Extreme wind speed retrieval from sar azimuth cut-off

2019 Living Planet Symposium, 13-17 May 2019, Milan, Italy

Nowadays, wind speed retrieval is a topic of great interest. Most of the remote-sensing satellite radar systems are able to provide sea-surface wind field information, and they can be considered the main near sea surface wind information source. Within this context, active microwave remote sensing, in particular the scatterometer and the Synthetic Aperture Radar (SAR), is worldwide recognized as one of the best suitable tools to perform a reliable sea-surface wind speed retrieval. Radar backscatter intensity and its statistical properties contain quantitative information about sea surface roughness and, therefore, can be used to derive sea-surface wind information. Empirically derived Geophysical Model Functions (GMF) are used to relate the calibrated radar return to the wind speed at a height of 10m. GMFs usually relate the radar backscatter to wind speed and wind direction. An alternative method to retrieve wind speed from SAR is represented by the spectral based approaches, i.e.; the azimuth cut-off procedure. When dealing with SAR microwave sensors, Doppler misregistration along the azimuth is induced by gravity wave orbital motion. This issue is the major responsible of a distortion of the imaged wave spectrum and of a strong cut-off in the azimuthal direction: this is the azimuth cut-off. Originally, this technique was proposed in [1] to retrieve significant wave height (SWH), without the knowledge of any a priori wind direction information. The theory that is at the basis of the azimuth cut-off method is related to the effect of the orbital motion related to the surface waves in the SAR imaging of the ocean surface. This orbital motion results in additional Doppler shifts that distort the phase history of the backscattered signal used to synthetize the resolution in azimuth. The result is a low-pass filtered SAR image in the azimuth direction. The azimuth cut-off well correlates to the SWH because of its sensitivity to the long waves. In this study, the ACF-based λc approach is extended to high wind speed regimes, e.g.; extreme weather conditions. The key issues that need to be tackled concern the tuning with respect to pixel spacing, box size and the homogeneity of the SAR imagery. Preliminary results obtained processing a large data set of Sentinle-1 SAR imagery collected under a broad range of wind conditions show that the box size and the median filter window should be set at 1 km × 1 km and 90-120 m, respectively. Then, tailored experiments have been undertaken an actual Sentinel-1 dataset collected under high wind regimes, i.e.; tropical cyclones. The obtained λc values are compared both with the ECMWF forecasted information and with wind speed maps provided by Ifremer. Preliminary results show that there is a good agreement between λc and wind speed in some areas of the cyclone, i.e.; further enough from the cyclone eye. [1] V. Kerbaol, B. Chapron, and P.W. Vachon, “Analysis of ERS-1/2 synthetic aperture radar wave mode imagettes”, Journal of Geophysical Reserarch, vol. 103, no. C4, pp. 7833-7846, 1998.