Geoscience Reference
In-Depth Information
0.1 mm h
−
1
Let us use Eq. (3) to calculate what RCS would cause an observed rain rate of
R
=
200
R
1.5
we find that
4cm
2
.
=
=
σ
=
(cf. Fig. 4) at a distance
D
13 km from the radar. Using
z
The RCS of wind turbines has been studied both experimentally and numerically (see, e.g.,
Greving & Malkomes (2006); Kent et al. (2008); Kong et al. (2011); Ohs et al. (2010); Poupart
(2003); Zhang et al. (2011)). These studies have shown that RCSs of wind turbines display
a sensitive dependence on yaw- and tilt angle. However, measurements of the RCS of large
wind turbines typically range between 20 to 30 dBsm (Kent et al., 2008; Poupart, 2003) which
is very far from what we obtained in the calculation. Using the RCS to calculate wind turbine
clutter in this simple way may therefore lead to erroneous results.
It has been argued that the RCS is not applicable to wind turbines (Greving & Biermann, 2008;
Greving et al., 2009; Greving & Malkomes, 2006; 2008). The reason is that the plane wave
condition does not hold for objects on the ground. From the calculation above it is clear that
more sophisticated models are needed in order to make a correct simulation of wind turbine
clutter.
For downrange clutter a simple, empirical model was constructed using an exponential
function to fit the limited amount of data available. The rain rate R behind a wind turbine
was modelled as
R
0
exp
C
2
x
N
=
−
R
(4)
where
R
0
is the rain rate in mm h
−
1
from the radar cell containing the wind turbine,
C
2
0.7
is an empirically determined constant,
x
is the distance behind the wind turbine in kilometres,
and
N
is the number of interfering wind turbines present in the radar cell. Observations and
model results are shown in Fig. 6b.
=
3.1.3 Mitigation concepts
Various concepts for mitigating wind turbine clutter have been suggested in different studies.
Some of these concepts are listed here.
• Placing wind turbines so that they are not in line of sight of a weather radar. Under normal
conditions a radar 's measurements will not be affected by objects that are not in the radar
line of sight. This method is therefore a certain way of limiting wind turbine clutter. It has
also been suggested that wind turbines should be arranged radially from the radar. Such
a formation probably does little to mitigate clutter since the blades of the different wind
turbines do not move synchronously.
• Reducing the wind turbines' RCS. It has been proposed that stealth materials can be
applied to wind turbines as a way of reducing the RCS (Appelton, 2005; Butler & Johnson,
2003). Studies of stealth coating wind turbine blades show that a reduction of more than
10 dB may be possible (Rashid & Brown, 2010), making it an interesting solution. An
alternative way of reducing the rotor blades' RCS is to modify their shape, but this is not
considered a realistic alternative as the shape of a rotor blade is optimized for efficiency.
• Adaptive clutter filters. Various filter techniques for removing or reducing effects of
wind turbine clutter have been suggested. Gallardo et al. (2008) suggested using an
image processing technique and Isom et al. (2009) proposed a multiquadratic interpolation
technique. Other signal processing techniques have also been proposed (Bachmann et al.,
