青藏高原东北部气候过渡带大气降水氢氧稳定同位素变化解析

Variation of hydrogen and oxygen stable isotopes in atmospheric precipitation in climate transition zone of the northeastern Tibetan Plateau

关键词:  大气降水  氢氧稳定同位素  氘盈余  典型降水事件  青藏高原东北部

Key words:  atmospheric precipitation  stable isotopes of hydrogen and oxygen  deuterium surplus  typical precipitation events  northeastern Tibetan Plateau

摘要:

大气降水中的氢氧稳定同位素是揭示水体循环过程的理想示踪剂，对于描述和评定区域气候变化及可持续发展具有重要指示意义。青藏高原东北部受高原天气系统与西风带天气系统的相互作用，形成了典型的气候过渡带，其水汽来源复杂，对气候变化的响应极为敏感。系统开展全球变化背景下青藏高原东北部大气降水氢氧稳定同位素特征研究，对揭示区域降水时空分布格局、促进水资源可持续利用与管理具有重要意义。本研究大气降水采样点布设在位于青藏高原东北部的青海湖北岸、距青海湖约 15 km 的刚察县气象局（37°20′N，100°08′E；3301.5 m），基于采样点逐次采集降水样品，2019年1—12月采样期间，共采集141个降水样品（其中，春季29个，夏季70个，秋季38个，冬季4个），包括100次降雨、 17次降雪和24次雨夹雪事件。结合气温、降水量等气象数据，分析了青藏高原东北部大气降水氢氧稳定同位素的变化特征，探讨了降水同位素的环境效应，并通过对比分析典型降水事件的大气降水线，揭示其影响因素。结果表明：青藏高原东北部降水中δ² H和δ¹⁸O具有夏季富集，冬季贫化的特征；当地大气降水线为δ² H=7.94δ¹⁸O+12.39，受到云下二次蒸发的影响。氘盈余值（d-excess）春夏秋季节高，冬季低， 年均值大于10‰；全年降水δ¹⁸O表现出显著的温度效应，而降水量效应不显著。持续降水事件中的δ² H 和δ¹⁸O值逐渐偏负，表现出淋溶效应；强降水事件大气降水线方程为δ² H=8.76δ¹⁸O+23.70，弱降水事件大气降水线方程为δ² H=7.88δ¹⁸O+11.10，表明弱降水事件发生时更容易受到云下二次蒸发的影响。观测期间，弱降水事件发生频次超过80%，因此云下二次蒸发对青藏高原东北部降水δ² H和δ¹⁸O有重要影响。

Abstract:

Background, aim, and scope The stable isotopes of hydrogen and oxygen in atmospheric precipitation are ideal tracers to reveal the water cycle process and have important indicative significance for describing and evaluating regional climate change and sustainable development. Particularly, in the northeastern Tibetan Plateau these isotopes provide insights into the interaction between plateau and westerly weather systems. This area, being a climate transition zone with multiple moisture sources, is extremely sensitive to climate change. A comprehensive study of the characteristics of hydrogen and oxygen stable isotopes of atmospheric precipitation, in the context of global change, is crucial for understanding the spatial and temporal distribution of regional precipitation, and for promoting sustainable water resource management. Materials and methods The precipitation sampling site of this study was set up at the Gangcha County Meteorological Bureau (37° 20′N, 100°08′E; 3301.5 m a.s.l.) in the northeastern Tibetan Plateau. Located on the north bank of Qinghai Lake and approximately 15 km away from the lake, the meteorological bureau served as the fixed sampling location where precipitation samples were collected on an event-based basis. A total of 141 precipitation samples were gathered during the sampling period from January 2019 to December 2019, including 29 samples in spring, 70 in summer, 38 in autumn and 4 in winter. These samples corresponded to 100 rainfall events, 17 snowfall events and 24 sleet events, respectively. Based on the successive precipitation samples, combined with the meteorological data such as temperature, precipitation, atmospheric precipitation in the northeastern Tibetan Plateau were analyzed change characteristics of hydrogen and oxygen stable isotopes. This paper discusses the environmental effect of precipitation isotopes, and through the analysis of meteoric water line in typical precipitation events, reveals its influence factors. Results The values of δ² H and δ¹⁸O ranged from −192.19‰ to 23.82‰, and from −25.18‰ to 1.58‰, respectively. The local meteoric water line (LMWL) is δ² H=7.94δ¹⁸O+12.39, with its slope ranging from 7.9 to 8.0. This indicates that the unique local circulation system in the northeastern Tibetan Plateau features distinct water vapor sources and evaporation patterns across different temporal and spatial scales.The deuterium surplus value was higher in spring, summer and autumn, but lower in winter, with an annual average of more than 10‰. The δ¹⁸O of annual precipitation shows a significant temperature effect, but the precipitation effect is not significant. Discussion The δ² H and δ¹⁸O values in continuous precipitation events are gradually negative, showing a leaching effect. The atmospheric precipitation line equation for heavy precipitation events is δ² H= 8.76δ¹⁸O+23.70, and that for weak precipitation events is δ² H=7.88δ¹⁸O+11.10, indicating that weak precipitation events are more susceptible to the influence of secondary evaporation under clouds. Conclusions During the observation period, the frequency of weak precipitation events is more than 80%, so secondary evaporation under clouds has an important effect on the precipitation δ² H and δ¹⁸O over the northeastern Tibetan Plateau. Recommendations and perspectives From the perspective of water vapor cycle, there are two factors affecting precipitation isotope: one is local, that is, meteorological elements such as temperature, precipitation intensity and relative humidity during precipitation analyzed in this paper; The other is the δ¹⁸O characteristics of water vapor itself, that is, the source of water vapor evaporation and the fractionation of δ¹⁸O in the transport process of precipitation. This article does not to water vapor source of atmospheric precipitation in the northeastern Tibetan Plateau and conveying process is analyzed, the current work is in progress, expect next water vapor sources and transmission of successive precipitation process tracking analysis, comprehensive understanding in atmospheric precipitation in the northeastern Tibetan Plateau the influence factors of hydrogen and oxygen stable isotopic variation characteristics.