Long-term monitoring of the solar and thermal infrared radiation fluxes between the atmosphere and the earth’s surface is important for climate research. High accuracy measurements make it possible to determine reference values for the energy budget of solar and thermal radiation. This allows assessing the evolution of radiation fluxes and identify deviations. The reconstruction of UV radiation level since 1926 shows that it mainly depends on ozone and sunshine duration.
Radiation is the driving force of energy exchanges between the atmosphere, the oceans and the surface of the earth. It is expected that the most direct effect of global warming will be a rise in infrared radiation from the atmosphere to the ground. Imbalances in the radiation budget are responsible for the dynamics of weather, and they also drive climate change. Minor differences between the incoming and outgoing radiation fluxes are already sufficient to start such processes. Consequently, long-term monitoring of surface radiation fluxes is an important component of climate change monitoring, which the weather service MeteoSwiss is actively cooperating to within the scope of the Global Atmosphere Watch Program (GAW).
Radiation over the course of the year
MeteoSwiss is measuring the surface radiation ranging from the ultraviolet to the visible and infrared at the stations of the SACRaM (Swiss Alpine Climate Radiation Monitoring) Network. The stations are located in Davos, Locarno-Monti, Payerne and at the Jungfraujoch.
UV radiation and its influencing factors
The intensity of UV radiation depends on many factors. They include the position of the sun, cloudiness and surface albedo (radiation reflected by the ground, strongly influenced by snow). The amount of atmospheric components interacting with UV, such as ozone, plays a key role as well. The predominant influence of the position of the sun is clearly shown when comparing the low UV radiation in winter with the much higher levels reached in summer. The highest-altitude station at the Jungfraujoch has the highest UV radiation. This is due among other things to the thinner atmosphere as well as the fact that a large part of the surrounding area is covered by snow year-round.
Every blue dot in the graphic chart represents a daily average in 2014. The red curves represent the moving averages of 2014, calculated for every day of the year using the values of the 31 surrounding days. The black curves depict the average annual cycle computed using data from the beginning of the series of measurements until 2013. The comparison of the monthly moving average for 2014 with the average annual cycle reveals the deviations from average . For example, it clearly shows that the UV radiation from mid-June until late August at most stations was significantly below the long-time average.
Impact of cloudiness on the shortwave and longwave radiation
The intensity of shortwave radiation is dependent on the position of the sun, cloudiness, surface albedo and the aerosol optical depth. The latter is a measure of atmospheric turbidity. At Jungfraujoch for instance, the aerosol optical depth is usually very low, although it is influenced by important perturbation such as Saharan dust or local emission from neighbouring valleys.
Cloudiness has a major impact on both shortwave (solar) as well as longwave (infrared, thermal) irradiance, but the effect on these two wavelength ranges is generally opposite. While clouds usually reduce the intensity of shortwave irradiance, they generally increase the longwave irradiance. However, longwave irradiance is likewise influenced by the atmospheric temperature and water vapour content. In 2014, the shortwave irradiance in January-February and from end of June to late August was below average, which is due to a higher degree of cloud cover. On the opposite, at Payerne and Locarno-Monti in June the shortwave irradiance is higher than average. The longwave irradiance in January and February was higher than average, which is clearly an effect of the higher level of cloud cover. On the other hand, from end of June to late August the longwave irradiance was not higher than usual, which may be due to parameters other than cloudiness influencing longwave radiation.
Longer-term data series on UV radiation
MeteoSwiss has been measuring ultraviolet radiation in Davos since 1995, at Jungraujoch since 1996, in Payerne since 1997 and in Locarno-Monti since 2001. In light of the considerable impacts of UV radiation on health and the ecosystem, there is a need for longer-term data series. Reconstruction methods can be used to infer the UV radiation for past periods if information is available on the amount of ozone, the position of the sun, the cloudiness and the surface albedo. Longer-term data series are available for these parameters. The European initiative COST 726 was launched with the purpose of establishing a reliable method that can be used to determine the climatology of the daily UV radiation since 1958. Within this framework, MeteoSwiss has specifically addressed the influence of cloudiness and albedo.
The UV radiation was reconstructed using series of measurements from Arosa (ozone) and Davos (cloudiness), which date back to 1926. The results show that the annual average fluctuation of the UV radiation ranges between 5-10 percent over the entire period. Ozone and sunshine duration have the greatest impact on the fluctuations, while the change in snow cover only has a minor effect. The depletion of the ozone layer is the main cause for the rise in UV radiation starting from the late 1970s.
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