  The mass balance of mountain glaciers is of interest for several applications
(local hydrology or climate projections), and turbulent fluxes can be an
important contributor to glacier surface mass balance during strong melting
events. The underlying complex terrain leads to spatial heterogeneity and
non-stationarity of turbulent fluxes. Due to the contribution of
thermally-induced flows and gravity waves, exchange mechanisms are fully
three-dimensional, instead of only vertical. Additionally, glaciers have their
own distinct microclimate, governed by a down-glacier katabatic wind, which
protects the glacier ice and interacts with the surrounding flows on multiple
scales. In this study, we perform large-eddy simulations with the WRF model
with dx=48 m to gain insight on the boundary-layer processes over an Alpine
valley glacier, the Hintereisferner (HEF). We choose two case studies from a
measurement campaign (August 2018) with different synoptic wind directions
(South-West and North-West). Model evaluation with an array of eddy-covariance
stations on the glacier tongue and surroundings reveals that WRF is able to
simulate the general glacier boundary-layer structure. Under southwesterly
airflow, the down-glacier wind is supported by the South-Western synoptic wind
direction, a stable boundary layer is present over the ice surface, and local
processes govern the turbulence kinetic energy production. Under northwesterly
airflow, a cross-glacier valley flow and a breaking gravity wave lead strong
turbulent mixing and to the subsequent erosion of the glacier boundary layer.
Stationarity analyses of the sensible heat flux suggest non-stationary
behaviour for both case study days, while non-stationarity is highest on the NW
day during the gravity-wave event. These results suggest that the synoptic wind
direction has, in addition to upstream topography and the atmospheric
stability, a strong impact on whether a local glacier boundary layer can form
or not, influencing whether a glacier is able to maintain its own microclimate.
