Dev Pravin Jeba, Shanmugam Palanisamy
Opt Express. 2017 Oct 30;25(22):27086-27103. doi: 10.1364/OE.25.027086.
Radiative transfer across the water-air interface has important implications for optics and remote sensing of natural waters. The upward radiance emerging from the water suffers a critical change when it passes through the water-air interface. Upwelling radiance transmittance τw,a is an optical process occurring at the water-air interface that determines the in-water radiances propagating through the interface. In previous studies, τw,a was successfully derived for determining the water-leaving radiances in open ocean waters, despite being oversimplified with a constant value. The constant τw,a value becomes rapidly invalid in high scattering and absorbing waters within nearshore and inland environments. In this study, we attempt to quantitatively solve the upwelling radiance transmittance τw,a (i.e., the percentage of in-water photons that escape through the water-air interface) for varying coefficients of scattering and absorption within the range of natural waters. The two important optical phenomena which are ignored in the previous studies have been fully accounted: (i) the particulate contribution to the refractive index (RI) of seawater and (ii) the multiple interactions of the upwelling photons with the water-air interface. As a result, this study leads to a new theoretical formulation of the upwelling radiance transmittance applicable to all natural waters. The effect and variation of the new formulation on the water-leaving radiance and remote sensing reflectance is further studied for coastal and inland waters. Particular attention is also focused on the conversion of sub-surface remote sensing reflectance (rrs) to above-surface remote sensing reflectance (Rrs), which is important for calibration and validation of the remote sensing algorithms. The results show substantial improvement in the ocean color quantities (Lw and Rrs) by up to factor 33% for scattering waters and <5% for absorbing waters.
辐射在水 - 气界面的传输对天然水体的光学和遥感研究具有重要意义。从水中射出的向上辐射在穿过水 - 气界面时会发生关键变化。向上辐射透过率τw,a是发生在水 - 气界面的一个光学过程，它决定了通过该界面传播的水中辐射。在以往的研究中，尽管τw,a被简化为一个恒定值，但它已成功用于推导开阔海域的离水辐射。在近岸和内陆环境中，具有高散射和吸收特性的水体中，τw,a的恒定值会迅速失效。在本研究中，我们试图针对天然水体范围内不同的散射和吸收系数，定量求解向上辐射透过率τw,a（即通过水 - 气界面逸出的水中光子的百分比）。以往研究中忽略的两个重要光学现象得到了充分考虑：（i）颗粒对海水折射率（RI）的贡献；（ii）向上光子与水 - 气界面的多次相互作用。因此，本研究得出了一个适用于所有天然水体的向上辐射透过率的新理论公式。对于沿海和内陆水体，进一步研究了新公式对离水辐射和遥感反射率的影响及变化。特别关注的还包括将水下遥感反射率（rrs）转换为水上遥感反射率（Rrs），这对于遥感算法的校准和验证很重要。结果表明，对于散射水体，海洋颜色量（Lw和Rrs）有显著改善，最多提高33%；对于吸收水体，最多提高5%。
