2017 Vol. 36, No. 11
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2017, 36(11): .
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2017, 36(11): 4-13.
doi: 10.1007/s13131-017-1120-1
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A new data set of observations by six cruises of ship-mounted acoustic doppler current profiler (SADCP) and three 40 d long bottom-mounted ADCPs (BADCPs) is employed to reveal the spatiotemporal variability of tidal and subtidal currents in the western Taiwan Strait (TWS) during winter season. The results confirm the existence of intense cotidal lines for M2 tidal current, which is located north of 25°N. In this case, no existence of an amphidromic point can be identified. It is also revealed that the counter-wind current (CWC) can extend through the whole western TWS and even occupy the entire water column during winter monsoon relaxation. However, this CWC is observed to be thoroughly overwhelmed by the downwind China coastal current (CCC) during the two big monsoon bloom events in the winter of 2007, and the CCC consequently extends southward throughout the western TWS instead. Most importantly, the variation of the spatial extent for the CWC and the CCC in the western TWS is found to be well explained by the first two modes of the vector empirical orthogonal function (VEOF) analysis, that is, it is mainly controlled by a wind-driven quasi barotropic current as the first mode and slightly modulated by a relatively weak background current with a first-order baroclinic structure as the second mode.
A new data set of observations by six cruises of ship-mounted acoustic doppler current profiler (SADCP) and three 40 d long bottom-mounted ADCPs (BADCPs) is employed to reveal the spatiotemporal variability of tidal and subtidal currents in the western Taiwan Strait (TWS) during winter season. The results confirm the existence of intense cotidal lines for M2 tidal current, which is located north of 25°N. In this case, no existence of an amphidromic point can be identified. It is also revealed that the counter-wind current (CWC) can extend through the whole western TWS and even occupy the entire water column during winter monsoon relaxation. However, this CWC is observed to be thoroughly overwhelmed by the downwind China coastal current (CCC) during the two big monsoon bloom events in the winter of 2007, and the CCC consequently extends southward throughout the western TWS instead. Most importantly, the variation of the spatial extent for the CWC and the CCC in the western TWS is found to be well explained by the first two modes of the vector empirical orthogonal function (VEOF) analysis, that is, it is mainly controlled by a wind-driven quasi barotropic current as the first mode and slightly modulated by a relatively weak background current with a first-order baroclinic structure as the second mode.
2017, 36(11): 14-20.
doi: 10.1007/s13131-017-1134-8
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A three-dimensional (3-D) ocean model is coupled with a two-dimensional (2-D) sea ice model, to revisit a nonlinear advection mechanism, one of the most important mesoscale eddy genesis mechanisms in the marginal ice zone. Two-dimensional ocean model simulations suggest nonlinear advection mechanism is more important when the water gets shallower. Instead of considering the ocean as barotropic fluid in the 2-D ocean model, the 3-D ocean model allows the sea ice to affect the current directly in the surface layer via ocean-ice interaction. It is found that both mesoscale eddy and sea surface elevation are sensitive to changes in a water depth in the 3-D simulations. The vertical profile of a current velocity in 3-D experiments suggests that when the water depth gets shallower, the current move faster in each layer, which makes the sea surface elevation be nearly inverse proportional to the water depth with the same wind forcing during the same time. It is also found that because of the vertical motion, the magnitude of variations in the sea surface elevation in the 3-D simulations is very small, being only 1% of the change in the 2-D simulations. And it seems the vertical motion to be the essential reason for the differences between the 3-D and 2-D experiments.
A three-dimensional (3-D) ocean model is coupled with a two-dimensional (2-D) sea ice model, to revisit a nonlinear advection mechanism, one of the most important mesoscale eddy genesis mechanisms in the marginal ice zone. Two-dimensional ocean model simulations suggest nonlinear advection mechanism is more important when the water gets shallower. Instead of considering the ocean as barotropic fluid in the 2-D ocean model, the 3-D ocean model allows the sea ice to affect the current directly in the surface layer via ocean-ice interaction. It is found that both mesoscale eddy and sea surface elevation are sensitive to changes in a water depth in the 3-D simulations. The vertical profile of a current velocity in 3-D experiments suggests that when the water depth gets shallower, the current move faster in each layer, which makes the sea surface elevation be nearly inverse proportional to the water depth with the same wind forcing during the same time. It is also found that because of the vertical motion, the magnitude of variations in the sea surface elevation in the 3-D simulations is very small, being only 1% of the change in the 2-D simulations. And it seems the vertical motion to be the essential reason for the differences between the 3-D and 2-D experiments.
