2014 Vol. 33, No. 5

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2014, Vol. 33, No. 5 Content
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2014, 33(5): .
Diurnal wind and nonlinear interaction between inertial and tidal currents in the South China Sea during the passage of Typhoon Conson
LIU Junliang, CAI Shuqun, WANG Shengan
2014, 33(5): 1-7. doi: 10.1007/s13131-014-0467-9
Diurnal wind (DW) and nonlinear interaction between inertial and tidal currents near the Xisha Islands of the South China Sea (SCS) during the passage of Typhoon Conson (2010) are investigated using observational data and a damped slab model. It is found that the DWs, which are dominated by clockwise wind components, are prominent at our observational site. The DWs increase after the passage of the typhoon from 1 to about 4 m/s, which may be due to the decrease of the sea surface temperature caused by the passage of the typhoon. Kinetic energy spectra and bicoherence methods reveal nonlinear interactions between the inertial currents and the 2MK3 tidal constituent at our observational site. The slab damped model reproduces the inertial currents successfully induced by the total observed winds, and it is shown that the inertial currents induced by DWs are positively proportional to the DWs speed. Even though the observed inertial currents are distinct, the proportion of inertial currents induced by DWs to those induced by the total observed winds is just 0.7%/4% before/after the passage of typhoon. This shows that the inertial currents induced by the DWs are unimportant near the Xisha Islands during the typhoon season.
Formation and transport of intermediate water masses in a model of the Pacific Ocean
LI Yangchun, XU Yongfu
2014, 33(5): 8-16. doi: 10.1007/s13131-014-0480-z
A basin-wide ocean general circulation model of the Pacific Ocean was used to investigate how the interior restoration in the Okhotsk Sea and the isopycnal diffusion affect the circulation and intermediate water masses. Four numerical experiments were conducted, including a run with the same isopycnal and thickness diffusivity of 1.0×103 m2/s, a run employing the interior restoration of temperature and salinity in the Okhotsk Sea with a time scale of 3 months, a run that is the same as the first run except for the enhanced isopycnal mixing, and a final run with the combination of the restoration in the Okhotsk Sea and large isopycnal diffusivity. Simulated results show that the intermediate water masses reproduced in the first run are relatively weak. An increase in isopycnal diffusivity can improve the simulation of both Antarctic and North Pacific intermediate waters, mainly increasing the transport in the interior ocean, but inhibiting the outflow from the Okhotsk Sea. The interior restoration generates the reverse current from the observation in the Okhotsk Sea, whereas the simulation of the temperature and salinity is improved in the high latitude region of the Northern Hemisphere because of the reasonable source of the North Pacific Intermediate Water. A comparison of vertical profiles of temperature and salinity along 50°N between the simulation and observations demonstrates that the vertical mixing in the source region of intermediate water masses is very important.
The bimodality of the Luzon Strait deep water
WU Qingsong, ZHAO Jianru, ZHANG Junbiao, SHI Weiyong, LIU Chunqiu
2014, 33(5): 17-27. doi: 10.1007/s13131-014-0475-9
Combined conductivity-temperature-depth (CTD) casts and Argo profiles, 3 086 historical hydrocasts were used to quantify the water column characteristics in the northern South China Sea (SCS) and its adjacent waters. Based on a two-dimensional "gravest empirical mode" (GEM), a gravitational potential (φ, a vertically integrated variable) was used as proxy for the vertical temperature profiles TG(p, φ).φ integrated from 8 MPa to the surface shows a close relationship with the temperature, except in the deep layer greater than 15 MPa, which was caused by the bimodal deep water in the region. The GEM temperature profiles successfully revealed the bimodality of the Luzon Strait deep water, that disparate hydrophic vertical profiles can produce distinct specific volume anomaly (δ) in the SCS and the western Philippine Sea (WPS), but failed in the Luzon Strait, where different temperature profiles may produce a same δ. A significant temperature divergence between the SCS water and the WPS water confirmed that the bimodal structure is strong. The deepwater bifurcation starts at about 15 MPa, and gets stronger with increasing depth . As the only deep channel connecting the bimodal-structure waters, water column characteristics in the Luzon Strait is in between, but much closer to the SCS water because of its better connectivity with the SCS. A bimodal temperature structure below 15 MPa reveals that there was a persistent baroclinic pressure gradient driving flow through the Luzon Strait. A volume flux predicted through the Bashi Channel with the hydraulic theory yields a value of 5.62×106 m3/s using all available profiles upstream and downstream of the overflow region, and 4.03×106 and 2.70×106 m3/s by exclusively using the profiles collected during spring and summer, respectively. No volume flux was calculated during autumn and winter because profiles are only available for the upstream of the Bashi Channel during the corresponding period.
