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Circulation and Wave Models

COSYNA runs a nested-grid modelling system in pre-operational mode for both ocean waves (WAM model) and ocean circulation (GETM model): The model grid resolution is 1 km for the German Bight and 3 NM for the North Sea. With this setup it is possible to compute three-dimensional current, temperature and salinity fields, as well as two-dimensional water elevation and wave parameter fields such as wave heights and mean periods. Model runs are calculated with a forecast horizon of 72 hours and made available in hourly time steps.

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Water elevation [m] computed with the GETM model

Data Assimilation

Ocean state estimates provided by numerical models contain errors that increase with prediction time. Errors are due to imperfect model dynamics as well as to uncertainties in the initial state and the forcing fields. Data assimilation is a technique to reduce estimation errors by dynamically consistent combination of numerical models and observations. In coastal areas data assimilation requires dedicated techniques to deal with the frequently short memory of the system. COSYNA applies different statistical and variational assimilation techniques to correct the model now- and forecasts. For assimilating HF-radar surface current measurements a spatio-temporal optimal interpolation (STOI) method was developed which allows for provision of short term surface current forecasts with hourly updates.

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Data Assimilation

Unstructured Grid Models

Global ocean circulation models do not properly resolve shelf seas dynamics due to limitations in grid resolution and physical parameterizations. Similar to global models, regional models often poorly resolve estuarine dynamics and are rather controlled by open boundary conditions. Nested systems are not able to address the question of tracer transport from the river mouth to the open ocean or are hampered by numerical limitations. To investigate ecosystem processes in estuaries at the required high spatial resolution while avoiding the problems associated with nesting solutions COSYNA started using unstructured-mesh physical ocean models (FESOM coastal version and SCHISM). coupled to biogeochemical models.

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Sample salinity distribution and mesh resolution in SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model)

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