Optical Properties Of Water Research Articles

A lidar method for determining internal wave characteristics

An analytical model of lidar imaging of pycnoclinic internal waves (IWs) is developed. The IW image is shown to represent a superposition of two images: reflective and shadow. The former reflects perturbations in the profile of the light backscattering coefficient in the IW field, and the latter reflects perturbations in the optical thickness of the water layer, in which the IW disturbed the horizontal uniformity of optical characteristics. Algorithms for reconstructing the IW field from these images are proposed. It is shown that the shadow image, unlike the reflective one, is insensitive to fine details of the profiles of hydrooptical characteristics and can be used for determining IW parameters on the basis of very rough data on optical properties of water. The possibility of determining the mode composition as well as the lengths and amplitudes of IW modes is demonstrated by using the Barents Sea as an example and invoking actual and simultaneously measured profiles of the water density and light attenuation coefficient.

Asp-15—A stationary device for the measurement of the optical water properties at the NT200 neutrino telescope site

The operation of large underwater neutrino telescopes requires the precise knowledge of the water parameters governing light absorption and scattering, as well as a continuous monitoring of these parameters. For this purpose, a stationary underwater device, ASP-15, has been developed by the Baikal collaboration. We describe the basic assumptions and formulae behind ASP-15, the methods how absorption length, scattering length and phase functions are determined, the design of the device, and give some results obtained over many years of operation in conjuction with the Baikal telescope NT200.

Effects of Degraded Optical Conditions on Behavioural Responses to Alarm Cues in a Freshwater Fish

Prey organisms often use multiple sensory cues to gain reliable information about imminent predation threat. In this study we test if a freshwater fish increases the reliance on supplementary cues when the reliability of the primary cue is reduced. Fish commonly use vision to evaluate predation threat, but may also use chemical cues from predators or injured conspecifics. Environmental changes, such as increasing turbidity or water colour, may compromise the use of vision through changes in the optical properties of water. In an experiment we tested if changes in optical conditions have any effects on how crucian carp respond to chemical predator cues. In turbidity treatments we added either clay or algae, and in a brown water colour treatment we added water with a high humic content. We found that carp reduced activity in response to predator cues, but only in the turbidity treatments (clay, algae), whereas the response in the brown water treatment was intermediate, and not significantly different from, clear and turbid water treatments. The increased reliance on chemical cues indicates that crucian carp can compensate for the reduced information content from vision in waters where optical conditions are degraded. The lower effect in brown water may be due to the reduction in light intensity, changes in the spectral composition (reduction of UV light) or to a change in chemical properties of the cue in humic waters.

Predicting the light attenuation coefficient through Secchi disk depth and beam attenuation coefficient in a large, shallow, freshwater lake

The diffuse attenuation coefficient of photosynthetically active radiation (PAR) (400–700 nm) (Kd(PAR)) is one of the most important optical properties of water. Our purpose was to create Kd(PAR) prediction models from the Secchi disk depth (SDD) and beam attenuation coefficient of particulate and dissolved organic matter (Ct−w(PAR), excluding pure water) in the PAR range. We compare their performance and prediction precision by using the determination coefficient (r2), relative root mean square error (RRMSE), and mean relative error (MRE). Our dataset comprised 1,067 measurements, including Kd(PAR), SDD, and Ct−w(PAR) taken in shallow, eutrophic, Lake Taihu, China, from 2005 to 2010. The prediction models of Kd(PAR) were based on the linear model with an intercept of zero, using the inverse SDD, and the nonlinear model using SDD. The linear model generated a slope of 1.369, which was not significantly different from 1.7, the index used worldwide, but significantly lower than the value of 2.26. The nonlinear model gave a slightly more reliable prediction of Kd(PAR) with a r2 of 0.804. Compared to the SDD, Ct−w(PAR) was more significantly correlated to Kd(PAR) based on the linear model, with a significantly higher r2 and lower RMSE and RE. Considering the measurement simplicity of Ct−w(PAR) and data acquisition feasibility from high-frequency autonomous buoys and satellites, our results demonstrated that this prediction model reliably estimates Kd(PAR), and could be used to significantly expand optical observations in an environment where the conditions for underwater PAR measurement are limited.

