Isotopic compositions of precipitation and cloud base raindrops in Taiyuan, China

In the transitional zone between a monsoon and an arid climate, isotopic composition of precipitation is sensitive to climatic changes. Isotopic compositions of cloud base raindrops (CBR) are directly related to water vapor transport and in-cloud processes, but have been seldom studied. In this study, we examined precipitation-event samples from a ten-year period in Taiyuan, a climatic transition region, and developed a simple method to determine the isotopic composition of CBR. We analyzed and compared the relationship between the isotopic composition of meteoric water and various climatic factors and processes. Results indicate that the alteration of moisture sources plays a crucial role in determining the intra-annual isotopic pattern of precipitation and CBR. During the humid summer monsoon periods, the rainfall amount effect dominates the isotopic variation of precipitation, while the temperature effect controls the isotopic variation under the impact of the winter continental air mass. The isotopic effects are specific to precipitation originating from the same moisture source. The below-cloud evaporation (BCEV) is particularly strong in autumn and causes a great difference in d-excess between precipitation and CBR, which limits applications of d-excess of precipitation as an indicator of hydrometeorological and hydroclimatic processes. The d-excess of CBR can more effectively characterize the oceanic sources of water vapor than that of precipitation. In addition, the fraction of recycled moisture derived from d-excess of precipitation could be underestimated compared to the estimates based on d-excess of CBR. The BCEV process can be divided into an initial moderate evaporation stage and a subsequent deep evaporation stage. We report for the first time that the rates of change for δ18O, δD and d-excess of falling raindrops are significantly accelerated during the deep evaporation stage.