The suitability of different surface melt models for long-term simulations of glacier response to climate change

Abstract:

Long-term studies of glacier mass balance and glacier projections into the 21st century are affected by the type of melt model applied. Previous works have investigated differences in performance among melt approaches of various complexity and concluded that models that are only dependent on air temperature might be oversensitive to temperature fluctuations compared to models with a stronger physical basis. However, the validity of those analyses has been restricted to the point-scale and to few seasons. We therefore investigate the performance of five distributed glacier melt models over a multi-decadal period in order to assess their ability to model long-term and future glacier response. The models range from a simple degree-day model based solely on air temperature to more sophisticated models including the full shortwave radiation balance. In addition to the empirical models, the performance of a physically-based energy-balance model (EB) is examined. The melt models are coupled to an accumulation and a surface evolution model and applied in a distributed manner to Rhonegletscher (Switzerland). The models are run for the period 1929-2012 and forced with hourly data from a nearby weather station. For calibration, seasonal mass balance measurements covering the period 2006-2012 are used. Decadal ice volume changes for six periods in the years 1929-2012 serve for model validation. Over the period 2006-2012, almost no differences in performance among the models are evident except for EB, which is less consistent with observations likely due to lack of meteorological in-situ input data. However, simulations over the long-term (1929-2012) reveal that models which include a separate term for shortwave radiation agree best with the observed ice volume changes, indicating that their melt relationships are robust in time and thus most appropriate for long-term modelling. In contrast, more empirical approaches considerably overestimate the observed changes and are only able to reproduce the ice volume changes of the most recent subperiods with sufficient accuracy. This seems to confirm their oversensitivity to temperature variations and indicates that care should be used when choosing surface melt approaches for long-term simulations and studies of glacier response to climate.