ALBERT

Description: The isotopic composition of present-day Antarctic snow is simulated for the period September 1980–August 2002 using a Rayleigh-type isotope distillation model in combination with backward trajectory calculations with 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data as meteorological input. Observed spatial isotopic gradients are correctly reproduced, especially in West Antarctica and in the coastal areas. However, isotopic depletion of snow on the East Antarctic plateau is underestimated, a problem that is also observed in general circulation models equipped with isotope tracers. The spatial isotope–temperature relation varies strongly, which indicates that this widely used relation is not applicable to all sites and temporal scales. Spatial differences in the seasonal amplitude are identified, with maximum values in the Antarctic interior and hardly any seasonal isotope signature in Marie Byrd Land, West Antarctica. The modeled signature of deuterium excess remains largely preserved during the last phase of transport, though the simulated relation of deuterium excess with δ18O suggests that parameterizations of kinetic isotopic fractionation can be improved.

Description: A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

Description: Measurements of the summer surface energy balance at Summit, Greenland, are presented (8 June–20 July 2007). These measurements serve as input to an energy balance model that searches for a surface temperature for which closure of all energy terms is achieved. A good agreement between observed and modelled surface temperatures was found, with an average difference of 0.45°C and an RMSE of 0.85°C. It turns out that penetration of shortwave radiation into the snowpack plays a small but important role in correctly simulating snow temperatures. After 42 days, snow temperatures in the first meter are 3.6–4.0°C higher compared to a model simulation without radiation penetration. Sensitivity experiments show that these results cannot be reproduced by tuning the heat conduction process alone, by varying snow density or snow diffusivity. We compared the two-stream radiation penetration calculations with a sophisticated radiative transfer model and discuss the differences. The average diurnal cycle shows that net shortwave radiation is the largest energy source (diurnal average of +61 W m−2), net longwave radiation the largest energy sink (−42 W m−2). On average, subsurface heat flux, sensible and latent heat fluxes are the remaining, small heat sinks (−5, −5 and −7 W m−2, respectively), although these are more important on a subdaily timescale.

Description: Measurements of the summer surface energy balance at Summit, Greenland, are presented (8 June–20 July 2007). These measurements serve as input to an energy balance model that searches for a surface temperature for which closure of all energy terms is achieved. A good agreement between observed and modeled surface temperatures was found, with an average difference of 0.45°C and an RMSE of 0.85°C. It turns out that penetration of shortwave radiation into the snowpack plays a small but important role in correctly simulating snow temperatures. After 42 days, snow temperatures in the first meter are 3.6–4.0°C higher compared to a model simulation without radiation penetration. Sensitivity experiments show that these results cannot be reproduced by tuning the heat conduction process alone, by varying snow density or snow diffusivity. We compared the two-stream radiation penetration calculations with a sophisticated radiative transfer model and discuss the differences. The average diurnal cycle shows that net shortwave radiation is the largest energy source (+61 W m−2 on average), net longwave radiation the largest energy sink (−42 W m−2). On average, subsurface heat flux, sensible and latent heat fluxes are the remaining, small heat sinks (−5, −5 and −7 W m−2, respectively), although these are more important on a subdaily timescale.

Description: A firn densification model is presented that simulates steady-state Antarctic firn density profiles, as well as the temporal evolution of firn density and surface height. The model uses an improved firn densification expression that is tuned to fit depth-density observations. Liquid water processes (meltwater percolation, retention and refreezing) are also included. Two applications are presented. First, the steady-state model version is used to simulate the strong spatial variability in firn layer thickness across the Antarctic ice sheet. Second, the time-dependent model is run for 3 Antarctic locations with different climate conditions. Surface height changes are caused by a combination of accumulation, melting and firn densification processes. On all 3 locations, an upward trend of the surface during autumn, winter and spring is present, while during summer there is a more rapid lowering of the surface. Accumulation and (if present) melt introduce large inter-annual variability in surface height trends, possibly hiding ice dynamical thickening and thinning.

Description: It is notoriously difficult to couple surface mass balance (SMB) results from climate models to the changing geometry of an ice sheet model. This problem is traditionally avoided by using only accumulation fields from a climate model, and deriving SMB by parameterizing the run-off as a function of temperature, which is often related to surface elevation. In this study, a new parameterization of SMB is presented, designed for use in ice dynamical models to allow a direct adjustment of SMB as a result of a change in elevation (Hs) or a change in climate forcing. This method is based on spatial gradients in the present-day SMB field as computed by a regional climate model. Separate linear relations are derived for ablation and accumulation regimes, using only those pairs of Hs an SMB that are found within a minimum search radius. This approach enables a dynamic SMB forcing of ice sheet models, also for initially non-glaciated areas in the peripheral areas of an ice sheet, and circumvents traditional temperature lapse rate assumptions. The method is applied to the Greenland Ice Sheet (GrIS). Model experiments using both steady-state forcing and more realistic glacial-interglacial forcing result in ice sheet reconstructions and behavior that compare favorably with present-day observations of ice thickness.

