Observational Study on the Ground-based UVI at Dangxiong of Tibet

Factors of solar zenith angle (SZA), cloud, ozone and snow cover modifying the surface ultraviolet index (UVI) in the Tibetan Plateau are presented by the TUV model simulations and the two-year continuous ground-based measurements at Dangxiong (30.48°N, 91.10°E, 4200 m a.s.l) from September 2009 to August 2011. Results show that the key factors affecting the UVI over the Tibetan region are SZA and cloud. Variations of cloud-free surface UVI can be characterized exactly as a simple empirical SZA function. The UVI cloud modification factor (CMF) generally reduces by 46% as the cloud amount increases from 0 to 100%. CMF might increase by 3%-6%, or even by 40% in individual cases if the solar disk is partly masked with broken clouds. The ozone valley enhances 12% summer surface UVI compared to plain regions of the similar latitudes. The winter short-term thin snow coverage could enhance 16% or less surface UVI, and this enhancement is lower than that from the simulations (23%). UVI attenuation caused by aerosols with optical depth of 0.02-0.1 is no more than 3%. As compared with UVI measured at Lhasa (29.67°N, 91.13°E, 3650 m a.s.l), Tuotuohe (34.22°N, 92.43°E, 4500 m a.s.l) and Waliguan (36.29°N, 100.90°E, 3810 m a.s.l), the difference of site-altitude is a key factor influencing the UVI. Under the same SZA conditions, the summer UVI at Lhasa is 7%-10% higher than that at Waliguan due to the southern thinner ozone layer, the ozone valley. Comparison with satellite-derived product shows that, averagely, the OMI-UVI is 80% higher than the ground-based values at Dangxiong but only 8.6% higher under cloud-free circumstances. The OMI-UVIs are all above 65% higher than those ground-based measurements at the other 3 sites but their cloud-free values are only 13%, 9% and 50% higher at Tuotuohe, Waliguan and Lhasa, respectively. Clouds and geographical mismatches between the satellite pixel and fixed ground-based observation site are supposed to be the main factors of higher space-based UVI values, particularly over Lhasa. As the ground-based UVI is more than 14, the coincident space-based UVI is generally 3% lower at Dangxiong owning to the broken clouds strengthening the in-situ radiation.