Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200–1000 m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by ~9–10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475 nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565 nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (⁴⁰K) indicates that the solar irradiance dominates over the ⁴⁰K-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400–500 m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding ~400–500 m.     

Estimation of chlorophyll concentration from the specific fluorescence at 685nm in the upward irradiance just below the sea surface

An algorithm for estimating the chlorophyll concentration from the fluorescence intensity at 685nm in upward irradiance observed just below the sea surface is presented and discussed. The observed chlorophyll concentration is closely related to the ratio of the fluoresced quanta to the total incident quanta (downward plus upward quanta) in the spectral range between 350nm and 600nm. The correlation between them, however, is not good and the rate of increase of the ratio decreases as the chlorophyll concentration increases. This is due to the fact that the number of fluoresced quanta varies with attenuation of the incident light by various materials in the sea.A better correlation is obtained between the ratio of the fluoresced quanta to the upward quanta at wavelengths around 480nm. This is explained by the fact that dividing by the upward quanta at the appropriate wavelength compensates for apparent variation of fluorescence intensity with attenuation by the sea water. In this method, upward quanta at the four wavelengths of 480nm, 640nm, 685nm and 740nm are necessary for estimation of chlorophyll concentration. The quanta at 640nm and 740nm are used for interpolating the quanta at 685nm in the absence of fluorescence.It is also shown that the correlation of the observed chlorophyll concentration with the ratio of upward quanta at the wavelengths of 685nm and 480nm is also good. Use of this ratio is recommended for remote sensing of chlorophyll concentration since the number of spectral channels required by the remote multispectral imager can be minimized.     

水下光谱辐射计光学特性的测试与分析

为适应高精度水下光学测量的需求，设计研制了多通道水下光谱辐射计。该辐射计可同步测量12波段的下行／上行光谱辐照度和7波段的上行光谱辐亮度。对该辐射计的主要性能指标，如余弦响应特性、浸没因子、光谱辐射精度、稳定性和重复性进行了室内测试和分析，用MonteCarlo方法模拟了阴影误差，给出并分析了现场实验数据及部分推导的物理量。结果表明，该光谱辐射计各波段的中心波长和带宽偏差小于1．6nm，稳定性优于5％，阴影误差可控制在5％之内，具有良好的余弦响应特性，满足当前定量化水色遥感现象光辐射测量的技术要求。     

太湖水中可溶性有机质,悬浮质和浮游藻类色素的光学特性的测定

本文着重分析了分光光度计测定的太湖水体中悬浮质、可溶性有机质和浮游藻类色素在可见光波段(400nm—700nm)的吸收;讨论了对池塘及两种高等植物色素吸收的对比测定;并利用迭代回归方法对测试样品的散射效应进行了订正。结果表明:(1)可溶性有机质吸收在短波部分较强;并且随着入射波长的增大,吸收几乎呈指数律减小,减小速率比国外海洋中的测量结果小。(2)水中悬浮质对光具有较强的散射作用,散射系数约按入射波长呈倒数变化。(3)不同植物中的色素的吸收光谱有明显差异。此外,利用所测数据,本文还对冬季太湖水体的反射光谱和向下光照衰减系数进行了数值计算,表明该水体对入射光照的反射和衰减与纯净水情况有很大差别,目此,太湖水光学特性研究中必须考虑水中物质的影响。     

Estimation of water quality parameters from irradiance reflectance using optical models

In order to evaluate near-surface water quality parameters from spectral irradiance reflectance observed just below the surface, models of both radiative transfer and optical properties of sea water are used. The computed water quality parameters are compared with those determined directly from concurrent surface water samples. In this comparison, efforts have been made to minimize the number of spectral irradiance reflectance used in the computation while ensuring that the computed water quality parameters fit well to the observed ones.The observed water quality parameters were found to be closely represented by those computed from the irradiance reflectance at wavelengths of 400 nm, 440 nm, 510 nm and 600 nm, and the fit is as good as the fit of observed water quality parameters to those computed from the irradiance reflectances at 151 wavelengths located between 400 nm and 700 nm at intervals of 2 nm. This implies that the water quality parameters can be evaluated from at least four radiances observed at high altitude, if the effects of the atmosphere and the sea surface can be correctly removed.     

