STABLEST - Stable boundary layer separation and turbulence

The boundary layer is the region of the atmosphere closest to the Earth`s surface and is characterized by highly turbulent air motions. Wave-induced boundary-layer separation (BLS) is the process by which near-surface flow detaches from the ground as a consequence of adverse pressure gradients, owing their origin to internal gravity waves that are generated as the air passes over a mountain. The occurrence of BLS is often associated with vigorous perturbations of the atmospheric flow, such as wave breaking, and the generation of atmospheric rotors, which pose a serious threat to air and ground transportation on the lee side of mountains. The interaction of internal-gravity waves generated by airflow over orography with the flow within the atmospheric boundary layer has been pointed out as one of the major outstanding problems for the improvement of mountain weather forecasting. Furthermore, besides being a challenging geophysical fluid dynamics problem, wave-induced BLS also has relevant practical implications. Beyond the aforementioned safety of air and ground transportation, BLS can exert a significant impact on the efficiency of wind power production in mountainous areas by generating intense turbulence, which normally reduces the yield of wind turbines. In the STABLEST project, high-resolution numerical simulation is combined with the analysis of observational data to gain insight into the factors governing BLS. Particular attention is devoted to those aspects of BLS which are currently not well understood, in particular its possible non-stationarity, its dependence on the heat exchanges between the ground and the atmosphere and its evolution when occurring over obstacles with a complex three- dimensional shape. Numerical simulations will be performed using large-eddy simulation (LES), the most advanced approach nowadays available to model turbulent flows in the atmosphere. Plans include both idealized simulations, where the conditions leading to the onset of BLS are deliberately simplified in order to improve the understanding of the phenomenon, and real case studies. The latter will initially refer to events for which high-resolution observations are already available. The field observations to be analyzed during STABLEST include airborne in situ measurements and wind speed estimates from a Doppler cloud radar, which were collected during recent campaigns by a team at the University of Wyoming and are available to the STABLEST investigators by virtue of an existing partnership. Research efforts in this context are mostly directed towards the estimation of turbulence parameters from the radar data, in order to enable their comparison with the results of numerical simulations for verification purposes. A distinctive feature of the STABLEST project is the wide network of international cooperation that it builds upon. Research partners include the US National Center for Atmospheric Research, the consortium of north-American universities participating in the MATERHORN project, the University of Wyoming and the University of Innsbruck. The proposed research connections will lead to the acquisition and analysis of novel observational evidence of BLS, both with a Doppler lidar in a natural environment and with particle image velocimetry in a laboratory.

The project STABLEST provided new knowledge on mountain waves, which form in the air as the wind blows over and across hills or mountain ridges. In particular, the project led to new observational evidence of these waves during their breaking stage and to advances in their theoretical description with mathematical models. All of us have enjoyed first-hand experience of waves in natural fluids. Many relate the idea of a wave to water, and recall waves observed by the seaside or in a pond. The concept of a breaking wave is more complex, but it is intuitively known to everyone that has observed waves approaching a beach, getting higher and steeper as they come ashore. When breaking occurs, the energy borne by the orderly motion of the fluid in the wave is transferred to more chaotic fluctuations, that is, to turbulence. One of the many physical processes that can cause the initiation and propagation of waves in the atmosphere is the flow of air over mountains. Mountain waves have been the subject of decades of research, which highlighted their important role in the global atmospheric circulation and in the formation of clear-air turbulence, a potential hazard for aviation. A little-explored aspect of mountain waves is their effect on the boundary layer, that is, on the portion of the atmosphere nearest to the ground. One possible effect of mountain waves on the boundary layer is wave-induced separation. This is the core topic of STABLEST. In cases of wave-induced boundary-layer separation, the wind initially blows parallel to the Earths surface towards the trough of a mountain wave, but it is suddenly subject to a strong vertical acceleration that lifts it off the ground. The phenomenon can be associated with a breaking wave and is typically responsible for high levels of turbulence, as well as for the emergence of particular vortices in the air, called atmospheric rotors. Highlights from STABLEST include: 1) The first continuous two-dimensional characterization of turbulence intensity within breaking waves and atmospheric rotors, obtained by means of in-cloud radar observations; 2) Physical understanding, achieved through observational analysis and numerical modelling, of the mechanisms that cause fast horizontal shifts in the position of atmospheric rotors; 3) Theoretical advances on modelling the properties of mountain waves, establishing analogies with water waves and partially relying on the results of water tank experiments in a hydraulic laboratory. Research activities within STABLEST largely benefited from data exchange with collaborating scientists, whose work was supported by a number of additional funding agencies, including the European Commission (7th Framework Programme) and, in the USA, the National Science Foundation and the National Aeronautics and Space Administration.