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The proxy for temperature (δ signal) in ice cores is stored in the snow/ice during precipitation events and hence reflects the temperature at which precipitation is formed (here approximated by the inversion temperature Ti) weighted with the accumulation. Results from a 14 year integration (1980–93) with a regional atmospheric model (RACMO, ΔX = 55 km) show that the annual mean accumulation-weighted inversion temperature (Ti,w) and the annual mean Ti are not covariant in time at four out of the five deep-drilling sites considered, mainly due to year-to-year variations in the seasonality of precipitation. As a consequence, the surface temperature (Ts,core) derived from RACMO output, using a method analogous to the retrieval of the surface temperature from ice-core δ signals, deviates from the directly modelled surface temperature Ts on interannual time-scales. Results from a 5 year sensitivity integration, forced with a 2 K temperature increase, show an 18% overestimation of the increase in Ts,core relative to the increase in Ts due to a change in the relationship between the inversion strength and the surface temperature in a different climate regime. Similar errors may occur in deriving the temperature difference between Last Glacial Maximum and present-day climate from δ signals in ice cores.
Ice rises play key roles in buttressing the neighbouring ice shelves and potentially provide palaeoclimate proxies from ice cores drilled near their divides. Little is known, however, about their influence on local climate and surface mass balance (SMB). Here we combine 12 years (2001–12) of regional atmospheric climate model (RACMO2) output at high horizontal resolution (5.5 km) with recent observations from weather stations, ground-penetrating radar and firn cores in coastal Dronning Maud Land, East Antarctica, to describe climate and SMB variations around ice rises. We demonstrate strong spatial variability of climate and SMB in the vicinity of ice rises, in contrast to flat ice shelves, where they are relatively homogeneous. Despite their higher elevation, ice rises are characterized by higher winter temperatures compared with the flat ice shelf. Ice rises strongly influence SMB patterns, mainly through orographic uplift of moist air on the upwind slopes. Besides precipitation, drifting snow contributes significantly to the ice-rise SMB. The findings reported here may aid in selecting a representative location for ice coring on ice rises, and allow better constraint of local ice-rise as well as regional ice-shelf mass balance.
The performance of a regional atmospheric climate model (RACMO) with a horizontal resolution of 55 km X 55 km is evaluated using measured temperature and humidity profiles. Parameterisations of the physical processes are taken from the EC-HAM4 general circulation model (GCM). Sea-surface temperatures and sea-ice mask in the model are prescribed from observations. The model is forced by re-analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) at the lateral boundaries.
We compared simulations for January 1993 with boundary-layer profiles measured at the Swedish research station Svca (Dronning Maud Land, Antarctica) and with radiosonde measurements made at the Georg von Neumayer (GvN) and South Polar stations for the same period. This comparison was performed in order to study some model characteristics before the model is used for mass-balance calculations. The vertical temperature gradient at Svea during the night is overestimated by RACMO, but corresponds much more closely to the observations than do the ECMWF re-analyses. in the re-analyses a decoupling of the lowest model layer from the higher atmosphere occurs. The differences between the absolute temperatures at the GvN and SP stations and the absolute temperatures at the model gridpoints corresponding most closely to these sites are less than 5°C. The humidity profiles indicate that the model generally underestimates the turbulent transport of moisture from the surface to higher levels.
To reduce the uncertainty in the calculation of Antarctic solid ice fluxes, the firn depth correction (Δh) in Antarctica is inferred from a steady-state firn densification model forced by a regional atmospheric climate model. The modelled density agrees well with observations from firn cores, apart from a site at the origin of fast flowing West Antarctic ice stream (Upstream B), where densification is anomalously rapid. The spatial distribution of Δh over Antarctica shows large variations, especially in the grounding line zone where large climate gradients exist. In places where the grounding line crosses ablation areas, Δh is zero. Along the remainder of the grounding line, Δh values range from typically 13 m in dry coastal areas (e.g. Dronning Maud Land) to 19 m in wet coastal areas (e.g. West Antarctica).
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