Modelling the contemporary liquid water balance of the Greenland Ice Sheet

Abstract

The Greenland Ice sheet (GrIS) stores ∼7.4 m of global mean sea level equivalent in water, which poses a threat to low-lying areas in case of continuing deglaciation. Since the mid-1990s, the ice sheets’ mass balance (MB) has become negative, owing to enhanced solid ice discharge through marine-terminating outlet glaciers and increased surface melt and subsequent runoff. Runoff does thereby not simply equal surface melt due to potential retention of liquid water in the ice sheet’s hydrological systems. The porous firn layer, which covers ∼90% of the GrIS, acts as a prominent meltwater buffer via refreezing or liquid storage in firn aquifers. To investigate the magnitude of liquid water retention in the firn layer, we applied a detailed physical snow/firn model (SNOWPACK) to the glaciated area of Greenland. SNOWPACK simulates snow/firn compaction and considers vertical water percolation and related processes. The model explicitly simulates the evolution of microstructural snow properties and links them to thermal and mechanical quantities. SNOWPACK is run for the period 1960–2014 with atmospheric forcing data from the regional climate model RACMO2.3. An evaluation of the model with various in-situ measurements and remote sensing data confirms the model’s capability of reproducing the general characteristics of the contemporary GrIS firn layer, such as the spatial variability in vertical density profiles, the horizontal extent of firn aquifers and the interannual trend in Greenland’s MB. Averaged over the ice sheet and the period 1960–2014, ∼47% of the liquid water production/input at the surface is refrozen in the firn layer. This percentage exhibits a high spatial variability and is lowest for the northern ice sheet (∼30%) and highest in the southeast (∼75%), where porous firn depth is large owing to high snowfall rates. In terms of interannual variability, modelled components of the liquid water balance (melt, runoff, refreezing, rainfall and evaporation/condensation) are relatively constant between 1960 and 1989. For the period 1990–2014, a spatially consistent and significant increase in melt is simulated, which also induces positive trends in runoff and refreezing. Trends in the former process are generally higher, which means that the majority of the additional melt is not buffered in the firn layer but runs off to the ocean. An exception is the southeastern GrIS, where 70% of the additional liquid surface production/input is retained, again due to the comparably high amounts of available pore space. Firn aquifer area exhibits a temporal inland and upward migration, which is, at least for southeast Greenland and the end of the simulated period, in line with observations. A comparison of simulated surface mass balance and gravimetry time series indicates a spatially consistent deviation in timing between the signals during the melt season. Part of this deviation can likely be attributed to the neglect of lateral transit times of runoff to reach the ocean. To consider this process, a routing model, forced by output from SNOWPACK/RACMO2.3, is applied. Accounting for runoff transit times generally improves the agreement but also reveals different limitations in the applied modelled and observational data.