Numerical simulations of glacier evolution performed using flow-line models of varying complexity

Abstract. The performance of two numerical models of different complexity, i.e., a Shallow Ice Approximation (SIA) and a Full-Stokes Model (FSM), is studied by analyzing glacier evolutions at various bed geometries and by applying different climatic forcings. Glacier bed geometry changes from a constant slope and a uniform width to a superimposed Gaussian bump or ice-fall on a constant slope and an exponentially varying width. Constant slopes of 0.1, 0.2 and 0.3 are chosen to study the evolution of a large, medium and small glacier, respectively. A specific mass balance serves as a climatic forcing. The steady state is reached 60, 30 and 10 years, respectively faster for large, medium and small glacier, when simulations are performed using SIA instead of FSM. Glaciers time response is studied by using step and periodic changes, and by imposing natural variability in the equilibrium-line altitude. Glacier length response time is up to 14 years longer when FSM is used compared to SIA. When periodic and natural variability are enforced, glaciers simulated using SIA lag in phase compared to the forcing up to 81.2° for glacier length and up to 56.5° for volume. Contrary to that, glaciers simulated with FSM show greater lag in phase compared to the forcing for glacier length and smaller lag for volume. The models differ in their treatment of glacier flow mechanics and differences in physical variables become apparent with increasing glacier bed slope and bed profile complexity.