The Isotopic Composition of Water Vapor and the Concurrent Meteorological Condition over the Northern Part of the Tibetan Plateau

The stable isotopic ratio of water vapor (ä18O and äD) was measured near July-1st Glacier (39.25N, 97.75E, 4250 m) in the northeast part of the Tibetan Plateau. From 11 to 17 August 2003, we collected and then analyzed atmospheric samples at July-1st Glacier (Qiyi) and a base camp (BC), that was 4km from Qiyi. While from 14 to 29 July 2004, we did the same experiment over the Qiyi area as well as the Silkroad stations (Lanzhou, Zhangye and Jichuan) those are located northern foot of the Tibetan Plateau and adjacent to the Taklimakan/Gobi desert. Then the temporal change of the isotopic composition was compared with the atmospheric circulation fields using and European Centre for Medium-range Weather Forecasts (ECMWF) objective analysis dataset. After a snowfall in the early afternoon of the 14 th, a strong northerly wind was observed in the evening. The isotopic analysis revealed a clear difference in the evening of 14th, which showed heavier äD (and ä18O ) and higher d-excess compared with those of the previous period. The snowfall and the strong northerly wind that followed were identified with the passing of a deep trough that was observed at 500 hPa and dry air from the northwest. A back trajectory analysis revealed that the dry air from the northwest came from the upper troposphere/lower stratosphere. We also found a diurnal change in the isotopic composition. As the day progressed from noon to late evening, the values of äD (and ä18O ) became smaller (lighter) and the values of d-excess became higher. These changes occurred during a northerly valley wind. We suggest here that these changes were observed because Qiyi (and BC) were located an atmospheric region where the Tibetan Plateau air mass mixes with that from the northerly arid regions. Air from the Tibetan Plateau has relatively high ä18O and äD values (relatively heavy water) and relatively low d-excess values. As the air from the northern desert has relatively small ä18O, relatively small äD values and relatively high d-excess values, the valley wind that prevails in afternoon through evening must be bringing air from the northern desert to our site. The more intensified observation in 2004 showed the similar diurnal variation in both Qiyi and the Silkroad, and confirmed the above diurnal variation of the airmass transport between the Plateau and desert is occurred. During the year 2004 observation period, we had extreme precipitation event at Jichuan (one of the Silkroad station) but no precipitation was observed in the Qiyi station in the night of 24th July, 2004. The characters in isotopic composition of the precipitation as well as the water vapor showed the different air mass transport system for the two areas. Concurrent meteorological conditions are under investigation. We believe that analysis of the isotopic ratio of water vapor should be useful to understand the continental and local hydrological circulation over mountaneous and desert terrains, and to interpret the ice core records in paleo-climatological studies.