Measured and Modeled Radiometric Quantities in Coastal Waters: Toward a Closure

Accurate radiative transfer modeling in the coupled atmosphere-sea system is increasing in importance for the development of advanced remote-sensing applications. Aiming to quantify the uncertainties in the modeling of coastal water radiometric quantities, we performed a closure experiment to intercompare theoretical and experimental data as a function of wavelength and water depth z. Specifically, the study focused on above-water downward irradiance Ed ((lambda), 0 ) and in-water spectral profiles of upward nadir radiance Lu (, z ), upward irradiance Eu ((lambda), z ), downward irradiance Ed ((lambda), z ), the Eu ((lambda), z )Lu ((lambda), z ) ratio (the nadir Q A (the irradiance reflectance). The theoretical data were produced with the finite-element method radiative transfer code ingesting in situ atmospheric and marine inherent optical properties. The experimental data were taken from a comprehensive coastal shallow-water data set collected in the northern Adriatic Sea. Under various measurement conditions, differences between theoretical and experimental data for the above-water Ed ((lambda), 0 ) and subsurface Ed ((lambda), 0 ) as well as for the in-water profiles of the nadir Q factor were generally less than 15%. In contrast, the in-water profiles of Lu ((lambda), z ), Ed ((lambda), z ), Eu ((lambda), z ) and of the irradiance reflectance exhibited larger differences to approximately 60% for Lu ((lambda), z ) and Eu ((lambda), z ), 30% for Ed ((lambda), z ), and 50% for the irradiance reflectance. These differences showed a high sensitivity to experimental uncertainties in a few input quantities used for the simulations: the seawater absorption coefficient; the hydrosol phase function backscattering probability; and, mainly for clear water, the bottom reflectance.