Analysis 01: OWF Effect on the Atmosphere - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 34

supersaturated mixture to develop and condense as fog or sea smoke. In contrast,
cold-water advection fog occurs when warm moist air flows across colder water and
the dew point temperature is reached such that fog forms (Hasager et al. 2013 ).
Such impact of OWF can play an important role for local climates regarding
moisture/cloudiness and temperature.
Christiansen identified the wind wake in evaluated radar images, which means
the reduction of wind speed downstream of the wind farm (Christiansen 2006 ).
While in the surrounding of Horns Rev the wind increases behind coastal area with
strong shadowing effect of land, behind the wind farm a long plume of reduced
wind speed was detected—the wind wake of the whole wind farm of more than
15-km lengths.
At this juncture, it becomes obvious that OWF effects on the wind field not only
are a local phenomenon but also impact an area being much bigger than the OWF
itself and have to be considered in models.
4.2 Modeled Effects
Modeling wind wake of wind turbines is a necessary tool for wind farm planning
because a wind farm layout depends on the main wind direction and on the size of
used rotor. To avoid turbulence from one turbine affecting another wind turbine,
wind farms are designated with a minimum distance between individual turbines of
around 7 rotor diameters (Jimenez et al. 2007 ; Meyers and Meneveau 2012 ) in the
main wind direction. Wake modeling with high-resolution models like Large Eddy
Simulations (LES) or Detached Eddy Simulation (DES) is widely in use. LES
studies about wind farm wake are done by Jimenez et al. ( 2007 , 2008 ), Wu and
Port ´ -Agel ( 2010 ), Port ´ -Agel et al. ( 2011 ), and Hasager et al. ( 2013 ), just to name
a handful of researches. Hasager, for example, did simulations with DES (Hasager
et al. 2013 ). Such simulations provide a complex view of turbulences behind one or
more turbines. Those details cannot be dissolved with mesoscale models like METRAS.
However there exists a trend of implementing wind turbines into mesoscale models to
evaluate OWF impact on the climate. The usage of mesoscale models with wind turbine
implementation provides the advantage to analyze their impact on the weather and
climate, but one has to deal with disadvantages regarding horizontal resolution.
The impact on the mesoscale is based on a theoretical approach after Brostr¨m
( 2008 ) and simulated results of the mesoscale model METRAS. Both approaches
are adapted to HAMSOM later.
4.2.1 Mesoscale 01: Brostro¨m
Brostr¨m( 2008 ) used a theoretical approach to analyze the influence of large wind
farms on the upper ocean circulation by changing wind stress. After Brostr¨m, a