Overview of wind turbine interference with weather radar

Figure 1: How weather radar works, including visual representation of RLOS (Radar's line of sight)
Figure 1: How weather radar works, including visual representation of radar’s Line of Sight (RLOS).

Image shows a radar antenna on the left transmitting a pulse of electro-magnetic waves.  The waves go out to an image of a cloud with descending precipitation to the right.  The waves are shown to bounce back as returning echoes to the radar antennae.

How do wind turbines affect meteorological radar?

The quality of the data received by the weather radars can be affected by many objects. Stationary objects that have zero velocity, such as buildings, mountains, trees and terrain, may block signals, which could result in the loss of the meteorological information behind the object (e.g., data indicating intensity of thunderstorm). The Doppler capability of the ECCC radars is typically able to identify and filter out this stationary blockage. However, the presence of wind turbines poses new challenges to weather radars due to their rotating blades. The rotating blades of wind turbines can impact meteorological radar in several ways ranging from contamination of the quality of the radar data to loss of meteorological data. More specifically, the presence of wind turbines in the radar’s Line of Sight (RLOS) can create the following types of interference with weather radar data: blockage, reflectivity, multi-path scattering and clutter, which have been well-documented by academia and the international meteorological community, based on studies and experiences in other countries.

Figure 2: Depiction of how wind turbines affect weather radar
Figure 2: Depiction of how wind turbines affect weather radar

Image shows an elevated radar antenna with a blue colored main beam travelling horizontally and a green colored side lobe beam travelling at a 45 degree downward angle.  In the path of these two beams are diagrams of 4 wind turbines of various heights and a large rock obstruction.  The image includes labels indicating three of the taller turbines that are visible by the main beam only, one turbine that is visible by the side lobes only and one turbine that is visible by both the side lobes and the main beam.  The image of the rock obstruction is labeled as an obstruction that may block the side lobes.

Blockage

Radar beam blocking is the most serious effect that wind turbines have on radar. Wind turbines are tall structures composed of a fixed tower and multiple rotating blades. If the wind turbines are located close enough to the radar, the tower could cause “blockage” meaning the radar is unable to “see” weather behind the turbine because it blocks the pulses of energy. Even partial blockage can severely affect the radar’s operational capability (e.g. storm/snow fall detection rate and lower level wind shear), which can in turn cause extreme weather conditions to go undetected.

Figure 3: Radar picture of the effects of wind farms.
Figure 3: Radar picture of the effects of wind farms

Image shows an example of raw radar data on a radar screen.  Precipitation is shown as red and blue pixels in the middle and to the left on the radar screen.  Wind farm locations are shown on the upper right of the image with red pixels which look similar to the precipitation pixels.  Outlines of land masses are also shown as blue lines.

Doppler reflectivity

Wind turbines can also cause “reflectivity” contamination. The rotating blades of wind turbines constantly change their orientation based on the wind direction, which can be detected by the Doppler radar as an object with velocity. Thus, the signals returned to the radar by the wind turbines cannot be filtered out in the same way as the reflections from the stationary objects (i.e. building, mountains). Such false information can be significantly misleading for forecasters under storm conditions. Numerous reflections from multiple wind turbines may appear similar to heavy or severe precipitation.

Similarly, the low elevation radial velocity data received from radar is typically used by forecasters to determine the prevailing wind direction or to track a storm, which can be a key predictor of tornados. Wind turbine rotating blades can impact a weather radar’s ability to detect this low level rotation.

Multi-path scattering

When turbines are clustered, a radar signal can be reflected between multiple turbines before it returns back to the radar. The radar processor determines the location of a target by how long it takes the signal to return. The extra reflected targets from the turbine blades cause the radar processor to create false signals. These multiple reflections appear as the radial spikes of reflectivity on a radar image and can cause data contamination many kilometers beyond the wind farm. The result will be the appearance of more precipitation than is actually present.

Clutter

Clutter is defined as unwanted echoes or reflected energy from wind turbines. The impact of clutter depends on the radar cross section (RCS) of the supporting structure, the nacelles and the cross section area of the rotating blades, which in turn, depends on the orientation of the turbine. The turbine can rotate 360 degrees azimuthally in order to align itself with the prevailing wind direction, changing the RCS, with a maximum possible RCS approximately equal to that of a 747 aircraft. Therefore, the energy reflected back to the radar system appears as clutter on the radar image, contaminating the reflectivity data.

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