Measuring Horizontal Shear and Turbulence in Mountain Valleys using UAS and Lidar

Turbulent mixing in complex terrain remains a major source of uncertainty for weather and climate models. Many processes within the planetary boundary layer (PBL) occur on spatial scales that numerical models cannot resolve explicitly and thus require parameterization. For complex terrain, however, the common mesoscale-model assumption that horizontal shear production of turbulent kinetic energy (TKE) is negligible no longer holds. This motivates the need to develop 3D PBL parameterizations that include horizontal shear production of TKE. However, observational datasets that quantify the relative contributions of horizontal versus vertical shear production are still lacking. We deployed a combined measurement strategy utilizing small uncrewed aircraft systems (UAS) and Doppler wind lidar stations to provide the missing high-resolution measurements and thus to improve the understanding of multi-scale exchange processes in mountainous regions.
The measurement strategy incorporated commercially available and automatically operating multi-rotor UAS equipped with fast-response meteorological sensors to collect high-resolution measurements of the 3D wind vector, temperature and humidity, with additional aerosol particle measurements. During the TEAMx 2025 summer Extended Observation Period, four UAS performed simultaneous in-situ measurements at multiple heights and key valley locations (valley floor, foot of sidewall, mountain slope and crest) along a valley transect in the Inn Valley at the TEAMx Radfeld supersite in Austria. This included vertical profiles up to 2 km above mean sea level and horizontal cross-sections through the valley. The vertical profile spacing was representative of the grid resolution of targeted operational weather forecast simulations and was coordinated with the locations of the three deployed Doppler wind lidar systems, which continuously measured vertical profiles of wind.
The combined measurements deliver a unique observational dataset of wind distribution in the Inn Valley, enabling a spatially and temporally highly-resolved analysis of horizontal and vertical wind shear. The UAS measurement systems resolve the turbulent scales of wind up to 3 Hz, which corresponds to a vortex size of about 3 m at a mean horizontal wind speed of 10 ms^-1, allowing the calculation of turbulent kinetic energy and turbulent fluxes. For up-valley winds, which are thermally driven and characteristic of the afternoon in mountain valleys, TKE increases in the horizontal direction from the valley center toward the mountain and reaches its vertical maximum near the mountain ridge. This observed rise in TKE coincides with strong horizontal wind shear, peaking at 0.01 s^-1 near the mountain ridge, with the horizontal wind speed decreasing toward the mountain. By combining UAS- and lidar-based measurements with model parameterization development within the TEAMx framework, we aim to make turbulence representation in high-resolution numerical weather prediction models both more accurate and physically grounded, leading to more reliable forecasts in mountainous regions.