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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  direction, ­
  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  increasing ­
  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  regionalization ­
  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  reanalysis ­
  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  classified ­
  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  gradient ­
  field.
In  winter  (see  Fig.  3.2.3,  January),  the  zonal  gradient  is  most  pronounced  and  the  curvature ­
  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