Changes in Below-Cloud Evaporation Affect Precipitation Isotopes During Five Decades of Warming Across China

Based on daily meteorological records for 651 sites across China during the period 1960-2018, we estimated the changes in isotopic variations in raindrops as they descend from cloud base to ground over past decades, and tested the sensitivity of isotopic variations to climate parameters like air temperature and relative humidity. Air temperature correlates positively and relative humidity correlates negatively with below-cloud isotopic variation. Generally, the below-cloud evaporation effect on precipitation isotopes in the arid and semi-arid regions of China is much greater than that in the humid and semi-humid regions, although the impact might be reduced under cold-arid or hot-humid conditions. With aridity increasing with distance from the coast, the continental effect of precipitation isotopes is modified due to the below-cloud evaporation. The seasonal pattern of the measured isotopic composition in precipitation near the ground and estimated at cloud base, is still similar in most regions, although the seasonal range is higher at the ground. During the last five decades, the below-cloud evaporation effect has enhanced for the cold and arid regions of China especially across Qinghai-Tibet Plateau and Inner Mongolia, due to combined effects of increasing air temperature and decreasing relative humidity. Although the below-cloud evaporation effect is not always the dominant factor influencing the variability of stable isotopes, it needs to be considered as one of the contributing factors. This enhanced effect may impact the interpretation of past climate based on stable water isotopes, particularly in paleoclimate studies using speleothems and tree rings.