Decadal variability of clouds, solar radiation and temperature at a high-latitude coastal site in Norway

Abstract

A B S T R A C TThe observed variability of shortwave (SW) irradiance, clouds and temperature and the potential connections between them is studied for the subarctic site Bergen (60.48N, 5.38E), located on the Norwegian west coast. Focusing on the quality and spatial representativity of the data, we compare observations from independent instruments and neighbouring measurement sites. The observations indicate that the decrease of sunshine duration and SW irradiance during the 1970s and 80s in Bergen is associated with the increasing frequency of clouds, in particular clouds of low base heights. We argue that the observed cloud changes are indicative of increased frequencies of storms in northern Europe. The annual mean observational time series show an increase in SW irradiance since 1990, which is not accompanied by a cloud cover (NN) decrease. This implies the influence of factors other than clouds, for example, decreasing aerosol emissions. Calculations of the aerosol optical depth (AOD) based on irradiance observations for hours when the sun is unobscured by clouds confirm a decreasing aerosol load after 1990, from 0.15 to 0.10 AOD which corresponds to 2Á6 Wm (2 of brightening.At the same time, a seasonal analysis reveals opposite changes in SW irradiance and NN during the months of strongest changes Á March, April and August Á also during the recent period of increasing SW irradiance. We conclude that the seasonally decreasing NN also contributes to the recent changes in SW irradiance. Finally, we address the relationship between temperature, SW irradiance and clouds. In winter (DecemberÁFebruary), the surface air temperature in Bergen is statistically linked to the warming influence of clouds. In all other seasons, the North Atlantic sea surface temperature variability has a more dominant influence on the air temperature in Bergen compared to local cloud and SW irradiance variability.

Impact of aerosols and clouds on decadal trends in all-sky solar radiation over the Netherlands (1966–2015)

“…While it is obvious that clouds are important, the difficulty here is that there are several factors that control their impact. For example there are considerable regional differences in fractional cloudiness (Norris, 2005): fractional cloudiness is constant in northern Europe (Parding et al, 2014) and in Germany before 1997 , well after the minimum in global radiation in 1984, and has declined in the period after 1997 in Switzerland and Germany, at least until 2010 (Ruckstuhl et al, 2010). Furthermore, cloud optical thickness changes can be the result of changes in microphysics or cloud thickness and current observations are not able to separate the two effects.…”

“…In the context of Europe there have been a considerable number of regional studies that focus on trends in global radiation and their attribution, such as in Germany (Liepert and Tegen, 2002;Liepert and Kukla, 1997;, in Germany and Switzerland combined (Ruckstuhl et al, 2008;Ruckstuhl and Norris, 2009;Ruckstuhl et al, 2010), in Estonia (Russak, 2009), in the general Baltic states (Ohvril et al, 2009), in Spain (Mateos et al, 2014), in Norway (Parding et al, 2014), northern Europe in general (Stjern et al, 2009) and in Italy (Manara et al, 2015). Even though there are regional differences, the summarized global or allsky radiation data from Europe combined display a minimum in [1984][1985] at the end of a "dimming" period with a subsequent return to higher values.…”

Validation of CM SAF Surface Solar Radiation Datasets over Finland and Sweden

“…The particular case of causes behind (in situ) SSR trends in Northern Europe has been investigated by Stjern et al [47] and Parding et al [48]. They found that cloudiness decrease appears to be the main driver for the 1990s-2000s brightening in Scandinavia, with likely additional contributions from decreasing aerosol concentrations.…”

Reassessment and update of long‐term trends in downward surface shortwave radiation over Europe (1939–2012)

“…Changes in the transparency of the atmosphere due to variations in anthropogenic aerosol emissions and/or cloudiness are considered as the major factors explaining this dimming/brightening phenomenon [ Stanhill and Cohen , ; Long et al ., ; Ohmura , ; Wild , ; Gan et al ., ; Parding et al ., ; Cherian et al ., ; Mateos et al ., ; Wang et al ., ]. However, analysis of the SSR series still poses some challenges that are necessary to overcome in order to better understand their trends during the last decades.…”