Optical properties of turbid coastal water and their remote sensing retrievals based on radiative transfer approaches

Abstract

Radiative transfer theory describes how light, regarding its spectral radiance, travels and varies along any specific path at a specified point in the water, and ties together the water apparent optical properties (AOPs) and inherent optical properties (IOPs) through the radiative transfer equations (RTEs). In this study, the author used the analytical 2SeaColor model, which is based on the approximate solution of RTEs using two-stream approach, to retrieve IOPs, diffuse attenuation coefficient (K d , in m -1 ), and the concentration of suspended particulate matter (C SPM , in mg/L) in the turbid Yangtze estuary. The application of 2SeaColor to the GOCI geostationary sensors has enabled a better understanding of the diurnal cycle of suspended sediment in the study area. The known linear relationship between C SPM and particulate backscattering coefficient (b bp , in m -1 ), was investigated, using 2SeaColor retrievals and field measurements. Although the validity of the linear relationship was confirmed for relatively clear waters, the author showed that in turbid waters, nonlinearity governs the relationship between C SPM and b bp . The two respective relationships between C SPM and b bp for relatively clear and turbid waters were further integrated using a sigmoid function to retrieve wide-range C SPM without the need for a switching scheme. Apart from SPM, colored dissolved organic matter (CDOM) also significantly reduces light transparency and contributes to the complexity of the optical properties in the Yangtze estuary. Thereby, the author has also investigated the seasonal variability of CDOM absorption properties. Finally, the author proposed a locally calibrated model to estimate the concentration of dissolved organic carbon (DOC). The summary of the major results obtained from this dissertation is briefly described in the following.The thesis is structured in six chapters, of which the first chapter describes the background of ocean color remote sensing in optically complex waters and the second chapter explains the physical principals of the 2SeaColor model. In Chapter 3, the author first proposed improved IOPs parameterizations to the analytical 2SeaColor model to account for the complex optical properties in the Yangtze estuary. The improved 2SeaColor model was later employed to derive the diffuse attenuation coefficient (K d , in m -1 ) from remote sensing reflectance (R rs , in sr -1 ). Retrievals from the improved model were then validated with three in-situ datasets and compared with an empirical (the Zhang model) and a semianalytical model (the Lee model). Statistics from the validation results show that the 2SeaColor model provides the best estimates of K d for the full range of Summary ii observations, with the largest determination coefficient (R 2 = 0.935) and the smallest root mean squared error (RMSE = 0.078 m -1 ). For clear waters, where K d (490) < 0.2 m -1 , the Zhang model provides the most accurate K d estimations, but results from the Lee model and the 2SeaColor are comparable. For ...