Observing Tropical Cyclone Morphology Using RADARSAT-2 and Sentinel-1 Synthetic Aperture Radar Images Journal of Atmospheric and Oceanic Technology

Since the 1960’s, meteorological satellites have been able to monitor tropical cyclones and typhoons. Their images have been acquired by passive remote sensing instruments that operate in the visible and infrared bands, where they only display the cloud-top structure of tropical cyclones and make it a challenge to study the air-sea interaction near the sea surface. On the other hand, active remote sensors, such as spaceborne microwave scatterometers and synthetic aperture radars (SARs), can “see” through clouds and facilitate observations of the air-sea interaction processes. However, SAR acquires images and provides the wind field at a much higher resolution, where the eye of a tropical cyclone at surface level can be identified. The backscattered signals received by the SAR can be processed into a high-resolution image and calibrated to represent the normalized radar cross-section (NRCS) of the sea surface. In this study, 33 RADARSAT-2 and 102 Sentinel-1 SAR images of Atlantic and Indian Ocean tropical cyclones and Pacific typhoons from 2016-2021, which display eye structure, have been statistically analyzed with ancillary tropical cyclone intensity information. To measure the size of the eye, a 34-kt contour is defined around it and the amount and size of pixels within the eye is utilized to provide its area in km2. Additionally, an azimuthal wavenumber for each shape of the eye was assigned. Results showed that eye areas increase with decreasing wind speed and increasing wavenumber and demonstrate that SAR-derived data is useful for studying tropical cyclones at the air-sea interface and provide results of these behaviors closely to data derived from best-track archives.

自20世纪60年代以来，气象卫星已具备热带气旋与台风的监测能力。其搭载的可见光与红外波段被动遥感仪器所获取的观测图像，仅能呈现热带气旋的云顶结构，这为近海面海气相互作用的研究带来了诸多挑战。与之相对，星载微波散射计、合成孔径雷达（synthetic aperture radars, SAR）等主动遥感传感器可穿透云层，为海气相互作用过程的观测提供支撑。不过，合成孔径雷达可获取分辨率更高的图像与风场数据，能够识别表层热带气旋的风眼结构。合成孔径雷达接收到的后向散射信号可经处理生成高分辨率图像，并经过辐射校准以表征海面的归一化雷达散射截面（normalized radar cross-section, NRCS）。本研究针对2016-2021年间获取的33幅RADARSAT-2与102幅Sentinel-1合成孔径雷达图像展开统计分析，这些图像均呈现了大西洋、印度洋热带气旋以及太平洋台风的风眼结构，并结合了热带气旋强度的辅助资料。为量化风眼尺寸，研究人员围绕风眼设定了34节等值线，并通过统计风眼内的像素数量与尺寸，计算得到其面积（单位：平方千米）。此外，研究为每个风眼形态分配了方位波数。研究结果显示，风眼面积随风速降低而增大、随方位波数升高而增大；同时证实，合成孔径雷达衍生数据可有效用于海气界面处的热带气旋研究，且所得结果与最佳路径数据集的相关结果高度吻合。