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winter and summer: The RCMs are in winter consistently warmer than ERA-40 (Fig. 
3.5.4), but colder in July (Fig. 3.5.6). 
All regional models show for the annual means a smaller variability than ERA-40 
(Fig. 3.5.7 and 3.5.8). This is also the case for 9 models in January and for 10 models 
in July (Figs. 3.5.9 and 3.5.11). 
The annual cycles of all models for the sub regions are presented in Fig. 3.5.13. The 
differences between the mean monthly temperatures of the RCMs and of ERA-40 are 
presented in Fig. 3.5.14. Nearly all RCMs are warmer than ERA-40 in winter and 
colder in summer. There is an influence from the land in the coastal ERA-40 grid 
points, too. Since comparisons with in-situ observations show similar results, it can 
be assumed, that the regional models are closer to the reality, mainly because of the 
increased spatial resolution. 
These findings are supported by the frequency distributions, displayed in Fig. 3.5.15. 
Here, clearly can be seen, that ERA-40 shows for temperatures lower than 5°C a 
higher frequency than the RCM. Between 5°C and 12°C in the RCM more often 
higher temperatures occur than in ERA-40. Above 12°C the ERA-40 distribution rep 
resents the average of the RCM with a warmer tendency towards the warm tail. As a 
consequence of that, the ERA-40 distribution is wider than those of the RCM. 
3.6 Global Radiation 
The downward short wave radiation at the seas surface has an important effect on the 
thermal structure and the dynamics of the ocean and thus, on the climate system of 
the Earth. This is because more than 90 % of the solar radiation irradiance that arrives 
at the sea surface is absorbed by the ocean. The knowledge of the radiation budget 
and its components at the sea surface is important for the evaluation of the sea surface 
temperature and, subsequently the temperature driven ocean currents influencing the 
heat exchange between ocean and atmosphere. On the other hand the radiation budget 
at sea steers the growth of phytoplankton and other organisms in the water column. 
The total of direct solar radiation and diffuse sky radiation received by a unit horizon 
tal surface is called global radiation. 
According to the varying annual sunshine duration and the solar altitude, all models 
show for the North Sea area an increase of global radiation from north to south (Fig. 
3.6.1). However, there are big differences in the gradients and the absolute amounts. 
Nevertheless, the maps of global radiation exhibit the expected pronounced seasonal 
cycle in all RCMs as does the ERA-40 reanalysis data set, which is illustrated with 
the fields for January and July in Figs. 3.6.3 to 3.6.6. Temporal variability at each 
grid point is much larger in the RCMs than in ERA-40 (Figs. 3.6.10 und 3.6.12). Var 
iations of the mean monthly values are mostly due to temporal variations in the 30 
year averaging period and to a lesser extend from spatial variations in the sub regions. 
Comparing 
meteorological 
fields of the 
Ensembles 
regional climate 
models with ERA- 
40-data over the 
North Sea