Mukherjee Soham, Hedley John D, Fichot Cédric G, Laliberté Julien, Bélanger Simon
Opt Express. 2023 Oct 9;31(21):35178-35199. doi: 10.1364/OE.501732.
In hydrological optics, "optical closure" means consistency between the apparent optical properties (AOPs) determined from radiometric measurements and those derived from radiative transfer modelling based on concurrently measured inherent optical properties (IOPs) and boundary conditions (sea and sky states). Good optical closure not only provides confidence in the data quality but also informs on the adequacy of the radiative transfer parameterization. Achieving optical closure in highly absorptive coastal waters is challenging due to the low signal-to-noise ratio of radiometric measurements and uncertainties in the measurements of IOPs, namely the spectral absorption and backscattering coefficients. Here, we present an optical closure assessment using a comprehensive set of in situ IOPs acquired in highly absorptive coastal waters optically dominated by chromophoric dissolved organic matter (CDOM). The spectral remote sensing reflectance, R(λ), was modeled using the software HydroLight (HL) with measured IOPs and observed boundary conditions. Corresponding in-water in situ R(λ) was derived from radiometric measurements made with a Compact Optical Profiling System (C-OPS; Biospherical). The assessment revealed that the inclusion of inelastic scattering processes in the model, specifically sun-induced CDOM fluorescence (f) and sun-induced chlorophyll fluorescence (SICF) from Chlorophyll-a ([chl]), significantly improved the optical closure and led to good agreement between measured and modeled R (i.e., for 440 ≤ λ ≤ 710 nm with no inelastic processes: R=0.90, slope=0.64; with inelastic processes: R=0.96, slope=0.90). The analysis also indicated that f and SICF contributed a substantial fraction of the green-red wavelength R in these waters. Specifically, f contributed ∼18% of the modeled R in the green region and SICF accounted for ∼20% of the modeled R in the red region. Overall, this study points out the importance of accounting for f in remote sensing applications in CDOM-dominated waters.
在水文光学中，“光学闭合”是指由辐射测量确定的表观光学特性（AOP）与基于同时测量的固有光学特性（IOP）和边界条件（海况和天空状况）通过辐射传输模型得出的表观光学特性之间的一致性。良好的光学闭合不仅能让人对数据质量有信心，还能说明辐射传输参数化的充分性。由于辐射测量的信噪比低以及IOP测量存在不确定性，即在光谱吸收和后向散射系数方面的不确定性，在高吸收性沿海水域实现光学闭合具有挑战性。在此，我们利用在以发色溶解有机物（CDOM）为主导的高吸收性沿海水域获取的一套全面的现场IOP数据进行光学闭合评估。利用软件HydroLight（HL）结合测量的IOP和观测到的边界条件对光谱遥感反射率R(λ)进行建模。通过紧凑型光学剖面系统（C-OPS；Biospherical公司）进行辐射测量得出相应的水中现场R(λ)。评估结果表明，在模型中纳入非弹性散射过程，特别是太阳诱导的CDOM荧光（f）和来自叶绿素a（[chl]）的太阳诱导叶绿素荧光（SICF），显著改善了光学闭合，并使测量的R和建模的R之间达成良好一致（即对于440≤λ≤710nm，无非弹性过程时：R = 0.90，斜率 = 0.64；有非弹性过程时：R = 0.96，斜率 = 0.90）。分析还表明，f和SICF在这些水域的绿 - 红波段R中占了很大比例。具体而言，f在绿色区域贡献了约18%的建模R，SICF在红色区域占建模R的约20%。总体而言，本研究指出了在CDOM主导水域的遥感应用中考虑f的重要性。
