The Second Hintereisferner Experiment (HEFEX II): Initial Insights into Boundary Layer Structure and Surface–Atmosphere Exchange Processes from Intensive Observations at a Valley Glacier

Abstract Mountain glaciers offer opportunities to observe boundary layer exchanges in conditions characterized by predominantly stable stratification, thermally driven winds, and varying surface roughness. Logistical challenges involved in instrumenting glacier surfaces mean that in situ observations remain relatively scarce, limiting the use of this outdoor laboratory. The second Hintereisferner Experiment (HEFEX II) was carried out on an Austrian Alpine glacier during summer 2023. This collaborative endeavor, involving 12 institutions from Austria, France, Germany, Switzerland, and the United Kingdom, represents an unprecedented set of observations of glacier microclimate. Instrumentation on the glacier surface consisted of eight 3-m and two 5-m weather stations equipped with multilevel eddy covariance systems and auxiliary instrumentation, and eight additional lower-specification weather stations. These operated successfully for 26 days with minimal data gaps. During a 3-day intensive observational period, additional instrumentation was deployed: a short-path ultrasonic anemometer installed very close to the glacier surface; a high-speed thermal camera capturing high-resolution boundary layer heat transport at the glacier centerline on a synthetic screen; 3D sampling of the glacier boundary layer using two meteorological UAVs; and a Streamline XR Doppler lidar capturing the structure of the above-valley atmosphere. These novel datasets are valuable for improving understanding of glacier–atmosphere exchange processes, the role of glaciers in valley circulation, and how both might be affected by continued climate change and glacier recession. Here, we detail the scientific goals and implementation of the campaign, describe the general weather conditions, and present first insights into what the observations reveal about the glacier boundary layer features observed during the campaign. Significance Statement The second Hintereisferner Experiment (HEFEX II) deployed extensive instrumentation on an Austrian glacier in summer 2023 to investigate glacier microclimate, turbulence properties, and multiscale atmosphere–glacier exchanges. Such observations are of value because (i) the stably stratified, persistent katabatic flow over glaciers offers a real-world laboratory in which to test turbulent theory; (ii) they enable refined quantification of the atmosphere–glacier exchanges that govern glacier meltwater production; and (iii) they allow verification of high-resolution simulations of the atmosphere that can subsequently be used to understand how ongoing glacier shrinkage will impact the wider valley circulation.