Abstract

Abstract. Since the original formulation of the positive-degree-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 present-day 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.

Highlights

  • Simulating the surface mass balance of polar ice sheets correctly is crucial to better understand climate and ice sheet feedbacks at decadal to multi-millennial time scales and to better predict the evolution of present-day ice sheets in the coming decades

  • 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

  • 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

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Summary

Introduction

Simulating the surface mass balance of polar ice sheets correctly is crucial to better understand climate and ice sheet feedbacks at decadal to multi-millennial time scales and to better predict the evolution of present-day ice sheets in the coming decades. With the exception of Reijmer et al (2012) who compared different refreezing schemes with outputs from a regional atmospheric climate model, no PDD inter-comparison has been undertaken so far It is, difficult to evaluate the performance and the efficiency of each PDD parameterisation for present-day ice sheets. The PDD approach is used as a tool to evaluate the relative importance of both icesheet initial configuration and ablation-related processes in the build-up of Northern Hemisphere ice sheets throughout this period To achieve this goal, we first test the sensitivity of the surface mass balance of Northern Hemisphere ice sheets to different PDD formulations and we evaluate the impact of each PDD parameter in this response. Through a set of sensitivity experiments, we investigate the relative impact of the ice-sheet feedbacks onto the climate (i.e., through a change of initial ice-sheet geometry) and, on the evolution of the ice system with respect to the way the ablation is computed

The standard PDD formulation
Dependency of degree-day factors on temperature
Revised treatment of meltwater retention
PDD formulations adopted in the present study
TP02 Model
FST09 Model
Models
Coupled transient climate-ice sheet simulations
Present-day simulations
Simulations of past periods
CLIMBER
Role of PDD parameters
Role of the ice-sheet geometry
Findings
Conclusions
Full Text
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