西藏当雄地基紫外线指数观测研究

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

基于西藏当雄2009年9月—2011年8月地基紫外线指数（UVI）观测,结合TUV辐射传输模式分析,该文检验太阳天顶角、云、臭氧、积雪和气溶胶对UVI的影响。结果显示,影响地面UVI主要因素是太阳天顶角和云。晴天地面UVI可简单用太阳天顶角拟合函数表征;地面UVI的云调制因子总体上随云量增加呈下降趋势,但间隙性、未遮蔽日面的云可增强太阳散射辐射,使云调制因子值平均增加约3%~6%,个别情形达40%。臭氧低谷使当雄UVI比同纬度平原地区增加约12%;冬季短期、浅层积雪使UVI增加16%或更低,也低于模式模拟值（23%）;气溶胶（光学厚度为0.02~0.1）对UVI衰减低于3%。因臭氧低谷导致青藏高原臭氧南北分布的差异,相同太阳天顶角下拉萨（海拔为3650 m）UVI较瓦里关（海拔为3810 m）偏高7%~10%。与卫星产品比较表明：OMI卫星UVI产品在当雄、沱沱河、瓦里关和拉萨较地基测值总体偏高65%以上,而晴天则平均分别偏高8.6%,13%,9%和50%。云、地基与卫星像元地理位置差异应是卫星UVI产品偏高的原因。当雄地基UVI测值大于14时,卫星UVI产品反而低3%应与间隙性云有效增强了地面辐射有关。

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.1） 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.1）,Tuotuohe（34.22°N,92.43°E,4500 m a.s.1） and Waliguan（36.29°N,100.90°E,3810 m a.s.1）,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.