1German Meteorological Service, Freiburg, Germany (emeritus)
2National Institute for Public Health and the Environment, Bilthoven, The Netherlands
3Aristotle University, Thessaloniki, Greece
4German Meteorological Service, Lindenberg, Germany
5Norwegian Radiation Protection Authority, Oesteraas, Norway
6Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland
Abstract. Cloud impacts on the transfer of Ultraviolet (UV) radiation through the atmosphere can be assessed using a cloud modification factor (CMF). The total global solar irradiation has proven to be a solid basis to derive CMF's for the UV radiation (UV_CMF). Total global irradiance is frequently measured and forecasted by numerical weather prediction systems. Its advantage compared to for instance cloud cover is that measured solar global irradiance contains already the effect of multiple reflection between cloud layers, reflection between the sides of the clouds, as well as the distinct difference whether the solar disc is obscured by clouds or not. In the UV range clouds decrease the irradiance to a lesser extent than in the visible and infrared spectral range; Rayleigh scattering in the atmosphere yields a larger fraction of scatter radiation compared to that of light and infrared, hence, obscuring the solar disc will not totally block out the irradiation. Thus the relationship between CMF's for solar radiation and for UV-radiation is not straight forwards, but will depend on e.g. the solar zenith angle (SZA) and wavelength band or action spectrum in the UV considered. Den Outer et al. (2005) provide a UV_CMF algorithm on a daily base accounting for these influences. It requires as input a daily CMF in total global radiation (SOL_CMF) and the SZA at noon. The calculation of SOL-CMF uses the clear sky algorithm of the European Solar Radiation Atlas to account for varying turbidity impacts. The algorithm's capability to derive hourly UV_CMF's based on the SZA at the corresponding hour and its worldwide applicability is validated using hourly resolved observational data retrieved from the databases of the COST-Action 726 on "Long term changes and climatology of UV radiation over Europe" and the USDA UV-B Monitoring and Research Program. The model performance for hourly resolution is shown to be comparable in absolute and relative deviations from a measured mean smoothed dependent on SZA with the well performing daily models of the COST-726 model intercomparison (Koepke et al., 2006).