Huaguo Zhang

Study of the propagation direction of the internal waves in the South China Sea using satellite images

Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.

Reconstruction of sand wave bathymetry using both satellite imagery and multi-beam bathymetric data: a case study of the Taiwan Banks

Sand waves are a widespread bed-form in the tidal environment. Their formation, migration, and other properties have great significance in the fields of geology and oceanography. Currently, research on sand waves is mainly based on multi-beam bathymetric (MB) data. For large sand wave regions, however, the acquisition ability of MB data becomes limited; instead, interval-line measurement is adopted but with the inherent problem that it cannot achieve the full spatial coverage required. Reconstructed sand wave bathymetry (SWB) using interpolation algorithms cannot reflect the real SWB either. In this article, we propose a new approach in using both satellite imagery and MB data to reconstruct SWB, which takes advantage of characteristic information (ripple and crest) of SWB rendered in satellite imagery and MB data. The new approach is exemplified by a case study of the Taiwan Banks. We use the imagery obtained by the Charge-Coupled Device from the Small Satellite Constellation for Environment and Disaster Monitoring and Forecasting, and the MB data from R2Sonic to reconstruct the digital elevation model. The results show that the root mean square error of the reconstructed water depth is 1.47 m (compared to the MB data not using this approach), suggesting that the new approach is effective in reconstructing SWB. This approach allows a reduction in MB data track density, which may lead to an improvement in efficiency.

Analysis on the coastline change and erosion-accretion evolution of the Pearl River Estuary, China, based on remote-sensing images and nautical charts

Abstract Coastline change, erosion-accretion evolution, and their relationship in the Pearl River Estuary (PRE) of China over the past 25 years are analyzed using six remote-sensing images from 1986 to 2011 and two nautical charts. Due to land reclamation in the period from 1986 to 2011, the total length of the PRE coastline increased by 149.2 km, which is equivalent to a growth of 0.57% per year, and the coastal land increased by 251.76     km 2 , which is equivalent to a growth of 0.23% per year; in addition, water depth change showed a trend in that foreshores became shallower, while deep channels became deeper. Areas where the coastline extended seaward had deposition to some extent, except for deepwater ports. Human activities played an important role in coastline change and erosion-accretion evolution in the PRE, which intervened with the natural variation of coastline and erosion-accretion. In addition, pollutants from the reclamation land became the major factors of coastal water pollution, which may significantly influence the environment of the PRE in a negative way.

SAR imaging of a topography-induced current front in a tidal channel

A quasi-linear dark-bright feature was observed on a TerraSAR-X synthetic aperture radar (SAR) image acquired in the area of Pearl River Estuary (PRE), China, on 25 October 2010. Examining the detailed local bathymetry chart, we find that the feature is collocated with major axis of Lingding Channel in the waterway of PRE. In the study, we first run a 3-D hydrodynamic model to simulate the tidal currents within PRE, and then used the simulated current and local wind data as input to run a radar simulation model to calculate the variation of normalized radar cross section induced by these parameters. Ocean model simulation shows that surface currents were parallel to the major axis of the channel at the satellite overpass time. Radar model simulation results show good agreement between the simulated and actual SAR images. The quasi-linear dark-bright feature on the SAR image was found to be due to the surface current convergence and divergence caused by the bathymetry-induced tidal current variation.

The brightness reversal of submarine sand waves in "HJ-1A/B" CCD sun glitter images

The brightness reversal of submarine sand waves appearing in the small satellite constellation for environment and disaster monitoring and forecasting (“HJ-1A/B”) CCD sun glitter images can affect the observation and depth inversion of sand wave topography. The simulations of the normalized sun glitter radiance on the submarine sand waves confirm that the reversal would happen at a specific sensor viewing angle, defined as the critical angle. The difference between the calculated critical angle position and the reversal position in the image is about 1′, which is excellent in agreement. Both the simulation and actual image show that sand wave crests would be indistinct at the reversal position, which may cause problems when using these sun glitter images to analyze spatial characteristics and migration of sand waves. When using the sun glitter image to obtain the depth inversion, one should take the advantage of image properties of sand waves and choose the location in between the reversal position and the brightest position. It is also necessary to pay attention to the brightness reversal when using “HJ-1A/B” CCD images to analyze other oceanic features, such as internal waves, oil slicks, eddies, and ship wakes.

Observation of sand waves in the Taiwan Banks using HJ-1A/1B sun glitter imagery

Abstract This study focuses on the large sand waves in the Taiwan Banks. Our goals are to observe the sand waves as completely as possible, to obtain their direction, wavelength, density, and ridge length, to analyze their spatial distributions, and to understand the effects of the current field and water depth on the sand waves. This study demonstrates the possibility of using HJ-1A/1B sun glitter imagery with a large swath width and rapid coverage in studying sand waves. Six cloud-free HJ-1A/1B optical images with sun glitter signals received during 2009 to 2011 were processed. The sand waves were mapped based on their features in the images; their direction, wavelength, density, and ridge length were measured and analyzed. We identified 4604 sand waves distributed in an area of 16,400     km 2 . The distributions of sand waves and their characteristics were analyzed, and the differences of sand waves between the northwestern subregion and the southeastern subregion are reported. Further analysis and discussion of the relationships between spatial distribution of the sand waves and both the tidal current field from a numerical simulation and water depth led to some interesting conclusions. The current field determines the orientation of the sand wave, while the hydrodynamic conditions and water depth influence the shape, size, and density of sand waves to a certain degree.

