Models, Data, and Methodology - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 17

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¼ equation of motion þ wind turbine mask ð 3 3 Þ

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Due to the wind speed depending on thrust coefficient, wind turbines switch on and
off autonomously, if wind speed is equal to the cut-in or cut-off velocity. Normally,
the cut-in and cut-off wind velocities used in METRAS are 2.5-17 m/s (personal
correspondence with M. Linde). Turbines and actuator disc are movable and are set
orthogonal to the wind. The influence of the tower is ignored so far that no real
obstacle is implemented. But to have an effect due to friction in case of less wind,
pressure in the grid of the rotor will increase in case wind speed is lower than cut-in
velocity. With these assumptions and the implementation of the ADC, several large
wind farms can be represented in the model domain.
Wind turbines are simulated using an active rotor disc, and the changed wind
field is parameterized adding a deficit term to the equation of motion.
In this study, the height of the tower counts 80 m, like the rotor diameter, which
is a normal size of wind turbines in offshore wind parks.
A restriction is the different scales between the wind turbine and the grid box.
Due to ratable consideration of wind change, the effect is a ratable projection on the
whole grid box where the wind turbine is placed within. Although the turbine does
not affect the whole box, a better solution is currently not available. This adaption
leads in turn to an overestimation of the wake size, which has to be considered. As
described in the SAR (synthetic aperture radar) data by the work of Li and Lehner
( 2012 ), wind farms often lead to one uniform wind wake, but under certain
conditions, single wakes behind each wind turbine can be seen. Such single wind
wakes cannot be considered here due to scales.

3.2 Data

Besides using METRAS data as forcing data, additional forcing data were neces-
sary. These data are reanalysis data from ECMWF. An additional data set of ocean
data comes from ship measurements, which are used to evaluate model results.
Further details can be found in following sections.

3.2.1 Climatological and Reanalysis Data

Climatological and reanalysis data were necessary for the simulation of the North
Sea and German Bight. METRAS uses ECMWF (European Centre for Medium-
Range Weather Forecasts) data as meteorological forcing, as well as NOAA
( National Oceanic and Atmospheric Administration ) Optimum Interpolation Sea
Surface temperatures for SST forcing.