Adinrichs et al. 
For BNSChydr and BNSCatm, there are two NetCDF-files 
available with the same dimensions as described in Table 4. They 
consist of a binary field, which determines whether a grid box 
corresponds to water (value = 1) or land (value = 0). The 
distinction between water and land was made based on the 
highly spatially resolved GEBCO bathymetry (Weatherall et al., 
2015) by estimating the fraction of GEBCO water boxes in one 
BNSC box. The threshold value for distinction wass set to 0.9 
for BNSChydr and to 0.6 for BNSCatm for this purpose. This 
means that more than 10% (40%) for BNSChydr (BNSCatm) 
water fraction determine the BNSC box to be water. In addition, 
the land sea fraction for each grid box is also provided. 
Table 5 lists the file names of the BNSC data product and 
gives an overview about the nomenclature of the file names in 
the individual data products. 
Finally, Table 6 gives an overview and short explanation of the 
different variables stored in the .nc-files. 
RESULTS 
As all results for the BNSC can be found under https://icdc. 
cen.uni-hamburg.de/projekte/bnsc- project/, we only show some 
chosen examples of the results. 
BNSCatm 
General Results 
The longtime climatology 1950-2015, from which the months 
‚anuary and July are shown in Figure 4 for the three atmospheric 
parameters, exhibits an acceptable spatial coverage for all 
months, with poor data coverage just in the north of the North 
Sea, and the Gulf of Bothnia, as well as west of Ireland. 
General climatological characteristics are well represented by 
the BNSC. The values in the air temperature and dew point 
temperature climatology range from —2 to19°C and —5 to 20°C, 
respectively. The results for January and July are shown in 
Figures 4A,B for mean air temperature and (D) and (E) for mean 
dew point temperature. As expected, the temperatures are higher 
in summer than in winter and have a tendency to decrease from 
south to north. The mean intra monthly standard deviations 
are smaller in summer than in winter and the values are in the 
range of 1 to 4K for both parameters. Mean values of the intra 
monthly standard deviations over all months are shown for air 
temperature in Figure 4C and dew point in Figure 4F. 
The values of the air pressure climatology range between 
‚000 and 1019 hPa and are higher in summer than in winter 
with a tendency to decrease from southeast to northwest. The 
results for mean air pressure in January and July are shown in 
Figures 4G,H. The standard deviation has higher values in winter 
than in summer and ranges between 6 and 16 hPa in the monthly 
data. It is smallest in the south of Great Britain and largest in the 
north west over the open water of the Atlantic Ocean. A mean 
of the standard deviations of the climatology product over all 
months is shown in Figure 4I. 
The four 30-years climatologies (not shown) exhibit a 
temporal evolution of data distribution, where the largest data 
gaps exist in the 1951-1980 climatology for all parameters, and 
rontiers in Earth Science | www.frontiersin.or 
Baltic and North Seas Climatology 
best spatial coverage has the climatology for the period 1961- 
1990. It can be suggested that the poorer data coverage in the 
early period is due to the setup of a monitoring network after 
the Second World War, whereas the gaps in later periods are due 
to changes in the measurement technologies, like replacements 
of old drifting buoys, less manual observations from ships, 
but an increasing number of automated observations. The dew 
point temperature has fewer observations than the other two 
parameters and therefore larger data gaps. The climatologies for 
all parameters show the expected behavior over the year. 
Sensitivity Study 
The sensitivity study yields regional fluctuation ranges caused 
by the random reduction of the basis of observations by 10%. 
As a result, the fluctuation ranges are listed in Table 3 (first 
three rows for BNSCatm) for the regions defined in section 
Sensitivity analysis. The values shown here are based on monthly 
mean fields and are the temporal mean differences between the 
spatial mean of the reference run (full data collection) and the 
upper and the lower envelope. For all three atmospheric BNSC 
parameters, the central North Sea exhibits the lowest and the 
region of the Gulfs of Finland and Bothnia the highest fluctuation 
ranges (marked with bold font weight in the table). This is 
most probably a direct consequence of the sampling density, 
which is very high in the central North Sea and rather low 
in the two greater gulfs of the Baltic Sea, also due to seasonal 
ice coverage. 
In most of the seven regions of the sensitivity study, the 
fluctuation ranges of all parameters exhibit a temporal evolution 
(not shown). Except for the region of the two gulfs and the 
central Baltic Sea, the fluctuation ranges are significantly higher 
in the period 1950-1960 than from 1960 onward for the 
BNSCatm data product: The fluctuation ranges of the earlier 
period are about two to seven times the ranges of the later 
period. This is probably also caused by the poor data base in the 
earlier periods. 
The climatological mean fields of BNSCatm are also subjected 
to this analysis, but the results are not listed in Table 3. For 
the 30-year climatological monthly mean fields the fluctuation 
ranges are in general of similar magnitude as the values for 
the time series of the monthly mean field. Similar to the 
findings for the time series of the monthly mean fields, a 
difference in the fluctuation ranges can be seen between the 
first 30-year-window (1951-1980) and the other three time 
periods. Lessons to be learned from the sensitivity analysis 
are that the data set of BNSCatm is more stable in the 
period from 1960 onward and in the later 30-year climatologies 
than before 1960 and in the first climatology from 1951 
to 1980. 
Comparison With KNSC 
For the purpose of comparison, monthly averages of the KNSC 
period from 1950 to 2010 are subtracted from those of the BNSC, 
resulting in a difference time series. 
For the mean air pressure, the largest differences between 
BNSC and KNSC seem to occur in areas and at times that have 
fewer observations than the surroundings. The differences in the 
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