ИСТИНА

UV monitoring at the Meteorological Observatory of Moscow State University (MO MSU) provides the longest UV time series within the spectral range of 300-380nm since 1968 by different kinds of UV –instruments, which are calibrated against the European standards. In this presentation we discuss seasonal and interannual UV variability and its reasons in Moscow according to 1968-2014 period of observations. The ratio of diffuse to global UV irradiance is analyzed for different seasons of the year: it changes from 96-99% in winter to 78-80% in summer conditions. The ratio of global UV irradiance 300-380nm to total radiation is about 4% and does not change significantly over the analyzed period. Special attention is paid to the effects of different geophysical factors influencing the UV irradiance at ground. We analyze the effects of cloudiness, aerosols and surface albedo on the level of UV irradiance 300-380nm. On average, the lowest losses due to cloudiness of about 23-27% are observed from May to August. Maximum losses (up to 44%) are in October-November. Changes in the global UV irradiance due to natural fluctuations of aerosol can reach 30-35% at the solar elevation h = 10° and 20-25% at h = 55° . In cloudless conditions, the snow cover increases global UV radiation by 8-12 % and is significantly enhanced in the presence of clouds. In order to reveal the reasons of year-to-year variability a special UV reconstruction model has been applied over the whole period of UV observations(Chubarova, ACP, 2008). We show that the changes in the effective cloud transmission plays the most important role in interannual UV variations (about plus-minus 8-10%). Variability of total ozone does not play a significant role since the effective wavelength of the UV irradiance 300-380nm is out of strong ozone absorption. The role of interannual UV variations due to changes in aerosol properties can also be seen but it is substantially smaller than the effects of cloudiness. However, the existing long-term decrease in aerosol loading leads to the increase of UV irradiance 300-380nm.