Deriving optical properties of Mahakam Delta coastal waters, Indonesia using in situ measurements and ocean color model inversion

The development of an operational water quality monitoring method based on remote sensing data requires information on the apparent and inherent optical properties of water (AOP and IOP respectively). This study was performed to determine the apparent and inherent optical properties of coastal waters of the Mahakam Delta, Kalimantan, Indonesia. Inherent optical properties (IOPs) were derived from above-water radiometric measurements and ocean color model inversion. Retrieved IOPs and measured concentrations show good agreement both for total suspended matter (TSM) and chlorophyll a (Chl a) (R2=0.72 and 0.80 respectively). The linear relationship between the retrieved IOPs and the measured concentrations was then used to estimate the specific inherent optical properties (SIOPs) using the basic equation of the Lambert–Beer law. The specific backscattering coefficient of TSM (bb,TSM∗(550)) was found to be 0.0087m2g−1, and the specific absorption coefficient of Chl a (aChl∗(440)) was found to be 0.023m2g−1 in the Mahakam Delta. The estimated values of SIOP for TSM and Chl a could be considered spatially constant for the Mahakam Delta, and resulted in reliable estimates of TSM and Chl a concentrations (R2=0.84 and 0.85 respectively). The specific backscattering coefficient of TSM found in this study is similar to that of the Barito Estuary (in the southern part of Kalimantan) but lower than that of the Berau Estuary (in the northern part of Kalimantan), whereas the specific backscattering coefficient of Chl a is similar to that found in the Berau Estuary. This study contributes to the development of an operational method based on remote sensing data to map water constituent concentrations in the Mahakam Delta, as well as to enrich the information about the optical properties of Indonesian waters.

Contributions from Particles in Europe (PiE) 2010, Villefranche-sur-Mer, France: an introduction

This special issue of Geo-Marine Letters presents selected contributions from the international conference Particles in Europe (PiE) 2010 organized by Sequoia Scientific, Inc., and the Laboratoire d’Oceanographie de Villefranche (LOV) on 15–17 November 2010 in Villefranche-sur-Mer, France, and guest-edited by Ole Mikkelsen, Malik Chami and David Doxaran. PiE was initiated in 2008, in order to promote and further our understanding of the importance of suspended particulate matter (SPM) for a very wide range of processes in the aquatic environment—from optics and acoustics, over sediment transport, to the global carbon balance. The papers in this special issue are in particular concerned with the interaction between SPM and water optical properties, as well as how to use optical proxy measurements to understand SPM processes. The next PiE conference is scheduled for 17–19 October 2012 in Barcelona, Spain.

Numerical Experiments of an Advanced Radiative Transfer Model in the U.S. Navy Operational Global Atmospheric Prediction System

AbstractA high-order accurate radiative transfer (RT) model developed by Fu and Liou has been implemented into the Navy Operational Global Atmospheric Prediction System (NOGAPS) to improve the energy budget and forecast skill. The Fu–Liou RT model is a four-stream algorithm (with a two-stream option) integrating over 6 shortwave bands and 12 longwave bands. The experimental 10-day forecasts and analyses from data assimilation cycles are compared with the operational output, which uses a two-stream RT model of three shortwave and five longwave bands, for both winter and summer periods. The verifications against observations of radiosonde and surface data show that the new RT model increases temperature accuracy in both forecasts and analyses by reducing mean bias and root-mean-square errors globally. In addition, the forecast errors also grow more slowly in time than those of the operational NOGAPS because of accumulated effects of more accurate cloud–radiation interactions. The impact of parameterized cloud effective radius in estimating liquid and ice water optical properties is also investigated through a sensitivity test by comparing with the cases using constant cloud effective radius to examine the temperature changes in response to cloud scattering and absorption. The parameterization approach is demonstrated to outperform that of constant radius by showing smaller errors and better matches to observations. This suggests the superiority of the new RT model relative to its operational counterpart, which does not use cloud effective radius. An effort has also been made to improve the computational efficiency of the new RT model for operational applications.

Influence of the solar zenith angle on the irradiance reflectance just beneath the water surface in Lake Taihu

恰在水面下辐照度比变化与吸收系数、散射系数和太阳天顶角存在着密切的关系,而太阳天顶角对太湖恰在水面下辐照度比的定量影响程度尚不明确.为此,本文基于2010年5月太湖27个样点的水体光学参数和理论数值模拟,对太湖水体的情况进行研究.理论数值模拟结果表明:当太阳天顶角分别在0°~25°和70°~89°区间内变化时,对恰在水面下辐照度比增幅影响较小,增幅相差不到10%;而当太阳天顶角在30°~70°之间时,对辐照度比增幅影响很大,增幅为13.13%~52.19%.同时,利用实测数据对理论结果进行线性相关验证表明,此数值研究整体上符合太湖的实际情况.因此,对水体光学特性以及光对水生生态系统驱动作用研究的时候,应充分考虑太阳天顶角的影响.;The irradiance reflectance just beneath the water surface is closely related to the absorption coefficient, scattering coefficient and solar zenith angle. However, the effect of solar zenith angle on the irradiance reflectance just beneath the water surface is not clear in Lake Taihu. The influence factors of irradiance reflectance just beneath the water surface in Lake Taihu were analyzed based on the optical parameters and numerical simulates using the data of total 27 sampling sites measured in May, 2010. The results show that while solar zenith angle is in the range of 0°-25ånd 70°-89°, it has little influence on the growth of irradiance reflectance just beneath the water surface which is less than 10%. While solar zenith angle is in the range of 30°-70°, the growth of irradiance reflectance is 13.13%-52.19%. The correlation of theoretical results and measured data show that such numerical study is correct for Lake Taihu. Therefore, full consideration should be given to the effect of solar zenith angle when studying on the optical properties of water and the driving mechanism of the light to aquatic ecosystems.

