Conference Agenda

It is presented in this paper the scientific objectives and some progresses of ESA-MOST China Dragon Cooperation Program “Synergistic Monitoring of Ocean Dynamic Environment from Multi-Sensors (ID. 58009)” including: (1) assimilation studies of wind, waves and sea level in the context of hurricanes forecasts; (2) the influence of swell on the studies of coastal extremes such as sea level rise, storm surges and extreme wave events; (3) studies of vortex Rossby waves, asymmetric tropical cyclone structures, rain bands, and sub-scale circulations by using high spatial resolution ocean wind data; (4) analysis of relationship between the above internal dynamical processes and tropical cyclone intensity changes; and (5) consistent monitoring of ocean surface current and internal waves using multi-source satellite data.

Tropical cyclones (TC) transfer kinetic energy to the upper ocean and thus enhance the ocean mixed layer (OML) currents. However, the quantitative link between near-surface currents and high wind speeds, under extreme weather conditions, remains poorly understood. In this study, we use multi-mission satellite and drifting-buoy observations to investigate the connections between TC surface winds and currents, including their spatial distribution characteristics. Observed ageostrophic current speeds in the OML increase linearly with wind speeds. The ratios of the ageostrophic current speeds to the wind speed are found to vary with TC quadrants. In particular, the mean ratio is around 2% in the left-front and left-rear quadrants with relatively small variability, compared to between 2% and 4% in the right-front and right-rear quadrants, with much higher variations. Surface winds and currents both exhibit strong asymmetric features, with the largest wind speeds and currents on the TC right side. In the TC eyewall region, high winds (e.g. 47 m/s) induce strong currents (2 m/s). The directional rotations of surface winds and currents are coupled and dependent of specific locations. Wind directions are approximately aligned with current directions in the right-front quadrant; a difference of about 90^o occurs in the left-front and left-rear quadrants. The directional discrepancy between winds and currents in the right-rear quadrant is relatively smaller. Reliable observations of the wind-current relation, including asymmetric features, will enhance our understanding of TC air-sea interactions.

Satellite altimeter is a fundamental important global ocean remote sensing technique to monitor the marine dynamic environment. Sentinel-3A/3B and Sentinel-6 satellite equipped with altimeters have been launched on 16 Feb. 2016, 25 Apr. 2018 and 21 Nov. 2020 in Europe, and HY-2B/2C satellite equipped Radar Altimeter were launched on 25 Oct. 2018 and 21 Sep. 2020 in China. The objectives of this research topic are to improve the retrieval of SSH and SWH of Sentinel-3 and HY-2 series altimeters in the Chinese seas by the waveform retracking method in the coastal areas. First we combine Sentinel-3 and HY-2 series and other altimeters data into high spatial resolution grid data in the China seas and western Pacific Ocean. Then we develop the retrieval method of sea surface current by combining the altimeter, sea surface wind and SST data in the Chinese seas and western Pacific Ocean. Subsequently we analyze the spatial-temporal variation characteristics of ocean waves, ocean current and mesoscale eddies in the Chinese seas and the western Pacific Ocean. In this study, Altimetry data of Sentinel-3A/3B, Cryosat-2, Sentinel-6 HY-2B/2C and CFOSAT SWIM data will be investigated in this study. Field data of tide gauge stations and buoys are used for data validation of SSH and SWH. Two master students and young scientist Dr. Wei Cui from the First Institute of Oceanography, MNR of China are involved in this study. For the first year of Dragon 5, waveform retracking processing of HY-2B altimeter in coastal areas of the China seas are carried out by different methods and the results are analyzed. The accuracy of HY-2B SSH and SWH data in the coastal area are improved by data reprocessing. Based on in situ data from the tide gauge station and buoy, the HY-2B altimeter SSH and SWH are evaluated, and the improvement of the HY-2B in the coastal area by the reprocessing is summarized. The European partners are mainly contributing to the data reprocessing and mean surface model of altimeters, and the Chinese partners are contributing to data reprocessing of altimeters and their applications in Marine dynamic environment monitoring.

