This paper demonstrates an atmospheric correction method to process MODIS/Aqua (Moderate-resolution Imaging Spectroradiometer) ocean color imagery over turbid coastal waters with the aid of concurrent CALIOP (Cloud-Ae...This paper demonstrates an atmospheric correction method to process MODIS/Aqua (Moderate-resolution Imaging Spectroradiometer) ocean color imagery over turbid coastal waters with the aid of concurrent CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization) aerosol data, assuming that there exists "nonturbid" water in the study area where MODIS aerosol optical properties can be retrieved accurately. Aerosol properties from CALIOP measurements were obtained and related to those from MODIS. This relationship, combined with CALIOP aerosol data, was extended to turbid water to derive MODIS aerosol properties, where atmospheric correction using MODIS data alone often fails. By combining MODIS and CALIOP data, aerosol signals were separated from the total signals at the satellite level, and water-leaving radiances in turbid waters were subsequently derived. This method was tested on several MODIS/Aqua ocean color images over South China turbid waters. Comparison with field data shows that this method was effective in reducing the errors in the retrieved water-leaving radiance values to some extent. In the Zhujiang (Pearl) River Estuary, this method did not overestimate the aerosol effects as severely, and provided far fewer negative water-leaving radiance values than the NASA (National Aeronautics and Space Administration) default methods that used MODIS data alone.展开更多
An airborne oceanographic lidar, with a frequency-tripled Q-switched Nd： YAG laser of 355 nm, has been designed to measure chlorophyll-a （Chl-a） concentration in the sea surface layer by the Ocean Remote Sensing In...An airborne oceanographic lidar, with a frequency-tripled Q-switched Nd： YAG laser of 355 nm, has been designed to measure chlorophyll-a （Chl-a） concentration in the sea surface layer by the Ocean Remote Sensing Institute, OUC. The field experiment was carried out in the bay which is located south of the Liaodong Peninsula on the 10th of September 2005. After the flight, the raw data were processed and analyzed by the fluorescence-to-Raman ratio method with seawater attenuation coefficients calculated from signal profiles. The results of Chl-a concentration sea water were also compared with those of Chl-a concentration by measurements by lidar are shown. The measurements in clear a Moderate Resolution Imaging Spectroradiometer （MODIS）.展开更多
A Multi-channel Oceanographic Fluorescence Lidar(MOFL), with a UV excitation at 355 nm and multiple receiving channels at typical wavelengths of fluorescence from oil spills and chlorophyll-a(Chl-a), has been develope...A Multi-channel Oceanographic Fluorescence Lidar(MOFL), with a UV excitation at 355 nm and multiple receiving channels at typical wavelengths of fluorescence from oil spills and chlorophyll-a(Chl-a), has been developed using the Laser- induced Fluorescence(LIF) technique. The sketch of the MOFL system equipped with a compact multi-channel photomultiplier tube(MPMT) is introduced in the paper. The methods of differentiating the oil fluorescence from the background water fluorescence and evaluating the Chl-a concentration are described. Two field experiments were carried out to investigate the field performance of the system, i.e., an experiment in coastal areas for oil pollution detection and an experiment over the Yellow Sea for Chl-a monitoring. In the coastal experiment, several oil samples and other fluorescence substances were used to analyze the fluorescence spectral characteristics for oil identification, and to estimate the thickness of oil films at the water surface. The experiment shows that both the spectral shape of fluorescence induced from surface water and the intensity ratio of two channels(I495/I405) are essential to determine oil-spill occurrence. In the airborne experiment, MOFL was applied to measure relative Chl-a concentrations in the upper layer of the ocean. A comparison of relative Chl-a concentration measurements by MOFL and the Moderate Resolution Imaging Spectroradiometer(MODIS) indicates that the two datasets are in good agreement. The results show that the MOFL system is capable of monitoring oil spills and Chl-a in the upper layer of ocean water.展开更多
Azimuth ambiguities （ghost targets） discrimination is of great interest with the development of a synthet- ic aperture radar （SAR）. And the azimuth ambiguities are often mistaken as actual targets and cause false ...Azimuth ambiguities （ghost targets） discrimination is of great interest with the development of a synthet- ic aperture radar （SAR）. And the azimuth ambiguities are often mistaken as actual targets and cause false alarms. For actual targets, HV channel signals acquired by a fully polarimetric SAR are approximately equal to a VH channel in magnitude and phase, i.e., the reciprocity theorem applies, but shifted in phase about ±π for the first-order azimuth ambiguities. Exploiting this physical behavior, the real part of the product of the two cross-polarized channels, i.e. （SHVSVH）, hereafter called A12r, is employed as a new parameter for a target detection at sea. Compared with other parameters, the contrast of A12r image between a target and the surrounding sea surface will be obviously increased when A12r image is processed by mean filtering algo- rithm. Here, in order to detect target with constant false-alarm rates （CFARs）, an analytical expression for the probability density function （pdf） ofA12r is derived based on the complexWishart-distribution. Because a value of A12r is greater/less than 0 for real target/its azimuth ambiguities, the first-order azimuth ambiguities can be completely removed by this A12r-based CFAR technology. Experiments accomplished over C-band RADARSAT-2 fully polarimetric imageries confirm the validity.展开更多
In order to obtain the three dimensional (3D) fluorescence spectra directly and rapidly by in situ and in vivo phytoplankton monitoring, a prototype ocean lidar system with the ability of measuring 3D fluorescence spe...In order to obtain the three dimensional (3D) fluorescence spectra directly and rapidly by in situ and in vivo phytoplankton monitoring, a prototype ocean lidar system with the ability of measuring 3D fluorescence spectra is developed, in which an optical parametric oscillator (OPO) and a grating spectrometer are employed. Using the system, the 3D fluorescence spectra of five typical red-tide species in the East China Sea are measured in lab. The preliminary experimental results show the feasibility of monitoring and classifying phytoplankton by this ocean lidar.展开更多