Time Calibration of the KM3NeT Neutrino Telescope

Abstract KM3NeT is a future deep-sea Research Infrastructure hosting a cubic kilometer-scale neutrino telescope and facilities for marine and earth sciences in the Mediterranean Sea. The consortium is made up of 40 institutes from 10 European countries, and includes all the groups that have developed the pilot projects, ANTARES, NEMO and NESTOR. The KM3NeT telescope will consist of a three-dimensional array of optical modules arranged on vertical detection units (DUs), anchored to the sea floor and held in tension by submerged buoys. The time resolution of this detector has to be known with great accuracy since the angular resolution of the track reconstruction depends on the accurate measurement of the relative arrival times of Cherenkov photons reaching the photon sensors. The intrinsic, unavoidable limitation in time resolution (chromatic dispersion and PMT transit time spread) imply that the calibration system of a water-based neutrino telescope must provide a precision at the nanosecond level. The experience with the ANTARES deep sea neutrino telescope has shown that a distributed system of external light sources illuminating the photon detectors with short (∼ 5 ns FWHM) time-referenced light flashes is very useful to ensure the time calibration of the detector and to measure water optical properties. Whilst the basic timing calibration concept applied in ANTARES will be retained, the larger spacing between photo-detectors required in a cubic-kilometer-scale detector results in modified requirements for the KM3NeT system. A three-dimensional system of optical emitters has been studied: several LED models have been tested, and four models preselected as suitable for use in KM3NeT were incorporated into ANTARES for in-situ testing. Based on the resulting data, several LED beacons will be integrated in the forthcoming deployment of a pre-production KM3NeT detection unit planned for autumn 2011. The design, optimization and construction of the KM3NeT optical time calibration devices are described.

The effect of Arabian Sea optical properties on SST biases and the South Asian summer monsoon in a coupled GCM

This study examines the effect of seasonally varying chlorophyll on the climate of the Arabian Sea and South Asian monsoon. The effect of such seasonality on the radiative properties of the upper ocean is often a missing process in coupled general circulation models and its large amplitude in the region makes it a pertinent choice for study to determine any impact on systematic biases in the mean and seasonality of the Arabian Sea. In this study we examine the effects of incorporating a seasonal cycle in chlorophyll due to phytoplankton blooms in the UK Met Office coupled atmosphere-ocean GCM HadCM3. This is achieved by performing experiments in which the optical properties of water in the Arabian Sea—a key signal of the semi-annual cycle of phytoplankton blooms in the region—are calculated from a chlorophyll climatology derived from Sea-viewing Wide Field-of-View Sensor (SeaWiFS) data. The SeaWiFS chlorophyll is prescribed in annual mean and seasonally-varying experiments. In response to the chlorophyll bloom in late spring, biases in mixed layer depth are reduced by up to 50% and the surface is warmed, leading to increases in monsoon rainfall during the onset period. However when the monsoons are fully established in boreal winter and summer and there are strong surface winds and a deep mixed layer, biases in the mixed layer depth are reduced but the surface undergoes cooling. The seasonality of the response of SST to chlorophyll is found to depend on the relative depth of the mixed layer to that of the anomalous penetration depth of solar fluxes. Thus the inclusion of the effects of chlorophyll on radiative properties of the upper ocean acts to reduce biases in mixed layer depth and increase seasonality in SST.

Modeling of the Suspended Solid Concentration of the Jin River Based on High-Spectral Resolution Data

This study used a spectrometer and the above-water sampling method to measure the water's optical properties of the Jin River.The correlation values between high spectral resolution reflectance data and suspended solid concentration(SSC),or between the first derivative of high-resolution reflectance and the SSC showed that the wavelength around 733±3nm was the best one for estimating the SSC of the Jin River.The regression analysis using several regression models revealed that the quadratic polynomial model was the best one to estimate the SSC of the river.It also found that the single-band regression model is more suitable than the first derivative regression model for estimating the SSC of the Jin River.

