Calibration and validation of the ocean color version-3 product from ADEOS OCTS

We present calibration and validation results of the OCTS’s ocean color version-3 product, which mainly consists of the chlorophyll-a concentration (Chl-a) and the normalized water-leaving radiance (_nL_w). First, OCTS was calibrated for the inter-detector sensitivity difference, offset, and absolute sensitivity using external calibration source. It was also vicariously calibrated using in-situ measurements for water (Chl-a and_nL_w) and atmosphere (optical thickness), which were acquired synchronously with OCTS under cloud-free conditions. Second, the product was validated using selected 17 in-situ Chl-a and 11 in-situ_nL_w measurements. We confirmed that Chl-a was estimated with an accuracy of 68% for Chl-a less than 2 mg/m³, and_nL_w from 94% (band 2) to 128% (band 4). Geometric accuracy was improved to 1.3 km. Stripes were significantly reduced by modifying the detector normalization factor as a function of input radiance.     

Validation of ADEOS-II GLI ocean color products using in-situ observations

The Global Imager (GLI) aboard the Advanced Earth Observing Satellite-II (ADEOS-II) made global observations from 2 April 2003 to 24 October 2003. In cooperation with several institutes and scientists, we obtained quality controlled match-ups between GLI products and in-situ data, 116 for chlorophyll-a concentration (CHLA), 249 for normalized water-leaving radiance (nLw) at 443 nm, and 201 for aerosol optical thickness at 865 nm (Tau_865) and Angstrom exponent between 520 and 865 nm (Angstrom). We evaluated the GLI ocean color products and investigated the causes of errors using the match-ups. The median absolute percentage differences (MedPD) between GLI and in-situ data were 14.1–35.7% for nLws at 380–565 nm, 52.5–74.8% nLws at 625–680 nm, 47.6% for Tau_865, 46.2% for Angstrom, and 46.6% for CHLA, values that are comparable to the ocean-color products of other sensors. We found that some errors in GLI products are correlated with observational conditions; nLw values were underestimated when nLw at 680 nm was high, CHLA was underestimated in absorptive aerosol conditions, and Tau_865 was overestimated in sunglint regions. The error correlations indicate that we need to improve the retrievals of the optical properties of absorptive aerosols and seawater and sea surface reflection for further applications, including coastal monitoring and the combined use of products from multiple sensors.     

空间变异对海洋叶绿素a质量浓度遥感产品精度验证的影响

海洋叶绿素a质量浓度遥感产品是海洋初级生产力与海洋生态系统固碳能力研究的重要数据源,为了保证数据的可靠性,对遥感产品进行精度验证以及验证误差的成因分析尤为重要.遥感产品的验证过程中,由于空间变异的存在,使得遥感像元尺度内的实测数据具有不同的离散程度和统计分布特征,并由此产生了不同的误差统计结果.本文选择MODIS-Aq...     

Ocean color products retrieval and validation around China coast with MODIS

Waters along China coast are very turbid with high concentrations of suspended sediment nearly all the time,especially at the Hangzhou Bay,the Changjiang (Yangtze) River Estuary and the shoal along Jiangsu Province.In these turbid and optically complex waters,the standard MODIS ocean color products tend to have invalid values.Because the water-leaving radiances in the near-infrared (NIR) are significant resulting from the strong scattering of suspended particles,the standard MODIS atmospheric correction algorithm often gets no results or produces significant errors.And because of the complex water optical properties,the OC3 model used in the standard MODIS data processing tends to get extremely high chlorophyll-a (Chl-a) concentrations.In this paper,we present an atmospheric correction approach using MODIS short wave infrared (SWIR) bands based on the fact that water-leaving radiances are negligible in the SWIR region because of the extreme strong absorption of water even in turbid waters.A regional Chl-a concentration estimation model is also constructed for MODIS from in situ data.These algorithms are applied to MODIS Aqua data processing in the China coastal regions.In situ data collected in the Yellow Sea and the East China Sea in spring and autumn,2003 are used to validate the performance.Reasonably good results have been obtained.It is noted that water-leaving reflectance in the NIR bands are significant in waters along the China coast with high sediment loadings.The satellite derived and in-situ reflectance spectra can match in the turbid waters along China coast,and there is relatively good linear relationship between satellite derived and in-situ reflectance.The RMSE value of Rrs(λ) is 0.0031 sr ?1 for all the nine ocean color bands (412 to 869 nm).The satellite-derived Chl-a value is in the reasonable range and the root mean square percentage difference is 46.1%.     

