Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 55

phenomenon can be established. That time, analysis comprises a run of 1 month
(30 days) with operating wind turbines. Again, here used assumption for ocean
simulation is a constant wind field (last time step of METRAS run) forcing the
ocean every 10 min by wind speed and direction for each time step. In reality,
meteorological conditions will be never as constant as used here, but that proceed-
ing allows an estimation of a possible OWF effect on the ocean by reaching an
equilibrium ocean change. Also, the approach is used to avoid additional effects due
to wind veering and gusts, for example. Such proceeding allows the best analysis of
the development of the effect as well.
Changes in the horizontal velocity field (VELH) due to the wind refer to the wind
forcing that incurs into the equation of motion as wind stress acting on the sea
surface and pictured in Fig. 5.5a . The area of wind wake downstream of wind farm
is projected on sea surface in the form of flow reduction from first time step (first
time step is given after 10 min of simulation, including a 10-min mean) with a
minimal speed of 0.01 m/s. With time, the wake flanks are even identifiable,
resulting in an increase of horizontal velocity of 0.07 m/s and more, Fig. 5.5 .
The direction of the horizontal velocity field at surface is veered by around 45 ,
compared to the wind direction due to friction and Coriolis force, so a southwesterly
wind direction in 10-m heights leads to a nearly westerly (NWW) ocean flow in
accordance with the Ekman theory. Although wind direction is constant with time,
the direction of VELH varies from west to northwest close to OWF. That is
connected with changes in the magnitude of velocity components, especially of
the v -component. While during the first hours of operating wind turbines the
horizontal velocity field looks like a fingerprint of the wind field, including a
wake area, a surge zone, and flanks, the structure slightly changes with time.
Hence, the u -component indicates the wind field, including an increase of magni-
tude with time. The change at velocity component is smaller, compared to the u -
component. With time, the v -component shows a similar structure to the u -compo-
nent, but intensified changes located around the OWF are stronger. Besides hori-
zontal velocities, another indicator for a change in ocean dynamics is the change in
surface elevation.
The reduction of the horizontal velocity field in the wake area affects the surface
elevation increases easterly of OWF first, Fig. 5.5 . Physically, the

reduced ocean flow ends in a reduced transport of water masses in the wake area,
which again results in a slack flow and so in an increase of surface elevation. Due to
the law of conservation of mass, a counterreaction to that is recognized westerly of
OWF; here,

ζ in a way that ζ

decreases. So OWF leads to a dipole formation of surface elevation.
The positive and negative cores of

ζ

changes move with time counterclockwise till
the final dipole position is reached having an increase of surface elevation north of
the wind farm and a decrease south of the wind farm.

ζ

changes are spread over the
whole model area. A separation of the model area into increase and decrease of

ζ

ζ is

defined by the separation line y ¼ 0.25 x 30 by setting a zero point within the

OWF. The positions of extrema at the beginning of the simulation (Fig. 5.5 d1) are

supported by the atmospheric pressure, presented in Sect. 4.2.2 , and the velocity
wake. The final distribution of the positive and negative

ζ

ζ changes depend on the