| Summary: | Accurate knowledge of wind directions plays a critical role in ocean surface wind retrieval and tropical cyclone (TC) research. Under TC conditions, apparent wind streaks induced by marine atmospheric boundary layer rolls can be detected in VV- and VH-polarized synthetic aperture radar (SAR) images. It suggests that though relatively noisy, VH signals may help enhance wind streak orientation magnitudes contained in VV signals and thus to achieve a more accurate wind direction estimation. The study proposes a new method for wind direction retrieval from TC SAR images. Unlike conventional approaches, which calculate wind directions from single-polarization imagery, the method combines VV and VH signals to obtain continuous wind direction maps across moderate and extreme wind speed regimes. The technique is developed based on the histogram of oriented gradient descriptor and Hann window function, accounting for the contribution of neighboring wind streak information (weighted by separation distances). As a case study, the wind directions over four TCs (Karl, Maria, Douglas, and Larry) are derived and verified by estimates from simultaneous dropsonde, ASCAT and ECMWF winds, showing a promising consistency. Furthermore, a more comprehensive statistical analysis is carried out with 14 SAR images, revealing that obtained wind directions have a correlation coefficient of 0.98, a bias of −6.07<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> and a RMSD of 20.24<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, superior to estimates from VV (0.97, −7.84<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, and 24.23<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, resp.) and VH signals (0.96, −10.46<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, and 29.53<inline-formula><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, resp.). The encouraging results prove the feasibility of the technique in SAR wind direction retrieval.
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