Preprints
https://doi.org/10.5194/amt-2022-316
https://doi.org/10.5194/amt-2022-316
 
06 Dec 2022
06 Dec 2022
Status: this preprint is currently under review for the journal AMT.

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

Marco Gabella1, Martin Lainer1, Daniel Wolfensberger1, and Jacopo Grazioli2 Marco Gabella et al.
  • 1Federal Office of Meteorology and Climatology MeteoSwiss, Locarno-Monti, CH-6605, Switzerland
  • 2Environmental Remote Sensing Laboratory, École polytechnique fédérale de Lausanne, Lausanne, Switzerland

Abstract. A still wind turbine (WT) observed with a fixed pointing radar antenna shows peculiar polarimetric signatures: during two minutes (from 17:08 to 17:10 UTC) of the first day (March 4, 2020) of the WT MeteoSwiss X-band radar campaign in Schaffhausen, the copolar correlation coefficient between the two orthogonal polarization states was persistently equal to 1. The reflectivity at vertical polarization was bounded between 38.5 and 41.5 dBz; however, the changes between two consecutive 64 ms values (retrieved by means of 128 transmitted pulses) were either 0 dBz or ±0.5 dBz. The 2-min median (mean) value was 40.0 (39.9) dBz over the 1875 echoes of this interval. The reflectivity at horizontal polarization was persistently equal to 56.5 dBz, which means no change exceeding ±0.25 dBz. The standard deviation (1874 degrees of freedom) of the differential phase shift, which in the absence of precipitation was, in fact, coincident with the dispersion of the differential backscattering phase shift, was as small as 3.0°. During two 10-min intervals (17:10–17:20 UTC and 17:30–17:40 UTC) the rotor has moved less than 1 revolution; however, this slow movement together with a change in blade angles and nacelle orientation was sufficient to cause large changes and significant variability in the polarimetric signatures, with two pairs of ZH consecutive values reaching the extreme of 78.5 dBz, which is the absolute reflectivity maximum reached in the whole campaign (March 4–21, 2020). Between 17:20 and 17:30 UTC, the rotor has accomplished 22.5 revolutions: the variability becomes smoother and softer in the central part of the interval (probably thanks to uniform rotor speed and “frozen” blade angles and nacelle orientation). It is desirable and recommended to extend this preliminary (32-minute) analysis (based on thirty thousand polarimetric measurables) to several other 10-min intervals with zero rotor speed.

Marco Gabella et al.

Status: open (until 18 Feb 2023)

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  • RC1: 'CommeComment on "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"nt on amt-2022-316', Anonymous Referee #1, 06 Jan 2023 reply

Marco Gabella et al.

Marco Gabella et al.

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Short summary
A still wind turbine observed with a fixed pointing radar antenna shows peculiar and stable polarimetric signatures: the correlation coefficient between the two orthogonal polarization states was persistently equal to 1. The reflectivity at both horizontal and vertical polarization was also quite persistent. The standard deviation of the differential backscattering phase shift was as small as 3.0 deg.