5th Swiss Geoscience Meeting, Geneva 2007 Short-term variability of erythemal UV radiation due to clouds Walker Daniel* **, Vuilleumier Laurent* * Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland (firstname.lastname@example.org) ** Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland This project is part of the ongoing COST action 726 whose goal is the reconstruction of erythemal ultraviolet radiation for time periods and locations where no measurements are available. Ultraviolet radiation (UV) is part of the solar spectra reaching the earth's surface, influencing human beings and therewith having an impact on public health. Erythemal UV radiation is a measure to quantify these health effects by weighting different wavelengths accordingly to their harmfulness for the human skin. Furthermore, UV radiation also shows impacts in biology, various ecosystems and materials sciences. Medics and policy makers request more information about the spatial distribution and temporal evolution of this part of solar radiation. In Switzerland, erythemal UV radiation is measured operationally at four locations in different climate regimes since mid to end of the 90ies. The available time series are spatially too sparse and temporally too short to answer current needs including trend analyses. Therefore, radiative transfer models are used to approach this data problem. However, simulating the effect of clouds on UV radiation is difficult and treating scattered cloud coverage is extremely difficult even with 3D-models. On the other hand, being able to handle not only clear-sky situations but various cloud conditions is actually the crucial part in UV reconstruction. An alternative way to describe the influence of clouds on UV radiation is the use of global shortwave radiation as a proxy. This parameter holds information about the transmittance of the atmosphere and describes the cloud effect over the whole solar spectra. Furthermore, the advantage of global shortwave radiation is the vast availability of measurements and the long time series available in Switzerland. Semi-empirical relationships between UV and global shortwave radiation are presented for the different locations in Switzerland where UV is measured. These relationships lead to cloud modification factors (CMF) which are used for estimating UV doses for all-sky conditions. The connection between the CMFs in the UV and shortwave radiation is linear and differs for almost overcast and partly cloudy skies. Furthermore, the dependencies of the derived relationships on the solar zenith angle and total ozone content are investigated. Correlations between CMFs in UV and global shortwave radiation up to 0.89 are found with higher values for heavy cloud coverage.