The BSH operates a comprehensive numerical model system for the computation of ocean, dispersion and water quality forecasts in the North Sea and Baltic Sea. With this model system water level, current, temperature, salinity and sea ice forecasts for the next days are generated and archived several times a day.
On demand based on these model results dispersion calculations for multiple application areas are carried out and a number of products are derived. These represent an important basis for the daily work of the BSH internal services such as the water level forecast and storm surge warning service or the ice service, as well as for some national institutions (e.g. Central Command for Maritime Emergency Germany, German Navy, German Maritime Search and Rescue Association (DGzRS)). In addition, the model results are made available to numerous users in various ways in the form of figures, tables or file downloads.
The model results are stored in an extensive data archive. On request, data can be extracted, images generated and delivered so that both internal and external customers can use the data for further evaluations.
Model system of the BSH
Meteorological drive data from the DWD models
The meteorological forecast data required for the atmospheric forcing of the BSH model are computed several times a day by the operational weather prediction models of the German Weather Service (DWD) and the European Centre for Medium-Range Weather Forecasts (ECMWF) and transferred to the BSH. In addition to wind and air pressure forecasts, the BSH models use air temperature, cloudiness and specific humidity above sea level to calculate heat flows between air and water. All data are delivered to a BSH computer in Hamburg as well as in Rostock in order to achieve maximum reliability.
Model area/model topography of the North Sea and Baltic Sea model. On the left the whole area, on the right the area with finer resolution - both areas are fully dynamically coupled.
The core of the model system is the three-dimensional circulation model, which calculates on several interactively coupled grids. In the German Bight and western Baltic Sea the horizontal grid resolution is about 0.9 km, in the rest of the North Sea and Baltic Sea about 5 km. In the area of the tidal Elbe, a very high resolution model with a horizontal lattice spacing of 90 m is additionally computed in order to better simulate the complex hydrographic processes in this area.
The water column is divided into a number of layers of different thicknesses (35 layers in the North Sea and Baltic Sea, 25 layers in the German Bight and the western Baltic Sea, 7 layers in the tidal Elbe area).
Since also the drying and flooding of the tidal flats is simulated with the model, processes in the manifold structured German coastal waters (tidal flats, sands, tidal channels and island chains) and the water exchange with the open sea can be reproduced realistically. To simulate the temperature, not only the water but also the seabed and the sea ice are considered, which plays a major role especially in the wintry Baltic Sea.
The forecasts computed by the circulation model for the North Sea and the Baltic Sea currently extend 72 hours into the future. As the calculations for the tidal Elbe are due to the high resolution computationally very intensive, only 48 hours forecasts are made.
The here presented curves show water level measurements (solid line) and subsequent forecasts (dashed line). This graph illustrates the predictions of the main circulation model (V4) of the BSH for various locations on the Baltic Sea coast. One recognizes the spatial differences of the water level curve.
The BSH uses either Lagrangian or Eulerian dispersion models for simulations of dispersion processes in the ocean.
The Lagrangian dispersion model
For the simulation of drift and dispersion of substances, the investigated substance is treated like a "particle cloud" in such kind of model (Lagrangian method). The particles drift under the influence of the ocean currents. Floating substances are additionally transported with a certain percentage of the wind speed. When simulating oil dispersion, the physical behaviour of different types of oil on the water surface and in the water column is also taken into account.
The Lagrangian model is used amongst others to support the Central Command for Maritime Emergency in current marine pollution. Next to dispersion and drift calculations for oil and water-soluble chemicals drift forecasts for persons and objects (e.g. boats and overboard cargo) are performed with the model. In addition, the model is often used to determine the area of origin of a substance. It is therefore an important tool for determining the cause of pollution.
Eulerian dispersion model
Eulerian dispersion model simulates the temporal development of concentration distributions. The transport algorithm used in the model allows the representation of strong concentration differences (fronts).
Studies on the dispersion of water-soluble substances and on the water quality of the North Sea and Baltic Sea are therefore mainly carried out with Eulerian dispersion model. The model can, for example, provide an answer to the question of how harmful substances that enter the sea through rivers spread and distribute.
In addition to the circulation models, the BSH computes with a two-dimensional (storm surge) model for the North Sea a forecast of the water level or the wind surge 4 times a day.
The horizontal grid spacing of this model is about 5 km. For forecast periods of up to 7 days, the results form an essential basis for the water level forecasts of the BSH.
Since the ecological status of the North Sea and the Baltic Sea is increasingly in the public eye, the BSH has also been operating an ecosystem model component for several years. This enables the BSH to provide up-to-date information on a daily basis on issues relating to the occurrence of toxic algal blooms in the Baltic Sea or the occurrence of oxygen-depleted marine regions as a result of high nutrient inputs.
The ecosystem model used in the BSH is called ERGOM (www.ergom.net) and calculates the basis of the complex food chain, limiting itself to the relationships between nutrients, oxygen, visibility depth, smallest free-floating algae and small free-floating organisms. The computation of these fundamental processes is already sufficient to describe water quality.
Oxygen saturation [%] at the seafloor of the North Sea before and after storm Sebastian has crossed the German Bight.
Data assimilation in development
In order to increase the quality of all predictions, we are working on creating a connection between the model simulations and the measured data. Methods of data assimilation (here Ensemble-Kalman filter) allow to link the model simulation optimally with measurements without disturbing the dynamics of the model.
Prediction errors with data assimilation, observation data used and prediction errors without data assimilation.