Modelling the optical properties of suspended particulate matter of coral reef environments using the finite difference time domain (FDTD) method

The optical scattering properties of non-spherical calcium carbonate particles were modelled using the finite difference time domain (FDTD) method. This algorithm is computationally expensive but an implementation running on a standard computer’s graphics processing unit (GPU) provided up to 40 times speed increase and made feasible several thousand model runs for particles up to size parameter 33 (4 μm for a wavelength of 376 nm in water). The model evaluates all coefficients of the single-particle Mueller matrix and, when applied to spheres in this size range, compares closely to Mie theory. The modelled unpolarised and polarised angular irradiance scattering from elongate calcium carbonate spicules displayed many differences from that of spheres of equivalent volume. Scattering features were observed that are not immediately obvious consequences of spicule geometry. The extinction efficiency of these particles is also less than that of spheres of equivalent volume. These results suggest both potential consequences of the non-sphericity of particles on instrument calibration and radiative transfer modelling, and opportunities for information extraction by polarimetry.     

A method for remote detection of oil spills using laser-excited Raman backscattering and backscattered fluorescence

A laser method for the remote detection of oil present as a pollutant in the sea water using Raman backscattering and backscattered fluorescence is discussed. The scattering spectra of oils obtained by using a laser Raman spectrometer that employs a CW Ar laser are described. The backscattering spectra of oils obtained by a laser radar technique in the laboratory and the field are also described. Furthermore, theoretical performance of a laser radar with a CW laser or a pulsed laser for the detection of Raman backscattering of kerosene is discussed.     

Variation of diffuse attenuation coefficient of downwelling irradiance in the Arctic Ocean

The diffuse attenuation coefficient(Kd) for downwelling irradiance is calculated from solar irradiance data measured in the Arctic Ocean during 3rd and 4th Chinese National Arctic Research Expedition(CHINARE), including 18 stations and nine stations selected for irradiance profiles in sea water respectively. In this study, the variation of attenuation coefficient in the Arctic Ocean was studied, and the following results were obtained. First, the relationship between attenuation coefficient and chlorophyll concentration in the Arctic Ocean has the form of a power function. The best fit is at 443 nm, and its determination coefficient is more than 0.7. With increasing wavelength, the determination coefficient decreases abruptly. At 550 nm, it even reaches a value lower than 0.2. However, the exponent fitted is only half of that adapted in low-latitude ocean because of the lower chlorophyll-specific absorption in the Arctic Ocean. The upshot was that, in the case of the same chlorophyll concentration, the attenuation caused by phytoplankton chlorophyll in the Arctic Ocean is lower than in low-latitude ocean. Second, the spectral model, which exhibits the relationship of attenuation coefficients between 490 nm and other wavelength, was built and provided a new method to estimate the attenuation coefficient at other wavelength, if the attenuation coefficient at 490 nm was known. Third, the impact factors on attenuation coefficient, including sea ice and sea water mass, were discussed. The influence of sea ice on attenuation coefficient is indirect and is determined through the control of entering solar radiation. The linear relationship between averaging sea ice concentration(ASIC, from 158 Julian day to observation day) and the depth of maximum chlorophyll is fitted by a simple linear equation. In addition, the sea water mass, such as the ACW(Alaskan Coastal Water), directly affects the amount of chlorophyll through taking more nutrient, and results in the higher attenuation coefficient in the layer of 30–60 m. Consequently, the spectral model of diffuse attenuation coefficient, the relationship between attenuation coefficient and chlorophyll and the linear relationship between the ASIC and the depth of maximum chlorophyll, together provide probability for simulating the process of diffuse attenuation coefficient during summer in the Arctic Ocean.     

我国海区SeaWiFS资料大气校正

利用光谱辐射传输理论，结合海上同步实测资料，开发出我国海区SeaWiFS资料大气校正模型。经卫星资料处理结果比对，本模型在一类水体，基本消除了412nm和443nm波段离水辐射率小于0的现象；在二类水体，利用临近一类水体的大气条件进行了有效的大气校正；同时建立了670nm,765nm,865nm波段的大气校正模型，这三个面适用于高浓度悬浮泥沙的信息提取。本模型用于处理我国海区的SeaWiFS资料比美国NASA模型更适合我国海区特定的大气和海洋环境，为SeaWiFS资料海洋水色信息提取和我国海洋一号（HY－1）及风云一号（FY－1C）卫星资料的大气校正研究提供了技术基础。     

Velocity and Sediment Concentration Profiles in Sediment-Laden Flows

A theoretical analysis of velocity profiles in sediment-laden flows is presented by means of Prandtl-Karman mixing length theorem. The study shows that the upward velocity of liquid-phase caused by settling sediment leads to the invalidity of the log-law and Rouse equation. The theoretical analysis takes into account the upward velocity and shows: 1) the mean velocity in sediment-laden flows follows the log-law, but the Karman constant reduces in the main flow region, 2) sediment concentration reduces the mixing length of fluid particles, 3) flow resistance reduces with the presence of sediment concentration, and 4) the sediment concentration profile deviates from the well know Rouse equation. The experimental data agree well with the equations derived on the basis of non-zero wall velocity. It is found that the wall-normal velocity should not be neglected for density gradient flows because it induces more than for pure water flows.     

