Sammendrag
COUPLED ATMOSPHERE - OCEAN WAVE MODELLING FOR WIND POWER APPLICATIONS
---Introduction---
Marine wind power production facilities are subjected to the effects of ocean waves, directly as well as indirectly. Waves act as roughness
elements, acting on the wind speed and turbulence profile. Waves also affect offshore structures directly, and via their influence on ocean
currents and sediment transport. The effect of waves on the atmospheric boundary layer may be important for wind resource assessment, and
we should be able to estimate this effect using a coupled atmosphere - ocean wave model system.
---Approach---
Our approach involves coupling together two numerical models: a mesoscale atmosphere model - the Weather Research and Forecasting
Model (WRF), and a spectral wave model (WAM) which employs radiative transfer equations on wave spectral components of different wave
frequencies and directions. The two models are coupled together using the Model Coupling Environmental Library (MCEL), which allows for
exchange of dynamical variables and their interpolation from and to differing spatial model grids and time-stepping schemes.
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The atmosphere model WRF is run on a polar stereographic domain of 9 km grid spacing, using a Mellor-Yamada-Janjic turbulent planetary
boundary layer scheme, and the wave model is run on a 0.08 degree latitude x 0.16 degree longitude geographical grid. Each model domain
has approximately 500 x 500 grid cells. The models are run simultaneously, and exchange of dynamical variables via MCEL is performed
every 60 model minutes.
The wave model WAM is driven by the computed wind speed at 10 metres height, and computes the wave-induced component of the air-sea
momentum flux, as a fraction of the total air-sea momentum flux, which is then fed back to the atmosphere model. The surface boundary-layer
scheme in WRF has been modified to compute the consequent modification of the surface roughness length (or the change in the Charnock
parameter relating the roughness length to the friction velocity).
Comparision of the results of a 4-week model run with radiosonde wind observations in the lowest 1000 metres of the amosphere, and with
QuikSCAT satellite radar observations of sea surface winds, indicate that the coupled model system may reduce the model bias and
root-mean square error, particularly over the North Sea, when compared with model results from a mesoscale atmospheric model simulation
alone.
---Conclusion---
The results of the coupled atmosphere-wave model simulations are promising, and may improve wind speed estimates in the boundary layer,
at least in the North Sea. It is possible to use different model domains and grid spacings for the two models.
Ongoing work involves refining the model system by nesting down to 1-2 km grid resolution, and extending the model run period to at least 12
months to encompass seasonal variations.
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