The second HinterEisFerner EXperiment (HEFEX II): Initial insights into boundary layer structure and surface-atmosphere exchange processes from intensive observations at a valley glacier

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 insitu 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 twelve institutions from Austria, France, Germany, Switzerland, and the UK, 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 three 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; 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 first insights into what the observations reveal about the glacier boundary layer features observed during the campaign.