HOTSPOT- HOrizonTal Shear Production Over complex Terrain - Uncrewed Aircraft Measurements of Turbulence for Numerical ModelParameterization

• Applicant: Dr. Andreas Platis
• Subject Area: Atmospheric Science
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 549349557

Turbulent exchange plays a crucial role for the transport of momentum, heat, moisture, and other atmospheric components between the earth’s surface and the atmosphere and within the atmospheric boundary layer. In operational numerical weather prediction models, turbulent motions are, however, not resolved because of too coarse grid spacing. The turbulent transport thus needs to be parameterized. These planetary boundary layer (PBL) turbulence parameterizations typically assume horizontally homogeneous conditions, neglecting the contribution from horizontal shear production in the prognostic equation for turbulence kinetic energy (TKE). Over complex terrain, several mountain-induced processes occur that invalidate this assumption of horizontal homogeneity. The objectives of HOTSPOT are to determine the magnitudes of horizontal wind shear and turbulence production due to horizontal wind shear in a valley cross-section during thermally driven flow conditions and to quantify the importance of horizontal shear production in PBL parameterizations for mesoscale simulations. Specific research questions will focus on the influence of the valley scale, stable versus convective conditions (up-and down valley winds), and the influence on the exchange of moisture and air pollutants. Measurements will be conducted with multiple uncrewed aircraft systems (UAS) in the Inn Valley, Austria, as part of the larger TEAMx program. The UAS flights are designed to capture the horizontal wind shear in the valley cross section and to allow the calculation of turbulent quantities, such as TKE and turbulent fluxes. The observations will be used to evaluate mesoscale and large-eddy WRF simulations. To determine the role of horizontal wind shear in the turbulence parameterizations, the horizontal shear production term will be added to the TKE equation of PBL parameterizations. Most of the existing turbulence data in complex terrain are limited to near-surface measurements. The UAS flights provide the unique opportunity to measure turbulence quantities above the surface layer at multiple locations and heights, thus allowing the evaluation of PBL parameterizations within the valley atmosphere. The model evaluation will focus on mesoscale simulations with a horizontal grid spacing on the order of a few hundred meters, which corresponds to the resolution of near-future operational regional forecast models. Andreas Platis will lead the project at the University of Tübingen and by Manuela Lehner at the University of Innsbruck. Andreas Platis has conducted UAS measurements in multiple field campaigns, measuring turbulent and non-turbulent transport of momentum, heat, gases, and particles in the boundary layer. Manuela Lehner has worked on modeling different boundary-layer processes over complex terrain, including thermally driven flows, nocturnal cold pools, turbulent exchange, and dynamically driven flows.

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partner: Dr. Manuela Lehner