KLIWAS 
Seite 16 
KLIWAS 
Project 1.03: 
Atmospheric and 
Oceanic Reference 
Data and Climate 
Projections for 
Coastal and Open 
Sea Areas 
The differences between the monthly mean wind speed of ERA-40 and the RCMs 
have no systematic behaviour (see Fig. 3.3.14): There is no seasonal cycle in the dif 
ferences and there are positive as well as negative deviations. 
However, the comparisons with ERA-40 do not provide the conclusion that wind 
fields of one specific RCM give more realistic results than others. One reason is the 
reliability of the ERA-40 wind fields over the North Sea. This is due to the fact that 
validations of wind speed over sea are difficult because of the lack of suitable obser 
vational data. One reason for that is the spatially varying friction, wherefore it is 
problematic to compare the point measurements from the observations with the spa 
tial averages provided by models (e.g. ERA-40 grid 1.125° x 1.125°). Wind fields 
derived from satellite data reflect the spatial pattern of observed wind fields quite 
well, However, they are not available for a period long enough to calculate reliable 
climatological averages. Nevertheless, the 10 year mean of wind speeds over the 
North Sea, calculated from QuikScat data show very similar structures and velocities 
compared to ERA-40 fields (see Fig. 3 in Hasager et al. 2010). Comparisons of wind 
speeds from satellite observations and in situ data for short periods of one year (Win- 
terfeldt et al.; 2010) show that root-mean-square errors of satellite data compared to 
buoy measurements are roughly in the range of 1.5 m/s - 2.36 m/s, depending on the 
satellite product. The differences between ERA-40 and RCM wind fields are in the 
same range, which means that the results are in the same range as the uncertainties the 
of the observed wind fields. 
3.4 Wind Direction 
Since wind is a vector property its direction has also to be considered in terms of im 
pact on the ocean state. Wind direction is the direction from which wind is blowing, 
contrary to ocean currents. It is reported in the normal geographical coordinate sys 
tem, e.g. south wind reported as 180°. As described in chapter 3.3, wind is caused by 
pressure differences. The wind blows roughly parallel to the isobars, surrounding 
high pressure systems clockwise and low pressure systems counter clockwise on the 
Northern hemisphere. However, it is directed away from the high and towards the low 
pressure region with an angle that depends on the roughness of the underlying sur 
face. 
The 30-year mean of the wind direction was evaluated from the means of wind com 
ponents of the model output (Fig. 3.4.1) Due to the lower mean pressure being cen 
tred in the northwestern part of the North Sea area (see Fig. 3.2.1), southwestern 
winds prevail in this region. This can roughly be found in the wind fields of all mod 
els (Fig. 3.4.1). However, some of the ENSEMBLES regional models deviate dis 
tinctly from the mean wind direction of ERA-40, as the prevailing wind direction in 
these models is west instead of southwest.