Based on the first order small slope approximation theory （SSA-I） for oceanic surface electromagnetic scattering, this paper predicts the Doppler shifts induced by wave displacements. Theoretical results from three ...Based on the first order small slope approximation theory （SSA-I） for oceanic surface electromagnetic scattering, this paper predicts the Doppler shifts induced by wave displacements. Theoretical results from three distinct hydrody- namic models are compared： a linear model, the nonlinear Barrick model, and the nonlinear Creamer model. Meanwhile, the predicted Doppler shifts are also compared with the results associated to the resonant Bragg waves and the so-called long waves in the framework of the two-scale model. The dependences of the predicted Doppler shifts on the incident angle, the radar frequency, and the wind speed are discussed. At large incident angles, the predicted Doppler shifts for the linear and nonlinear Barrick models are found to be insensitive to the wind speed and this phenomenon is not coincident with the experimental data. The conclusions obtained in this paper are promising for better understanding the properties of time dependent radar echoes from oceanic surfaces.展开更多
The Sea Level Anomaly-Torque(SLAT,relative to a reference location in the Pacific Ocean),which means the total torque of the gravity forces of sea waters with depths equal to the Sea Level Anomaly(S/A)in the tropical ...The Sea Level Anomaly-Torque(SLAT,relative to a reference location in the Pacific Ocean),which means the total torque of the gravity forces of sea waters with depths equal to the Sea Level Anomaly(S/A)in the tropical Pacific Ocean,is defined in this study.The time series of the SLAT from merged altimeter data(1993-2003)had a great meridional variation during the 1997-1998 E1 Nifio event.By using historical upper layer temperature data(1955-2003)for the tropical Pacific Ocean,the temperature-based SLAT is also calculated and the meridional variation can be found in the historical E1 Nifio events(1955-2003),which suggests that the meridional shifts of the sea level anomaly are also intrinsic oscillating modes of the E1 Nifio cycles like the zonal shifts.展开更多
The oceanic warm pool （OWP） defined by sea surface temperature （SST） is known as the ＂heat reservoir＂ in the ocean. The warmest portion in the ocean mirrors the fact that the wettest region with the largest accu...The oceanic warm pool （OWP） defined by sea surface temperature （SST） is known as the ＂heat reservoir＂ in the ocean. The warmest portion in the ocean mirrors the fact that the wettest region with the largest accumulation of water vapor （WV） in the atmosphere, termed atmospheric wet pool （AWP）, should be identified because of the well-known Clausius-Clapeyron relationship between SST and WV. In this study, we used 14-year simultaneous observations of WV and SST from January 1988 to December 2001 to define the AWP and investigate its coupling and co-variations with the OWE The joint examination of the area variations, centroid locations, and zonal migrations of the AWP and OWP lead to a number of interesting findings. The results hopefully can contribute to our understanding of the air-sea interaction in general and characterization of E1 Nifio/La Nifia events in particular.展开更多
The heat distributions in the upper layers of the ocean have been studied and some important low frequency oscillations （LFOs） are already found and quantified by using various characteristic factors. In this paper,...The heat distributions in the upper layers of the ocean have been studied and some important low frequency oscillations （LFOs） are already found and quantified by using various characteristic factors. In this paper, the ‘heat center＇ of a sea area is defined with a simple method. Then the temperature data set of the upper layer of the global ocean （from surface down to 400 m, 1955-2003） is analyzed to detect the possible LFOs. Not only some zonal LFOs, which were reported early, but also some strong LFOs of the vertical and meridional heat distribution, which might imply some physical sense, are detected. It should be noted that the similar vertical oscillation pattern can be found in the Pacific Ocean, Atlantic Ocean and Indian Ocean. Results from some preliminary studies show that the vertical LFO might be caused by the solar irradiance anomalies. This study may help reveal some unknown dynamical processes in the global oceans and may also benefit other related studies.展开更多
This paper proposes a speckle-suppression method for ocean internal solitary wave（ISW） synthetic aperture radar（SAR） images by using the curvelet transform.The band-shaped signatures of ocean ISWs in SAR images sh...This paper proposes a speckle-suppression method for ocean internal solitary wave（ISW） synthetic aperture radar（SAR） images by using the curvelet transform.The band-shaped signatures of ocean ISWs in SAR images show obvious scale and directional characteristics.The curvelet transform possesses a very high scale and directional sensitivity.Therefore,the curvelet transform is very efficient in analyzing wave signals in SAR images.A noisy ocean ISW SAR image can be decomposed at different scales,directions,and positions using the curvelet transform.The information of the ISWs is centralized in the curvelet coefficients of certain directions under certain scales,whereas the speckle noise is distributed in every scale and direction.By manipulating the curvelet coefficients,the signals of the ISWs can be extracted from the noisy SAR image.Finally,the speckle noise is suppressed and the ISW feature is enhanced by adding the signals of the ISWs back to the original SAR image.Experiments demonstrate the effectiveness of this method.展开更多
Based on the general framework of the second order small slope approximation （SSA-II）, Doppler shifts of backscattered fields from linear and nonlinear hydrodynamic sea surfaces are predicted. Comparisons are perfor...Based on the general framework of the second order small slope approximation （SSA-II）, Doppler shifts of backscattered fields from linear and nonlinear hydrodynamic sea surfaces are predicted. Comparisons are performed at incidence angles varying from near-nadir to moderate-grazing for different radar frequencies. The predicted Doppler shifts for nonlinear hydrodynamic model are somewhat larger than the results corresponding to linear surface. Meanwhile, the dependences of the Doppler shifts on the wind speed and radar polarizations are also analyzed, and the comparisons are shown to confirm the sensitivities to the polarization and wind speed. At the same time, the results of the small perturbation method （SPM） and geometrical optics approximation （GO） are also presented for comparisons.展开更多
