The atmospheric boundary layer (ABL) directly influences human life through weather and air quality. Much remains to be understood about this thin atmospheric layer, partly because it is strongly affected by variations in topography and land-surface characteristics. These variations can lead to thermally-forced flow such as valley winds under calm conditions and atmospheric rotors and lee waves under strong winds with topographic blocking. Understanding the physical dynamics of such flow features requires detailed field observations and high-resolution numerical simulations. Large-eddy simulation (LES) is one of the most promising numerical techniques for modeling ABL flow because it allows control of turbulent length scales through spatial filtering to separate large, resolved motions from subfilter-scale, turbulent motions. Although primarily used for simulations of idealized flow conditions, LES has the potential to become universally applicable to ABL studies over complex terrain.
Intellectual merit: This research will investigate key, innovative steps to extend the scalability of LES so it can be used from regional to very fine scales. This will allow LES to be effectively applied to flow over complex terrain and lead to greater insight into boundary layer flow processes. New strategies to control turbulent length scales, improve lateral boundary forcing, generate ensemble simulations, and reduce numerical errors due to subgrid surface roughness parameterizations and representation of steep topography will create a universal framework for general LES applications over complex terrain. This framework will be used together with observational data analysis to study terrain-induced flow features such as mountain waves and rotors and valley wind circulations. The PI has extensive prior experience with boundary layer flows over steep, mountainous terrain as well as with developing numerical methods and turbulence closure schemes for LES. An integrated education and outreach program is proposed to excite K-12 students and the general public about atmospheric boundary layer phenomena through an interactive modeling website and to provide numerical modeling experience to both undergraduate and graduate students. Students will participate in this public outreach program with UC Berkeley's Lawrence Hall of Science (LHS) through summer assistantships and a new graduate course on large-eddy simulation that will be directly coupled to the online exhibit. This course will also accelerate graduate research progress by providing hands-on training in numerical methods.
Broader impacts: The proposed universal framework for LES will make possible seamless integration of flow features at all scales through nested simulations from the mesoscale to the urban scale. New knowledge of complex flows gained with this new numerical framework will dramatically improve models used to predict weather, air pollution, contaminant dispersion, and regional climate. Furthermore, insights into atmospheric circulations and intense gravity waves that can occur in mountainous areas will have important implications for aviation safety. New computational methods implemented in an advanced research numerical model will reach a broad community of users through continued public releases of code updates. Two graduate students will be trained through this research and two undergraduate research assistantships will be provided for under-represented minority engineering students through the SUPERB program at UC Berkeley. Research results will be disseminated through journal publications and conference presentations. The availability of the proposed educational tools will be advertised through the main Lawrence Hall of Science (LHS) website, through the PI's lab and department websites, and through an article in the Bulletin of the American Meteorological Society. The interactive modeling website has the potential to reach thousands of individuals and can be used in classroom settings at all age levels to introduce students to weather processes and numerical modeling. Hundreds of additional visitors, primarily K-12 students, will attend LHS museum floor demonstrations given by the PI and students.
The atmospheric boundary layer (ABL) directly influences the lives of the entire world population through weather and air quality. Much remains to be understood about this thin atmospheric layer found directly above the earth’s surface, partly because it is so strongly affected by topography and land-surface forcing, such as changes in surface elevation, land cover type, and soil and vegetation properties. Our current knowledge of ABL flow is based primarily on studies performed over simple, flat terrain. Only recently have theoretical, field, and numerical studies begun to seriously consider the challenges of complex and heterogeneous terrain surfaces. These new studies provide insight into the influence of terrain, but are still isolated examples, and conclusions are often limited to the specific study site. This project developed new computational techniques to enable ABL studies over very steep terrain. We expanded the applicability of high-resolution modeling (called large-eddy simulation) to flow over complex terrain over a broad range of scales. This work investigates ABL behavior under the most complex conditions, considering the effects of steep terrain features and heterogeneous land surfaces, such as urban areas and the Sierra-Nevada mountains. The figure shows an isosurface (a surface with points of constant value) of potential temperature in Owens Valley, California, where cold air (blue) appears to be pouring in like a waterfall over the Inyo mountains on the right side of the valley. Internal waves are visible on this isosurface. Results from these simulations help explain mechanisms of complex terrain flow features observed in the field. Such simulations can be used to improve predictions of weather and air pollution dispersion. The research components of this work were linked to an education and public outreach program designed to increase understanding of atmospheric boundary layer processes (weather and air pollution events) and numerical models. This resulted in a public website with content on ABL flows designed by UC Berkeley students: http://scienceview.lhs.berkeley.edu/ (click on upper right menu).