Polarized light field under dynamic ocean surfaces: Numerical modeling compared with measurements

As part of the Radiance in a Dynamic Ocean (RaDyO) program, we have developed a numerical model for efficiently simulating the polarized light field under highly dynamic ocean surfaces. Combining the advantages of the three‐dimensional Monte Carlo and matrix operator methods, this hybrid model has proven to be computationally effective for simulations involving a dynamic air‐sea interface. Given water optical properties and ocean surface wave slopes obtained from RaDyO field measurements, model‐simulated radiance and polarization fields under a dynamic surface are found to be qualitatively comparable to their counterparts from field measurements and should be quantitatively comparable if the light field measurement and the wave slope/water optical property measurements are appropriately collocated and synchronized. This model serves as a bridge to connect field measurements of water optical properties, wave slopes and polarized light fields. It can also be used as a powerful yet convenient tool to predict the temporal underwater polarized radiance in a real‐world situation. When appropriate surface measurements are available, model simulation is shown to reveal more dynamic features in the underwater light field than direct measurements.

A neural-network model to retrieve CDOM absorption from in situ measured hyperspectral data in an optically complex lake: Lake Taihu case study

Coloured dissolved organic matter (CDOM) is an important water component that affects water colour and ecological environment under water. The remote estimation of CDOM is always a challenge in the field of water-colour remote sensing owing to its weak signal. To further study the CDOM-retrieval approach, field experiments, including water-quality analysis and spectral measurements, were carried out in Lake Taihu waters from 8 to 21 November 2007. On the foundation of analysing water-inherent optical properties, sensitive spectral factors were selected, and then neural-network models were established for retrieving CDOM. The results show that the model with 10 nodes in the hidden layer performs best, yielding a correlation coefficient (R) of 0.887 and a root-mean-square error of 0.156 m−1. Meanwhile, the predictive errors of the model developed here and the previously proposed algorithms were compared with each other. The mean value of the relative error of the former is 12.8% (standard deviation of 29.9%), and is much lower than its counterpart of other models, which indicates that the developed model has a higher accuracy for CDOM retrieval in Lake Taihu waters. Meanwhile, other datasets collected at different times were also imported into the model for applicability analysis; the derived errors suggest a relatively good performance of the model. This research firstly explores the CDOM retrieval in optically complex lake waters, and the corresponding findings support a technical framework for accurately extracting CDOM information in Lake Taihu waters, based on an adequate understanding of water optical properties.

Neural network approach to retrieve the inherent optical properties of the ocean from observations of MODIS

Retrieving the inherent optical properties of water from remote sensing multispectral reflectance measurements is difficult due to both the complex nature of the forward modeling and the inherent nonlinearity of the inverse problem. In such cases, neural network (NN) techniques have a long history in inverting complex nonlinear systems. The process we adopt utilizes two NNs in parallel. The first NN is used to relate the remote sensing reflectance at available MODIS-visible wavelengths (except the 678 nm fluorescence channel) to the absorption and backscatter coefficients at 442 nm (peak of chlorophyll absorption). The second NN separates algal and nonalgal absorption components, outputting the ratio of algal-to-nonalgal absorption. The resulting synthetically trained algorithm is tested using both the NASA Bio-Optical Marine Algorithm Data Set (NOMAD), as well as our own field datasets from the Chesapeake Bay and Long Island Sound, New York. Very good agreement is obtained, with R² values of 93.75%, 90.67%, and 86.43% for the total, algal, and nonalgal absorption, respectively, for the NOMAD. For our field data, which cover absorbing waters up to about 6 m⁻¹, R² is 91.87% for the total measured absorption.

Monitoring of optical properties of water near the Baikal neutrino telescope with the use of a submerged device ASP-15: Methods and results

Some spectral and angular dependences of optical parameters measured at different depths are presented. The submerged device ASP-15, designed for continuous monitoring of inherent optical properties of Baikal water in southern Lake Baikal near the Neutrino Telescope NT200+ at a depth of 1200 m, is described. Light field measurement and hydrooptical parameter recovery techniques are described.

Optical properties of water at high temperature

We calculate optical properties of water along the principal Hugoniot curve from ambient conditions up to temperatures of 130 000 K with density functional theory (DFT) and the Kubo-Greenwood formula. The effect of the exchange correlation functional is examined by comparing the generalized gradient approximation with a hybrid functional that contains Fock exchange. We find noticeable but moderate differences between the respective results which decrease rapidly above 80 000 K. The reflectivity along the principal Hugoniot is calculated and a good qualitative but fair quantitative agreement with available experimental data is found. Our results are of general relevance for calculations of optical properties with DFT at zero and elevated temperature.