High-resolution regional ocean dynamics simulation in the southwestern tropical Atlantic

The southwestern tropical Atlantic (05°S–25°S/20°W–47°W), where part of the South Equatorial Current (SEC) enters at its eastern border, is of particular interest as it is fed by many western boundary currents along the eastern Brazilian continental shelf. However, the long-term variability of the dynamics in this region, which are also important as they contribute to the climate over northeastern Brazil, is largely unknown. We use the Regional Ocean Model System (ROMS) here for the first time in this area to simulate the ocean circulation with an isotropic horizontal grid resolution of 1/12° and 40 terrain-following layers. As a primary evaluation of the ROMS configuration, we explore surface and vertical thermal structures, the surface mixed layer, and mass transports within the upper levels. Interannual variability results are compared with the first two-year series of observed thermal profiles derived from the three PIRATA-SWE moorings. The simulated thermal structure in the upper ocean layers agrees well with in-situ data. ROMS simulations point out a broad and relatively weak SEC flow composed of a sequence of more or less defined near-surface cores. The westward SEC transport for the upper 400 m along the PIRATA-SWE section, calculated from the ROMS simulation for 2005–2007, shows an average volume transport of 14.9 Sv, with a maximum observed in JFM (15.7 Sv), and a minimum during MJJ (13.8 Sv). ROMS results indicate that the 2005–2007 seasonal near-surface westward SEC transport is modulated by the zonal wind variability. Three zonal sections extending from the American continent to the PIRATA buoy sites confirm that stronger northward NBUC transport and decreasing BC transport were achieved during May 2006 and May 2007, i.e. at the time the sSEC bifurcation reaches its southernmost position. On the other hand, the maximum southward BC flow was verified during January 2006, January 2007 and March 2007, with a minimum northward NBUC flow in December 2005 and October/December 2006, corresponding to the period when the sSEC bifurcation reaches its lowest latitude (OND). Sea Surface Height (SSH) and the surface Eddy Kinetic Energy (EKE) derived from simulations and AVISO Rio05 product point out the highest surface meso-scale activity (EKE  50 cm² s⁻²) along the cSEC and NBUC/BC patches. Preliminary results provide additional ingredients in the complexity of the SEC divergence region and encourage us to conduct a more detailed exploration of the dynamics of this region using the ROMS. This also shows the need to continue, extend, and vertically upgrade the observational PIRATA-SWE array system, especially with more levels of salinity measurements and the installation of current measurements.     

A simulation of radiation imagery for ocean color satellite

The ocean color satellite is mainly applied to measure the water constituents such as chlorophyll, suspended material and yellow substance. The leaving water radiance is very small part of the total radiance arrival at the sensor about 3%-15%,and depends on the properties of the orbit and sensor. Before the satellite is launched on the orbit, it needs to simulate the radiation imagery in order to evaluate the quality and availability of the satellite data.
If the parameters of satellite orbit, the properties of sensor as well as the characteristics of the atmosphere and water have been known, the radiation scattered by air molecule and aerosol, reflected solar radiation, water leaving and total radiation arriving at the sensor can be simulated by the models of radiation transfer in the atmosphere, air/water and subsurface water. In this paper, the mechanism, models and procedures of the radiation simulation are first discussed and employed to simulate a series of imageries for the Chinese satellites FY-lA, FY-1B, and ROCSAT-1 (Taiwan, China), and American satellite Ses STAR. Their results show that the solar irradiance arrived at water surface and the sun glitter mainly affect the quality and availability of satellite data, which depend on the orbit properties. Finally, some suggestions are proposed to improve the quality and availability of a:ean color satellite data.     

Retrospective satellite ocean color analysis of purposeful and natural ocean iron fertilization

Significant effort has been invested in understanding the role of iron in marine ecosystems over the past few decades. What began as shipboard amendment experiments quickly grew into a succession of in situ, mesoscale ocean iron fertilization (OIF) experiments carried out in all three high nutrient low chlorophyll (HNLC) regions of the world ocean. Dedicated process studies have also looked at regions of the ocean that are seasonally exposed to iron-replete conditions as natural OIF experiments. However, one problem common to many OIF experiments is determination of biological response beyond the duration of the experiment (typically<1 month). Satellite-derived products have been used to address this shortcoming with some success, but thus far, have been limited snapshots of a single parameter, chlorophyll. Here, we investigate phytoplankton responses to OIF in both purposeful and naturally iron enriched systems using estimates of chlorophyll (Chl), phytoplankton carbon biomass (C_phyto), their ratio (Chl:C_phyto) and two fluorescence indices, fluorescence per unit chlorophyll (FLH:Chl) and the chlorophyll fluorescence efficiency (ϕ_f). These quantities allow partitioning of the biological response to OIF into that due to changes in biomass and that due to phytoplankton physiology. We find that relative increases in Chl (∼10–20x) following OIF far exceed increases in C_phyto (<4–5x), suggesting that a significant fraction of the observed Chl increase is associated with physiological adjustment to increased growth rates, photoacclimation, and floristic shifts in the phytoplankton community. Further, a consistent pattern of decreased satellite fluorescence efficiency (FLH:Chl or ϕ_f) following OIF is observed that is in agreement with current understanding of phytoplankton physiological responses to relief from iron stress. The current study extends our ability to retrieve phytoplankton physiology from space-based sensors, strengthens the link between satellite fluorescence and iron availability, and shows that satellite ocean color analyses provide a unique tool for monitoring OIF experiments.     

