Firn changes at Colle Gnifetti revealed with a high-resolution process-based physical model approach

Abstract. Our changing climate is expected to affect ice core records as cold firn progressively transitions to a temperate state. Thus there is a need to improve understanding and further develop quantitative process modeling, to better predict cold firn evolution under a range of climate scenarios. Here we present the application of a distributed, fully coupled energy balance model, to simulate high-alpine cold firn at Colle Gnifetti over the period 2003–2018. For the first time, we force such a model with high-resolution, long-term and extensively quality-checked meteorological data measured in closest vicinity of the firn site, at the Capanna Margherita (4560 m a.s.l.). The model incorporates the spatial variability of snow accumulation rates, and is calibrated using several, partly unpublished high-altitude measurements from the Monte Rosa area. The simulation reveals a very good overall agreement in the comparison with a large archive of firn temperature profiles. The rate of firn warming at 20 m depth is estimated at 0.44 °C per decade. Our results show that surface melt over the glaciated saddle is increasing by 3–4 mm w.e. yr⁻² depending on the location (29–36 % in 16 years), although with large inter-annual variability. Analysis of modeled melt indicates a marked tendency towards small melt events (< 4 mm w.e.), which collectively represent a significant fraction of the melt totals. Atmospheric humidity is found to be a prominent control over melt occurrence, with considerable amounts of sublimation happening in dry conditions. For future developments, we recommend implementing a physical simulation of meltwater infiltration, to be calibrated with more measurements of percolation in cold firn.