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Received：16 April 2021，
Published：07 April 2023
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北极海域的海浪与海冰相互作用是目前北极海冰变化研究的一个前沿问题。但观测数据的缺失，限制了对于该问题的深入研究。星载合成孔径雷达SAR（Synthetic Aperture Radar）具有独特的海面二维成像能力，是极地边缘区海浪—海冰相互作用研究重要的遥感手段。本文基于哨兵1号干涉宽幅模式IW（Interferometric Wide Swath）数据，利用SAR海浪非线性反演方法在格陵兰海及挪威海开展了海浪方向谱的反演实验，进而使用中法海洋卫星CFOSAT搭载的SWIM波谱仪的海浪谱数据和星下点有效波高数据对SAR反演结果进行了验证。结果表明，无论是海浪方向谱的结构、能量分布特征还是海浪方向谱的积分参数，SAR的反演结果都与SWIM的测量结果具有良好的一致性。其中，SAR海浪方向谱和SWIM斜率谱积分计算的有效波高的偏差和均方根误差为0.11 m和0.71 m，平均波周期的偏差和均方根误差分别为-0.52 s和0.62 s，谱峰传播方向与波长的偏差分别为-7.74 °和-0.56 m， 均方根误差分别为15.75°和52.73 m。SAR反演的有效波高与SWIM星下点测量的有效波高之间偏差为0.03 m，均方根误差为0.48 m，二者相关系数为0.95。本研究验证了利用SAR海浪非线性反演方法进行北极海域海浪方向谱及海浪参数反演的可行性，为后续的海浪向冰传播衰减过程研究奠定了数据基础和方法基础。
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.
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