Using the US National Aeronautics and space Administration （NASA） Earth Observing-1 Mission （EO-1） hyperion hyperspectral remote sensing data, we study the shallow-water bathymetry inversion in Smith Island Bay. T...Using the US National Aeronautics and space Administration （NASA） Earth Observing-1 Mission （EO-1） hyperion hyperspectral remote sensing data, we study the shallow-water bathymetry inversion in Smith Island Bay. The fast line-of-sight atmospheric analysis of spectral hypercubes module is applied for atmospheric correction, and principal component analysis method combined with scatter diagram and maximum likelihood classification is used for seabed classification. The diffuse attenuation coefficient Kd is derived using quasi-analytical algorithm （QAA）, which performs well in optically deep water. Kd obtained from QAA requires correction, particularly those derived in some coastal areas with optically shallow water and calculated by direct inversion based on radiative transfer theory to obtain the bathymetry. The direct inversion method derives the water depth quickly, and matches the results from optimized algorithm.展开更多
This paper proposes a hybrid method based on the forward-backward method （FBM） and the reciprocity theorem （RT） for evaluating the scattering field from dielectric rough surface with a 2D target above it. Here, th...This paper proposes a hybrid method based on the forward-backward method （FBM） and the reciprocity theorem （RT） for evaluating the scattering field from dielectric rough surface with a 2D target above it. Here, the equivalent electric/magnetic current densities on the rough surface as well as the scattering field from it are numerically calculated by FBM, and the scattered field from the isolated target is obtained utilizing the method of moments （MOM）. Meanwhile, the rescattered coupling interactions between the target and the surface are evaluated employing the combination of FBM and RT. Our hybrid method is first validated by available MOM results. Then, the functional dependences of bistatic and monostatic scattering from the target above rough surface upon the target altitude, incident and scattering angles are numerically simulated and discussed. This study presents a numerical description for the scattering mechanism associated with rescattered coupling interactions between a target and an underlying randomly rough surface.展开更多
Since January 2012,the National Satellite Ocean Application Service has released operational wind products from the HY-2A scatterometer(HY2-SCAT),using the maximum-likelihood estimation(MLE) method with a median filte...Since January 2012,the National Satellite Ocean Application Service has released operational wind products from the HY-2A scatterometer(HY2-SCAT),using the maximum-likelihood estimation(MLE) method with a median filter. However,the quality of the winds retrieved from HY2-SCAT depends on the sub-satellite cross-track location,and poor azimuth separation in the nadir region causes particularly low-quality wind products in this region. However,an improved scheme,i.e.,a multiple solution scheme(MSS) with a two-dimensional variational analysis method(2DVAR),has been proposed by the Royal Netherlands Meteorological Institute to overcome such problems. The present study used the MSS in combination with a 2DVAR technique to retrieve wind data from HY2-SCAT observations. The parameter of the empirical probability function,used to indicate the probability of each ambiguous solution being the "true" wind,was estimated based on HY2-SCAT data,and the 2DVAR method used to remove ambiguity in the wind direction. A comparison between MSS and ECMWF winds showed larger deviations at both low wind speeds(below 4 m/s) and high wind speeds(above 17 m/s),whereas the wind direction exhibited lower bias and good stability,even at high wind speeds greater than 24 m/s. The two HY2-SCAT wind data sets,retrieved by the standard MLE and the MSS procedures were compared with buoy observations. The RMS error of wind speed and direction were 1.3 m/s and 17.4°,and 1.3 m/s and 24.0° for the MSS and MLE wind data,respectively,indicating that MSS wind data had better agreement with the buoy data. Furthermore,the distributions of wind fields for a case study of typhoon Soulik were compared,which showed that MSS winds were spatially more consistent and meteorologically better balanced than MLE winds.展开更多
The line-of-sight velocity of scattering facets is related to the Doppler signals of X-band coherent marine radar from the oceanic surface. First, the sign Doppler Estimator is applied to estimate the Doppler shift of...The line-of-sight velocity of scattering facets is related to the Doppler signals of X-band coherent marine radar from the oceanic surface. First, the sign Doppler Estimator is applied to estimate the Doppler shift of each radar resolution cell. And then, in terms of the Doppler shift, a retrieval algorithm extracting the vertical displacement of the sea surface has been proposed. The effects induced by radar look-direction and radar spatial resolution are both taken into account in this retrieval algorithm. The comparison between the sea surface spectrum of buoy data and the retrieved spectrum reveals that the function of the radar spatial resolution is equivalent to a low pass filter, impacting especially the spectrum of short gravity waves. The experimental data collected by McMaster IPIX radar are also used to validate the performance of the retrieval algorithm. The derived significant wave height and wave period are compared with the in situ measurements, and the agreement indicates the practicality of the retrieval technology.展开更多
The SMOS(soil moisture and ocean salinity) mission undertaken by the European Space Agency(ESA) has provided sea surface salinity(SSS) measurements at global scale since 2009.Validation of SSS values retrieved from SM...The SMOS(soil moisture and ocean salinity) mission undertaken by the European Space Agency(ESA) has provided sea surface salinity(SSS) measurements at global scale since 2009.Validation of SSS values retrieved from SMOS data has been done globally and regionally.However,the accuracy of SSS measurements by SMOS in the China seas has not been examined in detail.In this study,we compared retrieved SSS values from SMOS data with in situ measurements from a South China Sea(SCS) expedition during autumn 2011.The comparison shows that the retrieved SSS values using ascending pass data have much better agreement with in situ measurements than the result derived from descending pass data.Accuracy in terms of bias and root mean square error(RMS) of the SSS retrieved using three different sea surface roughness models is very consistent,regardless of ascending or descending orbits.When ascending and descending measurements are combined for comparison,the retrieved SSS using a semi-empirical model shows the best agreement with in situ measurements,with bias-0.33 practical salinity units and RMS 0.74.We also investigated the impact of environmental conditions of sea surface wind and sea surface temperature on accuracy of the retrieved SSS.The SCS is a semi-closed basin where radio frequencies transmitted from the mainland strongly interfere with SMOS measurements.Therefore,accuracy of retrieved SSS shows a relationship with distance between the validation sites and land.展开更多
