Effects of ozone, cloud and snow on surface UV irradiance

Total solar irradiance (TSO), total UV irradiance (TUV) and erythemal UV irradiance (EUV) measured at King Sejong station (62.22°S, 58.78° W) in west Antarctica have been used together with total ozone, cloud amount and snow cover to examine the effects of ozone, cloud and snow surface on these surface solar irradiance over the period of 1998-2003. The data of three solar components for each scan were grouped by cloud amount, n in oktas (0≤ n<3, 3≤n<4, 4≤n<5, 5≤n<6, 6≤n<7 and 7≤n<8) and plotted against solar zenith angle (SZA over the range of 45° to 75°. The radiation amplification factor (RAF) is used to quantify ozone effect on EUV. RAF of EUV decreases from 1.51 to 0.94 under clear skies but increases from 0.94 to 1.85 under cloudy skies as SZA increases, and decreases from 1.51 to 1.01 as cloud amount increases. The effects of cloud amount and snow surface on EUV are estimated as a function of SZA and cloud amount after normalization of the data to the reference total ozone of 300 DU. In order to analyse the transmission of solar radiation by cloud, regression analyses have been performed for the maximum values of solar irradiance on clear sky conditions (0≤n<3) and the mean values on cloudy conditions, respectively. The maximum regression values for the clear sky cases were taken to represent minimum aerosol conditions for the site and thus appropriate for use as a normalization (reference) factor for the other regressions. The overall features for the transmission of the three solar components show a relatively high values around SZAs of 55° and 60° under all sky conditions and cloud amounts 4≤n<5 and 5≤n<6. The transmission is, in general, the largest in TUV and the smallest in EUV among the three components of the solar irradiance. If the ground is covered with snow on partly cloudy days (6≤n<7), EUV increases by 20 to 26% compared to snow-free surface around SZA 60° -65°, due to multiple reflections and scattering between the surface and the clouds. The relative difference between snow surface and snow-free surface slowly increases from 9% to 20% as total ozone increases from 100 DU to 400 DU under partly cloud conditions (3≤n< 6) at SZA 60°. The snow effects on TUV and TSO are relatively high with 32% and 34%, respectively, under clear sky conditions, while the effects changes to 36% and 20% for TUV and TSO, respectively, as cloud amount increases.