Skyrad - Sky-Radiance Models for Monte Carlo Radiative Transfer Applications

Photon-tracing can be initialized through sky-radiance (Lsky) distribution models when executing Monte Carlo simulations for ocean color studies. To be effective, the Lsky model should: 1) properly represent sky-radiance features of interest; 2) require low computing time; and 3) depend on a limited number of input parameters. The present scope of the study is to verify the satisfiability of these prerequisite by comparing results from different Lsky formulations. Specifically, two Lsky models are considered as reference cases because of their different approach among solutions presented in the literature. The first model, developed by the Harrisson and Coombes (HC), is based on a parametric expression where the sun geometry is the unique input. The HC model is one of the sky-radiance analytical distribution applied in state-of-art simulations for ocean optics. The coefficients of the HC model were set upon broad-band field measurements and the result is a model that requires a few implementation steps. The second model, implemented by Zibordi and Voss (ZV), is based on approximated physical expressions that accounts for the optical thickness of permanent gases, aerosol, ozone and water vapour as specific wavelengths.

Funding entity: CENTRIA - UNL.

Supervisor, jointly with Jose Paulo Santos, of the student Ines Macedo Santos

Inter-comparisons between normalized HC and ZV sky-radiance distributions (i.e., with unitary scalar irradiance) were discussed by means of individual polar maps and signed percent difference between sky-radiance distributions. Sky-radiance cross-sections were analyzed as well. Results have shown a significant convergence between HC and ZV results in the red region of the visible spectrum. Differences between models increase with the sun zenith and mostly with wavelength. For Instance, relative differences up to 50% can be observed in the sky-region symmetric (i.e., same zenith but opposite azimuth) to the sun position in the lower end of visible spectrum. The effects of these differences was afterwards investigated by analyzing how these models vary the sun and sky photon fraction in MC simulations that use the same diffuse-to-total irradiance ratio. In this case, differences can be up to 14%. The study recommendation is then using Lsky models that, like the ZV expression, accounts for the wavelength dependence of light interaction with atmospheric particles and molecule when initializing MC simulations for ocean color applications, mostly in the case of analyses including the blue region of the visible spectra.