1990–2016 surface solar radiation variability and trend
over the Piedmont region (northwest Italy)
Received: 5 February 2018 /Accepted: 20 May 2018
Springer-Verlag GmbH Austria, part of Springer Nature 2018
A new surface solar radiation database of 74 daily series is set up for the Piedmont region (northwest Italy) for the 1990–2016 period.
All the series are subjected to a detailed quality control, homogenization and gap-filling procedure and are transformed into relative
annual/seasonal anomaly series. Finally, a gridded version (0.5°×0.5°) of the database is generated. The resulting series show an
increasing tendency of about + 2.5% per decade at annual scale, with strongest trend in autumn (+ 4% per decade). The only exception
is winter, showing a negative but not significant trend. Considering the plain and mountain mean series, the trends are more intense for
low than for high elevations with a negative vertical gradient of about − 0.03% per decade per 100 m at annual scale and values up to −
0.07% per decade per 100 m in spring. Focusing on clear days only (selected by CM SAF ClOud fractional cover dataset from
METeosat first and second generation—Edition 1 satellite data over the 1991–2015 period), trend significance strongly increases and
both low and high elevation records exhibit a positive trend in all seasons. However, the trends result slightly lower than for all-sky days
(with the only exception of winter). The differences observed under clear-sky conditions between low and high elevations are more
pronounced in winter, where the trend shows a negative vertical gradient of about − 0.1% per decade every 100 m. Overall, this paper
shows how a high station density allows performing a more detailed quality control thanks to the higher performances in detecting the
inhomogeneities with higher data availability and capturing regional peculiarities otherwise impossible to observe.
The fraction of solar radiation that reaches the Earth’ssurface
(surface solar radiation—E
) is the primary energy source for
the Earth’s climate system governing a large number of phys-
ical and chemical processes (Stanhill 1983; Stephens et al.
2012; Wild 2016). Any possible variation is important not
only for scientific interests, but it has a lot of implications
for example for agricultural production and solar power gen-
eration (Stanhill 1983;Wildetal.2015). While it is widely
recognized that E
decreased between the 1950s and the
1980s of about 3–9Wm
(“Global dimming”), and in-
creased since the 1980s of about 1–4Wm
period”)(Wild2009), it is a debate whether these variations
are caused by aerosols or cloud effects, even if they are not
completely independent (Twomey et al. 1984;Albrecht1989;
Ramanathan et al. 2001; Lohmann and Feichter 2005).
Uncertainties are mainly connected to the lack of a full knowl-
edge of all the mechanisms that influence E
far as ground-based observations are concerned, the main
problem is connected to the low availability both of E
aerosol/cloud long-term quality-checked data (Wild 2009).
Some of the studies reported in literature point to the dominant
role of aerosols (e.g., Norris and Wild 2007; Ruckstuhl et al.
2008; Turnock et al. 2015), while other studies show how
cloud effects are more dominant contributors to E
(e.g., Mateos et al. 2014; Kambezidis et al. 2016; Boers et al.
* Veronica Manara
Institute of Atmospheric Sciences and Climate, ISAC-CNR, via
Gobetti 101, 40129 Bologna, Italy
Department of Forecasting Systems, Regional Agency for
Environmental Protection of Piedmont, Turin, Italy
Department of Environmental Science and Policy, Università degli
Studi di Milano, Milan, Italy
Theoretical and Applied Climatology