Analysis 01: OWF Effect on the Atmosphere - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 37

are ignored, like rotor diameter or turbine power. Also, it will be problematic to
adopt this approach to nonquadratic wind farm adjustments.
To account for such limitations and to describe such wakes more precisely, the
wind wake is simulated using the mesoscale atmosphere model METRAS.

4.2.2 Mesoscale 02: METRAS

This section deals with the results of METRAS simulations of TOS-01 (atmosphere
box) used for the theoretical analysis. Meteorological forcing for more realistic
atmospheric situations, as North Sea simulations, is explained in Chap. 6 .
METRAS advantage compared to, previously treated, Brostr¨m method is its
physically based model frame and the employed wind farm parameterization,
specified in Sect. 3.1.2 . The wake is not estimated by an empirical formula but
numerically solved. Therefore, METRAS forcing is deemed to be the better

alternative for simulations of the ocean, later due to an expected more realistic
wake illustration. On one hand, the commonly used actuator disc approach allows a
better definition of the form of the wake, as well as of its strength and dimension; on
the other hand, specific details of wind turbines can be considered. Further, this way
of wind forcing production provides data in the vertical and of all atmospheric
parameters.
At first, the OWF

'

s effect on the atmosphere as simulated in METRAS is
analyzed, followed by an analysis on different wind speeds and wind farms, to
estimate the wind wake based on different conditions and by an analysis of the
OWF operation. In the following, changes between the run with wind turbines
(OWFr) and the reference run without wind turbines (REFr) after 4 h of simulation
are considered.

'

4.2.2.1 Analysis of OWF ' s Effect on the Atmosphere in METRAS

In general, atmospheric changes in METRAS are based on a 12-turbine wind farm
and geostrophic wind with ug 8 m/s (run M_T012ug08 in Table 3.2 ). Results for

meteorological parameter pressure, wind, temperature, and humidity are illustrated
in Fig. 4.2 .
The main difference in the wind field , compared to Brostr¨m, is the form of the
wind wake. The change of the wind field can be separated into three areas: a surge
zone with weak decreased wind speed in front of the wind farm, the wind wake; a
plume of reduced wind speed behind the wind farm; and two flanks of increased
wind speeds flanking the wind wake. Here, the simulated wind wake is more than
120-km long with a width of 30 km and a maximum decrease of 4.42 m/s
conforming to a decline of 71.65 %.
In the vertical, the zone directly affected by the rotor is 40-120 m. But the wind
reduction occurs from the ground up to 250 m. In front of the wind farm exists a

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