Shengjie Wang, Rong Jiao, Mingjun Zhang*, Jagoda Crawford, Catherine E. Hughes, Fenli Chen. Changes in below-cloud evaporation affect precipitation isotopes during five decades of warming across China. Journal of Geophysical Research: Atmospheres, 2021, 126(7): e2020JD033075. DOI: 10.1029/2020JD033075

内容导读：

当雨滴在不饱和的大气中下落时，云下二次蒸发过程可能使得降水中的氢氧稳定同位素值升高，其变化程度受到气温和湿度等因素的综合影响。以往关于云下二次蒸发对降水同位素的影响评估限于雨滴直径等参数不易获取而多为局地案例，对大空间尺度的关注较少。在过去的几十年里，中国气温表现出显著的上升趋势，而相对湿度的变化趋势则不显著，在这样的气候背景下云下二次蒸发过程对降水同位素年际尺度的影响尚不明确，亟待开展大空间尺度和长时间序列的评估，这些评估对科学审视气候代用指标中的同位素信号具有参考价值。

西北师范大学地理与环境科学学院王圣杰副教授等通过1960～2018年中国651个气象站的观测资料，结合雨滴谱等数据优化了基于日尺度气象数据评估云下二次蒸发作用下云底至地面降水同位素变化量的计算流程，进而模拟了升温背景下云下二次蒸发过程对中国降水中氢氧稳定同位素的影响。研究发现，云下二次蒸发对降水同位素的影响程度表现出自东南沿海向西北内陆增强的趋势，特别是塔里木盆地东部和柴达木盆地降水同位素受到了云下二次蒸发的强烈改造，并且过去几十年中寒旱区大部分区域均呈现出影响程度增强的特点。尽管各自然区云下二次蒸发对降水同位素的影响程度存在季节差异，但是尚没有改变降水同位素的基本季节变化趋势，而经过云下二次蒸发订正后的云底雨滴同位素值相较于地面降水同位素值而言表现出了更为明显的大陆效应。此外，研究还针对气温、相对湿度、雨滴直径和云底高度等主要输入参数对模拟结果的影响进行了探讨。研究有助于深入认识气候变化背景下云下过程对大气降水以及气候代用指标中氢氧稳定同位素年际变化的影响程度与范围。

该成果受到国家自然科学基金和甘肃省杰出青年基金等项目的联合资助，发表在《Journal of Geophysical Research: Atmospheres》。

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

Shengjie Wang, Rong Jiao, Mingjun Zhang, Jagoda Crawford, Catherine E. Hughes, Fenli Chen

First published: 28 March 2021

https://doi.org/10.1029/2020JD033075

Abstract

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.