The coupling of models is a commonly used approach when addressing the complex interactions between different components of earth system. Here we present the development of a new, high -resolution, coupled atmosphere, ocean and wave model system for the North Sea and the Baltic Sea, which is part of the Geestacht COAstal model SysTem GCOAST. We focus on the nonlinear feedback between strong tidal currents and wind -waves, which can no longer be ignored, in particular in the coastal zone where its role seems to be dominant. The proposed coupling parameterizations account for the feedback between of the upper ocean on the atmospheric circulation by accounting for the effects of sea surface temperature and the sea surface roughness. The individual and collective effects of different coupling components are analysed and the performance of the coupled modelling system is illustrated for the cases of several extreme events. For example, the inclusion of wave coupling leads to decreases strong winds through wave dependent surface roughness or changes sea surface temperature, the mixing and ocean circulation; leading to better agreement with in -situ and satellite measurements. The satellite altimeter observations can be used to support further the regional and coastal oceanography. The model comparisons with data from satellite altimeter and in-situ observations showed that the implementation of wave model component into the coupled systems reduces the errors, especially under severe storm conditions. This justifies these further developments and implementation in the frame the Earth System Modelling (ESM) system.
Accurate coastal ocean simulations remain a challenging topic (e.g. for a coastal flooding) research, not least along the European shelf which is characterized by vast shallow tidal flats and a large coastal population. The increased demand for improved climate simulations and predictions requires further development and refinement of the physical processes represented by the hydrodynamical models to properly account for wave generated currents and the corresponding changes to the water level. The effect of coupling on model simulations becomes more important with increasing the grid resolution, which therefore emphasizes the need for coupling on the regional scales. Spatial and temporal changes in the wave and wave energy propagation are not yet sufficiently addressed in high-resolution regional models. The shallow water terms in the wave equations (depth and current refraction, bottom friction and wave breaking) play a dominant role near coastal areas, especially during storm events, where the wave breaking term prevents unrealistically high waves near the coast. The spray caused by breaking waves modulates the atmosphere boundary layer. Air-sea interaction is also of great importance in regional climate modelling. Understanding the wave-current interaction processes is important for the coupling between the ocean, atmosphere and waves in numerical models. Storm surges are meteorologically driven, typically by wind and atmospheric pressure. Waves combined with higher water levels may break dykes, cause flooding, destroy construction and erode coasts. Coastal flooding can be caused by the combined effects of wind waves, high tides and storm surges in response to fluctuations in local and remote winds and atmospheric pressure. The role of these processes can be assessed using high-resolution coupled models. However, in the frame of forecasting and climate modelling studies, the processes of wave and current interactions are not sufficiently exploited.
The circulation model NEMO (Nucleus for European Modelling of the Ocean), the LIM-3 sea-ice dynamics and thermodynamics package (Madec et al, 2008, Staneva et al., 2017). The wave model WAM (The WAMDI group, 1988; ECMWF, 2014) is a third-generation wave model, which solves the action balance equation without any a priori restriction on the evolution of spectrum (Staneva et al., 2015, 2017). The atmospheric model used in this study is the non-hydrostatic regional climate model COSMO-CLM (CCLM) version 4.8 (Rockel et al., 2008, Wahle et al., 2017,). Exchanged fields between the atmospheric CCLM and the wave model WAM are wind and sea surface roughness length and the models are coupled using the coupler OASIS3-MCT. Ocean waves influence the circulation through number of processes: turbulence due to breaking and non-breaking waves, momentum transfer from breaking waves to currents in deep and shallow water, wave interaction with planetary and local vorticity, Langmuir turbulence. The NEMO ocean model has been modified to take into account the following wave effects as described by Staneva et al. (2017): (1) The Stokes-Coriolis forcing; (2) Sea state dependent momentum flux; and (3) Sea state dependent energy flux. A schematic overview of these processes is shown on Fig. 1.
We found improved skills in the seasonal mean simulations as well as during the storm conditions when using a coupled wave-circulation model. In the periods of storm events, the ocean stress was significantly enhanced by the wind-wave interaction leading to an increase in the estimated storm surge (compared to the ocean-only integration) to values closer to the observed water level. The model comparisons with data from satellite altimeter and in-situ observations showed that the use of the fully coupled system reduces the errors, especially under severe storm conditions. This proves the further implementation of the coupled model systems for both, operational and climate research and development activities.
References
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