Sea surface winds （SSWs） are vital to many meteorological and oceanographic applications, especially for regional study of short-range forecasting and Numerical Weather Prediction （NWP） assimilation. Spaceborne se...Sea surface winds （SSWs） are vital to many meteorological and oceanographic applications, especially for regional study of short-range forecasting and Numerical Weather Prediction （NWP） assimilation. Spaceborne seatterometers can provide global ocean surface vector wind products at high spatial resolution. However, given the limited spatial coverage and revisit time for an individual sensor, it is valuable to study improvements of multiple microwave scatterometer observations, including the advanced scatterometer onboard parallel satellites MetOp-A （ASCAT-A） and MetOp-B （ASCAT-B） and microwave scatterometers aboard Oceansat-2 （OSCAT） and HY-2A （HY2-SCAT）. These four scatterometer-derived wind products over the China Seas （0°-40°N, 105°-135°E） were evaluated in terms of spatial coverage, revisit time, bias of wind speed and direction, after comparison with ERA-Interim forecast winds from the European Centre for Medium-Range Weather Forecasts （ECMWF） and spectral analysis of wind components along the satellite track. The results show that spatial coverage of wind data observed by combination of the four sensors over the China Seas is about 92.8% for a 12-h interval at 12：00 and 90.7% at 24：00, respectively. The analysis of revisit time shows that two periods, from 5：30-8：30 UTC and 17：00-21：00 UTC each day, had no observations in the study area. Wind data observed by the four sensors along satellite orbits in one month were compared with ERA-Interim data, indicating that bias of both wind speed and direction varies with wind speed, especially for speeds less than 7 m/s. The bias depends on characteristics of each satellite sensor and its retrieval algorithm for wind vector data. All these results will be important as guidance in choosing the most suitable wind product for applications and for constructing blended SSW products.展开更多
In this paper, we present a method of ROV based image processing to restore underwater blurry images from the theory of light and image transmission in the sea. Computer is used to simulate the maximum detection range...In this paper, we present a method of ROV based image processing to restore underwater blurry images from the theory of light and image transmission in the sea. Computer is used to simulate the maximum detection range of the ROV under different water body conditions. The receiving irradiance of the video camera at different detection ranges is also calculated. The ROV’s detection performance under different water body conditions is given by simulation. We restore the underwater blurry images using the Wiener filter based on the simulation. The Wiener filter is shown to be a simple useful method for underwater image restoration in the ROV underwater experiments. We also present examples of restored images of an underwater standard target taken by the video camera in these experiments.展开更多
Using monthly gridded ocean pathfinder Sea Surface Temperature （SST） data with a spatial resolution of 4kin from AVHRR, variations of SST over the Kuroshio region northeast of Taiwan Is. during the past two decades ...Using monthly gridded ocean pathfinder Sea Surface Temperature （SST） data with a spatial resolution of 4kin from AVHRR, variations of SST over the Kuroshio region northeast of Taiwan Is. during the past two decades （1985-2003） are studied. Some interesting findings are as follows. （1） The climatological SST field shows an expected pattern with southwest-northeast orientated isotherms, and this pattern is mainly dominated by solar irradiance and regional circulation. However, the interannual variation of this pattern is very notable, in particular along Kuroshio path. The most dynamic region is located in the east coast of Taiwan, where cold upwelling is very energetic. （2） Seasonal variation of SST over this region is mainly controlled by see-saw variation of solar irradiance between two hemispheres, but the strong interannual fluctuation of SST is found to be locked to boreal winter （January, February, and last December）, and the energetic region is identified along Kuroshio path. This phenomenon seems to closely connect with El Nifio＇s phase locking characteristics. （3） SST anomalies over Kuroshio region have a positive correlation with El Nifio-Southem Oscillation （ENSO）, which is dramatic due to the weak （strong） North Equatorial Current （NEC） during El Nifio （La Nifia） events, and the weak （strong） NEC is supposed to induce a same polarity of SST variation along the Kuroshio path. How the interannual variation and seasonal variation interact each other and what is the mechanism between ENSO and the thermal and thermodynamic processes over this region deserve our further analyses.展开更多
The variability of the air-sea system in the low-frequency time domain can be decomposed into several systematic climate modes, namely, the decadal variability （DV） mode, the El Nino Southem Oscillation （ENSO） mod...The variability of the air-sea system in the low-frequency time domain can be decomposed into several systematic climate modes, namely, the decadal variability （DV） mode, the El Nino Southem Oscillation （ENSO） mode, the annual cycle （AC） mode, the semiannual cycle （ SC ） mode and the intraseasonal variability （ ISV ） mode. The combination of these primary modes in the air - sea system orchestrates a complex climate system. The multi-mode low-frequency variability in SST is investigated based on 22 a SST records from 1982 through 2003. The variation of SST in the past two decades undergoes a different combination of these dominant climate modes over different regions, which leads to an interesting new classification of the global ocean based on the relative importance of these modes. The new classification can provide ideal locations for better monitoring of these low-frequency modes in the scientific proof sense. Moreover, two no-annual variation and 14 no-semiannual variation oceanic points, termed annual and semiannual amphidromes, have been well defined in the AC and SC phase maps. The formation of these nodal points is attributed to the couplings of climate modes in EOF analysis results.展开更多
基金Supported by the National Basic Research Program of China (973 Program, Nos. 2009CB723905, 2006CB701300)the National High Technology Research and Development Program of China (863 Program, No. 2007AA12Z161)+3 种基金the NSFC (Nos. 40676094, 40721001, 40706060)MOST, China (No. 2007BAC23B05)Open Fund of Nanchang University (No. Z03975)the Open Fund of Ocean University of China for visiting Ph. D students.
