Reduction by clouds
The figure shows the effect of clouds on spectral UV irradiance,
on a monthly basis. The measured monthly averaged
daily irradiation and the corresponding cloudless sky value
simulated by the model are plotted for each month of the year 1995.
The figure shows that surface irradiance is typically reduced by
about 30-40%. It is also obvious that, during the summer months,
the average reduction is independent of wavelength,
although this is not necessarily true for individual spectra, see
[Seckmeyer et al., 1996],
[Kylling et al., 1997],
[Mayer et al., 1998].
The wavelength dependence observed in the winter months
(December - March) is partly due to the special geometry at the
measurement site: mountains to the South, extending to
nearly 20° above the horizon, block the direct sun for a considerable
part of the day in winter. The shading of the direct sun results
in an increased attenuation towards longer wavelengths
because the fraction of the direct sun increases with increasing
wavelength. The effect of the mountains on the diffuse radiation
is less dramatic: only 3.8% of the cosine-weighted area of the sky
is obstructed by mountains.
The ratios are only shown down to 320 nm because at lower wavelengths,
an accurate measurement of the total ozone would be required
as model input which is problematic under cloudy conditions:
ground-based measurements of total ozone during overcast conditions
rely on scattered radiation rather than direct irradiance. As shown
e.g. by [Mayer et al.,
1998], such measurements may be subject to large errors.
The use of the such derived total ozone for the calculation
of the cloudless sky irradiance would be a circular argument,
leading to the conclusion that the attenuation by clouds in
the UVB range is independent of wavelength which is not
neccessarily true, see also the next paragraph.
Alternatively, satellite-derived total ozone could be used for the
analyses, but after the TOMS instrument on the Meteor3 satellite had
ceased operation in 1994, this had to be postponed until data
from the new TOMS and GOME missions became available (after 1996).
Kylling, A., A. Albold, and G. Seckmeyer.
Transmittance of a cloud is wavelength-dependent in the UV-range:
Physical interpretation. Geophys. Res. Lett., 24,1997.
Seckmeyer, G., R. Erb, and A. Albold.
Transmittance of a cloud is wavelength-dependent in the UV-Range
Geophys. Res. Lett., 23, 2753-2755, 1996.