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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-901
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
06 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Simultaneous determination of aerosol optical thickness and water leaving radiance from multispectral measurements in coastal waters
Chong Shi1,2,3 and Teruyuki Nakajima1 1Japan Aerospace Exploration Agency, Earth Observation Research Center, Tsukuba, Ibaraki , 305 - 8505, Japan
2Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanj ing, 210044, China
3Key Laboratory of Middle Atmosphere and Global Environmental Observtion, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
Abstract. Retrieval of aerosol optical properties and water leaving radiance over ocean is changeling since the latter mostly accounts for ~10% of satellite observed signal and can be easily contaminated by the atmospheric scattering. Such an effort would be more difficulty 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 (nLw) from multi-spectral 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 a full-physical nonlinear optimization method is adopted to retrieve AOT and nLw in one step. The algorithm is validated using Aerosol Robotic Network Ocean Color (AERONET-OC) products selected from eight OC sites distributed over different waters, consisting of observation cases covered both in and out of sun glint from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Results show a good consistency between retrieved and in situ measurements in each site. It is demonstrated that more accurate AOT are determined based on the simultaneous retrieval method, particularly in shorter wavelengths and sun glint conditions, where the averaged percentage difference (APD) of retrieved AOT generally reduce by approximate 10 % in visible bands compared with those derived from the standard atmospheric correction (AC) scheme. It is caused that 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 nLw, over atmospheric correction can be avoided to have a significant improvement for the inversion of nLw at 412 nm. Furthermore, generally better estimates of band ratios of nLw(443)/nLw(554) and nLw(488)/nLw(554), which are employed in the inversion of chlorophyll a concentration (Chl), are obtained using simultaneous retrieval approach with less root mean square errors and relative differences than those derived from the standard AC approach in comparison to the AERONET-OC products, as a result that the APD value of retrieved Chl decreases by about 5 %. On the other hand, the standard AC scheme yields a more accurate retrieval of nLw at 488 nm, prompting a further optimization of oceanic bio-optical module of current model.

Citation: Shi, C. and Nakajima, T.: Simultaneous determination of aerosol optical thickness and water leaving radiance from multispectral measurements in coastal waters, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-901, in review, 2017.
Chong Shi and Teruyuki Nakajima
Chong Shi and Teruyuki Nakajima
Chong Shi and Teruyuki Nakajima

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Short summary
An algorithm for the simultaneous retrieval of aerosol optical thickness (AOT) and normalized water leaving radiance (nLw) is proposed using multispectral satellite measurements. Retrieval accuracy is validated by the in situ measurements selected from eight sites over different waters. Results show more accurate spectral AOT and nLw are derived by current method than the standard atmospheric correction scheme, except nLw at 488 nm. Optimization of this approach is also required in future work.
An algorithm for the simultaneous retrieval of aerosol optical thickness (AOT) and normalized...
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