2017, 36(11): 21-26.
doi: 10.1007/s13131-017-1135-7
Abstract:
An operational three-dimensional oil spill model is developed by the National Marine Environmental Forecasting Center (NMEFC), State Oceanic Administration, China, and the model has been running for 9 a. On June 4 and 17, 2011, oil is spilled into the sea water from two separate oil platforms in the Bohai Bay, i.e., Platforms B and C of Penglai 19-3 oilfield. The spill causes pollution of thousands of square kilometres of sea area. The NMEFC's oil spill model is employed to study the Penglai 19-3 oil-spill pollution during June to August 2011. The wind final analysis data of the NMEFC, which is based on a weather research and forecasting (WRF) model, are analyzed and corrected by comparing with the observation data. A corrected current filed is obtained by forcing the princeton ocean model (POM) with the corrected wind field. With the above marine environmental field forcing the oil spill model, the oil mass balance and oil distribution can be produced. The simulation is validated against the observation, and it is concluded that the oil spill model of the NMEFC is able to commendably simulate the oil spill distribution. Thus the NMEFC's oil spill model can provide a tool in an environmental impact assessment after the event.
An operational three-dimensional oil spill model is developed by the National Marine Environmental Forecasting Center (NMEFC), State Oceanic Administration, China, and the model has been running for 9 a. On June 4 and 17, 2011, oil is spilled into the sea water from two separate oil platforms in the Bohai Bay, i.e., Platforms B and C of Penglai 19-3 oilfield. The spill causes pollution of thousands of square kilometres of sea area. The NMEFC's oil spill model is employed to study the Penglai 19-3 oil-spill pollution during June to August 2011. The wind final analysis data of the NMEFC, which is based on a weather research and forecasting (WRF) model, are analyzed and corrected by comparing with the observation data. A corrected current filed is obtained by forcing the princeton ocean model (POM) with the corrected wind field. With the above marine environmental field forcing the oil spill model, the oil mass balance and oil distribution can be produced. The simulation is validated against the observation, and it is concluded that the oil spill model of the NMEFC is able to commendably simulate the oil spill distribution. Thus the NMEFC's oil spill model can provide a tool in an environmental impact assessment after the event.
2017, 36(11): 27-37.
doi: 10.1007/s13131-017-1136-6
Abstract:
The eddy tracking approach is developed using the global nearest neighbor filter (GNNF) to investigate the evolution processes and behaviors of mesoscale eddies in the South China Sea (SCS). Combining the Kalman filter and optimal data association technologies, the GNNF algorithm is able to reduce pairing errors to 0.2% in tracking synthetic eddy tracks, outperforming other existing methods. A total of 4 913 eddy tracks that last more than a week are obtained by the GNNF during 1993-2012. The analysis of a growth and a decay based on 3 445 simple eddy tracks show that eddy radius, amplitude, and vorticity smoothly increase during the first half of lifetime and decline during the second half following a parabola opening downwards. The genesis of eddies mainly clusters northwest and southwest of Luzon Island whereas the dissipations concentrate the Xisha Islands where the underwater bay traps and terminates eddies. West of the Luzon Strait, northwest of Luzon Island, and southeast of Vietnam are regions where eddy splits and mergers are frequently observed. Short disappearances mainly distribute in the first two regions. Moreover, eddy splits generally result in a decrease of the radius and the amplitude whereas eddy mergers induce growing up. Eddy intensity and vorticity, on the contrary, are strengthened in the eddy splits and diminished in mergers.
