dinrichs et al. 
= 
'eC) 
ud 
12 
A 
Fa 
60, 
7 
2 
(A) | 
50 
T 
(°C) 0 20 
p 
| 7 
2 
P 
nr 
Zu 
SS 
(D)) 504 
F 
7 
(hPa) 0 20 
- 
1015 
1010 
1005 
1000 
7 
m 
ba = 
60 
J 
‘'G).- 
SU 
0 20 
E 
(BB) } 
0 20 
Y 
= 
& 
4 
{g) } 
020 
a 
— 
N 
(H) 
0 20 
60 
50 
60 
50 
5 
5( 
Baltic and North Seas Climatolog\ 
f°C) 
x 
=—_ 
/ 
{C) 
3 
', 
020 wc) 
Ü 
+. 
+ 
3 
EL 
{F): 
C 20 (hPa) 
15 
1? 
A 
0 
nS 
J 
(I) 
5 
3 
0 
020 
FIGURE 4 | BNSCatm Climatology for 1950 to 2015: means of the parameters 2 meter air temperature (A,B), 2 m dew point temperature (D,E), and sea leveı 
aressure (G,H) for the months January (A,D,G) and July (B,E,H) and the mean standard deviation of each parameter over the whole period (C,F,I). Boxes with 
insufficient data coverage are white 
extended winter months from December to March are in the 
interval —1.5 to 2 hPa. In the summer months, the deviations 
are slightly smaller in the interval —0.5 to 0.7 hPa. Due to the 
changes in the procedure, that were described in Data processing, 
the differences between BNSC and KNSC are quite large and 
the BNSC standard deviation is larger in most grid boxes. These 
differences are smaller in the summer than in the winter months 
and in the range —1 hPa to 15 hPa. 
The differences in air temperature climatology are very small 
and caused by the changes to the input data. The changes are of 
the order of a tenth of a degree for means as well as for standard 
deviations. Changes in the input data caused changes smaller 
than 1K in the dew point temperature climatology as well, that 
are spatially and temporally statistically distributed. 
Comparison With Reanalyses 
The time series of the monthly mean values of the BNSCatm 
parameters 2m air temperature and air pressure are compared 
with the corresponding parameters from the ERA-Interim 
reanalysis (Dee et al., 2011). For this purpose, in the first step, 
the ERA Interim data was transformed to the BNSC grid. 
Then climatologies for the period 1981-2010 of air pressure 
and air temperature are calculated from ERA-Interim data and 
compared with the corresponding climatology of the BNSCatm. 
rontiers in Earth Science | www.frontiersin.Ofru 
Figure 5 shows the results for the mean air pressure difference 
between BNSCatm and ERA-Interim climatologies for January 
(A) and July (B). The mean monthly differences are in the interval 
-2.2 hPa (November) to +2.7 hPa (February). In winter they 
are positive in most parts of the open sea, but negative close 
to most coastlines except for the Danish coast, the Skagerrak 
and Kattegat. This means that, especially during winter, the 
3RA-Interim reanalysis has higher pressure values near most 
of the coasts than the BNSCatm. A possible reason is that the 
BNSCatm only contains averaged data from observations above 
sea, whereas the reanalysis has grid boxes, where results above sea 
were mixed with land data that had higher pressure values. On 
the other hand, ERA-Interim has slightly lower pressure values 
over open water than BNSCatm and there are particularly large 
deviations in the north, where the number of observation data is 
lower. So it can be assumed that this is caused by sampling errors 
of the BNSCatm, especially as the largest differences tend to occur 
at the edges of the data field. 
A similar comparison on a monthly basis for sea level pressure 
was done for ERA-40 (Uppala et al., 2005) and ERA-Interim for 
the time period from 1979 to 2001, as well as with COSMO- 
REA6 (Bollmeyer et al., 2015) for the time period from 1995 to 
2015 (not shown). The comparison between sea-level pressure in 
BNSCatm, ERA-Interim and ERA-40 climatology showed mean 
Alk 9019 1 VMalııme 7 1 Article 15£