The investigations carried out by the European partners have focused in the first year on studying the effect of surface wave breaking on the radar imaging mechanism of internal waves. It is known since long time that the conventional radar imaging theory based on weak hydrodynamic interaction theory and Bragg scattering theory fails to describe the often observed strong co-polarization radar signatures of internal waves at C- and X-band and their weak dependence on look direction of the radar antenna. This calls for an improved radar imaging theory of internal waves, which includes scattering from breaking surface waves. To this end, we have analyzed a TerraSAR-X image of an internal wave packet acquired at HH and VV polarization and C-band Sentinel-1 SAR and L-band ALOS/PALSAR images of internal solitary waves (ISWs) acquired at co- and cross-polarizations. We found that in the case of co-polarized scattering (i.e., at HH and VV polarizations) the measured radar signature of large ISWs can only be explained by including non-polarized scattering from breaking waves into the scattering mechanism. Furthermore, we found that in the case cross-polarized scattering (i.e., at VH polarization), the cross-polarized radar signature of ISWs can be similarly strong as the co-polarization one. Furthermore, we have analyzed Sentinel-3 SAR altimetry data and found clear evidence of significant wave height (SWH) variations along the propagation paths of ISWs.

The investigations carried out by the Chinese partners have focused in the first year on improving models for the description of ISW propagation in the South China Sea.

Jianqiang Liu^1,2，Ying Xu^1,2，Daniele Hauser³,Jing Ding^1,2，Qingjun Song^1,2，Maohua Guo ^1,2，Xiuzhong Li⁴，Wenming Lin⁴， Lingling Xie⁵, François Schmitt⁶

¹(National Satellite Ocean Application Service, MNR, Beijing, China)

²(Key Laboratory of Space Ocean Remote Sensing and Application, MNR)

³(CNRS/LATMOS, Guyancourt, France)

⁴(Nanjing University of Information Science & Technology, Nanjing, China)

⁵(Guangdong Ocean University, Zhanjiang, China)

⁶(CNRS/Laboratory of Oceanology and Geosciences ,Wimereux, France)

Abstract

The China France Oceanography Satellite (CFOSAT) and Haiyang-2B (HY-2B) satellites were successively launched in China in 2018. As missions for measuring the dynamic marine environment, both satellites can measure the nadir significant wave height (SWH). In this project, the HY-2B altimeter and CFOSAT nadir SWHs have been validated against the National Data Buoy Center (NDBC) buoys and the Jason-3 altimeter SWH data, respectively, which resulted in CFOSAT nadir SWH having the best accuracy and HY-2B having the best precision. The SWHs of the two missions are also calibrated by Jason-3 and NDBC buoys. Following calibration, the root mean square error (RMSE) of CFOSAT and HY-2B are 0.21 and 0.27 m, respectively, when compared to Jason-3, and 0.23 and 0.30 m, respectively, compared to the buoys. Our results show that the two missions can provide good-quality SWH and can be relied upon as a new data resource of global SWH.

Using simultaneous observations of wind and wave fields by the CFOSAT, this project reports preliminary investigation results of the typhoon waves during the passage of super typhoon Lingling (2019) over the China offshore waters. The results show that the significant wave heights (SWHs) are over 5 m on the right side of the typhoon track for wind speeds over 14 m s-1, agreeing with the theoretical estimates. The dominant waves have wavelengths of 150 – 180 m, and propagate eastward for northwestward blowing winds. The misalignments of the wind and wave directions increase with the distance from the typhoon center, agreeing with theoretical prediction. We also present the typhoon monitoring results with multiple satellites such as CFOSAT, HY-2B and ASCAT.

HY-1D satellite which is China’s fourth series of ocean color satellites, was successfully launched in 2020. The overall objective of HY-1 serial satellite is to monitor global ocean color and SST (Sea Surface Temperature), as well as the coastal zones’ environment. Using HY-1 C/D data and Sentinel satellite data, this project investigates the sea ice, oil spill and green tide disaster in Bohai Sea and the Yellow Sea, red tide in East China Sea. The results show that combing HY-1 C/D and Sentinel satellite data have played an important role in ocean ecological disaster monitoring.