Below-cloud evaporation effect on stable water isotopes in precipitation at the eastern margin of Qinghai-Tibet Plateau

Quantitatively assessing the evaporation-caused stable water isotope fractionation in precipitation below the cloud base is of great significance to understand the modern hydrological circulation especially in a plateau climate. Based on the hourly meteorological data at eight meteorological stations of the Gannan Plateau at the eastern margin of Qinghai-Tibet Plateau in 2020, we examined the spatial and temporal variations of below-cloud evaporation effect on precipitation isotopes, and then discussed the factors influencing the below-cloud evaporation across the Gannan Plateau. The contemporary observation of precipitation isotopes at one site in the Gannan Plateau is also applied to understand the local meteoric water line associated with the below-cloud evaporation. From 9:00 to 21:00 Beijing Time, the impact of below-cloud evaporation is greater than that during the rest periods. Within the study region, the below-cloud evaporation in the central area is usually stronger than that of the surrounding area. The diurnal and monthly variability of relative humidity is highly consistent with that of the remaining raindrop mass fraction after evaporation as well as the isotope variation in raindrop below the cloud base. When the relative humidity is greater than 95%, there is a linear relationship between the remaining raindrop mass fraction and the below-cloud deuterium-excess variation of approximate 1.1 parts per thousand/%. Global warming will increase the below-cloud evaporation effect across the Gannan Plateau. The findings are useful to understand the below-cloud evaporation and isotope fractionation under a plateau climate.