678 
N.H. Schade et al.: Regional Evaluation of ERA-40 Reanalysis Data 
Meteorol. Z, 22, 2013 
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60°N 
57°N 
54°N 
51 °N 
48°N 
1 
2°E 
14000 
12000 
10000 
8000 
6000 
4000 
2000 
0 
Box 2, Air temperature observations 
per year, ICOADS / GZS 
III 
1965 1970 1975 19801985 1990 1995 2000 
Year 
Figure 2: North Sea area showing the two investigated grid boxes (left). Both boxes consist of four sub grid boxes, each referring to one 
ERA-40 grid point (grey plus). Also shown: Elistogram of air temperature observations by ICOADS (black) and GZS (grey) in Box 2 
(right). Base period: 1961-2000. 
of yearly mean air temperatures derived from monthly 
means show a good agreement between GZS and 
ICOADS in both boxes, both data sets should be compa 
rable here. The same arguments apply for sea level 
pressure observations which are in even better agreement. 
To address biases in marine observational data, infor 
mation about the metadata are needed. Kent et al. (2007) 
used the WMO Publication No .47 (e.g. WMO, 1994), 
which contain infomiation about the contributing VOS 
ships and the instruments used. Until the late 1980s, 
the thermometer type was mostly unknown, also was 
the method of exposure. Therefore, no correction has 
been applied. Biases related to changing measurement 
heights (about 0.01 K/m) due to the fact that ships 
became larger would lead to a cold bias in global air tem 
peratures of about 0.07 K in 2000, relative to 1970 val 
ues. In the North Sea area, the mean measurement 
height changed from the 1970-1979 to the 1995-2004 
period from 10 m to 20 m, which result in a 0.1 K cold 
bias (or 0.18 K, if corrected at 2 m). A height correction 
would therefore reduce the random measurement error by 
Kent and Berry (2005) to 1.1 (1.0) ± 0.3 K. 
Further, there is a variety of nations contributing to 
GZS, associated with specific biases. Some Russian ships 
are known to deviate about 2-3 K in air temperature 
observations (personal communication). Most of these 
substantial biases should have been sorted out by the 
“vicinity test”, unless there are no neighbouring observa 
tions. For the investigation only those observations were 
used that appear to be inconspicuous according to the 
HQC. Furthermore, the GZS data in both boxes were 
manually evaluated concerning possible biases caused 
by single vessels and fixed positions, but showed no 
noticeable inhomogeneities as to space and time, in spite 
of the fact that some vessels and platforms (prominently 
the Gemían fishing trawler “Walther Herwig III” and the 
Dutch oil rig “Maersk Endeavour”) in Box 2 have sub 
mitted a relatively high number of observations. Since 
Table 1: Total Sampling Error (TSU) for air temperatures (AT) and 
sea level pressure (SLP) for both North Sea boxes for the whole 
period 1961-2000 and the respective summer (August) and whiter 
(December) month. 
Box 1 
Box 2 
TSU AT 
±0.2 K 
±0.36 K 
TSU AT (summer) 
±0.32 K 
±0.25 K 
TSU AT (winter) 
±0.15 K 
±0.56 K 
TSU SLP 
±0.3 liPa 
±0.47 hPa 
TSU SLP (summer) 
±0.18 hPa 
±0.34 hPa 
TSU SLP (whiter) 
±0.51 hPa 
±0.77 hPa 
only time averaged values have been further used, and 
the exclusion of those vessels shows no substantial differ 
ences in the results, this fact can be considered negligible 
for our investigations (which of course does not mean it 
should be assumed to apply for other boxes and areas as 
well without prior investigation). 
Concerning sampling errors we followed the method 
of GULEV et al. (2007) and compared monthly means 
of our GZS-like sampled ERA-40 data with regularly 
sampled ERA-40 data to obtain the total sampling uncer 
tainties (TSU). These would be inherent in both datasets 
compared in this study. Results are shown in Table 1: 
TSU of air temperatures for the whole period 1961— 
2000 are within = 0.2 K for Box 1 and = 0.36 K for 
Box 2 with higher values in the summer (winter) in 
Box 1(2). Furthermore, the TSU winter values for 
Box 2 are slightly increasing towards the end of the per 
iod, whereas they remain constant in summer and 
throughout the whole period in Box 1. Sea level pressure 
data show TSUs of ± 0.3 hPa for Box 1 and = 0.47 hPa 
for Box 2. Here, TSU is close to zero since the late 80s in 
Box 1 but shows an increase up to = 1 hPa in winter in 
Box 2. Since we investigate GZS-like sampled 
ERA-40 data with GZS data, these sampling errors are 
not accountable for any differences between the two data