Surface solar radiation trends under all-sky and clear-sky conditions over Europe

Solar radiation is the primary source of energy for the climate system and a variety of biological processes on the planet. In that sense, understanding the radiative processes in the atmosphere and identifying the governing factors of these processes is key for climate diagnosis and prognosis. In this work, we use daily in-situ observations from 528 stations over Europe from the World Radiation Data Centre (WRDC) database, to analyze Surface Solar Radiation (SSR) trends from 1964 until 2018 in all regions of the continent. Statistical methods were applied to quality-control the dataset: detecting and removing outliers, homogenization and gap-filling of the time series. Two different statistical approaches for identification of clear-sky conditions were applied and compared. Observations in most of the regions on the European continent agree with previously observed negative trends (diming) until the 80’s, followed by positive SSR trends (brightening) from then on, continuing until recent years. However, the regime shifts and the intensity of the trends are not homogeneous within the continent, indicating that regional aspects have non-negligible impacts on the SSR behavior. The comparison between all-sky and clear-sky SSR observations helps to identify to what extent the clouds were a relevant factor in the observed trends in every part of the continent. With this type of analysis we intend to not only present the SSR trends over Europe, but also to expand the comprehension of their spatial heterogeneity across the continent, as well as their causes.