The attenuation effect on ultraviolet radiation caused by aerosol and cloud in Lhasa, Tibetan Plateau of China

Measurements of ultraviolet (UV) radiation and total radiation (G) in Lhasa, China during 2005–2013 are reported to explore the variations of UV radiation and the effects of aerosols and clouds on UV radiation under various cloud cover conditions. UV radiation increases from spring until summer and then decreases to lower values in winter; the annual average value is 0.91 MJ m−2d−1. The ratio of UV radiation to G (UV/G) exhibits an upward trend as cloud cover increases, especially for cloud cover higher than 5 octas conditions. The temporal variation displays an inverse relationship between the UV/G ratio and clearness index (Ks) under different cloud cover scenarios. Aerosols alone and aerosols combined with the cloud attenuation effect lead to distinct increasing trends in UV radiation from spring until the highest values in summer, after which the values gradually decrease until winter. The monthly average values of the UV cloud modification factor vary from approximately 73.9% in July to 93% in December; the annual average value is 82%.