中国地区蒸发量时空变化特征及其对全球变暖响应的研究

Evaporation plays a crucial role in regulating the global hydrological and energy cycle with implications ranging from climate prediction to climate change assessments, it is of great importance to research on changes in evaporation and the physical mechanism. However, the lack of direct observations hampers the efforts to identify and quantify the change of evaporation. Moreover, the hydrological cycle is among the most uncertain aspects of climate model predictions. As a result, there remains considerable uncertainty concerning the magnitude of evaporation in China response to a given increase in temperature. For now, the fundamental question of whether evaporation is increasing or decreasing, and, if so, where and when this is occurring, has not yet been comprehensively answered. The availability of the atmospheric reanalysis in recent years has provided a new opportunity to study not only the climatology, but also the variability of evaporation at different timescales. Therefore, we will extract the main spatial-temporal variation characteristic of evaporation in China and its response to global warming based on multi-reanalysis datasets. First, reanalyses are evaluated by comparing quantities with estimated evaporation obtained from the Mezentsev–Choudhury–Yang equation and the water budget equation. Differences in the total amount, spatial variability, and distribution of evaporation are analyzed in order to estimate the uncertainties and the possible causes incorporated in these reanalysis datasets. Next, spatial-temporal variation characteristic of evaporation will be analyzed in different scales of space and time. Finally, we will further study the predominant factors of evaporation to reveal its response to global warming. Contribution of the soil moisture conditions (induced by changes of precipitation), dynamic term (induced by changes of wind), thermodynamic term (induced by changes of specific humidity), and eddy flux term to evaporation changes and related uncertainty will be analyzed quantitatively by using the atmospheric moisture budget. This project aims to provide a scientific understanding of the effect of evaporation on climate change and the relationship between each other.

蒸发在全球能量、水分循环中发挥着关键的调节作用，系统研究蒸发变化规律及相关机理在气候预测和气候变化方面具有重要科学意义。但是由于观测资料匮乏，并且气候模式对水循环的模拟水平也非常有限，目前对于中国地区蒸发量的变化趋势、幅度、区域分布和影响因子等依然存在诸多不确定性。再分析资料的不断发展为解决上述问题提供了新的途径，本项目拟基于多套再分析资料研究中国地区蒸发量的时空变化特征及其对全球变暖的响应机理。首先，通过水热耦合模型和水汽水量平衡方程等理论公式综合评估各套再分析蒸发资料在中国地区的适用性，及其误差的可能原因；其次，从不同的时间尺度和空间尺度分析中国地区蒸发量变化的主要特征；最后，深入研究蒸发量与影响因子的关系，利用水汽平衡方程定量区分下垫面供水能力、热力作用、动力作用和湍流涡旋等因素对蒸发量变化的相对贡献，揭示蒸发对全球变暖的响应机理。旨在进一步科学认识蒸发在气候变化中的作用和联系。

本项目紧密围绕中国地区蒸发量变化的研究主题，基于多套再分析资料和理论经验公式揭示了中国地区蒸发量的时空变化特征，根据Budyko水热平衡理论分析了蒸发量对全球变暖的响应机制，定量评估了能量和水分供应条件与下垫面变化对蒸发量的影响，并通过多源蒸发数据的交叉对比诊断了各套再分析蒸发资料在中国地区的可信度和适用性。研究结果表明：1979~2015年中国地区蒸发量呈显著增加趋势，秋季增幅最大，夏季增幅最小；考虑到中国不同地区气候和下垫面有很大差异，利用干旱指数（aridity index）将中国划分为6个气候区，发现极端干旱、干旱和湿润区蒸发量都显著增加，而干旱半湿润和半湿润区蒸发量略有减少。对于极端干旱和干旱地区，蒸发量升高分别与气候变暖和人类活动引起的潜在蒸散发增加和冰川融水供应增多有关；全球变暖导致的潜在蒸散发增加是湿润区蒸发量增加的主要原因。另一方面，根据Budyko水热平衡理论定量分析了蒸发量与降水量、潜在蒸散发和下垫面变化之间的关系，发现东南地区蒸发量增加主要与潜在蒸散发有关；西南地区蒸发量增加是潜在蒸散发增加和下垫面变化共同作用的结果；东北和青藏高原东部蒸发量减少分别与降水量减少和下垫面变化有关；西北地区蒸发量的增加既与降水量增加有关，还受到了下垫面变化的影响，理论上冰川融水引起的下垫面供水条件变化对西北地区蒸发量增加的贡献量可以达到30%左右。本项目研究成果将为进一步科学认识全球变暖背景下中国地区蒸发量和气候变化的相互作用和联系提供参考。在项目的实施过程中，本项目共资助发表论文8篇，完成了原设的科学目标。