Albedo Influences on Surface UV Irradiance at the Sonnblick High-Mountain Observatory (3106-m Altitude)

In this work the influences of ozone, aerosols, and albedo on the clear sky UVA and UVB irradiance at a high-mountain station are investigated by using both routine spectral UV measurements from the high-mountain Sonnblick observatory in Austria (3106-m altitude) and theoretical simulations. The 501 single measurements and the model output show that calculations are on average higher than the measurements by 10%–12% at 305 nm, 8%–10% at 315 nm, and 5%–8% at 370 nm. The study of the fluctuations in UV irradiance constitutes the second part of this work. Columnar-ozone fluctuations lead to enhancements of UV irradiances of 560% at 305 nm and 69% at 315 nm as columnar ozone changes from 380 to 230 Dobson units. The radiative transfer model shows the same behavior with changes in columnar ozone and solar zenith angle as the measurements do. By using the measurement : model ratio, it therefore is possible to analyze the fluctuations in UV irradiance occurring at Sonnblick observatory that are not dependent on solar zenith angle or columnar ozone in order to trace the influence of albedo on UV irradiance. First, the maximum possible change in UV irradiance due to aerosols is simulated and shows that changes in aerosol optical depth could induce changes in UV irradiance of up to 5%. Second, the influence of various parameters on UV irradiance is examined. Cloud cover under the Sonnblick summit may enhance the UV irradiance on average by 6% in wintertime. The UV values in winter are on average 4.5% higher than the summer values for a fixed solar zenith angle. This result most probably is caused by changes in ground albedo resulting from larger areas being covered with snow during the winter. Simulations with the radiative transfer model suggest that the average albedo may be larger by 0.15–0.22 during wintertime as compared with summertime. Clouds under the summit may enhance the average albedo by 0.30 ± 0.15.