西藏羊卓雍错湖面蒸发与能量交换过程

Qinghai-Tibetan Plateau is sensitive to global environmental change for its special underlying surface and atmospheric processes. As an important and unique component over the plateau, alpine lakes can reflect regional and global climate change as well. The Yamzhog Yumco, located in the South Tibet and supplied both by rainfall and snowmelt, is one of the three holy lakes in Tibet. It is the largest inland lake with an area of 588.9 km2 in the northern foot of the Himalayas. According to the observations at Baidi hydrological station, the lake level has dropped significantly during 1974-2010. Hence it is of significance to explore the predominant causes for lake changes based on a balance between inflow (e.g. rainfall over lake, river inflows) and outflow (e.g. evaporation from lake). Particularly the lake surface evaporation of Yamzhog Yumco as one of these key processes needs to be clarified. Although the experiments of pan evaporation are carried out at the meteorological and hydrological stations in Yamzhog Yumco basin, the direct and precise measurements of the lake surface evaporation rate and energy exchange have not been performed yet. So this project is designed to measure the lake surface evaporation and energy balance of Yamzhog Yumco using the eddy covariance (EC) method. This study is aimed to analyze the diurnal, seasonal and annual variations of evaporation and their major environmental controls, to reveal the characteristics of energy partitioning and exchange at the atmosphere-water interface of the lake, and further to develop a method for calculating the real lake evaporation based on the pan evaporation measurements and corresponding meteorological data. It is also designed to characterize the water and energy exchange between the lake and the surrounding land. Hopefully the study will lay theoretical foundation for further analysis of hydrological cycle and its response to climate changes in alpine area on the plateau.

青藏高原特殊的下垫面和大气过程使其成为全球环境变化的敏感区与脆弱区。而湖泊是青藏高原地表系统重要而且独特的组成部分。被誉为"圣湖"的羊卓雍错是藏南最大的内陆湖泊。近年来湖泊迅速萎缩，严重影响到当地居民的生产和生活。因此，迫切需要研究羊卓雍错流域气象、水文过程和水-热循环过程。然而，目前作为重要环节的湖泊蒸发过程研究匮乏，特别是如何准确、便利地测定羊卓雍错的湖面蒸发速率和能量交换过程是当前羊卓雍错湖泊研究的瓶颈。为此，本项目拟基于涡度相关系统野外定点观测，分析羊卓雍错湖面蒸发速率不同尺度的时间变化及其主要控制因子，揭示湖面的能量分配和交换特征以及湖水-大气相互作用过程，探讨如何利用站测蒸发器观测资料和气象资料估算湖面实际蒸发量的方法，进而揭示湖陆之间的能水交换过程和特征，为进一步研究青藏高原高寒地区的水循环过程及其对气候变化的响应机制提供理论支撑。

基于涡动相关方法（Eddy Covariance，EC）的湖面水热通量观测数据是湖泊响应气候变化研究的基础科学依据。基于涡动相关观测系统，近地层湍流特征已在高原多地区陆地下垫面开展研究，而湖泊下垫面报道相对较少，仅限于青海湖、色林错、鄂陵湖及纳木错等，对高原南部湖泊的近地层湍流特征研究更是未有涉及，亟待扩展和加强高原湖泊大气湍流的观测分析和深入探究.羊卓雍错位于青藏高原南部，是雅鲁藏布江南岸和喜马拉雅山北麓最大的内陆湖泊. 最近10年该湖泊面积萎缩、水位急剧下降，大片湖床出露，已威胁到湖区及周围农牧民生产生活和生态环境安全，引起当地政府高度重视. 目前亟需对该湖泊水热循环及气候环境效应方面开展综合研究. 然而自然环境严酷导致的观测数据匮乏成为羊卓雍错湖泊研究的瓶颈. 依据本项目资助，首次获取2016-2018连续3年西藏羊卓雍错湖面涡动相关观测原始数据36.9 Gb，填补了羊卓雍错涡动相关观测数据空白。利用涡动相关计算软件EddyPro®(LI-COR®, Inc.)，我们对涡动相关系统的原始观测数据（包含三维风速、温度、水汽密度等）进行了涡动相关计算和足迹分析，并进行风速坐标轴旋转、超声虚温修正和Webb-Pearman-Leuning密度波动修正、全频滤波谱修正等一系列湍流动力学订正以及数据质量标定处理。在此基础上，利用人工神经网络插值法，对缺失的湖面水热通量数据进行了插补，提高了分析的时间分辨率。基于2016-2017年4-12月羊卓雍错湖面水热通量数据，我们已经完成并发表了羊卓雍错湖面大气湍流特征、湖面水热通量变化及其主控气候因子等研究。.上述珍贵的原始数据、加工数据以及初步分析将结果，将为青藏高原不同补给类型的高寒湖泊蒸发和水量平衡研究，提供重要的区域案例。同时，在数值预报模式和气候模式尺度精细化发展的趋势下，陆面过程模型参数化方案的优化依赖于更广泛和深入的湖泊-大气物质及能量交换观测研究。因此，羊卓雍错湖面水热通量数据可以为大气数值模式中陆面过程模拟和模型参数化方案提供必要的下垫面数据和参数。