The influence of snow sublimation and meltwater evaporation on δD of water vapor in the atmospheric boundary layer of central Europe

Abstract. Post-depositional fractionation of stable water isotopes due to fractionating surface evaporation introduces uncertainty to various isotope applications such as the reconstruction of paleotemperatures, paleoaltimetry, and the investigation of groundwater formation. In this study, we investigate isotope fractionation at snow-covered moisture sources by combining 17 months of observations of isotope concentration ratios [HD¹⁶O] ∕ [H₂¹⁶O] in low-level water vapor in central Europe with a new Lagrangian isotope model. The isotope model is capable of reproducing variations of the observed isotope ratios with a correlation coefficient R of 0.82. Observations from 38 days were associated with cold snaps and moisture uptake in snow-covered regions. Deviations between modeled and measured isotope ratios during the cold snaps were related to differences in skin temperatures (T_skin). Analysis of T_skin provided by the Global Data Assimilation System (GDAS) of the NCEP implies the existence of two regimes of T_skin with different types of isotope fractionation during evaporation: a cold regime with T_skin < T_subl,max = −7.7 °C, which is dominated by non-fractionating sublimation of snow, and a warmer regime with T_subl,max < T_skin < 0 °C, which is dominated by fractionating evaporation of meltwater. Based on a sensitivity study, we assess an uncertainty range of the determined T_subl,max of −11.9 to −2.9 °C. The existence of the two fractionation regimes has important implications for the interpretation of isotope records from snow-covered regions as well as for a more realistic modeling of isotope fractionation at snow-covered moisture sources. For these reasons, more detailed experimental studies at snow-covered sites are needed to better constrain the T_subl,max and to further investigate isotope fractionation in the two regimes.