Effects of cirrus cloudiness on solar irradiance in four spectral bands

The impact of cirrus cloudiness on solar irradiance is estimated with the use of a cloud climatology (coverage, optical thickness) and a radiative transfer model. The change in irradiance in four spectral bands (shortwave, photosynthetically active radiation and ultraviolet A and B integrals) due to cirrus clouds at different height levels between the cloud base and the ground is examined for different solar zenith angles. The disproportionate role of the cirrus cloud layer, due to the different contribution of the direct and the diffuse irradiance components, is revealed. The average effect of cirrus clouds on solar irradiance is, in many cases, comparable with the measuring uncertainties of remote sensing instruments. This result indicates that, the detection of cirrus clouds and the determination or parameterization of their optical properties will significantly reduce the uncertainty of tropospheric measurements of several atmospheric constituents.