6Liu J, Sun Z, Liang H, et al. Precipitable Water Vapor on the Ti- betan Plateau Estimated by GPS, Water Vapor Radiometer, Radio- sonde, and Numerical Weather Prediction Analysis and its impact on the Radiation Budget [ J ]. Journal of Geophysical Research,2005, 110, D 17106, doi : 10,1029/2004JD005715.