Sky radiance at a coastline and effects of land and ocean reflectivities
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 surro...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/14353/2017/acp-17-14353-2017.pdf |
| Summary: | 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 850 nm 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 650 nm. 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 15 km 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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| ISSN: | 1680-7316 1680-7324 |