Fenfen Liu

Retrieval of suspended sediment concentration in the Pearl River Estuary from MERIS using support vector machines

With the rapid industrialization and urbanization, more and more solid have been emitted into the Pearl River Estuary. The suspended sediment concentration is one of the most important water quality parameters. With in-situ optical data and suspended sediment data collected on four cruises from 2004 to 2006 in the Pearl River Estuary, analysis shows that with the increasing of the total suspended sediment (TSM) concentration, the intensive bands which have the best correlation relationship with the TSM concentration shift from Rrs(620) to Rrs(778). When the mean suspended concentration is 14.5 g.m−3, the Rrs(620) has best correlation with the suspended concentration. However, when the mean suspended concentration becomes more than 40g.m−3, the most correlated band shifts to 778nm. It seems that all of the Rrs(620), Rrs(665), Rrs(681), Rrs(708), Rrs(753), Rrs(760), Rrs(778) may be the most sensitive band for the different TSM concentration. This work investigates the possibility of using a new universal approximator-support vector machines (SVMs)-as the nonlinear transfer function between TSM concentration and remote sensing reflectance in the Pearl River Estuary. Experimental results show that the SVM performs better result than general empirical algorithms or the piecewise algorithm. The correlation coefficient between the in-situ and modeled TSM of the test dataset is 0.91 and the root mean squared error (RMSE) is 0.145. The algorithm based on the SVM is applied to MERIS satellite data in January 31, 2007. The distribution of TSM concentration was obtained and it shows that the algorithm could be a useful tool for the study of TSM distribution in Pearl River estuary.

Retrieval of chlorophyll a concentration from a fluorescence enveloped area using hyperspectral data

It is acknowledged that fluorescence line height (FLH) algorithms are still hampered by the uncertainty of fluorescence peak position. The fluorescence peak moves to longer wavelengths with the increase of chlorophyll a concentration. In this article, the fluorescence enveloped area (FEA), which integrates the fluorescence height and the fluorescence peak position, was used to estimate the chlorophyll a concentration in the coastal waters of the Pearl River Estuary. The FEA algorithm was developed from in situ data of chlorophyll a concentration, total suspended matter (TSM) concentration and above-water remote sensing reflectance, which were collected at 37 sampling stations in the Pearl River Estuary during two cruises. The results showed that the FEA algorithm made a better estimation of chlorophyll a concentration compared with the widely used FLH algorithm and moving fluorescence line height (MFLH) algorithm. These three algorithms were applied to the Pearl River Estuary for retrieval of chlorophyll a concentration from Hyperion data acquired on 21 December 2006. Compared with the FLH and the MFLH, the FEA algorithm showed a rational distribution of the chlorophyll a concentration in the Pearl River Estuary.

Remote sensing assessment of sediment variation in the Pearl River Estuary induced by Typhoon Vicente

A MODIS-based algorithm was developed to investigate the impact of Typhoon Vicente on the total suspended solid concentration in the Pearl River Estuary. This algorithm used two high resolution bands at 645 nm and 555 nm to map the concentrations. Regression between the remote sensing reflectance and in situ total suspended solid concentration showed good correlation (R2 = 0.91), indicating that the algorithm was valid for the high turbid waters in the Pearl River Estuary. MODIS-derived maps showed different total suspended solid concentration anomalies in different sub-regions of the estuary during the passage of Typhoon Vicente. In western inlets, the increase of total suspended solids (maximum values of 22.20 g m−3 before and 55.71 g m−3 after the typhoon) was possibly related to the larger rainfall discharge from the Pearl River. In Lingdingyang, the increase of total suspended solids (maximum values of 19.60 g m−3 before and 44.59 g m−3 after the typhoon) might be the result of typhoon-induced resuspension. In the southeastern portion of the Estuary, due to the typhoon-induced current, a significant decrease of total suspended solid area (decreased by 10 g m−3) was observed. Different changes were observed in different sub-regions under the influence of the typhoon, implying the complicated hydrological environment is an important feature in the Pearl River Estuary.

Remote sensing study of the seasonal distribution of phytoplankton groups in the South China Sea

Distribution of phytoplankton groups in the South China Sea (SCS) is retrieved from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data using a classification method named PHYSAT. Four major phytoplankton groups including haptophytes, Prochlorococcus, Synechococcus-like cyanobacteria and diatoms are distinguished. It is found that the Prochlorococcus and Synechococcus-like cyanobacteria are the dominant species over the deep basin in the SCS, while the haptophytes and diatoms prevail along the coastal area, which is consistent with the previous reports. We also compare monthly distribution of phytoplankton groups. From November to February, Prochlorococcus is the dominant over the entire SCS. The haptophytes is the dominant species along the coast of China. The dominant species is diatoms along the southeast of Vietnam. From March to June, both Prochlorococcus and Synechococcus-like cyanobacteria cover the entire SCS and the area dominated by haptophytes decreases. Instead of haptophytes, the diatoms become the dominant phytoplankton group along the coast of China. Large patches of Synechococcus-like cyanobacteria cover continental slope of the northwestern SCS during July and August. Haptophytes and Prochlorococcus accumulate in the east of Vietnam in August. The Synechococcus-like cyanobacteria starts to extend towards the southern of SCS during September.

Remotely sensed study of air-sea CO 2 fluxes variability in the Northern South China Sea

The CO2 source or sink status and variability in the coastal ocean are the subjects of highly debate. Satellite remote sensing offers an avenue for expanding observations and analyzing temporal and spatial variability of the environment of the ocean. In the present study, the algorithm for surface layer seawater partial pressure of CO2 (pCO2sw) of the northern South China Sea (NSCS) was applied to remote sensing products and the air-sea CO2 fluxes were calculated from salinity, wind speed corrected to 10 meters, sea surface temperature data obtained from remote sensing in the NSCS, for the period 2004–2007. The CO2 source and sink status in the NSCS were analysed and showed significant variability in time and space. In winter, the pCO2sw in most of the NSCS ranged from ~320–360µatm, indicative of undersaturation with respect to atmospheric partial pressure of CO2 (pCO2air). However, in summer, the pCO2sw increased to ~400–420µatm mainly due to sea surface temperature. The air-sea CO2 fluxes in summer have been increasing year after year which have the same trend with the pCO2sw. These upswings correlated with a rise in sea surface temperatures, showing that sea surface temperatures played key role in the CO2 source and sink status in NSCS. In regions of the Luzon Strait, the air-sea CO2 flux remained generally high likely associated with the remineralization of organic matter. The pCO2sw highly undersaturation conditions occurred on the western part of the Taiwan Strait in winter, which may be caused by the low temperature and low salinity China coastal current entering the Taiwan Strait driven by the northeast monsoon. The research showed that most of the NSCS represents weak sinks for atmospheric CO2 in winter and sources for atmospheric CO2 in summer.

Atmospheric Correction of the Hyperion Imagery for Turbid Estuary Water

Atmospheric correction of Hyperion imagery over aquatic environments is generally more demanding than over land because the signal from the water column is small. In this paper, an atmospheric correction algorithm designed for the Hyperion hyper spectral imagery based on the analysis of the atmospheric correction algorithm of FLAASH and the algorithm adopted by MODIS imagery. It makes use of band 111 and band 149 of the Hyperion hyper-spectral imagery to obtain the type of aerosol, then applied it to the near-infrared and visible light wave bands, thereby the atmospheric correction of the whole imagery is realized. The Hyperion hyper-spectral imagery applied the algorithm referred above is suitable to monitor the turbid estuary water.