The eddy tracking approach is developed using the global nearest neighbor filter (GNNF) to investigate the evolution processes and behaviors of mesoscale eddies in the South China Sea (SCS). Combining the Kalman filter and optimal data association technologies, the GNNF algorithm is able to reduce pairing errors to 0.2% in tracking synthetic eddy tracks, outperforming other existing methods. A total of 4 913 eddy tracks that last more than a week are obtained by the GNNF during 1993-2012. The analysis of a growth and a decay based on 3 445 simple eddy tracks show that eddy radius, amplitude, and vorticity smoothly increase during the first half of lifetime and decline during the second half following a parabola opening downwards. The genesis of eddies mainly clusters northwest and southwest of Luzon Island whereas the dissipations concentrate the Xisha Islands where the underwater bay traps and terminates eddies. West of the Luzon Strait, northwest of Luzon Island, and southeast of Vietnam are regions where eddy splits and mergers are frequently observed. Short disappearances mainly distribute in the first two regions. Moreover, eddy splits generally result in a decrease of the radius and the amplitude whereas eddy mergers induce growing up. Eddy intensity and vorticity, on the contrary, are strengthened in the eddy splits and diminished in mergers.
2017, 36(11): 38-50.
doi: 10.1007/s13131-017-1073-4
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The seasonal variability of the significant wave height (SWH) in the South China Sea (SCS) is investigated using the most up-to-date gridded daily altimeter data for the period of September 2009 to August 2015. The results indicate that the SWH shows a uniform seasonal variation in the whole SCS, with its maxima occurring in December/January and minima in May. Throughout the year, the SWH in the SCS is the largest around Luzon Strait (LS) and then gradually decreases southward across the basin. The surface wind speed has a similar seasonal variation, but with different spatial distributions in most months of the year. Further analysis indicates that the observed SWH variations are dominated by swell. The wind sea height, however, is much smaller. It is the the largest in two regions southwest of Taiwan Island and southeast of Vietnam Coast during the northeasterly monsoon, while the largest in the central/southern SCS during the southwesterly monsoon. The extreme wave condition also experiences a significant seasonal variation. In most regions of the northern and central SCS, the maxima of the 99th percentile SWH that are larger than the SWH theoretically calculated with the wind speed for the fully developed seas mainly appear in August–November, closely related to strong tropical cyclone activities. Compared with previous studies, it is also implied that the wave climate in the Pacific Ocean plays an important role in the wave climate variations in the SCS.
The seasonal variability of the significant wave height (SWH) in the South China Sea (SCS) is investigated using the most up-to-date gridded daily altimeter data for the period of September 2009 to August 2015. The results indicate that the SWH shows a uniform seasonal variation in the whole SCS, with its maxima occurring in December/January and minima in May. Throughout the year, the SWH in the SCS is the largest around Luzon Strait (LS) and then gradually decreases southward across the basin. The surface wind speed has a similar seasonal variation, but with different spatial distributions in most months of the year. Further analysis indicates that the observed SWH variations are dominated by swell. The wind sea height, however, is much smaller. It is the the largest in two regions southwest of Taiwan Island and southeast of Vietnam Coast during the northeasterly monsoon, while the largest in the central/southern SCS during the southwesterly monsoon. The extreme wave condition also experiences a significant seasonal variation. In most regions of the northern and central SCS, the maxima of the 99th percentile SWH that are larger than the SWH theoretically calculated with the wind speed for the fully developed seas mainly appear in August–November, closely related to strong tropical cyclone activities. Compared with previous studies, it is also implied that the wave climate in the Pacific Ocean plays an important role in the wave climate variations in the SCS.
2017, 36(11): 51-60.
doi: 10.1007/s13131-017-1137-5
Abstract:
Air temperature is a key index reflecting climate change. Air temperature extremes are very important because they strongly influence the natural environment and societal activities. The Arctic air temperature extremes north of 60°N are investigated in the winter. Daily data from 238 stations at north of 60°N from the global summary of the day for the period 1979-2015 are used to study the trends of cold days, cold nights, warm days and warm nights during the wintertime. The results show a decreasing trend of cold days and nights (rate of -0.2 to -0.3 d/a) and an increasing trend of warm days and nights (rate of +0.2 to +0.3 d/a) in the Arctic. The mean temperature increases, which contributes to the increasing (decreasing) occurrence of warm (cold) days and nights. On the other hand, the variance at most stations decreased, leading to a reduced number of cold events. A positive AO (Arctic Oscillation) index leads to an increased (decreased) number of warm (cold) days and nights over northern Europe and western Russia and an increased (decreased) number of cold (warm) days and nights over the Bering Strait and Greenland. The lower extent of Arctic autumn sea ice leads to a decreased number of cold days and nights. The occurrences of abrupt changes are detected using the Mann-Kendall method for cold nights occurring in Canada in 1998 and for warm nights occurring in northwestern Eurasia in 1988. This abrupt change mainly resulted from the mean warming induced by south winds and an increased North Atlantic sea surface temperature.
