Abstract
Abstract. Our knowledge on trends in surface solar radiation (SSR) involves uncertainties due to the scarcity of long-term time series of SSR, especially with records before the second half of the 20th century. Here we study the trends of all-sky SSR from 1885 to 2010 in Switzerland, which have been estimated using a homogenous dataset of sunshine duration series. This variable is shown to be a useful proxy data of all-sky SSR, which can help to solve some of the current open issues in the dimming/brightening phenomenon. All-sky SSR has been fairly stable with little variations in the first half of the 20th century, unlike the second half of the 20th century that is characterized also in Switzerland by a dimming from the 1950s to the 1980s and a subsequent brightening. Cloud cover changes seem to explain the major part of the decadal variability observed in all-sky SSR, at least from 1885 to the 1970s; at this point, a discrepancy in the sign of the trend is visible in the all-sky SSR and cloud cover series from the 1970s to the present. Finally, an attempt to estimate SSR series for clear-sky conditions, based also on sunshine duration records since the 1930s, has been made for the first time. The mean clear-sky SSR series shows no relevant changes between the 1930s to the 1950s, then a decrease, smaller than the observed in the all-sky SSR, from the 1960s to 1970s, and ends with a strong increase from the 1980s up to the present. During the three decades from 1981 to 2010 the estimated clear-sky SSR trends reported in this study are in line with previous findings over Switzerland based on direct radiative flux measurements. Moreover, the signal of the El Chichón and Pinatubo volcanic eruption visible in the estimated clear-sky SSR records further demonstrates the potential to infer aerosol-induced radiation changes from sunshine duration observations.
Highlights
Solar radiation incident at the Earth’s surface, known as surface solar radiation (SSR), is the fundamental source of energy in the climate system, and the source of life on our planet, due to its central role in the surface energy balance and for processes such as evaporation and the water cycle or plant photosynthesis (e.g. Wild, 2009, 2012)
It has been suggested that the dimming period may be linked to an increase in cloud cover detected in some regions (Trenberth et al, 2007; Wild, 2009)
It has been shown that sunshine duration (SD) records can be considered as an excellent proxy to estimate all-sky and clear-sky surface solar radiation (SSR) variations over regions such as Switzerland, for more than a century, which can help to reduce some of the open issues in the dimming/brightening phenomenon
Summary
Solar radiation incident at the Earth’s surface, known as surface solar radiation (SSR), is the fundamental source of energy in the climate system, and the source of life on our planet, due to its central role in the surface energy balance and for processes such as evaporation and the water cycle or plant photosynthesis (e.g. Wild, 2009, 2012). Tang et al (2011) recently pointed to deficiencies in the SSR series over China regarding the homogeneity of the series before 1994, which were solved estimating the SSR variations since the 1960s by using different parameters such as standard meteorological observations, the Angstrom turbidity coefficient, and the thickness of the ozone layer Overall, to overcome these limitations in the dimming/brightening issue, the analysis can be supported with the help of other related climate variables, or proxy data, such as sunshine duration (SD), cloudiness or visibility, which are much more widely available (Wild, 2009, 2012). In order to study clear-sky conditions for SD and SSR data, only the days with daily mean TCC equal or less than 25 % (or 2 oktas) have been selected (e.g. Sanchez-Lorenzo et al, 2009; Xia, 2010; Wang et al, 2012). For clear-sky SD and SSR, the monthly values were computed if at least one clear-sky day was available in this month
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