Ablation drivers over a cold-based ice cap in the Eastern Alps: a surface energy balance analysis

Glaciers are archives of past climatic conditions, reflected in the yearly amount of ice accumulation or depletion. Ice coring allows access to the information stored in glacial layers. However, discontinuities in an ice core create problems in reliably dating the core layers and questions regarding the origin of the discontinuity. The analysis of an ice core from 2017 on Weiseespitze, a cold-based ice cap located at 3498 m a.s.l. in Tyrol (Austria), could be explained by the presence of a discontinuity around 400 CE. Since the glacier is nowadays experiencing potentially similar mass loss conditions, this study analyses present day surface energy balance to identify the potential climatic drivers behind the supposed discontinuity. Energy balance at the core site is modelled with the COupled Snowpack and Ice surface energy and mass balance model in Python (COSIPY), forced with data collected between 2017 and 2022 by an automatic weather station on the glacier. COSIPY initialisation was optimised by comparing modelled and observed snowheight, the observed albedo was introduced as an input variable and the precipitation input was modified to better suit high altitude locations. Comparison of the modelled and observed snowheight shows minor mismatches, connected in part to the absence of a wind erosion parameterization in the model and in part to the overestimation of surface temperature from the energy balance optimization algorithm. Nevertheless, COSIPY ice melt gradient agrees very well with observations and the simulation of the ablation season is not deeply affected by such problems. Weiseespitze lost on average about 3 m of ice at the summit since 2018. The summer characterised by maximum ice melt in the observational period was 2022, where ablation stakes recorded on average 1.5 m of ice loss. On the contrary, summer 2020 was the only summer where most of the summit registered no ice loss. Comparison of energy balance components between the summer 2022 and 2020 showed that 2022 was characterised by positive and more intense sensible (9.8 W m-2 vs 5.8 W m-2) and latent (1.0 W m-2 vs -1.0 W m-2) heat fluxes and a lower outgoing shortwave energy flux (118.8 W m-2 vs 166.1 W m-2). The latter is caused by an abrupt albedo lowering at the beginning of July, which aligns with a period of uninterrupted positive air temperature of almost two weeks removing the snow from the previous winter. Therefore, air temperature and its impact on the glacier surface seems to be the main driver of ablation in 2022, which had about 20 positive temperature days more than 2020, resulting in an average air temperature 1.2 C warmer. These preliminary results will be shortly complemented by research in local archives of paleotemperature to verify whether the time of the hypothesised discontinuity was characterised by similar conditions.