Plateau du Golan | EPFL Graph Search

vignette|Éoliennes israéliennes sur le plateau du Golan annexé et le reste de la Syrie en arrière plan Le plateau du Golan (en arabe : الجولان, al-Jūlān, en hébreu : גולן, Golan), est un territoire syrien occupé depuis le 9 juin 1967, lors de la guerre des Six Jours, annexé par le gouvernement israélien de par la Loi du plateau du Golan votée par la Knesset (parlement) le 14 décembre 1981 et constitue un sous-district au sein du district Nord. L'annexion n'est pas reconnue par la communauté internationale, pour qui le plateau du Golan est un territoire syrien occupé. Cette annexion est considérée par la résolution n°497 du 17 décembre 1981 du Conseil de sécurité, comme étant . Cette loi israélienne est uniquement reconnue par les États-Unis depuis la déclaration du président Trump le 25 mars 2019. Depuis la conquête du territoire, le Golan a connu une évolution démographique importante ; de nombreux Israéliens se sont installés, lors de l'installation de colon

Investigation of snow melt dynamics and boundary layer processes over a melting snow surface

Sebastian Schlögl

In this thesis, we investigate the surface energy balance of a snow pack for a continuous and a patchy snow cover. Snow surface temperatures have been validated to assess the accuracy of the surface energy balance of a continuous snow pack for individual points. As the four radiation components contribute largest to the surface energy balance, accurate radiation measurements (typically only available for the shortwave radiation) are required for the model input. A model error (up to 5 K in snow surface temperatures) is introduced to the surface energy balance of a continuous snow pack by parametrizing the incoming longwave radiation, which is typically not measured. Turbulent heat fluxes are typically calculated in physics-based models with Monin-Obukhov bulk formulations and its parametrization contributes to model errors in the surface energy balance up to 2 K. The parametrization of the turbulent fluxes was found to be the largest error source in the case of measured incoming longwave radiation. Very stable atmospheric conditions over snow and a non-equilibrium boundary layer have a strong but very difficult to quantify influence on turbulent surface fluxes. Stability corrections were typically developed over non-snow surfaces and applied over snow in complex terrain, where several mandatory assumptions of the Monin-Obukhov bulk formulation are heavily violated. Therefore, our validation shows a much better model representation of the surface energy balance in idealized flat terrain in comparison with complex terrain. In this thesis, we develop new stability corrections over snow and assess the error of the Monin-Obukhov bulk formulation with 6 W m-2 and an additional error of 1-5 W m-2 due to state-of-the-art parametrizations of the stability correction. The energy balance of a snow pack significantly alters for heterogeneous land-surfaces in the late ablation period. Warm air from the bare ground can be efficiently transported over the snow patch and modifies the near-surface air temperature field. Terrestrial laser scanning measurements reveal that local snow ablation rates at the upwind edge of the snow patch are 25 % larger than further inside of the snow patch. The strong thermal contrast in surface temperatures in combination with calm wind conditions leads to the development of a stable internal boundary layer, which grows along the fetch and reduces turbulent mixing of warm air masses. Small-scale boundary layer dynamics are typically not resolved in hydrological models. Numerical simulations with the atmospheric model Advanced Regional Prediction System (ARPS) reveal a mean air temperature increase above the patchy snow cover of 2-5 K in comparison with a continuous snow cover, which could lead to an increase in daily mean snow ablation rates up to 30 %. Above-average snow ablation rates at the upwind edge of a snow patch could be resolved by the development of a temperature footprint approach. However, the effect of increasing near-surface mean air temperatures to snow ablation from lateral transport processes is small when considering an entire melting period. Uncertainties in measured precipitation, parametrized incoming longwave radiation and calculated turbulent fluxes with Monin-Obukhov bulk formulation lead to larger errors in modelled snow heights than neglecting lateral transport processes.

EPFL2018

The impact of variable building height on drag, flow and turbulence over a realistic suburban surface

Marc Calaf Bracons, Andreas Christen, Marco Giovanni Giometto, Marc Parlange

In realistic urban environments, where buildings experience variable heights, the tallest structures have a disproportional impact on drag, mean flow and turbulence. Although wind-engineering studies document well the effects of individual high-rise buildings on the immediate surrounding, the impact of varying building heights on the larger horizontally averaged flow has not been quantified systematically for realistic urban configurations. We use Large Eddy Simulation (LES) as a mean to study the fully developed turbulent flow over and within a 512 x 512m subset of truthful urban geometry in the city of Basel, Switzerland. A periodic LES domain is centered on the location of a tower, where measurements of turbulence were performed in 2001/02, which allows a direct validation of the LES at a specific location in the domain. The Lagrangian scale-dependent LES model is adopted to parametrize the subgrid stresses in the bulk of the flow and buildings are taken into account adopting a discrete-forcing-approach immersed boundary method (IBM), with the geometry taken from a highly accurate digital building model. A series of high-resolution LES runs are performed for various directions of the approaching flow, and with all buildings included and then buildings above a certain height threshold progressively removed, to isolate the impact of tall structures. Results show how the presence of isolated tall buildings strongly modifies the roughness properties of the entire urban roughness sublayer, causing an increase in resolved pressure forces, which contributes to several percentages of the overall surface induced drag. In presence of tall buildings the local structure of the roughness sublayer is partitioned in two regimes: fine scale wake turbulence and elongated, high speed streak-like motions, locked between the position of isolated structures, with their axis aligned in the streamwise direction. For arrays with variable building heights statistics significantly differ from standard surface layers’ ones and are strongly inhomogeneous in space. Dispersive momentum fluxes are shown to significantly increase with the presence of isolated tall buildings, mainly due to the spatial heterogeneity of the flow in the spanwise direction.

2014

Investigation of snow melt dynamics and boundary layer processes over a melting snow surface

Sebastian Schlögl

EPFL2018

The impact of variable building height on drag, flow and turbulence over a realistic suburban surface

Marc Calaf Bracons, Andreas Christen, Marco Giovanni Giometto, Marc Parlange

2014