Measuring and Modeling Interactions of the Turbulent Atmospheric Boundary Layer with Multiscale Ground Topology

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Flow and transport processes in the atmosphere are strongly influenced by orography that generates forces and causes complex flow distortions. At smaller scales, the atmospheric surface layer is also affected substantially by vegetation canopies. Most previous work has focused on effects of hills and vegetated terrain characterized by a single length scale, e.g. a single hill of a particular size, or canopies consisting of plants, often modeled using a prescribed leaf-area density distribution. It is well known, however, that flow obstructions such as mountain ranges and canopies are characterized by a wide range of length scales. Yet, it is not known how to parameterize the effects of such multi-scale objects on the lower atmospheric dynamics. This research addresses this issue with an integrated laboratory experimental and computational program, focusing on atmospheric boundary layer flow over fractal shapes. Fractals provide convenient idealizations of the inherently multi-scale character of mountain range and vegetation geometries, within certain ranges of scales. The experiments consist of laboratory model studies of flow structure and drag forces in an "optically index-matched" facility, where unobstructed, detailed flow and force measurements can be performed within the entire complex domain. Multi-plane particle image velocimetry measurements will provide all components of the stress and velocity gradient tensors. A key motivation for the experimental work is the need to validate and support further development and improvements of a new prediction tool, Renormalized Numerical Simulation (RNS). This technique models forces from unresolved features of the ground topology using drag coefficients determined from interrogation of the large scales that are explicitly resolved on the computational mesh, followed by dynamic rescaling. The RNS will be used to model the flow across fractal trees and mountain ranges, and compare the predicted forces and flow features with the measurements. With the detailed flow data available, causes for discrepancies will be identified and used for improvements.Research on improving scientific foundations of sub-grid parameterizations of land-atmosphere interactions, the subject of the project, has a broad impact on the infrastructure of atmospheric and climate sciences. Measurements using novel, optically index-matched, methods in the context of atmospheric flow phenomena provide the possibility of a quantum step in the level of detail with which flows can be mapped and understood. Moreover, the development of properly validated RNS applied to flow with fractal boundaries may yield broader impact in areas other than turbulent boundary layers over multi-scale ground topology. Fractals have been used as a descriptive tool in many disciplines, such as biology (branching blood network, pulmonary structures, corals), astrophysics (large-scale structure of the universe, intermittency of interplanetary magnetic fields), and other geosciences aspects (fractal coastlines, clouds). RNS extends the geometric idea of fractals to fluid dynamics. Educational impact of the work will focus on graduate education/training that stresses the interplay between physical experimentation and simulation. As part of the educational outreach effort, interactions with the Baltimore City School system will continue and strengthen. Specifically, senior high-school students from the Baltimore Polytechnic Institute will be involved in yearlong research experiences in our laboratory, as part of their required Research Practicum. Involvement in research on atmospheric flows over fractal boundaries will help motivate talented senior high-school students to consider future careers in this field.