Simultaneous determination of aerosol optical thickness and water-leaving radiance from multispectral measurements in coastal waters

Abstract. Retrieval of aerosol optical properties and water-leaving radiance over ocean is challenging since the latter mostly accounts for  ∼  10 % of the satellite-observed signal and can be easily influenced by the atmospheric scattering. Such an effort would be more difficult in turbid coastal waters due to the existence of optically complex oceanic substances or high aerosol loading. In an effort to solve such problems, we present an optimization approach for the simultaneous determination of aerosol optical thickness (AOT) and normalized water-leaving radiance (nL_w) from multispectral satellite measurements. In this algorithm, a coupled atmosphere–ocean radiative transfer model combined with a comprehensive bio-optical oceanic module is used to jointly simulate the satellite-observed reflectance at the top of atmosphere and water-leaving radiance just above the ocean surface. Then, an optimal estimation method is adopted to retrieve AOT and nL_w iteratively. The algorithm is validated using Aerosol Robotic Network – Ocean Color (AERONET-OC) products selected from eight OC sites distributed over different waters, consisting of observations that covered glint and non-glint conditions from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Results show a good consistency between retrieved and in situ measurements at each site. It is demonstrated that more accurate AOTs are determined based on the simultaneous retrieval method, particularly in shorter wavelengths and sunglint conditions, where the averaged percentage difference (APD) of retrieved AOT is generally reduced by approximate 10 % in visible bands compared with those derived from the standard atmospheric correction (AC) scheme, since all the spectral measurements can be used jointly to increase the information content in the inversion of AOT, and the wind speed is also simultaneously retrieved to compensate the specular reflectance error estimated from the rough ocean surface model. For the retrieval of nL_w, atmospheric overcorrection can be avoided in order to have a significant improvement of the inversion of nL_w at 412 nm. Furthermore, generally better estimates of band ratios of nL_w(443) / nL_w(554) and nL_w(488) / nL_w(554) are obtained using the simultaneous retrieval approach with lower root mean square errors and relative differences than those derived from the standard AC approach in comparison to the AERONET-OC products, as well as the APD values of retrieved Chl which decreased by about 5 %. On the other hand, the standard AC scheme yields a more accurate retrieval of nL_w at 488 nm, prompting a further optimization of the oceanic bio-optical module of the current model.