机载海洋激光雷达测量叶绿素a浓度、悬移质浓度和浅海深度的性能估计

以14:00时离水辐亮度作为背景光强度,模拟355nm激光雷达测量不同深度水层叶绿素a荧光的信噪比.分析体积衰减系数c、辐照度衰减系数k、散射系数b和透明度盘深度(SDD)的关系.最后模拟532nm激光雷达测量悬移质浓度的最大测量深度和误判率小于10%时的最大海底探测深度随SDD的变化.     

OCTOPUS,an Octo parameter underwater sensor,for use in biological oceanography studies

A realtime monitoring system for observing physical, biological, and optical oceanographic parameters in shallow waters was developed. It consists of a CTD with oxygen sensor, anin situ fluorometer, anin situ transmissometer, and anin situ quanta meter together with an on-deck quanta meter. The system is also equipped with a water sampling device. Water sampling can be done during the upward cast on the basis of depth profiles of temperature, salinity, oxygen,in situ fluorescence, transmittance, and underwater irradiance taken during the downward cast.     

海面光谱辐射计系统研制及遥感反射比的测量

本文报道海面光谱辐射计的研制、定标和海上现场实验测量。运用一个 2 5 6元光电二极管列阵传感器件测量海面离水辐亮度和向下辐照度的光谱分布 ,可计算得出遥感反射比。该仪器所测量的数据可以用于海洋方面相关专业的科学研究 ,并给出海上实验测量数据     

Effects of scattering and resonance on energy dissipation of an internal tide in a narrow shelf

The effects of scattering and resonance on the energy dissipation of an internal tide were investigated using a two-dimensional model which is a reassembled version of the theoretical generation model devised by Rattray et al. (1969) for internal tide. The basic character of the scattering process at the step bottom was first investigated with a wide shelf model. When the internal wave incited from a deep region (Region II) into the shallow shelf region (Region I), a passing wave into the shallow region, a reflected wave into the deep region, and a beam-like wave, i.e. a scattered wave (SW), emanated at the step bottom. The SW, which consists of the superposition of numerous internal modes, propagated upward/downward into both regions. The general properties of the SW were well expressed around the shelf edge, even in the present model with viscosity effect. The amplitude of the SW decreased dramatically when the depth of the velocity maximum of the incident internal wave in Region II corresponded with the depth of the shelf edge. In the narrow shelf model, where the decay distance of the internal wave in Region I is longer than the shelf width, the incident internal wave reflected at the coast to form a standing wave. When the internal wave in Region I is enhanced by the resonance, the energy of the SW in Region II is also intensified. Furthermore, the energy of the modes in Region II predominated when the velocity maximum is identical to that of the dominant mode in Region I. These results suggest that the spatial scale of shelf region is a very important factor governing the energy dissipation of the internal tide through reflection and scattering in a narrow shelf.     

Observations of natural fluorescence with an underwater radiometer

Measurements of subsurface irradiance spectra in the ocean consistently indicate intensities of long-wavelength visible light (589 nm) greater than could be derived from the penetration of sunlight. This can be attributed to natural fluorescence and/or to spectral crosstalk due to light leakage through the blocking filters of each discrete detector. A comparison of observed profiles with modelled contributions from these two possible sources indicates that both factors are contributing. At 671 and 694 nm wavelength, the excess signal appears to be predominantly fluorescence while that at 589 nm is mostly crosstalk. Both effects appear to be important at 625 nm although the amount of excess light is small compared to the other wavelengths.The above observations and interpretations are consistent with the optical design of the instrument used and the shape of natural irradiance spectra. Fluorescence efficiencies derived from the irradiance measurements correlate well with measuredin situ fluorescence.     

