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FIGURE 5 | Difference between BNSC and ERA Interim Climatology 1981-2010 for mean sea level pressure (A,B) and air temperature (C,D) for the months January 
{A.C) and July (B.D). 
monthly differences in the interval of —1.5 hPa and 1.5 hPa. 
The comparison with COSMOS-REA6 showed mean monthly 
differences in the interval of —1 hPa to 1 hPa overall mean 
difference. All reanalyses show similar patterns as described 
above, with negative values at the coast and slightly positive 
values at open water. The differences in winter are higher 
compared to differences in summer. 
One possible explanation for the average lower pressures of 
the re-analyses over open water is the “fair weather bias,” which 
means that ships avoid the bad weather conditions associated 
with low pressures, and therefore there are fewer measurements 
in these situations. These low pressure values are therefore 
missing in the mean values of the BNSCatm, but are included 
in the re-analyses. On the coasts, on the other hand, the re- 
analyses seems to have on average too high pressure values, 
which is probably again due to mixed land/sea grid boxes in 
the re-analyses. 
The mean 2m air temperature difference between the 
climatologies of BNSCatm and ERA Interim is shown in Figure 5 
for January (C) and July (D). The differences are between —2.1K 
(May) and 8 K (December), but this occurs only at a single box at 
the Norwegian coast, most differences are below 2 K. The average 
air temperatures of the BNSCatm are higher during winter on 
the coasts and especially over the Baltic Sea than in the ERA: 
[nterim data set. This is a well-known effect (Schade et al., 2013), 
as in ERA-Interim, during winter the cooler temperatures of the 
land points are mixed with the warmer ocean. During summer, 
this effect occurs in the opposite direction, but in this case the 
temperature differences between warm land and cold sea are 
rontiers in Earth Science | www.frontiersin.or 
smaller than in winter, since land is a worse heat storage. Again, 
the differences north of the UK are most likely related to a 
sampling error caused by the lower number of observations in 
this region, and most extreme values occur at the edges of the 
data field as well. 
Comparison With Station Data 
Data from coastal measurements are not included in the 
calculation of the BNSCatm, Therefore, these data can be used 
to investigate the differences between BNSCatm and reanalysis 
data at the coasts. The monthly mean of the measurements at a 
station and in the nearest grid cell in the BNSCatm and ERA- 
40 is compared for the overlapping time periods. Figure 6 shows 
the station names (A) and the mean difference of monthly means 
of BNSCatm and ERA-40 to the measurements (B). In most 
cases, the mean values of both BNSCatm and ERA-40 are lower 
than the values of the measurements. At the North Sea coast, 
the differences between BNSCatm and measurements are smaller 
than those between the reanalysis and the measurements. At the 
Baltic Sea coast, the reanalysis is closer to the observations than 
the BNSC. Nevertheless, these differences of both products lie 
within the range of uncertainty, as they are also due to the fact 
that we compared means over an area to a point measurement 
and that the areas of the corresponding grid cells were completely 
over sea and not over the coast. 
Comparison With Area Means and ICOADS 
The 14 results for the different areas showed in general 
excellent conformity with each other, only small differences 
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