The humidity effect, namely the markedly positive correlation between the stable isotopic ratio in precipitation and the dew-point deficit ATd in the atmosphere, is put forward firstly and the relationships between the δ18O in precipitation and ATd are analyzed for the Urumqi and Kunming stations, which have completely different climatic characteristics. Although the seasonal variations in δ18O and △Td exhibit differences between the two stations, their humidity effect is notable. The correlation coefficient and its confidence level of the humidity effect are higher than those of the amount effect at Kunming, showing the marked influence of the humidity conditions in the atmosphere on stable isotopes in precipitation. Using a kinetic model for stable isotopic fractionation, and according to the seasonal distribution of mean monthly temperature at 500 hPa at Kunming, the variations of the δ18O in condensate in cloud are simulated. A very good agreement between the seasonal variations of the simulated mean δ18O and the mean monthly temperature at 500 hPa is obtained, showing that the oxygen stable isotope in condensate of cloud experiences a temperature effect. Such a result is markedly different from the amount effect at the ground. Based on the simulations of seasonal variations of δ18O in falling raindrops, it can be found that, in the dry season from November to April, the increasing trend with falling distance of δ18O in falling raindrops corresponds remarkably to the great △Td, showing a strong evaporation enrichment function in falling raindrops; however, in the wet season from May to October, the δ18O in falling raindrops displays an unapparent increase corresponding to the small △Td, except in May. By comparing the simulated mean δ18O at the ground with the actual monthly δ18O in precipitation, we see distinctly that the two monthly δ18O variations agree very well. On average, the δ18O values are relatively lower because of the highly moist air, heavy rainfall, small ATd and weak evaporati
Previous studies found extremely high d-excess in both ice core and glacial melt water in Dasuopu glacier, Xixiabangma, middle of Himalayas. These values are much higher than the global average and those measured in southwest monsoon precipitation. The d-excess variation in over one year at Nyalam station will clarify this phenomenon. Studies show that the high d-excess is related to the seasonal variation of moisture transport to this region. The d-excess values are low during the southwest monsoon active periods, when moisture originated from the humid ocean surface. The d-excess values are higher in non-monsoon months, when moisture is derived from westerly transport. Winter and spring precipitation accounts for a substantial portion of the annual precipitation, resulting in higher d-excess in the yearly precipitation in the middle of Himalayas than other parts of the southern Tibetan Plateau. This finding reveals that the precipitation in the middle of Himalayas is not purely from southwest monsoon, but a large portion from the westerly transport, which is very important for ice core study in this area.
