KLIWAS 
Seite 
- 13- 
3 
Comparison of ENSEMBLES Regional dim a te 
Mo d e Is re suits to ERA-40 re a na lysis d a ta 
Comparing 
meteorological 
fields of the 
Ensembles 
regional climate 
models with ERA- 
40-data over the 
North Sea 
3.1 Overview 
Seven meteorological parameters, such as sea level pressure, wind speed and direc 
tion, air temperature, global radiation, cloud cover and precipitation of 12 regional 
models are compared to ERA-40 data. We investigate only meteorological elements 
with the scheme described in chapter 2.3. All mini pictures show absolute values and 
anomalies over the 30 years time period 1971-2000. Starting with the most prominent 
parameter “sea level pressure”, we point out the similarities and differences between 
each of the different RCMs and between the RCMs and ERA-40. 
3.2 Sea Leve 1 Pressure 
Air pressure is a basic atmospheric parameter. As the air pressure decreases with in 
creasing altitude, reduced data to sea level height - the sea level pressure (SLP) - is 
commonly used in meteorology. Sea level pressure varies widely on Earth, and these 
changes are of utmost importance in studying weather and climate. Changes in sea 
level pressure can cause subsequent changes in other parameters, e.g. atmospheric 
circulation and sea level. Thus, influences on the pressure fields by the regionaliza 
tion models compared to ERA-40 values are of utmost interest. 
Fig. 3.2.1 shows the annual mean values of sea level pressure of the ERA-40 reanaly 
sis and the ENSEMBLES RCMs for the period 1971-2000. A clear zonal structure 
can be seen in all model results with higher values in the SSE and lower values in the 
NNW. The deviations between the pressure fields of the regional climate models and 
the mother model (ERA-40) show a variety of different patterns. They can be classi 
fied into two characteristic types (see Fig. 3.2.2): Those with a nearly constant offset 
(positive and negative) and others with dominating differences in the pressure gradi 
ent field. 
In winter (see Fig. 3.2.3, January), the zonal gradient is most pronounced and the cur 
vature of the isobars prevailingly cyclonic. In summer (see Figs. 3.2.5 and 3.2.6, July) 
the pressure is higher than in winter and more evenly distributed. The isobars show 
anticyclonic curvatures. As in winter there are still considerable differences between 
the model fields. 
Fig.s 3.2.7, 3.2.9 and 3.2.11 display the fields of the standard deviations of the sea 
level pressure: An increased variability over the northern parts of the North Sea is