Modelling the isotopic composition of snow using backward trajectories: a particular precipitation event in Dronning Maud Land, Antarctica

Abstract

We consider a specific accumulation event that occurred in January 2002 in western Dronning Maud Land, Antarctica. Snow samples were obtained a few days after accumulation. We combine meteorological analyses and isotopic modelling to describe the isotopic composition of moisture during transport. Backward trajectories were calculated, based on European Centre for Medium-Range Weather Forecasts operational archive data so that the history of the air parcels transporting water vapour to the accumulation site could be reconstructed. This trajectory study showed that the air masses were not (super)saturated along most of the transport path, which is in contrast with assumptions in Lagrangian fractionation models and probably true for most precipitation events in Antarctica. The modelled fractionation along the trajectories was too limited to explain the measured isotopic content of the snow. It is shown that the observed isotopic composition of precipitation resulted from fractionation of initially more depleted water. This lower initial isotopic composition of water vapour might result from atmospheric mixing with more depleted air along the trajectory or from earlier condensation cycles, not captured by the trajectories. This is in accordance with isotope fields resulting from general circulation models, indicating a gradient in isotopic composition from the Equator to Antarctica.

Interannual variability of Greenland winter precipitation sources: 2. Effects of North Atlantic Oscillation variability on stable isotopes in precipitation

“…While this seems valid as a first approximation, the full transport and fractionation history of an air mass could be considered better by running MCIM as a box model along diagnosed distillation trajectories. Such an approach has recently been introduced by Helsen et al [2004]. In their Lagrangian approach based on the MCIM model and reanalysis trajectories, both for the whole of Antarctica [ Helsen et al , 2007] and for single sites [ Helsen et al , 2004, 2005], simulated levels of isotopic depletion are in good absolute correspondence with data near the coasts, but underestimate total depletion in the Antarctic interior.…”

“…Such an approach has recently been introduced by Helsen et al [2004]. In their Lagrangian approach based on the MCIM model and reanalysis trajectories, both for the whole of Antarctica [ Helsen et al , 2007] and for single sites [ Helsen et al , 2004, 2005], simulated levels of isotopic depletion are in good absolute correspondence with data near the coasts, but underestimate total depletion in the Antarctic interior. Their calculations however rely heavily on atmospheric monthly mean isotope data from isotope GCMs throughout the moisture transport process.…”

“…While we only extract the initial isotopic composition of the evaporating moisture from isotope GCM data, their method performs “isotopic recharge” [ Kavanaugh and Cuffey , 2003] every time moisture increase is detected in an air parcel. It therefore appears that the approach of Helsen et al [2004] currently takes an intermediate position between isotope GCMs and fully Lagrangian isotope box model calculations.…”

Ground‐based measurements of spatial and temporal variability of snow accumulation in East Antarctica

“…Case studies of chemical and physical anomalies in the firn can be based on individual accumulation events identified in the ultrasonic time series. In combination with AWS data, the accumulation/ablation time series of ultrasonic height rangers can also be used to force snowpack models at their upper boundary or serve as a starting point for atmospheric trajectory calculations [ Noone et al , 1999; Reijmer et al , 2002; Helsen et al , 2004]. Moreover, the temporal distribution of accumulation/ablation events is essential for validation of meteorological and/or mass balance models [ Gallée et al , 2001; van Lipzig et al , 2004a].…”

Importance of precipitation seasonality for the interpretation of Eemian ice core isotope records from Greenland

“…The moisture that is eventually deposited at an ice core site has taken different paths and originates from different latitudes, so no simple relation is expected between temperature and isotopic depletion (e.g. Jouzel et al, 1997;Helsen et al, 2004). After snow deposition, surface snow processes alter the isotopic signal, and diffusion damps the seasonal signal within the snowpack (Johnsen, 1977;Johnsen et al, 2000).…”