Reconstruction of daily global solar radiation under all-sky and cloud-free conditions in Badajoz (Spain) since 1929

This work analyses the long-term temporal variability of the annual and seasonal series of reconstructed global solar radiation for both all-sky and cloud-free conditions in Badajoz (Spain) over the 1929-2015 period. Specifically, daily values of global horizontal irradiation (GHI) for all-sky cases are derived from a semiempirical method based on the relationship between the cloud modification factor and sunshine duration records. Additionally, cloud-free situations are selected using cloud cover (CC) information recorded by surface observations. Regarding GHI linear trends for all-sky conditions, three periods are clearly identified: during the 1929-1950 period, there is a positive and statistically significant trend of +4.18 W center dot m(-2)center dot decade(-1). It is followed by a significant dimming with a trend of -3.72 W center dot m(-2)center dot decade(-1) between 1951 and 1984. GHI levels increase again from 1985 to 2015 with a statistically significant trend of +2.04 W center dot m(-2)center dot decade(-1). The seasonal trends are found to be statistically significant only in summer for all the three subperiods. With the goal to find out the possible causes of the reconstructed GHI trends, the temporal variability of the CC was also analysed. It was observed that CC has a statistically significant negative trend between 1985 and 2015 which may partially explain the GHI increase shown for this period. In contrast, not statistically significant trends were found in the annual and seasonal CC series before 1985. The long-term evolution of the GHI under cloud-free conditions exhibits the same pattern as all-sky conditions: an increase during 1929-1950, followed by a decrease in 1951-1984 and then a new increase from 1985 to 2015. Therefore, the positive (negative) linear trends in GHI reported in this study could be partially related to a decrease (increase) in the aerosol load during the analysed three subperiods.