Civil Engineering Reference
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downwind area reaching to 1,100-m heights. Behind the wind farm, above 250 m,
wind speeds are increased by maximal 0.62 m/s.
Differences in the horizontal fields in 10-m heights of
pressure, temperature and
relative humidity
are poor but show similar formations (Fig.
4.2
).
The wind farm district, in the middle of the model area, is in 10-m heights
0.02
C warmer and 0.2 % less moist due to operating wind turbines, and the
pressure in the OWFr is reduced by 1.5 Pa. Outside of the OWF district occurs a
circle with plume having opposite changes. There, the pressure is maximal 11.33 Pa
higher in the run OWFr and temperatures are 0.04
C lower and about 0.4 %
moister. Within the wind wake district, 15 km northeasterly of the OWF area,
temperature increases while moisture and pressure decrease. Near ground, the wind
farm leads to very weak changes in temperature and humidity, while the pressure
field changes at the surface at about
1 Pa. Strongest effects in wind direction of
temperature occur around 230 m with a reduction of 0.61
C, and the positive
extreme of 0.22
C is located around 130 m. The extreme values for changes in
relative humidity are an increase of 7.66 % in 230 m and a decrease of 2.72 % in
130 m. The change of temperature and humidity is linked to up- and downwind in
the vertical.
The warming of lower layers within the OWF and downstream of wind farm is
connected with stability conditions and transport of potential temperature
θ
. In case
of stable conditions, here (
0) for example, the vertical mixing due to OWF
brings warm air down and cold air up, leading to a cooling above hub height,
respectively at rotator disc, and a warming below. In case of unstable conditions
(
∂
θ
/
∂
z
0), induced turbulence would cause a mixing of cold air downward and
warm air upward, producing a cooler surface. The similar process occurs for
humidity. Dryer air is mixed down and moist air up, resulting in a frying below
hub height and a moistening aloft. In this connection, an OWF also triggers surface
fluxes. The ocean surface is colder than the above air, leading to a negative ocean-
atmosphere thermal gradient, which again means a negative sensible heat flux. An
increase of potential temperature due to OWF ends in a more negative sensible heat
flux, and so more sensible heat is transferred from the atmosphere to the ground.
The drying causes a positive ocean-atmosphere moisture gradient, which positively
affects evapotranspiration. That context is also proved in Baidya Roy (
2004
).
The combination of changes in pressure and temperature over the ocean can
favor cloud and fog formation. Here, any cloud and precipitation occur, perhaps due
to stable atmospheric conditions, although having moist conditions with 68 %
relative humidity.
In sum, the most important meteorological forcing parameters are the wind
speed and pressure, which drive the upper ocean. Therefore, following analysis is
concentrated only on changes in the wind field.
∂
θ
/
∂
z
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