Coastal sea surface current observation with GOCI imagery

Measurements of sea surface currents are important in understanding the dynamic of ocean processes especially in coastal waters, and satellite remote sensing has been used to derive sea surface advective velocities with visible or infrared imagery. Yet satellite remote sensing suffered from lack of high temporal imagery in dynamic coastal waters and from frequent cloud cover. In this study, using measurements from the Geo-stationary Ocean Color Imager (GOCI), we observed the sea surface currents in the East China Sea on 29 May 2011 by applying the Maximum Cross-Correlation (MCC) technique. The hourly ocean color images on 29 May 2011 (8 images per day from 8:30 am to 15:30 pm) showed water pattern movement and evolution through the course of a day, and 7 sequence sea surface velocity fields were derived. The results show that there were significant diurnal changes of the sea surface currents in the coastal waters. The average current velocity was 30~40 cm/s during the measurements, with maximum velocity of 50~60 cm/s observed between 9:30 and 10:30 am. The short-term changes of the surface velocities are mainly a result of the horizontal dilution due to tides. The case study here demonstrates the unique value of a geostationary satellite ocean color sensor in revealing short-term dynamics in coastal waters.

Satellite observation of a red tide in the East China Sea during 2005

Red tides occurred frequently in the East China Sea in recent years. A red tide dominated by Prorocendrum donghaiense and Karenia mikimotoi happened in the Zhejiang Coastal Waters in late-May 2005. EOS MODIS water color data were utilized in extracting the red tide information. Sea surface temperature data derived from NOAA AVHRR and EOS MODIS were also analyzed to understand the possible formation mechanism of this red tide. The relationships between the red tide and related oceanographic features were discussed based on image data. The results indicate that the red tide was associated with several oceanographic processes, such as coastal front. This study showed that the combination of remote sensing data of water color and sea surface temperature can be useful in studying and understanding the oceanography of red tides.

Preliminary Analysis of Chinese GF-3 SAR Quad-Polarization Measurements to Extract Winds in Each Polarization

This study analyzed the noise equivalent sigma zero (NESZ) and ocean wind sensitivity for Chinese C-band Gaofen-3 (GF-3) quad-polarization synthetic aperture radar (SAR) measurements to facilitate further operational wind extraction from GF-3 data. Data from the GF-3 quad-polarization SAR and collocated winds from both NOAA/NCEP Global Forecast System (GFS) atmospheric model and National Data Buoy Center (NDBC) buoys were used in the analysis. For NESZ, the co-polarization was slightly higher compared to the cross-polarization. Regarding co-polarization and cross-polarization, NESZ was close to RadarSAT-2 and Sentinel-1 A. Wind sensitivity was analyzed by evaluating the dependence on winds in terms of normalized radar cross-sections (NRCS) and polarization combinations. The closest geophysical model function (GMF) and the polarization ratio (PR) model to GF-3 data were determined by comparing data and the model results. The dependence of co-polarized NRCS on wind speed and azimuth angle was consistent with the proposed GMF models. The combination of CMOD5 and CMOD5.N was considered to be the closest GMF in co-polarization. The cross-polarized NRCS exhibited a strong linear relationship with moderate wind speeds higher than 4 m·s−1, but a weak correlation with the azimuth angle. The proposed model was considered as the closest GMF in cross-polarization. For polarization combinations, PR and polarization difference (PD) were considered. PR increased only with the incidence angle, whereas PD increased with wind speed and varied with azimuth angle. There were three very close PR models and each can be considered as the closest. Preliminary results indicate that GF-3 quad-polarization data are valid and have the ability to extract winds in each polarization.

An Improved Local Gradient Method for Sea Surface Wind Direction Retrieval from SAR Imagery

Sea surface wind affects the fluxes of energy, mass and momentum between the atmosphere and ocean, and therefore regional and global weather and climate. With various satellite microwave sensors, sea surface wind can be measured with large spatial coverage in almost all-weather conditions, day or night. Like any other remote sensing measurements, sea surface wind measurement is also indirect. Therefore, it is important to develop appropriate wind speed and direction retrieval models for different types of microwave instruments. In this paper, a new sea surface wind direction retrieval method from synthetic aperture radar (SAR) imagery is developed. In the method, local gradients are computed in frequency domain by combining the operation of smoothing and computing local gradients in one step to simplify the process and avoid the difference approximation. This improved local gradients (ILG) method is compared with the traditional two-dimensional fast Fourier transform (2D FFT) method and local gradients (LG) method, using interpolating wind directions from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis data and the Cross-Calibrated Multi-Platform (CCMP) wind vector product. The sensitivities to the salt-and-pepper noise, the additive noise and the multiplicative noise are analyzed. The ILG method shows a better performance of retrieval wind directions than the other two methods.