For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.
[1] Euphotic zone depth, z 1% , reflects the depth where photosynthetic available radiation (PAR) is 1% of its surface value. The value of z 1% is a measure of water clarity, which is an important parameter regarding ecosystems. Based on the Case-1 water assumption, z 1% can be estimated empirically from the remotely derived concentration of chlorophyll-a ([Chl]), commonly retrieved by employing band ratios of remote sensing reflectance (R rs ). Recently, a model based on water's inherent optical properties (IOPs) has been developed to describe the vertical attenuation of visible solar radiation. Since IOPs can be nearanalytically calculated from R rs , so too can z 1% . In this study, for measurements made over three different regions and at different seasons (z 1% were in a range of 4.3-82.0 m with [Chl] ranging from 0.07 to 49.4 mg/m 3 ), z 1% calculated from R rs was compared with z 1% from in situ measured PAR profiles. It is found that the z 1% values calculated via R rs -derived IOPs are, on average, within $14% of the measured values, and similar results were obtained for depths of 10% and 50% of surface PAR. In comparison, however, the error was $33% when z 1% is calculated via R rs -derived [Chl]. Further, the importance of deriving euphotic zone depth from satellite ocean-color remote sensing is discussed.
[1] Penetration of solar radiation in the ocean is determined by the attenuation coefficient (K d ()). Following radiative transfer theory, K d is a function of angular distribution of incident light and water's absorption and backscattering coefficients. Because these optical products are now generated routinely from satellite measurements, it is logical to evolve the empirical K d to a semianalytical K d that is not only spectrally flexible, but also the sunangle effect is accounted for explicitly. Here, the semianalytical model developed in Lee et al. (2005b) is revised to account for the shift of phase function between molecular and particulate scattering from the short to long wavelengths. Further, using field data collected independently from oligotrophic ocean to coastal waters covering >99% of the K d range for the global oceans, the semianalytically derived K d was evaluated and found to agree with measured data within $7-26%. The updated processing system was applied to MODIS measurements to reveal the penetration of UVA-visible radiation in the global oceans, where an empirical procedure to correct Raman effect was also included. The results indicated that the penetration of the blue-green radiation for most oceanic waters is $30-40% deeper than the commonly used euphotic zone depth; and confirmed that at a depth of 50-70 m there is still $10% of the surface UVA radiation (at 360 nm) in most oligotrophic waters. The results suggest a necessity to modify or expand the light attenuation product from satellite ocean-color measurements in order to be more applicable for studies of ocean physics and biogeochemistry.
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