KLIWAS 
Seite 6 
KLIWAS 
Project 1.03: 
Atmospheric and 
Oceanic Reference 
Data and Climate 
Projections for 
Coastal and Open 
Sea Areas 
Air temperature: Differences in temperature between all RCM and ERA-40 are rela 
tively small over the North Sea. For the annual means as well as for January and July 
nearly all regional models show a smaller variability than ERA-40. The RCMs are 
consistently warmer than ERA-40 in winter but colder in July. The fact that, along the 
coasts nearly all RCMs are warmer in winter and colder in summer than ERA-40 pos 
sibly depends on a continental influence onto the coastal grid points. Since compari 
sons with in-situ observations show similar results, it can be assumed, that the re 
gional models are closer to the reality, mainly because of their higher spatial resolu 
tion. 
Global radiation: All models show an increase of global radiation from north to 
south for the North Sea area. However, there are big differences in the gradients and 
the absolute range. Temporal variability at each grid point is much larger in the 
RCMs than in ERA-40. The RCM can basically be divided into two groups, those 
which overestimate the annual cycle of global radiation and those that underestimate 
it. All models share the tendency for growing differences to ERA-40 during the 
summer maximum, i.e. those which over estimate simulate a stronger seasonal cycle 
and those which under estimate a weaker seasonal cycle. 
Cloud cover: All RCMs simulate similar spatial patterns in cloud cover over the 
North Sea with an NW/SE oriented decrease but the differences to the ERA-40 cloud 
cover vary strongly between the models. Basically the same separation of the RCMs 
as in the simulation of global radiation exists for cloud cover. Models in which over 
estimation of global radiation was detected underestimate cloud cover; however, there 
are some exemptions. The annual cycles in the RCM vary in time and amplitude 
compared to ERA-40. The long-term standard deviations of the majority of the RCMs 
are larger than that of ERA-40. 
Precipitation: In the RCM precipitation shows a similar pattern as ERA-40. During 
the entire year the RCM simulate much more precipitation than ERA-40. The stan 
dard deviations of the RCM for the annual mean show higher variability than ERA- 
40, while in summer the variability is higher than that of ERA-40 and lower in winter. 
Therefore, the annual cycle is more pronounced in ERA-40 than in the RCM. 
Some general conclusions can be drawn from the analyses: A higher resolving grid is 
favourable close to the coasts. Distant from the coasts on the open sea there is hardly 
a difference to models with a lower spatial resolution.