Turbulent boundary layer measurements near the surface below the jet maximum in a katabatic wind along alpine slope.

Katabatic winds are gravity flows generated at night by radiative cooling at the ground surface. They are mainly observed in winter during anticyclonic weather episodes associated with stratification and temperature inversion in the lower troposphere. Although their intensity is relatively low, they play a major role in areas with complex terrain, as they systematically contribute to the generation of cold air pools in the valley. There are a large number of in situ observations of the katabatic process along gentle slopes such as those reported in valleys or glaciers but far fewer along steep alpine slopes in the mountains. The katabatic wind consists of a turbulent wall jet along the slope coupled to a turbulent thermal boundary layer cooled at the surface, both subject to the effects of gravity. Little is known about the flow region below the maximum jet velocity, very close to the surface. It is aimed here to deepen general knowledge of the turbulent properties of katabatic winds in this region, which is rarely observed in situ for technical reasons. We further propose and validate a set of laws of the wall necessary for the definition of surface boundary conditions for the appropriate use of regional models over complex terrain. A recent campaign of in situ measurements has been carried out on 3-15 February 2023 in the French Alps near Grenoble. A continuous set of measurements is available from the ground up to a height of 1m, i.e. close to the maximum height of the jet, using a multi-hole 3D velocity probe mounted on a high-precision vertical displacement system. Measurements were taken from z=2 mm close to the snow surface (z+ = 20) to z=1 m high (z+ = 10000). In addition, a thermocouple and an infrared radiation probe were used to determine the temperature profile at the same vertical positions. Firstly, direct measurement of the w velocity component normal to the slope shows a significant contribution towards the ground of the order of 10% of the maximum upslope velocity of the katabatic jet. The trend is well described by a simplified analytical model derived from the Navier-Stokes equations. This model reveals the main role of buoyancy close to the surface, quantified by the strong cooling at ground level and the steep slope. Secondly, measured temperature profile deviates from the logarithmic distribution expected on flat ground. This study will be transposed to a more comprehensive planned as part of the European alpine observation campaign TeamX in 2024-2026 in the Austrian Alps near Innsbruck. A pre-installation was carried out this winter 2024 on the slopes overlooking the Inn valley at InnBox station NF27.