Briançon case study | |
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Effects of topography & surface inhomogeneities on surface UV radiation |
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The MYSTIC (Monte Carlo code for the physically correct tracing of photons
in cloudy atmospheres) radiative transfer model has been used to calculate
the surface irradiance at three wavelengths for a solar
zenith angle of 60°. The sky is assumed to be cloudless and contains no
aerosols. The altitude information is taken from topographical maps generated
during the first year of the EDUCE project.
In the figure above, the leftmost panel shows the altitude map for an area of 201 x 201 km surrounding the measurement site in Briançon, France. The panels to the right give the irradiance enhancement relative to the centre pixel for the three wavelengths 305, 320 and 340 nm. Two features are immediately identified: First, the irradiance received at altitudes below and above the central measurement site is reduced and enhanced, respectively. Secondly, the altitude effect is largest for the shortest wavelength. The first effect is due to the increased optical depth the radiation must penetrate as the atmosphere gets thicker. The second effect is caused by the wavelength dependence of the Rayleigh scattering cross-section. (Note: the periodic boundary conditions introduce some edge effects). Similar results are available for Ispra and Sonnblick. |