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| Titel |
Sky-Radiance Models for Monte Carlo Radiative Transfer Applications |
| VerfasserIn |
I. Santos, D. Dalimonte, J. P. Santos |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065537
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| Zusammenfassung |
| 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)Â properly represent sky-radiance features of interest; 2)Â require low computing
time; and 3)Â depend on a limited number of input parameters. The present study verifies the
satisfiability of these prerequisite by comparing results from different Lsky formulations.
Specifically, two Lsky models were 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 physical expressions that accounts for the optical thickness of permanent
gases, aerosol, ozone and water vapour at specific wavelengths. Inter-comparisons
between normalized ^LskyZV and ^LskyHC (i.e., with unitary scalar irradiance)
are discussed by means of individual polar maps and percent difference between
sky-radiance distributions. Sky-radiance cross-sections are presented as well. Considered
cases include different sun zenith values and wavelengths (i.e., λ=413, 490 and
665Â nm, corresponding to selected center-bands of the MEdium Resolution Imaging
Spectrometer MERIS). Results have shown a significant convergence between ^LskyHC
and ^LskyZV at 665Â nm. Differences between models increase with the sun zenith
and mostly with wavelength. For Instance, relative differences up to 50% between
^
L
skyHC and ^
LskyZV can be observed in the antisolar region for λ=665 nm and
θ*=45°. 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 diffuse-to-total irradiance ratio. In this case, differences up to 14% have
been found for λ=665 nm and θ*=60°. The study recommendation is then using
Lsky models that, like the ZV expression, account 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. Dr. Giuseppe Zibordi, Prof. Pedro Vieira and Tamito
Kajiyama are duly acknowledged for valuable discussions. This study has been
partiallysupported by ESA under contract n. 12595/09/I-OL with FCT/UNL, Portugal. |
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