通用型海洋水色遥感精确瑞利散射查找表

当前对海洋水色遥感精确瑞利散射的计算均采用查找表方式进行,但由于这些查找表是针对特定遥感器生成的,无法直接应用于新的水色遥感器,给实际应用带来一定程度的麻烦,为此提出了一种通用的海洋水色遥感精确瑞利散射查找表.首先,详细地推导了加倍法解大气矢量辐射传输方程的基本关系式和实际的计算原理.通过与MODIS精确瑞利散射查找表计算结果比较,证明利用加倍法计算瑞利散射的精度优于0.25%,完全能够满足当前海洋水色遥感大气校正对瑞利散射计算精度的要求,并可以用来生成精确瑞利散射查找表.其次,利用加倍法解大气矢量辐射传输方程,生成了一个通用的海洋水色遥感精确瑞利散射查找表,验证结果表明该查找表可用于所有水色遥感器的精确瑞利散射计算,且计算精度优于0.5%.     

Short-term variability of suspended sediment and phytoplankton in Tampa Bay,Florida: Observations from a coastal oceanographic tower and ocean color satellites

We examined short-term phytoplankton and sediment dynamics in Tampa Bay with data collected between 8 December 2004 and 17 January 2005 from optical, oceanographic, and meteorological sensors mounted on a coastal oceanographic tower and from satellite remote sensing. Baseline phytoplankton (chlorophyll-a, Chl) and sediment concentrations (particle backscattering coefficient at 532 nm, bbp(532)) were of the order of 3.7 mg m⁻³ and 0.07 m⁻¹, respectively, during the study period. Both showed large fluctuations dominated by semidiurnal and diurnal frequencies associated with tidal forcing. Three strong wind events (hourly averaged wind speed >8.0 m s⁻¹) generated critical bottom shear stress of >0.2 Pa and suspended bottom sediments that were clearly observed in concurrent MODIS satellite imagery. In addition, strong tidal current or swells could also suspend sediments in the lower Bay. Sediments remained suspended in the water column for 2–3 days after the wind events. Moderate Chl increases were observed after sediment resuspension with a lag time of ˜1–2 days, probably due to release of bottom nutrients and optimal light conditions associated with sediment resuspension and settling. Two large increases in Chl with one Chl > 12.0 mg m⁻³ over ˜2 days, were observed at neap tides. For the study site and period, because of the high temporal variability in phytoplankton and sediment concentrations, a monthly snapshot can be different by −50% to 200% from the monthly “mean” chlorophyll and sediment conditions. The combination of high-frequency observations from automated sensors and synoptic satellite imagery, when available, is an excellent complement to limited field surveys to study and monitor water quality parameters in estuarine environments.     

Analysis of ocean color spectra (II)

A method for evaluating the radiance due to sky light reflected by the sea surface and radiance emerging from the sea is described. The calculation is made as a function of the sun altitude, sky condition and sea state for varying optical properties of sea water. As a result of the contribution of reflected sky light, the shape of the spectral distribution for radiance just below the surface is considerably distorted above the surface, especially when chlorophyll concentrations are high. Special attention is paid to the ratio of radiance emerging from the sea to total upwelling radiance at the wavelength of 670 nrn. The variation in the ratio with wind-speed is small and the ratio decreases with increase in the atomospheric turbidity factor.     

Analysis of ocean color spectra (I)

By using the two-flow optical model, the variation in the irradiance reflectance of the ocean just below the surface has been calculated for varying water optical properties, for the interpretation of remotely-sensed ocean color data. The input variables used in our model are the concentration of phytoplankton (chlorophyll-a), the absorption coeffiicient of yellow substance, the particle scattering coefficient, the ratio of the back-scattering coefficient to the total scattering coefficient of particles, and the ratio of the absorption to scattering coefficients for particles.The irradiance reflectance increases monotonically with the ratio of the back-scattering coefficient to the total scattering coefficient. Spectral changes occur in the irradiance reflectance for increases in chlorophyll-a concentration and yellow substance as well as the ratio of the absorption to scattering coefficients.Because slightly different mathematical expressions have been derived by other investigators using the two-flow model, an evaluation of the resulting calculation differences is presented and discussed.     

