Analysis of Azimuthal Cutoff Wavelength Derived from Sentinel-1 Synthetic Aperture Radar Image in Tropical Cyclones

The purpose of this study is to analyze the impact of upper ocean dynamics on velocity bunching, represented by azimuthal cutoff wavelength (i.e., sea surface wind, wave, and current). In this study, over 1400 dual-polarized (vertical-vertical (VV) and vertical-horizontal (VH)) Sentinel-1 (S-1) synthetic aperture radar (SAR) images collected in tropical cyclones (TC) are utilized. These images are combined with wind and rain observations from the stepped-frequency microwave radiometer (SFMR), wave simulations conducted using a third-generation numerical wave model, WAVEWATCH-III (WW3), and SAR-derived wind information collected from CyclObs winds. The WW3-simulated significant wave height (SWH) is validated against measurements from HY-2B altimeter taken in August and September 2021, yielding a root mean square error (RMSE) of 0.48 m and a correlation coefficient (COR) of 0.88. The SAR-based azimuthal cutoff wavelengths in VV polarization, which quantitatively represent the effect of velocity bunching, are compared with theoretical values calculated using WW3-simulated SWH. A notable relationship is observed between the difference in azimuthal cutoff wavelength and SAR-derived wind speed and WW3-simulated SWH. Analysis results show that the correlation between SAR-based azimuthal cutoff wavelength and SWH is stronger than that with wind and current. Finally, a machine learning algorithm is used to develop an algorithm aimed at simulating the azimuthal cutoff wavelength in TCs, including wind, wave, and incidence angle. This method yields an RMSE of 8.90 m, a COR of 0.91, and a scatter index of 0.04 for VV-polarization SAR.