The thesis is a study of outdoor ultraviolet (UV) radiation as a part of solar radiation – its spectral and temporal variability, doses, as well as dependence on solar elevation, cloudiness, column ozone and aerosols. All conclusions are sitespecific, based on measurements carried out at Tõravere, Estonia, since 2004. Special attention is paid to the influence of cloudiness as the main factor modifying solar radiation, including the UV region. Clear sky, totally cloudy and broken cloudiness conditions are intercompared as well as different cloud types. UV doses in overcast days are about 1/3 from the clear weather ones and cloudiness showed wavelength dependence with more radiation passing the clouds at the shorter end of UVA than at the longer end – cloud modification factor in case of overcast sky at 332 nm is 0.38 and at 386 nm 0.36, respectively.
Due to various effects of UV radiation on ecosystems (in particular, humans, animals, plants, microorganisms) and materials, there is a necessity to assess quantitatively UV daily exposures and describe their long-term variations. As spectral measurements of UV radiation are carried out only in short time period (since 2004 at Tõravere) and due to technical reasons, there are gaps in data, models for calculating daily doses for UVA and UVB were developed using freely available ARESLab and libRadtran software. Beside complementing UV database, the models also enabled a retrospective survey on the UV regime in the past, for years 1955–2003. Integrated time series showed a decrease in UVB and UVA yearly doses since the 1970s until the 1980s, mainly due to cloudiness.
In both wavelength bands doses have increased since the 1990s, UVB started the increase already in the 1980s. The variability of UV doses is linked with changes in cloudiness observed at Tõravere and changes in column ozone. The thesis is structured into 4 chapters: 1) introduction, an overview of UV radiation and the objectives of the thesis; 2) data and methods, description of the site, available data from Tartu–Tõravere meteorological station and UV measurements at Tartu Observatory; 3) results, the essential part of the thesis, committed to the results of analysing the influence of clouds and other variables on UV radiation as well as describing the reconstruction using the developed models; 4) conclusions, deems the main results, ongoing and future plans. I hope that my investigation helps to better understanding of Estonian radiation climate.
Aun, M., 2017. Dependence of UV radiation on climate factors. Reconstruction of UV doses in Estonia for past years (Doctoral dissertation).