Preprints
https://doi.org/10.5194/amt-2023-112
https://doi.org/10.5194/amt-2023-112
07 Jul 2023
 | 07 Jul 2023
Status: a revised version of this preprint was accepted for the journal AMT and is expected to appear here in due course.

Higher-Order Calibration on WindRAD scatterometer winds

Zhen Li, Ad Stoffelen, and Anton Verhoef

Abstract. WindRAD (Wind Radar) is a dual-frequency rotating fan-beam scatterometer instrument on the FY-3E (Fengyun-3E) satellite. Scatterometers are generally calibrated using the linear NOC (NWP Ocean Calibration) method, to control the main gain factor of the radar. While WindRad is stable, the complex geometry, the design of the instrument, and the rotating antenna make the backscatter (σ°) distributions persistently non-linear, hence NOC is insufficient. Therefore, a higher-order calibration method is proposed, called HOC. The CDF (Cumulative Distribution Function) matching technique is employed to match the CDF of measured σ°s to simulated σ°s. HOC removes the non-linearities for each incidence angle. However, it is not constructed to remove the anomalous harmonic azimuth dependencies caused by the antenna rotation. These azimuth dependencies are reduced by NOCant (NOC as a function of incidence angle and relative antenna azimuth angle). Therefore, the combination of HOC&NOCant is implemented to correct both anomalous σ° amplitude and azimuth variations. The wind retrieval performance is evaluated with NOCant, HOC, and HOC&NOCant combined. The wind statistics and the cone distance metric both show that HOC&NOCant achieves the optimal winds for C-band and Ku-band. The calibrations have been tested on two operational input data versions; HOC works well on both data versions and HOC&NOCant can achieve the optimal wind performance for both data versions. This confirms the usefulness of HOC calibration in the case of non-linear instrument gain anomalies.

Zhen Li et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-112', Anonymous Referee #1, 16 Jul 2023
  • RC2: 'Comment on amt-2023-112', Raj Kumar, 17 Jul 2023

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-112', Anonymous Referee #1, 16 Jul 2023
  • RC2: 'Comment on amt-2023-112', Raj Kumar, 17 Jul 2023

Zhen Li et al.

Zhen Li et al.

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Short summary
WindRAD is the first dual-frequency rotating fan-beam scatterometer in orbit. We observe non-linearity in the backscatter distribution. Therefore, Higher-Order Calibration (HOC) is proposed here, which removes the non-linearities for each incidence angle. The combination of HOC&NOCant is discussed, this combination can remove not only the non-linearly but also the anomalous harmonic azimuth dependencies caused by the antenna rotation, hence the optimal winds can be achieved.