Qiankun Zhu

Evaluation of the aerosol models for SeaWiFS and MODIS by AERONET data over open oceans

The operational atmospheric correction algorithm for Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) uses the predefined aerosol models to retrieve aerosol optical properties, and their accuracy depends on how well the aerosol models can represent the real aerosol optical properties. In this paper, we developed a method to evaluate the aerosol models (combined with the model selection methodology) by simulating the aerosol retrieval using the Aerosol Robotic Network (AERONET) data. Our method can evaluate the ability of aerosol models themselves, independent of the sensor performance. Two types of aerosol models for SeaWiFS and MODIS operational atmospheric correction algorithms are evaluated over global open oceans, namely the GW1994 models and Ahmad2010 models. The results show that GW1994 models significantly overestimate the aerosol optical thicknesses and underestimate the Ångström exponent, which is caused by the underestimation of the scattering phase function. However, Ahmad2010 models can significantly reduce the overestimation of the aerosol optical thickness and the underestimation of the Ångström exponent as a whole, but this improvement depends on the backscattering angle. Ahmad2010 models have a significant improvement in the retrieval of the aerosol optical thickness at a backscattering angle less than 140°. For a backscattering angle larger than 140°, GW1994 models are better at retrieving the aerosol optical thickness than the Ahmad2010 models.

In-orbit cross-calibration of HY-1A satellite sensor COCTS

The Chinese Ocean Color and Temperature Scanner (COCTS) is the main sensor launched in May of 2002 on the Chinese first ocean color satellite named HY-1A. In this paper, first, the properties of HY-1A and COCTS are introduced briefly. Second, the theory and methodology of cross-calibration for COCTS ocean color bands by using the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data are discussed in detail. Third, with the quasi-synchronized SeaWiFS data, we simulate the COCTS radiances at the top of atmosphere (TOA), and then obtain the calibration coefficients for COCTS ocean color bands. The results show that the cross-calibration TOA radiances are consistent with the in situ vicarious calibration TOA radiances at the COCTS ocean color bands. Finally, we compare the COCTS-retrieved water leaving radiances with SeaWiFS-retrieved ones and the results show that the cross-calibration method could provide reasonable accuracy for ocean color measurement.

Evaluation of Coastline Changes under Human Intervention Using Multi-Temporal High-Resolution Images: A Case Study of the Zhoushan Islands, China

Continued sea-level rise and coastal development have led to considerable concerns on coastline changes along inhabited islands. Analysis of long-term coastline changes of islands is however limited due to unavailable data and the cost of field work. In this study, high-resolution images taken from 1970–2011 at an interval of about 10 years and topographic maps were collected to determine coastline changes and their drivers in the Zhoushan Islands, China. Results show that nearly all inhabited islands appeared to have noteworthy seaward expansion during the past four decades. Coastline change rates varied among islands, and the annual change rate of Zhoushan Island (the main island) reached 12.83 ± 0.17 m/year during the same period. Since 2003, the study area has been dominated by artificial coast. The proportion of harbor/port and urban/industrial coast has significantly increased, while rocky coasts and shelter-farm coasts have shrunk greatly. Preliminary analysis of drivers for these coastline changes across the Zhoushan Islands highlights the roles of human policies during different periods as well as location, which were the dominant factors controlling the great spatial and temporal complexity of coastline changes of the major islands. Sediment supply from the Yangtze River decreased after the completion of the Three Gorges Dam in 2003; however, the Zhoushan coast rapidly accreted seaward during the last decade and the artificial siltation, coastal engineering, and harbor dredging materials could be responsible for the observed coastline changes. Pressured by rapid development of the port industry, the Zhoushan coast may face unprecedented challenges in coastal use in the near future. This research provides the basic background information for future studies on coastal protection and management.

