Aiqin Shi

Coastally trapped atmospheric gravity waves on SAR, AVHRR and MODIS images

Alternative dark–bright patterns on two ENVISAT Advanced Synthetic Aperture Radar (ASAR) images of the east coast of the Korean Peninsula acquired on 18 and 19 May 2004 are interpreted as atmospheric gravity waves (AGWs) on the basis of simultaneous multi‐satellite observations and atmospheric gravity wave theory. The AGWs appeared in the form of a wave packet containing several waves located between 50 and 200 km offshore. The wavelengths were ranging from 13 to 20 km. The lengths of AGW crests were from 20 to 150 km. An NOAA‐17 pass was received about 30 min after the ASAR pass. Its channel 4 infrared (IR) image clearly shows wave‐like moisture patterns. However, the sea surface temperature (SST) image derived after applying nonlinear calibration and split‐window atmospheric correction shows no wave patterns. A daytime true‐colour MODIS image taken about 14 h later still shows a cloud band of same AGWs, indicating the lifespan of the standing AGWs can be over half a day. Although oceanic internal waves (IWs) may also cause similar wave patterns imaged by spaceborne SAR as they modulate the ocean surface roughness, we provide evidence to eliminate this possibility in this case. The characteristics of satellite observed AGWs are in good agreement with these simulated by a linear coastal AGW model.

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 u2009 u2009 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.

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 u2009 u2009 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.

Observation of submarine sand waves using ASTER stereo sun glitter imagery

Signatures of sun glitter images strongly depend on the viewing angle. Stereo observation can provide more valuable information than observation from one angle. In this work, the sun glitter patterns caused by submarine sand waves were studied using stereo images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), which operates a high spatial resolution (15 m) optical and near-infrared sensor on board the Terra satellite. The previous imaging geometrical model has been improved, and the difference in the viewing angle among pixels is considered. We found that brightness reversal occurs in some stereo imagery of submarine sand waves. Based on the imaging geometry model, an interaction model of current topography, and a sun glitter radiance transfer model, a simulation model was developed for sun glitter from submarine sand waves at multiple viewing angles. The cases of nadir-looking and backward-looking were simulated with the model. The results show the following differences between the two viewing angles, which have also been observed in the ASTER images. Both tendency and extent of the simulated radiance are in good agreement with those in actual images. In the nadir-looking view, obvious differences have been observed between the normalized significant radiance from smooth and rough facets. This difference increases with increasing viewing angle, to a peak, and then decreases. However, in most cases, the difference in the back view is smaller and shows an opposite tendency regarding the viewing angle from the nadir-looking view. Thus, the sand-wave characteristics in the nadir-looking images seem to be more enhanced than that in the backward-looking images.

Bathymetric mapping of submarine sand waves using multiangle sun glitter imagery: a case of the Taiwan Banks with ASTER stereo imagery

Abstract. Submarine sand waves are visible in optical sun glitter remote sensing images and multiangle observations can provide valuable information. We present a method for bathymetric mapping of submarine sand waves using multiangle sun glitter information from Advanced Spaceborne Thermal Emission and Reflection Radiometer stereo imagery. Based on a multiangle image geometry model and a sun glitter radiance transfer model, sea surface roughness is derived using multiangle sun glitter images. These results are then used for water depth inversions based on the Alpers–Hennings model, supported by a few true depth data points (sounding data). Case study results show that the inversion and true depths match well, with high-correlation coefficients and root-mean-square errors from 1.45 to 2.46 m, and relative errors from 5.48% to 8.12%. The proposed method has some advantages over previous methods in that it requires fewer true depth data points, it does not require environmental parameters or knowledge of sand-wave morphology, and it is relatively simple to operate. On this basis, we conclude that this method is effective in mapping submarine sand waves and we anticipate that it will also be applicable to other similar topography types.

Whitecap features induced by submarine sand waves in stereo optical imagery

Under high wind speed conditions, waves break on the sea surface producing whitecaps, which can be recorded in optical satellite images. However, the breaking of waves is also influenced by submarine topography. In this study, we investigate the whitecap features induced by submarine sand waves using optical stereo images. We determine the whitecap coverage in images, and estimate the probabilities of wave breaking. The observed and estimated results are in agreement with some previous investigation results. A physical model is used to discuss the mechanisms of wave breaking along sand wave crests. The physical model is able to qualitatively explain the differences between the observed whitecap coverage and the estimated probability of wave breaking between the sand wave area (SWA) and the background area (BGA). Further analysis shows that the periodic sand wave system may play the role of a frequency-control device, and may result in more broken waves in the SWA than in the BGA. This study demonstrates that it is possible to observe submarine topography under high-wind conditions by using whitecap features.

