-40 over the whole North Sea area. Its frequency dis 
tribution shows wind speeds larger than 10 m/s more often than the distribution of 
ERA-40. For this single model, spatial and monthly means of wind speed are also 
higher than those of ERA-40 during the entire year and for all four North Sea regions 
(see Figs. 3.3.13 and 3.3.14). 
The spatial structures of the January wind fields are identical to the annual mean for 
both ERA-40 and RCM data; compare Fig. 3.3.1 and Fig. 3.3.3. Mean winds are ap 
proximately 2 m/s higher in January than on annual average. The deviations between 
individual RCMs and ERA-40 wind field are in the same range for the yearly means 
and for the monthly means of January, compare Fig. 3.3.2 and Fig. 3.3.4. For the Jan 
uary mean, seven RCMs have smaller wind speeds than ERA-40 over the entire North 
Sea area. There are two RCMs (ETHZ-CLM and DMI-HIRAM) with higher wind 
speeds than ERA-40 in most of the North Sea region. 
Monthly means of wind speed in July calculated from ERA-40 or the 12 RCMs show 
increased wind speeds in the eastern part of the North Sea compared to the central 
areas (see Fig. 3.3.5). For July, only one RCM has smaller mean wind speeds over 
most parts of the North Sea area and again DMI-HIRHAM is the only RCM with 
higher means than ERA-40 in the entire North Sea area (see Fig. 3.3.6). 
As shown in Fig. 3.3.7, the variability of the wind speed both from RCMs and ERA- 
40 for the period 1970 - 2000 decreases from north to south. Standard deviations of 
only one RCM (ETHZ-CLM) are larger in the entire North Sea region than those of 
ERA-40 (see Fig. 3.3.8). As expected for a winter month, standard deviations in Jan 
uary are larger (for both ERA-40 and all RCMs) than on annual average (see Fig. 
3.3.9) whereas they are smaller in summer (see Fig. 3.3.11). In July, the variability of 
the RCM wind fields is for most parts of the North Sea larger than that of ERA-40, 
see Fig. 3.3.12. 
Higher areal and monthly means of wind speeds than ERA-40 during the entire year 
can be found in the results of DMI-HIRHAM in the all 4 areas, ETHZ-CLM in both 
eastern areas, MPI-REMO in the north-eastern and ICTP-REGCM in the north 
eastern and southwestern areas (see Fig. 3.3.13). 
Comparing 
meteorological 
fields of the 
Ensembles 
regional climate 
models with ERA- 
40-data over the 
North Sea