Influence of bio-optical parameter variability on the reflectance peak position in the red band of algal bloom waters

article i nfo On the basis of field measurements, the quantitatively different relationships of peak position in the red band of the remote sensing reflectance vs. Chl concentration are found in the bloom waters of the diatom Skeletonema costatum and the dinoflagellate Prorocentrum donghaiense in coastal areas of the East China Sea. Model simulations of remote sensing reflectance, Rrs, accounting for the influence of variations in the bio-optical parameters such as chlorophyll fluorescence quantum efficiency, Φ, and specific absorption coef- ficient, aph , are carried out to analyze the characteristics of this spectral peak. The strong effect of fluorescence on the magnitude of Rrs results in the inhibition of the shift of the peak to longer wavelengths, increasing Φ enhances this effect. Increasing aph , specifically in the red-wavelength band, causes a sharper shift in the red peak position by decreasing the effect of the fluorescence. The dominant parameter governing the slope of the shift is aph . The analysis indicates that the higher aph of S. costatum in the red region is primarily respon- sible for the much higher slope of the peak shift than for that of P. donghaiense. We show that the relationship between the peak position and Chl concentration may be useful for discriminating S. costatum blooms from those due to P. donghaiense, although information about chlorophyll fluorescence quantum efficiency should be included. Finally, we show that using the band ratio Rrs(708 nm)/Rrs(665 nm) instead of Chl in the relation- ship with peak position can be useful for the practical identification of S. costatum blooms from hyperspectral measurements of remote sensing reflectance.

Optical detection of Prorocentrum donghaiense blooms based on multispectral reflectance

Prorocentrum donghaiense is one of the most common red tide causative dinoflagellates in the Changjiang (Yangtze) River Estuary and the adjacent area of the East China Sea. It causes large-scale blooms in late spring and early summer that lead to widespread ecologic and economic damage. A means for distinguishing dinoflagellate blooms from diatom(Skeletonema costatum) blooms is desired. On the basis of measurements of remote sensing reflectance [Rrs(λ)] and inherent optical parameters, the potential of using a multispectral approach is assessed for discriminating the algal blooms due to P. donghaiense from those due to S. costatum. The behavior of two reflectance ratios [R1 = Rrs(560)/Rrs (532) and R2 = Rrs(708)/Rrs(665)], suggests that differentiation of P. donghaiense blooms from diatom bloom types is possible from the current band setup of ocean color sensors. It is found that there are two reflectance ratio regimes that indicate a bloom is dominated by P. donghaiense: (1) R1 > 1.55 and R2 < 1.0 or (2) R1 > 1.75 and R2 ⩾ 1.0. Various sensitivity analyses are conducted to investigate the effects of the variation in varying levels of chlorophyll concentration and colored dissolved organicmatter (CDOM) as well as changes in the backscattering ratio (bbp/bp) on the efficacy of this multispectral approach. Results indicate that the intensity and inherent optical properties of the algal species explain much of the behavior of the two ratios. Although backscattering influences the amplitude of Rrs(λ), especially in the 530 and 560 nm bands, the discrimination between P. donghaiense and diatoms is not significantly affected by the variation of bbp/bp. Since aCDOM(440) in coastal areas of the ECS is typically lower than 1.0 m−1 in most situations, the presence of CDOM does not interfere with this discrimination, even as SCDOM varies from 0.01 to 0.026 nm−1. Despite all of these effects, the discrimination of P. donghaiense blooms from diatom blooms based on multispectral measurements of Rrs(λ) is feasible.

