HelioClim: a long -term database on solar radiation for Europe and Africa

An information system, called HelioClim, is available for answering the needs for long-term time-series of solar radiation data. Daily irradiation values are available over Europe, Africa and Atlantic Ocean, from 1985 onwards. This database is accessible through the Soda Web service (http://www.s oda-is.com) on a free basis. Meteosat satellites images are used to produce this climatological database. The method Heliosat-2 converts these images into maps of the global irradiation at ground level. A comparison was performed between irradiation values available in HelioClim and those measured by ground stations in Europe. A measuring station is not necessarily contained within a B2 pixel and an interpolation method is needed. The root mean square error (RMSE) increases as the irradiation increases and is less than 850 Wh/m², that is a relative error of 20 %, a satisfactory result compared to what can be achieved using sparse pyranometric stations only. INTRODUCTION Time series of solar radiation at ground level open the way to many applications for diffe- rent users. Biomass production and crop forecast, oceanography and limnology applica- tions, urban air quality studies, sizing of space borne sensors, solar energy engineering or even the optimization of daylight use in building applications, are among the various do- mains needing solar radiation information. Researchers and engineers are facing the problem of solar radiation data retrieving on va- rious parts of the world. Several studies assessed the user needs for solar radiation data (ESRA 2000). These needs consist generally in values of hourly or daily global irradiation and their derived quantities (diffuse component, spectral distribution, spatial structure of the radiation) for various part of the world. Accordingly, databases should be available that cover the whole earth surface for several years. These data should be available at high spatial resolution (about 10 km in size) with a relative accuracy less than 20 % in root mean square error (RMSE) for the daily irradiation. The data should present a convenient and low cost access. Even if several initiatives have been made to answer these require- ments (Fontoynont et al., 1998; Heidt et al., 1998; Cros and Wald, 2003), there is still a discrepancy between users needs requirements and availability of solar radiation data (Cros et al., 2002). We propose an answer by building an integrated information system called HelioClim. It contains a database of global irradiation routinely estimated from a times -series of Meteosat images from 1985 onwards and software to exploit it. This paper presents briefly the Heliosat-2 method, which converts satellite images into global irradiation maps. The method is applied to satellite images for the construction of a database of global irradiations. Comparisons are performed with ground measurements. THE METHOD HELIOSAT-2 FOR THE MAPPING OF THE SOLAR RADIATION The Heliosat method converts observations made by meteorologica l satellites into estimates of the global irradiation at ground level. The principle of the method, as well as most current methods, is that a difference in global radiation perceived by the sensor aboard the satellite is only due to a change in apparent albedo, which is itself due to an increase of the radiation emitted by the atmosphere towards the sensor. A key parameter is the cloud index n, resulting from a comparison of what is observed by