Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica

Gerit.Birnbaum [ at ]


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 iso- topic composition are required. This study presents contin- uous 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 gen- eral circulation models (AGCMs) equipped with water va- por 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. Mix- ing 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 oc- curs during sublimation and that vapor–snow exchanges can no longer be considered insignificant for the isotopic compo- sition of near-surface snow in polar regions.

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DOI 10.5194/tc-10-1647-2016

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Ritter, F. , Steen-Larsen, H. C. , Werner, M. , Masson-Delmotte, V. , Orsi, A. , Behrens, M. , Birnbaum, G. , Freitag, J. , Risi, C. and Kipfstuhl, S. (2016): Isotopic exchange on the diurnal scale between near-surface snow and lower atmospheric water vapor at Kohnen station, East Antarctica , The Cryosphere, 10 , pp. 1647-1663 . doi: 10.5194/tc-10-1647-2016

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