KLIWAS 
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Frequency distributions of daily mean wind directions were calculated with ERA-40 
and RCM-data for the four areas of the North Sea and the period 1971 - 2000. For all 
grid points in each of the four North Sea sub regions, the data of wind speed and di 
rection were separated into five classes for wind speed and eight for wind direction. 
Wind speed classes have a width of 4 m/s, the last class includes all wind speeds 
higher than 16 m/s. Wind direction classes cover an angle of 45 °. They are centred at 
the values 0°, 45 °, 90 °... 315 °. 
A comparison of wind roses from ERA-40 data (orange) and the 12 RCMs (blue) for 
the four sub regions of the North Sea area is shown in Figs. 3.4.2 - 3.4.5. The lengths 
of the spokes of the wind roses are proportional to the frequencies of the wind direc 
tion and their widths broaden with increasing wind speed class. 
For all sub regions the frequencies of wind directions of RCMs in all wind speed and 
wind direction classes differ from those of ERA-40. Largest deviations of the wind 
direction frequencies of all RCMs from those of ERA-40 are smaller than 7.6 %. 
In the worthwest sub region, for the ERA-40 and eight of the RCM wind fields 
southwest is the most frequent wind direction (Fig. 3.4.2). This is in agreement with 
the mean wind direction in Fig. 3.4.1 and the mean pressure field in Fig. 3.2.1. In the 
wind fields of the other four models, west is the most frequent direction. East is the 
least frequent wind direction in most models. 
In the northeast sub region, ERA-40 winds preferably show southwest direction (Fig. 
3.4.3). In five of the RCM southwest is also the prevailing wind direction, in five 
RCM fields it is west and in two fields south. Northeasterly winds are least likely in 
this area for 8 RCMs and ERA-40. For the resulting four RCMs wind directions, 
northeast and east are the less frequent directions (about equally shared). 
In the southwest and the southeast sub region, the most frequent wind direction in 
eight models and ERA-40 is southwest (Figs. 3.4.4 and 3.4.5), but west for the other 
four models. In the latter sub area, there is a special interest in winds with high speeds 
(storms) from westerly directions because they potentially can cause storm surges at 
the German coast (Muller-Navarra & Giese, 1999; Befort et al., 2012). For north 
westerly winds and high wind speeds (above 16 m/s) the frequencies of 10 RCMs are 
higher than those of ERA-40. The largest frequency among the results of all RCMs 
(0.52 %) is four times the value of ERA-40 (0.14 %). Storm surges occurring on both 
the Lower Saxony and the Schleswig-Holstein coasts can be induced by north 
westerly storms, whereas storms from west and southwest only can lead to surges 
along the coast of Schleswig-Holstein. ERA-40 wind speeds above 16 m/s from west 
have a frequency of 0.35 %. In five of the RCMs these wind conditions are more fre 
quent than in the ERA-40 results with a maximum frequency of 0.77 %. For storms 
from southwest there are six RCMs with higher frequencies of occurrence. In this 
case, the maximum RCM frequency is 0.45 %, compared to the 0.21 % for ERA-40. 
Comparing 
meteorological 
fields of the 
Ensembles 
regional climate 
models with ERA- 
40-data over the 
North Sea