Estimation of hourly and daily ultraviolet solar irradiation under various sky conditions at Sanya, Southern China

Total ultraviolet (UV) irradiation (290-400 nm) observed at Sanya (SY, 109 degrees 28'E, 18 degrees 13'N), China during 2005-2012 was used to investigate UV variability and its relationship with global solar radiation under various sky conditions in Southern China for the first time. Monthly mean daily UV irradiation generally increased from 0.585 MJ m(-2) in December to 0.965 MJ m(-2) in July with annual mean value being about 0.767 MJ m(-2). Monthly mean daily F-UV (ratio of UV to G) changed from 4.19% in December to 4.68% in July with annual mean being about 4.45 % at SY, China. By analyzing the dependences of hourly UV irradiation on optical air mass, clearness index, and cosine of solar zenith angle, respectively, under various sky conditions, three different UV estimation equations have been introduced and validated at SY, Inner Mongolia, and Lhasa. The statistical results indicated that estimation Eq. 3 (based on clearness index and cosine of solar zenith angle) produced the most satisfied estimates at Sanya: mean bias error, mean absolute bias error, and root-mean- square error were only -0.72 % (-0.23 %), 4.67 % (4.45 %), and 6.43 % (5.88 %), respectively, at hourly (daily) basis. Hourly and daily UV irradiation during 1992-2012 were then reconstructed, it was discovered that UV irradiation increased slightly during the study period and the most significant increases (0.904 MJ m(-2) day(-1) decade(-1)) were observed in summer.