Case-2 Waters Research Articles

Study on the correlation between chlorophyll maximum and remote sensing data

Based on the in situ optical measurements in the Bohai Sea of China, which belongs to a typical case-2 water area, we studied the characteristics of DCM (deep chlorophyll maximum) such as its spatial distribution, vertical profile, etc. We found that when the depth of the chlorophyll maximum is comparatively small, even in turbid coastal water regions, there is always a good correlation between the concentrations of chlorophyll maximum and the satellite-received signals in blue-green spectral bands; the correlation is even better than that between the surface chlorophyll concentrations and the satellite-received signals. The strong correlation existing even in turbid coastal water regions indicates that an ocean color model to retrieve the concentration of DCM can be constructed for coastal waters if a comprehensive knowledge of the vertical distribution of chlorophyll concentration in the Bohai Sea of China is available.

Detecting the red tide algal blooms from satellite ocean color observations in optically complex Northeast-Asia Coastal waters

Over the last few decades, the coastal regions throughout the world have experienced incidences of algal blooms, which are harmful or otherwise toxic because of their potential threat to humans as well as marine organisms, owing to accelerated eutrophication from human activities and certain oceanic processes. Previous studies have found that correct identification of these blooms remains a great challenge with the standard bio-optical algorithms applied to satellite ocean color data in optically complex coastal waters containing high concentrations of the interfered dissolved organic and particulate inorganic materials. Here a new method called the red tide index (RI) is presented which is capable of identifying potential areas of harmful algal blooms (HABs) from SeaWiFS ocean color measurements representing the typical Case-2 water environments off the Korean and Chinese coasts. The RI method employs the water-leaving radiances ( L w), collected from in-situ radiometric measurements of three SeaWiFS bands centered at 443 nm, 510 nm and 555 nm, to achieve derivation of indices that are then related to absorbing characteristics of harmful algae (i.e., L w at 443 nm) from which a best fit with a cubic polynomial function with correlation coefficient of R 2 = 0.91 is obtained providing indices of higher ranges for HABs and lower and slightly reduced ranges for turbid and non-bloom waters. Similar indices derived from the use of remote sensing reflectance ( R rs), normalized water-leaving radiance ( nL w) and combination of both are found rather inadequate to characterize the variability of the encountered bloom. In order to quantify the HABs in terms of chlorophyll (Chl), an empirical relationship is established between the RI and in-situ Chl in surface waters from about 0.4–71 mg m − 3 , which yields a Red tide index Chlorophyll Algorithm (RCA) based on an exponential function with correlation coefficient R 2 = 0.92. The established methods were extensively tested and compared with the performances of standard Ocean Chlorophyll 4 (OC4) algorithm and Local Chlorophyll Algorithm (LCA) using SeaWiFS images collected from typical red tide waters of Korean South Sea (KSS), East China Sea (ECS), Yellow Sea (YS) and Bohai Sea (BS) during 1999–2002. The standard spectral ratio algorithms, the OC4 and LCA, yielded large errors in Chl retrievals for coastal areas, besides providing false information about the encountered HABs in KSS, ECS, YS and BS waters. On the contrary, the RI coupled with the standard spectral ratios yielded comprehensive information about various ranges of algal blooms, while RCA Chl showing a good agreement with in-situ data led to enhanced understanding of the spatial and temporal variability of the recent HAB occurrences in high scattering and absorbing waters off the Korean and Chinese coasts.

Improved detection of turbid waters from ocean color sensors information

Various empirical techniques have been developed to identify turbid marine waters within satellite ocean color imageries. The present technique, which essentially rests on the enhancement of reflectance in the green part of the spectrum when sediments are present, accounts for the improved knowledge regarding the relationship between the chlorophyll concentration and the scattering coefficient in oceanic case-1 waters. It also accounts for the bidirectional character of the light field emerging from a water body and thus for the sensitivity of the water-leaving radiance to the observational conditions (viewing angle and sun angle). Therefore, the thresholds, above which the turbid waters can be detected, are not constant. They depend on the chlorophyll concentration and on geometry (on the zenith-sun angle). The practical way of implementing such a discriminating tool is explained. Examples of the application of this technique are provided for level-2 and level-3 satellite data (SeaWiFS and MERIS).

