Downwelling Diffuse Attenuation Coefficients from in Situ Measurements of Different Water Types (original) (raw)
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Diffuse attenuation coefficient of downwelling irradiance: An evaluation of remote sensing methods
Journal of Geophysical Research, 2005
1] The propagation of downwelling irradiance at wavelength l from surface to a depth (z) in the ocean is governed by the diffuse attenuation coefficient, K d (l). There are two standard methods for the derivation of K d (l) in remote sensing, which both are based on empirical relationships involving the blue-to-green ratio of ocean color. Recently, a semianalytical method to derive K d (l) from reflectance has also been developed. In this study, using K d (490) and K d as examples, we compare the K d (l) values derived from the three methods using data collected in three different regions that cover oceanic and coastal waters, with K d (490) ranging from 0.04to4.0mAˋ1.Thederivedvaluesarecomparedwiththedatacalculatedfrominsitumeasurementsoftheverticalprofilesofdownwellingirradiance.ThecomparisonsshowthatthetwostandardmethodsproducedsatisfactoryestimatesofKd(l)inoceanicwaterswhereattenuationisrelativelylowbutresultedinsignificanterrorsincoastalwaters.Thenewlydevelopedsemianalyticalmethodappearstohavenosuchlimitationasitperformedwellforbothoceanicandcoastalwaters.ForalldatainthisstudytheaverageofabsolutepercentagedifferencebetweentheinsitumeasuredandthesemianalyticallyderivedKdis0.04 to 4.0 m À1 . The derived values are compared with the data calculated from in situ measurements of the vertical profiles of downwelling irradiance. The comparisons show that the two standard methods produced satisfactory estimates of K d (l) in oceanic waters where attenuation is relatively low but resulted in significant errors in coastal waters. The newly developed semianalytical method appears to have no such limitation as it performed well for both oceanic and coastal waters. For all data in this study the average of absolute percentage difference between the in situ measured and the semianalytically derived K d is 0.04to4.0mAˋ1.Thederivedvaluesarecomparedwiththedatacalculatedfrominsitumeasurementsoftheverticalprofilesofdownwellingirradiance.ThecomparisonsshowthatthetwostandardmethodsproducedsatisfactoryestimatesofKd(l)inoceanicwaterswhereattenuationisrelativelylowbutresultedinsignificanterrorsincoastalwaters.Thenewlydevelopedsemianalyticalmethodappearstohavenosuchlimitationasitperformedwellforbothoceanicandcoastalwaters.ForalldatainthisstudytheaverageofabsolutepercentagedifferencebetweentheinsitumeasuredandthesemianalyticallyderivedKdis14% for l = 490 nm and $11% for l = 443 nm.
Spectral measurements of underwater downwelling radiance of inland water bodies
Tellus A, 2013
The apparatus exploited in this work is composed of an optical cable linked to a portable FieldSpec UV/VNIR that records the spectral downwelling radiance in underwater environment, allowing us to calculate the shortwave attenuation coefficient in water. Results for three inland water bodies are presented under different atmospheric conditions (sun zenith angle and wind speed) and water composition (chlorophyll a concentration and turbidity). We show that the spectral downwelling zenith radiance profiles under high sun elevations present a positive slope in the upper layers due to relatively high scattering of direct sunlight compared to attenuation. For deeper layers, attenuation overcomes the scattering of sunlight leading to a constant negative logarithmic slope. For low sun elevations, a negative slope is observed in the entire water column since the scattering of direct sunlight is always lower than attenuation. Whenever a negative logarithmic constant slope is observed, the attenuation coefficient was computed. A relation was observed between attenuation coefficient in the photosynthetically active radiation (PAR) spectral region and water turbidity, for the three water bodies under study.
Isprs Journal of Photogrammetry and Remote Sensing, 2005
To characterize the water column, the diffuse attenuation coefficient of downwelling irradiance, K d (z, λ) (m − 1 ) is one of the most important optical properties of seawater. The purpose of this research was to determine the downwelling diffuse attenuation coefficient of water around Roatan Island, Honduras. In situ K d analysis showed low attenuation coefficient values in green and blue and increased exponentially after 570 nm. The blue, green and red portion of the spectrum showed a K d value of 0.138, 0.158, and 0.503 m − 1 , respectively. Error analysis revealed a significantly high uncertainty in the red region (600-700 nm) and, as expected, low estimation uncertainty in blue and green. When compared with IKONOS derived K d (490 nm), it was observed that the differences were negligible, being 0.0084 and 0.0054 m − 1 for station #1 and #2, respectively. Based on the fact that 90% of the diffused reflected light from a water body comes from a surface layer of water of depth 1 / K d, the results showed that a typical satellite sensor (such as IKONOS) can penetrate up to 8 m in the blue band, 6 m in green, and 2 m in the red region.
The ocean color satellite can only sense a water column up to one optical depth. However, literatur regarding the depth of one optical depth is very limited to none. This study aimed to determine light propagation, attenuation coefficient (Kd), and the depth of one optical depth in different water types. We used in situ data of downwelling irradiance (Ed) with depths taken using the instrument of submersible marine environmental radiometer (MER) in the northeastern gulf of mexico (NEGOM) in April 2000. We also used SeaWiFS data such as water leaving radience (Lw), remote sensing refectance (Rrs), and chlorophyll-a concentration (Chla). The results showed that the light propagation pattern generally decreased with increasing depth. The reduction in light intensity with depth was very strong in the red wavelengths, lower in the green wavelengths, and the lowest in the blue wavelengths. In contrast, Kd values were generally found the lowest at the blue wavelengths, slightly increase at the purple and green wavelengths, and the highest at the red wavelengths. The depth of one optical depth in the case-1 waters was found as deep as 39.79 m (λ=475 nm), followed by intermediate water of 31.79 m (λ=475 nm), and in the case-2 waters of 16.08 m (λ=490 nm). Both Kd (490) in situ and modelled results showed a good correlation (r=0.83-0.84) and R 2 values of 0.68-0.71.
