Effects of inelastic radiative processes on the determination of water-leaving spectral radiance from extrapolation of underwater near-surface measurements.

Extrapolation of near-surface underwater measurements is the most common method to estimate the water-leaving spectral radiance, L_w(λ) (where λ is the light wavelength in vacuum), and remote-sensing reflectance, R_rs(λ), for validation and vicarious calibration of satellite sensors, as well as for ocean color algorithm development. However, uncertainties in L_w(λ) arising from the extrapolation process have not been investigated in detail with regards to the potential influence of inelastic radiative processes, such as Raman scattering by water molecules and fluorescence by colored dissolved organic matter and chlorophyll-a. Using radiative transfer simulations, we examine high-depth resolution vertical profiles of the upwelling radiance, L_u(λ), and its diffuse attenuation coefficient, K_{L_u}(λ), within the top 10 m of the ocean surface layer and assess the uncertainties in extrapolated values of L_w(λ). The inelastic processes generally increase L_u and decrease K_{L_u} in the red and near-infrared (NIR) portion of the spectrum. Unlike K_{L_u} in the blue and green spectral bands, K_{L_u} in the red and NIR is strongly variable within the near-surface layer even in a perfectly homogeneous water column. The assumption of a constant K_{L_u} with depth that is typically employed in the extrapolation method can lead to significant errors in the estimate of L_w. These errors approach ∼100% at 900 nm, and the desired threshold of 5% accuracy or less cannot be achieved at wavelengths greater than 650 nm for underwater radiometric systems that typically take measurements at depths below 1 m. These errors can be reduced by measuring L_u within a much shallower surface layer of tens of centimeters thick or even less at near-infrared wavelengths longer than 800 nm, which suggests a requirement for developing appropriate radiometric instrumentation and deployment strategies.