Gunter Liebsch

Variability of sea surface heights in the Baltic Sea: an intercomparison of observations and model simulations

Sea level changes in the Baltic Sea are dominated by internal, short-term variations that are mostly caused by the ephemeral nature of atmospheric conditions over the Baltic area. Tides are small and their influence decreases from western parts of the Baltic Sea to the Baltic Proper. Superimposed to the large short-term sea level changes (up to few decimeters from day to day) are seasonal and interannual variations (centimeters to decimeters). This study focuses on the comparison of sea surface heights obtained from observations and from a high resolution oceanographic model of the Baltic Sea. From this comparison, the accuracy of the modeled sea surface variations is evaluated, which is a necessary precondition for the further use of the oceanographic model in geodetic applications. The model reproduces all observed Baltic sea level variations very reliably with an accuracy of 5 to 9 cm (rms) for short-term variations (up to 2 months) and 8 cm (rms) for long-term variations (>2 months). An additional improvement of the model can be attained by including long-period sea level variations of the North Sea. The model performs well also in the case of extreme sea level events, as is shown for a major storm surge that occurred at the southern coast of the Baltic Sea in November 1995.

Comparison of Multimission Altimetric Sea-Surface Heights with Tide Gauge Observations in the Southern Baltic Sea

One possible technique to validate the observations of altimeter missions is the comparison with sea-surface heights measured by tide gauges. In our investigation, we compared observations of the two tide gauge stations, Sassnitz and Warnemünde, which are located at the southern coast of the Baltic Sea, with sea-surface heights obtained from the altimeter missions Geosat, ERS-1, ERS-2, and TOPEX/Poseidon. For this purpose, the compared sea-surface heights were related to a common reference system and extrapolated to a common location. GPS observations, leveling data, regional geoid information, sea-surface topography, and postglacial rebound were included in the analysis. Considering the uncertainties of all model components, a more reliable estimation of the error budget (source, type, and magnitude of the errors) was performed. The obtained absolute altimeter biases are (-243 - 32) mm for Geosat, (467 - 19) mm for ERS-1, (76 - 19) mm for ERS-2, and (13 - 18) mm for TOPEX.

Regional geoid determination in Tierra del Fuego including GPS levelling

A regional geoid model for the Argentine part of the Isla Grande de Tierra del Fuego, established in previous works on the basis of GPS levelling, suffers a lack of observation data in the remote south-western investigation area. In order to improve the data distribution in this region, the mean lake level of Lago Fagnano has been regarded as a natural indicator for the local geoid. Using a GPS buoy and pressure tide gauges, a method to determine the mean lake surface topography with respect to the ellipsoid has been developed. It is shown that the obtained lake level geometry is essentially controlled by the regional gravity field. The derived information on the mean lake level has been included in the geoid model, which results in a more detailed and plausible representation of the regional geoid.

Anomalous ocean load tide signal observed in lake‐level variations in Tierra del Fuego

[1] We demonstrate the application of a 100 km long lake as a sensor for studying the tidal effects on Tierra del Fuego main island. The lake-level variations observed in Lago Fagnano reflect both the direct response to the tidal potential and the indirect effect of the ocean tidal loading. Modeling both contributions explains the observed tidal signal in the lake to about 70%. Underestimated model load tide amplitudes are found to be probably responsible for the remaining difference. We interpret this discrepancy as a hint for regional elastic lithosphere properties differing substantially from those represented by currently available global models.

Combination of Sea-Level Observations and an Oceanographic Model for Geodetic Applications in the Baltic Sea

Satellite altimetry and tide gauges provide observations of the instantaneous sea surface. As a further source an oceanographic model of the Baltic Sea also provides sea-level heights. The different sea-surface heights are of comparable precision and complement each other well in terms of their spatial and temporal resolution and their information content.

Sea-Level Variations in the Baltic Sea: Consistency of Geodetic Observations and Oceanographic Models

Altimetric sea-surface heights from the ERS-1, ERS-2 and TOPEX/Poseidon missions as well as tide gauge observations in the Baltic Sea area were used for comparisons with the output of a high resolution oceanographic model. The general agreement of the in-situ sealevel measurements and the sea-surface heights obtained from the model was investigated. Special attention was paid to the analysis of the temporal and spatial sea-level variability as well as sea-level extrema (high/low fill level, extreme sea-surface slopes).

Validation of TOPEX/POSEIDON Measurements in the Southern Baltic Sea

In the present investigation altimetric sea-level heights from TOPEX/POSEIDON are compared with measurements of 8 coastal tide gauges in the southern Baltic Sea. The zero points of the tide gauges were determined by GPS measurements and by precise spirit levellings. Therefore, the instantaneous sea level above the ellipsoid and above an equipotential surface of the earth gravity field can be estimated only by means of tide gauge measurements.

Sea Level Variations Measured by TOPEX/POSEIDON and by Tide Gauges: A Validation Study in the Southern Baltic Sea

To compare sea level heights resulting from satellite altimeter observations and from tide gauge measurements it is necessary to know the tides, the sea surface topography and the geoid in the area under investigation. The paper discusses the characteristics of these components for a test area in the southern Baltic Sea. It is shown that the Baltic Sea is a proper area for such investigations. The method used for the comparison of the sea level heights as well as the results for a possible altimeter drift are presented.