KLIWAS 
Seite 14 
KLIWAS 
Project 1.03: 
Atmospheric and 
Oceanic Reference 
Data and Climate 
Projections for 
Coastal and Open 
Sea Areas 
typical in winter (January), whereas the variability is significantly lower in summer 
(July). 
The ratio between the standard deviation of the RCMs and ERA-40 are mapped in 
Fig. 3.2.8 (annual mean), 3.2.10 (January) and 3.2.12 (July). There are pronounced 
differences between the three distributions, with more extreme values (positive and 
negative) found in July. 
The annual cycles for the four North Sea sub regions demonstrate good agreement of 
the ENSEMBLES RCMs with ERA-40 and, the ensemble mean reflects the ERA-40 
course very well. Only one RCM shows a permanent positive bias (see Fig. 3.2.13 
and 3.2.14). This bias is also evident in the frequency distributions (see Fig. 3.2.15). 
Despite the differences in sea level pressure between the RCMs and ERA-40, spa 
tially as well as in the sign of difference, overall good agreement is apparent. The 
regional climate models thus follow to a great extent their driving model ERA-40. 
Similar results were shown for Continental Europe investigating the influence of dif 
ferent global models as boundary conditions for RCMs (KJELLSTROM et al., 2011). 
3.3 Wind Speed 
Wind results from horizontal differences in air pressure. It is primarily affected by the 
pressure gradient. If the pressure gradient is sharp the wind speed is high. The influ 
ences of other factors like the Coriolis force, the Centrifugal Force and the friction 
have altogether smaller impact on the wind speed. The friction of the sea surface dif 
fers in comparison to land surfaces in two respects: friction is mostly smaller over sea 
and it is variable in time, depending on the changing state of the sea. For marine areas 
it is the most important meteorological parameter as wind generates waves and drives 
the currents. At the coasts, it is the cause of one of the most dangerous natural hazard, 
the storm surge. The World Meteorological Organisation (WMO) recommends that 
wind measurements should commonly be performed at 10 m height above ground to 
avoid inhomogenities in the data sets due to the increase of wind speed with height in 
the lower atmosphere. 10 m-wind speeds are one of the output parameters from the 
RCMs. 
In Fig. 3.3.1, the long-term means of the 10 m wind speed in the North Sea area are 
displayed for the ERA-40 reanalysis and the ENSEMBLES RCMs for the period 
1971-2000. While the ERA-40 wind fields show a significant decrease from north to 
south this is much less pronounced in some of the RCMs results (5 out of the 12 wind 
fields). 
Only two of the RCMs (MPI-REMO and ETHZ-CLM) show similar west/east wind 
speed gradients to ERA-40 with lower wind speeds in the East. All RCMs agree with 
ERA-40 in an increase of wind speeds from the coasts towards the open sea Because