Anabatic flows are common phenomena in the presence of sloping terrains, which significantly affect the dynamics and the exchange of mass and momentum in the low-atmosphere. Despite this, very few studies in the literature have tackled this topic. The present contribution addresses this gap by utilising high-resolved large-eddy simulations for investigating an anabatic flow in a simplified configuration, commonly used in laboratory experiments. The purpose is to analyse the complex thermo-fluid dynamics and the turbulent structures arising from the anabatic flow near the slope. In such a flow, three main dynamic layers are identified and reported: the conductive layer close to the surface, the convective layer where the most energetic motion develops, and the outer region, which is almost unperturbed. The analysis of instantaneous fields reveals the presence of thermal plumes, which are stable turbulent structures enhancing vertical transport and mixing of momentum and temperature. Such structures are generated by thermal instabilities in the conductive layer that trigger the rise of the plumes above them. Their evolution along the slope is described, identifying three regions responsible for the plumes generation, stabilisation, and merging. To the best of the authors’ knowledge, this is the first numerical experiment describing the along-slope behaviour of the thermal plumes in the convective layer.

<p>This contribution aims at presenting results from the project “Atmospheric boundary-layer modeling over complex terrain”, a collaboration between the University of Trento, the University of Bolzano and the University of Innsbruck with the objective to evaluate the performance of turbulence and land surface parameterizations over mountainous terrain and to identify potential issues that have a large impact on model results and consequently on the quality of weather forecasts.</p><p>A set of Reynolds-averaged Navier-Stokes (RANS) simulations at 1 km horizontal resolution is performed in an idealized three-dimensional valley-plain topography, using typical geometrical features of a north-south Alpine valley, with ridges up to 1500 m above the valley floor and a distance of 20 km from crest to crest. Simulations are initialized with a linear and stable vertical profile of potential temperature, dry air and an atmosphere at rest. The aim of the modeling experiment is to evaluate the sensitivity of model results to planetary boundary layer (PBL) parameterizations, exploring the performance of the PBL schemes implemented in the Weather Research and Forecasting (WRF) model, including a newly developed k-ε closure. Results from the RANS simulations are compared against a large-eddy simulation (LES) with a resolution of 100 m, which is taken as the benchmark. A full diurnal cycle has been considered for the evaluation of numerical results, focusing on the development of along- and cross-valley thermally-driven circulations and on the associated thermal field both in the nighttime and in the daytime phases. The sensitivity of model results to the change of the PBL scheme is assessed using as key metrics the strength and the timing of the thermally-driven circulations, as well as the vertical profiles of mean and turbulent quantities, when available. Results show that in most cases there is a good agreement between RANS simulations and the LES considering the main features of both along- and cross-valley circulations and the diurnal evolution of the thermal stratification. In particular, the intensity of the along-valley wind is generally well-reproduced by all the RANS simulations, while higher discrepancies are found for the timing of the evening transition. On the other hand, RANS simulations are in good agreement with the LES considering the timing of slope winds, whereas the simulation of their intensity presents much more variability, especially during nighttime.</p>

<p>Land surface models (LSMs), i.e. parameterization schemes for evaluating surface-atmosphere exchange implemented in meteorological models, usually prove inadequate over complex terrain,where orography strongly influences atmospheric processes and their interaction with the surface. In particular, LSMs use several parameters to suitably describe the surface and its interaction with the atmosphere, whose determination is often affected by many uncertainties. To this date, the sensitivity of meteorological model results to these parameters has not yet been studied systematically in complex terrain.The purpose of this work, which lies in the context of the TEAMx-related project ASTER, funded by the EGTC European Region Tyrol-South Tyrol-Trentino, is to evaluate the sensitivity of simulations with the Weather Research and Forecasting (WRF) meteorological model to variation of parameters describing land cover. Specifically, an idealized three-dimensional topography consisting of a valley-plain system is adopted and the analysis of the results focuses on the development of thermally-driven circulations. The analysis considers both the sensitivity to the type of vegetation cover and to the systematic variation of surface parameters based on typical values found in the literature. In particular, this analysis is carried out using the Global Sensitivity Analysis (GSA) methodology in order to quantify the uncertainty associated with the variation of each parameter evaluated and to estimate the optimal computational effort required for this type of study. The outcome of this analysis allows to evaluate which are the parameters that most influence model results and therefore should be estimated with particular attention in order to obtain reliable simulations over complex terrain.</p>