23 Aug 2022
23 Aug 2022
Status: this preprint is currently under review for the journal ACP.

A comparison of two methods to quantitatively evaluate the effect of below-cloud evaporation on the precipitation isotopic composition in the semi-arid region of the Chinese Loess Plateau

Meng Xing1,2, Weiguo Liu1,2,3, Jing Hu1,2, and Zheng Wang1,2 Meng Xing et al.
  • 1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, 710061, China
  • 2CAS Center for Excellence in Quaternary Science and Global Change, Xi'an, 710061, China
  • 3University of Chinese Academy of Sciences, Beijing, 100049, China

Abstract. Below-cloud evaporation effect could heavily alter the isotope composition of the rain water as it travels from the saturated environment in the cloud towards the surface, especially in the arid and semi-arid regions, and accounts for misinterpreting the isotopic signal. To correctly understand the information contained in the precipitation isotopes, the first step is to qualitatively analyze the below-cloud processes that the raindrops have encountered during their falling, and then to quantitatively compute the below-cloud evaporation ratio of raindrops. Here, based on two-year observations of precipitation and water vapor isotopes in Xi’an, we systematically evaluated the variations of precipitation and water vapor isotopes caused by the below-cloud evaporation effect. The precipitation δ18O and δ2H values range from -18.2 ‰ to 8.8 ‰ and -131.7 ‰ to 61.2 ‰, respectively, while the water vapor δ18Ov and δ2Hv values range from -29.5 ‰ to -10.1 ‰ and -214.9 ‰ to -63.9 ‰. Our results suggest that the equilibrium method could be successfully used to predict the ground-level water vapor isotopic composition from precipitation isotopes in semi-arid climates, especially for the winter data. Moreover, by using ΔdΔδ-diagram, our data show that evaporation is the main below-cloud process of raindrops, while snowfall samples retain the initial cloud signal because of less isotopic exchange between vapor and solid phases. In terms of meteorological factors, both temperature, relative humidity, and precipitation amount affect the intensity of below-cloud evaporation. In arid and semi-arid regions, the below-cloud evaporation ratio computed by the mass conservation equation would be overestimated relative to the isotopic method, while relative humidity is the most sensitive parameter in computing the remaining fraction of raindrop mass after evaporation. In this study, the mean remaining fractions of raindrop mass calculated by the isotopic method respectively are 69.2 %, 74.5 %, 85.2 %, and 80.8 % in spring, summer, autumn, and winter. The raindrops are weakly evaporated in autumn and winter, and heavily evaporated in spring and summer. Based on water vapor and precipitation isotope compositions, we designed a set of effective methods to evaluate the below-cloud evaporation effect, and this will improve our understanding of the information contained in the isotopic signals of precipitation.

Meng Xing et al.

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Short summary
Below-cloud evaporation effect (BCE) on precipitation would largely impact its final isotopic composition. However, to determine the BCE effect remains poorly constrained. Our work used ΔdΔδ-diagram to well differentiate the happened below-cloud processes. Moreover, by comparing two different computing methods, we considered that the isotopic method is more suitable to calculate the remaining fraction of BCE. Overall, our work set an integrated and effective method to evaluate the BCE effect.