Insights into wind turbine reflectivity and radar cross-section (RCS) and their variability using X-band weather radar observations

Abstract

Abstract. The increasing need of renewable energy fosters the expansion of wind turbine sites for power production throughout Europe with manifold effects, both on the positive and negative side. The latter concerns, among others, radar observations in the proximity of wind turbine (WT) sites. With the aim of better understanding the effects of large, moving scatterers like wind turbines on radar returns, MeteoSwiss performed two dedicated measurement campaigns with a mobile X-band Doppler polarimetric weather radar (METEOR 50DX) in the northeastern part of Switzerland in March 2019 and March 2020. Based on the usage of an X-band radar system, the performed campaigns are up to now unique. The main goal was to quantify the effects of wind turbines on the observed radar moments, to retrieve the radar cross-section (RCS) of the turbines themselves and to investigate the conditions leading to the occurrence of the largest RCS. Dedicated scan strategies, consisting of PPI (plan position indicator), RHI (range–height indicator) and fixed-pointing modes, were defined and used for observing a wind park consisting of three large wind turbines. During both campaigns, measurements were taken in 24/7 operation. The highest measured maxima of horizontal reflectivity (ZH) and RCS reached 78.5 dBZ and 44.1 dBsm, respectively. A wind turbine orientation (yawing) stratified statistical analysis shows no clear correlation with the received maximum returns. However, the median values and 99th percentiles of ZH show different enhancements for specific relative orientations. Some of them remain still for Doppler-filtered data, supporting the importance of the moving parts of the wind turbine for the radar returns. Further, we show, based on investigating correlations and an OLS (ordinary least square) model analysis, that the fast-changing rotor blade angle (pitch) is a key parameter, which strongly contributes to the variability in the observed returns.

On the peculiar polarimetric signatures backscattered by a still or quasi-still wind turbine acquired by an X-band radar in stare mode at high temporal resolution (64 ms): preliminary investigations

“…A detailed technical overview of the radar system can be found in Neely et al (2018). Some key specifications are listed in Table 2, page 3543 of Lainer et al, 2021. The radar site was near the city of Schaffhausen (approximately coordinates: 284 km North and 692 km East, using the Swiss LV03 conformal reference system), at an altitude of 455 m. The three wind turbines of the small wind park located north of Schaffhausen are installed on a hill surrounded by forests.…”

“…The radar site was near the city of Schaffhausen (approximately coordinates: 284 km North and 692 km East, using the Swiss LV03 conformal reference system), at an altitude of 455 m. The three wind turbines of the small wind park located north of Schaffhausen are installed on a hill surrounded by forests. For the specifications and other properties (including geometry) of the wind turbines, the reader may refer to Table 1 in Lainer et al (2021). For the whole campaign in 2020, we observed only the wind turbine with the best visibility and maximum reflectivity value observed during the 2019 campaign: in the paper by Lainer et al (2021), it is labelled as WT1, here after it will be simply labeled as WT.…”

“…For the specifications and other properties (including geometry) of the wind turbines, the reader may refer to Table 1 in Lainer et al (2021). For the whole campaign in 2020, we observed only the wind turbine with the best visibility and maximum reflectivity value observed during the 2019 campaign: in the paper by Lainer et al (2021), it is labelled as WT1, here after it will be simply labeled as WT. The horizontal distance from its mast and the radar site is 7.76 km.…”

“…From a weather radar viewpoint, of particular interest are those discussing the issue of contamination of weather radar data (Hood et al, 2010;Angulo et al, 2015;Lepetit et al, 2019); for other sectors, the identification of adverse effects of wind turbines on the performance of air surveillance and marine radars is of great concern (Angulo et al, 2014;Cuadra et al, 2019). In general, wind turbine clutter reflectivity depends on various parameters such as wind turbine dimensions, incidence angle of radiation, rotor speed, blade pitch angles, nacelle orientation and radiation frequency (Gallardo-Hernando et al, 2011;Norin, 2015;Lainer et al, 2021). In literature, several papers about the reflectivity (and equivalent backscattering radar cross section) of the wind turbines can be found.…”

“…On the contrary, research dealing with other polarimetric signatures of the WT is rare. In a recent (March 4-21, 2020), unique stare mode campaign held in Schaffhausen (Lainer et al, 2021), the WT is continuously illuminated by a fixed-pointing antenna; as emphasized by Reviewer 1 (Interactive comment on Atmos. Meas.…”