The Impact of Three-Dimensional Effects on the Simulation of Turbulence Kinetic Energy in a Major Alpine Valley

Abstract

The correct simulation of the atmospheric boundary layer (ABL) is crucial for reliable weather forecasts in truly complex terrain. However, common assumptions for model parametrizations are only valid for horizontally homogeneous and flat terrain. Here, we evaluate the turbulence parametrization of the numerical weather prediction model COSMO with a horizontal grid spacing of for the Inn Valley, Austria. The long-term, high-resolution turbulence measurements of the i-Box measurement sites provide a useful data pool of the ABL structure in the valley and on slopes. We focus on days and nights when ABL processes dominate and a thermally-driven circulation is present. Simulations are performed for case studies with both a one-dimensional turbulence parametrization, which only considers the vertical turbulent exchange, and a hybrid turbulence parametrization, also including horizontal shear production and advection in the budget of turbulence kinetic energy (TKE). We find a general underestimation of TKE by the model with the one-dimensional turbulence parametrization. In the simulations with the hybrid turbulence parametrization, the modelled TKE has a more realistic structure, especially in situations when the TKE production is dominated by shear related to the afternoon up-valley flow, and during nights, when a stable ABL is present. The model performance also improves for stations on the slopes. An estimation of the horizontal shear production from the observation network suggests that three-dimensional effects are a relevant part of TKE production in the valley.

Influence of a valley exit jet on the nocturnal atmospheric boundary layer at the foothills of the Pyrenees

“…As a result, these thermally driven flows strongly influence the evolution of the atmospheric boundary layer at the foothill areas, departing from the idealized behaviour observed over flat regions (Goger et al ; Serafin et al ). Wind speed and direction have a clear diurnal cycle due to the influence of the thermally driven winds, with winds veering from an upslope to a downslope direction close to sunset, depending on the site (Banta et al ; Chrust et al ; Martínez‐Villagrasa et al ; Jiménez and Cuxart, ; Giovannini et al ).…”

“…The model experiences some difficulties in reproducing the vertical temperature gradient close to the surface (Figure ). This is a well‐known limitation of mesoscale models (Jiménez et al ), and it is related not only to the coarse vertical resolution but also to the poor representation of the physical processes that take place in the surface layer under stably stratified conditions (Zhong and Fast, ; Jiménez and Cuxart, ), especially in complex terrain regions (Goger et al ; Serafin et al ). At higher levels, above the surface temperature inversion the model results are closer to the observations than at lower levels, suggesting that the model limitations are more linked to the parameterizations (surface model and turbulent scheme) than to the model resolution (Zhong and Fast, ).…”

Turbulence kinetic energy budget in the stable boundary layer over a heterogeneous surface

“…Rotach et al ). On the other hand, the presence of a (tall) plant canopy modifies the boundary layer (BL) near the surface in a specific way through aerodynamic drag imposed on the flow and by turbulent motions created in wakes of a plant element, thus adding further terms to the balance of TKE (Raupach and Shaw, ; Dwyer et al ; Goger et al ).…”

Evaluation of the turbulence parametrization in the MOLOCH meteorological model

“…(). The turbulence closure in COSMO (COnsortium for Small‐Scale MOdelling: Baldauf et al ., ) mesoscale model also has been tested in the frame of the GABLS programme (Buzzi et al ., ) and recently in a study in complex terrain for the Inn Valley in Austria (Goger et al ., ). An investigation of the budget terms and production mechanism of turbulent kinetic energy in complex terrain was performed by Weigel et al .…”

“…The prediction of turbulent kinetic energy in the operational configuration of the Fifth-Generation Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model, (MM5: Dudhia, 1993) and WRF models, for their use in dispersion modelling suites, has been evaluated in Hanna et al (2010). The turbulence closure in COSMO (COnsortium for Small-Scale MOdelling: Baldauf et al, 2011) mesoscale model also has been tested in the frame of the GABLS programme and recently in a study in complex terrain for the Inn Valley in Austria (Goger et al, 2018). An investigation of the budget terms and production mechanism of turbulent kinetic energy in complex terrain was performed by Weigel et al (2007) with the Advanced Regional Prediction System model (ARPS: Xue et al, 2000), based on observations from the Mesoscale Alpine Programme (MAP)-Riviera Project (Rotach et al, 2004).…”