Description: Since the launch in 2002 of the Gravity Recovery and Climate Experiment (GRACE) satellites, several estimates of the mass balance of the Greenland Ice Sheet (GrIS) have been produced. To obtain ice mass changes estimates, data need to be corrected for the effect of deformation changes of the Earth's crust. This is usually done by independently modeling the Glaciological Isostatic Adjustment (GIA) trend and then by removing it from the data. Recently, Wu et al. (2010) proposed a new method to simultaneously estimate GIA and the present-day ice mass change, reporting an ice mass loss of around half of the previously published estimates and a general bedrock subsidence concentrated in the central parts of Greenland. This subsidence appears to be counterintuitive since the ice sheet is loosing mass at present. It was suggested by the authors that this could be a new evidence for additional net past ice accumulation. In this study, a 3-D ice-sheet model with a surface mass balance forcing based on a mass balance gradient approach has been used to: (a) analyze the bedrock response to changes in the ice load in order to evaluate whether bedrock subsidence and ice thinning can exist simultaneously; (b) study the magnitude and the pattern of the bedrock movement; and (c) evaluate if present-day bedrock subsidence could be the result of a net past mass accumulation. Under a sine forcing of the annual temperature, that mimics the temperature variations in the Holocene, mass changes yield a delay of the bedrock response of 200 years. Thinning of the ice as well as bedrock subsidence coexist during this period with an order of magnitude equal to the observations by Wu et al. (2010). Although, the resulting pattern of bedrock changes differs considerable: instead of the general bedrock subsidence reported before, we found areas of bedrock uplift as well as areas of bedrock subsidence. A simulation since the last glacial maximum (with the temperature represented as a linear increase from −10 K to present-day) yields a time lag of 1990 years for the bedrock response relative to the temperature forcing and an average uplift of 0.3 mm yr−1 for present-day. The spatial pattern of bedrock-change shows subsidence in the south and northwest as well as uplift in the center and northeast. We obtained these results assuming that the solid earth is a flat elastic lithosphere resting over a viscous relaxed asthenosphere (ELRA model). Using a more sophisticated Self Gravitational Viscoelastic (SGVE) model, we obtain qualitatively similar results: a 2200 years lag and an average uplift for present-day of 0.2 mm yr−1. The spatial pattern of bedrock movement is similar as well. Finally, results are shown for a temperature reconstruction based on ice core data confirming the deglaciation experiment. According to this study, a bedrock subsidence with a maximum in the central parts of Greenland cannot be that recent explained by a net past ice accumulation. This undermines results suggesting that recent loss is only half of the regular ice mass loss changes.

Description: It is notoriously difficult to couple surface mass balance (SMB) results from climate models to the changing geometry of an ice sheet model. This problem is traditionally avoided by using only accumulation from a climate model, and parameterizing the meltwater run-off as a function of temperature, which is often related to surface elevation (Hs). In this study, we propose a new strategy to calculate SMB, to allow a direct adjustment of SMB to a change in ice sheet topography and/or a change in climate forcing. This method is based on elevational gradients in the SMB field as computed by a regional climate model. Separate linear relations are derived for ablation and accumulation, using pairs of Hs and SMB within a minimum search radius. The continuously adjusting SMB forcing is consistent with climate model forcing fields, also for initially non-glaciated areas in the peripheral areas of an ice sheet. When applied to an asynchronous coupled ice sheet – climate model setup, this method circumvents traditional temperature lapse rate assumptions. Here we apply it to the Greenland Ice Sheet (GrIS). Experiments using both steady-state forcing and glacial-interglacial forcing result in realistic ice sheet reconstructions.

Description: Since the launch in 2002 of the Gravity Recovery and Climate Experiment (GRACE) satellites, several estimates of the mass balance of the Greenland ice sheet (GrIS) have been produced. To obtain ice mass changes, the GRACE data need to be corrected for the effect of deformation changes of the Earth's crust. Recently, a new method has been proposed where ice mass changes and bedrock changes are simultaneously solved. Results show bedrock subsidence over almost the entirety of Greenland in combination with ice mass loss which is only half of the currently standing estimates. This subsidence can be an elastic response, but it may however also be a delayed response to past changes. In this study we test whether these subsidence patterns are consistent with ice dynamical modeling results. We use a 3-D ice sheet–bedrock model with a surface mass balance forcing based on a mass balance gradient approach to study the pattern and magnitude of bedrock changes in Greenland. Different mass balance forcings are used. Simulations since the Last Glacial Maximum yield a bedrock delay with respect to the mass balance forcing of nearly 3000 yr and an average uplift at present of 0.3 mm yr−1. The spatial pattern of bedrock changes shows a small central subsidence as well as more intense uplift in the south. These results are not compatible with the gravity based reconstructions showing a subsidence with a maximum in central Greenland, thereby questioning whether the claim of halving of the ice mass change is justified.

Description: A firn densification model is presented that simulates steady-state Antarctic firn density profiles, as well as the temporal evolution of firn density and surface height. The model uses an improved firn densification expression that is tuned to fit depth-density observations. Liquid water processes (meltwater percolation, retention and refreezing) are also included. Two applications are presented. The steady-state model version is used to simulate the strong spatial variability in firn layer thickness across the Antarctic ice sheet. The time-dependent model is run for 3 Antarctic locations. It reveals a gentle upward motion of the surface during autumn, winter and spring, while during summer there is a more rapid lowering of the surface. Accumulation and (if present) melt introduce large inter-annual variability in surface height trends, possibly hiding ice dynamical thickening and thinning.