Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 85

In the hydrographic fields , the alternating effect cannot be observed at the surface;
see Fig. 5.21 . Here, it is clearly seen that the OWF effect on the ocean s SST acts

delayed, becoming obvious after turning off the OWF. The OWF leads to a stronger
and longer increase of SST than in the operation cases opc01 and opc02 after the first
power on period. Here, the cooling occurs around 5 h earlier due to stronger vertical
velocities supported by stronger start wake and switching on turbines one more time.
Operation case opc02, where the OWF is turned on only once, ends faster in
cooling but is not as strong as opc01, Fig. 5.21 . In the case of opc01, where the
OWF is turned on again after 7 h, an SST warming is longer kept within the OWF
influenced area. Similarities occur for salinity and density; see Fig. 5.21 . The
rotation of vertical cells around the OWF affects temperature not at the surface
but in the ocean depth. Figure 5.22 depicts the evolution of the vertical velocity
component w and the temperature at the thermocline in 12-m depth. With a delay in
the vertical velocity component, a significant temperature reaction occurs, but the
warming/cooling does not rotate around the OWF like the cells of vertical motion.
The warming (north of the OWF) and the cooling (south of OWF) only vary due to
the rotation of the vertical cells. Due to the inertial oscillation, the effect on
temperature can be nearly reduced at the turning point from positive vertical motion
to negative vertical motion (Fig. 5.22 ), but the effect on the temperature field is
strengthened again when the downwelling/upwelling cell takes effect in north/south
of the OWF.
The inertial oscillation avoids a quick reestablishment of starting conditions. It
also shows how sensible ocean dynamics are related to a wind field. Hence, even a
short operation of OWF can induce a mixing, which is connected with temperature
changes of 0.5 C up to 1.0 C within a couple of hours.

'

5.3.2 Analyzing OWF Effect on the Ocean Depending
on Wind Speed

The OWF induced wind wake depends on, besides size and power of the wind farm,
the wind speed. Stronger wind speeds result in different strength of wind wakes
behind the wind farm, which again leads to variation on the effect on the ocean.
To evaluate the wake effect on the ocean related to wind speeds, three wind
speed cases were analyzed. So far, only the wake effect in case of one wind speed
based on ug 8 m/s has been discussed. In the case of METRAS, wind turbines

operate in case of wind speeds at hub height of 2.5 up to 17.0 m/s (personal
correspondence with M. Linde). Thus, the sorts of wind speeds for analysis com-
prises ug

¼

16 m/s (UG16). The wind
forcing shows that in all three cases, the affected area is similar, but stronger wind
speed leads to a more intensified effect, Sect. 5.2.2 .
Simulations are based on TOS-01 with the wind farm of 12 turbines directly
impacting an area of 36 km 2 . Corresponding simulations for result presentation are

¼ 5 m/s (UG5), ug ¼ 8 m/s (UG8), and ug ¼