Variability and trends of the total cloud cover over Italy (1951-2018)

A new quality-checked and homogenised dataset of total cloud cover (TCC) series over Italy for the period 1951-2018 is presented and the variability and trends of the obtained regional series is discussed. The diurnal cycle highlights the relevant role of convection induced by solar radiation that, as expected, is more relevant at medium and high elevations and in summer. In parallel, the annual cycle presents a strong minimum in July and a maximum during winter for southern Italy, while it exhibits a more complex behaviour with strong differences between low elevations and high elevations in northern Italy. Moreover, the seasonal and annual TCC series are characterized by a significant negative trend over the whole considered period, which is mainly due to the 1951-1990 period. Even if small differences between northern and southern Italy can be observed, the two regions exhibit a coherent behaviour both for long-term trends and decadal time-scale variability suggesting that the causes of variability and trends of the Italian TCC records are more related to large scale factors rather than to local scale changes. Indeed, the comparison with sea level pressure and 500 hPa geopotential height data highlights that large-scale atmospheric circulation explains a relevant fraction of the signal of the Italian TCC records. Finally, the new TCC dataset shows that the long-term evolution of sunshine duration and surface solar radiation in Italy is only partially influenced by changes in TCC.

Automated summary

This study presents a quality-checked and homogenised dataset of total cloud cover (TCC) series over Italy from 1951 to 2018, and discusses the variability and trends of the regional series. The results show that convection induced by solar radiation plays a significant role, especially at medium and high elevations and in summer. The annual cycle exhibits a strong minimum in July and a maximum during winter for southern Italy, with more complex behavior and differences between low and high elevations in northern Italy. The TCC series of both seasons and years show a significant negative trend over the entire period, mainly attributed to the period from 1951 to 1990. Despite small differences between northern and southern Italy, both regions exhibit coherent behavior in long-term trends and decadal variability, indicating that the variability and trends of Italian TCC records are more related to large-scale factors rather than local-scale changes. Comparison with sea level pressure and 500 hPa geopotential height data suggests that large-scale atmospheric circulation plays a significant role in the Italian TCC records. Finally, the new TCC dataset shows that the long-term evolution of sunshine duration and surface solar radiation in Italy is only partially influenced by changes in TCC.