Surface solar radiation trends over Europe assessed from ground-based measurements and satellite imagery and their comparison with climate models

Downward surface solar radiation (SSR) is the main component in the surface energy balance and the climate system, as well as being the fundamental source of energy in various forms of solar and photovoltaic technologies. It is therefore of great importance to know in detail the spatio-temporal variation of SSR, as well as its long-term trends. Scientific evidence has shown that the amount of solar radiation incident on the Earth’s surface is not stable over the years, but undergoes significant variations every decade. Until recently, ground-based observations have been the most reliable data source for SSR monitoring. Nevertheless, satellite-derived SSR measurements have a better spatial and temporal coverage, though the scientific literature on the use of satellite imagery for the study of SSR is still limited.

This study covers several purposes. First, a direct comparison between ground-based observations and satellite-derived estimates has been carried out, to determine the capability of the latter to reproduce measurements from surface observations. Monthly averaged time series of 108 land stations from GEBA (Global Energy Balance Archive) dataset (ground observations) have been compared to those estimated from satellite imagery by the Solargis model over the same locations. Solargis is a company based in Bratislava, dedicated to the assessment of the solar resource worldwide, using GIS (Geographic Information Systems). SSR anomalies measured at the surface and estimated from satellite images have been compared over Europe for the period 1994-2019. Second, multiannual SSR trends have also been calculated in detail (station-averaged and station-separated) for both ground-based and satellite-derived datasets, in the period of study. Finally, SSR time series have been compared to several CMIP6 (Coupled Model Intercomparison Project Phase 6 ) climate models runs.

The results show that the method of estimating SSR from satellite images is able to reproduce around 94% of the variability of the SSR measured by ground-based methods in Europe. In addition, trend analysis shows a general increase of SSR over the continent in the period of this study, with an average trend of 3.5 Wm^-2decade^-1for the observational data and 1.7 Wm^-2decade^-1for the satellite estimations. This increase in SSR may be associated with changes in the transmission of the atmosphere due to variations in cloud properties and aerosols. Finally, CMIP6 time series average over all models for RCP8.5 scenario shows exactly the same trend as the satellite-derived dataset, which suggests there are still some variables not considered by satellite imagery methods and climate models.