HOTSPOT

HOrizonTal Shear Production Over complex Terrain – Uncrewed Aircraft Measurements of Turbulence for Model Parameterization

01/06/2025 to 31/05/2028

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 programme. 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 uniqueopportunity 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.