Air temperature is a key index reflecting climate change. Air temperature extremes are very important because they strongly influence the natural environment and societal activities. The Arctic air temperature extremes north of 60°N are investigated in the winter. Daily data from 238 stations at north of 60°N from the global summary of the day for the period 1979-2015 are used to study the trends of cold days, cold nights, warm days and warm nights during the wintertime. The results show a decreasing trend of cold days and nights (rate of -0.2 to -0.3 d/a) and an increasing trend of warm days and nights (rate of +0.2 to +0.3 d/a) in the Arctic. The mean temperature increases, which contributes to the increasing (decreasing) occurrence of warm (cold) days and nights. On the other hand, the variance at most stations decreased, leading to a reduced number of cold events. A positive AO (Arctic Oscillation) index leads to an increased (decreased) number of warm (cold) days and nights over northern Europe and western Russia and an increased (decreased) number of cold (warm) days and nights over the Bering Strait and Greenland. The lower extent of Arctic autumn sea ice leads to a decreased number of cold days and nights. The occurrences of abrupt changes are detected using the Mann-Kendall method for cold nights occurring in Canada in 1998 and for warm nights occurring in northwestern Eurasia in 1988. This abrupt change mainly resulted from the mean warming induced by south winds and an increased North Atlantic sea surface temperature.
2017, 36(11): 61-67.
doi: 10.1007/s13131-017-1138-4
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The South China Sea (SCS) and the Arabian Sea (AS) are both located roughly in the north tropical zone with a range of similar latitude (0°-24°N). Monsoon winds play similar roles in the upper oceanic circulations of the both seas. But the distinct patterns of chlorophyll a (Chl a) concentration are observed between the SCS and the AS. The Chl a concentration in the SCS is generally lower than that in the AS in summer (June-August); the summer Chl a concentration in the AS shows stronger interannual variation, compared with that in the SCS; Moderate resolution imaging spectroradiometer (MODIS)-derived data present higher atmospheric aerosol deposition and stronger wind speed in the AS. And it has also been found that good correlations exist between the index of the dust precipitation indicated by aerosol optical thickness (AOT) and the Chl a concentration, or between wind and Chl a concentration. These imply that the wind and the dust precipitation bring more nutrients into the AS from the sky, the sub-layer or coast regions, inducing higher Chl a concentration. The results indicate that the wind velocity and the dust precipitation can play important roles in the Chl a concentration for the AS and the SCS in summer. However aerosol impact is weak on the biological productivity in the west SCS and wind-induced upwelling is the main source.
The South China Sea (SCS) and the Arabian Sea (AS) are both located roughly in the north tropical zone with a range of similar latitude (0°-24°N). Monsoon winds play similar roles in the upper oceanic circulations of the both seas. But the distinct patterns of chlorophyll a (Chl a) concentration are observed between the SCS and the AS. The Chl a concentration in the SCS is generally lower than that in the AS in summer (June-August); the summer Chl a concentration in the AS shows stronger interannual variation, compared with that in the SCS; Moderate resolution imaging spectroradiometer (MODIS)-derived data present higher atmospheric aerosol deposition and stronger wind speed in the AS. And it has also been found that good correlations exist between the index of the dust precipitation indicated by aerosol optical thickness (AOT) and the Chl a concentration, or between wind and Chl a concentration. These imply that the wind and the dust precipitation bring more nutrients into the AS from the sky, the sub-layer or coast regions, inducing higher Chl a concentration. The results indicate that the wind velocity and the dust precipitation can play important roles in the Chl a concentration for the AS and the SCS in summer. However aerosol impact is weak on the biological productivity in the west SCS and wind-induced upwelling is the main source.
