Meteorologische Zeitschrift, Vol. 22, No. 6, 675-684 (published online January 2014) 
© by Gebrüder Bomtraeger 2013 
Open Access Article 
Regional Evaluation of ERA-40 Reanalysis Data with Marine 
Atmospheric Observations in the North Sea Area 
Nils H. Schade 1 ’*, Hartmut Heinrich 1 and Gudrun Rosenhagen 2 
1 Bundesamt fur Seeschifffahrt und Hydrographie, Hamburg, Germany 
2 Deutscher Wetterdienst, Hamburg, Germany 
(Manuscript received January 21, 2013; in revised form November 1, 2013; accepted November 15, 2013) 
Abstract 
An important task of the departmental research programme KLIWAS is the evaluation and assessment of 
climate model results by means of a comprehensive reference data set. For validation purposes, and to create 
a North Sea wide maritime atmospheric and oceanographic reference database, in-situ observations of the 
Centre for Global Marine Meteorological Observations (GZS) of the National Meteorological Sen’ice DWD 
have been compared to the ERA-40 reanalysis. ERA-40 is used as forcing for the hindcast runs of the 
ENSEMBLES regional climate models, which is used within the KLIWAS model chain. The GZS hosts a 
regularly updated, quality controlled, world-wide data bank of weather observations from the oceans. It 
includes data from all sorts of observation platforms as Voluntaiy Obsen’ing Ships (VOS), drifting and 
moored buoys, light vessels, and offshore platforms, either from real-time (RT) via the Global 
Telecommunication System (GTS) or from international exchange in delayed-mode (DM). In addition to 
the automated set of programs applied for high quality control, erroneous data are also manually corrected to 
a certain extent, if possible. To assure reliable statistics for the evaluation, the corrected observations are 
gridded to a resolution of 2.25 degree, so each grid box includes four ERA-40 reanalysis grid points. The 
temporal coverage of the grid boxes depends on shipping routes and the positions of automated systems. 
Observed air temperatures, covering a period of 40 years (1961-2000), show noticeable differences to the 
reanalysis data for all land influenced boxes, specifically in the winter months. The same differences can be 
found if ERA-40 data alone are compared between land- and sea facing boxes. They can not be found in GZS 
data. It can be assumed that the differences are not resulting from measurement errors or uncertain fraction 
variabilities, since they are small during the winter months. A comparison of the differences basing on the 
1981-2000 period to those of the 1961-1980 period shows an increase. Further investigations reveal that 
differences are largest in the landward part of the grid boxes, whereas in the parts facing to the open sea, 
observations and ERA-40 are in fair agreement. This leads to the conclusion, that the resolution of the ERA- 
40 reanalysis is not sufficient for detailed analyses of air temperatures near the coasts. However, this problem 
does not occur for the sea level pressure. The described land influence does not interfere with parameters, that 
are more or less insensitive to the land-sea distribution. 
Keywords: KLIWAS, ERA-40, Marine in-situ Observations, North Sea area, Regional Climate Modelling. 
1 Introduction 
KLIWAS is a research program of the German Federal 
Ministry of Transport, Building and Urban Development 
(BMVBS). It aims at the investigation of potential conse 
quences of climate change for navigation on inland and 
coastal waterways and to formulate appropriate strategies 
for adaptation to changed environmental conditions in the 
future. Basic objectives of the project are (1) the evalua 
tion and assessment of climate model results by means of 
a comprehensive reference database and (2) to make this 
data available for subsequent projects. 
Current climate models are not able to sufficiently 
reproduce the spatial variability in the North Sea area. 
Especially, no regional coupled Atmosphere-Ocean 
model (RCAOM) exists to describe the complex non-lin 
ear interactions of air/sea fluxes properly. Nevertheless, 
Corresponding author: Nils H. Schade, Bundesamt für Seeschifffahrt und 
Hydrographie, Bemhard-Nocht-Strasse 78, 20359 Hamburg, Germany, 
e-mail: nils.schade@bsh.de 
Schrum et al. (2003) showed promising results of a cou 
pled hindcast model run over a full seasonal cycle with 
clear improvements compared to the uncoupled run. 
Therefore, within KLIWAS three RCAOMs will be 
implemented. Until the results of these models can be 
investigated, the focus lies on the available uncoupled 
ENSEMBLES data. The ENSEMBLES project (VAN 
den Linden and Mitchell, 2009) ran from 2004 to 
2009, coordinated by the Met Office Hadley> Centre. 
Probabilistic projections of the future climate were pro 
duced for the European region, based on an ensemble 
of 15 regional climate models (RCMs), nested in 5 global 
models (GCMs) and forced by the A1B emissions sce 
nario. The hindcast runs have been perfomied using the 
ERA-40 reanalysis as model input. Results focussing 
on terrestrial regions have been discussed in the literature, 
e.g. by Lorenz and Jacob (2010), Haylock et al. 
(2008), or Klok and KLEINTANK (2009). 
Up to now, no respective investigations for the 
North Sea region exist, so the development of a frame 
work for the evaluation of a comprehensive marine 
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