Flow Regimes and Turbulence Structure on a Steep Slope in Winter: Findings from the TEAMx wEOP

During the TEAMx winter EOP, an extensive measurement campaign took place on a steep undulating slope in the Inn Valley, Austria. This six-week long campaign featured a suite of instrumentation, including a network of eight turbulence towers installed at two across-slope and an along-slope transects equipped with two levels of sonic anemometers, nano-barometers, and slow response sensors. In addition, four component radiation at two heights measured radiative flux divergence at a central location on the slope, a short-range Doppler wind lidar (Wind Ranger) at the bottom of the slope recorded wind speed and direction, while a fibre optic array at one along-slope and two-across slope transects, and two vertical sections complemented the set-up. Additional observations during intense observational periods included temperature profile measurements using a drone and wind speed and temperature observations using tethered balloon at the top of the slope.

Here we present the measurement campaign design and focus on the first results that highlight the spatio-temporal variability of the flow on the slope, tightly coupled with the synoptic forcing. During conditions of low synoptic forcing, persistent katabatic flows developed on the slope with acceleration down the slope and warmer conditions towards one side of this non-uniform slope. On the other hand, during foehn conditions, very large differences in the mean and turbulence characteristics can be observed between the upper across-slope transect and lower stations that are more exposed to foehn. These differences translate to distinct behaviour of similarity scaling relations, as well as the importance of different terms in the momentum and TKE budgets.