Nested high-resolution modelling of the Greenland Summit region

The dynamics and thermodynamics of the vicinity of Summit, the highest point of the Greenland ice sheet at 72 degrees 34' N, 37 degrees 38' W, is simulated over two climate cycles until the present with a high-resolution regional model coupled to a large-scale model of the entire Greenland ice sheet. For the computation of the age of ice, two different methods are applied, an Eulerian scheme which solves the advective age equation in a frame fixed in space and requires some artificial diffusion, and a Lagrangian particle-tracing scheme which follows the motion of ice particles and is diffision-free. The transient simulation is based on the shallow-ice approximation which neglects normal stress deviators and shear stresses in vertical planes. For the simulated modern ice sheet, the velocity and stress fields are then re-computed in the Summit region by a first-order algorithm which includes these stresses. The measured ice topography as well as the temperature profiles of the boreholes GRIP and GISP2 are reproduced very well. The simulated Summit motion of 16 ice thicknesses during the last 250,000 years gives a clue for understanding the origin of irregularities observed in the GRIP and GISP2 cores. In a 50 km region around Summit, all stresses are of the same order of magnitude, so that a very precise modelling of the ice dynamics, which is necessary for an accurate ice-core dating, requires that the shallow-ice approximation be locally abandoned.