Attenuation Coefficient For Irradiance Research Articles

Irradiance and beam transmittance measurements off the West Coast of the Americas

Measurements of total irradiance versus depth and beam transmission versus depth were made at stations near shore along the west coast of the North and South American continents. The water types at each station were optically classified according to the system of Jerlov (1976), thus providing additional information for the description of the distribution of the world's ocean water types. In addition, the parameter k/c, where k is the irradiance attenuation coefficient and c is the beam attenuation coefficient, has been shown to be a useful parameter for determining the relative particle concentrations of ocean water.

The bio‐optical state of ocean waters and remote sensing 1

The diffuse attenuation coefficient for irradiance, KT, is a physical measure of the bio‐optical state of ocean waters. From an analysis of irradiance, KT, and pigment concentration, the specific attenuation due to chlorophyll‐like pigments is found to be 0.016 ± 0.003 [m−1 (mg pigment m−3)−1]. The bio‐optical state of ocean waters can be remotely sensed by spacecraft sensors. KT is readily measured at sea and is highly correlated with and dependent on the chlorophyll‐like pigment concentration, CK. This pigment concentration and KT provide a measure of the fraction of radiant energy attenuated by phytoplankton. This fraction, in turn, is closely related to the production equations formulated by Bannister and can be directly incorporated into a general theory of phytoplankton dynamics. CK may also be used as an index of primary productivity. The determination of the bio‐optical state of ocean waters by surface vessel provides direct information concerning the productivity of these waters; to the extent that the bio‐optical state can be determined by satellite, it may be possible to examine important parameters of the marine ecosystem rapidly and repeatedly.

Optical classification of natural waters 1

A technique has been developed that leads to an optical classification of natural waters in terms of the dissolved and suspended biogenous material present. As a first approximation, this classification has been made in terms of the total chlorophyll‐like pigment concentration.A relationship between the spectral diffuse attenuation coefficient for irradiance and the chlorophyll‐like pigment concentration has been found with spectral irradiance data from diverse types of ocean waters. The specific spectral attenuation coefficient due to phytoplankton is shown to be consistent with laboratory measurements of the diffuse absorption coefficient of various lot cultures of phytoplankton.

The Estimation of the Yearly Cycle of Submarine Irradiance for Ecological Purposes

The yearly cycle of surface solar irradiance at Banyuls-sur-Mer is derived from meteorological data. Monthly irradiance attenuation coefficients of the seawater were obtained from Secchi disc readings, which enabled the authors to classify the monthly optical water types according to Jerlov's classification. The average daily values of submarine irradiance are calculated for each month, and for different depths. Calculations have been carried out for the visible spectrum (350-710 nm) subdivided into 20 nm bands. Submarine irradiance is given in energetic (cal cm-2 day-1) as well as quantum (μE m-2 ѕ-1) units. Some considerations are given for underwater stations with different degrees of exposition to available light.

Spectral variations in the volume scattering function at large angles in natural waters

The quasi-single-scattering approximation in which a δ function replaces the forward portion of the volume scattering function is applied to radiative transfer in the ocean. Immediately beneath the surface, the product of the reflectance R and the downwelling irradiance-attenuation coefficient K(−) is equal to an integral of the volume scattering function in the backward direction weighted by a geometrical factor. Spectral variations of the volume scattering function are revealed in K(−)R; this is used to examine the wavelength dependence of scattering in two very different natural waters. In the clear water of Crater Lake, the backscattering is proportional to λ−3 (λ = wavelength), whereas the turbid, productive waters of San Vicente Reservoir show a complex dependence of backscattering on wavelength, which is associated with anomalous dispersion due to the 670-nm absorption band of the chlorophyll that is contained in the suspended phytoplankton.

Optical Properties of the Arctic Upper Water

Optical properties of the Arctic Upper Water have been measured from Fletcher's Ice Island, T-3, in the Arctic Ocean. Beam transmittance for various wave-lengths and the upwelling and downwelling irradiance have been measured to a depth of 120 metres. In the spectral region of maximum transmittance, the beam transmittance was found to be 93.1% per metre and the diffuse attenuation coefficient for irradiance was 0.0444 per metre. The data show that the Arctic Upper Water, in early May before the snow cover on the ice has cleared, is optically uniform and very clear.

Predicted Optical Properties for Clear Natural Water*

This paper presents the results of an effort to predict the optical properties of a clean-water type as a function of wavelength. The prediction is based on extensive experimental spectral values for the irradiance-attenuation coefficient and the irradiance reflectance for clean, naturally occurring water, together with an analytical expression for the shape of the asymptotic radiance distribution and a synthesized table of spectral values for the total scattering coefficient based on recent experimental data obtained by Morel in the Mediterranean Sea. The values predicted for the total attenuation coefficient are found to compare favorably with recent data for the total attenuation coefficient obtained at several wavelengths by Austin, in the Sargasso Sea. Other optical properties predicted herein are found to be consistent with experimental values recently reported in the literature.

THE SECCHI DISK IN TURBID COASTAL WATERS1

Measurements of Secchi disk depth are correlated with beam transmittance (B) in turbid coastal water. Both k (the irradiance attenuation coefficient) and α (the beam attenuance coefficient) can be estimated from the Secchi depth either on an empirical basis or by using the Duntley‐Preisendorfer equation of contrast reduction. Such estimates possess relatively large standard errors of estimate.The effects of the size of Secchi disk and viewing techniques are discussed. An estimate of contrast threshold of the human eye under field conditions has been obtained. A modification of the factor in the Poole and Atkins equation for estimating k from Secchi depth is suggested.