Estimating the compensation irradiance in the ocean: The importance of accounting for non-photosynthetic uptake of inorganic carbon

The compensation irradiance, the irradiance at which net photosynthesis is zero over a 24-h period, was estimated at station ALOHA (22°45′N, 158°W) from analysis of ¹⁴C uptake rates measured from 8 January 1989 to 13 June 1990 at depths ranging from 5 to 175 m. The estimates were made on the basis of linear regressions of the difference between light bottle and dark bottle ¹⁴C uptake in the light-limited region of the euphotic zone and determination of the depth at which the difference between the uptake rates was zero. About half of the non-photosynthetic ¹⁴C uptake at the compensation irradiance could be attributed to chemolithoautotrophy; the remainder was presumably due to anaplerotic processes. Deriving the compensation irradiance by extrapolating dawn-to-dawn light-bottle uptake above the compensation irradiance to zero resulted in underestimation of the compensation irradiance by a factor of 2. We estimated the compensation irradiance at station ALOHA to be 0.054 mol-photons m⁻² d⁻¹, about 0.11% of surface 400–700 nm radiation and 1% of surface 475-nm (blue) light.     

基于遥感的海表紫外辐射强度反演方法

Ultraviolet(UV) radiation has a significant influence on marine biological processes and primary productivity;however, the existing ocean color satellite sensors seldom contain UV bands. A look-up table of wavelengthintegrated UV irradiance(280–400 nm) on the sea surface is established using the coupled ocean atmosphere radiative transfer(COART) model. On the basis of the look-up table, the distributions of the UV irradiance at middle and low latitudes are inversed by using the satellite-derived atmospheric products from the Aqua satellite,including aerosol optical thickness at 550 nm, ozone content, liquid water path, and the total precipitable water.The validation results show that the mean relative difference of the 10 d rolling averaged UV irradiance between the satellite retrieval and field observations is 8.20% at the time of satellite passing and 13.95% for the daily dose of UV. The monthly-averaged UV irradiance and daily dose of UV retrieved by satellite data show a good correlation with the in situ data, with mean relative differences of 6.87% and 8.43%, respectively. The sensitivity analysis of satellite inputs is conducted. The liquid water path representing the condition of cloud has the highest effect on the retrieval of the UV irradiance, while ozone and aerosol have relatively lesser effect. The influence of the total precipitable water is not significant. On the basis of the satellite-derived UV irradiance on the sea surface, a preliminary simple estimation of ultraviolet radiation's effects on the global marine primary productivity is presented, and the results reveal that ultraviolet radiation has a non-negligible effect on the estimation of the marine primary productivity.     

A numerical experiment on nearshore circulation in standing edge waves

The unsteady shallow-water vorticity equation dominating nearshore flow on a gently sloping plane beach has been solved by using the implicit finite difference technique under the assumption of constant viscosity over the flow field. The result of computation showed that pairs of nearshore circulation cells are generated through the nonlinear effect of flow in the boundary layer formed by the run-up movement of a standing edge wave along a shoreline and the paired cell has the spacing of half a wavelength of the edge wave. When the leaky-mode standing wave of Lamb with the same wave period as the edge wave and the wave crest parallel to a shoreline was superposed on the edge wave field, the alongshore spacing of circulation cell doubled and seaward flow in the cell concentrated in the narrow zone like a ‘rip current’. Although no effect of breaking waves is considered in the computation, such a mechanism may also generate some kinds of nearshore circulation systems observed in a sea.     

空中重力测量数据代表地面数据的误差分析

利用严密的向上延拓公式将地面重力数据上延至空中不同高度,而后与相应的地面重力数据比较从而得到不同高度的代表误差。通过两个不同地形区域的实际算例表明,对于地形平坦区域,在1km高度以下,5＇空中重力数据直接代表地面重力数据的误差小于1mGal,在满足测量规范要求下,空中重力数据可以不用向下延拓而直接使用。对于地形复杂区域,当空中测量高度大于1km时,空中重力数据的代表误差大于3.3mGal,因此必须考虑向下延拓的问题。     

拉曼后向散射在海面检测及海面与陆地区分中的应用

给出了光散射、后向散射以及拉曼后向散射等基本概念,推导了后向散射截面与后向散射微分截面的基本关系,以及散射微分截面的计算公式。同时还得出了振动拉曼散射、振动一转动拉曼散射、转动拉曼散射、瑞利散射之间的关系。提出了利用647nm红光的拉曼后向散射检测海面以及区分海面与陆地的方法,可以有效地提高机载激光海洋测深的精度。