Deuterium Excess of Water Vapor in the Atmospheric Boundary Layer

本研究探讨了利用水汽同位素观测将水汽来源归因于远源区、局地影响和大气边界层（ABL）的输送。ABL是大气中最低~1 km的层，是大气中水的大储存库。ABL水池支持对流云的形成。云层中的凝结会消耗掉大气中的水蒸气。在ABL顶部的夹带或湍流扩散将蒸汽输送到上层大气。虽然这两个过程都消耗了地面低空层的水分，但新的水汽从陆地蒸散发和来自遥远源区的水汽平流进入这一层。了解水如何在ABL、陆地和高层大气之间流动，对于准确预测降水过程和建立气候模型至关重要。氘和氧-18是氢和氧的稳定同位素。这项研究将使用水蒸气的“氘过量”（d-过量），一种测量水的氘和氧-18同位素相对丰度的方法，来跟踪三个池之间的水的运动，并确定当地陆地和偏远海洋来源对ABL蒸汽的贡献。该项目的一个基本前提是，远程水源和当地ABL过程都会对蒸汽d过量产生影响，但在非常不同的时间尺度上。研究人员假设，结合使用适合于这些不同时间尺度的分析工具，将揭示来自当地领域以外的水运对当地的影响。该提案认为，当地水循环由三个水库（陆地水、ABL中的水蒸气和自由大气中的水蒸气）组成，这些水库之间的交换将在d-excess的时间变化上留下明显的特征。研究方法包括数据分析和ABL建模。实验数据是在北美和亚洲的11个气候带用激光仪器每小时进行的观测。同位素陆地表面模式将量化局部蒸散发对蒸汽d过量的时间变化的影响。平衡边界层计算和大涡模拟将用于推断自由大气中蒸汽的d-过剩信号以及夹带对ABL d-过剩的影响。轨迹分析将识别来自远源地区的贡献。该项目将有助于同位素水文学和边界层气象学方面的能力建设。尽管最近基于激光蒸汽同位素测量的爆炸式增长，但目前用这些测量来解释水输送过程的能力仍然是初步的。该项目可以将该领域从描述性转变为定量。基于大涡模拟数据和简化的一维ABL模型设计的实践模块将加强ABL和同位素原理的课堂学习。这些模块将构成ABL技术开放在线课程的基础。一个新的数据网站将促进高频蒸汽同位素数据的共享。随着基于激光的测量量的持续增长，一个集中的数据存储将促进超越学科和地理界限的调查。

This research explores using water vapor isotope observations for attributing the sources of water vapor to remote source regions, local influences, and transport in the atmospheric boundary layer (ABL). The ABL is the lowest ~1 km of the atmosphere and is a large reservoir of atmospheric water. The ABL water pool supports convective cloud formation. Condensation in clouds depletes vapor from the ABL. Entrainment or turbulent diffusion at the top of the ABL, transports vapor to the upper atmosphere. While both these processes deplete moisture from the ABL, new vapor enters this layer from land evapotranspiration and advection of moisture from remote source regions. Understanding how water moves between the ABL, the land, and the upper atmosphere is essential for accurate prediction of precipitation processes and for climate modeling. Deuterium and oxygen-18 are stable isotopes of hydrogen and oxygen. This study will use "deuterium excess" (d-excess) of water vapor, a measure of the relative abundance of the deuterium and oxygen-18 isotopes of water, to track the movement of water among the three pools and to identify contributions of local land and remote oceanic sources to the ABL vapor. A fundamental premise of the project is that both remote water sources and local ABL processes will imprint influences on vapor d-excess, but at very different time scales. The investigators postulate combined use of the analytical tools appropriate for these different time scales will unravel local influences from those originating from water transport from outside of the local domain. The proposal conceptualizes that the local hydrological cycle is composed of three pools of water (terrestrial water, vapor in the ABL, and vapor in the free atmosphere) and that exchanges among these pools will leave distinct signatures on the d-excess temporal variations. The research methodology consists of data analyses and ABL modeling. The experimental data are hourly observations made with laser-based instruments in 11 climate zones in North America and in Asia. An isotopic land surface model will quantify local evapotranspiration effects on temporal variations in vapor d-excess. Equilibrium boundary layer calculations and large-eddy simulations will be used to infer d-excess signal of the vapor in the free atmosphere and the entrainment influence on the ABL d-excess. Trajectory analysis will identify contributions from remote source regions. The project will contribute to capacity-building in isotope hydrology and in boundary-layer meteorology. Despite the recent explosive growth of laser-based vapor isotope measurements, current ability to interpret water transport processes with these measurements remains preliminary. The project could transform the field from descriptive to quantitative. Hands-on modules, designed from large-eddy simulation data and a simplified one-dimensional ABL model, will enhance classroom learning on ABL and isotopic principles. These modules will form the basis of an open online course on ABL techniques. A new data website will promote sharing of high-frequency vapor isotope data. As the volume of laser-based measurement continues to grow, a centralized data depository will facilitate investigations that transcend disciplinary and geographic boundaries.