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 e.g. [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).

References

1

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.

2

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.