文摘This paper demonstrates an atmospheric correction method to process MODIS/Aqua (Moderate-resolution Imaging Spectroradiometer) ocean color imagery over turbid coastal waters with the aid of concurrent CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization) aerosol data, assuming that there exists "nonturbid" water in the study area where MODIS aerosol optical properties can be retrieved accurately. Aerosol properties from CALIOP measurements were obtained and related to those from MODIS. This relationship, combined with CALIOP aerosol data, was extended to turbid water to derive MODIS aerosol properties, where atmospheric correction using MODIS data alone often fails. By combining MODIS and CALIOP data, aerosol signals were separated from the total signals at the satellite level, and water-leaving radiances in turbid waters were subsequently derived. This method was tested on several MODIS/Aqua ocean color images over South China turbid waters. Comparison with field data shows that this method was effective in reducing the errors in the retrieved water-leaving radiance values to some extent. In the Zhujiang (Pearl) River Estuary, this method did not overestimate the aerosol effects as severely, and provided far fewer negative water-leaving radiance values than the NASA (National Aeronautics and Space Administration) default methods that used MODIS data alone.
基金supported by the National Natural Science Foundation of China(No.60578038)Project 985 of the Remote Sensing Laboratory of the Ministry of Education of China,Ocean University of China.
文摘An airborne oceanographic lidar, with a frequency-tripled Q-switched Nd： YAG laser of 355 nm, has been designed to measure chlorophyll-a （Chl-a） concentration in the sea surface layer by the Ocean Remote Sensing Institute, OUC. The field experiment was carried out in the bay which is located south of the Liaodong Peninsula on the 10th of September 2005. After the flight, the raw data were processed and analyzed by the fluorescence-to-Raman ratio method with seawater attenuation coefficients calculated from signal profiles. The results of Chl-a concentration sea water were also compared with those of Chl-a concentration by measurements by lidar are shown. The measurements in clear a Moderate Resolution Imaging Spectroradiometer （MODIS）.
基金supported by the National High Technology Research and Development Program (2006AA06Z415)the Global Change Research Program of China (2012CB955603)
文摘A Multi-channel Oceanographic Fluorescence Lidar(MOFL), with a UV excitation at 355 nm and multiple receiving channels at typical wavelengths of fluorescence from oil spills and chlorophyll-a(Chl-a), has been developed using the Laser- induced Fluorescence(LIF) technique. The sketch of the MOFL system equipped with a compact multi-channel photomultiplier tube(MPMT) is introduced in the paper. The methods of differentiating the oil fluorescence from the background water fluorescence and evaluating the Chl-a concentration are described. Two field experiments were carried out to investigate the field performance of the system, i.e., an experiment in coastal areas for oil pollution detection and an experiment over the Yellow Sea for Chl-a monitoring. In the coastal experiment, several oil samples and other fluorescence substances were used to analyze the fluorescence spectral characteristics for oil identification, and to estimate the thickness of oil films at the water surface. The experiment shows that both the spectral shape of fluorescence induced from surface water and the intensity ratio of two channels(I495/I405) are essential to determine oil-spill occurrence. In the airborne experiment, MOFL was applied to measure relative Chl-a concentrations in the upper layer of the ocean. A comparison of relative Chl-a concentration measurements by MOFL and the Moderate Resolution Imaging Spectroradiometer(MODIS) indicates that the two datasets are in good agreement. The results show that the MOFL system is capable of monitoring oil spills and Chl-a in the upper layer of ocean water.
基金The National Natural Science Foundation of China under contract Nos 41376179 and 41106153
文摘Azimuth ambiguities （ghost targets） discrimination is of great interest with the development of a synthet- ic aperture radar （SAR）. And the azimuth ambiguities are often mistaken as actual targets and cause false alarms. For actual targets, HV channel signals acquired by a fully polarimetric SAR are approximately equal to a VH channel in magnitude and phase, i.e., the reciprocity theorem applies, but shifted in phase about ±π for the first-order azimuth ambiguities. Exploiting this physical behavior, the real part of the product of the two cross-polarized channels, i.e. （SHVSVH）, hereafter called A12r, is employed as a new parameter for a target detection at sea. Compared with other parameters, the contrast of A12r image between a target and the surrounding sea surface will be obviously increased when A12r image is processed by mean filtering algo- rithm. Here, in order to detect target with constant false-alarm rates （CFARs）, an analytical expression for the probability density function （pdf） ofA12r is derived based on the complexWishart-distribution. Because a value of A12r is greater/less than 0 for real target/its azimuth ambiguities, the first-order azimuth ambiguities can be completely removed by this A12r-based CFAR technology. Experiments accomplished over C-band RADARSAT-2 fully polarimetric imageries confirm the validity.
