Characterization and mitigation of wind turbine clutter on the WSR-88D network

Wind power technology has been well-established for many years and the benefits are widely known in terms of the importance of renewable energy sources. However, the recent increase in the number of wind farms within the United States as well as an increase in the number of turbines per farm has prompted studies of their impact on weather radar operations near these facilities. In addition to the rapid growth of the number of wind farms, increases in wind turbine size have also had an impact on the complexity of the problem. Individual wind turbines can exceed overall heights of 130 m with blade lengths of 40 m, which can rotate at 17 RPM creating tip velocities near 70 ms^-1. Due to the motion of blades, current ground clutter mitigation techniques, such as notch filtering and the currently employed GMAP algorithm, fail to significantly mitigate the effect of the wind farms on WSR-88D radar products. In this work, we will examine the Doppler spectral content of the wind turbine clutter (WTC) signal in detail for two WSR-88D locations known to experience WTC. The Dodge City, Kansas, WSR-88D radar is located 35 km from a large wind farm comprised of 170 large wind turbines. The Great Falls, Montana, radar is only 6 km from 6 wind turbines, providing an example of inter-turbine scatter and multi-trip echoes. For both radar sites, extensive Level-I time-series data were collected during two field campaigns organized by the National Weather Service's Radar Operations Center (ROC). These data were analyzed using conventional periodogram-based spectral estimation techniques revealing the unique spectral content of the WTC. It will be shown that depending upon the wind turbine orientation, in comparison to the beam of the radar, the Doppler spectrum can exhibit an extremely wide velocity content. Given the expected tip velocity and the typical aliasing velocity of the WSR-88D, the WTC can be spread over all Doppler velocities. Therefore, the WTC signal is not significantly affected by conventional clutter filtering schemes. This work will also propose signal processing techniques to mitigate WTC, which make use of the spatial continuity (moments and spectra) of the weather signal and knowledge of the fixed locations of the wind turbines.