dinrichs et al.
INTRODUCTION
in comparison to other oceanic regions, the Baltic and the
North Sea is very well-sampled with respect to both marine
and meteorological observations. Different platforms (voluntary
observing ships [VOS], buoys (drifting and stationary), research
vessels, fire ships etc.) contribute to the wealth of in situ
observations that help to assess the conditions of the ocean and
the atmosphere and to analyze changes in those two important
components of the climate system. Numerical modeling studies
in climate science aim at reproducing complex and dynamic
processes in order to predict future climate change. Regional
climate models, however, depend on observational data as a
reference and validation, as was stated in the goals of both the
KLIWAS project (Kofalk et al., 2010) and the project “Network
of Experts” (both founded by the German Ministry of Transport
and Digital Infrastructure). The KLIWAS project (Climate Water
Navigation: Impacts of Climate Change on Waterways and
Navigation) looked for the effects of climate change on navigation
an coastal waterways and on coastal protection infrastructure
by running regional climate models. In this project, a gridded
climatological reference data set over the North Sea was required
to evaluate the quality of hindcast runs of these models. Since
there were no suflicient climatologies for the North Sea region
in the form of a long term data set, such a reference data set
based on in situ observations was created for meteorological
and hydrographic parameters on a compatible grid: the KLIWAS
North Sea Climatology [KNSC (Bersch et al., 2013, 2016)
(hydrographic part) and (Sadikni et al., 2013, 2018) (atmospheric
part)]. In order to serve as a validation reference, the input data
for these data sets had to be homogenized with respect to quality
as well as spatial and temporal dimensions.
Similar data products exist for the North and Baltic Sea: The
first to mention that includes atmosphere is Korevaar (1990),
a climatology of the North Sea for the time period 1961-1980
based on observations from ships and lightvessels, that was best
at that time. The atmospheric North Sea climatology for 1981-
1990 by Michaelsen et al. (1998) could be seen not only as
an update of the previous, but as well as a precursor of the
XNSC, as it, too, was produced in a cooperation between the
DWD and the University of Hamburg. Janssen et al. (1999)
created a hydrographic climatology of monthly mean fields for
both marginal seas based on observational data from 1900 to
1996; (Feistel et al., 2008) provide a time series of monthly
mean fields of various marine and atmospheric parameters up
to 2005 for the Baltic Sea. A similar data product, but solely
focussed on hydrographic parameters, was produced by Nüfiez-
Ziboni and Akimova (2015). But in contrast to the BNSC data
product presented in this paper, these products do not cover the
entire North and Baltic Seas for meteorological and hydrographic
parameters on a compatible grid.
Comparison between the atmospheric KNSC data product
and the output of regional climate models was performed by
3Zülow et al. (2014). The hydrographic part of KNSC was used
in an intercomparison study between various ocean models
in Pätsch et al. (2017). A comprehensive assessment of the
atmospheric KNSC and global reanalysis products was done
rontiers in Earth Science | www.frontiersin.Oru
Baltic and North Seas Climatology
by Schade et al. (2018). Here, the global reanalyses NCEP-1
(Kalnay et al., 1996), ERA-40 (Uppala et al., 2005), ERA-Interim
(Dee et al., 2011), MERRA (Rienecker et al., 2011), and 20CRv2
(Compo et al., 2011) were used.
The idea of the data product presented here is to pursue
the concept of a combined meteorological-hydrographical data
product realized in the KLIWAS project by creating the KNSC
data product, to update the climatologies for the ocean and the
atmosphere with recent observational data and to extend the data
product to the Baltic Sea.
The data and methods applied in this work are described
in section Data and Methods. The sources of data of the
atmospheric part of the BNSC (hereafter referred to as BNSCatm)
and changes in the creation procedure of the BNSCatm data
product with respect to the creation of KNSC are briefly
described in section Atmospheric BNSC.
The data and methods applied for the calculation of
the hydrographic BNSC data product (hereafter referred
to as BNSChydr) are elaborately described in section
Hydrographic BNSC.
In contrast to the KNSC data product, a sensitivity analysis
is applied to the BNSC data products which assesses the
zensitivity of the data product with respect to the chosen base of
observational data. The description of this study can be found in
section Hydrographic BNSC.
Additionally, both BNSC data products are compared to
several other data products. A description of the data used and
the methods applied is provided in section Comparison With
Other Data Products.
The description of the data products themselves (BNSCatm
and BNSChydr), the outcome of the sensitivity study and the
comparison with the other data products is presented in section
Results. It is followed by a summary and an outlook (section
Summary and Outlook). The data availability and description of
the data files and the provided variables are listed in section Data
Availability and Description.
DATA AND METHODS
Overview
The BNSC data product bases on marine observational data
in the longitudinal range of 15°W to 31°E. The lower limit of
the latitudinal range is set to 47°N; the upper limit is 66°N
for the Baltic and 65°N for the North Sea region. The data
product consists of temporally averaged fields of the following
atmospheric and hydrographic parameters:
eo sea level air pressure (SLP)
e air temperature at 2 m height (AT)
a dew point temperature at 2 m height (DP)
e sea water temperature (T)
® sea water salinity (S)
The latter two are available as 3-D fields.
The box size of the horizontal grids is set to:
e 1° x 1° for BNSCatm.
e 0.25° x 0.25° for BNSChydr
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