UV measurements in the 3000–5000 m altitude region in Tibet

Abstract

We present measurements of solar UV radiation performed with multichannel moderate‐bandwidth NILU‐UV filter instruments during winter and summer in 2003 in the altitude region from 3000 m to 5000 m at 29°N in the Lhasa region in Tibet. During summer the UV index was found frequently to exceed 15 on clear days and occasionally to exceed 20 on partially cloudy days. High altitudes, low ozone column amounts, clean atmospheres, and relatively low latitudes are factors that contribute to the high UV levels on the Tibetan plateau. UV index values of 12 were measured in late winter for a solar zenith angle of 40° at a snow‐covered 5000 m altitude site. This is a 35% increase compared to a corresponding snow‐free surface. Our measurements show that the solar UV radiation increases with altitude. For clear‐sky and snow‐free conditions the altitude increase is 7–8% per km for erythemal UV dose rates and 3% per km at a wavelength of 340 nm. Results from clear‐sky calculations using a multiple‐scattering radiative transfer model were found to agree within 5% with clear‐sky UV measurements. Radiative transfer calculations combined with measurements were used to estimate the influence of clouds on the UV radiation at the surface. On the average the variable cloud cover in Lhasa reduced the daily integrated erythemal UV dose by 25%. The NILU‐UV instruments also provide total ozone column amounts. The mean difference between daily total ozone column amounts derived from NILU‐UV measurements and from Earth Probe TOMS data was −1.4% ± 3.2% (1σ).