2017, 36(11): 68-78.
doi: 10.1007/s13131-017-1076-1
Abstract:
The Jiaozhou Bay is characterized by heavy eutrophication that is associated with intensive anthropogenic activities. Four core sediments from the Jiaozhou Bay are analyzed using bulk technologies, including sedimentary total organic carbon (TOC), total nitrogen (TN), the stable carbon (δ13C) and nitrogen (δ15N) isotopic composition to obtain the comprehensive understanding of the source and composition of sedimentary organic matter and further shed light on the environmental changes of the Jiaozhou Bay on a centennial time scale. Results suggest that the TOC and TN concentrations increase in the upper core, having indicated a probable eutrophication process since the 1920s in the inner bay and the 2000s in the bay mouth. The TOC and TN concentrations outside the bay have also changed since 1916 owing to the variation of terrigenous input. Considering TOC/TN ratio, δ13C and δ15N, it can be concluded there is a mixture of terrigenous and marine organic matter sources in the study area. A simple two end-member (terrigenous and marine) mixing model using δ13C indicats that 45%-79% of TOC in the Jiaozhou Bay is from the marine source. The environmental changes of the Jiaozhou Bay are recorded by geochemical proxies, which are influenced by the intensive anthropogenic activities (e.g., extensive use of fertilizers, and discharge of sewage) and climate changes (e.g., rainfall).
The Jiaozhou Bay is characterized by heavy eutrophication that is associated with intensive anthropogenic activities. Four core sediments from the Jiaozhou Bay are analyzed using bulk technologies, including sedimentary total organic carbon (TOC), total nitrogen (TN), the stable carbon (δ13C) and nitrogen (δ15N) isotopic composition to obtain the comprehensive understanding of the source and composition of sedimentary organic matter and further shed light on the environmental changes of the Jiaozhou Bay on a centennial time scale. Results suggest that the TOC and TN concentrations increase in the upper core, having indicated a probable eutrophication process since the 1920s in the inner bay and the 2000s in the bay mouth. The TOC and TN concentrations outside the bay have also changed since 1916 owing to the variation of terrigenous input. Considering TOC/TN ratio, δ13C and δ15N, it can be concluded there is a mixture of terrigenous and marine organic matter sources in the study area. A simple two end-member (terrigenous and marine) mixing model using δ13C indicats that 45%-79% of TOC in the Jiaozhou Bay is from the marine source. The environmental changes of the Jiaozhou Bay are recorded by geochemical proxies, which are influenced by the intensive anthropogenic activities (e.g., extensive use of fertilizers, and discharge of sewage) and climate changes (e.g., rainfall).
2017, 36(11): 79-85.
doi: 10.1007/s13131-016-1072-5
Abstract:
In estuarine and coastal areas, the seabed is in a constant process of dynamic change under marine conditions. Seabed sediment erosion and resuspension are important processes that safely control the geological environment. Field tripod observations conducted in the Jiaozhou Bay in China are reported, to investigate the effects of hydrodynamic conditions on the erosion and resuspension processes of the seabed. The observational results show that the maximum shear stress created by tidal currents can reach 0.35 N/m2, which is higher than the wave-induced shear stress during fair weather conditions. A seabed erosion frequently occurs during the flood tide, whereas a seabed deposition occurs during ebb tide. Waves can produce a bottom shear stress approximately equivalent to that induced by currents when the local wind reaches Force 4 with a speed of 5 m/s. When the wind reaches 7 m/s and the significant wave height reaches 26 cm, waves play a more significant role than currents in the dynamic processes of the seabed sediment resuspension and lead to a high value of turbidity that is approximately two to eight times higher than that in fair weather. These analyses clearly illustrate that periodic current-induced sediment erosion and resuspension are dominant in fair weather, whereas episodic high waves are responsible for significant sediment resuspension. Additional work is needed to establish a more thorough understanding of the mechanisms of sediment dynamics in the Jiaozhou Bay.
In estuarine and coastal areas, the seabed is in a constant process of dynamic change under marine conditions. Seabed sediment erosion and resuspension are important processes that safely control the geological environment. Field tripod observations conducted in the Jiaozhou Bay in China are reported, to investigate the effects of hydrodynamic conditions on the erosion and resuspension processes of the seabed. The observational results show that the maximum shear stress created by tidal currents can reach 0.35 N/m2, which is higher than the wave-induced shear stress during fair weather conditions. A seabed erosion frequently occurs during the flood tide, whereas a seabed deposition occurs during ebb tide. Waves can produce a bottom shear stress approximately equivalent to that induced by currents when the local wind reaches Force 4 with a speed of 5 m/s. When the wind reaches 7 m/s and the significant wave height reaches 26 cm, waves play a more significant role than currents in the dynamic processes of the seabed sediment resuspension and lead to a high value of turbidity that is approximately two to eight times higher than that in fair weather. These analyses clearly illustrate that periodic current-induced sediment erosion and resuspension are dominant in fair weather, whereas episodic high waves are responsible for significant sediment resuspension. Additional work is needed to establish a more thorough understanding of the mechanisms of sediment dynamics in the Jiaozhou Bay.