Global distribution of Case-1 waters: An analysis from SeaWiFS measurements

“Case-1” has been a term frequently used to characterize water type since the seventies. However, the distribution of Case-1 waters in global scale has been vague, though open ocean waters are often referred to as Case-1 in the literature. In this study, based on recent bio-optical models for Case-1 waters, an inclusive and quantitative Case-1 criterion for remote sensing applications is developed. The criterion allows Case-1 waters to have about two-fold variations of non-pigment absorption and particle backscattering around their exact Case-1 values, allowing a large range of waters to be classified as Case-1. Even so, application of this criterion to ocean color data from the SeaWiFS satellite sensor suggests that Case-1 waters occupy only about 60% of the global ocean surface. Regionally, more Case-1 waters are found in the southern hemisphere than in the northern hemisphere, and most Indian Ocean waters are found to be Case-1. The Case-1 percentage and spatial distribution change with season, and with the boundaries chosen in the criterion. Nevertheless, this study for the first time provides a quantitative and geographical perspective of Case-1 waters in global scale, and further demonstrates that many open ocean waters are not necessarily Case-1.

Estimation of ocean contribution at the MODIS near‐infrared wavelengths along the east coast of the U.S.: Two case studies

Atmospheric correction for the ocean color products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) uses two near‐infrared (NIR) bands centered at 748 and 869 nm. Ocean is usually assumed to be black at these two NIR wavelengths. For Case‐2 and high productive Case‐1 waters, however, ocean could have significant contributions in the NIR, leading to significant under‐estimation of the MODIS‐derived water‐leaving radiances. This is often the case in coastal regions. In this paper, measurements from the MODIS Terra at the short wave infrared (SWIR) wavelengths (1240 and 1640 nm) are used to evaluate the ocean contributions at wavelengths 748 and 869 nm. The ocean is black in coastal regions in these SWIR bands due to much stronger water absorption. Studies of two MODIS granules off the east coast of the U.S. show that the ocean could have reflectance values of ∼3.1% and ∼1.8% at the top of atmosphere (TOA) for bands 748 and 869 nm in the Outer Banks, while these values are ∼0.15% and ∼0.1% in the Chesapeake Bay region. Thus, for the turbid waters, it is important to accurately account for the ocean contributions in the NIR bands for the atmospheric correction of ocean color remote sensing.

Assessing the potential of SeaWiFS and MODIS for estimating chlorophyll concentration in turbid productive waters using red and near-infrared bands

Bio-optical algorithms for remote estimation of chlorophyll-a concentration ( Chl) in case-1 waters exploit the upwelling radiation in the blue and green spectral regions. In turbid productive waters other constituents, that vary independently of Chl, absorb and scatter light in these spectral regions. As a consequence, the accurate estimation of Chl in turbid productive waters has so far not been feasible from satellite sensors. The main purpose of this study was to evaluate the extent to which near-infrared (NIR) to red reflectance ratios could be applied to the Sea Wide Field-of-View Sensor (SeaWiFS) and the Moderate Imaging Spectrometer (MODIS) to estimate Chl in productive turbid waters. To achieve this objective, remote-sensing reflectance spectra and relevant water constituents were collected in 251 stations over lakes and reservoirs with a wide variability in optical parameters (i.e. 4 ≤ Chl ≤ 240 mg m − 3 ; 18 ≤ Secchi disk depth ≤ 308 cm). SeaWiFS and MODIS NIR and red reflectances were simulated by using the in-situ hyperspectral data. The proposed algorithms predicted Chl with a relative random uncertainty of approximately 28% (average bias between − 1% and − 4%). The effects of reflectance uncertainties on the predicted Chl were also analyzed. It was found that, for realistic ranges of R rs uncertainties, Chl could be estimated with a precision better than 40% and an accuracy better than ± 35%. These findings imply that, provided that an atmospheric correction scheme specific for the red-NIR spectral region is available, the extensive database of SeaWiFS and MODIS images could be used to quantitatively monitor Chl in turbid productive waters.

Bidirectional reflectance function for oceanic waters with varying chlorophyll concentrations: Measurements versus predictions

The bidirectional reflectance of the ocean is an important parameter in ocean color remote sensing. Model predictions for case-1 waters were compared with measurements over a large range of chlorophyll concentrations (0.1–10 mg m−3 Chl, where Chl represents the sum of chlorophyll a and phaeophytin a), but with restricted solar zenith angles. We used the measured chlorophyll concentration and a model to predict the shape of the upwelling spectral radiance distribution. We found that the model predicted the radiance in the view direction, normalized by the nadir radiance, to within 7%. We also found that Q(Eu/Lu) was predicted within 7%.

