Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 96

Figure 5.30 clarifies the fact that affected grid cells play a much more important
role for the effect on the ocean than the number of turbines. Extrema of the surface
elevation increase with OWF size and number of turbines. A comparison of the
surface elevation along the S-N cross section results in a good correlation of 0.90
up to 1.00 with an RMSD in order of 10 3 and 10 4 m for runs T012 and T048
(Fig. 5.30a ). Runs T012 and T048 (wind turbines are scattered over four grid cells)
show along y -cross-section from S to N at the surface, as well as in 12-m depth a
high correlation of 0.99 and better for the velocity components u , v , and w , as well
as for hydrographic variables (Fig. 5.30b-f ).
Independent of v -component, T080 and T160 are even well correlated for almost
all ocean variables by 0.99 and better. Discrepancies between T080 and T160 in the
velocity v -component (correlation of 0.77) relate to the difference of affected grids
(16 and 32) and discrepancies in the wind forcing field.
The important factor of the OWF size regarding grid cells is highlighted by the
comparison of runs with OWFs over four grid cells (T012&T048) with runs with
OWFs over more grid cells (T080&T160). Here, correlations, along representative
S-N cross section, are almost bad for velocity components u and v and tend often to
the statement of not being correlated. Horizontal variations due to the OWFs along
the cross-section S-N impact the correlation value in Fig. 5.30a-b . The simulations
of different OWFs result in good correlations for the variables velocity component
w , temperature, and salinity at 12-m depths along cross-section S-N (Fig. 5.30d-f ).
Here, the stronger agreement underlines the connection of formation of up- and
downwelling to surface elevation. The change of the

ζ dipoles more to the north for

ζ decrease is also identified in w -component and

hydrographic conditions. The distortion of the thermocline in dependence to the
OWF area is also documented in Fig. 5.30e based on the horizontal dimension of
the temperature excursion.

increase and to the south for ζ

Upshot
The comparison of the effect on the ocean due to different wind farm sizes results in
the assumption that the wind farm

s size does not impact vertical mixing in a direct
way. Different wind speed forcing cases more strongly impact on the vertical
stratification. But the wind farm size amplifies the effect in the horizontal. The
magnitude of vertical mixing slightly varies due to the different amount of affected
grid cells, and the maximal anomaly between T012 and T160 in the temperature
counts 0.25 C. The results of that analysis that greater OWFs have a comparable
effect on the ocean to smaller wind farms come in force. But based on issues of the
horizontal resolution and wind wake presentation, it must be considered that a finer
grid in the case of T012 may result in a horizontally smaller wake dimension
orthogonal to the wind direction. Hence, the effect on ocean would be weaker.

'

Therefore, the outcome of that analysis cannot be generalized at this point. A
more detailed wake illustration simulated by higher resolution would be necessary
for a final statement.
But results can be generalized for velocity wakes at ocean surface. A slightly
wider wake does not change stratification in the vertical but triggers impact in the