Overview of wind turbine interference with weather radar

Protecting Environment and Climate Change Canada’s weather radar monitoring

With the continued rapid growth of the wind energy industry, wind turbine interference with weather radar is an issue in Canada, and one which has been recognized by the international community, including by the World Wind Energy Association, the International Telecommunications Union, the World Meteorological Organization and EUMETNET. It is generally accepted by these organizations that this issue can be addressed through judicious siting of the two systems.

Environment and Climate Change Canada’s (ECCC) objective in addressing this issue is to enable weather radars and wind farms to co-exist in support of renewable energy development without compromising the Department’s ability to deliver on its public safety mandate to provide quality weather forecasts and warnings to Canadians. Respecting the impact zones established in these guidelines for judicious siting of wind turbines and weather radars is the most effective approach to achieve this objective.

ECCC recommends that the following guidelines - which are based on those developed by the World Meteorological Organization – be respected and adhered to by wind farm proponents at the earliest stage of planning (i.e. determining site location of wind farm). These guidelines may also be consulted to help inform provincial regulator decisions when approving wind farm projects.

Why are meteorological weather radars important?

Canadians rely on ECCC’s weather and environmental services 24 hours a day, 365 days a year. ECCC’s Meteorological Service of Canada (MSC) has been the primary agency responsible for the provision of meteorological services in Canada since 1871. ECCC’s authority with respect to meteorology is conferred by the Department of the Environment Act. Pursuant to this authority, ECCC provides weather services to Canadians. ECCC is Canada’s official source for severe weather alerts, issuing on average 15,000 severe weather warnings to Canadians per year so they can take steps to protect themselves and their property from harm.

ECCC’s weather services enable informed decision-making and support a broad range of users and stakeholders, including the private sector, other federal departments and agencies, such as NAV CANADA, National Defence, the Canadian Coast Guard to support specialized weather services to the aviation, defence and marine communities, respectively, as well as emergency responders, weather sensitive economic sectors and the general public. ECCC forecasts daily weather conditions and issues weather warnings for severe weather events, such as severe thunderstorms, tornadoes or snow squalls, which can cause loss of life, injury, and damage to property and businesses. Through these forecasts ECCC plays an integral role in promoting public safety, and therefore it is essential that weather forecasters have the necessary tools, systems and training to produce timely and accurate weather warnings.

Weather radars are one of the key tools used by ECCC’s meteorologists for the detection, short-term prediction and warning of impending severe weather. In addition to the important data supplied by weather radars, meteorologists use a variety of other tools to collect and analyze weather and environmental data including satellite images, surface weather stations, lightning detectors, and sophisticated computer models, as well as a network of weather watchers on the ground to detect and predict severe weather.

ECCC’s weather radar network spans across the country, covering the regions that represent the area where approximately 98 percent of the population lives. The radar network’s primary purpose is the early detection of developing thunderstorms and high impact weather, including snowsqualls and tornados, as well as the tracking of precipitation. Additional details on ECCC’s weather radar network are available, including a map of the current network.

How do meteorological radars work?

Weather radars transmit short pulses of energy out into the atmosphere. The narrow pulsed radio-frequency beam emitted from the weather radar sweeps the sky 360 degrees around the radar site, pointing at different elevation angles typically from zero degrees to about 24 degrees in elevation. The scans at the lowest elevation angles are the most beneficial for detecting the weather at a distance.

The energy emitted from the radar antenna strikes particles of precipitation, such as drops of water, snowflakes, hail and ice pellets, and a portion of that energy is reflected back to the radar. The measured energy provides information on the location and intensity of the target. The intensity of this energy (or “reflectivity”) is related to the number, size and type of the precipitation particles.

ECCC’s weather radars provide conventional radar coverage to detect the precipitation and intensity of the precipitation. The weather radars in Canada also have Doppler capability, which means the radial velocity of the target can be measured. Radial velocity indicates whether the target is moving away from or towards the radar and at what speed. The conventional and Doppler capability of ECCC’s weather radars, supplemented by other tools, enables meteorologists to accurately forecast daily weather and severe weather warnings.

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

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

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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Date modified:: 2018-04-23

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