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. Providing reliable projections of the ice sheet contribution to futuresea-level rise has become one of the main challenges of the ice sheetmodelling community. To increase confidence in future projections, a goodknowledge of the present-day state of ice flow dynamics, which is criticallydependent on basal conditions, is strongly needed. The main difficulty istied to the scarcity of observations at the ice–bed interface at the scale ofthe whole ice sheet, resulting in poorly constrained parameterisations in icesheet models. To circumvent this drawback, inverse modelling approaches canbe developed to infer initial conditions for ice sheet models that bestreproduce available data. Most often such approaches allow for a goodrepresentation of the mean present-day state of the ice sheet but areaccompanied with unphysical trends. Here, we present an initialisation methodfor the Greenland ice sheet using the thermo-mechanical hybrid GRISLI (GRenoble Ice Shelf and Land Ice) ice sheet model. Our approach is based on the adjustment of the basal dragcoefficient that relates the sliding velocities at the ice–bed interface tobasal shear stress in unfrozen bed areas. This method relies on an iterativeprocess in which the basal drag is periodically adjusted in such a way thatthe simulated ice thickness matches the observed one. The quality of themethod is assessed by computing the root mean square errors in ice thicknesschanges. Because the method is based on an adjustment of the slidingvelocities only, the results are discussed in terms of varying ice flowenhancement factors that control the deformation rates. We show that thisfactor has a strong impact on the minimisation of ice thickness errors andhas to be chosen as a function of the internal thermal state of the ice sheet(e.g. a low enhancement factor for a warm ice sheet). While the methodperformance slightly increases with the duration of the minimisationprocedure, an ice thickness root mean square error (RMSE) of 50.3 m is obtained in only 1320 modelyears. This highlights a rapid convergence and demonstrates that the methodcan be used for computationally expensive ice sheet models.

A rapidly converging initialisation method to simulate the present-day Greenland ice sheet using the GRISLI ice sheet model (version 1.3)