Flow structure and turbulence characteristics on a mid-latitude glacier

Mountain glaciers are a perfect laboratory to study the interaction between the mountain atmosphere, including the multiscale processes developing within it, and the stably stratified ice surfaces. Due to their setting within mountain valleys, the structure of the glacier boundary layers is a result of a complex interplay between the surface thermal forcing, the thermally and dynamically driven multiscale mountain flows and the larger scale flow aloft. This complex flow structure plays an important role in glacier microclimates and surface energy and mass balance of glaciers. However, few datasets of atmospheric measurements over the whole surface of a glacier are available to probe this complex interaction and spatio-temporal variability. In August and September 2023, the Second Hintereisferner Experiment (HEFEX II), a three-week measurement campaign took place on the Hintereisferner glacier in the Austrian Alps to address these challenges. The glacier was instrumented with 18 surface weather stations, of which 10 were equipped with two or three levels of turbulence measurements.

The data from this extensive dataset is used to characterize the surface atmospheric flow over the glacier and investigate its turbulent properties. Using a clustering method on the vertical profiles from one tower at the upper part of the glacier tongue, we show that different classes of katabatic flows, as well as some perturbed flows related to the impact of synoptic flows during strong synoptic winds periods, and the passage of a cold front take place during the campaign. We also show that these different types of flow show characteristic horizontal wind and temperature structure across the glacier tongue. The results thus suggest that it is possible to recover the type of flow from one multi-level measurement location and extend it consistently to the whole surface of the glacier, meaning that a well-chosen point on the glacier is correctly representing the spatial structure of the flow. The surface measurements are then used to explore the turbulence structure during the different flow regimes, and estimate the surface energy balance over the glacier and calculate the melt rate. The calculated melt rates are consistent with ablation measurements. The results indicated that the different clusters are associated with different melt rates and surface energy balance contributions, with katabatic flows having a large radiative contribution and synoptically perturbed flows having large sensible and latent heat contribution.