Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 98

horizontal. Although vertical cells become slightly wider, the magnitude of vertical
mixing stays nearly stable.

5.3.4 Analyzing OWF Effect on the Ocean due to Wind
Forcing Based on the Brostro ¨ m Approach

Section 4.2.1 introduces a simple approach to describe wind reduction behind one
cubic wind farm developed by Brostr¨m( 2008 ). That approach can be used for
relative big wind farms. The here used wind farm located over four grid cells, thus
comprising an area of 6 km 6 km, almost complies with a large wind farm,

compared to the used wind farm of 0.15 E (~9.56 km at 55 N) in the LOIZ report
2010 (Lange et al. 2010 ), where the Brostr¨m approach is employed. Although
effect on the ocean using the Brostr¨m approach for wake description is
documented, the approach is not fully evaluated so far. A comparison between
wind forcing considering OWF effect by the METRAS approach and the Brostr¨m
approach enables the identification of contrasts. Simulations considered for that
analysis are the master simulation using METRAS forcing T012ug08
TS01HD60F01 , here abbreviated and denoted as F01, and the run T012ug08
TS01HD60F04 , denoted as F04. Thus ocean conditions are the same, and only
forcing differs in dimension, formation, and intensification of wind wake.
The differences between run OWFr and REFr of ocean variables, surface
elevation

, velocity components u and w , and temperature, due to the two different
wind wake approaches after 1-day simulation, are documented in Fig. 5.31 .Itis
apparent that the Brostr ¨ m forcing (F04) also results in the common OWF effect on
the ocean. In the case of F04, the ocean shows the velocity wake, the dipole
structure of surface elevation

ζ

ζ

, and the up- and downwelling cells connected
with cooling, respectively warming. But the formation and dimension of the
occurred OWF impact on the ocean system differs obviously in comparison to the
METRAS forcing (F01).
In case of velocity u-component , the velocity wake by F04 is 0.092 m/s deeper
than in F01 based on extrema at surface, but the wake is spanned nearly along the
W-E cross section through the OWF (Fig. 5.31 ). A maximum reduction of the u -
component by F04 is placed within the OWF at the southeasterly grid box (OWF
consist of four grid boxes), but the wake trail is totally dislocated, compared to F01,
which has its maximum reduction in the northeasterly grid box of the OWF district.
In the case of F04, wake slightly tends to SW direction, but geostrophic effect is
limited due to the short wake length downstream of the wind farm. Based on
satellite data, mentioned in Chap. 4 , the Brostr¨m approach underestimates the
wake dimension. The stronger wake magnitude after 1 day of simulation is explain-
able by the compact and locally more limited effect on the ocean.