Models, Data, and Methodology - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 24

The second stratification setup, TS02 , only consists of two layers and is used to
clearly define diffusion and exchange processes at thermocline. Values for the
upper layer are 12.0 C and a salinity of 34.0 psu; for the lower layer, it is 7.0 C
and salinity of 35.0 psu. Here, the thermocline is even placed in 12 m.
The third stratification, TS03 , is based on CTD measurements around alpha
ventus in May 2013 and is used to make the simulation comparable with measure-
ments. In the case of TS03, the upper layer has a temperature of 8.0 C and 30.5 psu
and the lower one has 7.0 C and salinity of 32.8 psu. The broad thermocline is
located between 5 and 15 m.
Generally, HAMSOM is meteorologically forced with meteorological fields of
10-m winds, surface pressure, 2-m temperature, 2-m humidity, precipitation, and
existence of clouds.
One aim of the analysis of ocean box simulations is the determination of the
OWF s effect on the ocean due to the knowledge that wind turbines change

dominantly the wind field. Therefore, in the analysis, wind forcing and atmosphere
surface pressure were used only to detect the single effect of wind change. Two
different wind forcing was employed, the one simulated by METRAS, including the
wind farm

'

s effect based on a disc rotor approach, and one considering OWFs by
using an approach after Brostr¨m( 2008 ), which is explained in Sect. 4.1 .
Brostr¨m

'

s approach was applied on METRAS 10-m reference wind field without
OWF influence, which was first converted to wind stress.
Full meteorological forcing was used based on METRAS data. But due to
METRAS data availability, 10-m temperature and humidity fields were
implemented instead of 2-m fields. An interpolation was not reasonable.
HAMSOM

'

s ocean box is only meteorologically forced by METRAS data of last
time step where the atmosphere is balanced and the wind wake is stable. The
approach of using only one constant forcing wind field helps to define occurring
processes in the ocean by OWF

'

s wind wake. The forcing acts every 10 min on the
ocean, which is calculated for each minute. Besides the last time step of METRAS,
10-min mean wind values are used for analyzing the effect of OWF operation.
In the end, HAMSOM is not forced by the variable wind but by wind stress.
Therefore, wind values were transformed into wind stress by

'

ρ air

!

!

τ ¼

ρ ref

e 3 ð Þ

CD 1 : 3 : 4

1,026 kg/m 3 , !

1.25 kg/m 3 ,

with CD wind vector.

Wind forcing incurs into the equation of motion as wind stress acting on the sea
surface, but the turbulent shear stress is finally the actuating force for motion of the
ocean. So the stress is a negative momentum flux into the ocean working as
frictional force

¼ 0.0016, ρ air ¼ ρ ref ¼

: ¼

at the sea surface:

F