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2010a;b; Gallardo-Hernando & Pérez-Martínez, 2009; Nai et al., 2011). These methods all
use raw data as input, i.e., in- and quadrature phase (I/Q) data.
To speed up filtering, only radar cells containing wind turbines should ideally be
processed. This may be achieved by keeping maps of all wind turbines near a weather
radar or by using automatic detection schemes (Cheong et al., 2011; Gallardo-Hernando
et al., 2010; Hood et al., 2009; 2010).
• Gap-filling radars. Areas contaminated by clutter may be covered by a second, nearby
radar, a so-called gap-filler (Aarholt & Jackson, 2010; Department of Defense, 2006; Ohs
et al., 2010). This alternative may be a convenient solution for specific cases but could also
lead to even bigger problems since an introduction of additional radars introduces new
sites which also must be protected.
• Adaptation of the radar scan strategy. Changing the radar scan strategy to pass over areas
with wind turbines will limit the amount of clutter received. The drawback is that data
will be gathered from higher altitudes which may shorten the effective range of the radar.
3.2 Blockage
For a weather radar, blockage manifests itself as a reduction of the expected precipitation
echoes downrange from an obstacle. But, as we have seen in Section 3.1, obstacles in line of
sight of a radar do not only cause blockage, they also cause clutter. Stationary obstacles cause
static clutter which can be removed by a clutter filter. However, dynamic clutter, such as
echoes from rotating blades of wind turbines, is not removed by the clutter filter. Downrange
from such obstacles both clutter and blockage can appear. For wind turbines in line of sight
of a weather radar the increased echo strength from the clutter can often be as large, or larger,
than the reduction in echo strength due to blockage. Separating the effects of blockage and
clutter is therefore often impossible using data analysis. However, large wind farms may
cause substantial blockage and the effect may be visible for tens of kilometres downrange of
the farm.
3.2.1 Observations
Blockage caused by wind turbines is not always visible in radar reflectivity images. As
explained previously this is partly due to clutter tails but also because precipitation echoes
are not always spatially homogeneous.
One example of blockage caused by a wind farm near Dodge City, Kansas, is shown in Fig. 7.
In the figure a weak shadow can be seen behind a wind farm to the southwest of the weather
radar. Other examples of blockage caused by wind turbines can be found in Vogt et al. (2007a)
and Seltmann & Lang (2009).
As mentioned above, making a quantitative analysis of blockage behind wind farms is difficult
due to clutter tails and the spatial variation of precipitation echoes. Let us therefore instead
examine blockage caused by a stationary structure in line of sight of a weather radar.
The air traffic control tower of Arlanda Airport near Stockholm, Sweden, is located only
0.9 km from the Arlanda weather radar. The full width at half maximum of the radar beam is
0.9
◦
, which at the distance of the tower corresponds to approximately 14 m. The radar beam is
