Atmospheric correction of ocean color imagery over turbid coastal waters using active and passive remote sensing

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.

Field Experiments of Multi-Channel Oceanographic Fluorescence Lidar for Oil Spill and Chlorophyll-a Detection

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.

Field Detection of Chiorophyll-a Concentration in the Sea Surface Layer by an Airborne Oceanographic Lidar

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 experi- ment 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 measurements by lidar are shown. The measurements in clear sea water were also compared with those of Chl-a concentration by a Moderate Resolution Imaging Spectroradiometer (MODIS).

Targets detecting in the ocean using the cross-polarized channels of fully polarimetric SAR data

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.

The atmospheric wet pool:definition and comparison with the oceanic warm pool

海洋的温暖的水池(OWP ) 海运定义表面温度(SST ) 在海洋作为“热水库”被知道。在海洋的最温暖的部分反射事实有在空气的水蒸汽(WV ) 的最大的累积的最湿的区域，称为的大气的湿水池(AWP ) ，因为在 SST 和 WV 之间的著名 Clausius-Clapeyron 关系，应该被识别。在这研究，我们从 1988 年 1 月使用了 WV 和 SST 的 14 年的同时的观察到 2001 年 12 月定义 AWP 并且调查它与 OWP 联合和共变。AWP 和 OWP 的区域变化，矩心地点，和带的迁居的联合考试导致很多有趣的调查结果。结果能希望贡献我们特别地一般来说理解由空至海的相互作用和 El Niño/La Niña 事件的描述。

Low Frequency Oscillations of the Heat Distribution in the Global Upper Ocean Layers

在海洋的上面的层的热分布被使用各种各样的典型因素学习了，一些重要低频率摆动(LFO ) 已经被发现并且确定。在这篇论文，“一个海区域的热中心”与一个简单方法被定义。然后全球海洋的上面的层的温度数据集(从在 to400m 下面的表面， 1955-2003 ) 被分析检测可能的 LFO。不仅一些带的 LFO，早被报导，而且垂直、南方的热分发的一些强壮的 LFO 被检测。类似的垂直摆动模式能在太平洋，大西洋和印度洋被发现，这应该被注意。从一些初步的研究的结果证明垂直 LFO 可能被太阳的发光异例引起。这研究可以帮助在全球海洋揭示一些未知动态过程并且可以也有益于另外的相关研究。

Investigation on the Doppler shifts induced by 1-D ocean surface wave displacements by the first order small slope approximation theory:comparison of hydrodynamic models

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.

Meridional Variation of the 1955-2003 Sea Level Anomalies in the Tropical Pacific Ocean Associated with El Nifio Events

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 (SLA) 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 El Nio event. By using historical upper layer temperature data (1955-2003) for the tropical Pacific Ocean, the tempera- ture-based SLAT is also calculated and the meridional variation can be found in the historical El Nio events (1955-2003), which suggests that the meridional shifts of the sea level anomaly are also intrinsic oscillating modes of the El Nio cycles like the zonal shifts.

Speckle suppression in synthetic aperture radar ocean internal solitary wave images with curvelet transform

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.

Doppler Shifts of Backscattering Fields from One Dimensional Linear and Nonlinear Oceanic Surface by SSA-Ⅱ

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.

Solution of scattering from rough surface with a 2D target above it by a hybrid method based on the reciprocity theorem and the forward-backward method

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.

Preliminary validation of SMOS sea surface salinity measurements in the South China Sea

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.

The measurement of sea surface profile with X-band coherent marine radar

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.

Direct inversion of shallow-water bathymetry from EO-1 hyperspectral remote sensing data

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.

An improved wind retrieval algorithm for the HY-2A scatterometer

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.

Assessment of wind products obtained from multiple microwave scatterometers over the China Seas

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 scatterometers 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 Met Op-A(ASCAT-A) and Met Op-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.

SST variations of the Kuroshio from AVHRR observation

Using monthly gridded ocean pathfinder Sea Surface Temperature (SST) data with a spatial resolution of 4km from AVHRR, variations of SST over the Kuroshio region northeast of Taiwan Is. dur-ing the past two decades (1985-2003) are studied. Some interesting findings are as follows. (1) The cli-matological 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 varia-tion 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 Nio’s phase locking characteristics. (3) SST anomalies over Kuroshio region have a positive correlation with El Nio-Southern Oscillation (ENSO), which is dramatic due to the weak (strong) North Equatorial Current (NEC) during El Nio (La Nia) 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.

ROV Based Underwater Blurred Image Restoration

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.

A global analysis of multi-mode sea surface temperature pattern

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.

Performance evaluation of operational atmospheric correction algorithms over the East China Seas

To acquire high-quality operational data products for Chinese in-orbit and scheduled ocean color sensors, the performances of two operational atmospheric correction(AC) algorithms(ESA MEGS 7.4.1 and NASA Sea DAS 6.1) were evaluated over the East China Seas(ECS) using MERIS data. The spectral remote sensing reflectance R_(rs)(λ), aerosol optical thickness(AOT), and ?ngstr?m exponent(α) retrieved using the two algorithms were validated using in situ measurements obtained between May 2002 and October 2009. Match-ups of R_(rs), AOT, and α between the in situ and MERIS data were obtained through strict exclusion criteria. Statistical analysis of R_(rs)(λ) showed a mean percentage difference(MPD) of 9%–13% in the 490–560 nm spectral range, and significant overestimation was observed at 413 nm(MPD>72%). The AOTs were overestimated(MPD>32%), and although the ESA algorithm outperformed the NASA algorithm in the blue-green bands, the situation was reversed in the red-near-infrared bands. The value of α was obviously underestimated by the ESA algorithm(MPD=41%) but not by the NASA algorithm(MPD=35%). To clarify why the NASA algorithm performed better in the retrieval of α, scatter plots of the α single scattering albedo(SSA) density were prepared. These α-SSA density scatter plots showed that the applicability of the aerosol models used by the NASA algorithm over the ECS is better than that used by the ESA algorithm, although neither aerosol model is suitable for the ECS region. The results of this study provide a reference to both data users and data agencies regarding the use of operational data products and the investigation into the improvement of current AC schemes over the ECS.