A method to generate near real time UV-Index maps of Austria

A method is presented that combines individual ground based ultraviolet (UV) measurements from the Austrian UVB monitoring network and area-wide data of the distribution of clouds derived from satellite images to generate a UV-Index map all over the region. The Austrian UVB Monitoring network provides near real time ground based measurements of surface UV irradiance from fifteen selected locations throughout and in the vicinity of Austria. The amount of ultraviolet radiation passing through the atmosphere as measured by the UVB detectors is indicated in units of the UV-Index, the internationally agreed unit for erythemally weighted solar UV irradiance. Together with clear sky model calculations the measured UV-Index is used to determine the cloud modification factor (CMF), a scaling factor giving the reduction of radiation due to the presence of clouds. Moreover satellite images from MSG (Meteosat Second Generation) with a time resolution of 15 min and a spatial resolution of 0.05° are received. From the satellite images the CMFs for the area of Austria are obtained using an algorithm provided by Jean Verdebout. Then both independent data sets of cloud modification factors are checked for consistency by comparing satellite derived and ground based values at the positions of the monitoring stations. If necessary the satellite derived cloud modification factors are corrected by about ±20% according to the results of the ground based measurements. Afterwards realistic UV-Index maps of the whole area are generated by scaling model derived UV-Indexes with the corresponding cloud modification factors. Since all the data is available in almost real time, the calculated UV-Index maps are available in the web at www.uv-index.at/ with a time delay of about 30 min.