Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 57

increase of the velocity wake under geostrophic conditions and the Ekman trans-
port, which is explained later in Sect. 5.2.3 .
Connected with changes in the surface elevation, vertical motion w occurs
following the same formation process around the OWF, Fig. 5.5 . Two main cells
of opposite vertical velocities increase by time showing downward speeds in the
south of the OWF and an upward pointed w -component in the north. The reason for
the up- and downwelling will be later examined, in Sect. 5.2 . After 2 days,
additional areas are affected by vertical motion besides two main cells. Here,
especially, the area southwesterly of the OWF shows belts of vertical motion at
depths of the thermocline.
Changes in temperature and salinity , representative of density, occur slower
than changes in hydrodynamics, Fig. 5.5 . Strongest changes are detected at the
depth of the thermocline, while at the surface, effects are weaker. An increase/
decrease of temperature/salinity at the surface in the area of the wake is related to a
change in surface elevation and wind forcing. The effect on the surface grows by
time first, but the mostly warmer/less salty conditions are only a temporary effect.
At the depth of the thermocline the OWF effect on temperature and salinity is
caused by vertical velocities. Cooling and salinization dominate the model area
downstream of the OWF after 1 day.
Although changes are variable within the first 48 h of simulation, formation of
change is relatively constant after 2 days. Figures 5.6 and 5.7 illustrate changes
during a time period of 30 days along the y -section as representative.
The surface elevation increases quite consistently with time, Fig. 5.6a . The
analysis clarifies that maxima and minima stay stable after 27 days. The magnitude
of negative tilt is stronger with a 33.22-mm change, while the positive change only
counts 18.22 mm. The combination of horizontal and vertical compensation
motions avoids a symmetric dipole.
The trend of horizontal velocity field and components is not as smooth as in the
case of surface elevation. Over the first 2 days, a projection of wind field can be
identified in the change of ocean flow, but within the OWF, the flow shows an
increase from day three onward (see Fig. 5.6b ). The velocity wake, formed in the
beginning, is shifted more to the north and becomes weaker and horizontally
thinner with time. Hereby, an additional second region of flow reduction occurs
20 km south of the OWF from day 10 on. Therefore, the area of a southerly wake
flank spreads more to the south. Maximum changes in the flow are 0.18 m/s increase
and 0.09 m/s decrease at the beginning and 0.08 m/s from day 14 on. The upcoming
second flow wake is a result of an intensified positive v -component south of the
OWF (Fig. 5.6d ), as reaction to changed surface elevation. Within the OWF, the v -
component is more negative in the run OWFr than in REFr, while the u -component
is positive in both cases, with the exception of the OWF and wake area in OWFr.
That leads to a higher horizontal velocity within the OWF area in the case of OWFr,
compared to REFr. However, the u -component and v -component reach their con-
stant level earlier than the surface elevation. After 5 days, they have within the
OWF a reduction of 0.32 m/s for u -component and 0.12 m/s for v -component