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  seasonal ­
  cycle  and  those  who  under  estimate  a  weaker  seasonal  cycle  (Fig.  3.6.14).  Maximum ­
  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  radiation. ­
  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  variability ­
  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  radiation ­
  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.