The weather service MeteoSwiss is compiling climatological data series relating to the spatial and chronological variability of solar irradiation. The calculation method takes into account the complex Alpine terrain as well as the snow-related radiation effects.
The amount of solar radiation incident on the surface of the earth is an important climatological variable and of major significance for a number of applications relating to climate monitoring, solar energy and agriculture. For example, knowing the level of solar irradiance at a location is essential for planning and designing solar plants and energy-efficient buildings. Exposure-related radiation calculations likewise help improve the suitability mapping and income statement in viticulture.
Satellite data as basis
The weather service MeteoSwiss is compiling such data, using information from the “METEOSAT Second Generation“ satellite, a geostationary observation satellite. It is operated by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). The satellite data are reviewed by means of selective ground-based measurements. The global radiation, that is, all of the solar radiation incident on the horizontal surface of the earth, is recorded at the ground-based stations. In addition, the satellite data ideally supplement the ground-based measurements, as they likewise supply information where the ground-based data have gaps, for example in Alpine territory.
Consideration of the complex Alpine topography
The weather service MeteoSwiss is indirectly deriving the global radiation from the satellite data, using the so-called Heliosat method. This method was adapted to the special characteristics of the complex Swiss topography by MeteoSwiss during many years of scientific cooperation within the scope of the EUMETSAT Satellite Application Facility on Climate Monitoring (CM SAF). For this purpose, it is necessary to locate the satellite data in the terrain with an accuracy of 1-2 km (navigation and orthorectification). Furthermore, clouds are distinguished from snow-covered areas below them, requiring the use of a combination of near-infrared and infrared satellite channels. The high degree of backscattering from snow as well as the casting of shadows from nearby buildings are taken into account at every point.
The highest values of global radiation for 2012 derived from the satellite data are shown for the Cantons of Valais and Grisons. In flat terrain, the resulting monthly global radiation averages exhibit a maximum uncertainty of about 5 W per m2 and in the mountains of about 10 W per m2 and can also be converted to sloping areas by the weather service MeteoSwiss.