基金supported by the National Natural Science Foundation of China (No.40976060)the National High Technology Research and Development Program of China (No.2006AA09Z148)
文摘In order to obtain the three dimensional (3D) fluorescence spectra directly and rapidly by in situ and in vivo phytoplankton monitoring, a prototype ocean lidar system with the ability of measuring 3D fluorescence spectra is developed, in which an optical parametric oscillator (OPO) and a grating spectrometer are employed. Using the system, the 3D fluorescence spectra of five typical red-tide species in the East China Sea are measured in lab. The preliminary experimental results show the feasibility of monitoring and classifying phytoplankton by this ocean lidar.
基金supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 40906088)Specialized Research Fund for the Doctoral Program of Higher Education of China
文摘Based on the first order small slope approximation theory （SSA-I） for oceanic surface electromagnetic scattering, this paper predicts the Doppler shifts induced by wave displacements. Theoretical results from three distinct hydrody- namic models are compared： a linear model, the nonlinear Barrick model, and the nonlinear Creamer model. Meanwhile, the predicted Doppler shifts are also compared with the results associated to the resonant Bragg waves and the so-called long waves in the framework of the two-scale model. The dependences of the predicted Doppler shifts on the incident angle, the radar frequency, and the wind speed are discussed. At large incident angles, the predicted Doppler shifts for the linear and nonlinear Barrick models are found to be insensitive to the wind speed and this phenomenon is not coincident with the experimental data. The conclusions obtained in this paper are promising for better understanding the properties of time dependent radar echoes from oceanic surfaces.
基金This study is supported by the Doctoral Startup Foundation of 0cean University of China(2003)partly supported by the National Science Foundation of China(40506035)The altimeter products were produced by the CLS Space 0ceanography Division as part of the Environment and Climate EU ENACT project(EVK2-CT2001-00117)and with support from CNES.
文摘The Sea Level Anomaly-Torque(SLAT,relative to a reference location in the Pacific Ocean),which means the total torque of the gravity forces of sea waters with depths equal to the Sea Level Anomaly(S/A)in the tropical Pacific Ocean,is defined in this study.The time series of the SLAT from merged altimeter data(1993-2003)had a great meridional variation during the 1997-1998 E1 Nifio event.By using historical upper layer temperature data(1955-2003)for the tropical Pacific Ocean,the temperature-based SLAT is also calculated and the meridional variation can be found in the historical E1 Nifio events(1955-2003),which suggests that the meridional shifts of the sea level anomaly are also intrinsic oscillating modes of the E1 Nifio cycles like the zonal shifts.
基金Supported by the National Natural Science Foundation of China under projects (Nos.40730530,40675016,40706056)
文摘The oceanic warm pool （OWP） defined by sea surface temperature （SST） is known as the ＂heat reservoir＂ in the ocean. The warmest portion in the ocean mirrors the fact that the wettest region with the largest accumulation of water vapor （WV） in the atmosphere, termed atmospheric wet pool （AWP）, should be identified because of the well-known Clausius-Clapeyron relationship between SST and WV. In this study, we used 14-year simultaneous observations of WV and SST from January 1988 to December 2001 to define the AWP and investigate its coupling and co-variations with the OWE The joint examination of the area variations, centroid locations, and zonal migrations of the AWP and OWP lead to a number of interesting findings. The results hopefully can contribute to our understanding of the air-sea interaction in general and characterization of E1 Nifio/La Nifia events in particular.
文摘The heat distributions in the upper layers of the ocean have been studied and some important low frequency oscillations （LFOs） are already found and quantified by using various characteristic factors. In this paper, the ‘heat center＇ of a sea area is defined with a simple method. Then the temperature data set of the upper layer of the global ocean （from surface down to 400 m, 1955-2003） is analyzed to detect the possible LFOs. Not only some zonal LFOs, which were reported early, but also some strong LFOs of the vertical and meridional heat distribution, which might imply some physical sense, are detected. It should be noted that the similar vertical oscillation pattern can be found in the Pacific Ocean, Atlantic Ocean and Indian Ocean. Results from some preliminary studies show that the vertical LFO might be caused by the solar irradiance anomalies. This study may help reveal some unknown dynamical processes in the global oceans and may also benefit other related studies.
基金The National Natural Science Foundation of China under contract No.61601132
文摘This paper proposes a speckle-suppression method for ocean internal solitary wave（ISW） synthetic aperture radar（SAR） images by using the curvelet transform.The band-shaped signatures of ocean ISWs in SAR images show obvious scale and directional characteristics.The curvelet transform possesses a very high scale and directional sensitivity.Therefore,the curvelet transform is very efficient in analyzing wave signals in SAR images.A noisy ocean ISW SAR image can be decomposed at different scales,directions,and positions using the curvelet transform.The information of the ISWs is centralized in the curvelet coefficients of certain directions under certain scales,whereas the speckle noise is distributed in every scale and direction.By manipulating the curvelet coefficients,the signals of the ISWs can be extracted from the noisy SAR image.Finally,the speckle noise is suppressed and the ISW feature is enhanced by adding the signals of the ISWs back to the original SAR image.Experiments demonstrate the effectiveness of this method.
基金Supported by the Young Scientists Fhnd of the National Natural Science Foundation of China under Grant No 40906088, the National Natural Science Foundation of China under Grant No 60971067, and Specialized Research Fund for the Doctoral Program of Higher Education under Grant No 200804231021.
文摘Based on the general framework of the second order small slope approximation （SSA-II）, Doppler shifts of backscattered fields from linear and nonlinear hydrodynamic sea surfaces are predicted. Comparisons are performed at incidence angles varying from near-nadir to moderate-grazing for different radar frequencies. The predicted Doppler shifts for nonlinear hydrodynamic model are somewhat larger than the results corresponding to linear surface. Meanwhile, the dependences of the Doppler shifts on the wind speed and radar polarizations are also analyzed, and the comparisons are shown to confirm the sensitivities to the polarization and wind speed. At the same time, the results of the small perturbation method （SPM） and geometrical optics approximation （GO） are also presented for comparisons.