Playing with water drops: from wetting to optics through electrostatics

We present a consistent series of activities, including experiments and basic computational studies, investigating the shape and optical properties of water drops in connection with novel technological devices. Most of the work can be carried out with simple teaching equipment and is well suited to undergraduate students. Firstly, we show how the mass variations of a sessile drop can be used to control its curvature and hence to produce lenses with tunable focal distance. Alternatively, the shape of the drop can be varied using electrowetting on dielectric (EWOD). We propose a simple pedagogical approach to this phenomenon in connection with historical electrostatic apparatus. A detailed process for the preparation of an EWOD device is given, together with a focimetric method allowing the analysis of electrowetting effects in practical exercises. Finally, the manipulations of a commercialized variable focus lens illustrate that EWOD is at the heart of most recent technological developments, making practical work in optics more attractive than traditional exercises using conventional lenses.

Water optical properties of high mineralization in desert lakes

Lakes in desert areas are very different from large lakes in the eastern part of China,like Lake Taihu.The objective of this study was to find out the optical properties of desert lakes.Reflectance spectra were measured in October 2009 with ASD FieldSpec spectrometer.Concurrently,water samples were collected directly from the lake to be analyzed in laboratory.After correlation analysis between the TDS and concentration of chlorophyll-a,DOC,TSM,comparing absorption spectra of NPSS,CDOM,chlorophyll-a,and discussion of the remote sensing reflectance in different wave bands,the result shows TDS and water depth are the main influence of IOPs and AOPs.The TDS has influenced the absorption properties remarkably and has high correlation coefficient with the concentration of DOC and with absorption of CDOM in 240-300nm.Water depth is most important factor of remote sensing reflectance,and TDS plays a major role in 550-720nm within the same water depth ranges.These conclusions may provide useful RS basis to estimate water quality parameters in desert lakes.

Enhancement of solar radiation absorption using nanoparticle suspension

The radiation absorption characteristics of a Ni nanoparticle suspension were investigated by spectroscopic transmission measurement. It was demonstrated that the absorption coefficient of the nanoparticle suspension is much higher than that of the base liquid for visible to near-infrared wavelengths. Radiation characteristics predicted by the Mie theory showed good agreement with the increase of absorption coefficient in wavelengths where the base liquid is transparent. It was also confirmed that a new transmittance measurement technique for a liquid sample using a liquid cell with no spacer was quite useful for evaluating a material possessing an extremely strong absorption band. The proposed measurement method and successive Kramers–Kronig analysis were validated by measuring the optical properties of water. The measurement and prediction process of the thermal radiation properties of nanoparticle suspensions developed here could be used in developing direct absorption solar collectors.

Satellite observations of optical and biological properties in the Korean dump site of the Yellow Sea

With the standard near-infrared (NIR) atmospheric correction algorithm for ocean color data processing, a high chlorophyll- a concentration patch was consistently observed from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua platform in the middle of the Yellow Sea during the spring (end of March to early May). This prominent patch was not observed in the historical ocean color satellite imageries in late 1970s to early 1980s, and a location corresponding to this patch has been used as a Korean dump site since 1988. At the same time, MODIS chlorophyll- a concentrations derived using the shortwave infrared (SWIR) atmospheric correction algorithm developed for the ocean color satellite data in turbid coastal or high-productive ocean waters were significantly reduced. Comparison between in situ and MODIS chlorophyll- a measurements shows that the chlorophyll- a from the MODIS-Aqua products using the standard-NIR atmospheric correction algorithm is significantly overestimated. The images of the MODIS-derived normalized water-leaving radiance spectra and water diffuse attenuation coefficient data using the NIR-SWIR-based atmospheric correction approach show that absorption and scattering by organic and inorganic matter dumped in the Korean dump site have strongly influenced the satellite-derived chlorophyll- a data. Therefore, the biased high chlorophyll- a patch in the region is in fact an overestimation of chlorophyll- a values due to large errors from the standard-NIR atmospheric correction algorithm. Using the NIR-SWIR algorithm for MODIS-Aqua ocean color data processing, ocean color products from 2002 to 2008 for the Korean dump site region have been generated and used for characterizing the ocean optical and biological properties. Results show that there have been some important changes in the seasonal and interannual variations of phytoplankton biomass and other water optical and biological properties induced by colored dissolved organic matters, as well as suspended sediments.