Investigating the influence of changing ice surfaces on gravity wave formation and glacier boundary-layer flow with large-eddy simulations

Abstract. Mountain glaciers are located in highly complex terrain and their local micro-climate is influenced by mountain boundary layer processes and dynamically-induced gravity waves. Observations (turbulence flux towers) and simulations (large-eddy simulation) over the Hintereisferner (HEF) glacier in the Austrian Alps have shown that down-glacier winds are often disturbed by cross-glacier flow from the North-West associated with gravity waves. In this work, we explore how the glaciers located upstream of HEF influence the gravity wave formation and intensity and and the feedback this has on boundary layer flow over HEF. In semi-idealized large-eddy simulations, we explore the impact of changing surface properties on HEF's microclimate by removing the upstream glaciers only (NO_UP) and removing all ice surfaces (NO_GL). Simulations suggest that removing the upstream glaciers (which causes a change of boundary layer stratification from stable to unstable) leads to a weaker gravity wave breaking earlier than in the reference simulation and leading to enhanced turbulent mixing over HEF. As a consequence, this results in higher temperatures, sensible heat fluxes, and stronger warm-air advection over HEF tongue. Removing all glaciers results – as expected – in higher temperatures of up to 5 K over the missing ice surfaces, while the gravity wave pattern is similar as in the NO_UP simulation, indicating that the upstream boundary layer exerts dominant control over downstream response in such highly dynamic conditions. This study shows that a single glacier tongue is never isolated from its environment and that surrounding glaciers and local topography have to be taken into account when studying glacier boundary-layer processes. Furthermore, glaciers have a stabilizing effect on the boundary layer, impacting gravity wave formation and downslope windstorm intensity and their impact on the flow structure in valleys downstream.