2017, 36(11): 86-93.
doi: 10.1007/s13131-017-1139-3
Abstract:
A behavior-oriented formula is improved to calculate the annual coastal bathymetry evolution caused by a reclamation work. This formula is based on a simple hypothesis that, on the time scale of years, the bathymetry evolution is closely related to the change of tidal current field due to the reclamation work. A new coefficient, named the erosion reduction coefficient, is introduced to extend the original formula for applications in calculating bed erosion. A simplified relationship between the annual variation of the siltation/erosion rate and the water depth is introduced to more realistically represent the long-term process of the bathymetry evolution. The improved formula is applied to calculate the bathymetry evolution in 3 a following a reclamation project in the Xiaomiaohong Tidal Channel in Jiangsu coast in China. The results compare well with measurements and those obtained from a process-based numerical model, demonstrating the capability of the improved behavior-oriented formula in reproducing the impact of the reclamation project on the local bathymetry evolution.
A behavior-oriented formula is improved to calculate the annual coastal bathymetry evolution caused by a reclamation work. This formula is based on a simple hypothesis that, on the time scale of years, the bathymetry evolution is closely related to the change of tidal current field due to the reclamation work. A new coefficient, named the erosion reduction coefficient, is introduced to extend the original formula for applications in calculating bed erosion. A simplified relationship between the annual variation of the siltation/erosion rate and the water depth is introduced to more realistically represent the long-term process of the bathymetry evolution. The improved formula is applied to calculate the bathymetry evolution in 3 a following a reclamation project in the Xiaomiaohong Tidal Channel in Jiangsu coast in China. The results compare well with measurements and those obtained from a process-based numerical model, demonstrating the capability of the improved behavior-oriented formula in reproducing the impact of the reclamation project on the local bathymetry evolution.
2017, 36(11): 94-105.
doi: 10.1007/s13131-017-1140-x
Abstract:
An efficient and accurate prediction of a precise tidal level in estuaries and coastal areas is indispensable for the management and decision-making of human activity in the field wok of marine engineering. The variation of the tidal level is a time-varying process. The time-varying factors including interference from the external environment that cause the change of tides are fairly complicated. Furthermore, tidal variations are affected not only by periodic movement of celestial bodies but also by time-varying interference from the external environment. Consequently, for the efficient and precise tidal level prediction, a neuro-fuzzy hybrid technology based on the combination of harmonic analysis and adaptive network-based fuzzy inference system (ANFIS) model is utilized to construct a precise tidal level prediction system, which takes both advantages of the harmonic analysis method and the ANFIS network. The proposed prediction model is composed of two modules:the astronomical tide module caused by celestial bodies' movement and the non-astronomical tide module caused by various meteorological and other environmental factors. To generate a fuzzy inference system (FIS) structure, three approaches which include grid partition (GP), fuzzy c-means (FCM) and sub-clustering (SC) are used in the ANFIS network constructing process. Furthermore, to obtain the optimal ANFIS based prediction model, large numbers of simulation experiments are implemented for each FIS generating approach. In this tidal prediction study, the optimal ANFIS model is used to predict the non-astronomical tide module, while the conventional harmonic analysis model is used to predict the astronomical tide module. The final prediction result is performed by combining the estimation outputs of the harmonious analysis model and the optimal ANFIS model. To demonstrate the applicability and capability of the proposed novel prediction model, measured tidal level samples of Fort Pulaski tidal station are selected as the testing database. Simulation and experimental results confirm that the proposed prediction approach can achieve precise predictions for the tidal level with high accuracy, satisfactory convergence and stability.
