Tropical cyclone wind direction retrieval from sentinel-1 SAR imagery by a modified Improved Local Gradient (ILG-Mod) method

Tropical cyclones (TC) are the most intense interactions between the ocean and atmosphere. Synthetic aperture radar (SAR) can capture high resolution marine atmospheric and oceanic features. Therefore, high-resolution sea surface wind information can be retrieved from SAR imagery through various methods. In this study, a new modified local gradient (ILG-Mod) method is proposed for TC wind direction retrieval. It combines SAR image smoothing and local gradient computation in the spectral domain, and changes the way to obtain the main local gradient direction in the spatial domain. Nineteen TC images acquired from Sentinel-1 SAR have been collected for validation. The proposed method is compared with the local gradients (LG) method, modified local gradients (LG-Mod) method, and the improved local gradients (ILG) method. In addition, ERA5 (the fifth generation European Centre for Medium-Range Weather Forecasts) reanalysis data and CCMP (the Cross-Calibrated Multi-Platform) data are used for validation. Statistical results show that wind directions retrieved by the ILG-Mod method are highly consistent with the validated data. Comparisons with validated data reveal bias, root mean square error (RMSE), and correlation coefficient (r) values of 17.09 degrees, 19.63 degrees and 0.96, respectively. The proposed ILG-mod method is therefore suitable to retrieve wind direction of TCs.