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Volume 17, issue 23
Atmos. Chem. Phys., 17, 14353-14364, 2017
https://doi.org/10.5194/acp-17-14353-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 17, 14353-14364, 2017
https://doi.org/10.5194/acp-17-14353-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Dec 2017

Research article | 04 Dec 2017

Sky radiance at a coastline and effects of land and ocean reflectivities

Axel Kreuter1,2, Mario Blumthaler1, Martin Tiefengraber2,3, Richard Kift4, and Ann R. Webb4 Axel Kreuter et al.
  • 1Division for Biomedical Physics, Medical University of Innsbruck, Innsbruck, Austria
  • 2LuftBlick, Earth Observation Technologies, Mutters, Austria
  • 3Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria
  • 4School of Earth Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK

Abstract. We present a unique case study of the spectral sky radiance distribution above a coastline. Results are shown from a measurement campaign in Italy involving three diode array spectroradiometers which are compared to 3-D model simulations from the Monte Carlo model MYSTIC. On the coast, the surrounding is split into two regions, a diffusely reflecting land surface and a water surface which features a highly anisotropic reflectance function. The reflectivities and hence the resulting radiances are a nontrivial function of solar zenith and azimuth angle and wavelength. We show that for low solar zenith angles (SZAs) around noon, the higher land albedo causes the sky radiance at 20° above the horizon to increase by 50% in the near infrared at 850nm for viewing directions towards the land with respect to the ocean. Comparing morning and afternoon radiances highlights the effect of the ocean's sun glint at high SZA, which contributes around 10% to the measured radiance ratios. The model simulations generally agree with the measurements to better than 10%. We investigate the individual effects of model input parameters representing land and ocean albedo and aerosols. Different land and ocean bi-directional reflectance functions (BRDFs) do not generally improve the model agreement. However, consideration of the uncertainties in the diurnal variation of aerosol optical depth can explain the remaining discrepancies between measurements and model. We further investigate the anisotropy effect of the ocean BRDF which is featured in the zenith radiances. Again, the uncertainty of the aerosol loading is dominant and obscures the modelled sun glint effect of 7% at 650nm. Finally, we show that the effect on the zenith radiance is restricted to a few kilometres from the coastline by model simulations along a perpendicular transect and by comparing the radiances at the coast to those measured at a site 15km inland. Our findings are relevant to, for example, ground-based remote sensing of aerosol characteristics, since a common technique is based on sky radiance measurements along the solar almucantar.

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We have done measurements of the sky's brightness at the Italian coast and show the influence of the underlying surface: looking towards the land, the sky can be up to 50 % brighter than opposite viewing directions towards the ocean as a result of higher land reflectivity. At low solar elevations, the specular reflection from the ocean, or sun glint, increases the zenith brightness. Understanding these effects requires a 3-D model and is important when retrieving, e.g., aerosol properties.
We have done measurements of the sky's brightness at the Italian coast and show the influence of...
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