Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 73

a b

Fig. 5.15 Impact of diffusion (a) on velocity component w and impact of advection (b)on
velocity component w after 1 day of OWF operation by taking the differences between src54
(no vertical TS advection), src52 (no vertical TS diffusion), and src56 (no vertical TS advection/
diffusion). The diffusion supports up- and downwelling a little bit, while advection leads to control
vertical motion and reduces magnitudes. The OWF effect on w is stronger in the case of no
advection; in the case of diffusion, the effect is similar, compared to the normal run

which dominantly influences the upper layers. Based on the definition of the
vertical eddy diffusion coefficient, depending on A vc , the vertical diffusion of TS
is avoided in the case of a minimal A vc . But due to a less impact of the diffusion on
the final OWF effect on the ocean, that side effect will not much influence the
manner of A vc impact.
The sensitivity run src51 (no vertical exchange of momentum) leads to an
increased reduction of the velocity component u , while the velocity component
v is strengthened, compared to the normal run. The strong change in the horizontal
velocity is explained on one hand by a reduced Ekman transport due to the wake
and by a neglected transmission of momentum from the surface layer to the layers
below. Therefore, the velocity wake area becomes more intensified, as well as the
wake flank area. A more intense wake leads to a stronger effect in surface elevation,
which triggers a change in the v -component because of a reduction in the Ekman
transport (see explanations to Ekman transport in Sect. 5.2.3 ).
Figure 5.16 pictures the horizontal velocity components, the direction of the
horizontal velocity field at the surface, and the vertical velocity component w along
the cross section from S to N through the OWF for sensitivity run src60 (no vertical
TS diffusion & advection and no vertical exchange of momentum), sensitivity run
src56 (no vertical TS diffusion & advection), and the difference between the two to
capture the single impact of a normal-handled A vc coefficient after 1 day.
The direction of flow differs due to the difference in the velocity components and
hence in the Ekman transport. The horizontal velocity field in a run without vertical
exchanges (src60) causes a more intense vertical velocity component due to a more
dominant gradient in the horizontal velocity field.
Using a minimal A vc and also ignoring the vertical TS diffusion and advection
(src60) ,

the vertical component w ends in the strongest downwelling of all