Influence of wind-induced antenna oscillations on radar observations and its mitigation

Abstract

As Typhoon Goni (2015) passed over Ishigaki Island, a maximum gust speed of 71 m s-1 was observed by a surface weather station. During Typhoon Goni’s passage, mountaintop radar recorded antenna elevation angle oscillations, with a maximum amplitude of ~0.2∘ at an elevation angle of 0.2∘. This oscillation phenomenon was reflected in the reflectivity and Doppler velocity fields as Typhoon Goni’s eyewall encompassed Ishigaki Island. The main antenna oscillation period was approximately 0.21–0.38 s under an antenna rotational speed of ~4 rpm. The estimated fundamental vibration period of the radar tower is approximately 0.25–0.44 s, which is comparable to the predominant antenna oscillation period and agrees with the expected wind-induced vibrations of buildings. The re-flectivity field at the 0.2∘elevation angle exhibited a phase shift signature and a negative correlation of-0.5 with the antenna oscillation, associated with the negative vertical gradient of reflectivity. FFT analysis revealed two antenna oscillation periods at 0955–1205 and 1335–1445 UTC 23 August 2015. The oscillation phenomenon ceased between these two periods because Typhoon Goni’s eye moved over the radar site. The VAD analysis-estimated wind speeds at a range of 1 km for these two antenna oscillation periods exceeded 45 m s-1, with a maximum value of approximately 70 m s-1. A bandpass filter QC procedure is proposed to filter out the predominant wavenumbers (between 40 and 70) for the reflectivity and Doppler velocity fields. The proposed QC procedure is indicated to be capable of mitigating the major signals resulting from antenna oscillations.