KLIWAS 
Seite 20 
KLIWAS 
Project 1.03: 
Atmospheric and 
Oceanic Reference 
Data and Climate 
Projections for 
Coastal and Open 
Sea Areas 
ERA-40 data show similar annual cycles in all four North Sea boxes with average 
minimum values of 20 W/m 2 in December/January and maximum values of 220 
W/m 2 in June (Fig. 3.6.13). The RCM can basically be divided into two groups (Fig. 
3.6.14), 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 who over estimate simulate a stronger sea 
sonal cycle and those who under estimate a weaker seasonal cycle (Fig. 3.6.14). Max 
imum differences in summer time range from -60 W/m 2 to +40 W/m 2 . 
Table 2: List of RCMs that underestimate global radiation and, respectively, overestimate global ra 
diation. Asterisks mark models which do not display the behaviour throughout the entire annual cycle. 
RCM-name 
Underestimator 
Overestimator 
CAIRCA3* 
CNRM-RM 
DMI-HIRHAM 
HADRM3Q0 
EHTZ-CLM 
HADRM3Q3 
ICTP-RegCM3 
HADRM3Q16 
METNO-HIRHAM 
KNMI-RACM03 
MPIOM 
SMHIRCA* 
All RCM show a much higher temporal variability (Fig. 3.6.7 and 3.6.8) over land 
than the ERA-40 data because of the lower spatial resolution of the latter. The models 
that underestimate global radiation additionally have the tendency to show less vari 
ability in the northern North Sea. All three Hadley Centre models show pronounced 
variability over the continent; of these the most extreme is HadRM3Q3 which has 
standard deviations nearly twice the size of those in ERA-40 (Fig. 3.6.8). 
In January, according to the small values, the absolute differences between ERA-40 
and the RCM simulations are also small (Figs. 3.6.3 and 3.6.4). However, temporal 
variability in the RCMs (Figs. 3.6.9 and 3.6.10) is much higher compared to ERA-40. 
In the time of strong irradiation (July) differences between ERA-40 and the RCMs 
are considerably larger (Figs. 3.6.5 and 3.6.6). All RCMs from the underestimator 
group are exaggerating meridional gradients in global radiation. Again, none of the 
RCMs captures the localized radiation maximum of ERA-40 in the central North Sea. 
The frequency distributions of global radiation in the four North Sea sub regions (Fig. 
3.6.15), show two maxima, a strong maximum at low radiation and a second weaker 
maximum, most pronounced in the ERA-40 data, associated with summer time radia 
tion at 220-240 W/m 2 . No values in ERA-40 exceed 320 W/m 2 , while all of the 
RCMs do. The frequency distribution at the low end of global radiation differs in the 
northern and southern North Sea boxes in the ERA-40 data, but not in the RCMs.