Atmospheric sounding of the boundary layer over alpine glaciers using fixed-wing UAVs

Abstract. Glaciers are an integral part of the high mountain environment and interact with the overlying atmosphere and surrounding terrain in a complex and dynamic manner. The energy exchange between the glacier surface and the overlying atmosphere controls ice melt rates and promotes a characteristic microclimate, including the formation of a low-level katabatic jet that interacts with other, often thermally driven winds in alpine terrain. Information on the local circulation and structure of the atmospheric boundary layer over glaciers is crucial for studying cryosphere-atmosphere interactions and for investigating the characteristics of the katabatic wind, its broader cooling effect, and its susceptibility to be disrupted by strong valley or synoptic winds that promote heat advection from the ice- and snow-free periphery towards the glacier. While the number of ground-based measurements from weather stations and meteorological towers installed on glaciers for boundary layer research has increased in recent years, a lightweight and mobile measurement technique for atmospheric sounding over alpine glaciers has not yet been available. Here we describe a new measurement technique based on a low-cost and open-source fixed-wing UAV, which allows sounding the atmospheric boundary layer over glaciers up to several hundred metres above the surface. In the frame of a feasibility study in 2021, two half-day (16 June, 23 September) and two 24 h campaigns (9/10 July, 25/26 August), including nocturnal soundings, were performed, demonstrating the UAV's capability to reach heights of up to 800 m above the glacier surface. From these campaigns, 40 profiles of air temperature, specific humidity, wind speed, wind direction, and turbulence were derived. The results highlight characteristic features of the glacier boundary layer, including a persistent surface-based inversion, a cool and dry katabatic wind confined to the lowest 50 m a.g.l., an overlying shear layer, and a warmer, often more humid valley wind above 100–200 m a.g.l. These observations illustrate how the boundary layer responds to synoptic forcing and local winds and demonstrate the potential of UAV-based atmospheric soundings for advancing glacier meteorology in complex alpine terrain.