Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 120

The model depicts the OWF effect quite well, but due to the model restriction,
the thermocline disturbance is overestimated in the horizontal and underestimated
in the vertical. On one hand, discrepancies between the modeled and observed
ocean occur due to the simplified meteorological forcing (no temperature, humidity
forcing, etc.) and the negligence of tides in the simulations. On the other hand, the
wind wake itself results in differences in the OWF impact on the ocean. The
discrepancies in the technical wind turbine parameters (thrust coefficient, rotor
diameter, hub height, etc.) and in the operation mode of the wind turbines can
affect the wind wake description. The used forcing of a constant 10-m wind field
prescribed by a geostrophic wind of 8 m/s and constant wind direction can affect the
wind wake. And the horizontal model resolution of 3 km 3 km can affect the

dimension of the wind wake as well. For example, satellite analyses of the wind
wake behind alpha ventus show that the wind reduction can occur as several wind
wakes behind each turbine and not as one big wind wake (Li and Lehner 2012 )
downstream of the wind farm, like the one simulated by METRAS.
These mentioned model restrictions could result in a different wind wake behind
the OWF, compared to the actually predominating wind wake of May 12, 2013,
behind alpha ventus . Considering the horizontal resolution of the wind wake, we
can assume that the OWF effect on the ocean varies in dependence of the wind
wake dimension, as the analysis of the Brostr¨m approach in Sect. 5.3.4 shows.
Hence, the simulation can overestimate the horizontal dimension of the affected
areas in the ocean.
On the basis of a comparison between modeled SSTs and measured SSTs, the
issue of the horizontal resolution can be clarified. In the case of measurements, the
square around the OWF along the west, north, east, and south sections has a
distance to the OWF center of an average of 6 km. In assumption that the model
overestimates the horizontal dimension of the OWF effect on the temperature field,
compared to the CTD measurements, the modeled SSTs along that square around
the OWF, in 6-km distance to the OWF center, cannot fit the measurements.
Considering the horizontal resolution, the modeled SSTs along another square
around the OWF having a greater distance to the OWF center is provided.
Figure 5.46 exemplifies the situation of measured SSTs around the wind farm
alpha ventus along the mentioned 6-m-distance square on May 12, 2013. The line
plots summarize that the highest SST values were measured along the southern and
eastern sections, and the lowest values were detected at the west section. The cube
illustration helps to get a spatial idea of the SST along the square around the wind
farm.
The counterpart, the modeled SSTs around the OWF, is pictured in Fig. 5.47 .
Figure 5.47a shows the modeled sea surface temperature of the whole model area.
In the middle, the OWF district (four grid boxes) is marked with a solid black
square. The modeled SST along the corresponding square around the OWF with a
distance of 6 km to the OWF center illustrates that the modeled SSTs strongly differ
from the measured one.
Using a greater square provides a better agreement; moving the original 6 km-

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distance square more to the north and expanding it to the west and slightly to the

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