Estimation of vertical diffusion coefficient based on a onedimensional temperature diffusion equation with an inverse method
2014, 33(5): 28-36. doi: 10.1007/s13131-014-0472-z
Diapycnal mixing is important in oceanic circulation. An inverse method in which a semi-explicit scheme is applied to discretize the one-dimensional temperature diffusion equation is established to estimate the vertical temperature diffusion coefficient based on the observed temperature profiles. The sensitivity of the inverse model in the idealized and actual conditions is tested in detail. It can be found that this inverse model has high feasibility under multiple situations ensuring the stability of the inverse model, and can be considered as an efficient way to estimate the temperature diffusion coefficient in the weak current regions of the ocean. Here, the hydrographic profiles from Argo floats are used to estimate the temporal and spatial distribution of the vertical mixing in the north central Pacific based on this inverse method. It is further found that the vertical mixing in the upper ocean displays a distinct seasonal variation with the amplitude decreasing with depth, and the vertical mixing over rough topography is stronger than that over smooth topography. It is suggested that the high-resolution profiles from Argo floats and a more reasonable design of the inverse scheme will serve to understand mixing processes.
On the Fourier approximation method for steady water waves
ZHAO Hongjun, SONG Zhiyao, LI Ling, KONG Jun
2014, 33(5): 37-47. doi: 10.1007/s13131-014-0470-1
A computational method for steady water waves is presented on the basis of potential theory in the physical plane with spatial variables as independent quantities. The finite Fourier series are applied to approximating the free surface and potential function. A set of nonlinear algebraic equations for the Fourier coefficients are derived from the free surface kinetic and dynamic boundary conditions. These algebraic equations are numerically solved through Newton's iterative method, and the iterative stability is further improved by a relaxation technology. The integral properties of steady water waves are numerically analyzed, showing that (1) the set-up and the set-down are both non-monotonic quantities with the wave steepness, and (2) the Fourier spectrum of the free surface is broader than that of the potential function. The latter further leads us to explore a modification for the present method by approximating the free surface and potential function through different Fourier series, with the truncation of the former higher than that of the latter. Numerical tests show that this modification is effective, and can notably reduce the errors of the free surface boundary conditions.
Validation of the “HY-2” altimeter wet tropospheric path delay correction based on radiosonde data
WANG Jin, ZHANG Jie, FAN Chenqing, WANG Jing
2014, 33(5): 48-53. doi: 10.1007/s13131-014-0473-y
Wet tropospheric path delay (PD) is a highly variable term for the altimeter measurement of a sea surface height, caused by the refraction effect of atmospheric water vapor and cloud liquid water. In order to estimate PD values, the "HY-2" system includes a calibration microwave radiometer (CMR) operating at 18.7, 23.8 and 37 GHz. The PD data of the CMR were compared and validated by coincident radiosonde profiles from ten globally distributed radiosonde stations during October 2011 to August 2012. The temporal interval was 1 h. In order to avoid land contamination, different spatial intervals between these two data sets were tested. The empirical fit function of PD uncertainty and spatial interval was found and extrapolated to the ideal situation that the data of CMR and radiosonde were totally coincident. The stability of the brightness temperature of the CMR and its impact on the PD correction was also studied. Consequently, the uncertainty of the PD algorithm of the CMR was estimated to be 2.1 cm.
