Influence of ablation-related processes in the build-up of simulated Northern Hemisphere ice sheets during the last glacial cycle

Abstract

Abstract. Since the original formulation of the positivedegree-day (PDD) method, different PDD calibrations have been proposed in the literature in response to the increasing number of observations. Although these formulations generally provide a satisfactory description of the presentday Greenland geometry, they have not all been tested for paleo ice sheets. Using the climate-ice sheet model CLIMBER-GRISLI coupled with different PDD models, we evaluate how the parameterisation of the ablation may affect the evolution of Northern Hemisphere ice sheets in the transient simulations of the last glacial cycle. Results from fully coupled simulations are compared to time-slice experiments carried out at different key periods of the last glacial period. We find large differences in the simulated ice sheets according to the chosen PDD model. These differences occur as soon as the onset of glaciation, therefore affecting the subsequent evolution of the ice system. To further investigate how the PDD method controls this evolution, special attention is given to the role of each PDD parameter. We show that glacial inception is critically dependent on the representation of the impact of the temperature variability from the daily to the inter-annual time scale, whose effect is modulated by the refreezing scheme. Finally, an additional set of sensitivity experiments has been carried out to assess the relative importance of melt processes with respect to initial ice sheet configuration in the construction and the evolution of past Northern Hemisphere ice sheets. Our analysis reveals that the impacts of the initial ice sheet condition may range from quite negligible to explaining about half of the LGM ice volume depending on the representation of stochastic temperature variations which remain the main driver of the evolution of the ice system. The main findings of this paper underline the need for conducting studies with high resolution climate models coupled to detailed snow models to better constrain the temporal and spatial variations of the PDD parameters. The development of such approaches could improve the calibration of the PDD formulation which is still widely used in climate-ice sheet studies.

The influence of atmospheric grid resolution in a climate model-forced ice sheet simulation

“…4d). Several coupled climate-ice sheet experiments with a low-resolution atmospheric model have shown qualitatively similar results, for example , Charbit et al (2013) and Beghin et al (2014). The common denominator for these studies is that they all used CLIMBER-2 to produce the atmospheric forcing fields.…”

“…This response bears some structural similarity to the low-resolution model results shown in Beghin et al (2014) and Charbit et al (2013), and also the pre-LGM ice sheets in , and Bonelli et al (2009). In a similar fashion to the T42 and T31 cases, the T21 climate forcing is too warm (and presumably too dry) over the southwestern parts of the Eurasian ice sheet area to reproduce the LGM ice sheet reconstruction.…”

“…Following Charbit et al (2013), we test the sensitivity of the surface-mass balance scheme using three different PDD-based ablation models: the default parameterizations in SICOPOLIS (based on Reeh, 1991), plus the ones presented in Fausto et al (2009b), and Tarasov and Peltier (2002) (henceforth referred to as SICOdef, FST09, and TP02, respectively).…”

Deglacial ice sheet meltdown: orbital pacemaking and CO<sub>2</sub> effects

“…The standard deviation of the daily mean temperature from monthly means is assumed to be 5.5 • C and spatially homogeneous. The refreezing rate of melted snow is set to 60 % (for an intercomparison of different PDD models see Charbit et al, 2013).…”

Comparison of surface mass balance of ice sheets simulated by positive-degree-day method and energy balance approach

“…In turn, the results from the SEB simulations show that these changes in albedo might significantly affect the surface mass balance of ice sheets during glacial times (Krinner et al, 2006;Ganopolski et al, 2010) and in future climate change scenarios (Dumont et al, 2014;Goelles et al, 2015). Charbit et al (2013) discuss the effect of different PDD parameterizations on the NH ice sheet evolution, but a direct comparison between the PDD and SEB approaches in a transient simulation over the glacial cycle with a climate-icesheet model has not been performed yet. Using results from an ensemble of transient simulations of the last glacial cycle performed with an Earth system model of intermediate complexity, we undertake a systematic comparison of ice sheet surface mass balance simulated using the SEB and PDD approaches for different ice sheets and during different periods of the last glacial cycle.…”