The NEXRAD Program is executing a multi-pronged short-term strategy, and developing a long-term strategy, to mitigate the impacts of wind farms on NEXRAD data, products, and operations.

SHORT-TERM STRATEGY

a. Creating awareness of the problem.

We are conducting an outreach program to ensure the wind energy industry and developers are aware of NEXRAD locations and potential wind farm impacts on NEXRADs earlier in the development process so that they site developments at more favorable locations with respect to NEXRAD impacts. The Radar Operations Center (ROC) works with developers and suggests mitigation options to consider. We continue to learn about wind farm impacts on radars, weather forecast office operations, and other users where radars and wind farms are already in close proximity. Based on this information, we will develop training materials for radar operators and weather forecasters on how to identify, mitigate, and partially work around wind turbine impacts during forecast and warning operations. In the absence of predictive modeling software, we are also learning how observed impacts at one site can apply to similar proposals for evaluation.
 

b. Collaborating with other Federal Agencies

The NEXRAD radar is not the only radar affected by wind turbine clutter. The long-range Air Route Surveillance Radars (e.g. ARSR-4), used jointly by the DOD and the Department of Homeland Security (DHS) for tracking aircraft, have also been impacted by large wind farms. The ROC is working with the DOD/DHS Long-Range Radar Joint Program Office to build off of their experience with this issue. During 2008 the ROC took the following actions to collaborate with other federal agencies:

     (1) Briefed on NEXRAD efforts to mitigate wind turbine interactions at the JASON Technical Meeting on Wind Farms and Radar (January 2008). This high-level DOD scientific advisory board was investigating impacts of wind farms on DOD radars. Other federal agencies; AWEA; and private sector participated.
     (2) Participated in the DOD/DHS Long-Range Radar Joint Planning Office Annual Meeting (October 2008).
     (3) Exchanged information with other federal agencies operating radars.
 

c. Supporting Experimental Signal Processing Technique Investigations

The ROC is funding studies of potential signal-processing techniques by the Atmospheric Radar Research Center (ARRC http://arrc.ou.edu) at the University of Oklahoma (e.g., Isom et al 2009). One goal of these sophisticated signal processing methods is to automatically identify the turbine-corrupted data through spectral features, temporal continuity, etc., flag it, and potentially recover the underlying weather information. When this detection algorithm is finalized, the NEXRAD has a flexible, open architecture signal processor that would enable relatively low cost and timely implementation of the new signal processing technique.

In addition to detection, signal processing methods based on real-time, telemetry-based algorithms are being explored by the ARRC. These knowledge-based techniques would exploit wind turbine data of blade rotation rate, orientation, etc., and are a good example of the benefits of collaboration with wind farm operations. An initial phase is currently being conducted in a controlled laboratory environment using scaled models and scattering experiments (Fig 1). Further, electromagnetic simulations are being conducted to determine expected radar cross-section from turbines, which will be used to validate laboratory measurements and to enhance adaptive signal processing algorithms.

Finally, more advanced radar designs are being studied as a long-term solution to wind turbine interference. In particular, OU’s ARRC is investigating the possibility of using adaptive null steering with phased array radars as a means of mitigating clutter from moving targets (Palmer et al 2009). Promising results have already been obtained using sophisticated numerical simulation techniques previously developed by the ARRC (Cheong et al 2008). The potential exists for a possible solution to this difficult problem, but further research and additional funding sources are needed.

Success of these ARRC studies could depend on partnering with other federal agencies and/or the wind energy industry. However, these techniques, if achievable, will take several years to operationally implement. For further details, see the related paper entitled Spatial Filtering of Wind Turbine Clutter Using Adaptive Phased Array Radars (8B.6).

LONG-TERM STRATEGY

Our current (short-term) strategy is labor-intensive and the work-load will increase as more developers seek to place wind farms on land with the best wind resource, which is often near weather radars. A more effective approach will be needed to ensure this critical, national radar asset is not significantly degraded. In cooperation with the wind industry and other federal agencies, the ROC is exploring two possible areas: new national guidelines for the wind industry and new funding, federal and/or industry, to find technical solutions to the interference problem.

A. New National Guidelines:

New national guidelines might include one or more of the following:

 (1) A Memorandum of Understanding (MOU) between the wind energy industry and federal agencies, similar to an existing British MOU, with agreement on:

   a. No-build zones very near radars
   b. Notification of federal agencies with Doppler radar assets
   c. Consultation to mitigate impacts
   d. Halting wind turbine blades during significant weather events

(2) A National “clearing house” for developers to submit wind farm proposals to all federal agencies with radar assets – DHS, DOD, FAA, and NOAA. This clearinghouse would work similar to the FAA’s Obstruction Evaluation Office for determining obstructions in navigable airspace; refer to FAA Regulation Part 77

(3) Federal Statutory Authority

  a. Mandatory developer notification of projects and government evaluation of potential wind farm
      impacts, similar to FAA Regulation Part   77
  b. Define no-build zones very near radars

 b. New Funding:

New funding might be used to help develop radar-based and/or wind-turbine based solutions:

(1) Radar-based mitigation funding to:

  a. Develop modeling software that produces estimated radar impacts
  b. Develop signal processing technology that eliminates wind   turbine clutter – a difficult technical
      challenge for which there may not be a solution
  c. Build additional radars for an alternate view of impacted areas
  d. Build taller radar towers to see over wind turbines
  e. Move existing radars, where more favorable sites are available and wind turbine development is
      unlikely

(2) Wind turbine-based mitigation funding to develop radar-friendly “stealthy” wind turbine blades and towers.