Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Comparison of Observed and Modelled Sea-Level Heights in order to
Validate and Improve the Oceanographic Model
Kristin Novotny 1 , Gunter Liebsch 1,3 , Reinhard Dietrich 1 , Andreas Lehmann 2
1
Technische Universität Dresden, D-01062 Dresden, Germany
2
Institut für Meereskunde an der Universität Kiel, Düsternbrooker Weg 20, D-24105 Kiel, Germany
3
now at Bundesamt für Kartographie und Geodäsie, D-04105 Leipzig, Germany
contact: novotny@ipg.geo.tu-dresden.de
Abstract
Sea-surface heights are one important quantity in the
understanding and modelling of oceanographic processes.
One advantage of the sea-level height is its easy accessibility
for measurements especially at the coast lines. At the Baltic
Sea, some of the oldest tide gauge records of the world can
be found that represent very valuable information about the
long-term behaviour of the Baltic Sea sea level. Nowadays,
satellite altimetry provides additional precise sea-surface
height information with a high spatial resolution in the open
sea.
In this study, observed sea-level heights were compared with
modelled heights in order to validate an oceanographic
model. The model used here is a high resolution coupled sea
ice-ocean model of the Baltic Sea (Lehmann 1995). It is
forced by realistic atmospheric conditions taken from the
SMHI meteorological data base and river runoff (Bergström
and Carlsson 1994). Among other parameters the model
prognoses 2D surface elevations, which are read out in 6
hourly time steps. At its western boundary a simplified
North Sea is connected, and at the most western boundary
the sea level is adjusted to a constant reference value.
For the comparison of the modelled sea-surface heights with
sea-level observations monthly means at several tide gauge
stations around the Baltic Sea and hourly observations at its
south-western coast were available. The comparison showed
that the general variations of the Baltic Sea sea level are well
reflected by the model. Especially the characteristic increase
of the amplitudes of the low frequent sea-level variations
towards the north (Ekman 1996) is found in the modelled
variations. Instantaneous tilts of the Baltic Sea surface as
seen by satellite altimetry are also of comparable order in
the model (Novotny et al. 2002).
However, the amplitudes of the low frequency variations,
namely the seasonal and long-term variations, are in general
found to be too small in the model. The missing signal part
shows a uniform behaviour within the whole Baltic Sea,
indicating that the Baltic Sea fill level has a too
small variation in the oceanographic model. Moreover,
this missing signal component shows a high correlation
with sea-level observations in the North Sea. Therefore,
the model was improved by a modification of its boundary
condition. Monthly mean heights and daily variations
provided by tide gauge observations in the North Sea were
used to introduce a variation of the sea level at the model's
most western boundary.
The modelled sea-level heights generated by this modified
model show a better agreement with observations in the
Baltic Sea. The seasonal variation of the sea level got
much stronger after the modification, removing a great
part of the missing signal component. Also for sea level
variations with periods of some days to weeks, the
differences of observed and modelled heights became
smaller, see Figure 1.
References
Bergström, S. and Carlsson, B., River runoff to the Baltic
Sea: 1950-1990, Ambio, 29(4-5), pp. 280-287, 1994
Ekman, M. , A common pattern for interannual and
periodical sea-level variations in the Baltic Sea and
adjacent waters, Geophysica, 32(3), pp. 261-272, 1996
Lehmann, A., A three-dimensional baroclinic eddyresolving
model of the Baltic Sea, Tellus, 47A, pp.
1013-1031, 1995
Novotny, K., Liebsch, G., Dietrich, R., and Lehmann, A.,
Sea-level variations in the Baltic Sea: Consistency of
geodetic observations and oceanographic models. In
Adam, J. and Schwarz, K.-P. (editors), Vistas for
Geodesy in the New Millenium, pp. 493-498, Springer
2002
Figure 1. Figure 1: Comparison of tide gauge observations with modelled sea-level heights before and after the
model modification for two months in 1995