Abstract
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.
Highlights
Thanks to the design of mass spectrometers and their application to stable water isotopes since the 1950s, precipitation has long been sampled for laboratory stable isotope analyses in order to trace atmospheric processes related to the hydrological cycle (e.g., Dansgaard, 1964)
We report the parallel surface snow sampling over 35 h and introduce simulations performed with two atmospheric general circulation models (AGCMs) equipped with stable water isotopes, LMDZ5Aiso and ECHAM5-wiso
Continuous measurements of temperature, humidity mixing ratio and water vapor isotopes were performed during summer 2013/2014 at Kohnen station in East Antarctica
Summary
Thanks to the design of mass spectrometers and their application to stable water isotopes since the 1950s, precipitation has long been sampled for laboratory stable isotope analyses in order to trace atmospheric processes related to the hydrological cycle (e.g., Dansgaard, 1964). Ice cores form one of the most direct records of the isotopic compo-. F. Ritter et al.: Isotopic exchange at Kohnen station sition of past precipitation. In Antarctica, stable water isotope measurements are central for past climate reconstructions from these ice cores, through atmospheric distillation processes connecting temperature, condensation and isotopic composition (e.g., Masson-Delmotte et al, 2008). The relationship between precipitation isotopic composition and climate is complex, as it is affected by fractionation taking place at most phase transitions, at evaporation, during atmospheric transport, and at condensation
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