Horizontal shear production over complex terrain

Turbulent motions play a crucial role in the transport of momentum, heat, moisture, and other atmospheric components between the earths surface and the atmosphere and within the atmospheric boundary layer, that is, the lowest layer of the atmosphere. Representing turbulent transport correctly in weather and climate models is thus critical for producing accurate forecasts. Current operational weather forecast models typically have a horizontal grid spacing on the order of 1 to 10 km. Small-scale processes, such as turbulent motions, thus cannot be resolved explicitly by the model and need to be parameterized, that is, represented by empirical relationships derived from observations. Turbulence in the atmospheric boundary layer is produced by wind shear and, during daytime, by buoyancy due to the heating of the surface. During nighttime, on the other hand, cooling of the surface and the resulting stable stratification of the atmosphere suppress turbulence. Several planetary boundary-layer (PBL) parameterizations have been developed that compute the turbulent transport in the boundary layer using different levels of simplification. One of the simplifications that these PBL parameterizations typically apply is to neglect turbulence production due to horizontal wind shear. While horizontal wind shear can be considered negligible compared to vertical wind shear over flat and homogeneous terrain, several processes occur in the atmosphere over mountainous terrain that may invalidate this assumption. For example, thermally driven valley winds are typically characterized by both horizontal and vertical wind shear with the highest wind speeds observed above the surface near the center of the valley. In HOTSPOT, we want to determine (i) the magnitude of horizontal wind shear in the cross section of an Alpine valley compared to vertical wind shear, (ii) whether a state-of-the-art weather model can reproduce the observed horizontal wind shear correctly, (iii) the role of horizontal wind shear for the total turbulence production, and (iv) the importance of including turbulence production from horizontal wind shear in the model for representing the exchange of, for example, moisture and air pollutants in the boundary layer. Measurements will be conducted with multiple uncrewed aircraft systems (UAS) in the Inn Valley, Austria. While most of the existing turbulence measurements in mountainous terrain are limited to near-surface observations, the UAS flights allow sampling of the flow field and turbulence at multiple locations and heights above the surface layer. The observations will be used to evaluate simulations with the Weather Research and Forecasting (WRF) model, using a horizontal grid spacing of a few hundred meters, which corresponds to the grid resolution of near-future operational regional weather forecast models.