KLIWAS 
Seite 22 
KLIWAS 
Project 1.03: 
Atmospheric and 
Oceanic Reference 
Data and Climate 
Projections for 
Coastal and Open 
Sea Areas 
All RCMs simulate over the North Sea similar spatial patterns with an NW/SE ori 
ented decrease in cloud cover, but the differences to the ERA-40 cloud cover vary 
strongly between the models (Figs. 3.7.1 and 3.7.2). The entire group of Hadley Cen 
tre models shows large-scale underestimations of the mean cloud cover. Even more 
extreme differences to ERA-40 than in the annual means exist in July (Fig. 3.7.6). 
HadRM3Q3 has a completely different seasonal cycle (Figs. 3.7.13 and 3.7.14) and 
thus shows huge differences in January (Fig. 3.7.4) and July (Fig. 3.7.6). 
The long-term standard deviations of the majority of the RCMs (Fig. 3.7.7) are larger 
than that of ERA-40. Here, extreme cases are the Hadley Centre models and DMI- 
HIRHAM which nearly have twice the variability of ERA-40 in the long-term mean 
(Fig. 3.7.8). DMI-HIRHAM shows large variability in cloud cover as well in winter 
as in summer (Figs. 3.7.10 and 3.7.12) and especially the summer variability is far 
outside of those of the other RCMs. 
3.8 Precipitation 
Precipitation at sea plays an important role within the hydrological cycle. However, 
in-situ precipitation sampling is sparse and the satellite observations are supported 
only by a few validation sites on islands and coastal locations, and by a few rain- 
gauge equipped surface moorings. Measurements aboard ships are, like all other 
measurements at sea very problematic and data can have large errors, mainly because 
of wind flow distortion around the ship. The amount of precipitation is usually ex 
pressed as millimetres or inches of liquid water deposited on a horizontal surface at a 
given point over a specified time interval. 
The annual mean precipitation in the ERA-40 reanalysis shows for the period of 
1971-2000 a region of high values at the Scandinavian coast (Fig. 3.8.1), with a max 
imum of nearly 1400 mm/year as a consequence of the orographic effect. By contrast, 
due to the “lee-effect” there is a minimum east of Great Britain with only 300-400 
mm/year. 
In the ENSEMBFES regional models precipitation shows a similar pattern like ERA- 
40, with lower values in the south western parts, and higher values in the north of the 
North Sea, with a maximum near the Norwegian coast. Since the spatial resolution of 
the RCMs is about five times higher than that of ERA-40, it provides an improved 
representation of the land-sea distribution and the mountain ranges. Overall, the 
RCMs simulate much more precipitation than ERA-40 during the entire year, as well 
in winter as in summer. Summing up annually, the totals are more than 400 mm (i.e. 
50 %) higher than those of ERA-40, (Fig. 3.8.2). For most of the RCMs the differ 
ences are smaller in winter (Fig. 3.8.4) than in summer (Fig. 3.8.6).