基金supported by the Key Funds Programof the National Natural Science of China under Grant No. 60638020
文摘Using the US National Aeronautics and space Administration （NASA） Earth Observing-1 Mission （EO-1） hyperion hyperspectral remote sensing data, we study the shallow-water bathymetry inversion in Smith Island Bay. The fast line-of-sight atmospheric analysis of spectral hypercubes module is applied for atmospheric correction, and principal component analysis method combined with scatter diagram and maximum likelihood classification is used for seabed classification. The diffuse attenuation coefficient Kd is derived using quasi-analytical algorithm （QAA）, which performs well in optically deep water. Kd obtained from QAA requires correction, particularly those derived in some coastal areas with optically shallow water and calculated by direct inversion based on radiative transfer theory to obtain the bathymetry. The direct inversion method derives the water depth quickly, and matches the results from optimized algorithm.
基金Project supported by the National Natural Science Foundation of China (Grant No 60571058)the National Defense Foundation of China
文摘This paper proposes a hybrid method based on the forward-backward method （FBM） and the reciprocity theorem （RT） for evaluating the scattering field from dielectric rough surface with a 2D target above it. Here, the equivalent electric/magnetic current densities on the rough surface as well as the scattering field from it are numerically calculated by FBM, and the scattered field from the isolated target is obtained utilizing the method of moments （MOM）. Meanwhile, the rescattered coupling interactions between the target and the surface are evaluated employing the combination of FBM and RT. Our hybrid method is first validated by available MOM results. Then, the functional dependences of bistatic and monostatic scattering from the target above rough surface upon the target altitude, incident and scattering angles are numerically simulated and discussed. This study presents a numerical description for the scattering mechanism associated with rescattered coupling interactions between a target and an underlying randomly rough surface.
基金Supported by the National High Technology Research and Development Program of China(863 Program)(No.2013AA09A505)the Shandong Joint Fund for Marine Science Research Centers(No.U1406404)+1 种基金the National Natural Science Foundation of China(No.41106152)he National Key Technology R&D Program of China(No.2013BAD13B01)
文摘Since January 2012,the National Satellite Ocean Application Service has released operational wind products from the HY-2A scatterometer(HY2-SCAT),using the maximum-likelihood estimation(MLE) method with a median filter. However,the quality of the winds retrieved from HY2-SCAT depends on the sub-satellite cross-track location,and poor azimuth separation in the nadir region causes particularly low-quality wind products in this region. However,an improved scheme,i.e.,a multiple solution scheme(MSS) with a two-dimensional variational analysis method(2DVAR),has been proposed by the Royal Netherlands Meteorological Institute to overcome such problems. The present study used the MSS in combination with a 2DVAR technique to retrieve wind data from HY2-SCAT observations. The parameter of the empirical probability function,used to indicate the probability of each ambiguous solution being the "true" wind,was estimated based on HY2-SCAT data,and the 2DVAR method used to remove ambiguity in the wind direction. A comparison between MSS and ECMWF winds showed larger deviations at both low wind speeds(below 4 m/s) and high wind speeds(above 17 m/s),whereas the wind direction exhibited lower bias and good stability,even at high wind speeds greater than 24 m/s. The two HY2-SCAT wind data sets,retrieved by the standard MLE and the MSS procedures were compared with buoy observations. The RMS error of wind speed and direction were 1.3 m/s and 17.4°,and 1.3 m/s and 24.0° for the MSS and MLE wind data,respectively,indicating that MSS wind data had better agreement with the buoy data. Furthermore,the distributions of wind fields for a case study of typhoon Soulik were compared,which showed that MSS winds were spatially more consistent and meteorologically better balanced than MLE winds.
基金The National Natural Science Foundation of China under contract Nos 41376179 and 41106153the National Natural Science Foundation of China-Shandong Joint Fund for Marine Science Research Centers under contract No.U1406404
文摘The line-of-sight velocity of scattering facets is related to the Doppler signals of X-band coherent marine radar from the oceanic surface. First, the sign Doppler Estimator is applied to estimate the Doppler shift of each radar resolution cell. And then, in terms of the Doppler shift, a retrieval algorithm extracting the vertical displacement of the sea surface has been proposed. The effects induced by radar look-direction and radar spatial resolution are both taken into account in this retrieval algorithm. The comparison between the sea surface spectrum of buoy data and the retrieved spectrum reveals that the function of the radar spatial resolution is equivalent to a low pass filter, impacting especially the spectrum of short gravity waves. The experimental data collected by McMaster IPIX radar are also used to validate the performance of the retrieval algorithm. The derived significant wave height and wave period are compared with the in situ measurements, and the agreement indicates the practicality of the retrieval technology.
基金Supported by the National Natural Science Foundation of China(Nos.41006110,41106155)
文摘The SMOS(soil moisture and ocean salinity) mission undertaken by the European Space Agency(ESA) has provided sea surface salinity(SSS) measurements at global scale since 2009.Validation of SSS values retrieved from SMOS data has been done globally and regionally.However,the accuracy of SSS measurements by SMOS in the China seas has not been examined in detail.In this study,we compared retrieved SSS values from SMOS data with in situ measurements from a South China Sea(SCS) expedition during autumn 2011.The comparison shows that the retrieved SSS values using ascending pass data have much better agreement with in situ measurements than the result derived from descending pass data.Accuracy in terms of bias and root mean square error(RMS) of the SSS retrieved using three different sea surface roughness models is very consistent,regardless of ascending or descending orbits.When ascending and descending measurements are combined for comparison,the retrieved SSS using a semi-empirical model shows the best agreement with in situ measurements,with bias-0.33 practical salinity units and RMS 0.74.We also investigated the impact of environmental conditions of sea surface wind and sea surface temperature on accuracy of the retrieved SSS.The SCS is a semi-closed basin where radio frequencies transmitted from the mainland strongly interfere with SMOS measurements.Therefore,accuracy of retrieved SSS shows a relationship with distance between the validation sites and land.
