Sunshine duration and surface solar radiation variability and trends in Italy and underlying causes

A dataset of 104 daily Italian sunshine duration (SD) [1] and 54 surface solar radiation (SSR) [2] records have been set up collecting data for the 1936-2013 and 1959-2013 periods, respectively. The datasets were subjected to a daily quality control and homogenization in order to eliminate nonclimatic signals in the raw series. In addition, SSR series under clear-sky conditions were obtained considering only the cloudless days by comparison with ground-based total cloud cover (TCC) series. Then, the records were interpolated onto a regular grid and clustered into two regions (north and south). Finally, the records were averaged in order to get regional all-sky SSR/SD and clearsky SSR series. Both SSR and SD regional records, in agreement with that reported in literature, show a decreasing tendency (i.e. global dimming) from the 1950s to the 1980s and a subsequent increasing tendency (i.e. brightening period) since the 1980s. Moreover, considering the length of the available series, SD records show also some evidence of an increasing tendency (i.e., early brightening) between the 1930s and 1950s. A comparison of SD with TCC records shows a positive correlation during the dimming period suggesting that there is an important fraction of SD evolution that cannot be explained by TCC, but it should depend on other factors, such as, changes in atmospheric aerosol concentration. This is also confirmed by SSR records that show stronger tendencies under clear-sky than under all-sky conditions during the dimming period in all seasons and during the brightening period in winter and autumn. In addition, the clear-sky SSR dimming is stronger in the south than in the north, suggesting a significant contribution of mineral dust and so of natural aerosols to the SSR variability