The extremely high concentration of suspended particulate matter in Changjiang Estuary detected by MERIS data

The Changjiang River is the third largest river of the annual flux around the world, which has a great impact on the ecosystem of the East China Sea and adjacent areas. Because of the shallow water, tide mixing and the runoff of the Changjiang River and Qiantang River, the suspended particulate matter (SPM) concentration is extremely high in the Changjiang Estuary, which is ever up to 2000mg/L. Due to the large water-leaving radiance at the near-infrared wavelength, the operational atmospheric correction algorithm for the open ocean can not be applied to this region, and the existing remote sensing algorithms for SPM may not be applicable for this extremely high turbidity waters. In this paper, we firstly apply the blue wavelength atmospheric correction algorithm to MERIS Level-1 data to get the reasonable spectral water-leaving radiances in the Changjiang Estuary. Based on the winter cruise data in 2007, a regional SPM algorithm was developed using the bands ratio of the normalized water-leaving radiances between 779nm and 560nm. This algorithm was validated by the summer cruise data in 2006, and the results show that the performance of the algorithm was very well, and there was good agreement between the retrieved data and in-situ measured concentrations of the SPM in the Changjiang Estuary, with the correlation coefficient of 0.98 in the logarithm form and the averaged absolute relative error of 27.2%, and the standard deviation of 20.8mg/l in the linear form. Finally, the seasonal variations of the SPM in the Changjiang Estuary were analyzed by the MERIS SPM maps retrieved by the algorithms developed in this paper.

Remote Sensing of Sea Surface pCO(2) in the Bering Sea in Summer Based on a Mechanistic Semi-Analytical Algorithm (MeSAA)

The Bering Sea, one of the largest and most productive marginal seas, is a crucial carbon sink for the marine carbonate system. However, restricted by the tough observation conditions, few underway datasets of sea surface partial pressure of carbon dioxide (pCO2) have been obtained, with most of them in the eastern areas. Satellite remote sensing data can provide valuable information covered by a large area synchronously with high temporal resolution for assessments of pCO2 that subsequently allow quantification of air-sea carbon dioxide 2 flux. However, pCO2 in the Bering Sea is controlled by multiple factors and thus it is hard to develop a remote sensing algorithm with empirical regression methods. In this paper pCO2 in the Bering Sea from July to September was derived based on a mechanistic semi-analytical algorithm (MeSAA). It was assumed that the observed pCO2 can be analytically expressed as the sum of individual components controlled by major factors. First, a reference water mass that was minimally influenced by biology and mixing was identified in the central basin, and then thermodynamic and biological effects were parameterized for the entire area. Finally, we estimated pCO2 with satellite temperature and chlorophyll data. Satellite results agreed well with the underway observations. Our study suggested that throughout the Bering Sea the biological effect on pCO2 was more than twice as important as temperature, and contributions of other effects were relatively small. Furthermore, satellite observations demonstrate that the spring phytoplankton bloom had a delayed effect on summer pCO2 but that the influence of this biological event varied regionally; it was more significant on the continental slope, with a later bloom, than that on the shelf with an early bloom. Overall, the MeSAA algorithm was not only able to estimate pCO2 in the Bering Sea for the first time, but also provided a quantitative analysis of the contribution of various processes that influence pCO2.

Assessment of satellite-based chlorophyll-a retrieval algorithms for high solar zenith angle conditions

Abstract. Numerous empirical algorithms have been operationally used to retrieve global ocean chlorophyll-a (Chla) concentrations from ocean color satellite data, for example, the OC4V4 algorithm for sea-viewing wide field-of-view sensor and the OC3M algorithm for moderate-resolution imaging spectroradiometer. However, the algorithms have been established and validated based on in situ data mainly measured under low to moderate solar zenith angle (SZA) (<70  deg). Currently, with the development of geostationary satellite ocean color remote sensing, enabling observations from early morning to late afternoon, it has become necessary to know whether the empirical Chla algorithms can be applied to high SZA. The performances of seven widely used Chla algorithms (i.e., OC2, OC3M, OC3V, OC4V4, Clark, ocean-color index, and Yellow Sea Large Marine Ecosystem Ocean Color Work Group) under high SZAs were evaluated using the global in situ ocean color dataset (NASA bio-optical marine algorithm dataset). The results show that the performances of all seven algorithms decreased significantly under high SZAs compared with those under low to moderate SZAs. For instance, for the OC4V4 algorithm, the relative percent difference (RPD) and root-mean-square error (RMSE) were 13.78% and 1.66  μg/L for the whole dataset and 3.95% and 1.49  μg/L for SZAs ranging from 30 deg to 40 deg, respectively. However, RPD and RMSE values increased to 30.45% and 6.10  μg/L for SZAs larger than 70 deg. Adjusting the coefficients of the algorithms using the in situ dataset with high SZA can only slightly improve the performance. The bidirectional remote sensing reflectance can explain the underestimation of the retrieved Chla under high SZA, and the low sensitivity of the blue–green ratio is responsible for the relatively larger scatter between the retrieved and the in situ Chla under high SZA.

