The Analysis of Variation Characteristics and Establishing of Estimating Equation for Ultraviolet Radiation in Lhasa

The temporal variation of ultraviolet (UV) radiation and the ratio of UV radiation to broadband solar radiation were analyzed in Lhasa using the radiation observations during 2005-2010, the results indicate that the UV radiation has consistent variation characteristics with the solar radiation. The highest value of daily UV radiation was measured at noon while the lowest value occurred in the morning or night, the highest value of monthly UV radiation was measured in June while the lowest value occurred in January. The annual mean daily UV radiation was 0.87 MJ·m^-2·d^-1 during this period, with a decreased trend. The ratio of UV radiation to solar radiation has obvious diurnal variation characteristics, namely, the highest value appeared at noon while the lowest value occurred in the morning or night, the seasonal highest value occurred in summer while the lowest value occurred in winter. The mean ratio of UV radiation to solar radiation was 0.0418, which also has a decreased trend in the six years. Finally, an experimental estimating equation was established by using the optical air mass and the clearness index in 2010. This model can estimat UV radiation with high accuracy at the site where the model was developed. The mean bias error was close to zero, and the relative errors were less than 8.66% and 5.5% for instantaneous and daily UV radiation, respectively.