Exploratory research on the retrieval of internal wave parameters and sea surface current velocity based on TerraSAR-X satellite data
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摘要: 本文利用TerraSAR-X(TSX)卫星于2010年4月22日在南海东沙岛附近海域获取的数据进行海洋内孤立波动力要素和海表流速信息的提取研究。基于TSX数据的后向散射强度信息，利用经验模态分解法得到内孤立波半波宽度，再利用两层模型法和参数化法计算得到内孤立波振幅和相速度。反演结果显示，利用参数化方法得到的振幅(约21～39 m)和两层模型法得到的相速度(约1.07 m/s)与历史实测资料较为一致。进而利用TSX的顺轨干涉数据获取研究海域内的多普勒速度，再分别采用M4S模型法和直接分离法处理，进而提取海表流速。结果显示，两种方法得到的海表流速的全场平均值较为一致，均为1.10 m/s左右。M4S模型法对流速最大值的改变量较大而直接分离法对流速最小值的改变量较大。M4S模型对内孤立波波峰线区域海表流速的修正大于无内孤立波的海域。最后，基于KdV方程计算得到内孤立波引起的表面流的流速约为0.28 m/s，对反演出的海表流速贡献占比23%。Abstract: In this paper, we conducted a study on deriving internal wave parameters and sea surface current velocity in the vicinity of Dongsha Island from spaceborne SAR data of TerraSAR-X (TSX), acquired on April 22, 2010. Based on the intensity data of the TSX scene, characteristic half width of the internal wave was obtained through the empirical mode decomposition method. Then amplitude and phase speed of the internal solitary wave were calculated using the two-layer model method and the parameterization method, respectively. The results suggested that the amplitude (approximately 21–39 m) acquired by the parameterization method and the phase speed (approximately 1.06 m/s) obtained by the two-layer model method were consistent with the historical measurements. Furthermore, the Doppler velocity in the research area was calculated using along-track interferometry data of the TSX scene. The M4S model and the direct-separation method were used to extract the sea surface current velocity. The mean sea surface current field of the study area obtained by the two methods were consistent, which were both of approximately 1.10 m/s. The M4S model led to more adjustments to maximum of the surface velocity whereas more adjustments to the minimum values were found in the direct-separation method. Moreover, it was found that the adjustment of surface current retrieval using the M4S model were more in the region of wave crest of the internal waves. Finally, based on the KdV equation, the velocity of internal-wave-induced surface flow is about 0.28 m/s, which contributes 23% to the retrieved sea surface current velocity.
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图 2 TSX卫星在东沙岛附近获取的快视图（2010年4月22日22:13 UTC）（a），对应该快视图的雷达后向散射强度（b），降噪处理后对应快视图的相位差（c）
海域大小约30 km（距离向）×110 km（方位向），图a中右下部所示明亮区域为东沙岛和东沙环礁；图b中5个断面用于内波参数提取的计算；图c用于后续海表流场的反演
Fig. 2 Quicklook of the TSX scene acquired over Dongsha Island at 22:13 UTC on April 22, 2010 (a); the corresponding radar backscatter intensity of the TSX scene (b); the derived phase difference from the ATI data of the TSX scene (c)
Sea area size is 30 km (range direction)×110 km (azimuth direction), the bright areas are Dongsha Island and Dongsha Atoll in a; the 5 transects in b are used to extract parameters of the internal wave; c is further used to retrieve the sea surface current
图 3 沿内孤立波传播方向的断面1平均后的后向散射强度数据（a）；b−i为经验模态分解法得到的8个本征模态
Fig. 3 Distribution of the averaged backscatter intensity of Transect 1 along the propagation direction of the internal wave (a); b−i are 8 intrinsic modes
图 4 研究海域的水深分布（a）和断面1所在海域的浮性频率剖面（b）
Fig. 4 Bathymetry of the research area (a) and profile of buoyancy frequency of Transect 1 (b)
图 5 顺轨干涉SAR海表运动速度提取示意图
Fig. 5 Diagram of derivation of sea surface velocity using ATI-SAR
图 6 利用TSX的ATI数据得到的多普勒速度的空间分布（a）和对应的多普勒速度值统计直方图（b）
Fig. 6 Spatial distribution of the derived Doppler velocity from the TSX ATI data (a) and statistic histogram of the corresponding Doppler velocity (b)
图 7 M4S模型法得到的海表流速结果（雷达视向）（a），直接分离法得到的海表流速结果（雷达视向）（b），两者的差值（c）
Fig. 7 Sea surface current speed (component in the line of sight) retrieved using the M4S model (a), sea surface current speed (component in the line of sight) derived using the separation method (b); differences between the two retrievals (c)
表 1 TSX获取的SAR图像的基本信息
Tab. 1 Technical specification of the TerraSAR-X ATI data
产品级别 工作模式 极化方式 天线接收模式 空间分辨率 单视斜距复
影像条带成像 HH DRA 约2 m 
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表 2 不同模态的归一化方差
Tab. 2 Normalized variance of the 8 intrinsic modes
模态 1 2 3 4 5 6 7 8 归一化方差 0.058 0.142 0.103 0.286 0.161 0.109 0.052 0.089
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表 3 图2b中各断面内孤立波特征半宽度
Tab. 3 Characteristic half width of internal solitary wave in each transect in Fig.2b
断面 1 2 3 4 5 特征半宽度/m 749.8 663.3 687.3 713.6 709.3
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表 4 利用两层模型法和参数化方法提取得到的图2b中5个断面内孤立波的振幅和相速度
Tab. 4 Amplitude and phase speed of internal solitary waves in the 5 transects marked in Fig. 2b, obtained using the two-layer-model and the parameterization method
断面 两层模型法 参数化法 振幅/m 相速度/m·s–1 振幅/m 1 6.72 1.07 38.69 2 4.83 1.06 21.45 3 4.97 1.08 27.33 4 4.84 1.10 29.70 5 5.27 1.11 32.25
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表 5 利用M4S模型法和直接分离法得到的速度及原多普勒速度对比
Tab. 5 Comparison of the velocities obtained by the M4S model method and direct-separation method and the Doppler velocity
原多普勒速度 M4S模型法 直接分离法 流速最大值/m·s–1 2.71 1.83 2.30 相对变化比率 –32.33% –15.12% 流速最小值/m·s–1 0.48 0.43 0.19 相对变化比率 –11.71% –61.27% 流速平均值/m·s–1 1.39 1.10 1.11 相对变化比率 –20.81% –19.96%
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