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We compare results for the UV index (UVI), the total ozone column (TOC), and the radiation modification factor (RMF, being 1 in the absence of clouds and aerosols) at four sites on the Tibetan Plateau. The results were obtained by analyzing ground measurements by multichannel moderate-bandwidth filter instruments for the period July 2008–September 2010, and radiative transfer modeling was used to aid the interpretation of the results. The highest UVI of 20.6 was measured in Tingri (28.7°N; 4335 m). For July, monthly mean UVI values were 14.5 and 12.9 in Tingri and Lhasa (29.7°N; 3683 m), respectively. Generally, the UVI levels in Tingri and Lhasa were higher than in Nagchu (31.5°N; 4510 m) and Linzhi (29.7°N; 2995 m), due to less cloud cover at the former two sites. In 2009, the annual mean UVI and RMF values were 6.8 and 0.7 for Linzhi, 8.8 and 0.92 for Lhasa, 10.5 and 0.92 for Tingri, and 6.7 and 0.7 for Nagchu. Radiative transfer simulations indicate that the latitude difference would correspond to an increase in the UVI of about 0.3 from Nagchu to Tingri; whereas, the altitude difference would correspond to a reduction of about 1.5%, implying that the observed difference is due to the difference in cloud cover. The annual mean TOC values were found to be 260–264 Dobson units (DU) in Lhasa, Linzhi, and Nagchu, and 252 DU in Tingri. TOC values in Lhasa were found to agree within 3% with those derived from Ozone Monitoring Instrument (OMI) measurements.

From ground-based measurements with NILU-UV multichannel, moderate-bandwidth filter instruments deployed at altitudes from 3,000 m to 5,000 m on the Tibetan Plateau (TP) values for the UV index (UVI), the total ozone column amount (TOCA), and the cloud modification factor (CMF, defined to be 1 for a clear-sky atmosphere) were derived for the measurement period from June 2008 to September 2010. The values of these three geophysical parameters were compared at four different study sites on the TP: Linzhi (29.66°N, 94.37°E, 2,995 m), Lhasa (29.65°N, 91.18°E, 3,683 m), Tingri (28.66°N, 87.13°E, 4,335 m), and Nagchu (31.47°N, 92.06°E, 4,510 m). For 2009, the annual mean UVI values at the four study sites were found to be 6.8 for Linzhi, 8.8 for Lhasa, 10.5 for Tingri, and 6.7 for Nagchu; the annual mean TOCA value was found to be in the range of 260-264 Dobson units (DU) in Lhasa, Linzhi, and Nagchu, but only 252 DU in Tingri; and the annual mean CMF values were found to be 0.70 for Linzhi, 0.83 for Lhasa, 0.92 for Tingri, and 0.70 for Nagchu. The low UV radiation level found in Linzhi was due to its low altitude among the four study sites and a quite cloudy sky. The highest UVI of 19.2 was measured in Tingri, where the high UV radiation level mainly was due to the high CMF value of 0.92 and a relatively high albedo associated with a gray ground surface and surrounding snow-covered mountains. By use of radiative transfer calculations the UVI differences between the four study sites due to different TOCA values, latitudes, and altitudes were estimated. The TOCA values derived from the ground measurements in Lhasa were found to be in good agreement with those derived from OMI measurements.

We present solar ultraviolet (UV) irradiance measurements at four sites in Tibet (Lhasa, Linzhi, Nagqu, and Tingri) for the period between July 2008 and September 2010. Erythemal UV dose rates and irradiances of solar UVA, UVB for all four sites are presented. Days with clear sky cloud transmittance (CLT) larger than 100% are particularly selected and analyzed for the four sites. Comparisons between the four sites are also carried out, and the clear-sky data analyses for the summer time show that Tingri (near Mt. Everest) has the highest UV irradiance, Lhasa the second highest, while Nagchu (Northen Tibet) and Linzhi (Eastern Tibet) have a bit lower values. Tingri has the strongest UV level among the four sites around the year, mainly due to increased albedo caused by snow-covered surroundings. UV data for Lhasa for the period from 2004 to 2010 are also analyzed, showing that the UV instantaneous irradiance trend for the recent six-year period is stable. However, the number of clear-sky days in Lhasa has increased over the recent six years, causing an increase in the yearly-integrated total UV irradiance. DOI: http://dx.doi.org/10.3126/jie.v8i3.5934 JIE 2011; 8(3): 75-86