Empirical Algorithm for Significant Wave Height Retrieval from Wave Mode Data Provided by the Chinese Satellite Gaofen-3
Abstract
:1. Introduction
2. Description of Data Sets
2.1. GF-3 SAR WM Data
- (1)
- Polarization. Different from the single-polarized WM on ERS-1/2 SAR, Envisat ASAR, and Sentinel-1A/B, GF-3 WM can acquire imagettes in quad-polarization (VV+HH+VH+HV).
- (2)
- Incidence angle. WM imagettes from European SAR satellites are acquired at one or two specific incidence angles. In contrast, although a certain fixed incidence angle was adopted for a specific orbit, the incidence angle could be switched from 20 to 50° for WM on GF-3 for the period of ten months on which we focused. Hence, we categorized the GF-3 WM data into six groups with respect to incidence angle. Here, they are called incidence angle modes of WV01 for 21°–25°, WV02 for 28°–32°, WV03 for 33°–37°, WV04 for 38°–42°, WV05 for 42°–46°, and WV06 for 46°–50°, and details are listed in Table 1.
- (3)
- Geographic distribution. Traditional European SAR WM could monitor swells in the open ocean at a global scale e.g., [4]. However, currently, the GF-3 SAR payload can only work in WM for up to 50 min in one acquisition, owing to power limitations [31]. Thus, as shown in Figure 2, GF-3 WM measurements are not globally distributed for any of the 6 incidence angle modes in the period from January to October 2017.
- (1)
- Homogeneity check. Here, the homogeneity quality control is performed using the parameter of normalized variance () computed from VV-polarized imagettes, defined as the imagette variance normalized with mean intensity:
- (2)
- Ice rejection. For the SAR images acquired in the ice; for instance, Figure 1d presents homogeneous features but should be obviously excluded for our SWH retrieval. Because the check mentioned above fails to reject these cases ( = 1.37 in Figure 1d), we further discarded the GF-3 WM acquisitions in high latitudes (>60°) to avoid sea ice near the polar regions.
2.2. Reference SWH Data
2.2.1. Wavewatch III Hindcast Data
2.2.2. Altimeter SWH Data
2.2.3. Buoy SWH Data
3. Development of Empirical Wave Retrieval Model: QPCWAVE_GF3
3.1. Radar Incidence Angle
3.2. Normalized Radar cross Section (NRCS)
3.3. Image Normalized Variance
3.4. Azimuth Cut-Off, Peak Wavelength and Direction
3.5. Tuning of the Empirical Model QPCWAVE_GF3
3.6. SWH Retrieval Scheme for GF-3 WM Data
4. Algorithm Validation
4.1. Comparison with Independent WW3 Hindcast
4.2. Comparison with Altimeters and Buoys
5. Discussions
5.1. Cross-Polarized NRCS Contribution to SWH Empirical Model
5.2. Case Studies
5.2.1. Case of 31 January 2017 from GF-3 WM Data
5.2.2. Case of 29 October 2017 from GF-3 QPSI Data
6. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
Abbreviations
ASAR | Advanced Synthetic Aperture Radar |
COR | CORrelation coefficient |
CWAVE_ENV | C-band WAVE algorithm for ENVisat wave mode |
CWAVE_ERS | C-band WAVE algorithm for ERS wave mode |
CWAVE_S1A | C-band WAVE algorithm for Sentinel-1A wave mode |
DLR | German Aerospace Center |
ECMWF | European Centre for Medium-Range Weather Forecasts |
GF-3 | Gaofen-3 |
GMF | Geophysical Model Function |
IFREMER | Institut Français de Recherche pour l’Exploitation de la Mer |
IOWAGA | Integrated Ocean Waves for Geophysical and other Applications |
NDBC | National Data Buoy Center |
NRCS | Normalized Radar Cross Section |
OLS | Ordinary Least Squares |
QPCWAVE_GF3 | Quad-Polarized C-band WAVE algorithm for GaoFen-3 wave mode |
QPSI | Quad-Polarization Strip I |
RMSE | Root Mean Square Error |
SAR | Synthetic Aperture Radar |
SI | Scatter Index |
SLC | Single Look Complex |
SWH | Significant Wave Height |
UTC | Universal Time Coordinated |
WM | Wave Mode |
WW3 | WaveWatch III |
XWAVE | X-band WAVE algorithm |
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ID | Incidence Angle | Number of GF-3 WM Data | |||||
---|---|---|---|---|---|---|---|
Range | Mean | Standard Deviation | Total | Rejected | Tuning | Validation against WW3 | |
WV01 | 21–25° | 22.27° | 1.03° | 988 | 106 | 180 | 702 |
WV02 | 28–32° | 29.92° | 1.02° | 1228 | 188 | 209 | 831 |
WV03 | 33–37° | 35.80° | 0.74° | 4192 | 440 | 748 | 3004 |
WV04 | 38–42° | 41.06° | 0.97° | 4742 | 545 | 836 | 3361 |
WV05 | 42–46° | 44.08° | 1.08° | 1620 | 204 | 284 | 1132 |
WV06 | 46–50° | 47.40° | 1.20° | 1758 | 170 | 319 | 1269 |
WV01 | WV02 | WV03 | WV04 | WV05 | WV06 | |
---|---|---|---|---|---|---|
−3.8082 | −9.0969 | 1.5534 | −19.5166 | −10.4568 | −9.4693 | |
0.0015 | 0.1906 | 0.2429 | 0.1698 | 0.0988 | 0.4062 | |
−0.6635 | −0.8883 | −0.7318 | 0.9653 | −1.5123 | −0.2300 | |
0.0007 | 0.0017 | −0.0024 | 0.0005 | −0.0041 | −0.0021 | |
1.5233 | 5.9697 | −0.1145 | 1.7617 | 1.9145 | 5.9112 | |
−0.2459 | −0.6458 | −0.4577 | −1.2828 | −0.6397 | −1.0020 | |
4.2210 | 11.3454 | 3.6351 | 19.2854 | 14.5511 | 15.8545 | |
0.0012 | 0.0010 | 0.0022 | 0.0002 | 0.0033 | 0.0014 | |
2.0985 | 1.2722 | 1.0585 | −0.3443 | 1.6726 | 0.8500 | |
−0.0110 | 0.0370 | 0.1652 | 0.0616 | 0.0352 | 0.0476 | |
−3.0297 | −5.0699 | 0.8747 | −0.3453 | −3.5451 | −5.5485 | |
0.1713 | 0.3660 | 0.1349 | 0.9692 | 0.5105 | 0.5614 |
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Wang, H.; Wang, J.; Yang, J.; Ren, L.; Zhu, J.; Yuan, X.; Xie, C. Empirical Algorithm for Significant Wave Height Retrieval from Wave Mode Data Provided by the Chinese Satellite Gaofen-3. Remote Sens. 2018, 10, 363. https://doi.org/10.3390/rs10030363
Wang H, Wang J, Yang J, Ren L, Zhu J, Yuan X, Xie C. Empirical Algorithm for Significant Wave Height Retrieval from Wave Mode Data Provided by the Chinese Satellite Gaofen-3. Remote Sensing. 2018; 10(3):363. https://doi.org/10.3390/rs10030363
Chicago/Turabian StyleWang, He, Jing Wang, Jingsong Yang, Lin Ren, Jianhua Zhu, Xinzhe Yuan, and Chunhua Xie. 2018. "Empirical Algorithm for Significant Wave Height Retrieval from Wave Mode Data Provided by the Chinese Satellite Gaofen-3" Remote Sensing 10, no. 3: 363. https://doi.org/10.3390/rs10030363