Geoscience Reference
In-Depth Information
Borely (2010); Butler & Johnson (2003); Davies (1995); Department of Defense (2006); Frye et al.
(2009); Jago & Taylor (2002); Lemmon et al. (2008); Ousbäck (1999); Poupart (2003); RABC &
CanWEA (2007); Sparven Consulting (2001); Summers (2001); Webster (2005a;b)). From these
and other studies it became clear that wind turbines in general cause three types of problems
for Doppler radars: clutter, blockage, and erroneous Doppler measurements.
• Clutter consist per definition of unwanted radar echoes. For a military surveillance radar
clutter can for example consist of precipitation echoes whereas for a weather radar echoes
from, e.g., aircraft are unwanted. Echoes from wind turbines are considered clutter by
most radars.
• Blockage occurs when obstacles such as buildings or terrain obscure the radar line of sight.
Measurements behind such obstacles become incomplete or non-existing. Wind turbines
located near a radar may block a substantial part of the radar 's measurement region.
• Doppler radars not only measure the echo strength of their targets but also their radial
velocities. The motion of the rotor blades of a wind turbine is detected by the radar and
may be interpreted as a moving target.
Clutter originating from the ground or from stationary buildings is often filtered out by
built-in clutter filters that remove echoes with zero or low radial velocities. Echoes from
the tower of a wind turbine have zero velocity and can therefore easily be removed but the
turbine's rotating blades can have very large and variable velocities, escaping the clutter filter.
Echoes from rotating wind turbine blades may therefore, for example, be mistaken for aircraft
by an air traffic control radar.
Weather radar problems related to wind turbines were recognised early by Hafner et al. (2004)
and Agence National des Fréquences (2005). These works have since been followed by many
studies (e.g. Brenner et al. (2008); Donaldson et al. (2008); Haase et al. (2010); Hutchinson
& Miles (2008); Toth et al. (2011); Tristant (2006a;b); Vogt et al. (2011; 2007a; 2009)). The
increased awareness of the problems wind turbines may cause weather radars led both the
World Meteorological Organization (WMO) and the Network of European Meteorological
Services (EUMETNET) to issue general guidelines for the deployment of wind turbines, based
on the distance from the radar (OPERA, 2010; WMO, 2010). These guidelines are summarised
in Table 1 and Table 2.
3. Wind turbine interference
In the sections below we examine the three identified problems wind turbines can cause to
Doppler weather radars: clutter, blockage, and erroneous wind measurements.
3.1 Clutter
For a weather radar, clutter refers to all non-meteorological radar echoes. Typical examples
of clutter include echoes from terrain, buildings, and clear-air targets (e.g. insects, birds,
atmospheric turbulence). Clutter can further be divided into two categories: static and
dynamic. Static clutter typically originates from terrain and buildings whereas dynamic
clutter is caused by moving targets such as clear-air returns. Static clutter has zero or
near-zero radial velocity and can be removed by a built-in clutter filter whereas dynamic
