Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland
Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland
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DOI：
10.5194/acp-12-4259-2012
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发表时间：
2012-01-01
影响因子：
6.3
通讯作者：
Blunier, T.
中科院分区：
文献类型：
--
作者：
Buizert, C.;Martinerie, P.;Blunier, T.
空气采样的多孔积雪层在NEEM网站在北方格陵兰岛。我们使用一个合奏的10个参考示踪剂已知的大气历史来计算该网站的传输特性。通过分析数据和参考气体大气历史中的不确定性，我们可以客观地为用于深度扩散率重建的每种气体分配权重。我们定义了一个客观的均方根标准，最小化模型调整过程。每个示踪剂约束的积雪剖面不同，通过其独特的大气历史和自由空气扩散率，使我们的多示踪剂表征方法比常用的单示踪剂调谐明显改善。六个雪空气传输模型调整到NEEM网站，所有模型成功地再现1西格玛高斯分布内的数据。两个重复的钻孔之间的比较，钻了64米，显示出测量的混合比剖面，超过实验误差的差异。我们发现的证据表明，扩散并不完全消失在锁定区，因为通常假设。雪冰过渡处的冰河时代-气体年龄差（Delta年龄）计算为182（-9）（+3）yr。我们进一步提出了第一次比较研究的积雪空气模型，在那里我们介绍了诊断方案，旨在探讨模型物理的具体方面。我们的研究结果表明，模式处理平流输送的方式有很大的差异。此外，扩散分馏的同位素在积雪的约束不好的模型，这有后果的尝试重建示踪气体的同位素组成的时间使用积雪空气和冰芯记录。
Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1 sigma Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Delta age) at the firn-ice transition is calculated to be 182(-9)(+3) yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.