Analysis of ultraviolet radiation in Central China from observation and estimation

Measurements of UV (ultraviolet) and global solar radiation in Central China during 2006–2012 were first reported to investigate the UV radiation variability in different time scales and its UV fraction under different sky conditions. Both UV irradiation and UV fraction showed similar features that peaked in values at noon during summer (July) and reached their lowest in winter (January) with annual mean values being about 0.49 MJ m−2 d−1 and 4.35%, respectively. It was also discovered that there were inverse relationships between UV fractions and clearness indexes at all sky conditions; clouds, water vapor and seasonality were main factors causing the daily variations of UV irradiations. The maximum UV irradiances decreased by 51.14% (33.49%) in overcast days when compared to clear days in summer (winter). By analyzing the dependence of UV irradiances on cosine of solar zenith angle and clearness index, an efficient all-sky model has been developed for estimating UV values in Central China, which has also been tested at Sanjiang and Lhasa and produced satisfied estimations. UV dataset from 1961 to 2011 in Central China was then reconstructed and annual mean daily UV irradiation was about 0.488 MJ m−2 d−1. There was a significant decreasing trend (−0.018 MJ m−2 d−1 per decade) during the last 50 years, the decreases were sharpest in summer (−0.027 MJ m−2 d−1 per decade) with smallest decreases being observed in autumn (−0.001 MJ m−2 d−1 per decade). Meanwhile, it was also revealed that UV energy began to increase since 1990s (0.003 MJ m−2 d−1 per year).