An efficient and accurate prediction of a precise tidal level in estuaries and coastal areas is indispensable for the management and decision-making of human activity in the field wok of marine engineering. The variation of the tidal level is a time-varying process. The time-varying factors including interference from the external environment that cause the change of tides are fairly complicated. Furthermore, tidal variations are affected not only by periodic movement of celestial bodies but also by time-varying interference from the external environment. Consequently, for the efficient and precise tidal level prediction, a neuro-fuzzy hybrid technology based on the combination of harmonic analysis and adaptive network-based fuzzy inference system (ANFIS) model is utilized to construct a precise tidal level prediction system, which takes both advantages of the harmonic analysis method and the ANFIS network. The proposed prediction model is composed of two modules:the astronomical tide module caused by celestial bodies' movement and the non-astronomical tide module caused by various meteorological and other environmental factors. To generate a fuzzy inference system (FIS) structure, three approaches which include grid partition (GP), fuzzy c-means (FCM) and sub-clustering (SC) are used in the ANFIS network constructing process. Furthermore, to obtain the optimal ANFIS based prediction model, large numbers of simulation experiments are implemented for each FIS generating approach. In this tidal prediction study, the optimal ANFIS model is used to predict the non-astronomical tide module, while the conventional harmonic analysis model is used to predict the astronomical tide module. The final prediction result is performed by combining the estimation outputs of the harmonious analysis model and the optimal ANFIS model. To demonstrate the applicability and capability of the proposed novel prediction model, measured tidal level samples of Fort Pulaski tidal station are selected as the testing database. Simulation and experimental results confirm that the proposed prediction approach can achieve precise predictions for the tidal level with high accuracy, satisfactory convergence and stability.
2017, 36(11): 106-114.
doi: 10.1007/s13131-017-1141-9
Abstract:
Since the atmospheric correction is a necessary preprocessing step of remote sensing image before detecting green tide, the introduced error directly affects the detection precision. Therefore, the detection method of green tide is presented from Landsat TM/ETM plus image which needs not the atmospheric correction. In order to achieve an automatic detection of green tide, a linear relationship (y=0.723x+0.504) between detection threshold y and subtraction x (x=λnir-λred) is found from the comparing Landsat TM/ETM plus image with the field surveys. Using this relationship, green tide patches can be detected automatically from Landsat TM/ETM plus image. Considering there is brightness difference between different regions in an image, the image will be divided into a plurality of windows (sub-images) with a same size firstly, and then each window will be detected using an adaptive detection threshold determined according to the discovered linear relationship. It is found that big errors will appear in some windows, such as those covered by clouds seriously. To solve this problem, the moving step k of windows is proposed to be less than the window width n. Using this mechanism, most pixels will be detected[n/k]×[n/k] times except the boundary pixels, then every pixel will be assigned the final class (green tide or sea water) according to majority rule voting strategy. It can be seen from the experiments, the proposed detection method using multi-windows and their adaptive thresholds can detect green tide from Landsat TM/ETM plus image automatically. Meanwhile, it avoids the reliance on the accurate atmospheric correction.
Since the atmospheric correction is a necessary preprocessing step of remote sensing image before detecting green tide, the introduced error directly affects the detection precision. Therefore, the detection method of green tide is presented from Landsat TM/ETM plus image which needs not the atmospheric correction. In order to achieve an automatic detection of green tide, a linear relationship (y=0.723x+0.504) between detection threshold y and subtraction x (x=λnir-λred) is found from the comparing Landsat TM/ETM plus image with the field surveys. Using this relationship, green tide patches can be detected automatically from Landsat TM/ETM plus image. Considering there is brightness difference between different regions in an image, the image will be divided into a plurality of windows (sub-images) with a same size firstly, and then each window will be detected using an adaptive detection threshold determined according to the discovered linear relationship. It is found that big errors will appear in some windows, such as those covered by clouds seriously. To solve this problem, the moving step k of windows is proposed to be less than the window width n. Using this mechanism, most pixels will be detected[n/k]×[n/k] times except the boundary pixels, then every pixel will be assigned the final class (green tide or sea water) according to majority rule voting strategy. It can be seen from the experiments, the proposed detection method using multi-windows and their adaptive thresholds can detect green tide from Landsat TM/ETM plus image automatically. Meanwhile, it avoids the reliance on the accurate atmospheric correction.
