Modelling the Vadret da Morteratsch

Abstract

The Vadret da Morteratsch (VdM), a glacier complex in southeastern Switzerland, was modelled using an ice flow model based on the Shallow Ice Approximation (SIA) coupled to a surface mass balance model. To solve the problems with mass conservation arising from steep slopes at the VdM, several methods were investigated at a test topography, and ultimately a restriction in ice flow towards grid points at which a negative ice thickness was created proved to be most successful. This method was applied to the VdM topography. The coupled model was forced with observations of surface air temperature and precipitation at Segl Maria starting in 1864 until 2015, and it was calibrated to glacier length observations over the same period. Changing the ice flow parameters and comparing the simulated ice thickness in 2001 to the ice thickness observations in Zekollari et al. (2013), it was found that the SIA cannot model the ice thickness and the response time correctly at the same time. An increase in flow parameters led to a response time more in line with earlier work, but the glacier became thin in 2001 compared to observations, and vice versa. Furthermore, for a low value of flow parameters a significant larger correction in absolute value to the mass balance was necessary to simulate the observed glacier length record. The resulting ice thickness distributions in 2015 were used as an initial state for runs that simulate the glacier until 2100 for three different climate scenarios (+1.5 °C, +3.0 °C and +4.5 °C by 2100 with respect to the 1986-2015 average), and a control case with the 1986-2015 average. Precipitation was increased differently for each scenario (wetter winters, drier summers) and a range of +/- 15% was used to take the large uncertainties in precipitation projections into account. It was found that the effect of a different climate forcing becomes evident after approximately 25 years, indicating that the glacier's response in the near future primarily is an effect of current and past climate conditions. All scenarios indicate a separation of Vadret da Morteratsch from Vadret da Pers. Only fragments of ice at high altitude remain by 2100 for the +3.0 °C and +4.5 °C scenario. The feasibility of saving the VdM by artificially generated snow deposition over an area of 0.8 km² at an altitude around 2450 m was investigated by adding snow to the model in the melting season (May-September) for the different climate scenarios. For the +3.0 °C and +4.5 °C scenarios saving the VdM was found to be virtually impossible. For lower temperature increase scenarios this deposition shows promise, provided that there is enough water (and money) available to do so.