Coastal wind field retrieval from polarimetric synthetic aperture radar
ZHANG Yi, JIANG Xingwei, SONG Qingtao, LIN Mingsen, XIE Xuetong
2014, 33(5): 54-61. doi: 10.1007/s13131-014-0479-5
Coastal winds are strongly influenced by topology and discontinuity between land and sea surfaces. Wind assessment from remote sensing in such a complex area remains a challenge. Space-borne scatterometer does not provide any information about the coastal wind field, as the coarse spatial resolution hampers the radar backscattering. Synthetic aperture radar (SAR) with a high spatial resolution and all-weather observation abilities has become one of the most important tools for ocean wind retrieval, especially in the coastal area. Conventional methods of wind field retrieval from SAR, however, require wind direction as initial information, such as the wind direction from numerical weather prediction models (NWP), which may not match the time of SAR image acquiring. Fortunately, the polarimetric observations of SAR enable independent wind retrieval from SAR images alone. In order to accurately measure coastal wind fields, this paper proposes a new method of using co-polarization backscattering coefficients from polarimetric SAR observations up to polarimetric correlation backscattering coefficients, which are acquired from the conjugate product of co-polarization backscatter and cross-polarization backscatter. Co-polarization backscattering coefficients and polarimetric correlation backscattering coefficients are obtained form Radarsat-2 single-look complex (SLC) data.The maximum likelihood estimation is used to gain the initial results followed by the coarse spatial filtering and fine spatial filtering. Wind direction accuracy of the final inversion results is 10.67 with a wind speed accuracy of 0.32 m/s. Unlike previous methods, the methods described in this article utilize the SAR data itself to obtain the wind vectors and do not need external wind directional information. High spatial resolution and high accuracy are the most important features of the method described herein since the use of full polarimetric observations contains more information about the space measured.This article is a useful addition to the work of independent SAR wind retrieval. The experimental results herein show that it is feasible to employ the co-polarimetric backscattering coefficients and the polarimetric correlation backscattering coefficients for coastal wind field retrieval.
A new algorithm based on the background field for red tide monitoring in the East China Sea
XU Xiaohui, PAN Delu, MAO Zhihua, TAO Bangyi
2014, 33(5): 62-71. doi: 10.1007/s13131-014-0404-y
Remote sensing has been proven to be an effective means of monitoring red tides. The spectral information is an important basis for establishing a model to monitor red tides. The spectral curves of red tide events are analyzed and compared with multiyear monthly averaged spectral curves based on MODIS data from July 2002 to June 2012, as well as spectral differences at the same location during red tide presence and absence. A red tide monitoring algorithm is developed based on the background field to extract the red tide information of the East China Sea (ECS). With the application of the algorithm in the ECS, the results reveal that the developed model can effectively determine the location of red tides, with good correspondence to the results from an official bulletin. This demonstrates that the algorithm can effectively extract the red tide information.
Interannual variability of remotely sensed chlorophyll α during an autumn monsoon transitional period in the Taiwan Strait
ZHANG Caiyun, HONG Huasheng
2014, 33(5): 72-80. doi: 10.1007/s13131-014-0477-7
The time series of multiple sources of satellite data are used to examine the interannual variability of chlorophyll α concentration (Chl α) and its relation to the physical environment during the autumn monsoon transitional period in the Taiwan Strait (TWS). The satellite data included the Chl α concentration and sea surface temperature (SST) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS)/ Aqua as well as the multi-sensors merged wind products from 2002 to 2012. The results show that the average Chl α concentration of the whole TWS is mainly contributed by the northern TWS. The average Chl α in the northern TWS is 3.6 times that in the southern TWS. The maximum variability of Chl α is located in the frontal regions between the cold Zhe-Min Coastal Water and the strait warm water. The temporal change of Chl α concentration is different in the northern and southern TWS. The changes in the relative strength of the cold and warm water masses is suggested to be the dominant processes in controlling the phytoplankton growth in the northern TWS, while there is wind-induced mixing in the southern TWS. Additionally, La Niña events exhibited complex effects on the interannual variability of Chl α concentration in autumn. The longterm time series of physical and biological observations are especially needed to better understand how the TWS complex ecosystem responds to climate variations.
