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 the formation of a low-level katabatic jet that interacts with other, often thermally driven winds in alpine terrain. Information on the structure of the atmospheric boundary layer over glaciers is crucial for studying the characteristics of the katabatic jet, its broader cooling effect, and its susceptibility to be broken up 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. Vertical profiles of air temperature, humidity, pressure, wind speed, wind direction and turbulence can be derived from the meteorological and flight recorder data collected by the UAV. The results of a measurement campaign on the Kanderfirn in the Swiss Alps on 16 June 2021 demonstrate the potential of the technique and highlight typical features of the boundary layer above a melting glacier surface. The soundings reveal a persistent low-level katabatic jet, characterised by a pronounced surface-based inversion, relatively dry air, high wind speeds and enhanced turbulence, and a warmer and more humid valley wind aloft.