Abstract
Abstract. The trends of meteorological parameters and surface downward shortwave radiation (DSR) and downward longwave radiation (DLR) were analysed at four stations (between 370 and 3580 m a.s.l.) in Switzerland for the 1996–2015 period. Ground temperature, specific humidity, and atmospheric integrated water vapour (IWV) trends were positive during all-sky and cloud-free conditions. All-sky DSR and DLR trends were in the ranges of 0.6–4.3 W m−2 decade−1 and 0.9–4.3 W m−2 decade−1, respectively, while corresponding cloud-free trends were −2.9–3.3 W m−2 decade−1 and 2.9–5.4 W m−2 decade−1. Most trends were significant at the 90 % and 95 % confidence levels. The cloud radiative effect (CRE) was determined using radiative-transfer calculations for cloud-free DSR and an empirical scheme for cloud-free DLR. The CRE decreased in magnitude by 0.9–3.1 W m−2 decade−1 (only one trend significant at 90 % confidence level), which implies a change in macrophysical and/or microphysical cloud properties. Between 10 % and 70 % of the increase in DLR is explained by factors other than ground temperature and IWV. A more detailed, long-term quantification of cloud changes is crucial and will be possible in the future, as cloud cameras have been measuring reliably at two of the four stations since 2013.
Highlights
Downward shortwave radiation (DSR) and downward longwave radiation (DLR) are important terms in the surface radiation budget and are fundamental in understanding the climate effect of increasing greenhouse gas concentrations (Wang and Dickinson, 2013)
downward shortwave radiation (DSR), from earlier less reliable measurements, over Europe was observed to decrease in the 1950s to 1980s (“dimming”) and was followed by an increase (“brightening”) to the present, which has been attributed to changes in cloud cover and/or aerosol concentrations (e.g., Wild, 2009; Wang and Dickinson, 2013; Wild, 2016a; and references therein)
The one that seems most consistent with our findings postulates that an increase in DLR is related to an increase in integrated water vapour (IWV), since we found stronger IWV changes at mountain stations in relative terms
Summary
Downward shortwave radiation (DSR) and downward longwave radiation (DLR) are important terms in the surface radiation budget and are fundamental in understanding the climate effect of increasing greenhouse gas concentrations (Wang and Dickinson, 2013). DLR has been observed to increase during the 1973–2008 period (Wang and Liang, 2009) and since the 1990s (Wild, 2016b), but the reliable observational record is in general only several decades long at present In support of these international efforts, the Alpine Surface Radiation Budget (ASRB) network was established in 1994–1995 at 11 stations in Switzerland to monitor regional radiation fluxes (Philipona et al, 1996; Marty, 2000; Marty et al, 2002).
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