A dynamic sediment model based on satellite‐measured concentration of the surface suspended matter in the East China Sea

The concentration of total suspended matter (TSM) at the sea surface is derived from satellite data using a complex proxy TSM model in East China Sea from 1997 to 2008. The structure of the mean TSM image is similar to that of the topography, indicating that the distribution of the surface concentration is strongly related to the water depth. A dynamic sediment model (DSM) is constructed to relate the TSM concentration at the sea surface with suspended sediment at the benthic boundary layer, the Rouse number, and the water depth. The DSM model is improved through iteration with a convergence identified by the mean relative difference between two adjacent bottom TSM images which becomes smaller with the more iterations and the value is less than 1% after 50 iterations. The performance of the DSM model is validated by satellite-measured concentration with a mean relative error of 5.2% for the monthly mean images. The DSM model is used to deduce the bottom TSM concentration at the benthic boundary layer and the distribution of the Rouse number. The spatial distribution of the sea surface TSM concentration is determined predominately by both the bottom suspended sediment concentration and water depth. The temporal variation of the sea surface concentration mainly depends upon the Rouse number in the water column. Our result shows that the discharge of the Changjiang River can change the distribution of the Rouse number to form a band-shaped region in the Changjiang Estuary. The DSM model provides a framework for understanding some of the mechanisms of the formation and variation of the primary TSM plume and the secondary plume in the ECS. The primary TSM plume corresponds approximately to the region with depth shallower than 20 m and the secondary plume corresponds to the region with depths between 20 and 50 m.

Seasonal and interannual variability of sea surface wind over the China seas and its adjacent ocean from QuikSCAT and ASCAT data during 2000-2011

A total of 12 years (2000–2011) of sea surface wind data from QuikSCAT and ASCAT are employed to examine the seasonal and interannual variability over the China Seas and its adjacent ocean. The wind speed and direction, strong wind frequency, monsoon onset and transition time are revealed by statistic analysis. The result shows that (1) the wind speed is highest in winter and lowest in summer in the study area. The Southeastern Vietnam, Luzon Strait, Taiwan Strait and the eastern ocean of Japan are the regions with the higher wind speed throughout the year, especially more than 12 m/s in winter. (2) The monsoon prevails in the China Seas, southwesterly in summer and northeasterly in winter, while southeast wind also occurs all the seasons except winter. The northwest wind only occurs in the north of 27°N in winter. (3)The strong wind more than 10.8 m/s exceeds 140 days per year in the Taiwan Strait while only 30 days in the Bohai Sea and Yellow Sea. In addition, the Luzon Strait can reach 120 days with strong wind larger than 10.8 m/s, ranking the second after the Taiwan Strait. (4) The time of monsoon onset changes from year to year. Generally, the southwest wind in the low latitudes begins in May and ends in September or October, then turns to northwest wind completely which almost have no transition time. For the mid-latitudes zone, the southwest wind begins in June and ends in August. May and September are the transition time. Then the northeast wind begins to prevail. (5) The trend of sea surface wind speed in the study area is increasing from 2000 to 2011, except the Southeastern Vietnam. The annual mean wind speed is lowest in 2002 and 2010 during the period of. 2000–2011