The vertical attenuation of irradiance as a function of the optical properties of the water
Limnology and Oceanography, 2003
Average vertical attenuation coefficients, K(av), for irradiance calculated by linear regression of ln E(z) on z through the euphotic zone or from two irradiance values in a certain depth interval, are useful but somewhat arbitrary procedures for estimating these important apparent optical properties of the ocean. A more fundamental approach is to calculate an irradiance-weighted coefficient, w K(av), integrated over the whole water column, in which for each increment of depth, the corresponding irradiance value is used to weight the estimate of the irradiance coefficient in accordance with w K(av) ϭ # K(z)E(z) dz/# E(z) dz. Attenuation coefficients calculated in this way exhibit
Three methods of determination of underwater irradiance in PAR region of the spectrum are con- sidered: (1) measurements in situ (alternative possibility is theoretical calculations using spectral values of radiation) (2) computations using the depth-averaged diffuse attenuation coefficient (mean value for PAR region), (3) computations using depth-dependent diffuse attenuation coefficient. The results by methods (1) and (2) are compared for optically homogeneous water column (model calculations), the methods (1), (2) and (3) are compared using in situ measurements in Estonian and Finnish lakes and corresponding calculations. The depth averaged diffuse attenuation coefficient (determined by means of a semilog plot of irradiance vs. depth) describes rather well the optical contrasts between the different water bodies and allows satisfactory estimation of 1 % depth, but it is not suitable for determination of the real vertical profiles of the underwater irradiance. Essentially better r...
In situ methods for measuring the inherent optical properties of ocean waters
Limnology and Oceanography, 1995
In situ attenuation (488 and 660 nm), absorption (488, 676, and 750 nm), backscattering (at 488 nm), and stimulated fluorescence were determined as functions of depth in oceanic and coastal waters off Oregon, using both commercial instruments and recently developed prototypes. The inability to perfectly constrain scattered light in these instruments necessitates correction to retrieve accurate optical coefficients. Because scattering is usually one order of magnitude higher than absorption, the correction is most critical for absorption coefficients. A correction procedure based on values obtained with the absorption meter in the infrared region (750 nm) was used to correct the measured spectral absorption coefficients. Realistic values of the backscattering coefficient were obtained from a prototype instrument, although accurate calibration could not be performed for this study. These measurements resulted in the first-ever data set of in situ profiles of attenuation, absorption, scattering, backscattering, and the backscattering ratio. The blue and red attenuation coefficients were tightly correlated, with a relationship varying from site to site. The red-to-blue absorption ratio varied both from site to site and vertically, indicating changes in the relative concentrations of chlorophyllous (phytoplankton) and nonchlorophyllous (biogenous or mineral) particles. The backscattering ratio also appeared to be very sensitive to vertical changes in particle composition. The in situ scattering coefficients were compared to those estimated with a Mie scattering model, using measured particle size distributions and particulate absorption coefficients as input parameters. By allowing the real part of the refractive index of particles to vary over the range of realistic values, convergence between measured and modeled values was obtained. Light penetration in the ocean is ruled by the scattering and absorption properties of the various substances. These two inherent optical properties are, therefore, with the attenuation coefficient (c, the sum of the scattering, b, and absorption, a, coefficients), the fundamental parameters in radiative transfer studies and in the interpretation of apparent optical properties. More specifically, the absorption and backscattering (bb) coefficients directly determine the diffuse reflectance of the ocean and thus are integral to the interpretation of remote sensing data (see Gordon and Morel 1983). The absorption capacity of living phytoplankton, which now can be derived from
Radiance transmittance measured at the ocean surface
Optics Express, 2015
The radiance transmittance (Tr) is the ratio of the water-leaving radiance (L w (0 + )) to the sub-surface upwelling radiance (L u (0 -)), which is an important optical parameter for ocean optics and ocean color remote sensing. Historically, a constant value (~0.54) based on theoretical presumptions has been adopted for Tr and is widely used. This optical parameter, however, has never been measured in the aquatic environments. With a robust setup to measure both L u (0 -) and L w (0 + ) simultaneously in the field, this study presents Tr in the zenith direction between 350 and 700 nm measured in a wide range of oceanic waters. It is found that the measured Tr values are generally consistent with the long-standing theoretical value of 0.54, with mean relative difference less than 10%. In particular, the agreement within the spectral domain of 400-600 nm is found to be the best (with the averaged difference less than 5%). The largest difference is observed for wavelengths longer than 600 nm with the average difference less than 15%, which is related to the generally very small values in both L u (0 -) and L w (0 + ) and rough environmental conditions. These results provide a validation of the setup for simultaneous measurements of upwelling radiance and water-leaving radiance and confidence in the theoretical Tr value used in ocean optics studies at least for oceanic waters.
Measuring Light Attenuation in Shallow Coastal Systems
Journal of Ecosystem & Ecography, 2013
Photosynthetic Active Radiation (PAR) was measured using single planar and two-bulb spherical light sensors. The attenuation coefficient (K d ) was found to vary significantly during the year. The highest K d values were obtained in the station with higher influence of currents and run-off. Our data suggested a reflection of 50% of light that reaches the bottom, which is associated with a decrease in the K d value obtained with the spherical sensor of 0.15 m -1 . This means that flat sensors may underestimate PAR and that spherical sensor may underestimate K d . This is a critical issue given that knowledge on light attenuation is essential for modeling approaches and quality assessments.