Tropical Cyclone Ocean Winds and Structure Parameters Retrieved from Cross-Polarized SAR Measurements

Spaceborne synthetic aperture radar (SAR) can provide unique capabilities to measure ocean surface winds under tropical cyclones (TCs), on synoptic scales, and at a very high spatial resolution. In this paper, we first discuss the accuracy and reliability of SAR-retrieved TC marine winds. The results show that wind retrievals from SAR images are in good agreement with Stepped Frequency Microwave Radiometer (SFMR) measurements, with root-mean-square error (RMSE) and correlation coefficient (CC) of 3.52 m s⁻¹ and 0.91, respectively. Based on the marine winds retrieved from SAR images, a relatively simple method is applied to extract the storm intensity (maximum wind speed) and wind radii (R34, R50, and R64 kt) from 234 cross-polarized SAR images, in the Northwest Pacific Ocean from 2015 to 2023. The SAR-retrieved TC wind radii and intensities are compared with the best-track reports, with RMSEs for R34, R50, and R64 being 48.32, 41.88, and 38.51 km, and CCs being 0.87, 0.83, and 0.65, respectively. In terms of TC intensity, the RMSE and bias between SAR estimates and best-track data are 7.32 and 0.38 m s⁻¹, respectively. For TC Surigae (2023), we found that employing a combination of multi-platform SARs, acquired within a short time interval, has the potential to simultaneously measure the  intensity and wind structure parameters. In addition, for a storm with a long life cycle, the multi-temporal synergistic SARs can be used to investigate fine-scale features of the TC ocean winds, as well as the evolution of TC surface wind intensities and wind structures.