Platform perturbations in above-water radiometry

A comparison of above- and in-water spectral measurements in coastal (but predominantly Case-1) conditions has shown that the uncertainty in above-water determinations of water-leaving radiances made from an offshore tower depends on the proximity of the above-water measurement with respect to the side of the platform. For purposes of this study the proximity of the sampling platform is parameterized as the perpendicular distance (denoted x) from the side of the sampling platform to the center of the area on the sea surface observed by the sea-viewing sensor, the so-called surface spot, which is set by the field of view of the radiometer (or the overlapping fields of view of a multiaperture sensor). Two above-water data processing methods were used to create a diagnostic variable (formulated for Case-1 waters only but also applicable to Case-2 conditions over short time scales) to quantify the presence of superstructure reflections. Based on the height of the tower, H, the analyses were partitioned into near- and far-field data sets (x < H and x > H, respectively). The primary conclusions of the radiometric intercomparisons are as follows: (a) the maximum perturbations occur very close to the tower ( x/H << 1), and, as x/H increases and approaches 1 (i.e., as the surface spot becomes as far away as the platform is high), the platform perturbations converge toward smaller and smaller values, and (b) within the far field (x > H) the platform perturbation is negligible, and a remote sensing 5% absolute accuracy objective can be satisfied.

ATMOSPHERIC CORRECTION OF THE LANDSAT SATELLITE IMAGERY FOR TURBID WATERS

This paper describes methods for the correction of the atmospheric effects in the Landsat VIS/NIR imagery in relation to the retrieval of meaningful information about the ocean color, especially from Case-2 waters around Korean peninsula. Three atmospheric correction (AC) methods implemented and examined, using the TOA radiance or reflectance data, are 6S radiative transfer model, spectral shape matching (SSMM) and path-extraction methods. The results show that overall shape and magnitude of radiance or reflectance spectra of the atmospherically corrected Landsat VIS/NIR imagery by SSMM appears to have very good agreement with the in-situ spectra collected for clear and turbid waters, while path-extraction over turbid waters though often reproduces in-situ spectra, but yields significant errors for clear waters due to the invalid assumption of zero values for the black ocean pixels of the Landsat VIS/NIR bands. Because of the standard atmosphere with constant aerosols and models adopted in 6S model, a large error is possible between the retrieved and in-situ spectra. Validation suggests that accurate the retrieval of water-leaving radiance is not feasible with the invalid assumption of classical AC algorithms, but is feasible with SSMM.

An analytical model for subsurface irradiance and remote sensing reflectance in deep and shallow case-2 waters

Subsurface remote sensing signals, represented by the irradiance re fl ectance and the remote sensing re fl ectance, were investigated. The present study is based on simulations with the radiative transfer program Hydrolight using optical properties of Lake Constance (German: Bodensee) based on in-situ measurements of the water constituents and the bottom characteristics. Analytical equations are derived for the irradiance re fl ectance and remote sensing re fl ectance for deep and shallow water applications. The input of the parameterization are the inherent optical properties of the water - absorption a(lambda) and backscattering bb(lambda). Additionally, the solar zenith angle thetas, the viewing angle thetav , and the surface wind speed u are considered. For shallow water applications the bottom albedo RB and the bottom depth zB are included into the parameterizations. The result is a complete set of analytical equations for the remote sensing signals R and Rrs in deep and shallow waters with an accuracy better than 4%. In addition, parameterizations of apparent optical properties were derived for the upward and downward diffuse attenuation coefficients Ku and Kd.

Seasonal changes in colour ratios and optically active contituents in the optical Case-2 waters of the Menai Strait, North Wales

A time series was conducted in the Menai Strait from March to December 1996 combining optical measurements by a colour sensor with measurements of light-absorbing constituents in the water. In order to improve estimations of pigment concentration in Case-2 waters, an optical model was used to synthesize the spectrum of sea-leaving radiance from the absorption properties of pure sea water, yellow substances, mineral suspended solids and phytoplankton. The original model was tested and improved by using a different set of parameters. Multiple regression was used to empirically relate colour ratios to pigments and total suspended solids.