Closing the Isotope Hydrology at Summit: Measurements of Source Regions, Precipitation and Post-deposition Processes, Greenland, 2011-2014

The stable isotopic records from the Greenland Ice Sheet are the gold standard for understanding climate variations in the Arctic on decadal to millennial scales. While the basic tenets that underlie interpretation of isotopic information appear robust in a mean sense, meteorological and glaciological processes can confound simple interpretations. Processes of concern are variations in moisture sources, cloud processes, surface ablation, blowing snow and vapor diffusion in the firn. The project objectives are to resolve fundamental uncertainties in the controls on the isotopic composition of the ice sheet through a 3-year measurement campaign at Summit, Eureka and Reykjavik. The project will use measurements and modeling to evaluate 1) the degree to which oxygen isotopic composition and deuterium excess of snow capture variations in moisture sources versus cloud microphysical conditions, and 2) the degree to which blowing snow and vapor diffusion within the firn confound accurate interpretation of variability in the isotopic record. Continuous measurements of the isotopic composition of water vapor and daily measurements of the isotopic composition of freshly-fallen and blowing snow will be made at Summit, Eureka and Reykjavik. These will be combined with measurements of the amount, size distribution, and approximate habit of falling and blowing snow, turbulence measurements to evaluate snow lofting, surface latent heat flux (ablation and frost) and energy balance, and remote sensing of polar clouds and atmospheric structure. High-resolution firn cores will be drilled to reconcile the detailed isotopic measurements and modeling with glaciological records. The new isotope measurements will jump start an emerging international pan-Arctic cooperative network of isotope measurements, which complements Arctic observations under existing Arctic Observing Network activities. The advanced measurements at Summit enhance the site as a comprehensive observatory for monitoring and understanding Arctic change.格陵兰冰盖（Greenland Ice Sheet）的稳定同位素记录是理解北极地区十年至千年尺度气候变异性的黄金标准。尽管同位素信息解译的核心基本原则在平均意义上看似稳健，但气象与冰川学过程会干扰简单的解译工作。值得关注的过程包括水汽源（moisture sources）变化、云过程（cloud processes）、表面消融（surface ablation）、吹雪（blowing snow）以及粒雪层（firn）中的水汽扩散。本项目的目标是通过在萨米特（Summit）、尤里卡（Eureka）以及雷克雅未克（Reykjavik）开展为期三年的测量工作，厘清冰盖同位素组成控制机制中存在的根本性不确定性。本项目将通过测量与建模手段，评估两大核心问题：其一，积雪的氧同位素组成（oxygen isotopic composition）与氘盈余（deuterium excess）在多大程度上能够反映水汽源变化与云微物理条件（cloud microphysical conditions）；其二，吹雪以及粒雪层内的水汽扩散会在多大程度上干扰同位素记录变异性的精准解译。研究团队将在萨米特（Summit）、尤里卡（Eureka）以及雷克雅未克（Reykjavik）开展连续的水汽同位素组成测量，以及每日的新鲜降雪与吹雪同位素组成测量。这些测量数据将与降雪量、粒径分布（size distribution）、降雪与吹雪的大致晶体形态测量、用于评估雪粒扬散的湍流测量、表面潜热通量（surface latent heat flux，含消融与霜形成）与能量平衡（energy balance）测量，以及极地云与大气结构的遥感（remote sensing）数据相结合。本项目还将钻取高分辨率粒雪芯，以将精细的同位素测量与建模结果匹配至冰川学记录。这些全新的同位素测量将助力新兴的泛北极国际同位素测量合作网络的发展，该网络可补充现有北极观测网络（Arctic Observing Network）框架下的北极观测工作。在萨米特（Summit）开展的先进测量工作，将进一步将该站点打造为监测与理解北极气候变化的综合观测台站。