Preprints
https://doi.org/10.5194/amt-2022-316
https://doi.org/10.5194/amt-2022-316
06 Dec 2022
 | 06 Dec 2022
Status: a revised version of 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 Gabella, Martin Lainer, Daniel Wolfensberger, and Jacopo Grazioli

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: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • 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
    • AC1: 'Reply on RC1', Martin Lainer, 11 Apr 2023
  • RC2: 'Comment on amt-2022-316', Anonymous Referee #2, 05 Mar 2023
    • AC2: 'Reply on RC2', Martin Lainer, 11 Apr 2023

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.