A Study On The Accurate Correction Of Case ? Water Absorption Coefficient And Its Remote Sensing Inversion

Light absorption of water mass plays an important role in the spatio-temporal variability of light attenuation,mixed-layer heating,primary production,and remote sensing of phytoplankton biomass.Measurements of water absorption coefficient can be obtained by three methods,mainly including laboratory filtering analysis,in situ observation by corresponding meters and inversion based on remote sensing reflectance model.Optical properties are very complex in Case II coastal waters of China and show a wide range of spatial and temporal variability.The substances in water have significant absorption in the near infrared absorption spectra,and the absorption spectra of phytoplankton,the main component of water,are affected by package effect and pigment composition and other factors.All of those restrict the accuracy of obtaining absorption coefficient.Therefore,it is of great realistic significance to conduct an analysis on these problems to improve the accuracy of absorption coefficient and water color remote sensing.1)The spatio-temporal distribution characteristics of coastal waters opticsThis study presented classification work for data sets that were collected in the Yellow Sea during six cruises.Specifically,we analyzed the classification features of Rrs spectra and obtained spatio-temporal characteristics of reflectance and bio-optical properties in the coastal waters.Application of the spectral classification scheme to Medium Resolution Imaging Spectrometer(MERIS)imagery revealed long time series variations in the data,which suggested that the optical characteristics had distinctive temporal and spatial distributions.2)In situ measurement of absorption coefficientIn this study,the scattering contribution was evaluated by a modified exponential fitting approach using the nine-wavelength absorption and attenuation meter(AC9)measurements and was applied in the scattering correction.It was applied to in situ data representative of typical coastal and inland waters.The results suggested that the absorption coefficients level in the red and NIR were significantly higher than those obtained using standard scattering error correction procedures.The absorption values obtained using the exponential method were comparable to spectrophotometer measurements and thus could be applied in optical closure validation.3)The absorption spectra of phytoplankton filtering in laboratoryThis study conducted an analysis of Gaussian decomposition and pigment composition decomposition on the absorption spectra with correction of the package effect using the data of the concentration of pigments and the phytoplankton absorption spectra from East China Sea and Yellow Sea cruises at spring and autumn in year 2003.The decomposition results indicated that the majority of residual errors of the Gaussian decomposition of the absorption spectra could reach the level of less than 0.001 m-1.The pigment composition results and the measurement by HPLC revealed high consistency with the scatter plots,although it was of prominent seasonal variations.4)Inversion based on remote sensing reflectance modelQuasi-analytical algorithm could successfully inverse the total absorption coefficient and backscattering coefficient without water to a certain water types in Yellow Sea and Bohai Sea,but it failed to the phytoplankton dominated water and highly turbid coastal water and had big relative deviation.The inversion values of the absorption coefficient improved the accuracy by the modified coefficient of quasi-analytical algorithm with the spectrophotometer measurements absorption coefficient.The validation result was verified through the comparison between the in situ measurement and the modified inversion algorithm.The results of this study contribute to studies on the optical absorption properties of coastal water as well as the ecological and biogeochemical characteristics of the upper ocean in global shelf sea.