Determining a climatology of UV radiation for Switzerland

Ultraviolet solar radiation has strong effects on human health, ecosystems and materials. Concerns have been expressed about a potential increase in the UV radiation level due to ozone depletion, and about enhanced UV exposure caused by changing patterns in population activities (e.g., recreational open-air activities) and the popularity of sun tanning. In Switzerland as in the rest of the world, the number of diagnosed skin cancers increased markedly during the last decades and the causes of such an increase are still investigated. MeteoSwiss participation in COST 726 aims at determining a Swiss UV climatology that will help answering such questions.

Considerable resources have been applied in Europe to improve our knowledge of UV changes. The emphasis has been placed on calibration and maintenance of existing UV observing systems and instrument developments. However, difficulties involved in the routine operation and maintenance of these instruments have limited the length of reliable data records to about two decades, and the number of places where they were measured, resulting in a set of observations too short and too sparse for a good understanding of past UV changes.
In Switzerland, continuous monitoring of UV radiation has been performed at four stations of the MeteoSwiss CHARM network using broadband UV erythemal radiometers. Two stations are located in the Alps at Davos and the Jungfraujoch, and the others are located in the plains on the north and south side of the Alps (Payerne and Locarno-Monti). The earliest continuous CHARM UV monitoring started in May 1995 at Davos. In addition, since 1988, Brewer spectroradiometers measure the UV spectrum at Arosa, mainly for determining the atmospheric ozone column.

A solution, called UV reconstruction, was developed for supplementing the lack of UV observational records by using semi-empirical models linking UV measurements to proxy data indicating the level of the most influential parameters for UV. Typically, information on the atmospheric ozone content, on cloudiness and on the surface reflectivity (albedo) is sought. While UV measurements are relatively sparse and of limited duration, it is possible to find information with better spatial coverage and of longer time span for these proxy parameters. Such a method allows inferring the amount of UV radiation at time when, or locations where, direct measurements are not available.

COST action 726 aims at "advancing the understanding of UV radiation distribution under various meteorological conditions in Europe in order to determine the UV radiation climatology and assess UV changes over Europe". In this collaboration, UV reconstruction techniques are developed and studied in a common framework. COST 726 coordinates many tasks including building an inventory of available UV radiation data and other data sets used as proxies, testing and improving available models, assessing needs for a better understanding of UV influence on public health and the ecosystem, and improving the comparability of UV measurements from instruments of different European networks.

Switzerland is well suited for testing such UV reconstruction techniques in various Alpine and plains conditions: first, continuous time series of UV measurements exist for different environments; second, the ancillary data used as proxies are measured with one of the densest network in Europe; finally, at some locations, the record of these proxy data are among the longest of the world. The COST-726 project will allow using observational records from Arosa to verify whether an increase in UV level linked to the ozone decline since the 80's can be confirmed with real observations. In addition, proxy data are available with a very good spatial coverage in Switzerland. This suggests the possibility of extending the reconstruction technique to many locations in Switzerland, eventually allowing the determination of a climatology of UV radiation for the entire Switzerland.

The UV evolution was reconstructed in the region of Davos and Arosa using total ozone column, sun duration and snow cover. At this location, these proxy data are available since 1926. The results show UV level variations of the order of 5-10% (for the yearly averages) throughout the whole period of the study. Most of the variation in the estimated UV level is explained by the changes in total ozone column and sunshine duration (linked to cloudiness), while the influence of variations in the snow cover is minor. Variations prior to 1980 are to a large extent due to changes in cloudiness, while the main reason for the increasing UV level since the late 1970s is the diminution of the total ozone column. Additional tests showed the estimated UV doses to be in good agreement with real measurements and demonstrated the reliability of the method.

Applications:
Real-time estimation of ground UV radiation level, support for epidemiological studies, warning to the population, tourism.

Contact:
Laurent Vuilleumier, MeteoSwiss, Atmospheric Data department, Les Invuardes, CH-1530 Payerne, Switzerland.