Stephan Dick

Water transport and mixing in the North Frisian Wadden Sea — Results of numerical investigations

A three-dimensional baroclinic circulation model and a Lagrangian dispersion model are presented which were used to compute water transports and mixing times in sub-areas of the North Frisian Wadden Sea. Both models were used to simulate tide-induced water motions forced by the principal lunar semidiurnal tide M2, and two selected periods taking into account current meteorological and thermodynamic conditions. Wind speeds were low during the simulation period in summer 1994, while gale-force winds prevailed during the winter 1994 simulation period. The circulation model was used for determining water transports across selected transects in the Wadden Sea. For sub-areas of the Wadden Sea, flushing times were computed on the basis of water transports and volumes of the sub-areas. For a first rough estimate, turnover times were derived from a fixed percentage of the tidal prism. More precise data for the turnover times were determined by means of the dispersion model. When comparing the mixing times computed by different methods, only partial correspondence was found. Parameterization of the turnover time by means of the tidal prism was found to produce some unrealistic results. The turnover times computed with the dispersion model for the summer period ranged between 1 and 7 tidal periods in most sub-areas of the North Frisian Wadden Sea. During the simulation period in winter, both water transports and turnover times deviated substantially from the data of the other case studies.

Exchange of matter and energy between the Wadden Sea and the coastal waters of the German Bight-Estimations based on numerical simulations and field measurements

Based on numerical simulations and high resolution measurements, budgets have been computed for transports of water and heat as well as dissolved and particulate matter between the coastal waters of the German Bight and the North Frisian Wadden Sea. p]During 3 measuring campaigns in summer, spring and winter, current conditions and water transports differed only slightly. In spite of different meteorological situations, inflow and outflow of North Sea water to and from the Wadden Sea were largely balanced. A net heat export from the North Frisian Wadden Sea was determined both in summer 1994 and spring 1995, i. e. the Wadden Sea represented a heat source for the adjacent North Sea waters.

Combined analysis of field and model data: A case study of the phosphate dynamics in the German Bight in summer 1994

The intention of this paper is to analyse a specific phenomenon observed during the KUSTOS campaigns in order to demonstrate the general capability of the KUSTOS and TRANSWATT approach, i.e. the combination of field and modelling activities in an interdisciplinary framework. The selected phenomenon is the increase in phosphate concentrations off the peninsula of Eiderstedt on the North Frisian coast sampled during four subsequent station grids of the KUSTOS summer campaign in 1994. First of all, a characterisation of the observed summer situation is given. The phosphate increase is described in detail in relation to the dynamics of other nutrients. In a second step, a first-order estimate of the dispersion of phosphate is discussed. The estimate is based on the box model approach and will focus on the effects of the river Elbe and Wadden Sea inputs on phosphate dynamics. Thirdly, a fully three-dimensional model system is presented, which was implemented in order to analyse the phosphate development. The model system is discussed briefly, with emphasis on phosphorus-related processes. The reliability of one of the model components, i.e. the hydrodynamical model, is demonstrated by means of a comparison of model results with observed current data. Thereafter, results of the German Bight seston model are employed to interpret the observed phosphate increase. From this combined analysis, it was possible to conclude that the phosphate increase during the first three surveys was due to internal transformation processes within the phosphorus cycle. On the other hand, the higher phosphate concentrations measured in the last station grid survey were caused by a horizontal transport of phosphate being remobilised in the Wadden Sea.

Gezeitenströmungen um Sylt Numerische Untersuchungen zur halbtägigen Hauptmondtide (M2)

Vorgestellt wird ein hochauflosendes numerisches Modell (Gitterabstand: 1/3 sm) des nordlichen nordfriesischen Wattenmeeres und des davorliegenden Seegebietes. Die durch die M2-Gezeit bedingten Stromungen um Sylt werden zunachst anhand von Zeitreihen des Betrags und der Richtung der Geschwindigkeit beschrieben. Da die Untersuchung der Zeitreihen in Teilen des Modellgebiets gezeitenbedingte Wasserversetzungen erkennen last, werden fur das gesamte Modellgebiet mittlere Transporte und Reststrome bestimmt. An der Westkuste Sylts treten die grosten gezeitenbedingten Reststrome in Gebieten erhohten Abbruchs auf.

Ein Vergleich von GEOSAT-Altimeterdaten mit Ergebnissen numerischer ozeanographischer Modelle

GEOSAT-Altimeterdaten der Meeresoberflachentopographie und der signifikanten Wellenhohen werden mit Ergebnissen eines hydrodynamisch-numerischen Stromungsmodells fur die Nordsee und eines Seegangsmodells fur den Nordeuropaischen Schelf verglichen. Fur eine Verifikation des Stromungsmodells werden Daten von Kustenpegeln in der Deutschen Bucht herangezogen. Beim Vergleich der GEOSAT-Wasserstandsdaten mit Modellergebnissen zeigen sich grose Abweichungen, die in erster Linie auf die ungenaue Kenntnis der radialen Bahnkomponente zuruckzufuhren sind. Nach einer Korrektur dieser Abweichungen betragt dieRMS-Differenz (Gl. 6.4) noch ca. 55 cm. Sie ist grostenteils auf Geoidvariationen zuruckzufuhren, die mit dem von der NOAA (National Oceanic and Atmospheric Administration) benutzten Geoidmodell nicht erfast werden. Es wird ein Verfahren vorgestellt, wie dieser Geoidfehler mit Hilfe eines Stromungsmodells verringert werden kann. Nach einer Korrektur des Geoids betragt dieRMS-Differenz zwischen Satelliten- und Modelldaten im Untersuchungszeitraum weniger als 11 cm. Das Seegangsmodell wird mit Seegangsmessungen an der Forschungsplattform Nordsee verifiziert. Der Vergleich von Rechnung und Messung ergibt dort eineRMS-Differenz von 0,53 m. Eine ahnlicheRMS-Differenz wird beim Vergleich von Satelliten-und Modelldaten berechnet. GrosereRMS-Differenzen sind auf die ungenaue Erfassung meteorologischer Phanomene sowie auf den nichterfasten in das Untersuchungsgebiet einlaufenden Seegang zuruckzufuhren.

Currents at the light-vessel “Deutsche Bucht”: A comparison between ADCP measurements and the BSH forecast model

Between June 15 and August 10,1999, an Acoustic Doppler Current Profiler (ADCP) and a water level recorder (WLR) were deployed in the close vicinity of the unmanned light-vessel “Deutsche Bucht” in the German Bight. Together with locally recorded meteorological and hydrographical data (salinity and temperature), this in-situ data set is compared to the forecasts of the BSH circulation model “BSHcmod”. The meteorological measurements are used to assess the meteorological model input generated by an atmospheric model of the German Weather Service. Generally, we observed a good agreement between in-situ and model data. Differences between measured and predicted currents were observed at the surface and at mid-depth. They were caused mainly by missing ADCP data within the first 3 metres under the surface and by the coarse vertical grid spacing of the model which prevents the representation of sharp vertical gradients.

Verschmutzungen durch alkylierte Phenole in der Deutschen Bucht

Im Juni und Juli 1991 waren in der gesamten Deutschen Bucht erhebliche Verschmutzungen durch alkylierte Phenole (tert. Butylphenole) aufgetreten. Ahnliche Verschmutzungen waren bereits zwei Monate vorher an der danischen Nordwest-Kuste beobachtet worden. Mittels Driftmodellrechnnungen und detaillierter Untersuchung der chemischen Zusammensetzung der Verschmutzungen wurde versucht, die Herkunft des Produkts festzustellen.