Determination of the wind field from SAR in coastal areas

The possibility to get the wind field from the Synthetic Aperture Radar (SAR) images is extremely important in regional seas and coastal areas, only partially covered at present by satellite wind data. Here, the wind fields are often not well reproduced by both global and regional atmospheric models (Accadia and Zecchetto, 2007, Zecchetto and Accadia, 2014), probably because of the interaction between the wind flow and the orography and effects of the local sea water temperature. Furthermore, in small basins the atmospheric model winds are seldom validated, due to the lack of in-situ winds observations. To retrieve the wind field from SAR two things are mandatory: a reliable radar-backscatter versus wind speed algorithm and a suitable estimate of the wind direction. A method based on the 2D Continuous Wavelet Transform has been developed and applied to Sentinel-1 SAR images to retrieve the wind direction. The SAR derived wind directions have been compared with in-situ data the variability of which, expressed as the wind direction variance, has been found similar to that obtained from SAR (Zecchetto et al., 2016) and dissimilar to that of the WRF regional atmospheric model. Thus, it may be taken as a proxy for the reliability of the SAR wind directions. The problem of the wind direction determination from SAR is thus virtually solved by the 2D-CWT method without any external information, and the resulting wind fields have been analysed and compared with those derived using WRF wind directions and with the OWI ESA products. Possible uses of the SAR derived wind fields (wind vorticity and energy spectra) are also introduced and commented.