The CROSSINN Field Campaign on the Three-Dimensional Flow Structure in the Inn Valley, Austria: Overview and Selected Results

The atmospheric boundary layer over mountainous terrain (MoBL) is highly variable in space and time, as the exchange of mass, momentum and energy is not only effectuated by vertical turbulent mixing as over horizontally homogeneous and flat terrain, but also by advection due to mesoscale thermally and dynamically driven flows and their interactions. The experimental detection of the relevant processes and structures requires area-wide measurements with appropriate high spatial and temporal resolution. We conducted a 2.5 month-long field campaign in the Inn Valley, Austria, within the framework of the CROSSINN (Cross-valley flow in the Inn Valley investigated by dual-Doppler lidar measurements) project to capture the three-dimensional flow structure in the valley and its impact on the MoBL. For this purpose, we combined a set of ground-based remote sensing instruments, including six Doppler lidars, and in situ tower measurements with upper air information from radio soundings and a research aircraft. Three of the Doppler lidars were aligned along a line across the valley and performed synchronized range-height-indicator scans, which allowed retrieving the two-dimensional wind field in a vertical plane across the valley with 1 min temporal resolution and 50 m spatial resolution throughout the whole campaign.

In this contribution, we will give an overview of the field campaign and measurement strategy and show first results, which clearly demonstrate that the combination of the different sources of information allows to gain insight into the spatial variability of the MoBL and to link the vertical MoBL structure to the three-dimensional flow. Besides expected flow features such as thermally driven valley and slope winds, we identified an unforeseen persistent cross-valley circulation in the data set with upward motion in the outer part of the slightly curved valley, which formed in the presence of strong upvalley wind and strongly influenced the thermodynamic MoBL structure. The outcomes of the CROSSINN field campaign will facilitate the planning and conduction of future campaigns, such as the upcoming large international program and experiment on multi-scale transport and exchange processes over mountains (TEAMx), the experimental part of which will partly take place in the Inn Valley.