北极海域SAR海浪方向谱反演及其与中法海洋卫星CFOSAT/SWIM数据的比较

The interaction between ocean waves and sea ice in the Arctic ocean has received significant attention. However, the study on this issue is significantly limited due to the lack of observation data. Synthetic Aperture Radar (SAR) plays an important role in the research of ocean wave and sea-ice interaction because of its unique capability of imaging sea surface in two dimensions. Sentinel-1 (S1), which consists of Sentinel-1A (S1A) and Sentine-1B (S1B), can cover the entire Arctic Area within 2 days. The Interferometric Wide Swath (IW) mode, one of the main imaging modes of S1 in the Arctic, has been providing SAR images with a high resolution of 10 m. The ocean wave spectra are derived using the S1 IW data. The spectra are likely to provide vital observation for studying the interaction between sea-ice and waves as they present the distribution of wave energy and their variations in different frequencies and directions. Meanwhile, the retrieved ocean wave spectra are an excellent validation data source for SWIM, which is onboard the China-France Oceanography Satellite and provides measurements of ocean wave spectra in the global ocean.In this study, we use the sub-images from the S1 IW image with a size of 1024 pixels × 1024 pixels (10.24 km × 10.24 km) to retrieve ocean wave spectra by using a nonlinear retrieval method (i.e., MPI scheme). The retrieved ocean wave spectra and wave parameters are compared with SWIM measurements acquired at an incidence angle of 10°, and the Significant Wave Height (SWH) is measured at nadir. The SWIM spectrum covers a large area by approximately 70 km × 90 km. To make a comparison, all the SAR sub-image spectra within a SWIM beam coverage are averaged to calculate a new observed SAR spectrum, which is inputted into the MPI inversion scheme. Then, the retrieved SAR ocean wave spectrum is compared with the SWIM spectrum. The footprint size of the SWIM nadir beam is 18 km, which is comparable to the S1 sub-image size. Accordingly, a SAR sub-image is extracted at SWIM nadir, and the corresponding ocean wave spectrum is retrieved. The SWH is calculated by integrating the retrieved wave spectra to compare with the SWIM nadir measurements of SWH. The experiment was carried out using the data acquired in September 2020 in the Greenland Sea and Norwegian Sea, where the ocean waves generated in the North Atlantic and propagating vast distances to the ice-covered area in the Arctic ocean can be frequently observed. Fifty-four ocean wave spectra were retrieved from 25 IW data and are compared with the SWIM slope spectra. The comparison shows that the SAR-retrieved spectra are consistent with SWIM spectra in terms of structure and energy distribution. Good agreements are also found between the integral parameters of the SAR ocean wave spectra and SWIM slope spectra. The comparison yields a bias and an RMSE of 0.11 and 0.71 m for SWH and a bias and an RMSE of -0.52 s and 0.62 s for mean wave period. The comparison of the dominant wave parameters yield a bias of -7.74° and an RMSE of 15.75° for the dominant wave direction and a bias of -0.56 m and an RMSE of 52.73 m for the dominant wavelength. Furthermore, 5075 data pairs of S1-retrieved SWH and SWIM nadir SWH were collocated and compared. The comparison result yields a bias of 0.03 m, an RMSE of 0.48 m, and a correlation of 0.95. The comparison between the S1 retrieved results and the SWIM measurements suggests that ocean wave information can be effectively retrieved from S1 IW data by using the MPI method in the Arctic ocean. Although the MPI method relies on prior information, it is still an effective method for obtaining ocean wave spectra in high resolution. The spectra retrieved from S1 are likely to show the energy attenuation of ocean waves in different frequencies and directions when propagating toward an ice-covered area. This finding will be of great support for the further study on the interaction between sea ice and ocean waves. © 2023 National Remote Sensing Bulletin. All rights reserved.

Retrieval of ocean wave spectra from Sentinel-1 SAR data and comparison with the CFOSAT/SWIM data in the Arctic ocean