Upwelling region wind speed correction method for wind retrieval from synthetic aperture radar imagery in the East China Sea

The satellite-borne synthetic aperture radar (SAR) has been proven to be a valuable tool for high resolution ocean surface wind measurements. However, oceanic surface phenomena observed by SAR and oceanic processes which can cause the change of backscatter in SAR imagery will influence the SAR wind retrieval. Upwelling is one of the main factors, and it is prevalent in the East China Sea. It smooths the sea surface which results in a lower backscatter cross section in SAR imagery. In this study, using sea surface temperature and chlorophyll2-a data derived from Earth Observing System (EOS) MODIS, the low backscatter features in ENVISAT advanced synthetic aperture radar (ASAR) imagery are analyzed. A CMOD4 algorithm is adopted to retrieve the sea surface wind speed from SAR imagery. Results show that the wind speed is negatively biased due to the low normalized radar cross section associated with the upwelling. In order to resolve the impact of coastal upwelling on SAR wind retrieval, combined with high resolution numerical meteorological model wind field data, a wind speed correction method is proposed by using linear robust regression. To demonstrate the applicability of this method, underestimated wind speeds retrieved from ENVISAT ASAR images in the upwelling areas of Zhejiang coast and the sea area near northeast of Taiwan are corrected. Results show that the accuracy of upwelling region SAR wind retrieval data has been improved.

Wetland landscape pattern analysis with remote sensing images in Ximen Island special marine protected area

This paper focuses on the wetland of Ximen Island special marine protected areas in Yueqing Bay, Zhejiang, China. In this paper, four remote sensing images from Landsat-7, SPOT-4, SPOT-5 and WorldView-2 satellites are collected. These images are used for wetland investigation and analysis. Wetland information of island and tidal flat is derived from the remote sensing images. Wetland in island includes aquaculture water, pond water, paddy fields and reservoirs. Tide wetland includes vegetation areas, breeding areas, mud tide flat and water. The results mainly showed that the area of island wetland is 1,281,973.04 square meters, accounting for 18.09% of the whole island area, and that the mangroves communities distribute along the coast of Ximen Island.

Remote sensing monitoring of coastal change in Tangshan with Landsat imagery

Coastal zone is the interaction area between the ocean and the land, and it is one of the most important residential areas of human. Coastal area management and planning is necessary in utilizing coastal space and resources. Coastal zone changed rapidly in recent decades in Tangshan, China. In this research, a total of 11 Landsat images were selected for studying the coastal change in Tangshan during the last 35 years. Results showed that the coastline length increased by 114.05 km, while land area increased by 449.76km2 from 1975 to 2010. The main period of coastal increasing in Tangshan occurred during 2005–2010, and the primary area changes happened in Caofeidian District and Jingtang Port. The main reason of the rapid coastal changes in Tangshan was the human activities of industrial and commercial district construction and harbor construction.

Simulation and analysis on SAR imaging of channel topography changes in the Pearl River Estuary

According to SAR imaging mechanism of underwater bathymetry, a 3-D hydrodynamic model based on SELFE (Semi-implicit Eulerian-Lagrangian Finite Element) is applied to demonstrate the temporal variations of the channel bathymetry changes. A microwave radar imaging of oceanic surface’s program of M4S is used to simulate the variation of normalized radar cross section (NRCS) induced by the ocean surface current. The simulation is carried out to study the effect of water depth changes and current variations on SAR imaging of channel bathymetry. The depth of channel is defined between 5 m to 17 m with an interval of every two meters. Six time of tide are used in the current variation simulation. The NRCS variation demonstrated in the simulated images are compared with in situ data, historical bathymetric maps and calibrated SAR images. All the four kinds of images manifest similar changes of channels, which proves that there is agreement between the simulation model used in this paper and other data. Simulation results also show that bigger depth change induces bigger NRCS variation. In the time of maximum ebb tide, simulated relative NRCS is bigger than others.