Ocean surface radial Doppler Velocity (DV) signatures of Tropical Cyclones (TC) at moderate incidence angles are analyzed using a semi-empirical DV model. This model, originally named KaDOP, is based on the physics of Ka-band Doppler radar backscattering from the sea surface and represents the DV as a sum of components due to surface currents, long surface waves, Bragg scattering, and wave breaking. The latter two components were modified in this study to fit the model to C-band observations at strong winds. This modified model, referred to as a TC-DOP, requires several environmental input parameters, including TC winds and translation velocity, waves, and currents. For known winds and TC translation velocity, the surface currents are simulated using a model of the upper ocean response to TC passage. Waves are modeled using a self-similar description for the energy-containing TC-generated local wind waves and emanating swell. Given known winds, waves, and currents, the DV is calculated by the TC-DOP independently at each grid point. Calculations are performed for scanner-like observations that detect only the radial (cross-track) DV component. It is shown that at storm-strength winds and radar incidence angle, , the main contribution to the total DV is provided by the surface current and wave breaking DV components (1 m s). The latter component has strong azimuth asymmetry and maximizes in the upwind sector. Next in magnitude is the contribution from wind waves (0.5 m s), followed by Bragg waves (0.2 m s), while the contribution from swell is less important. Comparisons of the TC-DOP calculations with the C-band Sentinel-1A SAR radial DV data show qualitatively good agreement. All observed radial velocity images in TC storm areas have a dipole-like feature similar to that predicted by the simulations, with a zero DV area roughly aligned with the cross-wind direction. The TC-DOP model prediction is found generally consistent with calculations utilizing an empirical geophysical model function (known as the CDOP). The residual differences are attributed to specific features of radar scattering mechanisms in different radar wavelength bands (Ka- vs C-band), as well as possible shortcomings of the Ka-band Modulation Transfer Function (MTF) originally derived from platform-based measurements.

中文翻译：

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SAR 海洋径向多普勒速度中的热带气旋特征


使用半经验 DV 模型分析中等入射角的热带气旋 (TC) 的海洋表面径向多普勒速度 (DV) 特征。该模型最初名为 KaDOP，基于 Ka 波段多普勒雷达从海面反向散射的物理原理，并将 DV 表示为由于表面电流、长表面波、布拉格散射和破波而产生的分量之和。本研究对后两个组成部分进行了修改，以使模型适合强风下的 C 波段观测。这种修改后的模型称为 TC-DOP，需要多个环境输入参数，包括 TC 风和平移速度、波浪和洋流。对于已知的风和热带气旋平移速度，使用上层海洋对热带气旋通过的响应模型来模拟表面流。使用自相似描述对含能量的 TC 产生的局部风浪和涌浪进行波浪建模。给定已知的风、波浪和水流，DV 由 TC-DOP 在每个网格点独立计算。对仅检测径向（跨轨道）DV 分量的类似扫描仪的观测进行计算。结果表明，在风暴强度风和雷达入射角 下，对总 DV 的主要贡献是由表面电流和波浪破碎 DV 分量 (1 m s) 提供的。后一个分量具有很强的方位不对称性，并且在逆风区域最大化。其次是风浪（0.5 m s）的贡献，其次是布拉格波（0.2 m s），而涌浪的贡献则不太重要。 TC-DOP 计算与 C 波段 Sentinel-1A SAR 径向 DV 数据的比较显示出质量上良好的一致性。 台风风暴地区所有观测到的径向速度图像都具有与模拟预测类似的偶极子特征，零 DV 区域大致与侧风方向一致。发现 TC-DOP 模型预测与利用经验地球物理模型函数（称为 CDOP）的计算基本一致。残余差异归因于不同雷达波长带（Ka 与 C 波段）中雷达散射机制的特定特征，以及最初源自基于平台的测量的 Ka 波段调制传递函数（MTF）可能存在的缺陷。