基金Supported by the Shandong Joint Fund for Marine Science Research Centers(No.U1406404)the National High Technology Research and Development Program of China(No.2013AA09A505)the National Basic Research Program of China(973 Program)(No.2012CB955600)
文摘Sea surface winds （SSWs） are vital to many meteorological and oceanographic applications, especially for regional study of short-range forecasting and Numerical Weather Prediction （NWP） assimilation. Spaceborne seatterometers can provide global ocean surface vector wind products at high spatial resolution. However, given the limited spatial coverage and revisit time for an individual sensor, it is valuable to study improvements of multiple microwave scatterometer observations, including the advanced scatterometer onboard parallel satellites MetOp-A （ASCAT-A） and MetOp-B （ASCAT-B） and microwave scatterometers aboard Oceansat-2 （OSCAT） and HY-2A （HY2-SCAT）. These four scatterometer-derived wind products over the China Seas （0°-40°N, 105°-135°E） were evaluated in terms of spatial coverage, revisit time, bias of wind speed and direction, after comparison with ERA-Interim forecast winds from the European Centre for Medium-Range Weather Forecasts （ECMWF） and spectral analysis of wind components along the satellite track. The results show that spatial coverage of wind data observed by combination of the four sensors over the China Seas is about 92.8% for a 12-h interval at 12：00 and 90.7% at 24：00, respectively. The analysis of revisit time shows that two periods, from 5：30-8：30 UTC and 17：00-21：00 UTC each day, had no observations in the study area. Wind data observed by the four sensors along satellite orbits in one month were compared with ERA-Interim data, indicating that bias of both wind speed and direction varies with wind speed, especially for speeds less than 7 m/s. The bias depends on characteristics of each satellite sensor and its retrieval algorithm for wind vector data. All these results will be important as guidance in choosing the most suitable wind product for applications and for constructing blended SSW products.
基金supported by the National Natural Science Foundation of China(No.60178017)
文摘In this paper, we present a method of ROV based image processing to restore underwater blurry images from the theory of light and image transmission in the sea. Computer is used to simulate the maximum detection range of the ROV under different water body conditions. The receiving irradiance of the video camera at different detection ranges is also calculated. The ROV’s detection performance under different water body conditions is given by simulation. We restore the underwater blurry images using the Wiener filter based on the simulation. The Wiener filter is shown to be a simple useful method for underwater image restoration in the ROV underwater experiments. We also present examples of restored images of an underwater standard target taken by the video camera in these experiments.
基金Cosponsored by the NSFC (No. 40545018) and the National Basic Research Program of China (No. 2005CB422308).
文摘Using monthly gridded ocean pathfinder Sea Surface Temperature （SST） data with a spatial resolution of 4kin from AVHRR, variations of SST over the Kuroshio region northeast of Taiwan Is. during the past two decades （1985-2003） are studied. Some interesting findings are as follows. （1） The climatological SST field shows an expected pattern with southwest-northeast orientated isotherms, and this pattern is mainly dominated by solar irradiance and regional circulation. However, the interannual variation of this pattern is very notable, in particular along Kuroshio path. The most dynamic region is located in the east coast of Taiwan, where cold upwelling is very energetic. （2） Seasonal variation of SST over this region is mainly controlled by see-saw variation of solar irradiance between two hemispheres, but the strong interannual fluctuation of SST is found to be locked to boreal winter （January, February, and last December）, and the energetic region is identified along Kuroshio path. This phenomenon seems to closely connect with El Nifio＇s phase locking characteristics. （3） SST anomalies over Kuroshio region have a positive correlation with El Nifio-Southem Oscillation （ENSO）, which is dramatic due to the weak （strong） North Equatorial Current （NEC） during El Nifio （La Nifia） events, and the weak （strong） NEC is supposed to induce a same polarity of SST variation along the Kuroshio path. How the interannual variation and seasonal variation interact each other and what is the mechanism between ENSO and the thermal and thermodynamic processes over this region deserve our further analyses.
基金This research was jointly supported by the National Basic Research Program of China under contract N0.2005CB422308the National Natural Science Foundation of China under Contract N0.40545018the National Key laboratory of Remote Sensing Sciences.
文摘The variability of the air-sea system in the low-frequency time domain can be decomposed into several systematic climate modes, namely, the decadal variability （DV） mode, the El Nino Southem Oscillation （ENSO） mode, the annual cycle （AC） mode, the semiannual cycle （ SC ） mode and the intraseasonal variability （ ISV ） mode. The combination of these primary modes in the air - sea system orchestrates a complex climate system. The multi-mode low-frequency variability in SST is investigated based on 22 a SST records from 1982 through 2003. The variation of SST in the past two decades undergoes a different combination of these dominant climate modes over different regions, which leads to an interesting new classification of the global ocean based on the relative importance of these modes. The new classification can provide ideal locations for better monitoring of these low-frequency modes in the scientific proof sense. Moreover, two no-annual variation and 14 no-semiannual variation oceanic points, termed annual and semiannual amphidromes, have been well defined in the AC and SC phase maps. The formation of these nodal points is attributed to the couplings of climate modes in EOF analysis results.
