A set of methods to quantitatively evaluate the below-cloud evaporation effect on precipitation isotopic composition: a case study in a city located in the semi-arid regions of Chinese Loess Plateau
Abstract. Below-cloud evaporation effect heavily alters the initial precipitation isotopic composition, especially in the arid and semi-arid regions, and leads to misinterpreting the isotopic signal. To correctly explore the information contained in the precipitation isotopes, the first step is to qualitatively analyze the falling raindrops encountered below-cloud processes, and then to quantitatively compute the below-cloud evaporation ratio of raindrops. Here, based on two-year precipitation
... nd water vapor isotopic observations in Xi'an, we systematically evaluated the variations of precipitation and water vapor isotopes caused by the below-cloud evaporation effect. Our results suggest that the equilibrium method could be successfully used to predict the ground-level water vapor isotopic composition in semi-arid climates, especially for the winter data. Moreover, by using △d△δ-diagram, our data showed that evaporation is the mainly happened below-cloud process of raindrops, while snowfall samples retained 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 evaporation. In the Chinese Loess Plateau (CLP) city, raindrops are weakly evaporated in autumn and winter, and heavily evaporated in spring and summer, and in the meantime, the evaporation intensity is related to the local relative humidity. Our work sets an integrated and effective method to evaluate the below-cloud evaporation effect, and it will improve our understanding of the information contained in precipitation isotopic signals.