Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica
Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica
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东南极洲 Kohnen 站近地表雪与低层大气水蒸气之间的日尺度同位素交换
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发表时间：
2016
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通讯作者：
S. Kipfstuhl
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作者：
F. Ritter;H. Steen‐Larsen;M. Werner;V. Masson‐Delmotte;A. Orsi;M. Behrens;G. Birnbaum;J. Freitag;C. Risi;S. Kipfstuhl
抽象的。量化影响地表雪同位素组成的沉积后过程的大小对于更准确地解释冰芯数据至关重要。为了实现这一目标，需要对低层大气水蒸气和地表雪的同位素组成进行高时间分辨率的测量。这项研究介绍了2013年12月至2014年1月在东南极洲（Kohnen站）使用激光光谱仪进行的水蒸气同位素连续测量。观测结果与两个配备有水汽同位素的大气环流模式（AGCM）的输出进行了比较：ECHAM 5-wiso和LMDZ 5Aiso。在观测期间，2 m气温T、湿度混合比q和水汽同位素δD和δ 18 O的信号主要是日变化的。两个AGCM模拟相似的昼夜周期，平均振幅比观测值低30 - 70%，可能是由于对表面能量平衡和边界层动力学的模拟不正确。同时，在35小时内每小时收集一次雪表面样品，取样深度为2-5 mm。雪表面的同位素组成的日周期与水蒸气的相位一致，在24小时内达到3 ‰的峰值到峰值幅度（相比之下，水蒸气中的δD为36 ‰）。本文提出了一个简单的封闭系统箱模型来研究雪库与空气之间的水分子交换。在蒸汽中，箱模型模拟显示与观测相比同位素消耗太多。与其他来源（平流，自由对流层）的混合必须包括在内，以适应观测。在雪表面，模拟的同位素值接近观测与雪水库的0.5mm深（雪样品深度的范围）。我们的分析表明，分馏过程中发生的升华和气-雪交换不能再被认为是微不足道的近地表雪在极地地区的同位素组成。
Abstract. Quantifying the magnitude of post-depositional processes affecting the isotopic composition of surface snow is essential for a more accurate interpretation of ice core data. To achieve this, high temporal resolution measurements of both lower atmospheric water vapor and surface snow isotopic composition are required. This study presents continuous measurements of water vapor isotopes performed in East Antarctica (Kohnen station) from December 2013 to January 2014 using a laser spectrometer. Observations have been compared with the outputs of two atmospheric general circulation models (AGCMs) equipped with water vapor isotopes: ECHAM5-wiso and LMDZ5Aiso. During our monitoring period, the signals in the 2 m air temperature T, humidity mixing ratio q and both water vapor isotopes δD and δ18O are dominated by the presence of diurnal cycles. Both AGCMs simulate similar diurnal cycles with a mean amplitude 30 to 70 % lower than observed, possibly due to an incorrect simulation of the surface energy balance and the boundary layer dynamics. In parallel, snow surface samples were collected each hour over 35 h, with a sampling depth of 2–5 mm. A diurnal cycle in the isotopic composition of the snow surface is observed in phase with the water vapor, reaching a peak-to-peak amplitude of 3 ‰ for δD over 24 h (compared to 36 ‰ for δD in the water vapor). A simple box model treated as a closed system has been developed to study the exchange of water molecules between an air and a snow reservoir. In the vapor, the box model simulations show too much isotopic depletion compared to the observations. Mixing with other sources (advection, free troposphere) has to be included in order to fit the observations. At the snow surface, the simulated isotopic values are close to the observations with a snow reservoir of ∼ 5 mm depth (range of the snow sample depth). Our analysis suggests that fractionation occurs during sublimation and that vapor–snow exchanges can no longer be considered insignificant for the isotopic composition of near-surface snow in polar regions.