Analysis of ocean color spectra (III)

The spectral measurements of the attenuation coefficient of sea water and the absorption coefficient of suspended particles and of yellow substance were made with a spectrophotometer for the sea waters sampled in Tokyo Bay. These parameters can be used to compute the expected spectral irradiance reflectance. Some relationships between the optical properties of sea water and the concentration of dissolved organic carbon, the dry weight of suspended matter and chlorophyll-a concentration are presented. On the basis of these results, the possibility of modelling the relationship between optical and material properties is discussed.     

基于两种半分析算法的水体吸收系数反演

本文基于42组不同年份不同季节获得的遥感反射率、水体各组分吸收系数的实测数据,对QAA（Quasi-Analytical Algorithm）和GSM（Garver-Siegel-Maritorena）算法在寡营养的南海和富营养的福建沿岸两种不同类型水体的吸收系数反演进行了检验。以水样测量值为参考,两种算法在本研究水体中的反演成效与他人在其它水域的研究结果相当。QAA算法在南海的反演成效高于福建沿岸水体。对于443 nm的总吸收系数（a443 ）,南海的对数均方根误差（RMSE ）为0.046,平均相对误差为7.9%,对数平均偏差为0;福建沿岸水体的对数均方根误差（RMSE ）为0.194,平均相对误差为30.6%,对数平均偏差为-0.167 。GSM算法在两类水体的反演成效类似,A443 之RMSE和平均相对误差,南海分别为0.161和27.7%,福建沿岸分别为0.149和32.1%,但从A443的对数平均偏差值看,其在南海反演值低于实测值（对数平均偏差为-0.142 ）,在福建沿岸则略呈高于实测值（对数平均偏差为0.016）。两种算法中的部分经验参数与实测值之间的差异是产生反演误差的主要原因,为了提高反演精度,对算法中经验参数的更进一步区域化调整可能是必要的。     

POLDER水色大气校正算法研究

基于POLDER多角度观测数据,对其大气校正算法进行了研究。该大气校正算法中,大气分子散射利用多次散射加偏振的精确瑞利散射来计算,同时,对太阳耀斑及白帽散射进行了校正。由本文大气校正算法可以反演得到气溶胶光学厚度以及海水反射率,与POLDER二级产品相比表明,利用该算法反演所得产品效果较好,相对误差在10%左右,从而为未来中国多角度水色遥感的发展提供一些参考。     

基于DINEOF的静止海洋水色卫星数据重构方法研究

静止轨道海洋水色成像仪(Geostationary Ocean Color Imager, GOCI)提供了时间分辨率达小时级的海洋水色数据,使得对海洋环境的逐时变化监测成为可能。然而受到海洋上空云、雾和霾的影响,数据出现连续高缺失率甚至完全缺失的情况,使得数据使用价值大大降低。在经验正交函数重构法(Data INterpolating Empirical Orthogonal Functions, DINEOF)的基础上,突出时间要素在重构中的地位,运用异常像元检测、拉普拉斯平滑滤波和时间模态2次分解插值,提出了适用于静止海洋水色卫星数据的重构方法——DINEOF-G。利用此方法对杭州湾2017年的GOCI总悬浮物质量浓度数据进行重构,结果表明该方法相比经典方法在重构精度上提高了8%,数据重构率提高了36%,且重构结果较好地反映了杭州湾总悬浮物质量浓度的季节变化规律和空间分布特征。     

A general purpose exact Rayleigh scattering look-up table for ocean color remote sensing

1 Introduction In the total radiance received by an ocean color remote sensor, over 80% radiance is the contribution being scattered by air molecule and aerosol, and a wa- ter-leaving radiance carrying the ocean color informa- tion is only a small part. T…     

LIC color texture enhancement algorithm for ocean vector field data based on HSV color mapping and cumulative distribution function

Texture-based visualization method is a common method in the visualization of vector field data. Aiming at adding color mapping to the texture of ocean vector field and solving the ambiguity of vector direction in texture image, a new color texture enhancement algorithm based on the Line Integral Convolution (LIC) for the vector field data is proposed, which combines the HSV color mapping and cumulative distribution function calculation of vector field data. This algorithm can be summarized as follows: firstly, the vector field data is convoluted twice by line integration to get the gray texture image. Secondly, the method of mapping vector data to each component of the HSV color space is established. And then, the vector field data is mapped into HSV color space and converted from HSV to RGB values to get the color image. Thirdly, the cumulative distribution function of the RGB color components of the gray texture image and the color image is constructed to enhance the gray texture and RGB color values. Finally, both the gray texture image and the color image are fused to get the color texture. The experimental results show that the proposed LIC color texture enhancement algorithm is capable of generating a better display of vector field data. Furthermore, the ambiguity of vector direction in the texture images is solved and the direction information of the vector field is expressed more accurately.