A biooptical model of retrieving petroleum concentration in seawater
HUANG Miaofen, SONG Qingjun, XING Xufeng, ZHAO Zulong
2014, 33(5): 81-85. doi: 10.1007/s13131-014-0478-6
A biooptical modeling, which is based on a radiation transfer model, can be employed to simultaneously retrieve the concentration of various colour factors by multi-spectral remote sensing data, after connecting inherent optical properties (absorption coefficient and backward scattering coefficient) of colour factor with apparent optical properties (remote sensing reflectivity). At present, this method has been used in a relatively wide range of applications in the inversion of a conventional water colour factor concentration in the case II water body: applications such as chlorophyll, suspended sediment, yellow substance. On the basis of extensive field testing data of water quality, correspondingly apparent optical properties, and the full use of the existing parametric model of colour factor inherent optical properties (with the parametrization of petroleum substance inherent optical properties established in the project) the remote recognition model for oil concentration is established by introducing oil as a new water colour factor into a biooptical remote algorithm. The estimated value of the oil concentration was obtained by solving the biooptical model, using the data measured in May 2008 and August 2009 and June 2010 in seawater. The highly accurate inversion result promises to estimate the oil concentration in water for remote sensing.
Optical and SAR image registration based on improved nonsubsampled wavelet transform for sea islands
SHI Wei, SU Fenzhen, WANG Ruirui, LU Yongduo
2014, 33(5): 86-95. doi: 10.1007/s13131-014-0474-x
Homologous feature point extraction is a key problem in the optical and synthetic aperture radar (SAR) image registration for islands. A new feature point extraction method using a threshold shrink operator and non-subsampled wavelet transform (TSO-NSWT) for optical and SAR image registration was proposed. Moreover, the matching for this proposed feature was different from the traditional feature matching strategies and was performed using a similarity measure computed from neighborhood circles in low-frequency bands. Then, a number of reliably matched couples with even distributions were obtained, which assured the accuracy of the registration. Application of the proposed algorithm to SPOT-5 (multi-spectral) and YG-1 (SAR) images showed that a large number of accurately matched couples could be identified. Additionally, both of the root mean square error (RMSE) patterns of the registration parameters computed based on the TSO-NSWT algorithm and traditional NSWT algorithm were analyzed and compared, which further demonstrated the effectiveness of the proposed algorithm. The algorithm can supply the crucial step for island image registration and island recognition.
The wave motion over a submerged Jarlan-type perforated breakwater
LIU Yong, XIE Luqiong, ZHANG Zhehan
2014, 33(5): 96-102. doi: 10.1007/s13131-014-0471-0
The wave motion over a submerged Jarlan-type breakwater consisting of a perforated front wall and a solid rear wall was investigated analytically and experimentally. An analytical solution was developed using matched eigenfunction expansions. The analytical solution was confirmed by previously known solutions for single and double submerged solid vertical plates, a multidomain boundary element method solution, and experimental data. The calculated results by the analytical solution showed that compared with double submerged vertical plates, the submerged Jarlan-type perforated breakwater had better wave-absorbing performance and lower wave forces. For engineering designs, the optimum values of the front wall porosity, relative submerged depth of the breakwater, and relative chamber width between front and rear walls were 0.1-0.2, 0.1-0.2, and 0.3-0.4, respectively. Interchanging the perforated front wall and solid rear wall may have no effect on the transmission coefficient. However, the present breakwater with a seaside perforated wall had a lower reflection coefficient.
Research Notes
An improved method of absolute calibration to satellite altimeter:A case study in the Yellow Sea, China
LIU Yalong, TANG Junwu, ZHU Jianhua, LIN Mingsen, ZHAI Wanlin, CHEN Chuntao
2014, 33(5): 103-112. doi: 10.1007/s13131-014-0476-8
An improved absolute calibration technology based on indirect measurements was developed through two probative experiments, the performance of which was evaluated by applying the approach to in situ sea surface height (SSH) at the Tianheng Island (tidal gauge) and the satellite nadir (GPS buoy). Using Geoid/MSS (mean sea surface) data, which accounted for a constant offset between nadir and onshore tidal gauge water levels, and TMD (tidal model driver), which canceled out the time-varying offsets, nadir SSH (sea surface height) could be indirectly acquired at an onshore tidal gauge instead of from direct offshore observation. The approach extrapolated the onshore SSH out to the offshore nadir with an accuracy of (1.88±0.20) cm and a standard deviation of 3.3 cm, which suggested that the approach presented was feasible in absolute altimeter calibration/validation (Cal/Val), and the approach enormously facilitated the obtaining SSH from the offshore nadir.