3D Higher-Order modelling of Vadret da Morteratsch, Switzerland

We have developed a three-dimensional glacier flow model for the Vadret da Morteratsch, Engadin, Switzerland, in order to better understand the strong retreat of the glacier since 1860 and to project its further retreat under an intensifying warming trend. The ice dynamics model is a state-of-the-art higher-order model implemented on a 25 m horizontal resolution and considers both ice deformation and basal sliding. It is coupled to a two-dimensional energy balance model forced by monthly temperature and precipitation data. Both models make use of a comprehensive dataset collected over the past decade (2001-2011) that includes ice thickness, surface mass balance and surface velocity measurements. The ice thickness and bedrock topography are derived from a series of ground-penetrating radar (GPR) transects which are inter- and extrapolated over the entire glacier using the plastic flow assumption for central regions and an inverse quadratic distance interpolation to obtain U-shaped cross-sections. The observed present-day velocity field can be reproduced closely (RMSE around 15 m a-1), by simultaneous tuning of the rate factor of Glen's flow law and the sliding factor of a Weertman-type of sliding law. If present-day climate persists, the tongue of the glacier retreats by some 1.5 km and the Morteratsch glacier and its main tributary, the Pers glacier, disconnect. Mainly due to glacier orientation, the retreat is stronger for the Pers glacier, while the Morteratsch glacier still has a well-developed glacier tongue down to an elevation of 2300 m. Based on length and volume preservation experiments, a temperature forcing of between -1°C and -2°C is needed to maintain the glacier in its actual shape. Several future scenarios are adopted and in all cases a strong retreat is projected, which cannot be counteracted by a realistic increase in precipitation. If present-day temperatures increase by 3 to 4°C by the end of the century, only isolated ice patches remain at high elevations (above 3500m). These are largely stagnant and precede the almost total demise of the Morteratsch glacier complex if those climate conditions were sustained beyond that period.